7 Jan
2017
7 Jan
'17
6:37 p.m.
Evening all,
I wish I had the ability to take a board layout and turn it into a logically
laid out schematic but as yet I don't. Video sync on my Executel 3910 is
still running me round in circles so could one of you fine folk take a look
at this board layout drawn as best I can:
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
...and let me know what it does please? The chip on the left is a Plessey
MR9735 teletext video driver wired to run permanently in 'off hours' mode -
you can see 'Sync In' is just pulled high so the chip doesn't detect any
incoming sync and is supposed to generate its own, which it doesn't even
though all external clocks are present.
Resistor values are correct as far as measuring them with a DMM goes, the
diodes aren't actually 1N4001s but I can't read the printing on them. Both
transistors are BC548 NPN. The area labelled 'MON' is a 14-way ribbon cable
that goes off to the TV driver board.
I'm feeling pretty dumb at this point. Logic and truth tables are more my
thing, current flow is something I'm learning slowly.
Cheers!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
8 Jan
8 Jan
2:31 a.m.
On Sat, Jan 7, 2017 at 11:37 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
Evening all,
I wish I had the ability to take a board layout and turn it into a logically
laid out schematic but as yet I don't. Video sync on my Executel 3910 is
still running me round in circles so could one of you fine folk take a look
at this board layout drawn as best I can:
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
...and let me know what it does please? The chip on the left is a Plessey
I don't believe it's correct as drawn. For one thing you have V333, an NPN
transistor, with the collector grounded (and no -ve supplies on the circuit. For
another, R322 is ridiculously low. And I would expect the anodes of the
3 diodes on the RGB outputs to go somewhere other than a resistor to
ground.
-tony
6:10 a.m.
On 08/01/2017 07:31, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
On Sat, Jan 7, 2017 at 11:37 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
Hm, ok, that's today's task then. For V333 it could be that I just have the
pin numbers reversed but I'll double check. The diodes bothered me too and
I'm trying to think of a better way of tracing unknown lines on the bottom
of the board. My sponge-wrapped-in-tin-foil method isn't infallible.
Cheers!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
Evening all,
I wish I had the ability to take a board layout and turn it into a logically
laid out schematic but as yet I don't. Video sync on my Executel 3910 is
still running me round in circles so could one of you fine folk take a look
at this board layout drawn as best I can:
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
...and let me know what it does please? The chip on the left is a Plessey
I don't believe it's correct as drawn. For one thing you have V333, an NPN
transistor, with the collector grounded (and no -ve supplies on the circuit.
For
another, R322 is ridiculously low. And I would expect the anodes of the
3 diodes on the RGB outputs to go somewhere other than a resistor to
ground.
9:01 a.m.
On Sun, Jan 8, 2017 at 11:10 AM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
No, it's more than that. The emitter returns to ground too. There is no way
of getting a voltage across colletor and emitter.
On 08/01/2017 07:31, "Tony Duell"
<ard.p850ug1 at gmail.com> wrote:
I don't
believe it's correct as drawn. For one thing you have V333, an NPN
transistor, with the collector grounded (and no -ve supplies on the circuit.
For
another, R322 is ridiculously low. And I would expect the anodes of the
3 diodes on the RGB outputs to go somewhere other than a resistor to
ground.
Hm, ok, that's today's task then. For V333 it could be that I just have the
pin numbers reversed but I'll double check. The diodes bothered me too and I'm trying to think of a better way of tracing
unknown lines on the bottom
of the board. My sponge-wrapped-in-tin-foil method isn't infallible.
I've been known to test to every connection on the board. Or to desolder all the
components I know it goes to and then do resistance checks to prove it goes
somewhere else. Yes it takes a long time,but...
Knowing what to expect, what is possible, helps a lot too. But that only comes
with practice.
-tony
11:18 a.m.
>>
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
> I don't believe it's correct as drawn. For one thing you have V333,
> an NPN transistor, with the collector grounded (and no -ve supplies
> on the circuit.
