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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 20th Sep 2022, 10:38 pm   #41
BrackenFix
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by SiriusHardware View Post
Quote:
The thin legs broke off, there is the larger part of the leg is still there
In that case you might be able to rescue them without using a socket, by soldering 'donor' pins from another IC to what's left of the original pins. The attached crude sketch illustrates how, with the new / replacement pin highlighted in red soldered to the inside of the upper half of the original pin - this makes for a fairly invisible repair when the chip is inserted back into a socket.

Of course you can also solder the replacement pin to the outside of what is left of the original pin but then the repair is a bit more more obvious and not quite so strong.
Ok, Ill try that. Ill find a replacement pin

How exciting!

Edit. Actually, I'll use a pin from one of the sockets I have coming on Friday
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Old 20th Sep 2022, 10:49 pm   #42
SiriusHardware
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Default Re: 1978 Commodore PET project

That won't work too well, it really has to be a complete pin taken from another dead / scrap / surplus IC with a wide bit at the top to solder to the inside back of the wide bit on the injured IC pin.

It's a shame you don't live near Tim(bucus), he has loads of duff ICs which you could harvest spare pins from. The idea of using a pin from another IC to carry out a nice looking pin repair was originally his idea.
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Old 20th Sep 2022, 11:02 pm   #43
Mark1960
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Default Re: 1978 Commodore PET project

I took a quick look at the schematic Alan posted. If my guess based on the symptoms was correct the places to look would be as follows.

LSD5
F10 pin 23 (on the 6316 character generator, fingers crossed that IC is OK)
F9 pin 15 (74ls373 very common easy to find)

SD5
F9 pin 14
F7 pin 13 (2114 RAM, getting rare now)
E7 pin 4 and 5 (74ls244, not too rare)

Less likely is BD5 as I think you would not get the start up message displayed
E7 pin 15 and 16 (74ls244)
E10 pin 15 and 16 (74ls244)
I11 pin 13 and 14 (74ls244)

If you aren’t able to get access to a scope and decide to try swapping parts then I would suggest starting with E7 and then F9 before F7.

It would be worth getting a few spare 74ls244 if not too expensive, as they seem to be a problem in PETs based on Colin’s PET marathon repair.

If you do decide to start removing ICs the I would recommend considering the pcb itself as the most valuable part, consider the ICs disposable. Remove them by cutting the legs close to the body of the IC, then remove the legs one at a time with a soldering iron, then clear the holes with desolder braid or a solder sucker. Take extra care with solder sucker, try to move the soldering iron tip away before you release the solder sucker, as the shock when it releases can hammer the soldering iron against the pcb and damage traces.

Last edited by Mark1960; 20th Sep 2022 at 11:05 pm. Reason: Add pin numbers for E7, and typo
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Old 20th Sep 2022, 11:19 pm   #44
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Default Re: 1978 Commodore PET project

Agree with everything above, but if BrackenFix feels confident enough to remove ICs in one piece then I think removal / socketing and swapping over of the display RAMs UF7 and UF8 would be the lowest cost next step as it wouldn't involve buying any new parts (apart from sockets, which are already on the way). If the fixed characters displayed then change to alternative fixed characters, at least one of the RAMs is faulty. If swapping them over does not change the initial display, then the video RAMs are ruled out and can stay in place.

Making the video RAMs removable would also allow us to impose test logic levels on the SD0-SD7 lines going from the display RAMs to the video latch and onwards to the character generator ROM, so we could test both of those out without having to unsolder the 74LS373 latch.

Before any of that though, and only after the ROM is repaired, I still think it is worth measuring the voltages on the display RAM SD0-SD7 lines in the hope that we may see a clearly stuck bit.
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Old 21st Sep 2022, 6:54 am   #45
BrackenFix
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Default Re: 1978 Commodore PET project

As I don't have any spare ICs, I'll either try to buy some cheap or I'll solder the broken IC to the new socket as described earlier I think.

I have seen videos of people removing ICs without breaking but never done it myself so may buy a scrap board to practice on before risking this one ��
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Old 21st Sep 2022, 7:27 am   #46
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Default Re: 1978 Commodore PET project

That's a good idea but your 'practice' PCB needs to be a double sided PCB (tracks on the top side and the bottom side). Once you have removed all the solder from around the pins on the lower (track) side of the PCB, the IC will still be attached to the upper pads by small traces of solder where the wide parts of the pins rest on the upper side pads.

The thing you must never ever do at that point is to try to lever the IC out of the PCB by inserting a screwdriver or something similar underneath one end of the IC - it will come out all right, but probably with two or three of the top side PCB pads attached to it.

