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DOA PETs. (2001-8, 4032 and 8032)

Eudimorphodon said:
Anyway, I'd *really* like to get either the 4032 or 8032 working, preferably the 4032. Unfortunately my practical electronics experience is weak and the only tool I have on hand is an auto-ranging DVM,
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With a voltmeter and the 4032 schematics Zimmers, there are a few things you can do.

A static garbage screen usually means (with the non CRTC board) that the system is not running in program or perhaps not running at all. You checked the voltages, now check a few signals to see if anything is running. The CPU clock (phase 2) being a square wave should indicate about 2 V on a DC meter. The signal 'Sync' on the CPU should indicate 1 V if the system is fetching instructions.

If those signals are there then you may have bad ROMs, bad lower 16K RAMs or bad 40 pin I/O chips.

Are you lucky and have the ROMs on sockets? They can be replaced with EPROMs. I'm assuming they are the 24 pin 2332 style or are they the older 6540s?
 
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MikeS said:
A more or less standard 2716 will work in those ROM sockets without any adapter.
...
Like I said, it doesn't have to be the same model, or even have socketed ROMs; you can put your ROMs in one of the spare sockets and read/dump/checksum them. An EPROM programmer or even another system that uses 2532-compatible ROMs would do as well, of course.
Those aren't really the classic bad-ROM symptoms though.

How about some pictures of the motherboards and the numbers on those ROMs, so we have a better idea of what we're talking about?
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Doh! It looks like you're right. I've been laboring under the impression that I needed an adapter to replace oddball MOS 6540 roms, but apparently that's only for the original non-N PET. I just looked at the schematics for the 2001-N and it appears that it does indeed match up with a 2532.

(Honestly my gut tells me that "bad ROMs" is grasping at straws, but with so little to work with I've got to at least try to think up things I could brute-force test.)

I can take some pictures later if that would be any help. (I did write down the model numbers of the ROMs but forgot to bring them with me today.) Despite its labeling as a "4032" the machine seems by all appearances to really be a 2001-N-32. The motherboard is basically the same as this one, sans the punched-out RAM area. All ROMs are socketed.

http://www.classiccmp.org/dunfield/pet/h/p200108i.jpg
 
Eudimorphodon said:
If you end up with multiple working (or even "mostly working") 4032s I'll happily trade you the 8032 I snagged for one. It's not super-pretty but it's not terrible. It's the later lower-profile model.
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All the 4032 I got are dead. Even the one 8032 I had that appeared to be working has lost sound all of a sudden. It was doing the startup chirp just fine before but, no longer does. It still appears to work otherwise.
 
leaknoil said:
All the 4032 I got are dead. Even the one 8032 I had that appeared to be working has lost sound all of a sudden. It was doing the startup chirp just fine before but, no longer does. It still appears to work otherwise.
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That's too bad. Are any of the 4032s you got the small-screen models? (2001-N style motherboard?) I still might be interested in trading my sick-but-it-beeps 8032 for another machine that matches mine so I'll have a spare motherboard/parts mule.
 
dave_m said:
With a voltmeter and the 4032 schematics Zimmers, there are a few things you can do.

A static garbage screen usually means (with the non CRTC board) that the system is not running in program or perhaps not running at all. You checked the voltages, now check a few signals to see if anything is running. The CPU clock (phase 2) being a square wave should indicate about 2 V on a DC meter. The signal 'Sync' on the CPU should indicate 1 V if the system is fetching instructions.
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Thanks, I'll try that.

I think my DVM has a frequency counter but I'm not sure how fast it can handle. Perhaps probing those lines with that might be informative if it's capable enough.
 
Eudimorphodon said:
...I've been laboring under the impression that I needed an adapter to replace oddball MOS 6540 roms, but apparently that's only for the original non-N PET.
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Correct.
I just looked at the schematics for the 2001-N and it appears that it does indeed match up with a 2532.
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...Which is pin-compatible with the more common 2716 (the lower half only of course); in fact, the E ROM and the character generator are 2K 2716-compatible ROMs.
Despite its labeling as a "4032" the machine seems by all appearances to really be a 2001-N-32.
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AFAIK the only difference is BASIC4 vs. BASIC 2/3 (and of course the "new and improved" model number!). The 2001-N/4032 was also known as a 30xx-N (mostly in Europe), and of course the 12" FAT40 was also a 4032-N (note the 'N'), just to confuse the issue.
 
MikeS said:
AFAIK the only difference is BASIC4 vs. BASIC 2/3 (and of course the "new and improved" model number!). The 2001-N/4032 was also known as a 30xx-N (mostly in Europe), and of course the 12" FAT40 was also a 4032-N (note the 'N'), just to confuse the issue.
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"Are you keeping up with Commodore?" :^) Their marketing department never was the sharpest tool in the shed.