Bipolar transistors are to some extent symmetric under interchange of
collector and emitter. Simplified pictures showing a bar of
semiconductor with the ends being emitter and collector and a thin base
layer in the middle are, well, simplified, but there is _some_ truth
lurking in them.
However, the only reason I could see that being done deliberately is if
the circuit is analog and the transistor's behaviour is relevantly
different from the normal way around. This circuit doesn't look like
that to me. Redrawing it a bit less confusingly with V333 E/C swapped
makes it look as though V333 and V326 are being used in their switching
regions, not their linear regions.
The lack of negative supplies is hardly conclusivein itself; negative
voltages could be developed in any of many ways. However, V325, R322,
and V326 would make it difficult for the R335/V325/V333 junction point
to get too far below 5V (admittedly this is much less true if R322's
value is actually significantly higher).
I also question the way R311/R310/R309 are all different. I would
expect the red, green, and blue circuits to be electrically more or
less identical, and different pulldown values does not fit with that.
> For another, R322 is ridiculously low.
Indeed.
I don't know how much you know about electronics at the relevant level,
so it's possible what I'm about to say is unnecessary or has already
been considered.
Did these resistor values come from reading bands, or measuring
in-circuit, or what? Note that R322 is in parallel with the C-B diode
of V326; a simple ohmmeter put across it may end up measuring the diode
drop instead. (To test this theory, switch ranges. A diode drop will
usually give you approximately the same digits in each range - eg, 6.7
ohms on the 0-10 range, 67 ohms on the 0-100 one, 670 ohms for the 0-1K
range, etc. Also, swapping the probe leads will give a different
reading if that's what's behind this.)
> And I would expect the anodes of the 3 diodes on
the RGB outputs to
> go somewhere other than a resistor to ground.
At least with R312 as high as 10K. If it were much lower, the diodes
could be clipping diodes; a diode drop is not too far from about the
right swing for a video signal.
/~\ The ASCII Mouse
\ / Ribbon Campaign
X Against HTML mouse at rodents-montreal.org
/ \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B
11:36 a.m.
On Sun, Jan 8, 2017 at 4:18 PM, Mouse <mouse at rodents-montreal.org> wrote:
I also question the way R311/R310/R309 are all
different. I would
expect the red, green, and blue circuits to be electrically more or
less identical, and different pulldown values does not fit with that.
Well, I wonder if it's producing a weighted sum of R,G,B to make a
monochrome luminance signal. Not at all uncommon.
-tony
12:14 p.m.
On 08/01/2017 16:18, "Mouse" <mouse at Rodents-Montreal.ORG> wrote:
Yes, I got the emitter and collector swapped. The software I'm using
(Fritzing) is excellent given it costs nothing, but occasionally you can
make a change then alter something else in the properties of the item and
the change is lost.
>>
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
> I don't believe it's correct as drawn. For one thing you have V333,
> an NPN transistor, with the collector grounded (and no -ve supplies
> on the circuit.
Bipolar transistors are to some extent symmetric under interchange of
collector and emitter. Simplified pictures showing a bar of
semiconductor with the ends being emitter and collector and a thin base
layer in the middle are, well, simplified, but there is _some_ truth
lurking in them.
However, the only reason I could see that being done deliberately is if
the circuit is analog and the transistor's behaviour is relevantly
different from the normal way around. This circuit doesn't look like
that to me. Redrawing it a bit less confusingly with V333 E/C swapped
makes it look as though V333 and V326 are being used in their switching
regions, not their linear regions. The lack of negative supplies is hardly conclusivein
itself; negative
voltages could be developed in any of many ways. However, V325, R322,
and V326 would make it difficult for the R335/V325/V333 junction point
to get too far below 5V (admittedly this is much less true if R322's
value is actually significantly higher).
