27 Feb
2007
27 Feb
'07
11:35 p.m.
Some of you may reacll that, back in July, I wrote about an
oscilloscope whose power transformer failed in service. Well, today I
was going through my tuit collection and found some round ones. I've
now clipped the transformer free of the circuit and removed it.
I then applied an ohmmeter to the transformer, now that it's free of
the machine, which gave me good guesses at what winding is what. Then
I got out the current-limiting light bulb rig and hooked the primary up
(well, half the primary; it has a split primary, for 115/230 mains
switching). Then I measured the primary voltage and the voltages on
various interesting windings. Based on all of this, I think I have a
good guess what's what:
- Input mains: four wires, two windings, which are connected in
parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
on the 115V-nominal mains primary, this measured 176V, or ~778V when
running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes. (There are some
12V-heater tubes, but they all have centre-tapped heaters.)
- There is one more wire unaccounted for. On opening the transformer
case, I find it is a case ground.
The HV supply winding appears to be fried. I didn't bother with all
the filament windings (though I did measure the heater winding, which
appeared to be intact); the B+ supply winding seemed intact, but the HV
supply winding showed ~2V instead of the 200-300V I would expect with
~25V on the primary.
So now I'm thinking of finding a half-dozen different transformers to
run the six different pieces off of....
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
28 Feb
28 Feb
5:19 a.m.
- Input mains: four wires, two windings, which are
connected in
parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
on the 115V-nominal mains primary, this measured 176V, or ~778V when
running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes. (There are some
12V-heater tubes, but they all have centre-tapped heaters.)
- There is one more wire unaccounted for. On opening the transformer
case, I find it is a case ground.
They are probably old/collectable enough now that you wouldn't
want to part out a "good" one, but if you can find a dead/partial
one, old B&W console televisions often have nice transformers
with several 6.3v filament windows, a CT B+ winding, and often
several others thrown into the mix. Later model "portable" sets
with hot-chassis don't, but some of the old consoles do. Probably
won't give you everything you need, but you might get away with
only two xfmrs.
You won't get the HV winding, but you may be able to generate this
with a voltage multiplier - I built my first scope "from scratch",
and used an old TV transformer, and a tripler from a 600v winding
to get the 2kv needed for the CRT. It worked very well.
Dave
PS: I have a friend who's into restoration of old tube type gear,
I'll forward your posting to him on the off-chance that he might
have something similar, or an idea of where to find one.
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
5:53 a.m.
> - Input mains: four wires, two windings, which
are connected in
> parallel for 115V operation and series for 230V operation.
> - 5VAC filament winding for B+ rectifier tube.
> - ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
> on the 115V-nominal mains primary, this measured 176V, or ~778V when
> running normally.)
> - Filament winding for one HV rectifier tube.
> - Filament winding for the other HV rectifier tube.
> - HV supply winding.
> - 6.3VAC CT winding to run the heaters for most tubes. (There are some
> 12V-heater tubes, but they all have centre-tapped heaters.)
> - There is one more wire unaccounted for. On opening the transformer
> case, I find it is a case ground.
One further note - on a scale of 1-10, how authentic do you want the
repaired scope to be?
If the answer is 0-2, consider replacing the HV and B+ rectifiers with
solid state - this eliminates three filament windings - assuming you
generate the HV via another means as I suggested in my last posting,
all you need now is the main filiment windings and the CT B+ windings
which should not be all that hard to find in a single transformer.
If the answer is 10, you could consider rewinding the transformer -
I can provide some more information on this, but if you've never done
it, it's not all that much fun (it's not fun if you have done it before
either). Btw - I just did a google search on "transformer rewinding"
and there are a few people advertising that they do this... Might not
hurt to make a few inquiries.
Dave
--
dave06a (at) Dave Dunfield
dunfield (dot) Firmware development services & tools: www.dunfield.com
com Collector of vintage computing equipment:
http://www.classiccmp.org/dunfield/index.html
8:02 a.m.
One further note - on a scale of 1-10, how authentic
do you want the
repaired scope to be?
