Commodore pet flickering screen

[genobombino]

I really need help with this.

I have an original 2001-8 commodore pet with 6540 rom and 6550 ram chips. I bought it off of ebay for a ridiculous price with the description reading (screen powers on and functions fine). Yet, since Ive plugged it in, all i have a problem with IS the screen.

So here's the problem
the screen will randomly flicker, jump, distort, and whatever else you wanna call it. I had the commodore prompt actually working after moving around some chips and reseating the important ones, only to have the screen flicker throw it back into a garbled screen. Ive tried reslodering the monitor board, reseating the boot, replaced some capicitors, replaced the on board diodes, deoxit'd the whole board, even reseating the plunger, and have had no luck.

Im at a complete loss, any help would be appreciated immensely.

 

[genobombino]

Theres a link to a video of it flickering

 

[Hugo Holden]

That screen photo you have posted is non specific, because you are talking of a time domain event with terms like "flicker" and "jump"

So a "snapshot" in time, like a photo, cannot help anyone help you, as the meaningful information is not there to see is it ?

Can you post a video instead of a .jpg photo, it might help everyone get to the point of where the fault is.

What you have presented reminds me of an episode of Dr. Who. Where there were concrete statues of Angels.

They could not be animated at the time, because the BBC at that time, did not have the CGI technology to do it, like Pixar Laboratories now have.

So they took still photo, moved the Angel statue to another position, and re-took the photo and strung them together in a sequence. The Angels appeared to jump from place to place in time & space. A very clever and economical way to do the special effects, without computer assistance.

 

[genobombino]

That screen photo you have posted is non specific, because you are talking of a time domain event with terms like "flicker" and "jump"

So a "snapshot" in time, like a photo, cannot help anyone help you, as the meaningful information is not there to see is it ?

Can you post a video instead of a .jpg photo, it might help everyone get to the point of where the fault is.

What you have presented reminds me of an episode of Dr. Who. Where there were concrete statues of Angels.

They could not be animated at the time, because the BBC at that time, did not have the CGI technology to do it, like Pixar Laboratories now have.

So they took still photo, moved the Angel statue to another position, and re-took the photo and strung them together in a sequence. The Angels appeared to jump from place to place in time & space. A very clever and economical way to do the special effects, without computer assistance.

I put a video in the replies.

 

[Hugo Holden]

For some reason, it might be the server I'm on blocking it, I cannot access the video link. Currently on a service with limited access to video.

Anyway, what I was getting at with that still frame method (referred to in the Dr. Who show) if you are stuck and cannot make a video of something, you can convey the information probably, with a series of screen shots spaced in time, perhaps 5 or 6 .jpg sequential photos would work.

 

[daver2]

As these are your first posts, welcome to VCFED.

Don't forget to add your location to your profile and that you will be under moderation fir your first 10 posts, so your posts may be delayed a bit.

Do you have any test equipment (in particular an oscilloscope) - or access to one?

You really need to isolate the problem to either the logic board or the monitor first by monitoring the HDRIVE and VDRIVE signals between the two to see which end of the cable is at fault.

Dave

 

[genobombino]

As these are your first posts, welcome to VCFED.

Don't forget to add your location to your profile and that you will be under moderation fir your first 10 posts, so your posts may be delayed a bit.

Do you have any test equipment (in particular an oscilloscope) - or access to one?

You really need to isolate the problem to either the logic board or the monitor first by monitoring the HDRIVE and VDRIVE signals between the two to see which end of the cable is at fault.

Dave

I dont have an oscilloscope unfortunately, nor would i even know how to use one. Is there one you would reccomend? Id like to solve this problem any way I can

 

[daver2]

A logic probe perhaps?

I am just wondering whether you may be able to see a voltage change on a cheap analogue multimeter?

Can anyone else chip in with any ideas while I go to sleep...

Dave

 

[genobombino]

I really need help with this.

I have an original 2001-8 commodore pet with 6540 rom and 6550 ram chips. I bought it off of ebay for a ridiculous price with the description reading (screen powers on and functions fine). Yet, since Ive plugged it in, all i have a problem with IS the screen.

