12 Oct
2008
12 Oct
'08
9:08 p.m.
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
as being from the "Commodore Pet Users Club of England - Newsletter
issues 1 & 2, page 9". It's the variant with three NOR gates ('LS02),
with a few resistors, capacitors, and a diode. The part that I'm
curious about is how much variation is allowed in the caps.
The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around. The output of that gate (which NORs the inverted
vertical sync with the massaged horizontal sync) feeds unto a "47mf"
tantalum cap (then pulled down to ground by a diode, and wire-ored to
inverted video out/composite out via a 470 Ohm resistor). I have 47uF caps,
but that seems pretty large to me.
I have a basic understanding of RS-170-type video, and I know this circuit
is probably going to produce a signal that "modern" IC-based video inputs
will not like (much the same way the classic RCA CDP1861 video behaves),
but fortunately, I have an old B&W security-type monitor to plug this into
(which has already been tested with a CDP1861).
I'm just curious why the caps are so large. I think I get what's going
on with this circuit (the way it combines the syncs and video), and I've
heard of similar circuits being used successfully by others, so with
luck, it'll work the first time.
Oh... in case anyone is wondering why I'm bothering, I have a couple of
naked PET boards here, no chasses, no CBM monitors. I did cobble up a
DC-based harness (+5, +12, -5V, no tape motor voltage, but then no tape
drive, either) for testing, and I do get "interesting" looking pips on
a 'scope when I check for video, so I'm hopeful that the boards are
working well enough to at least give me a screen full of _something_,
even if it's the classic "garbage" screen you get when the machine
isn't running well enough to zero video RAM.
So has anyone here made an external PET video adapter and have any
experiences to share about what worked and what might not have worked?
Thanks,
-ethan
P.S. - I found another type of composite circuit using a CD4011 as an
inverter and a CD4066 to mix the signals, but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular. One interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 03:50 Z
South Pole Station
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13 Oct
13 Oct
12:23 a.m.
On 13/10/2008 05:08, Ethan Dicks wrote:
The schematic shows the horizontal sync coupled to a
gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around. The output of that gate (which NORs the inverted
vertical sync with the massaged horizontal sync) feeds unto a "47mf"
tantalum cap (then pulled down to ground by a diode, and wire-ored to
inverted video out/composite out via a 470 Ohm resistor). I have 47uF caps,
but that seems pretty large to me.
I'm not familiar with that circuit, but I wonder if the scan or JPG has
made an 'n' look like an 'm' and turned 2200nf into 2200mf? 2.2mfd (ie
2.2uF) is not uncommon for a non-polarised cap, being used in some
telephone circuits. On the other hand, 47uF is more reasonable than
47nF for a tantalum.
--
Pete Peter Turnbull
Network Manager
University of York
1:47 a.m.
On Mon, Oct 13, 2008 at 08:23:07AM +0100, Pete Turnbull wrote:
On 13/10/2008 05:08, Ethan Dicks wrote:
I can't be certain, of course, but enlarging the scan to ludicrous size
seems to show a clearly-rendered central descending stroke on the 'm'...
it _really_ looks like 2200mf and 47mf.
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 08:40 Z
South Pole Station
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The schematic shows the horizontal sync coupled to
a gate via a "2200 mf"
non-polarized cap....
a "47mf" tantalum cap...
I'm not familiar with that circuit, but I wonder if the scan or JPG has
made an 'n' look like an 'm' and turned 2200nf into 2200mf? 2.2mfd (ie
2.2uF) is not uncommon for a non-polarised cap, being used in some
telephone circuits. On the other hand, 47uF is more reasonable than
47nF for a tantalum. 
12:38 a.m.
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
Do you have a URL for said scheamtic?
as being from the "Commodore Pet Users Club of
England - Newsletter
issues 1 & 2, page 9". It's the variant with three NOR gates ('LS02),
with a few resistors, capacitors, and a diode. The part that I'm
curious about is how much variation is allowed in the caps.
The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
IO think we can instantly eliminat 'mf' == millifarad (which is what it
should be!), since that's then 2.2F, which is rediculously large
2200uF (microfarads) is also IMHO too large for a non-polarised
capacitor, I've not seen non-polarised electrolytics that large, and
anyhting else would be essentially hopeless. 2200uF as an aluminium
electrolytic would be very possible.
2200nF (==2.2uF) would be possible, but it's a very odd way to write it.
2200pF (==2.2nF) is quite possible, for some reason some people
(particularly in the States) don't like the unit 'nF'. That would also
seem to be a suitable value for coupling a 15kHz signal.
large lying around. The output of that gate (which
NORs the inverted
vertical sync with the massaged horizontal sync) feeds unto a "47mf"
tantalum cap (then pulled down to ground by a diode, and wire-ored to
Well, tatalum caps tend to be in the range 0.47uF to 100uF (yes, they
exist outside that range, but they're less common). so 47uF would be
possible.
inverted video out/composite out via a 470 Ohm
resistor). I have 47uF caps,
but that seems pretty large to me.
I have a basic understanding of RS-170-type video, and I know this circuit
is probably going to produce a signal that "modern" IC-based video inputs
will not like (much the same way the classic RCA CDP1861 video behaves),
but fortunately, I have an old B&W security-type monitor to plug this into
(which has already been tested with a CDP1861).