I also question the way R311/R310/R309 are all different. I would
expect the red, green, and blue circuits to be electrically more or
less identical, and different pulldown values does not fit with that.
There's been a couple of resistor failures with this board so I'm at the
stage of starting with a DMM, swapping probes to hopefully get a consistent
reading then reading the bands and using the calculator here for ones I
don't instantly recognise:
http://www.allaboutcircuits.com/tools/resistor-color-code-calculator/
If the two don't match I take it out of circuit.
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
9 a.m.
On 08/01/2017 07:31, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
On Sat, Jan 7, 2017 at 11:37 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
You were right, I'd drawn out R335 wrongly and flipped V333. All the anodes
go via R312 to the collector of V333 along with R309-R311. Base for V333 is
the inverted sync output of the MR9735. R322 is 6.8k
(blue-grey-black-brown). All diodes are 1N4148.
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit2.jpg
Thanks for looking!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
Evening all,
I wish I had the ability to take a board layout and turn it into a logically
laid out schematic but as yet I don't. Video sync on my Executel 3910 is
still running me round in circles so could one of you fine folk take a look
at this board layout drawn as best I can:
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit.jpg
...and let me know what it does please? The chip on the left is a Plessey
I don't believe it's correct as drawn. For one thing you have V333, an NPN
transistor, with the collector grounded (and no -ve supplies on the circuit.
For
another, R322 is ridiculously low. And I would expect the anodes of the
3 diodes on the RGB outputs to go somewhere other than a resistor to
ground. 
9:06 a.m.
On Sun, Jan 8, 2017 at 2:00 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
You were right, I'd drawn out R335 wrongly and
flipped V333. All the anodes
go via R312 to the collector of V333 along with R309-R311. Base for V333 is
the inverted sync output of the MR9735. R322 is 6.8k
(blue-grey-black-brown). All diodes are 1N4148.
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit2.jpg
That makes more sense. I am surprised that the anodes of the diodes don't go
to a +ve power line (maybe +12V), though. It could be right as drawn,
but I would
check it.
When you power up, what voltage and/or signal (use a 'scope) do you see on the
anodes of said diodes?
-tony
12:05 p.m.
On 08/01/2017 14:06, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
On Sun, Jan 8, 2017 at 2:00 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
I photographed both sides of the board and merged the pictures together to
make a front/back composite. The trace for the cathodes of the 3 diodes
links together on the back of the board and doesn't go anywhere else. The
front trace from V302 cathode goes straight to R312. The anodes DO link with
the monitor driver board through the connection marked MON though.
You were right, I'd drawn out R335 wrongly
and flipped V333. All the anodes
go via R312 to the collector of V333 along with R309-R311. Base for V333 is
the inverted sync output of the MR9735. R322 is 6.8k
(blue-grey-black-brown). All diodes are 1N4148.
http://www.binarydinosaurs.co.uk/STCExecutelSyncCircuit2.jpg
That makes more sense. I am surprised that the anodes of the diodes don't go
to a +ve power line (maybe +12V), though. It could be right as drawn,
but I would check it. When you power up, what voltage and/or signal (use a
'scope) do you see on the
anodes of said diodes?
Looks to be 4V on all 3 lines for RGB:
http://www.binarydinosaurs.co.uk/STCExecutelRGBsignal.jpg
One of the reasons I'm asking these questions is I'm trying to see if
there's another source of composite sync given that the MR9735 itself
doesn't do it, even though it should.
Cheers!
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
12:21 p.m.
I photographed both sides of the board and merged the
pictures together to
make a front/back composite. The trace for the cathodes of the 3 diodes
links together on the back of the board and doesn't go anywhere else. The
front trace from V302 cathode goes straight to R312. The anodes DO link with
the monitor driver board through the connection marked MON though.
OK, probably some kind of luminance signal. Was there ever a monochrome
version of this unit?
One of the reasons I'm asking these questions is
I'm trying to see if
there's another source of composite sync given that the MR9735 itself
doesn't do it, even though it should.
