Repairing a SuperPET / PET 8032

[ral-clan]

Hi,

I recently purchased a SuperPET (see thread here for details of acquisition):
http://sleepingelephant.com/ipw-web/bul ... f=4&t=8083

However, it has some hardware problems which I've narrowed down to the main PET 8032 motherboard, rather than the SuperPET "expansion" daughterboard (the SuperPET is a stock 8032 with a daughterboard that gives it special abilities).

The SuperPET has an external toggle which allows you to boot in regular 6502 mode, or 6809 CPU mode. With the toggle switch in 6809 cpu mode I get the Waterloo languages menu. However, I am unable to make any choices from the menu, as below the menu the number "6" starts appearing over and over, as if someone were holding down that key. Very occasionally, a few other characters show up ("b" and "n"). This occurs even when the keyboard is physically disconnected from the motherboard. So it isn't a stuck key or keyboard matrix problem.


(click to make bigger)

When the external toggle switch is set to 6502 CPU mode the computer will not start. Black screen only. I know that the monitor has power though because occasionally on startup there is a phosphor dot or line in the middle of the screen which fades within seconds.

• - I've tried spraying contact cleaner in the toggle switch, and I'm pretty sure the switch is clean, so that's not the culprit.
  - I've swapped the 6502 on the daughterboard with a spare with no improvement (still black screen with toggle in 6502 mode).
  - I've removed the entire SuperPET daughterboard, effectively turning this computer into a standard PET 8032. I made sure to install a 6502 into the CPU slot (normally a ribbon cable goes from this socket to the SuperPET daughterboard). Black screen on startup.

Can anyone offer assistance, suggestions, or recommend a PET forum I should join to help repair this machine? Thanks.

[eslapion]

I don't have a schematic for that specific machine but that really looks like a damaged communication chip.

I think these machines also used 6522 VIAs. It so happens, thanks to the WDC, they are still in production.

[ral-clan]

I don't have a schematic for that specific machine but that really looks like a damaged communication chip.

I think these machines also used 6522 VIAs. It so happens, thanks to the WDC, they are still in production.

I have a bunch of those so it's worth a try. Unfortunately I think they are soldered on, which is a pain.
But would that explain the absolute no-boot (black screen) when booting into 6502 mode. Removing the entire SuperPET daughterboard and making this a stock PET 8032 also results in black screen no-boot (again, it's now a 6502 machine by default).

As far as getting schematics - it's an 8032 at its heart.

[sjgray]

Your 6502 mode problem could be a bad Kernal or Editor ROM. It seems like nothing is initializing the CRTC controller. Check socket continuity or swap in new roms.

Steve

[eslapion]

@Ral-Clan
Are you able to tell which version of the 8032 mainboard you have ? These are supposed to be 8032081 or 8032087.

Looking on Zimmers', it seems the keyboard scan is performed by a 6520 through a 74LS145 BCD to decimal decoder IC.

If you're lucky, only the LS145 is at fault. If the 6520 is a problem, I have no clue how/where to get a chip like that.

If the problem lies with one or more of the ROMs, the 8032 board uses standard 2332 and one single 2316 ROM. These can be easily replaced with 2732 EPROMs and a bit of adapting.

[ral-clan]

Hello,

I found a SuperPET technical (service) manual here:
http://classiccmp.org/dunfield/pet/sptech.pdf

It says that the PET8032 motherboard is assembly number Assembly Number 32090 (a page or two after that it mentions the 32087).

However, in looking at the photos I took of my motherboard it looks like I have Assembly Number 32089. I need to double check this though by opening the PET again as the photos are a bit to low-res to read this number 100%.

First thing's first - I'm going to check the voltages on the power supply.
Does anyone have a pinout diagram with voltages for the PET power supply?

[eslapion]

...
It says that the PET8032 motherboard is assembly number Assembly Number 32090 (a page or two after that it mentions the 32087).

However, in looking at the photos I took of my motherboard it looks like I have Assembly Number 32089. I need to double check this though by opening the PET again as the photos are a bit to low-res to read this number 100%.

First thing's first - I'm going to check the voltages on the power supply.
Does anyone have a pinout diagram with voltages for the PET power supply?

Here is what you need:
http://www.zimmers.net/anonftp/pub/cbm/ ... index.html

[mrr19121970]

Maybe this can help ?
http://blog.tynemouthsoftware.co.uk/201 ... s.html?m=1

[ral-clan]

Before I start digging into this PET...

I know the flyback transformer in the monitor is dangerous. Thankfully the monitor seems fine and I don't have to go into it.
However, is the big, green barrel capacitor just south of the power supply in PET computers safe to work around?

[eslapion]

However, is the big, green barrel capacitor just south of the power supply in PET computers safe to work around?

Yes it is. At the most, it will carry about 20Vdc when the machine is powered. It discharges very quickly when the machine is turned off.

[Jogi]

Hi ral-clan,

is the Funktion Superpet OK
8032 Modus OK.?

or can help you

Gruß Jogi

[ral-clan]

Hi Jogi,

Sorry about the late reply. I have stripped this SuperPET down to be a simple 8032 (removed SuperPET daughter board).
You can follow my progress trying to repair it at the VCF forum:

http://www.vcfed.org/forum/showthread.p ... k-SuperPET

I have already determined the power supply is supplying good values to all chips I have tested. I am suspecting a bad IC at this point but need to narrow down which one.

[ral-clan]

Just a quick note that with the help of the folks on the Vintage Computer Forum, I was able to repair this SuperPET.
Great to have the VIC-20 beside it's bigger brother (uncle?).

The complete repair thread is here:
http://www.vcfed.org/forum/showthread.p ... k-SuperPET