I'm just curious why the caps are so large. I think I get what's going
Whenever I've wanted to combine syncs and video, I've used a circuit
rouhly based on the TRS-80 Model 1 video output and/or the IBM PC CGA
composite output. No capacitors involved. An XOR gate to combine syncs, a
couple of transistors and a resistor chain. Never had any problems with it.
-tony
1:57 p.m.
On Mon, Oct 13, 2008 at 08:38:30AM +0100, Tony Duell wrote:
I do not. I have a jpg file that I acquired some time ago from I know
not where.
Of course. I never thought it was millifarad.
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
Do you have a URL for said scheamtic? The schematic
shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
IO think we can instantly eliminat 'mf' == millifarad (which is what it
should be!), since that's then 2.2F, which is rediculously large 2200uF (microfarads) is also IMHO too large for a
non-polarised
capacitor, I've not seen non-polarised electrolytics that large, and
anyhting else would be essentially hopeless. 2200uF as an aluminium
electrolytic would be very possible.
Agreed.
2200nF (==2.2uF) would be possible, but it's a
very odd way to write it.
Yes. This was also written in the 1970s, so I'm trying historical
interpretation as well as what's obviously written.
2200pF (==2.2nF) is quite possible, for some reason
some people
(particularly in the States) don't like the unit 'nF'. That would also
seem to be a suitable value for coupling a 15kHz signal.
2200pF does sound reasonable. I am one of those who never grew up
using nF, so I don't tend to use it (I learned milli-microfarad from
my father for pF, he, in turn, was a Ham in the 1950s).
Well, tatalum caps tend to be in the range 0.47uF to
100uF (yes, they
exist outside that range, but they're less common). so 47uF would be
possible.
Possible, but it seems large.
Whenever I've wanted to combine syncs and video,
I've used a circuit
rouhly based on the TRS-80 Model 1 video output and/or the IBM PC CGA
composite output. No capacitors involved. An XOR gate to combine syncs, a
couple of transistors and a resistor chain. Never had any problems with it.
This is probably a transistor-less version of a similar circuit. It has
NORs rather than XORs (though I've seen that one) and a resistor chain,
but no transistor.
I don't know why there's a cap and a pull-down diode between the sync-combiner
and the sync-and-video combiner gates, but it's there. I was guessing the
cap was on the horizontal circuit, along with a pull-down resistor, to act
as an RC delay of some sort to allow one to tweak the horizontal sync slightly
to match up with whatever external monitor one is using, but I could be
wrong about that.
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 20:50 Z
South Pole Station
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Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
2:55 p.m.
On 13/10/2008 21:57, ethan.dicks at usap.gov wrote:
2200pF does sound reasonable. I am one of those who
never grew up
using nF, so I don't tend to use it (I learned milli-microfarad from
my father for pF, he, in turn, was a Ham in the 1950s).
I hope you didn't, because it would be wrong :-) One milli-microfarad
is one nanofarad; I think you meant to write micro-microfarad, or more
likely you meant to write nF instead of pF.
--
Pete Peter Turnbull
Network Manager
University of York
3:11 p.m.
On Mon, Oct 13, 2008 at 10:55:52PM +0100, Pete Turnbull wrote:
On 13/10/2008 21:57, ethan.dicks at usap.gov wrote:
Ah, yes... micro-microfarad. My bad. I hadn't heard the term since I
was about 10 and just mis-remembered it. I have seen old (1970s)
construction articles in "Popular Electronics", etc., refer to "mmf"
on occasion - presumably micro-microfarad.
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 22:00 Z
South Pole Station
PSC 468 Box 400 Temp -55.7 F (-48.7 C) Windchill -84.0 F (-64.4 C)
APO AP 96598 Wind 8.1 kts Grid 39 Barometer 667.9 mb (11088 ft)
Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
2200pF does sound reasonable. I am one of those
who never grew up
using nF, so I don't tend to use it (I learned milli-microfarad from
my father for pF, he, in turn, was a Ham in the 1950s).
I hope you didn't, because it would be wrong :-) One milli-microfarad
is one nanofarad; I think you meant to write micro-microfarad, or more
likely you meant to write nF instead of pF. 
14 Oct
14 Oct
1:47 a.m.
On Mon, 13 Oct 2008, Ethan Dicks wrote:
On Mon, Oct 13, 2008 at 10:55:52PM +0100, Pete
Turnbull wrote:
An analogy for you Americans comes to mind, as Obelix's "Ils sont fous,
ces romains!" ;-))
Christian
On 13/10/2008 21:57, ethan.dicks at usap.gov
wrote:
Ah, yes... micro-microfarad. My bad. I hadn't heard the term since I
was about 10 and just mis-remembered it. I have seen old (1970s)
construction articles in "Popular Electronics", etc., refer to "mmf"
on occasion - presumably micro-microfarad. 2200pF does sound reasonable. I am one of those
who never grew up
using nF, so I don't tend to use it (I learned milli-microfarad from
my father for pF, he, in turn, was a Ham in the 1950s).
I hope you didn't, because it would be wrong :-) One milli-microfarad
is one nanofarad; I think you meant to write micro-microfarad, or more
likely you meant to write nF instead of pF.
11:07 a.m.
The schematic shows the horizontal sync coupled to a
gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
IO think we can instantly eliminat 'mf' == millifarad (which is what it
should be!), since that's then 2.2F, which is rediculously large 2200nF
(==2.2uF) would be possible, but it's a very odd way to write it.
