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Dead 2001-16N

S

ScottishColin

Experienced Member
Joined
May 16, 2012
Messages
153
Location
Perth, Scotland
A bit of background - I first got a Commodore Pet when I was 15 in 1979 and it started me off on what I laughingly call my career. I spent amny a long night typing in programs from magazines in the UK and working out what was wrong with them and learnt a great deal and learnt to be inquisitive too.

Therefore, I couldn't resist buying a non-working Pet locally (well, two and a half hours away) in Scotland for twenty five pounds. It has power - I can hear it. The monitor is getting power - I can see it glow orange at the back. All I need to do is get it booting........

So far - nothing on the screen at all.

I'm starting from scratch. My electronics limits stretch to taking out chips and re-seating them. I can solder. I have a multi-meter.

Can someone point me off in the right direction to start please? I know I;m being vague about symptoms, but that's because I don't really have any.

Thanks for any advice.

Colin.
 
Colin,
Start by checking power supply. You need a volt meter of some sort to do that. Let us know the readings. You can measure from the supply directly and from certain board locations.
Bill
 
Posting a picture of the motherboard of the system would be helpful. PETs came with a number of different boards and the angle from which you start troubleshooting a "blank screen" problem varies depending on the board.
 
Is that dust or corrosion around the left of that motherboard pic? Might take a brush or something to it and get some of that dust outta there if that's all it is. Yes that thread you found should have a good start to checking power. After that your next assumption of re-seating chips comes in :) Welcome to the forums, btw.
 
Welcome, and good luck!

What are the voltages on the six pins of the cable going up to the monitor?

Got any friends with an oscilloscope?
 
ScottishColin said:
Thanks for the quick response. I'm assuming that this thread is good for checking the supply?

http://www.vintage-computer.com/vcf...08-Commodore-PET-2001-Substitute-Power-Supply

Colin.
Click to expand...
Not very; it's mostly about replacing the transformer on the older model, not yours.

You could start by checking voltages on pins 8, 9 and 1 of the RAM chips near the front, referenced to pin 16. You should see +12, +5 and -5 volts respectively.

There's also something odd about the 24 pin socketed ROM chips; you may have an unneeded option ROM in there, or one or more is in the wrong socket.

Please list the numbers of the chips D9 to D3 for us (assuming it's D3 and D5 that are empty). Maybe F10 as well while you're at it, and which 40 pin chips are in which socket just in case someone's swapped them around.

And don't forget the voltages on the monitor connector, the problem could be in the monitor or the connecting cable; try wiggling it at both ends.
 
Last edited:
Been busy cleaning - everything looks a lot healthier now. Sadly, still nothing on the screen.

I have reseated every chip that could be removed and reseated. I too was wondering about the 'extra' ROM - it has no markings on the top to identify it so I have taken it out but still nothing.

In terms of what is left, the chips are marked

D9 - 901465-03
D8 - a chip that is very very difficult to read but it might be 901447?
D7 - 901465-02
D6 - 901465-01
D5, D4 and D3 are now empty.

Off to check voltages next.

Thanks for all the assistance so far.

Colin.
 
ScottishColin said:
Voltages cheacked as per the other thread:

4 - 6 is ~16VAC
4 - 5 is ~8VAC
5 - 6 is ~8VAC
7 - 8 is ~16VAC

Colin.
Click to expand...
Can't hurt, but as I said it's a different model and power supply and we're not really interested in those voltages anyway. What we want is the voltages on the RAM chips; since the CRT lights if those voltages are OK then the transformer must be good.

Also the voltages on the video cable as I asked so we at least know whether the problem's on the board or in the monitor.

You also didn't answer whether you have a friend with an oscilloscope.
 
Will check the voltages on the chips tomorrow.

Any reccomendations on a low priced scope that'll do the job as I'm pretty sure I'm going to have to get one for this.

And will check with friends tomorrow re scope.

Thanks.

Colin.
 
Sorry - with a multi-meter, what's the easiest way to check the voltages on the video cable? Stuff one probe in the monitor end of the cable and one on the motherboard?

Apologies for the numpty questions.

Colin.
 
ScottishColin said:
Sorry - with a multi-meter, what's the easiest way to check the voltages on the video cable? Stuff one probe in the monitor end of the cable and one on the motherboard?
Click to expand...
That comes next ;-)

Pin 6 is missing, pins 2,4,7 are gnd; measure 1-2, 3-4, 5-7. If you've got the monitor open anyway, check that the connector is making good contact.

If you can read schematics, you can find them here:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html
 
MikeS said:
That comes next ;-)

Pin 6 is missing, pins 2,4,7 are gnd; measure 1-2, 3-4, 5-7. If you've got the monitor open anyway, check that the connector is making good contact.