I'm a bit Tonyish on this: I'd take unauthentic but working over
authentic but broken any day. :-)
I'm mostly looking for a working 'scope cheap, basically. If that
means an external pod with a half-dozen transformers in it, well, so be
it. It's not as if I'd be making it impossible to rip that out and
install a single replacement if I manage to luck into one.
If the answer is 0-2, consider replacing the HV and B+
rectifiers
with solid state - this eliminates three filament windings - assuming
you generate the HV via another means as I suggested in my last
posting, all you need now is the main filiment windings and the CT B+
windings which should not be all that hard to find in a single
transformer.
I was thinking about something like this, yes. I haven't traced the
circuit enough to be certain, but I think that if I leave the rectifier
tubes out of their sockets, I should be able to drive the B+ and HV
lnies with pretty much anything that can supply the required voltages
and handle the current draw.
I was actually considering replacing most of the circuitry with
solid-state stuff, leaving just the last couple of tubes to drive the
deflection plates.
Btw - I just did a google search on "transformer
rewinding" and there
are a few people advertising that they do this... Might not hurt to
make a few inquiries.
That's something I've been considering too, finding someone who does
custom transformers. With proper equipment it should be pretty easy,
I'd think, but you sorta have to do it as a business to amortize the
cost of the gear over many jobs to make it worthwhile.
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
3:41 p.m.
One further note - on a scale of 1-10, how
authentic do you want the
repaired scope to be?
I'm a bit Tonyish on this: I'd take unauthentic but working over
authentic but broken any day. :-)
4:13 p.m.
One further note - on a scale of 1-10, how authentic
do you want
the repaired scope to be?
I'm a bit Tonyish on this: I'd take unauthentic
but working over
authentic but broken any day. :-) For example I probably wouldn't replace valve
rectifiers with
semiconductors.
I'm not sure I'd *replace* them, but I'm giving real thought to putting
in parallel power-supply circuitry that uses solid-state rectifiers and
feeds the B+ and HV power lines. But that depends on the correctness
of my guess that removing the rectifier tubes will be enough to isolate
the B+ and HV power so I can feed them externally.
I owuld consider re-winding the transformer (not
original) or useing
serveral separate transformers to replace it (even less original, but
at least the design is basically unchagned).
Yeah. Put it this way: if I were to somehow get an exact replacement
transformer, I'd like to be able to do a little unsoldering and
resoldering and have it in basically original condition. But this is
not inconsistent with using solid-state rectifiers, if my guess about
the rectifier tubes is correct.
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
4:27 p.m.
On 28 Feb 2007 at 23:41, Tony Duell wrote:
Well, I like things to work, but equally, I do try to
maintain the
original design. For example I probably wouldn't replace valve rectifiers
with semiconductors.
Assuming that the load characteristcs allowed it, I'd do it in a
heartbeat, particulary with noisy gas rectifiers (such as OZ4).
There used to be a small market for direct plug-in solid state
replacements for the more common ones, such as 80, 5U4, 5Y3, 6Z4,
etc. But the old auto-radio vibrator supplies really improved with
decreased noise when one replaced the old cold-cathode gas rectifier.
The filament for a 5U4 is what, 5 volts at 3 amps? That's 15W of
heat that you don't have to worry about.
I'd also replace old selenium rectifiers with silicon diodes. Many
years ago, IR offered cross-referenced silicon replacements for
selenium rectifiers. Seleniums are pretty stinky when they went
kablooie.
As far as anode supplies for CRTs, go, you may want to consider
several junkbox solutions other than voltage multipliers. You can
cobble up a small inverter with not much more than a transistor or
two and a small transformer. There are also packaged versions of the
same that can be found by scavenging old photocopiers. You can also
find HV supplies in air filters, bug zappers and a whole host of
other things.
Cheers,
Chuck
1 Mar
1 Mar
12:26 a.m.
I've just spent a couple of hours tracing out the power-supply
circuitry in this 'scope, particularly the HV supply driving many of
the CRT terminals.
