Commodore PET Monitor Brightness

[SiriusHardware]

And that was it? Well done.

That's a new one for the notebook then, we'll have to write it down for future reference.

Normally if those resistors fail it puts the screen off completely. We aren't sure of the failure mode but we've had one here where the resistors were replaced and immediately started smoking because a capacitor on the downstream side was breaking down under real operational conditions (ie, with a lot of voltage across it) although it seemed not to be short circuit or low-resistance when checked beforehand with a meter.

I've seen some other threads where the capacitor had to be replaced, but many more where just replacing the resistors got the monitor working with no further action required.

[ortek_service]

Good to hear that fixed it. Most faults like this in monitors do tend to be resistors going high / open-circuit - But usually ones > 100k, where high-voltages across them tend to cause this. So they sometimes put multiple ones in series to reduce the high voltage across each one.

Yes, it is uncommon for fairly-low value ones to go high in value, as not much voltage across these to cause partial failure of the resistive-element. And much-more likely for these to just burn-out and go totally open-circuit - usually due to excessive current / power.

So it looks like Commodore put two in-series to spread the power across two standard 56R 0.25W resistors as it was > 0.25W across total 112R resistance (Rather than being in-series to spread the voltage)
Although it may have been more logical for them to have just used a single 110R etc 0.5W resistor (or put two 220R 0.25W in-parallel)

However, if the two 56R 0.25W ones do frequently fail on these, then maybe they didn't reduce the power in each enough below 0.25W, and should have either put 3off 36R / 4off 27R etc 0.25W in series, or used 0.5W ones.
You could maybe retro-fit 0.5W ones, for better reliability - or use metal-film ones where you can get 0.6W rated Vishay MRS25000C... (ex Philips MRS25T...) ones, in a standard 0.25W Carbon Film size.

Although maybe the original? resistors lasting >40years isn't too bad, and it might be they just eventually fail even at <0.25W.

[ortek_service]

BTW, I've never tried to measure the EHT so far - Although I did get a proper EHT probe a couple of years ago (for £1 from a rally, as I think tip may be a bit damaged).
Back in the 80's, I was planning to build the Hobby Electronics (30kV FSD?) EHT Meter, which used 10off? 100Meg High-voltage resistors in-series as an potential-divider attenuator into 10Meg. So could have used it with a standard DMM, but back then they used an integrated-in-the-box analogue meter.
And I think I did buy some special High-voltage ones.

Trying to find-out what the EHT voltage should actually be on this CRT was quite-difficult, as manual doesn't give. And trying to find data on the CRT was difficult (Although if you did need a replacement, it seems the CRT is equivalent to ones in the TRS80 / an Orion one used in old Amstrad GT65 Green screen monitors (which you can pickup fairly-cheap at the moment). Whereas I saw the original CRT is now around $200 for a NOS one from a US Company.

However, I knew the EHT on a mono / especially smaller-screen CRT's was rather-less than the often 25kV used on many > 20" Colour CRT's.
And it seems that around 10-12kV is used on 12" mono CRT's.

[SiriusHardware]

If you look upthread, centrespot also has another similar PET which happens to use the same monitor PCB, and that one was considered to be working properly so it would have been a handy source from which to measure the 'right' voltages for future reference, or for the purpose of fixing this one - however that seems not to be necessary now.