So here's the problem
the screen will randomly flicker, jump, distort, and whatever else you wanna call it. I had the commodore prompt actually working after moving around some chips and reseating the important ones, only to have the screen flicker throw it back into a garbled screen. Ive tried reslodering the monitor board, reseating the boot, replaced some capicitors, replaced the on board diodes, deoxit'd the whole board, even reseating the plunger, and have had no luck.

Im at a complete loss, any help would be appreciated immensely.

For some reason, it might be the server I'm on blocking it, I cannot access the video link. Currently on a service with limited access to video.

Anyway, what I was getting at with that still frame method (referred to in the Dr. Who show) if you are stuck and cannot make a video of something, you can convey the information probably, with a series of screen shots spaced in time, perhaps 5 or 6 .jpg sequential photos would work.

 

[genobombino]

A logic probe perhaps?

I am just wondering whether you may be able to see a voltage change on a cheap analogue multimeter?

Can anyone else chip in with any ideas while I go to sleep...

Dave

I dont have a logic probe either, but while measuring the transformer voltage (on my klein mm700) the voltage jumps from 15.9v ac all the way up to 16.6v ac during it cutting in and out
Im not sure if thats the cause of, or effect from

 

[mykrowyre]

I'm not familiar with working on pets but it sounds like a bad connection, cracked trace, cracked solder joint etc on the power to the crt. Voltage is jumping because load is dropping my guess. I had the same issue with many TRS 80 Model 4's all due to cracked traces at the pin headers.

 

[Hugo Holden]

It appears that there is a stable H drive and therefore H scan on the VDU, then transiently it becomes unstable.

The main question therefore becomes, is the H. drive pulse itself from the computer that is transiently going unstable, or is the fault in the H. drive/H scan section section of the VDU. The usual thing to do would be to scope the H. drive signal arriving at the VDU.

But since you don't have a scope, initially at least, check the connector at the VDU end on the VDU pcb, where the pins of the connector (that connects the main computer board to the VDU) have good soldering. These are notorious for getting 360 degree ring cracks around the solder to the connector pins on the VDU board and there could be an intermittent connection there.

I'm trying to think of a method to check the regularity of the H, drive signal, from the computer, without a scope. There are a couple of possible ways;

How old are you ? (so I can have a guess at your high frequency hearing ability) young people can hear the 15.7KHz of this H. drive signal, if it is applied via a resistor to a small speaker, so as to listen for any irregularities. I am not able to hear up there, these days myself.

 

[genobombino]

It appears that there is a stable H drive and therefore H scan on the VDU, then transiently it becomes unstable.

The main question therefore becomes, is the H. drive pulse itself from the computer that is transiently going unstable, or is the fault in the H. drive/H scan section section of the VDU. The usual thing to do would be to scope the H. drive signal arriving at the VDU.

But since you don't have a scope, initially at least, check the connector at the VDU end on the VDU pcb, where the pins of the connector (that connects the main computer board to the VDU) have good soldering. These are notorious for getting 360 degree ring cracks around the solder to the connector pins on the VDU board and there could be an intermittent connection there.

I'm trying to think of a method to check the regularity of the H, drive signal, from the computer, without a scope. There are a couple of possible ways;

How old are you ? (so I can have a guess at your high frequency hearing ability) young people can hear the 15.7KHz of this H. drive signal, if it is applied via a resistor to a small speaker, so as to listen for any irregularities. I am not able to hear up there, these days myself.

Im 20

 

[ajgriff]

My guess is that you have an intermittent connection between the transformer and the VDU. The voltage rise will reflect a change from load to no load due to the intermittency. Clean the connection to the VDU board and see what happens. The random character screen is a different issue but at least indicates that the character ROM and videos RAMs on the main board are ok.

Alan

 

[Hugo Holden]

20 Eh, your hearing is likely good to 18KHz at least.

So if you could find a small speaker, like a transistor radio speaker or similar, put a 330 Ohm resistor in series with it and connect that to the H drive signal that feeds the VDU, does it sound like a continuous, very even, super high pitched tone, or are there irregularities in it ?