Turns out the winding I thought was the HV winding, only shorted out,
is actually the CRT filament/heater winding. (I'm quite sure I've got
the CRT filament pins correct; they are the only two CRT pins that show
DC continuity.) I'm not sure whether it's a heater or filament;
there's a pin that could be the cathode (connected to one side of the
heater/filament, the most negative CRT pin found so far), but there is
an odd circuit which makes sense to me only as an attempt to compensate
for using one side of a filament as a cathode - there's a voltage
divider across one half of the 6.3V heater winding, with a HV cap
coupling it to the "cathode" side of the CRT filament/heater,
presumably to avoid the slight mains-frequency flicker that would
otherwise result.
The HV winding appears intact, and runs between one side of the B+
winding and the filament of one of the HV rectifiers;
===================
_oooo_oooo_oooo_oooo_
A B C D E
A-B-C is the centre-tapped B+ winding I found before. B is grounded
(the A and C points go to the plates of the B+ rectifier) and, based on
scaling up from the voltages I see, the A-B (and B-C) voltage is about
375VAC. D-E is the filament winding for one of the HV rectifiers. C-D
is a winding I was not previously aware existed; its design voltage is
about 500VAC (under the same assumptions about scaling).
The transformer's ground wire has DC connection to this winding, about
150 ohms to either D or E and something like twice that to C. Since
both this ground wire and B were grounded to the chassis when the
transformer was in-circuit, I conjecture that this is part of the
fault, and the ground wire is supposed to be isolated from all the
windings.
Interestingly, in view of the comments about how instead of a high
positive final anode voltage CRTs tend to run with a high negative
cathode voltage, it looks as though this does both: the final anode is
about as positive with respect to chassis ground as the filament is
negative.
I think I now understand all the power-transformer windings:
============================================================
_ooo_ooo_ooo_ooo_ _ooo_ _ooo_ _ooo_ _ooo_ooo_ _ooo_ _ooo_
A B C D E F G H J K L M N P Q R S T
A-B-C-D-E is as above. F-G, H-J, and K-L are the filament windings for
the B+ rectifier, the CRT, and the other HV rectifier. M-N-P is the
heater winding for most of the tubes. Q-R and S-T are the incoming
mains power windings (series for 230V, parallel for 115V). Add in the
chassis ground wire and this accounts for all the power transformer
wires.
I also suspect there is some effect operating to make my voltage
estimates come out high. The unregulated B+ supply (before the
resistor and 0B2) has a smoothing cap of 80uF 475V. This means the B+
winding, end-to-CT (A-B or C-B above) can't be higher than 335VAC,
probably less to allow some headroom, while my estimate put it at
nearly 400VAC, definitely high enough to overvoltage that cap.
Oh, and, yes, with the rectifier tubes pulled and the transformer out
of the circuit, I could feed it externally-provided power no problem.
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
12:46 p.m.
I've just spent a couple of hours tracing out the power-supply
circuitry in this 'scope, particularly the HV supply driving many of
the CRT terminals.
Turns out the winding I thought was the HV winding, only shorted out,
is actually the CRT filament/heater winding. (I'm quite sure I've got
the CRT filament pins correct; they are the only two CRT pins that show
DC continuity.) I'm not sure whether it's a heater or filament;
It's always possible (you'd better hope not!) that there's an internal
inter-electrode short in the CRT. But why you;d have those 2 electrodes
connected directyly to a transformer winding is beyond me.
Often (but not always), the 2 heater pins are the ones either side of the
locating key on the CRT base in a 'scope CRT.
there's a pin that could be the cathode (connected
to one side of the
heater/filament, the most negative CRT pin found so far), but there is
an odd circuit which makes sense to me only as an attempt to compensate
for using one side of a filament as a cathode - there's a voltage
It's very uncommon to have a directly-heated CRT, almost always there's a
seprate heater and cathode. But the insulation between the heater and
cathode won't stand much voltage, so it's normal to connect the heater to
the cathode (or at least to some point in the external circuit at about
the same voltage as tha cathode) to prevent breakdown.
divider across one half of the 6.3V heater winding,
with a HV cap
coupling it to the "cathode" side of the CRT filament/heater,
presumably to avoid the slight mains-frequency flicker that would
otherwise result.