Interestingly, my PET 2001N-32 also has BASIC 4.0 ROMs. But all of the PETs rescued from Albany Computer Center seem to have been repaired or modified in one way or another. Both my 2001 and 4032 had Superbus II cards in them, for example (which I removed and replaced with the original ROMs instead)

-Twylo
 
dave_m said:
now check a few signals to see if anything is running. The CPU clock (phase 2) being a square wave should indicate about 2 V on a DC meter. The signal 'Sync' on the CPU should indicate 1 V if the system is fetching instructions.
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So... on pin 39 of the CPU I see about 2.2v. Pin 7 however doesn't show anything. I tried holding the meter there while cycling the power and I think it may of surged very briefly but I can't be positive. (Typical for cheap autoranging DVMs it's always jittering around and showing a few millivolts even when the probes aren't touching anything.)

Would "sync" stop showing activity if the CPU were "crashed" (because of a bad ROM? Or some other hardware?) or is the lack of a reading there diagnostic of a CPU-related problem?
 
Twylo said:
Both my 2001 and 4032 had Superbus II cards in them, for example (which I removed and replaced with the original ROMs instead)

-Twylo
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Did you happen to see how the card for the superbuss II connected to the motherboard ? I took one off and it will go back four different ways.
 
Eudimorphodon said:
Would "sync" stop showing activity if the CPU were "crashed" (because of a bad ROM? Or some other hardware?) or is the lack of a reading there diagnostic of a CPU-related problem?
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The fact that there is no Sync signal is troubling. It could mean that there is a continuous low on the /Reset, the CPU is dead, or perhaps the CPU read an illegal instruction that hung it up. It is called the "KIL" instruction. That would imply a bad ROM. Am I correct that your ROMs are not on sockets except for the E000 Editor?

The fact that Phase 2 clock is being generated by the CPU seems to indicate that at least part of the 6502 may be OK. I suppose it would not hurt to try another if you have one.

Check the /Reset to make sure it is a good high (4 V). It should only go low for a few tenths of a Second at power up but you will not be able to detect that without a scope.
 
dave_m said:
The fact that there is no Sync signal is troubling. It could mean that there is a continuous low on the /Reset, the CPU is dead, or perhaps the CPU read an illegal instruction that hung it up. It is called the "KIL" instruction. That would imply a bad ROM. Am I correct that your ROMs are not on sockets except for the E000 Editor?

The fact that Phase 2 clock is being generated by the CPU seems to indicate that at least part of the 6502 may be OK. I suppose it would not hurt to try another if you have one.
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So, in a possibly "interesting" twist I found that I actually get very inconsistent readings from the sync pin. If I power cycle the machine I'll sometimes find as low as .3v, as high at .7v. (Apparently I may of mistook a 0.xx reading the first time I tried this as being one of its spurious mv readings.)

All ROMs are socketed, so making (an undoubtedly bad) guess I tried pulling the F000 ROM (that is where a PET vectors to on powerup, right?). With the ROM unplugged I did get an approximately 1v reading on sync. I'm not entirely sure that's consistent, however.

Check the /Reset to make sure it is a good high (4 V). It should only go low for a few tenths of a Second at power up but you will not be able to detect that without a scope.
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I'm getting a good 4.8v on RESET.
 
Eudimorphodon said:
If I power cycle the machine I'll sometimes find as low as .3v, as high at .7v.
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This may imply that the PET gets stuck in different loops and that the 6502 is OK.
All ROMs are socketed
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Lucky boy! The first thing to try would be to perform checksums on each of them as Mike has suggested. For that you need access to a working PET or an old PROM burner.
 
dave_m said:
This may imply that the PET gets stuck in different loops and that the 6502 is OK.
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Well, I must say that this is a novel way of testing the ROMs, but you may have something there; maybe we should compile a table of sync voltages with different ROM configurations ;-)

A working 2001 shows 1.02V on pin 7.
Removing the F8 ROM gives .02V (and a garbage screen)
Removing the F0 ROM also gives .02V (and a blank screen)
Removing both, same as F8 - .02V and garbage

Don't quite know what that means in regards to your PET though...
 
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dave_m said:
This may imply that the PET gets stuck in different loops and that the 6502 is OK.
Lucky boy! The first thing to try would be to perform checksums on each of them as Mike has suggested. For that you need access to a working PET or an old PROM burner.
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I'll see if I can either beg/borrow some time with my local electronics guru's EPROM reader/programmer or get a PET diagnostic session put together with Twylo and peek at the ROMs. In the meantime I think I need to do more reading and see if I can get a better understanding of what the PET's power-on sequence is actually like. (Perhaps I could make some educated guesses about *if* the ROMs were OK what other failure-prone hardware the CPU might tickle on its way up that would cause it to crap out before clearing the screen.)