Yes. This was also written in the 1970s, so I'm trying historical
interpretation as well as what's obviously written. 2200pF (==2.2nF) is quite possible, for some
reason some people
(particularly in the States) don't like the unit 'nF'. That would also
seem to be a suitable value for coupling a 15kHz signal.
2200pF does sound reasonable. I am one of those who never grew up
using nF, so I don't tend to use it (I learned milli-microfarad from
my father for pF, he, in turn, was a Ham in the 1950s). 
1:39 p.m.
Tony Duell wrote:
Incidentally, I saw a 2.7kF (no, not a typo!)
low-voltage electrolytic in
a catalogue recently. And to think that when I was at school the physics
'teacher' told us that a 1F capacitor would be 'larger than this room'
and that we'd never seen on. About a week later I dropped a square plasic
object on his deck -- a 1F capacitor that I'd bought from Farnell or
somewhere.
I remember having a similar conversation with my 'O' Grade Physics
teacher in high school, about how a 1F capacitor would be huge and
impractical. "Hang on a sec", I thought, "I've got 1000uF capacitors
and they're about the size of my thumb. My Dad's got 10000uF capacitors
for his audio amps, and they're the size of a coke can. I reckon 100
such capacitors would fit in my schoolbag, at worst..."
And that's how I learned that school wasn't going to teach me anything
useful, or basically true.
Gordon
1:53 p.m.
Gordon JC Pearce MM3YEQ wrote:
I remember having a similar conversation with my 'O' Grade Physics
teacher in high school, about how a 1F capacitor would be huge and
impractical. "Hang on a sec", I thought, "I've got 1000uF capacitors
and they're about the size of my thumb. My Dad's got 10000uF
capacitors for his audio amps, and they're the size of a coke can. I
reckon 100 such capacitors would fit in my schoolbag, at worst..."
And that's how I learned that school wasn't going to teach me anything
useful, or basically true.
All charged up for the the teacher to hold... ;)
All
Gordon
1:56 p.m.
My Dad's got 10000uF capacitors for his audio
amps, and they're the
size of a coke can. I reckon 100 such capacitors would fit in my
schoolbag, at worst..."
That's a big schoolbag. I regularly carry five dozen 341-ml aluminum
cans in my backpack (they're iced tea cans, not pop cans, but they're
standard pop-can size), and it's a big backpack and they fill it right
full up. 100 would be five-thirds as much, more like a sports
equipment bag.
And, of course, it depend heavily on the working voltage. a 1Fd 5V cap
is generally smaller than a 1Fd 50V cap. (And, now that supercaps have
shown up, low-voltage caps have gotten even smaller.)
And that's how I learned that school wasn't
going to teach me
anything useful, or basically true.
Well, you'd already learned to sanity-check claims you hear, which is
arguably a more valuable lesson. :-)
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15 Oct
15 Oct
2:38 a.m.
der Mouse wrote:
That's about right. I used to carry a lot of fairly random stuff about...
My Dad's
got 10000uF capacitors for his audio amps, and they're the
size of a coke can. I reckon 100 such capacitors would fit in my
schoolbag, at worst..."
That's a big schoolbag. I regularly carry five dozen 341-ml aluminum
cans in my backpack (they're iced tea cans, not pop cans, but they're
standard pop-can size), and it's a big backpack and they fill it right
full up. 100 would be five-thirds as much, more like a sports
equipment bag. And, of course, it depend heavily on the working
voltage. a 1Fd 5V cap
is generally smaller than a 1Fd 50V cap. (And, now that supercaps have
shown up, low-voltage caps have gotten even smaller.)
Now *that* is true...
Gordon
And that's how I learned that school
wasn't going to teach me
anything useful, or basically true.
Well, you'd already learned to sanity-check claims you hear, which is
arguably a more valuable lesson. :-) 
14 Oct
14 Oct
3 p.m.
On 14 Oct 2008 at 21:39, Gordon JC Pearce MM3YEQ wrote:
I remember having a similar conversation with my
'O' Grade Physics
teacher in high school, about how a 1F capacitor would be huge and
impractical. "Hang on a sec", I thought, "I've got 1000uF capacitors
and they're about the size of my thumb. My Dad's got 10000uF capacitors
for his audio amps, and they're the size of a coke can. I reckon 100
such capacitors would fit in my schoolbag, at worst..."
Some schoolbag that holds most of 17 six-packs of Coke!
To be fair to your teacher, some capacitor banks are very large. I
do not know the working voltage or capacitance of the following:
http://www.rheinmetall.de/index.php?fid=1805&lang=3
Cheers,
Chuck
15 Oct
15 Oct
10:39 a.m.
And that's how I learned that school wasn't
going to teach me anything
useful, or basically true.
Ah,m another one. I learnt _absolutely nothing_ at school. Fortunately
I'd already taught myself the things I thought I'd find useful.
Everything they tried to teach me was either woefully incomplete or so
wrong as to be laughable.
I rememebr having a long arguement with a physics teacher who insisted a
bridge circuit couldn't be used to measure capacitance, but only to
compare capacitors. I am strill trying to find a definition of
'measurement' which does not involve comparison to a standard.
-tony
4:42 p.m.
I rememebr having a long arguement with a physics
teacher who
insisted a bridge circuit couldn't be used to measure capacitance,
but only to compare capacitors. I am strill trying to find a
definition of 'measurement' which does not involve comparison to a
standard.