I doubt it.... Why would there be?
Is the MR9735 doing anything? Is it accessing memory, for example?
I still wonder if it has to be initialised (say to vewdata rather than teletext
mode) and if this is not occuring.
-tony
1:11 p.m.
On 08/01/2017 17:21, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
The whole unit is monochrome, it only has a little 5" B&W TV as its output
device -
http://www.binarydinosaurs.co.uk/STCExecutel01.jpg
However, two of us bought units at the same time and the other one has a
little bridgeboard containing a 74LS244 that sends the RGB signals to a DIN
socket on the back of the case for an external monitor. I also can't see a
luminance line on the connection to the TV board on mine, just
red-green-blue with sync and inverse sync which go to the horizontal driver
chip and vertical driver chip respectively. You may remember I was asking
questions about these chips a couple of months ago because that TV board
suffered some component rot and I thought the wavy picture was because of
that.
Clutching at straws :)
I photographed
both sides of the board and merged the pictures together to
make a front/back composite. The trace for the cathodes of the 3 diodes
links together on the back of the board and doesn't go anywhere else. The
front trace from V302 cathode goes straight to R312. The anodes DO link with
the monitor driver board through the connection marked MON though.
OK, probably some kind of luminance signal. Was there ever a monochrome
version of this unit? 
One of the
reasons I'm asking these questions is I'm trying to see if
there's another source of composite sync given that the MR9735 itself
doesn't do it, even though it should.
I doubt it.... Why would there be? Is the MR9735 doing anything? Is it accessing memory,
for example?
I still wonder if it has to be initialised (say to vewdata rather than
teletext mode) and if this is not occuring.
Its address and data buses are active as is the pair of 2114s that are
acting as a page store. There's a 74LS240 buffering the data bus and that's
active.
You could be right though, I'm not convinced the unit is actually running
any code yet despite the ROMCS lines working and I can see activity in all
16 RAM chips. Data bus and address bus at CPU and ROM chips looks happy from
a 'doing something' point of view. All the control lines on the CPU are
pulsing with the exception of IO/M but if all the IO the chip is doing is to
memory then that's to be expected.
I've disassembled the ROM code so I'll have a look through that for an init
routine, also the datasheet should tell me what its expecting at powerup.
Cheers,
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
1:20 p.m.
On Sun, Jan 8, 2017 at 6:11 PM, Adrian Graham
<witchy at binarydinosaurs.co.uk> wrote:
OK.. My dislike of 2114s is legendary, but probably not the problem here....
On 08/01/2017 17:21, "Tony Duell"
<ard.p850ug1 at gmail.com> wrote:
The whole unit is monochrome, it only has a little 5" B&W TV as its output
device -
OK, was there ever a colour version :-)
I
photographed both sides of the board and merged the pictures together to
make a front/back composite. The trace for the cathodes of the 3 diodes
links together on the back of the board and doesn't go anywhere else. The
front trace from V302 cathode goes straight to R312. The anodes DO link with
the monitor driver board through the connection marked MON though.
OK, probably some kind of luminance signal. Was there ever a monochrome
version of this unit?
http://www.binarydinosaurs.co.uk/STCExecutel01.jpg
However, two of us bought units at the same time and the other one has a
little bridgeboard containing a 74LS244 that sends the RGB signals to a DIN
socket on the back of the case for an external monitor. I also can't see a
And in a sense there was...
luminance line on the connection to the TV board on
mine, just
red-green-blue with sync and inverse sync which go to the horizontal driver
Do you mean that the common cathode connection of those diodes goes to
the monitor connector but nowhere then on the monitor PCB?
Is the MR9735
doing anything? Is it accessing memory, for example?
I still wonder if it has to be initialised (say to vewdata rather than
teletext mode) and if this is not occuring.
Its address and data buses are active as is the pair of 2114s that are
acting as a page store. There's a 74LS240 buffering the data bus and that's
active.
