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Nice PET but garbage screen

carlsson said:
Time for the power on test - will it smoke, go BANG or work as expected?
Click to expand...

Yes, I know that feeling of anticipation and dread :)

carlsson said:
Update: Ok, two motherboards tested OK. Since the latter one is equipped with Basic 4.0 and lower case characters, I installed it into the 4032 and will trade the other first one.
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Good one Anders! Another PET up and running. Any Pics?

Tez
 
I pondered taking some pictures this afternoon, but I was swamped with tools and never went up to grab the camera. Certainly I could take some after pictures.

Luzur: Too bad about your PET. I have one more motherboard, mostly working but may be missing some chip. Should I investigate for you or will you trouble shoot first?
 
you what it is then? it works as i can print stuff "behind" that "curtain" of garbled graphics, so it must be something to do with some videchip, right?

i have no clues on PET 2001 computer schematics so i dont know where to start.
 
Me neither, but I'd bet on partial RAM problems. I would look up which 4116 RAM chip holds the screen matrix. If it is bad, the computer will read garbage from memory when it generates the display.

For schematics, look here. I believe yours is a 2001N, a.k.a. 3000/4000.
http://zimmers.net/anonftp/pub/cbm/schematics/computers/pet/index.html

By the way, I believe the 6502 CPU generates the video display on the early PETs. Latter models has a 6545 video generator which helps quite a bit, but is nothing compared to even latter video chips like VIC and so on.
 
Luzur said:
so it could be the 6502 CPu thats making the fuss then?
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Maybe. Seems like it could be lots of things. This is problem I've found with my PET. There seems to be so many things that might cause the "garbage screen" symptom. RAM seems to be the "most likely" but the effort to unsolder and solder in a whole new bank of RAM begs a more definitive diagnosis.

The question is "where do you start?" If there were some quick tests which could be done to isolate the problem to one part of the circuitry, it would make the problem a lot less daunting.

Anyone know of any type of "diagnostic tree-diagram" for early Pets using a logic probe and/or voltmeter, which might assist people diagnosing faults in these beasts, or at least getting closer to what part of the circuit is not working?

Tez
 
well if its soldering involved im screwed, was never a solder master either, i better wait til my brother comes home then try something myself.

one thing i noticed was that UD4 was empty, no chip in place, is that normal?
 
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Yes, if I understand correctly the whole row of UD2 to UD9 are ROM slots. Well, maybe not UD2 as it contains a SN74154N. The upper four are (without checking) Kernel, Basic and Character ROMs. The remaining three: UD3, UD4 and UD5 would be option ROMs, e.g. for expansion software. The board I'm sending to Tezza has all three empty.

If you can type in a program in blind and execute it, there is probably nothing wrong with the CPU. Of course it means you need to trust that the keypresses register. On the other hand if there would be a CPU fault I doubt you would get any display at all.

While the board in front of me is not a 2001N, I suppose you have a row of 4116 RAM chips at one end. How many and at what locations? It should be U12 to U19, with room for expansion at UJ2 to UJ9. Possibly you also have 32K RAM so all positions would be filled. I don't know if the two small capacitors next to each RAM chip can go bad and cause the fault you're observing. Since you probably don't need to use your PET tomorrow, I'd wait for your brother to have some time to look at it, in combination with schematics. I have some stuff on paper in the basement too, but I believe it all is identical to what's on the web.
 
carlsson said:
UD3, UD4 and UD5 would be option ROMs, e.g. for expansion software. The board I'm sending to Tezza has all three empty.
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UD3 and UD5 have ROMS in them, HN462716 and 2516JOD or JOL (hard time reading what it says cuz someone has made a dot with a marker pen there)

carlsson said:
If you can type in a program in blind and execute it, there is probably nothing wrong with the CPU. Of course it means you need to trust that the keypresses register. On the other hand if there would be a CPU fault I doubt you would get any display at all.
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havent tried typing any command in yet, perhaps i should, but i wouldnt have a clue if it worked though :)

carlsson said:
While the board in front of me is not a 2001N, I suppose you have a row of 4116 RAM chips at one end. How many and at what locations? It should be U12 to U19, with room for expansion at UJ2 to UJ9. Possibly you also have 32K RAM so all positions would be filled. I don't know if the two small capacitors next to each RAM chip can go bad and cause the fault you're observing. Since you probably don't need to use your PET tomorrow, I'd wait for your brother to have some time to look at it, in combination with schematics. I have some stuff on paper in the basement too, but I believe it all is identical to what's on the web.
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U12 to U19 have HM4716A-4N in full rows, likewise with UJ2-UJ9and all capacitators looks fine my human eyes.
 
Tantalums (the little yellow caps) are notorious for going short-circuit - should be easy to check with a meter if you have one. (Cold check) If not maybe bridging the cap using an LED in series with a coin cell.



BG
 
I'm helping out Tezza by attempting to repair his PET. This 3032 is in excellent condition & as PETs are so rare here in New Zealand - it's mightily deserving in having attention thrown at it.

Summary so far: When turned on, the screen displays garbage. Terry has cleaned the CPU board and removed nearly all rust visible in the photos (at beginning of this thread). He has removed, cleaned pins & re-inserted all socketed ics. He tried piggy-backing the 4116 RAM chips one at a time watching for any change. No consistent change.

