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Old 17th Jun 2022, 8:26 am   #101
SiriusHardware
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Default Re: Just got myself a commodore pet setup but not working

Just to comment further on the video which Pud has mentioned a couple of times:-

1) The presenter observes that the monitor can not run by itself, as most monitors can - it needs the Horiz-drive and Vert-Drive signals to even start up. This is true. Early measurements made by Pud show us that there are signals present on the video, horiz-drive and vert-drive outputs from the mainboard and that would suggest that the mainboard IS working and that the monitor is not. That is our default position until we investigate further.

2) The presenter makes the further observation that on his example the frequency of his Horiz-drive signal from the mainboard, although present, is completely wrong, so far out that it does not properly drive the horizontal stage of the monitor, and that is why his monitor does not light up. He then goes on to investigate why it is that the horiz-drive signal has the wrong frequency.

It is for this reason that we are asking for the scope readings from the video, horiz-drive and vert-drive to be taken using the exact scope settings specified in the service manual, so we can see whether they are not only present or absent, but also at the right frequency. Pud could also check this in a different way if he has a frequency meter, or a multimeter which has a 'Hz' (Frequency) measurement setting.
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Old 17th Jun 2022, 8:50 am   #102
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Default Re: Just got myself a commodore pet setup but not working

(Thanks to AJ for the clearer view of the V525 controls).

One other thing I should have mentioned in the general scope setup summary - The Volts/Div controls have outer end knobs which rotate independently of the main controls. These should all be turned fully clockwise (where there is usually a 'click' stop) so that the 'UNCAL' LED associated with the control is NOT lit, and left at that setting. They should also be pushed in, not pulled out.

The Time / Div control also has a SWP VAR control just down and right of the main Time / Div control, this should also be rotated fully clockwise to the click stop so that its 'UNCAL' LED is NOT lit.

Last edited by SiriusHardware; 17th Jun 2022 at 9:08 am.
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Old 18th Jun 2022, 6:52 pm   #103
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Default Re: Just got myself a commodore pet setup but not working

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Originally Posted by SiriusHardware View Post
I did once find a PET monitor component layout which included what was supposed to be a transparent 'mask' showing the PCB tracks ......

As this thread seems to have stalled for the moment I thought I’d post the attached image. It’s the closest I’ve come to finding a transparent track mask on a PET VDU board diagram although it’s not the same monitor as the one being discussed here.

The image is extracted from some PET documents kindly scanned by Dave Moll after they’d been generously donated by Dennis M. If anybody missed out on these documents they can be found in this thread from last year.

Alan
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Old 20th Jun 2022, 12:01 pm   #104
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Default Re: Just got myself a commodore pet setup but not working

Hi all
Just an update and good news I took the crt board out of the monitor to have a look around and I had found a post on the vintage computer federation site about a 8032 pet with no picture and someone said a common issue was a couple of resistors by the flyback transformer which act like a fuse blows.
So I checked and replaced them and now we have the screen working.
Some of the keys are not working which is the next job to try and sort out and who knows why the resistors blew so we may not be out of the woods yet.
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Old 20th Jun 2022, 1:14 pm   #105
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Default Re: Just got myself a commodore pet setup but not working

Hi all
Busy afternoon stripped the keyboard down cleaned it now all keys are working would just like to give out a massive thanks to all the help I have recieved I could not have done it without you.
Now to get the disk drive working I will start a new post and look forward to talking to you again and see if we can save another vintage item.
Many Thanks
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Old 20th Jun 2022, 2:32 pm   #106
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Default Re: Just got myself a commodore pet setup but not working

After any fault that causes overheating of the flyback transistor I would change the power supply decoupling capacitor on the line driver transformer that feeds the base drive.
It was a common fault on VGA screens in there day.
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Old 20th Jun 2022, 11:00 pm   #107
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Default Re: Just got myself a commodore pet setup but not working

Quote:
someone said a common issue was a couple of resistors by the flyback transformer which act like a fuse blows.
That's great news Pud, well done, I sort of got the impression you were a little bit frustrated by our deciding that the monitor was probably faulty so I am relieved we were not leading you down the wrong path after all. Is there any chance you can say which resistors you are referring to (which 'R' numbers?), as it will be useful information for anyone who surfs onto this thread in future.

Edit: Ah, I think I've found the reference, was it R752 and R753?

Last edited by SiriusHardware; 20th Jun 2022 at 11:19 pm.
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Old 21st Jun 2022, 3:00 pm   #108
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Default Re: Just got myself a commodore pet setup but not working

Quote:
Originally Posted by pudwink View Post
and someone said a common issue was a couple of resistors by the flyback transformer which act like a fuse blows.
So I checked and replaced them
Well done for finding the fault. If these really are fusible resistors, they are fusible for a reason. They should be replaced by the correct type. Yes ordinary resistors will work but if a fault develops, it might not be the resistors that fail or they might not fail quickly enough to protect whatever circuit they are supposed to be protecting.

The usual convention on the circuit diagram is to show fusible resistors (or any safety component) with a triangle next to it or an exclamation mark (and sometimes a little footnote explaining that 'components marked with a '!' or (triangle) must be replaced with the correct part for continued safety'.

It might not apply to your monitor but worth checking.
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Old 21st Jun 2022, 5:24 pm   #109
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Default Re: Just got myself a commodore pet setup but not working

Hi all
Here is the picture of the area from the schematic of the two resistors they are on standoffs one end so commodore must have had an idea.
according to the guys on the vintage computer federation site they needed to be replaced by metal film resistors which I have done
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Old 22nd Jun 2022, 8:53 am   #110
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Default Re: Just got myself a commodore pet setup but not working

If the resistors were mounted 'floating' above the PCB on ceramic standoffs that usually means they were expected to run hot, and resistors which fail usually go high resistance or open circuit.

Thanks for confirming the parts in question, this is useful info for anyone else who ever ends up reading this thread.
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Old 22nd Jun 2022, 8:01 pm   #111
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Default Re: Just got myself a commodore pet setup but not working

So glad the PET is up and running. Always pleasing when a repair thread is reported as having a happy ending. Well done.

Alan
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Old 22nd Jun 2022, 9:39 pm   #112
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Default Re: Just got myself a commodore pet setup but not working

Maybe also add a note of the value for those two resistors as anyone needing to replace burnt resistors might not be able to read the colour codes, depending on the severity of the burning.
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Old 22nd Jun 2022, 9:54 pm   #113
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Default Re: Just got myself a commodore pet setup but not working

Looking at the schematic I think the resistors in question are both 56Ω.

Alan
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