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PET 2001-8 Repair Assistance Needed

Derpy730

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Looking for someone to advise/assist in troubleshooting and repair of a PET 2001-8. Device boots but screen display is garbled (but READY prompt is somewhat legible). Key presses are recognized but display produces a distorted character.
 

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Gary C

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What test equipment do you have and do you have the ability to burn an EPROM

First thing I would do is install Daver2's pettest V4 rom as your machine is almost working and that would probably give an idea of whats happening.

Looking deeper, the machine is obviously running, but is having problems with data to or from the video rams through to the Character ROM and out through the Parallel to serial shift register..

if you look here you can find the drawings for your system (2001N by the look)

Sheet 8 shows the video data section

Looking at it, its not the video RAM's as even if they presented corrupt data, whole characters should still be created from the data in the Character ROM.

So maybe the E11 shift register is corrupting what its being presented with or something in the logic.

Need to think but I'm wondering about G10 or G9 as thats to do (I think) with the inverse video. If the output of the 8bit latch is so noisy that the character generator is producing random outputs as the data is being clocked into the shift register and also flipping LSD7 ?
 
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daver2

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That's quite a 'funky' display you have there...

It is interesting that there are some partial characters (e.g. a 'zero' with a cut-off base and either the READY has a cut-off top or the characters on the line above have some additional lines).

It is also interesting with the bands of alternate inverse and none inverse lines.

These 'patterns' can't be produced by software/firmware to/from the video RAM, so (as Gary has stated) must be within the video display logic somewhere.

Dave
 

oldpcguy

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If I were troubleshooting this the first thing I would check is the RAM. The RAM in these systems is prone to failure and thus it would be my starting point. This might be a handy diagnostics tool for you (if you know basic soldering as it's sold as a kit):

ROMulator 6502 Kit
 

Nivag Swerdna

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In Post #1 if you squint you can read...

### COMMODORE BASIC ###
7167 Bytes Free

So this PET is almost working already. The issue seems to be in the video generation only (since it can clearly write to 8000- since that's where the message comes from)

Is the screen static or changing? Maybe remove, deoxit and replace the character generator ROM?

The chargen is A2 in http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320008.pdf on page 3

Another thing to try would be to swap the 2 video RAMs for 2 system RAMs... I would suggest swapping... C3 & C4.... with I8 and J8

Let us know what you see!
 
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ajgriff

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Isn't this a dynamic PET with soldered RAM chips? If so swapping chips around isn't that easy unfortunately.

Alan
 

Hugo Holden

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Another thing to try would be to swap the 2 video RAMs for 2 system RAMs... I would suggest swapping... C3 & C4.... with I8 and J8

Let us know what you see!
I think the two video RAMs are 2114 static RAMs (18 pin devices) and the main ram is 4116 dynamic RAM(16 pin devices) so the IC's are not interchangeable.

This is the Zimmers schematic for this board (I think)


It looks like this board has been subjected to the inexcusable vandalism at Commodore where some lunatic took to the pcb with a drill, to prevent the additional DRAM IC's from being added. I think the best way to fix this would be to take a photo of each hole and design a custom pcb plug to fill each one with the missing tracks and holes and they could be glued in with 254hr epoxy resin and the track edges linked with solder. Or one other option could be to add additional RAM on the expansion connector.
 
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Hugo Holden

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I have thought of another way to fix the damaged pcb in the DRAM area. A replica of the section of the pcb, of both sides, just around the perimeter of the 8 DRAM IC's could be made, so a rectangle of pcb. Two separate single sided very thin pcb's perhaps only about 0.5 to 0.7 mm thick could be made to overly each side of the board. This would; 1) cover the holes and regenerate the normal trackwork on both sides. Once these were made (the Gerbers) then this method could become the standard repair tool to neatly fix this damage, regardless of exactly where the vandal holes were drilled. Though the pcbs could have through holes and pads around the holes on the face that approximated the pcb surface, but they would not need tracks on that face. It might require a couple of links to its trackwork depending on the extent of the damage. It would effectively create a 4 layer pcb in the area.
 
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ajgriff

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I think the two video RAMs are 2114 static RAMs (18 pin devices) and the main ram is 4116 dynamic RAM(16 pin devices) so the IC's are not interchangeable.

It looks like this PET is displaying available RAM in line with that of an 8k machine in which case the system chips are likely to be 4108 dynamic RAMs (half faulty 4116s). Also the holes are consistent with Commodore's policy of making it difficult to circumvent their pricing structure by upgrading a less expensive PET more cheaply than buying a more highly specified machine. I agree that the video RAMs will be the static type and are most likely 2114s. The board quite clearly has the layout of a 2001N dynamic PET and Nivag's suggestion would only work for an earlier 2001-8 PET with static RAM throughout.

Like Dave (daver2) I think the problem rests within the video display circuitry. In terms of finding the fault much depends on what diagnostic equipment is available.

Alan
 

Hugo Holden

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That seems to be the perceived wisdom Hugo but I'm only going by comments gleaned from the web such as this article:


Seems plausible.

Alan
That is very interesting.

I knew this sort of thing happened with transistors where after they were manufactured & tested different suffix numbers for gain were added and sometimes even different part numbers used.

But I had never heard of this happening with IC's before. Mind you I had not considered the opportunity to do it with memory IC's, where it could be possible to label defective ones for a part number with a lower bit or byte capacity.
 

Gary C

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Yes, they are either A or B types and either need to be replaced with A or B, or just normal 4116's

But, I dont think its a RAM fault.
 

Gary C

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The PET is jumpered for type A or B (ie fault in bottom half or top half) and they all have to match too.
 

Hugo Holden

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I have been buying and collecting various brand 4116's to keep as spares for my PET and some S-100 boards I have that use them. Trying and testing ones from various manufacturers, Intel,TI, ITT etc etc. I found some interesting and very attractive looking unbranded ceramic ones in Sarajevo, Bosnia. I think they probably originated from Russia many years ago. (Picture attached). Every one of a sample I tested worked perfectly, so I bought a swag of them for my IC collection.
 

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