The HV winding appears intact, and runs between one side of the B+
winding and the filament of one of the HV rectifiers;
That suggests to me a -ve EHT supply (output taken from the anode of the
rectifier).
===================
_oooo_oooo_oooo_oooo_
A B C D E
A-B-C is the centre-tapped B+ winding I found before. B is grounded
(the A and C points go to the plates of the B+ rectifier) and, based on
scaling up from the voltages I see, the A-B (and B-C) voltage is about
375VAC. D-E is the filament winding for one of the HV rectifiers. C-D
is a winding I was not previously aware existed; its design voltage is
about 500VAC (under the same assumptions about scaling).
The transformer's ground wire has DC connection to this winding, about
150 ohms to either D or E and something like twice that to C. Since
both this ground wire and B were grounded to the chassis when the
transformer was in-circuit, I conjecture that this is part of the
fault, and the ground wire is supposed to be isolated from all the
windings.
THat would make siese. This sounds like an insulation breakdown in the
trasnformer, probable somwhere in the HV (C-D) section.
Interestingly, in view of the comments about how instead of a high
positive final anode voltage CRTs tend to run with a high negative
cathode voltage, it looks as though this does both: the final anode is
about as positive with respect to chassis ground as the filament is
negative.
How is the final anode supplied?
It's normal to run a en electrostaticly-deflected CRT with the deflection
plates close to ground potential (a few hundred volts is OK, so you can
run them from the anodes of a valve amplifier with the cathodes of said
valves close to ground. That means the cathode and control grid need to
be at a high -ve voltage, and if there's a 'post deflection
acceleration' electrode, it will be at a high +ve voltage (often higher
than the voltage on the cathode, if you see what I mean).
-tony
28 Feb
28 Feb
3:36 p.m.
Some of you may reacll that, back in July, I wrote about an
oscilloscope whose power transformer failed in service. Well, today I
was going through my tuit collection and found some round ones. I've
now clipped the transformer free of the circuit and removed it.
I then applied an ohmmeter to the transformer, now that it's free of
the machine, which gave me good guesses at what winding is what. Then
I got out the current-limiting light bulb rig and hooked the primary up
(well, half the primary; it has a split primary, for 115/230 mains
switching). Then I measured the primary voltage and the voltages on
various interesting windings. Based on all of this, I think I have a
good guess what's what:
- Input mains: four wires, two windings, which are connected in
parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
on the 115V-nominal mains primary, this measured 176V, or ~778V when
running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
The type of those valves will give you a good idea as to the voltage
required of course.
Almost certainly the EHT rectifier vavles are directly-heated, so the
filament will be at some high voltage wrt earth. These windings will need
good insulation.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes. (There are some
12V-heater tubes, but they all have centre-tapped heaters.)
I assume things like 12AX7s...
It's intereting. for vavles with centre-tapped heaters, the US code gives
the voltage for the 2 parts in series (as 12AX7), whereas thte European
code gicves the voltage for parallel connections (ECC83 -- E = 6.3V heater)
- There is one more wire unaccounted for. On opening
the transformer
case, I find it is a case ground.
It may also go to an inter-winding shield.
The HV supply winding appears to be fried. I didn't bother with all
the filament windings (though I did measure the heater winding, which
appeared to be intact); the B+ supply winding seemed intact, but the HV
supply winding showed ~2V instead of the 200-300V I would expect with
~25V on the primary.
My first concern is that the primary voltage is very low -- in other
words this transformer draws excessive input current even when unloaded.
This suggests to me shorted turns, probably in the HV winding (which
would alos explain the very low outptu voltage there.
So now I'm thinking of finding a half-dozen different transformers to
run the six different pieces off of....
A couple of ohter thoughts. You might consider rewinding the original
transformer, although getting the insulation to withstanda a few kV would
be 'interesting.
I don't know how easy it would be to remove the shorted HV winding (is it
near the outside of the windings?). If you can get it off and put the
rest of the windings back with the right insulation, you could keep the
exisitng transformer fo the heaters, HV rectifiers filaments, B+ line,
etc and only have to provide the HV input. Maybe a seprate transformer
(or a transformer + voltage multiplier) for that.
-tony
4 p.m.