Yes, but that standard does not have to be another of the type of thing
being measured.
In the case of capacitance, I think it's fair to draw a distinctino
between measuring capacitance in the form of "this capacitance is 12.7
times that one" and measuring capacitance in the form of, say, "it
takes 137ms for a 10mA current to charge this capacitance up to 50mV"
or "with a 50kohm resistor this capacitance exhibits an R-C time
constant of 27ms".
You will not, however, catch me defending said teacher's insistance
that "measure" is an inappropriate verb for one of those. :-)
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16 Oct
16 Oct
12:34 p.m.
I rememebr having a long arguement with a physics
teacher who
insisted a bridge circuit couldn't be used to measure capacitance,
but only to compare capacitors. I am strill trying to find a
definition of 'measurement' which does not involve comparison to a
standard.
Yes, but that standard does not have to be another of the type of thing
being measured. In the case of capacitance, I think it's fair to
draw a distinctino
between measuring capacitance in the form of "this capacitance is 12.7
times that one" and measuring capacitance in the form of, say, "it
takes 137ms for a 10mA current to charge this capacitance up to 50mV"
or "with a 50kohm resistor this capacitance exhibits an R-C time
constant of 27ms".
I would agree...
However, M.G.Scroggie's 'Radio Laboratory Handbook' makes the probably
valid point that in general it is more accurate to measure something
against a standard of the same time (there are fewer 'conversions' to
different physical quantities, each of which may not be exact). Said book
(of course) treats assorted bridge circuits as methods of measuring
'circuit constants'.
As does the _excellent_ 'Measurements in Radio Engineering' by Professor
Terman (I am sure everyone here knows who he is. Now, I am sure there are
people who know more about electrical measurements than Professor Terman,
but I don't think such people are exactly common, and I am darn sure said
physics teacher wasn't one of them.
-tony
3:03 p.m.
bfranchuk at jetnet.ab.ca wrote:
Antonio Carlini wrote:
What? That answer is nonsense.
Voltage and energy are two very different things, as you must know.
The correct answer can be found by using the equation E=mc^2. How
massive is this 2.7kF capacitor? :-)
Tony Duell wrote:
I have no idea , but 2 volts feels right.
Incidentally, I saw a 2.7kF (no, not a typo!)
low-voltage
Don't these potentially "self-charge" to some degree?
Just how much energy can one of these hold (what is its
rated voltage?)
Antonio
3:34 p.m.
Jim Battle wrote:
bfranchuk at jetnet.ab.ca wrote:
What? That answer is nonsense.
The answer wasn't stated so it cannot be nonsense.
Or maybe, like the cat in the box, it both is and isn't
until you look :-)
Voltage and energy are two very different things, as
you must know.
The correct answer can be found by using the equation
E=mc^2. How
massive is this 2.7kF capacitor? :-)
As long as the capacitor isn't moving or changing shape or
composition[*],
then you might find 0.5*C*V*V to be the energy.
C was given, I was looking for a possible V.
Maybe we're drifting too far off topic. For how long could
such a capacitor, fully charged, keep a ZX80 running, assuming
a 100% efficient inverter?
Antonio
[*] ISTR a problem stated in an old issue of an IEE magazine:
Take an oil filled jar used as a capacitor of capacitance C.
Charge it to voltage V. Disconnect the voltage source. Drain the
oil into an identical capacitor (initially uncharged). Describe
the final disposition of charges, voltages, energy, anything else
that takes your fancy.
4:03 p.m.
[*] ISTR a problem stated in an old issue of an IEE
magazine:
Take an oil filled jar used as a capacitor of
capacitance C. Charge
it to voltage V. Disconnect the voltage source. Drain the oil into an
identical capacitor (initially uncharged). Describe the final
disposition of charges, voltages, energy, anything else that takes
your fancy.
Ouch. That makes the electrostatics part of my brain hurt. :) I'm
reminded of the time I asked someone "energy has mass, right? so why
don't batteries get heavier when you charge them?".
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4:17 p.m.
New subject: Supercaps (was PET composite video adapter)
Hi,
I have designed products using 150F 2.7 volts caps, and like a lot of these
things the devil is in the details. The supercaps take a while to charge --
we limit the current to 2 amps. The charge is not like most capacitors --
they have a sort of dielectric soak effect, where charge doesn't reach the
remote areas for a while -- maybe 10's of minutes. We use them down to
0.5volts, and would like to go lower, but it's hard work.
Simply turn joules into watts for x seconds to work out the energy
available. In practice we get 300 joules out of the 550 stored in the above
capacitor -- the rest is lost to self discharge, the bit below 0.5volts,
convertor losses when the voltage gets low, convertor losses stepping up to
3.3 volts etc. The normal discharge and charge curves are quite different
on these supercaps -- due to all this hidden energy.
From memory the simple formulas were JoulesNeeded =
(VoltageWorking +
VoltageMinimum)/2 * LoadCurrent * Time. In my case
(2.7+0.5)/2*0.05amps*1800seconds = 144 joules needed.
Then Capacitance = 2 * VoltageWorking * LoadCurrent * Time /
(VoltageWorking^2 - VoltageMinimum^2), in this case 41 Farads.
Factor in a whole lot of losses and I needed 150 - 170 Farads to give 50mA
for 30 minutes.
