Screen Wobble on Commodore PET 4032 12" CRT

[jtjacques]

Just wanted to update the thread, for others with similar issues. I installed a closely fitting 1mm copper band around the transformer. The wobble has improved significantly, however, it is still quite visible. Having seen a rock solid picture with a jury-rigged power supply, I will probably end up swapping out the transformer entirely.

 

[Hugo Holden]

Just wanted to update the thread, for others with similar issues. I installed a closely fitting 1mm copper band around the transformer. The wobble has improved significantly, however, it is still quite visible. Having seen a rock solid picture with a jury-rigged power supply, I will probably end up swapping out the transformer entirely.

Do you also have the metal shield plate? Usually the combination of the two things nails it. Unless the transformer is a really bad case. Have you tried lowering the Line voltage ?

 

[jtjacques]

Do you also have the metal shield plate? Usually the combination of the two things nails it. Unless the transformer is a really bad case.

As I mentioned, it is a plastic unit without a plate. The case doesn't have any mounting holes for a plate, either. I suppose I could reuse the monitor/pcb mounting bolts, but they are recessed/setback in this model which makes this more of a pain. I'm open to suggestions on materials, thickness, etc. It does feel a little like treating the symptom, rather than cause, at this point though.

Have you tried lowering the Line voltage ?

Unfortunately, as mentioned, I don't have a variac to hand at the moment. More importantly, the machine will be deployed variously by others. Something that can be misconfigured or requires tuning each time it is used might not be ideal.

Thanks again for the comments.

 

[Hugo Holden]

As I mentioned, it is a plastic unit without a plate. The case doesn't have any mounting holes for a plate, either. I suppose I could reuse the monitor/pcb mounting bolts, but they are recessed/setback in this model which makes this more of a pain. I'm open to suggestions on materials, thickness, etc. It does feel a little like treating the symptom, rather than cause, at this point though.


Unfortunately, as mentioned, I don't have a variac to hand at the moment. More importantly, the machine will be deployed variously by others. Something that can be misconfigured or requires tuning each time it is used might not be ideal.

Thanks again for the comments.

I'd try a temporary plate first , to see if it had any useful effect then figure out how to mount it.

If there was still an issue and you are right about treating the symptoms rather than the cause, is to accept that the transformer has excessive magnetic radiation.

The magnetic field radiation does drop in level fairly abruptly (non-linearly) with reduced line voltage because of the non-linearity of the B-H curve of the core material, especially flattening out in the saturated area on peaks. You could use a bucking transformer in series with its primary. A Line voltage to 9V or 12V or 15V transformer, a small one is ok because the 9 or 12 or 15V winding only has to support the primary current of the PET's transformer. You could use a 1A or 2A part, not too large in size. You wire the low voltage winding in the correct phase so as to subtract 9 or 12 or 15V from the voltage that gets applied to the primary of the PET transformer. That would probably cure it, in conjunction with the copper shield that you now have. (of course it would be good to confirm that with the Variac first, but you don't have one, I think it will work, because of the nature of the issue going on in the PET transformer's core)

Currently, what does your meter report as the AC voltage applied to the primary of your PET's transformer ? Ideally we would get it to close to 100V with a suitable bucking transformer.

 

[jtjacques]

Currently, what does your meter report as the AC voltage applied to the primary of your PET's transformer ? Ideally we would get it to close to 100V with a suitable bucking transformer.

To answer your question, this was 120V with the step-down (2:1). However, I've gone ahead and swapped the transformer for a modern supply.

 

[Hugo Holden]

To answer your question, this was 120V with the step-down (2:1). However, I've gone ahead and swapped the transformer for a modern supply.

That doesn't surpise me at all. If you had added a bucking transformer to subtract say 15 to 20V from the Line voltage, I think your problem would have vanished.

When you say a "modern supply" you have to be very careful.