The type of those valves will give you a good idea as
to the voltage
required of course.
Yes. I need to look them up; they don't appear in the reference book I
have at ready hand. ("Had", actually; it's at home and I'm not.
That's also why I don't give the valve type number here.)
Almost certainly the EHT rectifier vavles are
directly-heated,
Yes. All the rectifier valves (including the B+ one) have filaments
rather than heaters.
(There are
some 12V-heater tubes, but they all have centre-tapped
heaters.)
I assume things like 12AX7s... My first concern is that the primary voltage is very
low -- in other
words this transformer draws excessive input current even when
unloaded.
Yes - as I mentioned back in the summer. This is just further
confirmation of it. I was using a 40W light bulb in series with the
primary, as a current limiter, which is why the primary voltage was 25V
instead of ~120V. The bulb was glowing (not quite full power, but
close).
This suggests to me shorted turns, probably in the HV
winding (which
would alos explain the very low outptu voltage there.
I agree; this is why I suspect the HV winding of being the culprit.
You might consider rewinding the original transformer,
although
getting the insulation to withstanda a few kV would be 'interesting.
Quite. It also would be "interesting" to get it apart far enough to
rewind. It's a laminated core made up largely of E-shaped pieces, and
getting it apart would mean either cutting them (= destructive),
solvents to unstick the tar/shellac/resin/whatever that's holding them
together (= likely damage to the other windings' insulation), or a
*lot* of patience working them apart. And unwinding a winding
nondestructively without taking the core apart...well, it better be the
outermost winding, and would require a lot of patience to feed the wire
through over, and over, and over again.
To just pull off the HV winding, if it's outermost, I could cut most of
the turns to simplify the work, since that winding I don't care about
preserving. (I should also check out the three filament windings,
mostly for completeness...and I really need to trace out a schematic of
at least the power-supply portions of the thing.)
I don't know how easy it would be to remove the
shorted HV winding
(is it near the outside of the windings?).
I don't remember.
If you can get it off and put the rest of the windings
back with the
right insulation, you could keep the exisitng transformer fo the
heaters, HV rectifiers filaments, B+ line, etc and only have to
provide the HV input. Maybe a seprate transformer (or a transformer
+ voltage multiplier) for that.
I'll look at that when I get home. Certainly there seems to be some
goodness happening between the primary and the heater winding, ditto
the B+ winding.
I imagine the HV winding is either innermost or outermost, simply
because it's easiest to insulate that way. But of course that's a
guess, and in this case it'll be easy to look and find out.
Actually, come to think of it, looking at your (natural enough) use of
"valve" where I'd write "tube", I'm curious - are CRTs called
CRTs on
your side of the pond, or something else (CRV?)?
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
4:31 p.m.
On 28 Feb 2007 at 19:00, der Mouse wrote:
Actually, come to think of it, looking at your
(natural enough) use of
"valve" where I'd write "tube", I'm curious - are CRTs called
CRTs on
your side of the pond, or something else (CRV?)?
This has come up before--it's "tube" if I recall. The same applies
to terms suchl as TWT.
The only difference is that some pronounce it "chube" on the other
side of the pond.
:<)
Cheers,
Chuck
1 Mar
1 Mar
4:57 a.m.
On 3/1/07, Chuck Guzis <cclist at sydex.com> wrote:
On 28 Feb 2007 at 19:00, der Mouse wrote:
^^^^^^
"all". No-one British that I've ever heard pronounces it "toob".
People would look at you funny if you did ;)
Ed.
Actually, come to think of it, looking at your
(natural enough) use of
"valve" where I'd write "tube", I'm curious - are CRTs called
CRTs on
your side of the pond, or something else (CRV?)?
This has come up before--it's "tube" if I recall. The same applies
to terms suchl as TWT.
The only difference is that some pronounce it "chube" 
6:07 a.m.
listmailgoeshere at gmail.com wrote:
"tewb" here, just FYI :-)
This has come
up before--it's "tube" if I recall. The same applies
to terms suchl as TWT.
The only difference is that some pronounce it "chube"
^^^^^^
"all". No-one British that I've ever heard pronounces it "toob".