 

[daver2]

Nice to have a 'youngster' repairing a PET!

What drew you to the PET out of interest? I grew up with these PETs when I was 20 or so (but they were brand new then!).

When you said a "ridiculous price" I assume you mean a ridiculously low price (I hope)!

Are you hoping to get into repairing vintage equipment as a 'hobby' - or is this PET a one-off? Just judging whether you need to be building up a supply of test equipment over the coming years...

Dave

 

[Hugo Holden]

+1

Nice to see a "youngster" interested in the PET , by that I mean that most familiar with the PET computer are older, for example I am 64. But interestingly I only discovered the PET recently myself, but it was oddly familiar, because it was composed of 74 series logic IC's and VDU technology that I grew up with, when I was in my 20's. So I sort of warmed to it right away.

 

[Hugo Holden]

I have been able to view the video now,

The fault is affecting both the H & V deflection, so most likely it is a power supply issue affecting both the H & V drive signals at the same time,

So work for now on making sure that the connections/connector is good and when the VDU has cut out, check the output of the voltage regulator in the VDU. And check the 3 voltage regulators in the PET, there are usually two +5V regulators and one +12 regulator, also a negative regulator for the memory not as important for now.

One could expect to see a small change on the AC side (the transformer output) with the change in current drawn by the VDU, if the H & V signals from the computer were disappearing at times.

The speaker method I suggested can also work to listen for the presence of the V drive signal which would sound like a sharp buzzing sound as it is 60Hz.

It may well be worth getting a small starter entry level scope if you plan to undertake repairs in the future too as it will help you get to the bottom of troubles much more quickly,

 

[genobombino]

Nice to have a 'youngster' repairing a PET!

What drew you to the PET out of interest? I grew up with these PETs when I was 20 or so (but they were brand new then!).

When you said a "ridiculous price" I assume you mean a ridiculously low price (I hope)!

Are you hoping to get into repairing vintage equipment as a 'hobby' - or is this PET a one-off? Just judging whether you need to be building up a supply of test equipment over the coming years...

Dave

I hope to make it a hobby. I just find the older technology so facinating and charming. Ive repaires several c64's (most of them just memory issues) but the pet is something i cant find a lot of information on like the pet, nobody seems to share the same problem im having

 

[daver2]

Don't forget to add your location into your profile.

When the monitor displays the screenful of random characters - this indicates to me that the CPU is not functioning correctly either. However, it does indicate that the clock is working (otherwise you wouldn't have got the random characters displayed).

The first thing is to hop over to Bo's website at http://www.zimmers.net/anonftp/pub/cbm/schematics/computers/pet/2001/index.html and download the schematics related to your particular machine.

If your monitor is playing up, I would suggest disconnecting it for the time being to prevent further damage. You can do this via the plug and socket at the monitor PCB - not on the main logic board (the power is fed directly to the monitor from the transformer).

The second thing to do is to measure all of the DC voltage rails to ensure that they are within specification (+/-5% of nominal voltage). This would be between 4.75V and 5.25V for a +5V rail. However, I would expect it to be realistically between 4.9V and 5.1V.

I think (for your machine) there should be four (4) +5V voltage regulators - each providing a supply to part of the logic.

The next thing is to measure the ripple voltage - using a low voltage AC range on your multimeter. The ripple should be within +/- 0.1V.

Just a point of note, all measurements are taken relative to the 0V/GND rail.

After that, you need to really purchase a logic probe. These are quite cheap. Have a look at the "Laser 5263". This will work with TTL or CMOS devices and indicates logic HIGH, logic LOW and has a PULSE catching option. Ask Santa as Christmas is coming...

You can take some measurements around the CPU with your multimeter, but I can tell you how to do that after you have looked at the power rails.

You can also work on the monitor in parallel - by connecting it and disconnecting it. However, you are aware of the high voltages on the monitor I assume and how to work safely on a monitor? If not, please ask for further advice. Safety First! Also, I wouldn't operate the monitor for too long with the fault as at present.

Dave