Hello,
I have a PET 2001-N-32 that needs repair. Can anyone recommend diagnostic books, tools, and part sources for the PET. The problem is with the monitor. It displays scrambled characters only. I would like to learn how to repair my PET myself. Any help you could provide would be greatly appreciated. Thanks!
Mark
If anyone specializes in repairing PETs, please let me know.
PET repair
-
Personable85
- Posts: 63
- Joined: Tue Oct 10, 2006 10:27 am
- Location: Warren, Michigan USA
-
j1mt3hcop13rwr3nch
- Posts: 28
- Joined: Thu May 25, 2006 5:00 pm
- Location: Hamilton, ON
- Contact:
Hi Mark.
When you say scrambled characters...
Is the rest of the 'puter apparently functioning correctly? Ie:you could still type a program and run it but the characters themselves would be messes up
Or is the whole thing locked down into a mode with properly formed but "garbage" characters on the screen - like you'll momentarily see with a normal warm restart?
Is the problem intermttent or always there?
Either way, old equipment does get - well old. I suggest personally before going wild to do some "housecleaning" - resocket and reseat what can be. and don't overlook the fact that electrolytic capacitors don't age well and can act like wonky RAM.
When you say scrambled characters...
Is the rest of the 'puter apparently functioning correctly? Ie:you could still type a program and run it but the characters themselves would be messes up
Or is the whole thing locked down into a mode with properly formed but "garbage" characters on the screen - like you'll momentarily see with a normal warm restart?
Is the problem intermttent or always there?
Either way, old equipment does get - well old. I suggest personally before going wild to do some "housecleaning" - resocket and reseat what can be. and don't overlook the fact that electrolytic capacitors don't age well and can act like wonky RAM.
Stranded in time. Surrounded by evil. low on gas.
-
Personable85
- Posts: 63
- Joined: Tue Oct 10, 2006 10:27 am
- Location: Warren, Michigan USA
Pet Repair
The monitor screen is completely covered in perfectly formed characters. The system is locked up. The problem occurs as soon as the computer is turned on and in not intermittent. Any help you could provide in diagnosing the problem and offering repair suggestions would be greatly appreciated. Thanks!
Mark
Mark
-
Personable85
- Posts: 63
- Joined: Tue Oct 10, 2006 10:27 am
- Location: Warren, Michigan USA
Video ROM?
I was thinking that the problem may be with the transformer. It does buzz very loudly. My theory is that the monitor is working just fine. After all, the screen is covered in perfectly formed characters. I believe the monitor is simply not getting a signal from the computer. It is my theory that the mother board is not getting any power from the transformer. I'm trying to test my theory now. If the problem is with the Video ROM, I don't know what I'll do. Can anyone recommend a User's Group for help? I originally bought the unit for parts, maybe that will be its ultimate destiny... Thanks!
Mark
I'm in Warren, Michigan. I have two PET 2001-N-32s. One is working and one is not. I'm looking for a good part source.
Mark
I'm in Warren, Michigan. I have two PET 2001-N-32s. One is working and one is not. I'm looking for a good part source.
-
j1mt3hcop13rwr3nch
- Posts: 28
- Joined: Thu May 25, 2006 5:00 pm
- Location: Hamilton, ON
- Contact:
IIRC a bad video rom would usually either add or remove "lines" from the 8x8 matrix that makes up the character, no? if the characters are formed correctly the vid rom should be good.
Really could be just about anything, Old TTL's are pretty tough customers, so personally after checking that you have clean board power and all of the proper voltages (especially the -5V at the RAM banks) I'd head toward the RAM and MOS IC's first. The MOS proms are I think all socketed, but the ram isn't - which can be a bit of a pain. The biggest of which is that god only knows where you could still find (2114?) dram IC's (However, lemme know where if you can) The bright side is if you're half decent with an iron the PET boards tolerated desoldering and socket population quite well.
Do you have the upgrade daughter board on there?
You emailed that the CRT was making some noise, but I doubt this is your problem. Although I've seen CRT driver board issues locking up PETS this was usually a more intermittent problem.
Trafos can get buzzy too as they get old and the shellac allows the windings to vibrate but it's unlikely this is your problem. Noisy doesn't necessarily mean bad - just noisy. Although a dying main filter electrolytic very well may be loading the trafo (which may make it louder) AND causing your problem.
A few places it's probably not...
Master clock. Gross clock generator errors will make the characters on the screen bigger or smaller, so prolly not here.
Uh. lemme try to find a schemmy to come up with a few more idears and reference the IC numbers (board location). It's been a very long time.
Got a logic probe?
Just noticed you posted again before I submitted this. You have a working machine so swap out some parts...
Video Rom is the socketed twenty something (24, 26?) pin socketed IC closest to the connector going to the CRT.
The filter cap is connected via two screws.
If the boards are identical - swap the proms one at a time. The CPU is socketed also. If they're not: ie - one upgrade one not - then you can still "swap" everything socketed, just not one at a time.
Take ESD precautions.
Really could be just about anything, Old TTL's are pretty tough customers, so personally after checking that you have clean board power and all of the proper voltages (especially the -5V at the RAM banks) I'd head toward the RAM and MOS IC's first. The MOS proms are I think all socketed, but the ram isn't - which can be a bit of a pain. The biggest of which is that god only knows where you could still find (2114?) dram IC's (However, lemme know where if you can) The bright side is if you're half decent with an iron the PET boards tolerated desoldering and socket population quite well.
Do you have the upgrade daughter board on there?
You emailed that the CRT was making some noise, but I doubt this is your problem. Although I've seen CRT driver board issues locking up PETS this was usually a more intermittent problem.
Trafos can get buzzy too as they get old and the shellac allows the windings to vibrate but it's unlikely this is your problem. Noisy doesn't necessarily mean bad - just noisy. Although a dying main filter electrolytic very well may be loading the trafo (which may make it louder) AND causing your problem.
A few places it's probably not...
Master clock. Gross clock generator errors will make the characters on the screen bigger or smaller, so prolly not here.
Uh. lemme try to find a schemmy to come up with a few more idears and reference the IC numbers (board location). It's been a very long time.
Got a logic probe?
Just noticed you posted again before I submitted this. You have a working machine so swap out some parts...
Video Rom is the socketed twenty something (24, 26?) pin socketed IC closest to the connector going to the CRT.
The filter cap is connected via two screws.
If the boards are identical - swap the proms one at a time. The CPU is socketed also. If they're not: ie - one upgrade one not - then you can still "swap" everything socketed, just not one at a time.
Take ESD precautions.
Stranded in time. Surrounded by evil. low on gas.