People would look at you funny if you did ;)
10:48 a.m.
The type of those valves will give you a good
idea as to the voltage
required of course.
Yes. I need to look them up; they don't appear in the reference book I
have at ready hand. ("Had", actually; it's at home and I'm not.
That's also why I don't give the valve type number here.)
Almost certainly the EHT rectifier vavles are
directly-heated,
Yes. All the rectifier valves (including the B+ one) have filaments
rather than heaters. (There
are some 12V-heater tubes, but they all have centre-tapped
heaters.)
I assume things like 12AX7s... my emails from the summer, they include a valve
complement list.)
Well, even with a 40W bulb in series, I would expect a transformer with
no load on the secondaries to give almost full output if there were not
shorted turns.
My first concern is that the primary voltage is
very low -- in other
words this transformer draws excessive input current even when
unloaded.
Yes - as I mentioned back in the summer. This is just further
confirmation of it. I was using a 40W light bulb in series with the
primary, as a current limiter, which is why the primary voltage was 25V
instead of ~120V. The bulb was glowing (not quite full power, but
close). This suggests to me shorted turns, probably in
the HV winding (which
would alos explain the very low outptu voltage there.
I agree; this is why I suspect the HV winding of being the culprit.
You might consider rewinding the original
transformer, although
getting the insulation to withstanda a few kV would be 'interesting.
Quite. It also would be "interesting" to get it apart far enough to
rewind. It's a laminated core made up largely of E-shaped pieces, and Actually, come to think of it, looking at your
(natural enough) use of
"valve" where I'd write "tube", I'm curious - are CRTs called
CRTs on
your side of the pond, or something else (CRV?)?
No, we call them 'Cathode Ray Tubes' too (at least unless you're talking
about the RAF 60 years ago ;-)).We also have travelling wave tubes,
photomultiplier tubes, nixie tubes, and so on. Also a 'television tube'
is what an RCA manual would call a 'kinescope' -- that is a TV receiver CRT
Other terms that are relevant and different include :
UK US
HT+ (High Tension Positive) B+
LT A supply (heater supply)
EHT (Extra High Tension) HV (high voltage to the CRT, etc)
Anode Plate (of a tube)
Also, in the UK, CRT electrodes are named 'cathode', 'grid', 'first
anode', 'second anode',...'final anode'. I believe you call them
'cathode', 'first grid', 'second grid',...'anode'.
Oh, the comment about the RAF perhaps needs explaining. In WW2, the UK
armed services all had their own valve numbering systems. The RAF one had
codes like
VUxx (Valve, Unidirectional) for rectifiers
VRxx (Valve, receiving) for small-signal valves
VTxx (Valve, transmitting) for transmitter valves -- this one caused, and
causes, endless confusion with the US Services 'VTxx' designations, here
standing, of course, for 'Vacuum Tube'. The numbers are not the same.
and
VCRxx (Valve, cathode ray) for CRTs.
To tie this in to classic computing, I believe the original 'Williams
Tube' store used an ex-radar CRT (easily available after the war) called
a VCR97.
-tony
6:43 p.m.
[replying to two messages in the same thread together]
Well, yes - since it would be drawing (to a first approximation) zero
current. That the bulb glows at all with all the secondaries open is
to me a strong indication that something is shorted.
Actually, on reflection, that makes more sense - without the I pieces,
there'd be some gaps.
Unlikely. Those two pins show about three ohms resistance. I'd expect
a failure-mode short to be either well under 1 ohm or at least several
hundred ohms.
Then it probably is a heater, with that odd circuit serving some other
purpose. (There is a "Z input" on the front panel, which is a
brightness control; it is capacitively coupled to this possibly-cathode
pin, which would make some sense....)
I now believe there is a -ve EHT supply and also a +ve EHT supply.
