So D7 looks suspicious to me. It has three clock signals but no output. Is it worth replacing it? They're all low power devices so it's difficult to tell, but it does seem to be a degree or two warmer than the surrounding chips. And the surface markings are completely gone (all the other chips have visible codes).

I measured D6 and found:

pin 1 = 15.625KHz, 64µs, 3.4Vrms
pin 2 = 1.78V
pin 3 = 3.97V
pin 4 = 3.97V
pin 5 = 1.7V
pin 6 = 1.7V
pin 7 = 0V
pin 8 = 0.08V
pin 9 = 3.16V
pin 10 = 3.97V
pin 11 = 3.97V
pin 12 = 15.625KHz, 64µs, 3.56V
pin 13 = 15.625KHz, 64µs, 3.48V
pin 14 = 5.1V

SN74177 is obsolete now and extremely hard to find, but I managed to find an Ebay seller in The Netherlands who sells packs of untested original stock 54177, which apparently is an equivalent. So I've ordered a few.

https://www.datasheets360.com/pdf/-4129922497183814808

Well this is very odd, I'm now seeing some kind of pulsed signal on the vertical sync line. That wasn't there before. CRT still showing a horizontal line. It's nowhere near 50/60Hz whatever it is, it looks more like about 50KHz to me.

I'm tempted to leave it all powered on for a few hours, with the brightness on the monitor down and the lid down, just to see if anything changes. D7 hasn't changed at all.

I left it powered up, screen disconnected, for a couple of hours this morning and that signal (above) is still there. But at one point, while I was measuring it, I had an unstable but perfectly obvious 15.625KHz signal on vsync at the correct voltage, but with those spikes in the troughs. That went away, leading me to wonder if there's a dry joint somewhere (I've wiggled a few of the ceramic caps and pushed a few ICs around to no avail).

I think I'll bring it upstairs, where the house is warmer, to see how it behaves. And I might just reflow some of those caps, especially as some have been pushed down onto the board over the years.

Just a reminder to never trust your instruments 100%. What you saw might be some sort of artifact, for instance if your probe happened to have both a (very poor) electrical connection to VSync and a capacitive connection to a nearby HSync trace. I know that sounds contrived -- and it is. But as a general rule, when you observe something flaky, in the back of your mind it's good to ask yourself, is this real ?

Right. Or a crack in the board... ?

If there's a difference, I hope you plan to continue troubleshooting in whichever location makes the problem worse. Have fun, and keep us posted

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Thanks, there's definitely a signal of some description now where there wasn't before. I wonder if I've kicked something into life while measuring, who knows. No cracks in the board that I can see, it seems to have been well looked after. I randomly checked all the ceramic capacitors and none of them seem short circuit.

Another thought I've just had about D7 is that although it's receiving three clock signals, one of them is from D6 (an identical chip right above it) and that signal is decidedly on the weak side. Which makes me wonder if D7 isn't counting because the clock generated by D6 isn't high enough. Fortunately the two are identical so if required I can replace both. I'm also toying with the idea of socketing both while I await new components, and swapping them to see what changes.

/edit: just checked the datasheet and apparently and output of 2.4-3.4V is typical for that IC, and a minimum of 2V input voltage.

(I think I gather that old electrolytics can reform under voltage- could that have been happening?)

Electrolytic capacitors do reform when a DC voltage is applied, the leakage current should fall from an unacceptable level to a low value. But this will happen quickly on supply decoupling capacitors.

Ceramic capacitors may drift in value, but generally don’t tend to go short circuit unless physical damaged. Tantalum capacitors on the other hand, will go short circuit if they are subjected to high inrush currents or voltage spikes.

It would be a VERY weird fault if the horizontal / line frequency was to appear on the vertical / field sync signal

If in doubt, test again. If still unsure, leave it for a while (at least a couple of hours) then return with a clear head, but don’t jump in where you left it, but attack from a different place (if you can).

