Analog oscilloscope repair - Philips pm 3234

[ccx]

Hi,
I have recently bought my first secondhand analog oscilloscope and I would really like some advice on repairing it.

Here are the symptoms:
After I turn it on, it seems to be working fine. However, in about 30 minutes the trace starts becoming dimmer and eventually fades. While this is happening the power consumption slightly increases and the supply voltages are dropping. The 12V rail goes from 12V to 11.5V. This doesn't seem that much to me but the service manual claims it should stay within 30mV of 12V.

Based on these observations, I suspect that something might be overloading the switching power supply. What I cannot make sense of is why it only happens after a while and not right away.
Or could the supply be overheating and limiting the output? Looking at the schematic, I think the only way it could notice that is by one of its components failing explosively.

What kind of faults should I be looking for and where? All suggestions are greatly appreciated.

[HB9EVI]

Looks like a thermic problem; wildly guessing I'd check the caps first, but the use of an non-inflammable cooling spray could help tracing the problems origin; used the same technique on my old Tek to locate a similar issue

[ccx]

Cooling spray is a clever idea. 
Thanks

[grumpydoc]

Hairdryer with a cool air setting might be more use than freezer spray, just to narrow it down to one area of the 'scope.

Phillips 'scopes run hot and are prone to component failures - I've never found them that repair friendly either. I only really like the Combiscope range (but I do really like those )

[ccx]

Finding a hairdryer turns out to be more difficult than I would have expected. 

I don't know much about repair friendliness due to lack of anything to compare to. But I have to admit I got my hopes up when I saw that all the transistors are socketed. To me it suggests that the designers must have put at least some thought into serviceability.

[shakalnokturn]

I have a couple of Philips PM3212 that need repair, same symptoms only they happen faster, you should also notice while displaying a waveform that the display blooms before disappearing.
Although I haven't got round to troubleshooting mine I'm suspecting the H.V. transformer or multiplier will be the cause, there are several reports around the WWW that they do fail this way.

[akimpowerscr]

Check and monitor the voltage of -1500V when the fault occurs.

Also check the temperature rise of the TS806 and TS807 transistors and compare with the temperature in normal operation.

NB: you should never buy an analog oscilloscope with a storage cathode-ray tube.

This is not a good technology, storage crt's are fragile, have a limited lifespan and require complicated control circuits that are easily prone to failure.

I have a PM3233, it is a simple oscilloscope with two beams (not two channels) which can display two signals in real time.

The main drawback of this oscilloscope are the chronic bad contacts in the switches because Philips did not use reed relays in this oscilloscope to switch the signals.

[ccx]

I spent a couple of hours monitoring the current draw with a fan blowing on different parts of the scope. The current stays under 300 mA (as it should according to the manual) only when the airflow is directed right at th HV multiplier so I'm pretty sure it's the cause of the problem.

It is a cockcroft-walton multiplier right? The schematic doesn't say much about it other than that it is not supposed to be serviced.

NB: you should never buy an analog oscilloscope with a storage cathode-ray tube.

This is not a good technology, storage crt's are fragile, have a limited lifespan and require complicated control circuits that are easily prone to failure.

I will take the advice going forward. Thank you.

[shakalnokturn]

It is a cockcroft-walton multiplier right? The schematic doesn't say much about it other than that it is not supposed to be serviced.

Right.
On schematic it is noted that it's a "fourfold voltage multiplier" going from 1.5kV pk to 7kV.
You may be able to find one for a colour TV although x4 models were not the most common, otherwise it will be quicker making your own and reusing the HV wire than trying to de-pot the original design.
If you're careful enough you can test it in open air before potting it, some scopes even have un-potted multipliers up to 12kV although potting them makes them smaller and less dangerous when servicing. (Less repairable also.)

[ccx]

Unfortunately I haven’t had much time to devote to the scope but I have come up with a bunch of questions.

First I’d like to make sure that my understanding of the voltage multiplier is correct. I’ve attached a drawing of what I think a fourfold one should look like. All the voltages are normalized to the peak of the input AC. They only apply in a steady, unloaded state. Everything is referenced to ground at the bottom left except the voltages across the capacitors.
It seems to make sense but please tell me if I made any mistakes.

The main reason I started doubting myself is that in the schematic of the scope the input peak is 1.5 kV and the output is 7 kV. But 4*1.5kV is only 6kV and losses haven’t even been considered. And we know for sure that the input is 1.5 kV because the same winding of the transformer is rectified with a diode to obtain the -1500 V rail.
If you know how this could be reconciled, please share your knowledge.

About the implementation:
I’ve got a multiplier from an old color TV. It is almost certainly like the one described here:
http://www.kronjaeger.com/hv/hv/src/tv/
If the D and focus connections are not used, it is a fivefold multiplier like shown in my second drawing. 5*1.5kV = 7.5kV minus the losses is much closer to the nominal 7kV. On that note, how tolerant are CRTs to variations in the acceleration voltage?

Another solution is to build one myself as suggested by shakalnokturn. In this case I have questions regarding pretty much every part of the circuit.

Diodes
Is it ok to connect several 1kV diodes (say uf4007) in series to create a hv diode? My intuition is that it should work fine. If one of them has a higher resistance than the others, its operating point starts advancing in the knee of reverse breakdown. This makes its resistance go down and basically regulates the voltage sharing.

Capacitors
Connecting them in series to increase the voltage rating seems a bit more risky. If one of them exceeds its rating even for a moment it will likely be damaged.

Insulation
The circuit needs to be potted to prevent arcing, corona discharge and to make it safer. What kind of potting compound do you recommend? So far I’ve only got two candidates that don’t exceed the budget of the repair: candle wax and tar. Though I’m not sure if their use is a good idea.

TL;DR
How sensitive are CRTs to higher acceleration voltage?
Can you connect diodes or capacitors in series to make a high voltage one?
Please recommend affordable potting compounds.

[Wallace Gasiewicz]

Multiple diodes in series have been used for 100 years for high voltages.
I Have an old wood case voltmeter that has several diode discs stacked for high voltage.
Used in tube amps for high volts also.
Many high volt diodes are actually stacked diodes, You can ascertain how many diodes are in the case by measuring the voltage drop (using low volts)

[TurboTom]

@ccx:
if you're still looking for PM 3234 spares, I may be able to help. I've got a parts mule here (broken CRT) that I used to get other such units going again (mostly for cosmetic issues). The HT cascade should be serviceable as well as the PSU and other parts.

It's true that storage CRTs are quite fragile, also from a mechanical point of view (especially so the highly sophisticated Philips variable persistance variety). On the other hand, this fact was known already when they were released for the first time, and these instruments were really expensive. As a consequence, you can expect to basically find these scopes in two conditions -- either in alsmost pristine shape and well cared for, or completely beaten up.

That's why I wouldn't recommend to stay away from analog storage CRT instruments completely, but rather to have a close look at what you get, and only get one if the price is really reasonable that a damaged CRT unit wouldn't leave you feeling depressed. Treated reasonably and carefully, these units do last. I've got two nice PM3234 and a tiny little TEK / Sony 314 in my collection. Still looking for a top-of-the-range PM3266, which must contain one of the most complex direct-view storage CRTs ever made.

IMO, their delicate nature and sparseness makes them perfect collector's items, if kept in decent shape. Perfect for celebrating the pinnacle of half a century's past high-tech. If you're on a budget and looking for an inexpensive scope as a "daily driver", no questions, get a recent entry-level digital one, and if that's still going to break the bank, go for a third-hand  non-storage analog CRT scope.