There are two HV rectifiers; if I draw the rectifier valves as
semiconductor diodes instead (to simplify the ascii-graphics), omitting
their filaments for simplicity, the circuit looks something like:
===================
.oooo.oooo.oooo.oooo. +--> resistor chain (see below)
| | | | +--> one end of CRT heater
| GND | | 100K | 220K
| | | +--/\/\/\/---+--/\/\/\/--> CRT cathode(?)
| | | +--|<--+ | .5uF/2KV 47
+->|-+-|<-+ | | +---||--GND +---||---+--/\/\/\/--GND
| +----+ .5uF/2KV +--/\/\/\/--3.15VAC
| | .5uF/2KV 91
| ==== +-->|---/\/\/\/--+-----||----GND
| .oooo. 100K +--/\/\/\/--GND
+--+ +--+ | 5M
| +---------> CRT final anode
+--> B++ +--> B+
| |
GND---)|--+--/\/\/\/-+-| . |---GND
80uF/475V 0B2
(The unconnected final winding of the transformer is actually the
filament winding for the rectifier whose cathode is shown connected to
one end of that winding.)
The "resistor chain" is a series of resistors to ground:
See above: from a half-wave rectification of the EHT AC supply.
Add in the four deflection electrodes and this accounts for all but one
of the connections to the CRT. That one is driven by a circuit I do
not understand which appears to have something to do with the "Y LIN"
internal adjustment - this may make more sense once I've traced more of
the circuit.
/~\ The ASCII der Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
I was using a
40W light bulb in series with the primary, as a
current limiter, which is why the primary voltage was 25V instead of
~120V. The bulb was glowing (not quite full power, but close).
Well, even with a
40W bulb in series, I would expect a transformer
with no load on the secondaries to give almost full output if there
were not shorted turns. It also would
be "interesting" to get it apart far enough to rewind.
It's a laminated core made up largely of E-shaped pieces, [...]
It's more
normal to have 'E's and 'I's arranged alternately. Turns out the
winding I thought was the HV winding, only shorted
out, is actually the CRT filament/heater winding. (I'm quite sure
I've got the CRT filament pins correct; they are the only two CRT
pins that show DC continuity.)
It's always possible (you'd better hope
not!) that there's an
internal inter-electrode short in the CRT. Often (but not always), the 2 heater pins are the ones
either side of
the locating key on the CRT base in a 'scope CRT.
That's another indication that they're the heater, then.
there's a
pin that could be the cathode (connected to one side of
the heater/filament, the most negative CRT pin found so far), but
there is an odd circuit which makes sense to me only as an attempt
to compensate for using one side of a filament as a cathode - [...]
It's very
uncommon to have a directly-heated CRT, The HV winding
appears intact, and runs between one side of the B+
winding and the filament of one of the HV rectifiers;
That suggests to me a -ve
EHT supply (output taken from the anode of
the rectifier). From above
|
(internal adjustment)
| Intensity Focus "SW DC LEVEL"
| 48K 100K 88K 100K 100K 100K 9K
+--/\/\/\/--/\/\/\/--/\/\/\/--/\/\/\/--/\/\/\/--/\/\/\/--/\/\/\/--GND
| ^ ^ ^
| | 15K +--> a CRT pin not traced yet
+-||--+--/\/\/\/----+------> a CRT pin
.01uF GND---||---+
.5uF/2KV
Interestingly,
in view of the comments about how instead of a high
positive final anode voltage CRTs tend to run with a high negative
cathode voltage, it looks as though this does both: the final anode
is about as positive with respect to chassis ground as the filament
is negative.
How is the final anode supplied?
2 Mar
2 Mar
2:58 p.m.
[replying to two messages in the same thread together]
Well, yes - since it would be drawing (to a first approximation) zero
current. That the bulb glows at all with all the secondaries open is
Not so. There will be an out-of-phase current due to the inductance of
the primary winding. And that current, if sufficiently large, would cause
the lamp to glpw
_But_ my experience suggests that for small-ish power transformers, the
sort of thing we're talking about here on no-load, a normal mains lamp in
series with the primary will not glow. You could trie a higher-wattage
bulb, thoguh.
IIRC, though, you've got a leak (or short) to earth from some point in
the HV widning, and another point on that winding (the centre-tap of the
B+ part) is deliberately earthed. That can't be right. And I'll bet that
short-to-reath is not just on one turn, in other words you also have
shorted turns there.