Mark

Still working my way through this, but this is my analysis of the circuit so far (assuming board assembly 32008 as on /pet/2001/32008-3.gif):

C9 74LS93 binary counter
Pin 1 - 8MHz clock in
Pin 9 - 4MHz out (not used)
Pin 8 - 2MHz out
Pin 11 - 1MHz out
Pin 14 - same as pin 11
Pin 12 - 500kHz

B5 74LS93 binary counter
Pin 14 - 500kHz clock in
Pin 12 - 250kHz
Pin 1 - same as pin 12
Pin 9 - 125kHz
Pin 8 - 62.5kHz
Pin 11 - 31.25kHz

C7 74LS107 JK flip-flop
Pin 9 - 31.25kHz
Pins 8, 10, 11 all Vcc +5V
Pin 5 - 15.625kHz
Pin 6 - 15.625kHz (but opposite phase to pin 5)

C6 74LS08 AND
Pin 1 - 15.625kHz (input)
Pin 2 - 31.25kHz (input)
Pin 3 - logical AND of pin 1 and 2 (output)
Pin 4 - 15.625kHz (input)
Pin 5 - 62.5kHz (input)
Pin 6 - logical AND of pin 4 and 5 (output)

C5 74LS107 JK flip-flop
Pin 9 - 500kHz clock input
Pin 8 - same as C6 pin 6 (J input)
Pin 11 - same as C6 pin 3 (K input)
Pin 5 - Q output (line labelled as “DIS. ON” which I presume means “display on”).
Pin 6 - as pin 5 but opposite phase (/Q output) (line labelled as “DIS. OFf” which I presume means “display off”).
Pin 10 - Vcc +5V

Mark

I was thinking the same thing as Jeff.

You're monkeying with a 40 year old PCB. Materials and the way in which PCBs were manufactured in those days were not at the quality level seen today. I've fixed my share of old electronics over the years whose problems proved to be nothing more than PCB "rot," especially around via.

I would carefully inspect all traces and via in the vicinity of the suspect signal under brightly-lit magnification for a hairline crack. Be sparing in your use of heat to resolder anything, lest the foil separate from the fiberglass. If you decide to replace any electrolytics use extreme caution in desoldering the old ones. You could end up pulling the through-hole right out of the board.

_________________
x86?  We ain't got no x86.  We don't NEED no stinking x86!

Right -- and here's an example, from my brother's Roland HP330 Digital Piano which failed last year. At first I didn't notice that a cap had leaked its electrolyte... and it didn't seem like a big deal anyway. The cap itself remained more or less functional. But the electrolyte was corrosive enough to open-circuit a via... which was "conveniently" located underneath the cap (removed, in this photo).

_________________
In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

Correct, it's a 320008.

It's a good idea so I did it. I used a magnifying glass under sunlight, all the traces, solder joints and PCB look as good as they did the day they left the factory. There isn't a mark or scratch anywhere, save some little scratches on various solder joints where I've been using the probes to get a reading.

The top of the board is generally just as good, although this has seen all the dust, plus a few liquid spills. The chips at the back have, therefore, all seen better days. In fact probably 50% of the chips have slightly rusty pins. I took some macro shots which look horrifying, but it's all surface rust (most of the worst falls off under my thumbnail) and I think that the connections are all probably fine. The worst are on the B row and are 74LS244N's. There's one IC that's been badly positioned before being soldered, it must have been a Friday afternoon when that one was done.

The CPU pin 20 looks rusty too, extending down into the socket. I'm slightly concerned that by lifting it out and cleaning it, I might be fixing a problem there that doesn't yet exist, and opening myself to the risk of damaging something that's currently working. So unless the CPU on this model affects video timing, I say leave it alone for now.

See attached pics for gore.

Look closer, it is not shown as a NOR gate. It is shown as a negative active input OR gate which is the same as a NAND gate. It's can be confusing if you are not used to it, but is is saying: "If any input goes low, the output goes high". The schematic was trying to tell you how the gate was being used (OR function), but it caused you a little confusion.

EDIT: I see that this was answered earlier - sorry.