PET 4032 Keyboard Circuit Board

[Mike Brixius]

On my pet 4032 there are 3 or 4 keys, all in the lower left corner that are very unreliable. This is a Graphics Keyboard

The stems are good and work perfectly in other positions (They read about 200 ohms)

The board has continuity (I had to repair 3 of the overlay traces)

Here's the rub, the pads look perfect and if I hold a stem and press on the board pads it reads great except on these pads. In their cases I have to press the pad a little lower than centered or it doesn't read. I am guessing (I know) that the traces for one side of the circuit are a little thicker than the other preventing contact?

All the problem pads are ion the lower left centered on the C key which is the worst

I dunno it is VERY weird. Ive thought of tinning the contact trace where the pad contacts but there is no going back from that.

Mike

 

[robert_sissco]

It sounds like the pads may be compressed to me, if they work when you press them against the exposed bored but when inserted to the try you need to press hard/at an angle.

You can try painting them with conductive paint/using a pencil to see if that helps, otherwise if you look you can find PET keyboard plungers with a bit of regularity. I had to replace 4 on mine due to a similar issue you are having (worked find out of the try, issues in it) and this is what I did when the pain failed on these 4 but restored functionality to the others.

 

[Hutch]

Possible the PCB is slightly warped?
200 ohms should be fine. Have you tried a mild abrasive to remove the hardened rubber surface and make the pads more flexible?
There are some chemical rejuvenation methods too <https://youtu.be/n9gGz2n-sBU/>

 

[Mike Brixius]

Possible the PCB is slightly warped?
200 ohms should be fine. Have you tried a mild abrasive to remove the hardened rubber surface and make the pads more flexible?
There are some chemical rejuvenation methods too <https://youtu.be/n9gGz2n-sBU/>

I am certain its not the pads. They read ok and work perfectly on all the other positions on the board. Any pad has issues on these few contact points on the board. Flexing the board a little in either direction doesn't help. THe only thing that has been helping is pressing the pad down so that the edge is about 1/3 to 1/2 the contact position diameter lower on the board than normal.

I am headed back out to the shop soon and will get some images.

 

[Mike Brixius]

I took the closest pictures I could then tried wearing 3x reading glasses along with a magnifier "Helmet of Goober" with both lenses in place and I still cannot see anything off.

Out of desperation I burnished the contacts with 5000 grit sand paper (For polishing car paint) and, indeed, the right portion of the contact was burnished but the two fingers that go to the center on the left are not getting sanded. I was unable to get a picture that shows this. I guess I am going to have to abraid the higher contact points down a bit.

I had no idea this was that touchy and have no idea what could have caused it. Maybe it was always a little off so students pounded the C key a lot to get it to work causing further damage?

Mike

 

[Mike Brixius]

It sounds like the pads may be compressed to me, if they work when you press them against the exposed bored but when inserted to the try you need to press hard/at an angle.

You can try painting them with conductive paint/using a pencil to see if that helps, otherwise if you look you can find PET keyboard plungers with a bit of regularity. I had to replace 4 on mine due to a similar issue you are having (worked find out of the try, issues in it) and this is what I did when the pain failed on these 4 but restored functionality to the others.

This contact point on the board has the same issue with all my plungers. On most key contact I can press a plunger sloppily and quickly and get fast characters while on these, and especially the one, any plunger is hard to get a press out of. After messing with it I think that the left contact traces going into the center of the contacts are thinner than the ones from the right. I uploaded some images on another post.

 

[Mike Brixius]

I burnished it well and then cleaned with alcohol. Obviously 5000 grit doesn't take much material off. I swapped posts with keys that were working well and put it back together. It's a bit better but not much. The problem keys are still c a s d & f so it's not the posts or the issue would have moved. I'm going to have to try a bit more major surgery but was wondering if there is a snowballs chance in hell of getting a replacement board and black plastic frame if things go poorly. That area of the frame is funky as seen in my restoration post.

Mike

 

[Tdillon]

On my pet 4032 there are 3 or 4 keys, all in the lower left corner that are very unreliable. This is a Graphics Keyboard

The stems are good and work perfectly in other positions (They read about 200 ohms)

The board has continuity (I had to repair 3 of the overlay traces)

Here's the rub, the pads look perfect and if I hold a stem and press on the board pads it reads great except on these pads. In their cases I have to press the pad a little lower than centered or it doesn't read. I am guessing (I know) that the traces for one side of the circuit are a little thicker than the other preventing contact?

All the problem pads are ion the lower left centered on the C key which is the worst

I dunno it is VERY weird. Ive thought of tinning the contact trace where the pad contacts but there is no going back from that.

Mike

I have several keys not working on a 4016.I ordered CaliKote44 based on others' suggestions.

 

[Hugo Holden]

I wouldn't apply any abrasive paper to the pcb, even 5000. The most abrasive , if you want to clean off surface films is some A4 paper with contact cleaner , it would roughly be equivalent to 20,000 grade paper if there was such a thing.

You need to look at the conductivity of what the key plunger applies to the pcb.

I have not examined this keyboard. But I have others including types with either conductive pads or capcitance pads. In the case of conductive pads the material degrades and loses conductivity at its surface. Sometimes cleaning helps, but many times makes it worse.

However, many sizes of conductive rubber pads have been produced and are for sale on ebay, they were primarily designed for vintage calculator repairs. This sort of thing, but there are many more:

450Pcs Conductive Buttons Rubber Remote Control Pads Keypad Repair Kit 2-8mm | eBay

50pcs x 8mm. Thickness of pads: various, between approx. 50pcs x 6mm. 50pcs x 5mm. 50pcs x 4mm.

www.ebay.com

 

[Mike Brixius]

The 5000 grit was not used on the rubber pads (I do use paper for that). That was on the circuit board itself. The issue here is that the keys that are unreliable do not change when plungers are swapped. The conductive rubber all read well within spec with a meter, but the keys in the lower left always have an issue no matter which plungers I use. I am leaning towards the issue being with the black plastic frame that the plungers mount in. It looks like it did not get sufficient material into the die when it was molded but probably passed QC because it worked and because... Commodore.

There are pictures of it on another thread here https://forum.vcfed.org/index.php?t...perate-need-of-some-love.1240755/post-1306977

Mike