You could be right though, I'm not convinced the unit is actually running
any code yet despite the ROMCS lines working and I can see activity in all
16 RAM chips. Data bus and address bus at CPU and ROM chips looks happy from
What are the RAMs? If they're more 2114s I wonder if they have problems
that are confusing the CPU (perhaps corrupting the stack?)
a 'doing something' point of view. All the
control lines on the CPU are
pulsing with the exception of IO/M but if all the IO the chip is doing is to
memory then that's to be expected.
What's the CPU? 8085? I've had those fail in odd ways.
I've disassembled the ROM code so I'll have a look through that for an init
routine, also the datasheet should tell me what its expecting at powerup.
Personally, I'd stick a logic analyser on it and start tracing the code. See
if it is executing anything correctly.
-tony
2:05 p.m.
On 08/01/2017 18:20, "Tony Duell" <ard.p850ug1 at gmail.com> wrote:
Yep. I'm pleased we found that mod in the other unit because I made my own
bridgeboard and hooked the display up to a Microvitec CUB, I got exactly the
same picture (in colour) on that as I did in mono on the 5" TV so I knew the
analogue board and TV were OK.
No, in trying to trace some of the lines along the bottom of the board I
took the monitor cable off and traced the pins. They are:
1 - 5V
2 - NC
3 - looped to 5
4 - Sync out
5 - looped to 3
6 - emitter of V326 via R336
7 - GREEN
8 - RED
9 - Inverse sync (which also goes via R332 to the base of V333)
10 - BLUE
11 - GND
12 - GND
13 - 12V
14 - 12V
OK.. My dislike of 2114s is legendary, but probably not the problem here....
They're OK too, surprisingly. I took them off the board and put them in a
PET8032 as video RAM.
4116-2 wired as 2 banks of 8.
Yeah, 8085A. Thanks to list members I have some spares to try.
I've only got a cheap-ish Saleae compatible 16 channel analyser. Watching
code is a bit above my current level of self-paced learning :)
--
Adrian/Witchy
Binary Dinosaurs creator/curator
Www.binarydinosaurs.co.uk - the UK's biggest private home computer
collection?
However, two
of us bought units at the same time and the other one has a
little bridgeboard containing a 74LS244 that sends the RGB signals to a DIN
socket on the back of the case for an external monitor. I also can't see a
And in a sense there was... luminance line
on the connection to the TV board on mine, just
red-green-blue with sync and inverse sync which go to the horizontal driver
Do you mean that the common cathode connection of those diodes goes to
the monitor connector but nowhere then on the monitor PCB? Is the MR9735 doing anything? Is it accessing memory,
for example?
I still wonder if it has to be initialised (say to vewdata rather than
teletext mode) and if this is not occuring.
Its address and data buses are active as is the pair of 2114s that are
acting as a page store. There's a 74LS240 buffering the data bus and that's
active. You could be
right though, I'm not convinced the unit is actually running
any code yet despite the ROMCS lines working and I can see activity in all
16 RAM chips. Data bus and address bus at CPU and ROM chips looks happy from
What are the RAMs? If they're more 2114s I wonder if they have problems
that are confusing the CPU (perhaps corrupting the stack?) a 'doing
something' point of view. All the control lines on the CPU are
pulsing with the exception of IO/M but if all the IO the chip is doing is to
memory then that's to be expected.
What's the CPU? 8085? I've had those fail in odd ways. I've
disassembled the ROM code so I'll have a look through that for an init
routine, also the datasheet should tell me what its expecting at powerup.
Personally, I'd stick a logic analyser on it and start tracing the code. See
if it is executing anything correctly.
2736
days inactive
2737
days old
13 comments
3 participants
participants (3)
-
ard.p850ug1＠gmail.com -
mouse＠Rodents-Montreal.ORG -
witchy＠binarydinosaurs.co.uk