I was
using a 40W light bulb in series with the primary, as a
current limiter, which is why the primary voltage was 25V instead of
~120V. The bulb was glowing (not quite full power, but close).
Well, even with a
40W bulb in series, I would expect a transformer
with no load on the secondaries to give almost full output if there
were not shorted turns. Turns out the winding I thought was the HV winding,
only shorted
out, is actually the CRT filament/heater winding. (I'm quite sure
I've got the CRT filament pins correct; they are the only two CRT
pins that show DC continuity.)
It's always possible (you'd better hope
not!) that there's an
internal inter-electrode short in the CRT.
Then it probably is a heater, with that odd circuit serving some other
purpose. (There is a "Z input" on the front panel, which is a
brightness control; it is capacitively coupled to this possibly-cathode
pin, which would make some sense....)
Sure. The Z-modulation input is an external brightness control input,
used for some special applications. In general it's capacitively-coupled
to either the cathode or control grid of the CRT.
The HV
winding appears intact, and runs between one side of the B+
winding and the filament of one of the HV rectifiers;
That suggests to me a -ve
EHT supply (output taken from the anode of
the rectifier). one end of that winding.)
Add in the four deflection electrodes and this
accounts for all but one
of the connections to the CRT. That one is driven by a circuit I do
not understand which appears to have something to do with the "Y LIN"
internal adjustment - this may make more sense once I've traced more of
the circuit.
It may well be an internal shield between the defleciton plates. Altering
the voltage on it wil lcontrol the shape of the beam-deflction .vs.
voltage between the defleciton plates characteriostic, thuse it will
LINearise the Y defleciton.
-tony
4 Mar
4 Mar
8:23 p.m.
Well, yes -
since it would be drawing (to a first approximation)
zero current.
Not so. There will be an out-of-phase current due to the
inductance
of the primary winding. IIRC, though, you've got a leak (or short) to
earth from some point
in the HV widning, and another point on that winding (the centre-tap
of the B+ part) is deliberately earthed.
Yes - when it was in-circuit. Not when it's sitting disconnected,
which was the circumstance under which I observed the excessive draw.
That can't be right.
Agreed.
And I'll bet that short-to-reath is not just on
one turn, in other
words you also have shorted turns there.
I agree, especially in view of the "open-circuit" draw. :)
The Z-modulation input is an external brightness
control input, used
for some special applications.
I once built one. :) (It was a 1Kx1 RAM with circuitry to scroll
through all 1K bits, effectively turning it into a 1Kbit shift
register, with the data bit controlling the Z input on the 'scope and
the X sweep generator triggered by the carry-out from the address
counter. Get the clock frequencies about right and hook the input up
to a ham's CW sending key and you get a nice little graphic Morse
display scrolling past.)
(The
unconnected final winding of the transformer is actually the
filament winding for the rectifier whose cathode is shown connected to
^^^^^^^
Don't you mean anode (plate to you) here? 
3:51 p.m.
der Mouse wrote:
- Input mains: four wires, two windings, which are
connected in
parallel for 115V operation and series for 230V operation.
- 5VAC filament winding for B+ rectifier tube.
- ~800VAC CT (400-0-400) winding which is rectified for B+. (With 26V
on the 115V-nominal mains primary, this measured 176V, or ~778V when
running normally.)
- Filament winding for one HV rectifier tube.
- Filament winding for the other HV rectifier tube.
- HV supply winding.
- 6.3VAC CT winding to run the heaters for most tubes. (There are some
12V-heater tubes, but they all have centre-tapped heaters.)
- There is one more wire unaccounted for. On opening the transformer
case, I find it is a case ground.
See:
http://www.electra-print.com/
\ / Ribbon Campaign
X Against HTML mouse at rodents.montreal.qc.ca
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
.
6278
days inactive
6283
days old
18 comments
7 participants
participants (7)
-
ard＠p850ug1.demon.co.uk -
bfranchuk＠jetnet.ab.ca -
cclist＠sydex.com -
dave06a＠dunfield.com -
julesrichardsonuk＠yahoo.co.uk -
listmailgoeshere＠gmail.com -
mouse＠Rodents.Montreal.QC.CA