Washing a PET 2001 motherboard?

[Oswald]

Hello,

I recently got my hands on a Commodore PET 2001. Cosmetically it is in excellent condition but it doesn't seem to work very reliably. Loading from disk only occasionally works and a few hours after I first turned it on the keyboard stopped working reliably. Sometimes it prints the wrong characters, repeatedly prints wrong characters for as long as the key is pressed, or just doesn't work at all. The problem is NOT the keyboard mechanism itself as I have two working keyboards and the same thing happens on either.

The motherboard was pretty dirty and has some rust and corrosion. I decided to reseat all the removable chips, so I took them all out and gently cleaned every socket with isopropyl alcohol. When it dried however I noticed the entire board started streaking and now it looks even worse than it did before...

I have washed Macintosh stuff in a dishwasher or by hand before, but never something this old. I'm particularly worried about the ceramic capacitors, because some of them are pretty old and crusty looking.

Does anyone have any recommendations on how to proceed? I haven't powered it up since I applied the alcohol, in case the residue is conductive or something.

Attached is a before and after (pic without socketed chips is after).

 

[GiGaBiTe]

Isopropyl alcohol is great for causing smears and streaks, it's pretty bad as a cleaner.

I would recommend CRC Lectra Clean, you can pick it up at Home Despot/Lowes. Just hose the board down with it and scrub it with a good brush. Let the fluid drip off the board before it evaporates so it carries the contaminants off the board.

For cleaning sockets, Isopropyl is pretty bad at that too. I'd recommend Deoxit Gold or CRC 2-26, the latter of which can also be picked up at Home Despot/Lowes.

The white crust you're seeing is most likely leftover solder flux that's been oxidized and destroyed by the alcohol. That residue isn't conductive, unless the board was also contaminated with something else that was conductive. The coating on ceramic capacitors is pretty tough, it can survive cleaning, just not hard impacts.

As for stuff misbehaving, it looks like your board only has single wipe sockets, where the socket only has metal on one side of the pin. These are notorious for having connection issues. I'm not sure about the TI branded sockets, but those may be problematic as well. I would recommend replacing all of those single wipe sockets with dual wipe, or go through every one of them with the chips installed and make sure they're making proper continuity. If you find any disconnected pins, or pins with high resistances, go back to replacing the sockets.

 

[Oswald]

Isopropyl alcohol is great for causing smears and streaks, it's pretty bad as a cleaner.

I would recommend CRC Lectra Clean, you can pick it up at Home Despot/Lowes. Just hose the board down with it and scrub it with a good brush. Let the fluid drip off the board before it evaporates so it carries the contaminants off the board.

For cleaning sockets, Isopropyl is pretty bad at that too. I'd recommend Deoxit Gold or CRC 2-26, the latter of which can also be picked up at Home Despot/Lowes.

The white crust you're seeing is most likely leftover solder flux that's been oxidized and destroyed by the alcohol. That residue isn't conductive, unless the board was also contaminated with something else that was conductive. The coating on ceramic capacitors is pretty tough, it can survive cleaning, just not hard impacts.

Thank you GiGaBiTe. I know people have washed VIC-20s and C64s successfully, and this PET board looks pretty similar to those.

 

[daver2]

+1 that the unreliability is probably the IC sockets.

Looking at your photograph I would suggest the ROM sockets are to blame - or other sockets of the same construction.

If you try my PETTESTER it might identify which sockets are being problematic (especially the ROM sockets as my PETTESTER checksums each ROM). Any poor ROM socket should result in an unstable checksum value being displayed.

Dave

 

[Oswald]

+1 that the unreliability is probably the IC sockets.

Looking at your photograph I would suggest the ROM sockets are to blame - or other sockets of the same construction.

If you try my PETTESTER it might identify which sockets are being problematic (especially the ROM sockets as my PETTESTER checksums each ROM). Any poor ROM socket should result in an unstable checksum value being displayed.

Dave

Are you the same Dave from Tynemouth Software? I love your work! My MiniPET 40/80 kit is still one of my favorite things I own, even now that I have a real PET. I actually recently ordered a second MiniPET for this machine (to use in tandem with the original logic board).