The really neat trick would be to have put two in series -- CV^2 helps here,
because (obviously) the capacitance halves, but the voltage squares. I
played around for ages trying to balance the charge on the two caps --
isolated chargers -- the whole bit, but never got anything to work whose
self discharge current wasn't more than the gain from having 5.4 volts.
Maybe we're drifting too far off topic. For how
long could
such a capacitor, fully charged, keep a ZX80 running, assuming
a 100% efficient inverter?
Antonio
[*] ISTR a problem stated in an old issue of an IEE magazine:
Take an oil filled jar used as a capacitor of capacitance C.
Charge it to voltage V. Disconnect the voltage source. Drain the
oil into an identical capacitor (initially uncharged). Describe
the final disposition of charges, voltages, energy, anything else
that takes your fancy.
Intrigued by this -- any answers ?
Regards,
Gavin Melville.
3:46 p.m.
Just how much energy can one of these hold (what is
its rated
voltage?)
I have no idea , but 2 volts feels right.
15 Oct
15 Oct
10:50 a.m.
Just how much energy can one of these hold (what is
its rated
voltage?)
I have no idea , but 2 volts feels right. 
16 Oct
16 Oct
7:07 a.m.
Tony Duell schrieb:
nF was common in Europe in the 70's, much less common in the States, I think.
Please don't speak for Europe when talking about GB. In Germany, pF, nF,
and uF are still common nowadays, and everyone uses them where
appropriate. I had to learn about circuits (mostly from US magazines)
that contained rather uncommon values like 2200000pF or 0.000001uF which
would be better understandable when written with less zeros.
>>> The schematic shows the horizontal sync
coupled to a gate via a "2200 mf"
>>> non-polarized cap. I don't know that I have any disc or ceramic caps
that
>> IO think we can instantly eliminat 'mf' == millifarad (which is what it
>> should be!), since that's then 2.2F, which is rediculously large
> Of course. I never thought it was millifarad.
I think in this case it was just sloppyness (or even lack of
understanding, or in best case just a typo) of the circuit designer to
specify 2200mf.
Well, it's what it _should_ be. Mind you,
millifarad is not a
commonly-used unit for some unknown reason (I routinely see capacitors
marked 100000uF rather than 100mF or even 0.1F), although i have seen it
used in, I think, an HP manual
mF has been uncommon because with the exception of gold caps, you rarely
find capacitors with such large capacity, and in common circuits, you
won't need these sizes. For the traditional power supply, I have
seldomly used larger caps than 10000uF; if one needed more, it is more
likely the wrong design has been chosen.
Incidentally, I saw a 2.7kF (no, not a typo!)
low-voltage electrolytic in
a catalogue recently. And to think that when I was at school the physics
'teacher' told us that a 1F capacitor would be 'larger than this room'
and that we'd never seen on. About a week later I dropped a square plasic
object on his deck -- a 1F capacitor that I'd bought from Farnell or
somewhere.
Been there, having done and seen that, most teacher with pure physics
background have no knowledge of practical electronics, so no surprise.
2200nF (==2.2uF) would be possible, but it's a
very odd way to write it.
Yes. This was also written in the 1970s, so I'm
trying historical
interpretation as well as what's obviously written. Err, millimicrofarad = nanofarad. The picofarad is the
micromicrofarad.
I've seen uuF (where the 'u' is actually $\mu$) in old manuals.
Am I the only person to remember the millimicron as an equivalent for nm
when talking about optical wavelengths, etc?
Reminds me of describing speeds as angstrom per fortnight. Infact,
instead of nm, angstrom are frequently still used for wavelengths, but
SI units are supposed to ever have only one single prefix pico, nano,
micro, milli etc. and not combining them as in milli-micro-farad.
OTOH, it might be a selling argument for a CPU with 2 MilliPeta-Hz, and
surely a common distance is more challengingly written as 1000000000000
femtokilometers.
--
Holger
17 Oct
17 Oct
12:13 p.m.
Well, it's
what it _should_ be. Mind you, millifarad is not a
commonly-used unit for some unknown reason (I routinely see capacitors
marked 100000uF rather than 100mF or even 0.1F), although i have seen it
used in, I think, an HP manual
mF has been uncommon because with the exception of gold caps, you rarely
find capacitors with such large capacity, and in common circuits, you
won't need these sizes. For the traditional power supply, I have
seldomly used larger caps than 10000uF; if one needed more, it is more likely the wrong design has been chosen.
I've seen values up to around 100000uF in classic computer supplies.
2200nF
(==2.2uF) would be possible, but it's a very odd way to write it.
Yes. This
was also written in the 1970s, so I'm trying historical
interpretation as well as what's obviously written. Am I the only
person to remember the millimicron as an equivalent for nm
when talking about optical wavelengths, etc?
Reminds me of describing speeds as angstrom per fortnight. Infact, 
21 Oct
21 Oct
6:41 a.m.
10000uF = 10mF, of course. And 1000uF (a very common
value) = 1mF. But
for some reason they're rarely written that way.
I suspect that the abbreviation (outside the usual system of unit
symbols and prefixes) MFD for microfarad had already become so common
that nobody dared.
It occurs to me that the mF in the video circuit might have been a
mistranscribed uuF
Reminds me of
describing speeds as angstrom per fortnight. Infact,
'Furlongs per fortnight', please :-) Or, of course, the infamous 'attoparsec per
microfortnight', which is
close to the 'inch per second' and is thus useful for measuring tape
speeds :-)
I tend to normalise that with the multiplier prefix only in the
denominator, as is recommended for SI units, but no-one else seems to.