Being sort of an idiot I found myself fantasizing that if I had direct access to an EPROM writer I could try writing some code that would fill the screen with something and die, just to tell me if there was life. For cheap thrills I banged this together in a 6502 simulator:

Code: 
    sei
    lda #$00
    ldx $00
loop:
             tax
             sta $8000,X
             sta $8100,X
             sta $8200,x
             sta $8300,x
             inx
             txa
             jmp loop

If I were smart enough to figure out where in the ROM to put a jump to that in theory at least I think it would work to spew the complete ASCII character set into video RAM four times without worrying if any of the other peripheral systems or even system RAM is working. (I wrote it to just use two registers. But it's also the first 6502 code I've ever written so it may just inadvertently trigger an HCF.)

Of course, even if that would work it'd probably be a complete waste of time when I could just be burning replacement ROMs. :^b
 
Well, if you have the necessary writer then you could presumably also read the ROMs, but if they turn out to be OK then it might be a useful idea indeed.

Another useful tool (if you have a scope) is the NOP adapter discussed elsewhere, basically a modified socket sandwiched between the 6502 and its socket that just continually cycles through all 65536 addresses; it will help find any bad buffers or RAM/ROM chips pulling down address lines etc.

I think it's time for sharing a cool beer or two with your fellow PET doctor...
 
MikeS said:
I think it's time for sharing a cool beer or two with your fellow PET doctor...
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Yeah, I think I've pretty much hit that wall.

Since I'm pretty sure I'm at a dead end with the 4032 until I can swap some parts or get some expert advice last night I decided to investigate the theory that my 8032 might have a bad flyback transformer or cold solder joints. After making a discharge tool out of a big fat megaohm resistor and a length of wire wrapped around a cold water pipe... I'm not so sure I like the flyback theory anymore. It all looked clean inside the monitor case, the tube heater is lighting up, and I can hear/feel the tube taking a static charge when it's powered up. Without an oscilloscope and a better idea of where to start when doing TV repair I'm boned trying to figure that out myself.

(And of course I don't know if the motherboard in my 8032 is even outputting anything for the monitor to sync with.)
 
Eudimorphodon said:
Without an oscilloscope and a better idea of where to start when doing TV repair I'm boned trying to figure that out myself.

(And of course I don't know if the motherboard in my 8032 is even outputting anything for the monitor to sync with.)
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OK, two things to try with a voltmeter.

Test the voltage regulator on the monitor board for +12V. Be careful not to touch high voltage.

On the system board, if there is no horizontal or vertical drive signals, they will be above 4.0V. if somewhat under, they are probably running. I'm not sure of the video signal DC average level, but I'm guessing it is around 3 V.
 
Pin 1 (Video): 0.4V
Pin 3 (Vertical): 4.6V
Pin 5 (Horizontal): 3.6V

But measuring average voltages with a digital meter isn't necessarily very accurate.
 
MikeS said:
Pin 1 (Video): 0.4V
Pin 3 (Vertical): 4.6V
Pin 5 (Horizontal): 3.6V

But measuring average voltages with a digital meter isn't necessarily very accurate.
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Boy, I guess not. But now that I think about it, it would be hard to tell the difference between no vertical drive and a skinny negative signal at a 60 Hz rate. And apparently, there is not much difference between no video and a mostly blank screen. So these tests would be inconclusive.
 
So, as of today I have a few more tools in my arsenal... maybe. (Might give me a few more things to do until I can arrange a PET beer party. I wish this weekend wasn't going to be the circus it's turning out to be.)

They guy I got the original-keyboard PET for came bearing gifts. I now have a rather antique-looking 25Mhz "Circuitmate LP25" logic probe at my disposal, and perhaps more usefully (or perhaps not) I also have an osziFOX Probescope. I was playing with it a little and I doubt it'll be much good at actually matching a waveform shape to a reference, but it should at least be able to give me an idea if there's a signal present someplace. And in the trunk of my car I have a new 6502, two 6520As, and a set of eight 4116s, and on order there's enough 2532 EPROMs to replace every ROM in all three PETs. All I need to complete my "big chip" collection is a 6522. (And a CRTC for the 8032?)

(The ROM thing actually brings up a question... I read in another thread here that there was a problem with replacing the 2K E000 ROM with a 2532 because putting a 4k ROM in that socket will cause a clash with the I/O space in the E800 range. If that's the case could that be avoided by bending up the last address pin on the 2532 when socketing it to essentially render it a 2k ROM?)

So... keeping in mind the resolution limitations of a "probescope" would there be something I could do with that to get an idea if there's actually video coming out of the 8032? Or is there anything that might be worth poking with a scope or logic probe on the 4032 for that matter, while I'm waiting for ROMs?

(One thing my "Google-Foo" is really failing me on is finding a complete service manual for the Commodore PET that might help. The Zimmers archive has those scanned pages of board schematics and some sample oscilloscope traces for some probe points on the monitor board, but they don't seem to add up to a complete manual. I've been trying to absorb as much as I can from reading old threads about the subject, such as recommendations to check to see if address busses are stuck or cycling in various locations, etc.)
 
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