My PET came with your red 6502 ROM/RAM board preinstalled. I configured the DIP switches for the built-in test program and as far as I could tell everything seemed fine (screen 1 had all Gs, lot of "snow" on screen 2 unless reset was held down but apparently that's normal - on an unrelated note, I had no idea the 2001 PETs had problems with snow... reminds me of the TRS-80 model I).

As for the board, I want to try soap and water but am afraid the water might permeate into some of the components and damage them. Does anyone have any feedback/comments on this?

 

[daver2]

I am not.

I am a different Dave. We are as common as muck!

That PETTESTER in there is not mine, it is either another Dave's or Eudi's.

Yep, 2001 machines had snow!

Dave

 

[Oswald]

Oh... well this is a little embarrassing

 

[daver2]

Don't be, I aren't!

Dave

 

[Hugo Holden]

I never wash boards with anything containing soaps or detergents, Very rarely with water (once exception where it has to be water see below).

Your board in its current state is easy to restore to a normal and good looking condition. Simply treat the board with Inox's MX-3.Then with a combination of cue tips and clean cotton rags progressively start wiping it off everything (don't worry about the amount trapped around the IC socket claws, the retained lubrication helps). Slowly as you wipe the MX-3 away from the surfaces including the ceramic capacitors and patchy looking board's coating, IC surfaces etc, the surface finish will restore to normal and those patchy retained areas of flux in the surface coating will disappear. You may need to do the wipe off over a couple of days and a few times, as some MX-3 will later bleed out from under the IC sockets.

Interestingly, even though the idea might not seem like a permanent fix, it seems to be very long lasting, I have had pcb's in terrible cosmetic condition and even years after doing this, they still look great. I think the inox displaces the debris in the surface pits of the coating or at least alters those deposits to make them invisible.

The time to use water is when electrolyte has spilled from a capacitor onto the board surface, especially when it is a surface with the matte rather than gloss pcb coating. The electrolyte is not very soluble in standard pcb cleaners, and it needs a running stream of warm to hot water over the surface for at least half an hour to and hour leach out the ions, then the application of pcb cleaner after that.

I had a pcb once with electrolyte contamination and a flat coating. The circuit was a high Z arrangement and malfunctioning. All the components were removed, Two adjacent tracks exhibited properties of leakage and charge storage and behaved as a low value electrolytic capacitor. The coating was acting like a porous matrix holding the electrolyte. It was not altered by pcb cleaners at all, only the hot water leaching worked.

A lot of the time on computer boards with low Z circuits the leakage after a capacitor spill gets tolerated without malfunction. But it is better to get rid of all the electrolyte or it causes ongoing corrosion. An electrolytic cap may look like it may have not leaked in the past but has. With radial types the way to tell is to check with the meter if the surface of the rubber bung in the base is conductive on the meter. It always is if they have once leaked.

 

[Oswald]

Thanks for the recommendations, everyone. I ended up washing the board. It looks substantially better now and when I first powered it up it seemed to be working. But within a few minutes the keyboard issue came back (attached is a picture of what happens when you hold the L key) and the disk drive still isn't working reliably (I'm using a PET MicroSD). Also, the amount of RAM I see with the stock configuration changes from start up to start up. Sometimes 8k is reported free, sometimes 6k. Anyone know what might be the matter?

 

[GiGaBiTe]

If you haven't changed the single wipe sockets, you need to do that. The random memory amounts could be bad sockets, or it could be bad memory.

 

[Oswald]

If you haven't changed the single wipe sockets, you need to do that. The random memory amounts could be bad sockets, or it could be bad memory.

If the problem is the sockets what are the odds that the exact same issues would manifest after reseating all the chips and a complete cleaning?

 

[GiGaBiTe]

The problem with single wipe sockets is that the contacts can become ever so slightly deformed to where they won't contact the pins on the IC properly, no matter how much you clean them. Dual wipe contacts grab the pin on both sides and hold the IC much more securely in a way it has more chance of staying properly connected.

 

[Hugo Holden]

One thing about the single wipe socket design (and the notorious TI socket too),the plastic shroud can be levered off. If you do it though, you need to protect the pcb surface, don't just use a screwdriver or it will dig into the pcb surface and damage track work, it requires a protective layer of cardboard usually works.

Once the plastic shroud is removed, you then have two options:

One is to clean & re-tension the spring contacts and re-fit the shroud.