ISTR the conversion between ips and picoparsecs per fortnight is about
1.004, but I can't remember which way.
In similar vein, I tend to quote lengths of magnetic tape spools in stadia.
Philip.
13 Oct
13 Oct
2:29 p.m.
On Mon, Oct 13, 2008 at 08:38:30AM +0100, Tony Duell wrote:
Found it...
Under http://oldcomputers.net/pet2001.html
http://oldcomputers.net/pics/pet-video.jpg
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 21:20 Z
South Pole Station
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Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
Do you have a URL for said scheamtic? 
18 Oct
18 Oct
8:05 p.m.
On Monday 13 October 2008 17:29, Ethan Dicks wrote:
On Mon, Oct 13, 2008 at 08:38:30AM +0100, Tony Duell
wrote:
Found it...
Under http://oldcomputers.net/pet2001.html
http://oldcomputers.net/pics/pet-video.jpg
Oh my. I'd say that 2200pF is indeed the correct value for that cap...
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
Do you have a URL for said scheamtic?
13 Oct
13 Oct
1:21 a.m.
Ethan Dicks wrote:
I'm just curious why the caps are so large. I
think I get what's going
on with this circuit (the way it combines the syncs and video), and I've
heard of similar circuits being used successfully by others, so with
luck, it'll work the first time.
Because that's what the original "designer" had lying around? If
they're just coupling caps, they're probably not terribly critical.
Even if they're timing caps you probably won't need to be *too* picky.
Old CCTV monitors work just great even with really horrible video.
Gordon
9:46 a.m.
On 13 Oct 2008 at 4:08, Ethan Dicks wrote:
The schematic shows the horizontal sync coupled to a
gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around. The output of that gate (which NORs the inverted
vertical sync with the massaged horizontal sync) feeds unto a "47mf"
tantalum cap (then pulled down to ground by a diode, and wire-ored to
inverted video out/composite out via a 470 Ohm resistor). I have 47uF caps,
but that seems pretty large to me.
That 2200mf is off by at least 3 orders of magnitude. 0.22 uF is
probably more like it. The 47 uF might be okay, but I suspect that
4.7 uF may also work just fine. (But I don't have your circuit, so
it's really difficult to say for certain).
It's been a long time since I wired up one of these, but ISTR that I
didn't find component values to be very critical.
Cheers,
Chuck
11:48 a.m.
On Mon, Oct 13, 2008 at 09:46:01AM -0700, Chuck Guzis wrote:
On 13 Oct 2008 at 4:08, Ethan Dicks wrote:
I can entirely believe that. What I'm wondering is if the original
article should have read 2200pF.
The schematic shows the horizontal sync coupled
to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around. The output of that gate (which NORs the inverted
vertical sync with the massaged horizontal sync) feeds unto a "47mf"
tantalum cap (then pulled down to ground by a diode, and wire-ored to
inverted video out/composite out via a 470 Ohm resistor). I have 47uF caps,
but that seems pretty large to me.
That 2200mf is off by at least 3 orders of magnitude. 0.22 uF is
probably more like it. The 47 uF might be okay, but I suspect that
4.7 uF may also work just fine. (But I don't have your circuit, so
it's really difficult to say for certain).
Agreed.
It's been a long time since I wired up one of
these, but ISTR that I
didn't find component values to be very critical.
Glad to hear it. I'm going to build the first version on a proto board
so it will be easy to swap out caps.
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 18:40 Z
South Pole Station
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Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
12:21 p.m.
Ethan Dicks wrote:
P.S. - I found another type of composite circuit using
a CD4011 as an
inverter and a CD4066 to mix the signals,
Sounds like the circuit I have here from "The PET Revealed".
but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular.
It's not labeled on the diagram in the book, nor does the text say what
value it is.
One interesting variation with that one is that the
'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
The text in the book says "You may encounter problems with the
horizontal hold, but these can usually be cured by adjusting R1", and
goes on to speak of low quality monitors or converted TVs...
Cheers.
Phill.
--
Phill Harvey-Smith, Programmer, Hardware hacker, and general eccentric !
"You can twist perceptions, but reality won't budge" -- Rush.
12:26 p.m.
On Mon, Oct 13, 2008 at 08:21:11PM +0100, Phill Harvey-Smith wrote:
That would explain why my schematic doesn't have it, then.
Ah. Thanks much for that. I have plenty of small PCB pots... easy to
fit on in.
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 13-Oct-2008 at 19:20 Z
South Pole Station
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APO AP 96598 Wind 8.0 kts Grid 52 Barometer 668.4 mb (11069 ft)
Ethan.Dicks at usap.gov http://penguincentral.com/penguincentral.html
Ethan Dicks wrote:
That is very likely, though I probably found a copy of just the
diagram somewhere on the web. I have a copy of the book at home,
but I didn't bring it with me.
P.S. - I found another type of composite circuit
using a CD4011 as an
inverter and a CD4066 to mix the signals,
Sounds like the circuit I have here from "The PET Revealed". but for
some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular.
It's not labeled on the diagram in the book, nor does the text say what
value it is. One
interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
The text in the book says "You may encounter problems with the
horizontal hold, but these can usually be cured by adjusting R1", and
goes on to speak of low quality monitors or converted TVs...
1:25 p.m.
