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Pet 2001-8 monitor - Any checks I should do before I try powering it?

A

arcaic

Experienced Member
Joined
Apr 1, 2015
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Location
Milton Keynes, UK
Hi all,

I have a PET 2001-8 that has not been used in <insert guess here> years. I have the PCB mostly working (possible VSYNC issue) but displays a good picture through an RGBTOHDMI adaptor.

I know I have good voltages from the transformer to the CRT so do I just plug it in and give it a go or are there any steps/actions I should/could take with the monitor to minimise the chances of doing it any harm?

One note I will add is that when I got the PET 2001-8 it had a label round the mains lead with the words "No heater". No idea whether this true, mistaken or has any bearing on testing it.

I haven't tried turning on the monitor at all so far. All the work I have done on the PCB has been on the bench with a bench PSU.

Thanks in advance.

Andy
 
If it says "no heater" then that possibly means that the CRT heater is dead and it isn't going to work.

You can do a simple test with the machine powered off. Disconnect the connector from the rear of the CRT and test across the two pins of the heater with a multimeter set to measure resistance.

A reading of infinite Ohms means that the CRT heater is dead and there is no point in powering it up, because it isn't going to work.

There are multiple types of monitor used in the PET, so you need to identify the correct schematic.

There are some fusible resistors in the monitor. It is worth testing these first to see if any have gone open circuit and replacing them. They have a tendency to die...

The worst case scenario is that the voltage regulator has gone short circuit and will apply full DC Volts to the monitor. This is a low probability event. But, if you are concerned and have a bench power supply, you can use this in place of the transformer AC supply. Bring up the voltage slowly and ensure that the monitor voltage regulator is working. The bench power supply should indicate the current that is being drawn.

Obviously, check for black smoke either way! Using a bench power supply means that you can limit any black smoke...

Dave
 
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There are three (3) slightly different schematics for the monitors, but all schematics have the same heater pins for the CRT.

The CRT heater seems OK to me.

Give it a go...

Dave
 
Well the monitor works!!!
pet-2001-8-crt-0.jpg

but notice how as on my other thread the top half of the first row of characters is being chopped off/lost. Also the picture is shifted to the right as we look at it.

pet-2001-8-crt-1.jpg
This looks exactly the same as the results I get using the RGB2HDMI.

I'm thinking I should start another thread with this problem as it would seem to be nothing to do with the monitor and nothing to do with the RGB2HDMI.

Agree Dave?
 
Excellent (regarding the monitor).

Did you chase up those references I gave you for the PET state machine?

Dave
 
Yes I have the references. I have an rough understanding of how the monitor timing/display works but its a struggle for someone who has never done this to grasp the detail.
I'm going to go through the PETREPAIR1 PDF now and see if I can work out what my board is doing against the images/diagrams in that. Bit tricky that the PDF is for a later board but I think there is enough there to hopefully work things out.

Ta.
 
Hi... haven't been around for a while but that warms the cockles of my heart... VOSSI is what I first used on my PETunia...

The display looks perfect to me except it is a truncated in the vertical direction.. chopping off the top... couldn't that be a monitor problem?

The ROMs also look good from VOSSI... and the RAM... oh yes now you're talking

:)

Regards

PS
What does the display look like when the CPU is removed? (or the Kernel ROM if that is easier)
 
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Hi Nivag (G), Andy from Milton Keynes here. ;-)

The PET is running with all your gear in it (using roms and ram on the Romulan PET Ramulator at the moment). Thanks again for sorting those out.

fr_3615_size640.jpg
or it was running fine until this evening and now only the VOSSI test will run... but that's another problem.

This is what the video looks like using an RGB2HDMI adaptor...
2001-RGBTOHDMI-PCB.jpg
As you can see the top line of the image is only showing half of it. Its also slightly offset to the right of the screen. Exactly the same as when its on the PET monitor/crt so I don't think it's the monitor.

I have tried the RGBTOHDMI with a working PET 2001-N I have and it displays a complete picture perfectly centrally. I mention this because I wonder if the offset to the right is also part of the problem. Something off with video the timing????
 
Yes, video timing - so the state machine that I have pointed you to...

To get the ball rolling, check that you have signals on all of the pins of G1 (74100), D6 and D7 (both 74177). Use the schematic at: https://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/320008-3.gif.

Any pin identified on the schematic that does not have a valid signal on it is a candidate for further investigation.

I am also still a bit concerned about the strange 'noise' we saw.

Dave
 
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OK... well you could switch to PETTESTE2KV02 which would help see if the bottom edge is affected? (And would make daver2 have a warm fuzzy feeling).

daver2 and hugo are the real experts... but I would suggest a quick swap of C3 and C4 as a layman's test.
 
OK... just played catch up with...
A

Thread 'Pet 2001 - Display using RGBToHDMI - Missing top half of chars on first line.'

Hi All - I have a Pet 2001 which I have been getting going.

At the moment I have a Romulan Pet Ramulator replacing the 6550 RAM (all apart from 1 tested bad) and 6540 ROMs (3 are bad), I have replaced the character rom which was bad with a 6540 to 2716 adapter.
So no original ram or rom in it at the moment.
The board is powered from a bench PSU supplying 9v.
I hooked it up to my HDMI LCD Panel using an RGBTOHDMI convertor and held my breath and powered it up and it looked good.
I had the PetTester ROM enabled and initially it was showing the whole screen and all checked out ok.

I decided...

Do you have any good 6550s lying around? I just don't want you to waste your time if there is a problem with the Video RAM replacement.

But I agree... that mostly it looks like the characters are being produced fine... is there any truncation on the left at all?
 
Just checked signals on E6, D6 and D7 and all signals that should be seem to be active. I assumed you meant E6 (74100) and not G1 which is a 74154.
Many of the signals on G1 look very weird but are these reliable when I have the RPR installed and taking over the RAM?
 
Yes, E6 (sorry about that).

Now follow the signals 'downwards' on the schematic through the various gates of E8 and D8.

Dave
 
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