The other is to remove the pins one by one. Add a little fresh solder to each pin on the solder side, stand the board vertically on one edge, heat the solder/pin until it is definitely melted and holding onto the pin with forceps, withdraw the pins one by one. This is less stressful on the plated through holes and tracks than trying to remove the socket in one go after de-soldering because sometimes some of the pins stick on the inside wall of part of the plated through hole's circumference. Then solder suck all the holes, which is very easy with no pin inside them. Clean up the residual flux with IPA and then fit a dual wipe socket rather than a machine pin one. Also buy new dual wipe sockets, not old stock as some stored badly can have oxidized pins that are difficult to solder as one forum member found out a while back. It also pays to use good 60:40 solder, Ersin Multi-core or the equivalent made by Loctite in the USA. Don't use Lead free solder.

 

[Oswald]

I understand these sockets are bad but I honestly find it hard to believe multiple chip-reseatings and scrubbings with soap and water wouldn't have had any discernable effect if they were really the source of the problem. Which is not, of course, to say I think it's impossible it's the sockets, but I'd like to explore more options before I commit to desoldering and replacing practically every socket on the board.

 

[GiGaBiTe]

Every time you reseat a chip, every time you scrub a socket, you change it slightly. You also remove material from the IC leg scraping the contacts. You are getting a different result every time, it's just not a consistent result, nor will it ever be a consistent result.

You have nearly 50 year old clapped out sockets, nothing lasts forever, they need to be changed. Vintage computers are a pay to play hobby, they're going to be cranky and require lots of maintenance, time and money.

 

[Oswald]

You are getting a different result every time, it's just not a consistent result, nor will it ever be a consistent result.

I'm not. The PET is manifesting the exact same issues it had before the cleaning/reseating.

I suspect one of the chips is dying. I can't think of any other reason why the machine usually works fine on first power up only to get progressively less stable as the components heat up.

 

[Hugo Holden]

I'm not. The PET is manifesting the exact same issues it had before the cleaning/reseating.

I suspect one of the chips is dying.

Yes, of course it is. The signs of the fault were not suggestive of a definite socket issue. More like an issue inside one of the IC's, specifically the 6520 Keyboard PIA. Except that now you have soap residues inside your IC sockets that are very difficult to remove. You could try applying some freeze spray to the PIA and see if that alters it. In one of my machines it had a "Ghost" in the PIA, metaphorically speaking of course. Out of the blue random letters would start typing themselves on the screen, almost as if somebody was pressing keys, like the scene in the movie Ghost. It was the PIA. So they can get some very interesting faults.

( Notice I said signs of the fault, not symptoms. Machines cannot have symptoms, that is a Human thing. A sign is what is observed by the examiner and symptoms are what is experienced & reported by the patient. Though one wonders, with AI, whether a computer one day might pipe up and say to its service technicians; "I have symptoms of memory failure in bank B.....Dave" )

 

[Oswald]

Thanks Hugo. Unfortunately I think(?) my 6520 PIA may already have been replaced, probably a sign the previous owner had been having the same problem. Instead of a MOS 6502 there’s a Rockwell R6520-26. Nevertheless I’ll try replacing it with a spare from one of my Atari computers and see what happens.

 

[Hugo Holden]

Thanks Hugo. Unfortunately I think(?) my 6520 PIA may already have been replaced, probably a sign the previous owner had been having the same problem. Instead of a MOS 6502 there’s a Rockwell R6520-26. Nevertheless I’ll try replacing it with a spare from one of my Atari computers and see what happens.

As noted on the other thread, you can swap the PIA's for the test, that is if the other PIA is there. Rockwell chips are usually pretty good. But in this case, the PIA has to be eliminated as the cause, before moving on to other possibilities.

The PIA's seem to have a moderately high failure rate. Though in the PET, the most common failures are the memory IC's, perhaps then followed by the PIA's and sometimes corrupted ROMs and then by the 74 logic IC's. Oddly, despite the complexity of the chip, the CPU's appear to very seldom fail. It is like the old saying with a computer fault "It is never the CPU" but it must be on rare occasions.

We have seen some cases on the forum where multiple IC's have failed in the distribution field of one of the 5V regulators. In these cases likely somebody has made a mistake hooking up the PET board to an alternate power supply. Normally the standard 5V regulators and the hard wiring of the original power supply system does a good job of globally protecting the IC's from over-voltage and reverse polarity accidents.