What you do instead is push the IC from side to side with something like the square end of the wooden handle of a wire brush, first from the right, then the left, then the right, over and over until the remaining solder cracks. At that point you should be able to lift the IC out without any force, just using your fingers.

If you are in any doubt as to whether you can remove ICs from the PET PCB in one piece, don't try - as Mark said the PCB is irreplaceable, the ICs, for the most part, are replaceable so in your situation it is better to cut the IC pins off near the body of the IC and then individually desolder the pins and remove them. You can get very small side cutters sold as 'cuticle nippers' which are ideal for cutting individual IC pins.

Repair the ROM however you choose to, then put it back in and verify that you are back to where you were before.

Then, measure the overall supply voltages (as per earlier info) and then we'll ask you to measure the voltages on the video RAM data pins. Let us know when you get to that point.
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Old 21st Sep 2022, 8:07 am   #47
ScottishColin
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Default Re: 1978 Commodore PET project

Here's a couple of 'new' legs from my project - one was the character ROM and the other one of the BASIC ROMs, both of which had snapped legs (in my defence, I only broke one of them myself....).

For the character ROM, I snipped off a leg of a potentiometer and used that - it's a little wider than the other legs, but it works in the original socket.

For the other, I did as suggested above - used a leg from a new socket donor.

The job wasn't particularly difficult (even for me who has never done anything like this before) - just fiddly to hold the chip, hold the solder, hold the leg in the right place and hold the iron at the same time.

For the new chips I bought, I used Cricklewood Electronics - they seemed reasonably priced to me although I expect if you hunt you can get them cheaper; they were quick and the web site was up to date and I would use them again.

In terms of desoldering, I started as suggested by cutting each leg and removing one leg at a time, but over time even as a complete novice I found I could with patience get whole chips out one leg at a time by using a solder sucker and the iron on just enough heat to flow the solder - any more heat and I lifted a couple of pads which gave me problems to deal with. I suspect the odd pad lifting is inevitable on older electronics, but I tried my hardest to avoid them.

Getting old sockets out I found harder and the advice I took from here was to cut the socket's plastic case before I desoldered one pin at a time. I expect that can easily be done without butchering the old socket with the right tools and technique, but as I say, I had never done anything like this before.

Good luck.

Colin.
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Old 21st Sep 2022, 9:14 am   #48
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Default Re: 1978 Commodore PET project

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Originally Posted by BrackenFix View Post
Do you have any recommendations for a logic analyser?
Firstly, I will say that when I was young and even more foolish than I am now, I thought a logic analyser (which no way could I afford then) would be the answer to all my prayers. When I did finally get a second-hand one (an old Gould Biomation K100D) I realised that while it's a very useful instrument, experience is worth a lot more.

I can only recomend the instruments I use on my bench, all of which are old. They are the Gould K100D (16 channels, 100MHz) which is enough for most work on older computers. The HP1630 which has rather more channels (but splits them between 'timing' and rather slower 'state' channels) which is the last HP series to have circuit diagrams in the service manual (that matters a lot to me) and the HP LogicDart, a handheld 3-channel instrument that is very convenient for quick checks.

There are many other excellent analysers available though. Some of the more modern ones are USB peripherals to a PC, I've heard good things about some of them but as I don't have a PC anywhere near my workbench they are not useful to me.

NEVER buy a logic analyser without the probes/pods. You will not find them separately, for just about every instrument the pods are much rarer than the unit itself. And they normally contain high-speed electronics (often as thick-film hybrid circuits) so making a reproduction is a major project, not something you want to do when you are starting out.

As for the PET, I notice your video RAM is 2114s. Those have much the same reputation with me as RIFA metalised paper capacitors. OK, they don't emit clouds of evil-smelling smoke, but they are not reliable. I regard them as 'replace on sight' and see if the problem goes away.

I'll second the recommendation for Cricklewood Electronics. I've bought many components from them without problems. In my case they are near enough for me to collect in person which is a major bonus.
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Old 21st Sep 2022, 9:21 am   #49
SiriusHardware
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Default Re: 1978 Commodore PET project

Conventional sockets are often easier to remove from double sided PCBs than ICs because they don't have the wide upper pin section which sits on the upper pad, like the pins of ICs do.

