Phillips 3212 Repair

[ColeLamothe]

I purchased a Phillips 3212 oscilloscope a few months ago pretty cheap but broken, either I get a cheap working oscilloscope with a few components or I learn about some old school circuit design. I have seen that there are a few topics around the web, on this site and elsewhere with people who have done similar but most of them are either unrelated to my problem or have no followup and are dead ends.

I have the manual and full schematics which has been very useful. When I first opened it up I found a burnt-out transistor that I was able to purchase a direct replacement for(BD237 replaced the pair even though only one was damaged). When I turned it on I got a blip of power, a fizzle sound and nothing more. Since then I went through and measured all of the power supply output voltages and found that they were lower than they should be and the previously burnt out transistor heats up very rapidly. I have gone through and tested nearly every component on the board, found a number of bad electrolytics and decided that if I was going to replace a few I may as well replace the lot of them. I have lifted a leg on every diode on the board, pulled the small transformer and checked for shorts/breaks, pulled the transistors and put them in a unit tester (mega328 style, it is the best I have for much of this testing) everything seems to be testing alright.

The problem I still have, beyond the power transistor getting hot enough to cook on, is that the voltages are still well below spec and the oscillator circuit seems to be collapsing periodically as I can see from my single-channel handheld scope.

This is a bit of an interesting scope as it can take an AC or a DC power input, the AC transformer was shot when I got it so I am powering it from my bench power supply, it takes that DC input, creates an AC signal at the frequency it needs and pushes it through a transformer with an entertainingly large number of secondaries. I have that transformer desoldered currently so I can try and test it but I honestly am not sure how to be completely sure it is alright, I am not seeing any shorts between primary and secondary coils, I cracked a portion of the housing off so I could see and smell the inside and nothing looks/smells burnt out. Additionally, I was hoping to try and test the circuit before the transformer with it removed from the circuit and the oscillator does not seem to work without it in and I am only getting straight DC.

I am at a bit of a loss at this point and not sure how to proceed, I am certain that the problem is on the power supply board but cannot find anything that is clearly shot. I am hoping that someone here has a direction they can point me in.

[JKKDev]

Just a few double checks...

1) Are you feeding it +30VDC in correct polarity (notice -30V on schematic, ie. that line is 0VDC and the other one is +30VDC)?
2) Are the voltages around BC55... what they are supposed to be?
3) Are +20VDC and -20VDC present in relation to the point A on the schematic on the second full bridge rectifier in?

[ColeLamothe]

1. According to the diagram and the rest of the manual that I can post if desired it should be a 22-27VDC supply, which is a part of the schematic that does not make much sense to me. Where is the extra voltage coming from? I have been supplying it with 24V and that seems to be the ideal area for it from the manual and from a lovely video series from a channel called joulespercoulomb that did a repair on one (different problem unfortunately)

2. When I last measured the output of that portion of the circuit I was getting ~25-30V AC which is the appropriate output for the oscillator from my understanding of the diagram.

3. I cannot recall off the top of my head, I will need to get it put back together tomorrow and double-check. I can say that points 13 & 14 shown in the diagram have an ideal oscilloscope readout in the manual which is basically two clipped sine waves 90 degrees out of phase with ~40VPP. I get something similar but with a fairly heavy DC offset and in the picture of my cheap scope you can see a line coming into frame, that would be the sine wave collapsing into my DC supply voltage.

[shakalnokturn]

What seems to be a very common problem on these is the HV winding of the transformer failing.
I have a couple around with the problem, need to look into a substitute solution one day. I came across a page somewhere here or WWW un general refering to the problem on the 3212 or one of the similar models (same PS) where the owner cut into the HV winding to disable it and used something seperate for the HV.

The symptoms usually begin with a CRT that fades away within minutes from a cold start.

[ColeLamothe]

Do you know how I would go about testing that to confirm?

Currently, I don't have any power to the CRT, the indicator light doesn't even turn on when the power is switched on. I measured the voltage to the CRT multiplier and what should be ~1000VPP is between 6-800 VPP so it is well below the spec but that is about the same proportion that the rest of the voltages are low so I am not sure.

[WirelessESD]

I have a 3250x (3252) and the hv tranformer on mine measures a death short. Not sure how to fix that though but at least it seems it is a common problem.

[ColeLamothe]

Finally had a chance to take a serious look at the multi-tap transformer. From what I can tell the HV secondary is measuring at 275 ohms which seems very high to me even without having a spec to compare it to. I am thinking I will isolate the HV winding from the circuit and hook it back up to see if I still get the loading issue. The only part of this that I am not sure how to handle is replacing the functionality of a 1000V AC supply for the CRT, from what I can tell it seems to be fairly low current considering the manual has the max current draw for the entire scope at 1.1 A. I am not concerned about maintaining authenticity or anything of that nature. Any advice on materials to consider for sourcing or designing something of that nature? I am comfortable enough around HV to not die but don't have any experience in designing this type of circuit.