The D7 counter appears bad (74177). It has pulsing load, clear and clock pulses but no outputs.

Thanks Dave, and I'm glad you found my post here because as good a resource as the other forum is, waiting days to have my posts authorised isn't very useful!

I've ordered the following which I think are an equivalent for the 74177:

https://www.ebay.co.uk/itm/220557070293

And once they arrive I'll remove D7 and socket the replacement. Kind of wondering if I shouldn't buy a desoldering station but if it's only the one chip I think I'll get by with my regular iron, solder sucker and braid.

You may want to consider cutting all the legs off the 74177 close to the body before unsoldering, and then unsoldering one leg at a time. This will put minimum heat into the board. The Commodore board has a tendency to lift solder pads and feed through holes.

Every day is another day where buying a Dremel seems like a better idea

Depends on how many PETs you are going to repair in the future.

I second dave_m: the old PCBs might be getting damaged quite easy,
traces and plated-through holes probably would prefer to stick at the chip pins rather than on the PCB.
A destroyed chip is less problematic than a destroyed PCB.
Avoid mechanical\thermal stress to the PCB "at all cost".


74197\74LS197 seems to be a faster version of the 74177 counter, it's out of production, too.
Nowadays TTL counters all seem to have a different pin assignment than the 74177.

I just use a small pair of diagonal cutters whose tips are sharp enough to get between the pins and cut the tops, right against the body of the IC. I have a Dremell tool, but I can't imagine getting that close with it, let alone without getting metal shavings under something else where they might hide and short things out.


We had these when I worked at TEAC in the early 1980's, but they were more trouble than they were worth, always getting clogged. I use a spring-loaded hand desoldering pump decades ago. I should have gotten more than one, because I want it to last my lifetime as it's better than the ones you buy today.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Gets clogged? I'm not sure quite what you're thinking of, but when I hear "desoldering station" I think of an SMD rework station, or at least the hot-air part of one.

I bought a Dremel and some very fine cutting discs. It's pretty easy, I just masked off the area surrounding the chip and very gently cut across. There's a change in tone when the disc nears the end of the metal, so it's pretty simple to avoid damaging anything else. Got my portable vacuum cleaner to pick up the relatively big bits and then brushed everything else off.

Some of the desoldering stations I've seen are excellent, but a £40 Dremel that I can use for all kinds of things vs a minimum £100 station I'll use once in a blue moon is an easy decision. That said, my father (who died years ago) taught himself electronics servicing and used to repair many of the TVs in my home town, through the 70s, 80s and 90s. I picked up a lot of basic info from watching him work (and some more while at college) and I can easily see myself getting more interested in repairing old things like this.

A part of me wonders, if the PCB is that delicate, would I be better off just soldering the socket to the (cleaned up) stumps of these legs?

Well, a skilled repair worker would be soldering the legs out of the PCB and replace them with a precision socket,
just in case one of the NOS 74177 chips might be defective. //I had seen defective TTL chips fresh from the factory.

Weller is building some neat desoldering stations, but be warned that they tend to cost (at least) an arm and a leg. //Industry grade stuff.

In the end it wasn't so bad, I used my old Ampex 25W iron, a magnifying glass and my dad's ancient solder sucker (whose nozzle has seen better days). I probably used slightly too much solder but it all took well.
No short circuits and everything still going where it should so now I just have to wait for the replacements to arrive later this week.

Just need to get some alcohol to remove that flux.

The context here is thru-hole; so the ones I'm talking about are like a soldering iron with a hollow tip and an electric vacuum pump that would suck the melted solder up the tip when you press the button. The tips would get clogged, and the filter that kept the molten solder out of the upstream tubing and the pump was always needing to be cleaned out or replaced. I don't remember the details anymore. I think I just took my own solder sucker in after getting tired of fighting with and maintaining the expensive desoldering pump. Sometimes the simpler tool is the better one.

_________________
http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

Well it isn't D7, I bought a job lot of the 74177 equivalent chips, 54177 (15 of them) and tried 4, all the same - no output. That leads me to suspect that E6 is at fault, so I'm going to look at that in a bit more detail.

I've changed E6 now, I bought two old stock SN74100Ns from an Ebay trader. The odd thing is, unless both of the chips I bought are faulty (they're apparently unused) E6 is no longer doing much of anything, and it seems that this is because C2 pin 8 is now stuck high at 4.3V. It goes without saying that there's still no vertical sync.

C2 - pin 8 - 4.3V (before I changed E6 it was 15.625KHz, 64µs, 1.4Vrms)
C2 - pin 9 - 15.625KHz @ 5v
C2 - pin 10 - high

So I went back from C2 pin 10 to B1 pin 8, which of course is 4.3V high. B1 is fed by A1, which I'd previously measured at:



And now, A1 is showing no outputs on 8, 9, 11 and 12:

1 - 15.625KHz
2 - 15.625KHz (at about 1v)
3 - 5.1V
4 - 0V
5 - 5V
6 - 0V
7 - 0V
8 - 0V
9 - 0V
10 - 0V
11 - 0V
12 - 0V
13 - 0V
14 - 4.5V

I'm at a loss to explain this behaviour. It seems unlikely that A1 has failed suddenly.

In addition, this is what I'm now seeing on the vertical pin on the video connector:

https://www.youtube.com/watch?v=2o-fH2zxoWU

C2 pin 10 high.
C2 pin 9 15.625KHz @ 5v.

There is supposed to be a signal at the output pin 8 of the C2 NAND gate.
Check, if a short circuit between C2 pin 8 and +5V "went into the board by accident" while soldering.

I don't trust that SN74100.
Would suggest to remove the SN74100 out of the socket, then to check if there is a signal at C2 pin 8.
If there is a signal at C2 pin 8, check if the signal still is there if C2 pin 8 is tied to GND or to +5V by using a 470Ohm resistor.

Another option would be to take a DIP24 precision socket, to wire a 74LS573 to it, then to plug that contraption into the SN74100 socket...