If you can read schematics, you can find them here:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html
Click to expand...

Bought a better multimeter today and will try tomorrow.

In the meantime, has anyone tried anything like this?

http://www.yann.com/en/diy-turn-your-gnulinux-computer-into-a-free-oscilloscope-29/09/2010.html

Colin.
 
Unfortunately a "sound card" frequency scope won't be of much help in troubleshooting a PET. For the digital board you'll need something that's comfortable with Mhz-frequency signals. Unless you have "a connection" you're probably looking at around the $200-$300 (ballpark) to get something that really does the job. (That $200 might buy you a decent used CRT scope, or could go towards a cheap handheld or USB digital sampling scope.)

Some cheaper things that can really come in handy:

1: Someone gave me one of these; If you can find one for cheap in a bargain bin or at a swap meet it was fast enough for probing most of the digital circuitry in a PET:

http://www.linuxtoys.org/pscope/pscope.html

It claims (at least) a higher sampling rate then most of the sub-$100 digital "micro-Scopes" that are out there, and I can speak from experience that you can at least get a feel for the waveform shape of 1mhz+ digital signals. It's not any good probing the video circuitry, however.

2: This cheap little logic analyser:

http://www.saleae.com/logic/

Is *really* fun to use. (I was loaned one of the 8-channel ones.) Having it made it positively trivial to check out theories I came up while staring at the schematics. Again, it's probably not much use if you're chasing a problem in the monitor but it's completely awesome for work on the motherboard.

For your voltage readings be sure to sample the supply legs of several ICs around the board; most everything runs on +5v. (The only exception is the RAM chips.) I think the main concern on people's minds here is that a "blank screen" is actually a somewhat unusual failure mode for PETs equipped with the motherboard yours has. The video circuitry doesn't require any software initialization to run, so unless something is specifically broken in the video scanning or output phase (or the monitor itself has problems) those machines should at least display a screenful of garbage when turned on.

Digital board problems we seem to be pretty good at solving around here. "TV repair" is a touchier subject.
 
Apologies for the delay

Apologies for the delay

MikeS said:
That comes next ;-)

Pin 6 is missing, pins 2,4,7 are gnd; measure 1-2, 3-4, 5-7. If you've got the monitor open anyway, check that the connector is making good contact.

If you can read schematics, you can find them here:
http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001N/index.html
Click to expand...

Moving house so things are a bit slower at the moment.

1-2 is a shade over 4 volts
3-4 a shade under 4
5-7 barely registers, about 0.1 volts.

Not sure that helps.....

Colin
 
MikeS said:
Not very; it's mostly about replacing the transformer on the older model, not yours.

You could start by checking voltages on pins 8, 9 and 1 of the RAM chips near the front, referenced to pin 16. You should see +12, +5 and -5 volts respectively.

There's also something odd about the 24 pin socketed ROM chips; you may have an unneeded option ROM in there, or one or more is in the wrong socket.

Please list the numbers of the chips D9 to D3 for us (assuming it's D3 and D5 that are empty). Maybe F10 as well while you're at it, and which 40 pin chips are in which socket just in case someone's swapped them around.

And don't forget the voltages on the monitor connector, the problem could be in the monitor or the connecting cable; try wiggling it at both ends.
Click to expand...

Sorry - another daft question. Pin 1 used to be marked on the motherboard when I was younger but I cannot see it here. Which one is it please? And so I dont ask again, I assume pin 16 is diagonally opposite it at the other end of the chip yes?

Thanks for your patience.

Colin.
 
ScottishColin said:
Sorry - another daft question. Pin 1 used to be marked on the motherboard when I was younger but I cannot see it here. Which one is it please? And so I dont ask again, I assume pin 16 is diagonally opposite it at the other end of the chip yes?

Colin.
Click to expand...
No; Looking from the top, IC pins are numbered counterclockwise starting at the end with a notch or dimple, so pins 1 and 16 are in fact across from each other; see:

http://hardware.speccy.org/datasheet/4116-15.pdf
 
ScottishColin said:
Moving house so things are a bit slower at the moment.

1-2 is a shade over 4 volts
3-4 a shade under 4
5-7 barely registers, about 0.1 volts.

Not sure that helps.....

Colin
Click to expand...
Well, measuring video signals with a DMM can be misleading, but those readings do suggest that the problem is in the monitor; rotate and wiggle the brightness control a few times, and check the connectors again at both ends of the cable.

Failing that, if you carefully remove the 24-pin chip at F10 (with power off, of course) does the voltage at pins 1-2 of the video connector drop to about 2.5 - 3 volts?
 
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