I enter the scene. With a scope I have verified Power Supply is fine. All voltages are correct & show no signs of problem ripple. RESET signal at CPU appears fine at start-up. Stays active (lo) for nearly a second after power up. There is also a good CLK signal coming in, plus RDY is active.

Address lines appear all active (signals present on all), however I believe I have found a fault on the Data bus - D4 appears stuck LO.

Referring to circuit diagram:

Pin 14 of F7 (Video Ram 6114) appears stuck HI. There is slight sign of a signal trying to work there, but 98% stuck HI. All other Data pins to F7 & F8 are good signal. Following SD4 thru the buffer (E7) to pins 17,18 - BD4 is stuck LO.

Now, I'm a newbie to Commodore PET design & have had little experience in faultfinding CPU boards. I ask where to from here? Am I correct in assuming D4 is truly stuck? If so, how do I go about determining exactly what is pulling it down? Lastly, am I correct in thinking the 6114 Video Ram can't be 'sticking' it, otherwise the signal would be fine on the otherside of the buffer ic (E7)?

Thanks for any help
Philip
 
Hopefully, a good working board should be on its way and arrive in a few days of time. Once there, you should be able to check the jumper settings, then pop it in and go. It'll be configured with Swedish characters though, but it is just a change of character ROM; I think the kernel is identical.

But yes, it would be nice to diagnose the fault on the existing board. By the way when you say 6114, do you mean 4116 or are there chips of both numberings?
 
carlsson said:
Hopefully, a good working board should be on its way and arrive in a few days of time. Once there, you should be able to check the jumper settings, then pop it in and go. It'll be configured with Swedish characters though, but it is just a change of character ROM; I think the kernel is identical.

But yes, it would be nice to diagnose the fault on the existing board. By the way when you say 6114, do you mean 4116 or are there chips of both numberings?
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Yea, it would be good to get this board fixed too. That what we are aiming for.

Either way, that PET is gonna be going soon! :)

Tez
 
pavery said:
Pin 14 of F7 (Video Ram 6114) appears stuck HI. There is slight sign of a signal trying to work there, but 98% stuck HI. All other Data pins to F7 & F8 are good signal. Following SD4 thru the buffer (E7) to pins 17,18 - BD4 is stuck LO.
Click to expand...

Sounds like you've narrowed it down Philip. Good going!

Not sure how many old ICs you've got in your workshop there? I've got a few logic chips here from the Apple Haul if it comes to the point where we need to subsitute a chip or two. The two systems seem to have a few ICs in common.

Hopefully someone will have an idea of how to proceed from here. There seems to be a lot less web material on the PET compared to the Apple as far as details of the electronics go, but maybe I just haven't looked hard enough.

Tez
 
Latch

Latch

SD0 ends up driving the LSB of the character ROM. Are you getting anything on pin 5 (A0) of F8 (6114) ?

You might want to try connecting SD1 to SD0 ( pin 13 to pin 14 ) and see if there's any difference on the monitor, and if the signal on SD1 collapses.
None of this stuff is socketed, right ?

patscc
 
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patscc said:
SD0 ends up driving the LSB of the character ROM. Are you getting anything on pin 5 (A0) of F8 (6114) ?

You might want to try connecting SD1 to SD0 ( pin 13 to pin 14 ) and see if there's any difference on the monitor, and if the signal on SD1 collapses.
None of this stuff is socketed, right ?

patscc
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Thanks Patscc

Strange. The second paragraph of your post only became visible when I replied.

I think you might be looking at the other 6114 (F8 ) instead of F7 where SD4 is. However I tried shorting SD4 with SD5 on F7 and yes, the monitor changes most chars. Eg where 'X' are, these change to 'H' (see original monitor photo). SD5 signal does NOT collapse. Nope, none of this socketed.

Philip
 
Sd4

Sd4

That's because I went back and editied to add a question. F7, F8 close enough. Never could count real well anyway. But you got it anyway.

Well, what I'd do is desolder SD4 (pin 14 on F7) from the board, and then do the same thing you did before (SD5 to the SD4 on the board). If you get the same behavoiur, I'd say dead 6114.

patscc
 
patscc said:
Well, what I'd do is desolder SD4 (pin 14 on F7) from the board, and then do the same thing you did before (SD5 to the SD4 on the board). If you get the same behavoiur, I'd say dead 6114.

patscc
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Ok Patscc. Yes - I get the same behaviour. With SD4 (pin 14 on F7) desoldered and SD5 & 4 linked - I get the 'H's. SD4 on the board now has good signal (as it's now tied to SD5), so I assume nothing was actually pulling it up - it was just that line (D0) of F7 is dead. So I need to replace that 6114 - yes?

Hey Tezza. You gotta 6114 ic (or 2114 as that is what this one is actually)? I ain't.

Question: If this is the only fault in this PET, wouldn't the PET boot up normally and just have a less strange screen than this? I'd expect it to have a mostly clear screen once booted (normally), and that in this case the clear parts of the screen should have the same (albeit wrong) character displayed.

So I ask: Is there something I can type at the keyboard which would activate a latch/port which I could measure without screen feedback?

Philip
 
Load

Load

Does this have the cassette ?
Try LOAD "foo",1

I think that's the load command.
You should also be able to POKE a memory location to make it beep or write a character to screen, but this varies with model & ROM version.

If you have a manual, the memory locations might be in there.

patscc
 
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