Ethan Dicks wrote:
P.S. - I found another type of composite circuit using
a CD4011 as an
inverter and a CD4066 to mix the signals, but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular. One interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
It's the 4011/4066 circuit that I recall. From (very rusty) memory:
The circuit as given in "The Pet Revealed" by Nick Hampshire doesn't
work, but I can't off-hand remember why not. Maybe he got two signal
names swapped.
You use a 4011 to combine the syncs, and maybe a second gate to invert
the composite sync. The output of this drives the control input of the
4066, which gates the video line from the PET to the monitor.
You may need a pull-dowm for when the 4066 is high impedance. I think
the only capacuitor (apart from on the supply lines to the chips) should
be on the composite output - the syncs that appear on the PET user port
are at logic levels; only the video output is at video levels.
I don't think any component values are critical as long as you don't try
and draw too much current. The pull-down should be large compared to
the on-resistance of the transmission gate, so several K ohms.
Before you ask, I do possess copies of TPR and circuit diagrams for the
PET. If you want me to find them, it may take some time to dig them
out, since I have no idea where they are.
HTH
Philip.
18 Oct
18 Oct
7:44 p.m.
On Monday 13 October 2008 00:08, Ethan Dicks wrote:
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
as being from the "Commodore Pet Users Club of England - Newsletter
issues 1 & 2, page 9". It's the variant with three NOR gates ('LS02),
with a few resistors, capacitors, and a diode. The part that I'm
curious about is how much variation is allowed in the caps.
The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around.
That value definitely doesn't sound right to me. Maybe 2200 nF?
The output of that gate (which NORs the inverted
vertical sync with the
massaged horizontal sync) feeds unto a "47mf" tantalum cap (then pulled down
to ground by a diode, and wire-ored to inverted video out/composite out via
a 470 Ohm resistor). I have 47uF caps, but that seems pretty large to me.
For a coupling cap? Maybe not. Particularly if that's coupling vertical
sync.
I have a basic understanding of RS-170-type video, and
I know this circuit
is probably going to produce a signal that "modern" IC-based video inputs
will not like (much the same way the classic RCA CDP1861 video behaves),
but fortunately, I have an old B&W security-type monitor to plug this into
(which has already been tested with a CDP1861).
I'm just curious why the caps are so large. I think I get what's going
on with this circuit (the way it combines the syncs and video), and I've
heard of similar circuits being used successfully by others, so with
luck, it'll work the first time.
With the right parts in there, yeah. The abbreviation "mf" is not common on
stuff coming out of Europe, it sounds more like early us-based tech, 1960s
or earlier, but that doesn't agree with the rest of what you say here about
the origins of the circuit.
(Snip)
P.S. - I found another type of composite circuit using
a CD4011 as an
inverter and a CD4066 to mix the signals, but for some reason I don't
have all the components labelled on my drawing - the cap on hsync in
particular. One interesting variation with that one is that the 'LS02
circuit has a 1.5K pulldown on hsync between the cap and the gate,
while the CD4011/CD4066 circuit has a 10K pot. How much adjustment on
hsync is likely to be needed between the PET and the first gate of the
composite circuit?
I suspect that pot might be to set black level?
It's really hard to say much more without looking at the schematic you're
looking at.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
8:05 p.m.
On Sat, Oct 18, 2008 at 10:44:42PM -0400, Roy J. Tellason wrote:
Having now built it, it seems to work, but I still think it's awfully large.
On Monday 13 October 2008 00:08, Ethan Dicks wrote:
As has been hashed over before, I'm more inclined to think some hand-drawn
original was marked 2200pF (and then poorly transcribed to a formal print
drawing). The graphic clearly shows "mf", though, not "nf" or
"nF".
In practice, though, since I didn't have anything quickly at hand, I
gave the circuit a try with no cap (based on Tony's comments about a
similar circuit for the TRS-80 that had no RC on hsync). I did get
recognizable video, but I couldn't get a perfect horizontal lock. The
screen slowly (at best) slid left or right. The text was perfect and
the pixels were perfectly aligned, at least. I'm beginning to think
(based on another circuit) that this cap and the attendant pull down
resistor are a simple delay circuit to allow one to phase-shift the
hsync relative to video-out, but perhaps that's an incomplete
understanding of it.
Hi, All,
I'm just finishing up assembling a PET "Video Mixer" from an old JPG
of a much older scan of said circuit. This particular one is annotated
as being from the "Commodore Pet Users Club of England - Newsletter
issues 1 & 2, page 9". It's the variant with three NOR gates ('LS02),
with a few resistors, capacitors, and a diode. The part that I'm
curious about is how much variation is allowed in the caps.
The schematic shows the horizontal sync coupled to a gate via a "2200 mf"
non-polarized cap. I don't know that I have any disc or ceramic caps that
large lying around.
That value definitely doesn't sound right to me. Maybe 2200 nF? The output of
that gate (which NORs the inverted vertical sync with the
massaged horizontal sync) feeds unto a "47mf" tantalum cap (then pulled down
to ground by a diode, and wire-ored to inverted video out/composite out via
a 470 Ohm resistor). I have 47uF caps, but that seems pretty large to me.
For a coupling cap? Maybe not. Particularly if that's coupling vertical
sync. > I have a basic understanding of RS-170-type
video, and I know this circuit
> is probably going to produce a signal that "modern" IC-based video inputs
> will not like (much the same way the classic RCA CDP1861 video behaves),
> but fortunately, I have an old B&W security-type monitor to plug this into
> (which has already been tested with a CDP1861).
