I picked up a SuperPET with some problems. It has a 8032089 Universal Dynamic PET motherboard (click on any of the images below to enlarge them).
When started in 6809 mode, sometimes I get the language startup menu, with some corrupted characters, and no ability to do or choose anything.
When started in 6502 mode, I just get a black screen with no startup beep, or sometimes a glowing phospor dot that may fade or get brighter (so I shut it off quickly). So....judging from this I know:
1. the monitor works (I sometimes see the menu displayed)
2. the MOS6545 Cathode Ray Tube Controller chip probably works.
3. the Character ROM probably works.
4. the SuperPET daughterboard with 6809 probably works (somewhat)
Because of the unwillingness for the machine to respond at all when switched to 6502 mode, it seems likely there is a problem with the main motherboard. So, I've removed the SuperPET daughterboard, temporarily turning this machine into a stock 8032 PET. I put a spare 6502 into the CPU socket because the original 6502 is on the (now removed) SuperPET daughterboard.
Turning the machine on like this results in a black screen, or sometimes a green phosphor dot.
I should mention that there is a strange jumper wire on the underside of the main motherboard. It runs from PIN 35 of the 6502 CPU to PIN 1 of a SN74LS393N at location UD3. Apparently this latter chip is a 4-bit binary counter. The confusing part is that pin-35 of the 6502 is listed as "NC" (not connected), but maybe this line is used to transfer timing from the counter, up the ribbon cable that normally sits in the motherboard CPU socket, to the SuperPET daughterboard for use by the 6509? Really I have no idea as I'm out of my league on this.
Not sure if a stock 8032 has this jumper. I've heard rumours that there might be a small mod in SuperPETs in regards to where CPU gets its timing from - I'm not sure what this means exactly - but maybe it's related to this? Also, perhaps because of this, a SuperPET won't run as a regular 8032 PET unless this mod is reversed?
First, I want to test the power supply to see if it's outputting proper voltage. I disconnected the power supply connector from the motherboard at location J8 with the intent to test voltages with a multimeter. However, I cannot find a proper voltage pinout diagram or table for this connector. I've found some confusing information about the 8032's power connector online, but some of these mention an 11-pin connector. The power connector on my SuperPET seems to be a 9 hole connector with one hole (hole 3) blocked. It connects to a 9-pin header on the motherboard ( location J8 ) with pin-3 intentionally missing. I've also downloaded the 8032 and SuperPET service manual (technical guide). It has a diagram of the power supply, but there's nothing on there that makes sense to me or matches the connector I see physically in the machine:
Secondly, I've heard there are some AC voltages on this connector. Okay....I'm used to dealing with PC/C64/VIC-20 power supplies that are all putting out DC, but I can probably handle testing the AC voltages if I have a pinout.
Thirdly, I've heard that some of the PET's power supply is actually on the motherboard - I can see the large capacitor and couple of circular voltage regulators on the black metal section, so am guessing that's part of it.
Once I have determined that the power supply is good, I can move on to the next suspect: a possible bad EDIT ROM - apparently with CRTC video chip PETs if the EDIT ROM is bad the machine will appear black on power up. But first I have to eliminate the power supply as the culprit.
Can anyone help me out, or guide me through this? I should mention that I have experience repairing old computers, and can do fine electrical work / component replacement. However, I am self-taught. I am no electrical engineer and cannot diagnose problems that are not obvious (i.e. burnt component) or easily measurable with a multimeter / logic probe.