It is important to keep the 5V analog pre-regulator voltages as low as possible in the range of 7.5 to 8.5V.

If you bypassed the Analog regulators (as some people have) to feed in a 5V output from a modern SMPS, then, as the old saying goes "God have mercy on you".

This is because the protections for over-voltage and over-current events in the Generic SMPS's are not a patch on what the Vintage analog regulators provide.

Although the foolish slag off the heat generating Analog regulators, but they do an admirable job of protecting the downstream IC's and pcb tracks in the event of various failures such as Tant cap shorts etc, and they never have a failure mode that over-voltages the IC's. In short they are an "inefficient miracle" prepared to give up their own lives to protect the multiple IC's on the mobo.

Unless the SMPS is specifically designed for computer use with over voltage and over current detection circuits (which most generic ones are not, except say look at the PSU For the IBM5155 computer) they don't come even close to the safety level of the Analog regulator parts that National created. Plus, you are in the UK right ? Isn't a little bit of extra heat generation useful ?

 

[daver2]

>>> Isn't a little bit of extra heat generation useful ?

It certainly is at the moment Hugo!

Dave

 

[jtjacques]

If you had added a bucking transformer to subtract say 15 to 20V from the Line voltage, I think your problem would have vanished.

This seems to depend on careful balance against the input voltage. As I won’t be the only one deploying this, and it may not always be in the same location, it seems like a bit of a hack to try and get precisely the right output. The UK ranges from 216 to 253. Assuming a fairly tight input range seems problematic. As a concrete example, my household and office voltage have approximately a 16V differential.

If you bypassed the Analog regulators (as some people have) to feed in a 5V output from a modern SMPS, then, as the old saying goes "God have mercy on you".

I’m simply providing the required voltages into the existing header (and to the CRT board), via the existing regulators (and capacitor).

FWIW, I would have preferred to keep the transformer, but practicalities of how this machine will be used (interactive exhibit) have dictated this one. Thanks for the feedback though. Good info for others.

 

[DrAlis]

Howdy, I also have some wobbly PETs that I never found the cause for. I'd say 1 out of 4 Where can I find the transformer version?

 

[Hugo Holden]

Where can I find the transformer version?

I don't know what this question means ? Can you elaborate ?

 

[daver2]

>>> Can you elaborate ?

Manufacturer and part number, possibly?

Dave

 

[DrAlis]

I learned from Hugo's post that the transformer design 50/60 Hz matters. Since I also have the problem I wanted to check which ones are used in my machines. However, I did not find any identification (manufacture, part number, frequency, etc.) on my PET transformers. I have not disassembled them or taken them out completely, so maybe there is a part number somewhere that I just did not find. Apologies for the confusion.

 

[Hugo Holden]

I learned from Hugo's post that the transformer design 50/60 Hz matters. Since I also have the problem I wanted to check which ones are used in my machines. However, I did not find any identification (manufacture, part number, frequency, etc.) on my PET transformers. I have not disassembled them or taken them out completely, so maybe there is a part number somewhere that I just did not find. Apologies for the confusion.

If you have the problem, then you are likely in a 50Hz country and the transformer was made for 60 Hz. Especially of it looks like the problem was too objectionable to have been tolerated from new.

If that is not the case, and there is objectionable interference, say if you are in a 60Hz country, the first move would be to lower the line voltage and see if that helps. High range line voltages aggravate the problem, regardless of which frequency the transformer was initially designed for and what country it is run in, it is just a lot worse with the combination of high range line voltage combined with a 60Hz designed transformer running off 50Hz.

 

[DrAlis]

I am in a 50 Hz country. So what to do with low key equipment? Mount copper shield around transformer or replace transformer, right?

 

[daver2]

Or use a Bucking Transformer (as Hugo has described elsewhere on VCFED) to reduce the primary voltage into the transformer. This should be a much cheaper solution than replacing the transformer - and less fiddly than manufacturing a copper band.

Dave