Turned-pin sockets on the other hand are a nightmare to remove from double sided PCBs because the 'fat' upper part of their pins completely seals the hole on the upper side making it impossible to draw solder through the hole with a solder sucker. The only safe way to take those out is to slice through the plastic frame with a hot wire or hot knife until all of the pins are standing alone with a little bit of plastic frame attached, at which point they can be individually desoldered and removed.
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Old 21st Sep 2022, 10:00 am   #50
BrackenFix
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Default Re: 1978 Commodore PET project

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Originally Posted by SiriusHardware View Post
That won't work too well, it really has to be a complete pin taken from another dead / scrap / surplus IC with a wide bit at the top to solder to the inside back of the wide bit on the injured IC pin.

It's a shame you don't live near Tim(bucus), he has loads of duff ICs which you could harvest spare pins from. The idea of using a pin from another IC to carry out a nice looking pin repair was originally his idea.
I have just bought some ICs so I can harvest the legs
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Old 21st Sep 2022, 10:01 am   #51
BrackenFix
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Default Re: 1978 Commodore PET project

I assume I cant use a hot air station to remove soldered ICs as the heat will damage other components?
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Old 21st Sep 2022, 10:22 am   #52
SiriusHardware
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Default Re: 1978 Commodore PET project

More likely to damage the board itself - definitely not recommended on PCBs of this era, sorry, because you may burn the pads off the PCB or even blister the PCB itself.

If you have money to burn, an electric pump assisted desoldering gun is what you really need.

We are getting a little bit ahead, though, it's still possible that you could use nothing more than your meter to see if there is a fault on the video RAM ICs without even removing them. First things first, repair that ROM and get back to where you were before the pins fell off.
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Old 21st Sep 2022, 10:25 am   #53
BrackenFix
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Default Re: 1978 Commodore PET project

I do fancy a desoldering gun, I will admit. Will come in use for lots of channel projects.

I bought a hot air station a few months back but assumed it wouldnt work for this application.

Should it ever be helpful later in diagnostics, i have a bench power supply too
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Old 21st Sep 2022, 10:35 am   #54
SiriusHardware
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Default Re: 1978 Commodore PET project

Quote:
I do fancy a desoldering gun, I will admit.
If you ever do use one you won't ever be able to go back to a hand desolder pump. Not long ago they used to cost in the mid hundreds for units by Weller or Metcal, possibly still do, but you can get quite acceptable temperature controlled ones for around the 80 upwards mark which are still way better and less liable to cause damage than hand desolder pumps.

This would be a typical example, I don't have one of these myself but there are a few people on the forum who do have this or a similar model. You can probably find them cheaper in other places, not necessarily with the same brand on them.

https://cpc.farnell.com/duratool/d00...ring%20station
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Old 21st Sep 2022, 10:48 am   #55
BrackenFix
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Default Re: 1978 Commodore PET project

Perfect, thats the one I have in my cart!
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Old 21st Sep 2022, 10:58 am   #56
SiriusHardware
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Default Re: 1978 Commodore PET project

Oh - well then, I hope you like it. Maybe you can do a review on your channel, for the benefit of anyone else considering buying one.

There is one minor issue with them where the end pieces which hold the glass tube in place don't move far enough to release the tube easily, there are apparently some simple mods you can make to the handset to increase the range of travel of the sliding part so the tube can be dropped in and removed more easily. You'll see what I mean when you get it.
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Old 21st Sep 2022, 11:55 am   #57
BrackenFix
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Default Re: 1978 Commodore PET project

Amazing. Thank you

I will absolutely do a review, awesome idea
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Old 21st Sep 2022, 1:27 pm   #58
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Default Re: 1978 Commodore PET project

This may be of interest to anyone following this thread:

http://brackenfix.co.uk/commodore-pe...val-episode-1/

Alan
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Old 21st Sep 2022, 1:41 pm   #59
BrackenFix
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Default Re: 1978 Commodore PET project

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Originally Posted by ajgriff View Post
This may be of interest to anyone following this thread:

http://brackenfix.co.uk/commodore-pe...val-episode-1/

Alan
Thank you

Didn't know if I was allowed to self-promote so didn't want to overstep as a new joiner.

The video may give purists rage if I did anything silly so i apologies if I did

Last edited by BrackenFix; 21st Sep 2022 at 1:47 pm.
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Old 21st Sep 2022, 4:27 pm   #60
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Default Re: 1978 Commodore PET project

Quote:
Originally Posted by BrackenFix View Post
Quote:
Originally Posted by ajgriff View Post
This may be of interest to anyone following this thread:

http://brackenfix.co.uk/commodore-pe...val-episode-1/

Alan
Thank you

Didn't know if I was allowed to self-promote so didn't want to overstep as a new joiner.

The video may give purists rage if I did anything silly so i apologies if I did
If we were to exclude anybody who did anything silly this would be a very, very quiet place - well done so far
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