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CBM PET 2001 RESTORATION

Desperado

Desperado

Veteran Member
Joined
Nov 25, 2017
Messages
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Good morning, I bought this model in pretty bad shape. I still don't know if it works, I haven't even turned it on because it's very dirty. I would like to restore it and repaint it and possibly repair it, I wanted to create this post to describe the various jobs and receive your help where I can't. Thanks to everyone in advance!
 

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One school of thought is not to restore it.

The reason being that a lot of people who want to buy vintage computers enjoy the challenge of doing it themselves. If others have been tampering with it, it makes it a much less attractive option. Interestingly this applies to vintage transistor radios too. The ones that sell for the most money to collectors are in original condition & untouched by restorers.
 
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Thanks for the advice...I'll just clean it up, put the label on the front and see if it works! Do you recommend not repainting it?
 
Update:
I cleaned the board well and tried to turn it on, what I get is a horizontal white line in the middle of the screen.
This line is only visible if I increase the contrast almost to the maximum. I checked the video connector with the oscilloscope and I only have the horizontal signal of 15KHz.. The voltages seems look fine!
 
Welcome back my friend. Another PET!

You have a healthy H DRIVE signal (15.625 kHz).

I do see a V DRIVE pulse - but only one due to the timebase you have selected. On order for the oscilloscope (or a human come to that) to measure the frequency, you require a minimum of two (2) pulses on the screen... Slow down the oscilloscope timebase a bit to see if you can get something like four (4) pulses on the screen. You may find you have a V DRIVE signal then! If so, the vertical scan issue is in the monitor.

I don't see any video signal from the PET. However, let's solve the V DRIVE / vertical scan issue first.

Remember: find an issue, fix an issue. Repeat as many times as necessary...

Dave
 
daver2 said:
Welcome back my friend. Another PET!

You have a healthy H DRIVE signal (15.625 kHz).

I do see a V DRIVE pulse - but only one due to the timebase you have selected. On order for the oscilloscope (or a human come to that) to measure the frequency, you require a minimum of two (2) pulses on the screen... Slow down the oscilloscope timebase a bit to see if you can get something like four (4) pulses on the screen. You may find you have a V DRIVE signal then! If so, the vertical scan issue is in the monitor.

I don't see any video signal from the PET. However, let's solve the V DRIVE / vertical scan issue first.

Remember: find an issue, fix an issue. Repeat as many times as necessary...

Dave
Click to expand...
Hi my friend!
Yes, unfortunately i have another dead Pet :(
Ok when i come back from my job i check again the V drive signal!

Thanks!
 
I can t see Vertical drive also with slowly time div
 

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I can see two (2) yellow pulses on the oscilloscope screen. Why can't you?

However, the V DRIVE frequency should be 50 Hz or 60 Hz. A 50 Hz frequency corresponds to a 20 ms cycle time. Your oscilloscope timebase (if I am correct and it is 200 us/div) gives us approximately 12.5 divisions * 200 us/div = 2,500 us = 2.5 ms (which is way too quick - but your timebase selection will not observed the 20 ms (or so) V DRIVE signal anyhow.

Dave
 
As I have stated in my last post, the cycle time between pulses appears to be 2.5 milliseconds. This is much too fast for the V DRIVE signal. However, your oscilloscope timebase is still too fast to see the expected V DRIVE period of 20 milliseconds (if it is present).

Slow the darn timebase down!

Dave
 
Correct now?
 

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I can't make any sense of what I am seeing I am afraid.

What does the 6 ms indicate?

What is the oscilloscope timebase (in milliseconds/div)?

Dave
 
As I said previously, none of this is making sense again.

You should be able to convert between frequency and period and select a timebase by now. Oscilloscope training course 101...

Post #10 is 12.5 divisions at 200 us/div = 2,500 microseconds = 2.5 milliseconds = 400 Hz.

Post #14 is 8.5 divisions at 6 ms/div = 51 milliseconds = 19.6 Hz.

The VDRIVE signal should be 50 (or 60) Hz = 20 millisecond period for 50 Hz. Set the timebase to 20 ms/div. You should then observe a pulse every major division on the horizontal axis of the oscilloscope. You then 'tweak' the timebase setting whilst looking at the signal to observe what it is you want to see.

Based on the fact that the two posts (#10 and #14) give different results, either the PET is changing the signal or we have a measurement issue.

Are you proficient at converting frequency into period and period into frequency or not? If not, you will get nowhere with an oscilloscope. You need to do some homework...

Dave
 
So, look at the PET schematic at the VDRIVE signal and either work backwards until you find a gate that is wrong or look at the inputs to the circuit and work forwards until you find a fault.

Look around to find where this circuit has already been analysed. For example, check @Hugo Holden website as a starter for ten!

Which 2001 board do you have? There are a few different ones.

Dave
 
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