To confirm - yes. This circuit produced video on a security monitor, but
a modern LCD wouldn't sync to it.
With the right parts in there, yeah. The
abbreviation "mf" is not common on
stuff coming out of Europe, it sounds more like early us-based tech, 1960s
or earlier, but that doesn't agree with the rest of what you say here about
the origins of the circuit.
It's clearly designed for the PET, so 1960s is right out, unless that
was the mindset and sematic context of the designer.
I suspect that pot might be to set black level?
I would doubt that, since the pot is long before anything that mixes
with video-out. Based on comments from another design, it's part of
a horizontal sync compensation circuit (along with that "2200 mf" cap,
I suspect).
It's really hard to say much more without looking
at the schematic you're
looking at.
I posted the URL in a followup once I rediscovered where I found it...
http://oldcomputers.net/pics/pet-video.jpg
As I've said - that circuit basically works, but I do get a slight
horizontal roll with the one monitor I have access to (and its
h.hold knob is quite twitchy - probably a dirty pot).
-ethan
--
Ethan Dicks, A-333-S Current South Pole Weather at 19-Oct-2008 at 02:50 Z
South Pole Station
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APO AP 96598 Wind 11.1 kts Grid 18 Barometer 679.8 mb (10636 ft)
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10:54 p.m.
On Saturday 18 October 2008 23:05, Ethan Dicks wrote:
Probably fussier about what it was being fed.
That "black level" comment was just a guess based on what I'd read, before
I
saw the diagram.
On Sat, Oct 18, 2008 at 10:44:42PM -0400, Roy J.
Tellason wrote:
> On Monday 13 October 2008 00:08, Ethan Dicks wrote:
> > Hi, All,
> >
> > I'm just finishing up assembling a PET "Video Mixer" from an old
JPG
> > of a much older scan of said circuit. This particular one is annotated
> > as being from the "Commodore Pet Users Club of England - Newsletter
> > issues 1 & 2, page 9". It's the variant with three NOR gates
('LS02),
> > with a few resistors, capacitors, and a diode. The part that I'm
> > curious about is how much variation is allowed in the caps.
> >
> > The schematic shows the horizontal sync coupled to a gate via a "2200
> > mf" non-polarized cap. I don't know that I have any disc or ceramic
> > caps that large lying around.
>
> That value definitely doesn't sound right to me. Maybe 2200 nF?
That should've said "2200pF" there...
As has been hashed over before, I'm more inclined
to think some hand-drawn
original was marked 2200pF (and then poorly transcribed to a formal print
drawing). The graphic clearly shows "mf", though, not "nf" or
"nF".
In practice, though, since I didn't have anything quickly at hand, I
gave the circuit a try with no cap (based on Tony's comments about a
similar circuit for the TRS-80 that had no RC on hsync). I did get
recognizable video, but I couldn't get a perfect horizontal lock. The
screen slowly (at best) slid left or right. The text was perfect and
the pixels were perfectly aligned, at least. I'm beginning to think
(based on another circuit) that this cap and the attendant pull down
resistor are a simple delay circuit to allow one to phase-shift the
hsync relative to video-out, but perhaps that's an incomplete
understanding of it.
Having now built it, it seems to work, but I still think it's awfully
large.
Yes, perhaps. Now that I've seen the thing maybe figuring some time
constants is in order. (But it's awfully late here at the moment. :-)
The
output of that gate (which NORs the inverted vertical sync with the
massaged horizontal sync) feeds unto a "47mf" tantalum cap (then pulled
down to ground by a diode, and wire-ored to inverted video
out/composite out via a 470 Ohm resistor). I have 47uF caps, but that
seems pretty large to me.
For a coupling cap? Maybe not. Particularly if that's coupling vertical
sync. > I have a
basic understanding of RS-170-type video, and I know this
> circuit is probably going to produce a signal that "modern" IC-based
> video inputs will not like (much the same way the classic RCA CDP1861
> video behaves), but fortunately, I have an old B&W security-type
> monitor to plug this into (which has already been tested with a
> CDP1861).
To confirm - yes. This circuit produced video on a security monitor, but
a modern LCD wouldn't sync to it. With the right
parts in there, yeah. The abbreviation "mf" is not
common on stuff coming out of Europe, it sounds more like early us-based
tech, 1960s or earlier, but that doesn't agree with the rest of what
you say here about the origins of the circuit.
It's clearly designed for the PET, so 1960s is right out, unless that
was the mindset and sematic context of the designer.
I suspect that pot might be to set black level?
I would doubt that, since the pot is long before anything that mixes
with video-out. Based on comments from another design, it's part of
a horizontal sync compensation circuit (along with that "2200 mf" cap,
I suspect).
It's really hard to say much more without
looking at the schematic you're
looking at.
I posted the URL in a followup once I rediscovered where I found it...
http://oldcomputers.net/pics/pet-video.jpg As I've said - that circuit basically works, but I
do get a slight
horizontal roll with the one monitor I have access to (and its
h.hold knob is quite twitchy - probably a dirty pot).
Or nowhere near enough horizontal sync amplitude.
No cap in there now? That'd give you some different waveform coming out,
probably.
--
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space, a critter that can
be killed but can't be tamed. --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James
M Dakin
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39 comments
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