Hey everyone
I bought a PM 3315 125MHz DSO from eBay for £30 - on account of it having a 'power supply fault'.
So I powered the unit up, and no surprises it doesn't work. The LEDs were flashing intermittently and the character displays flashed 'E' (for error, presumably). However, when I turned it off, I did see a flash on the CRT, so something's working at least.
Opening up the unit, I saw that there were two power boards, one feeding into the other. The secondary one has four transformers in a line, presumably for the high voltage CRT supply. I believe the fault lies with the primary board. See attached pics.
I was certain the DC filter caps would have blown, but they seem fine to me. The 'dints' you can see there appear to be regular, and thus by design. The rectifier tests OK.
Now, the big transformer to the right was emitting a high pitched intermittent noise, fairly loud, although I could probably deal with it. Apparently this can show degradation, but it looks fine to me. Maybe a few hairline cracks in the laminating material, but this thing is 20+ years old.
Moving to the far left, the mains cap there is definitely busted. Huge cracks all over it, however it is just a noise filter cap, it shouldn't affect the operation of the scope too much. And it is of course self healing, there's no short across it.
That DIP to the left is the switch-mode controller - a TDA1060. I can find these on ebay, so if it is busted, I can replace it without too much trouble. And that's certainly a possibility - on the underside, around a couple of passive pins connected to the chip (the transistor output and ground, as it turns out) the solder mask is cracked and swollen. Thoughts?
Between the two transformers there's a load of passives. The underside of the board is blackened around there, however I have tested the components to the best of my abilities and found them to be working.
One problem I think, is that on the main output header, all rails of which appear to come from the transformer output, a lot of the pins are shorted to one another. Is it possible for a transformer to catastrophically short? Wouldn't that blow a fuse somewhere?
On the output header, left to right on the topside, the pins are shorted in groups of 4, 8, 3, 1.
The solder shorts on the underside suggest they should be grouped 2, 2, 1, 1, 1, 1, 1, 1, 2, 3, 1.
On the connector to the secondary board, the matching pins are grouped 2, 2, 1, 1, 1, 1, 1, 1, 6.
Of course, this could be a feature of it being tapped, in which case I need not worry.
If it turns out the transformer is busted, I have no idea where to get a replacement. Maybe I'll just have to jury-rig several with the same ratios. Ideally I'd replace it anyway, as that whine, while bearable, is somewhat distracting. Maybe I can find some (fireproof, non-conductive) noise insulation foam somewhere.
It has some markings on it: 4022 369 68571, then DY 565. The first three groups look like a Philips part number, judging from the service manual of the 3305 (by the way, the service manual for the 3315 would be a great help, but I can't seem to find it anywhere). DY 565 seems to be a part number, but I can't find any reference to it.
So that's where I'm at. I've taken some pics from the other boards as well, for your viewing pleasure - it really is a work of art inside.
My current equipment consists of a £5 pocket meter - AC, DC, resistance/continuity, capacitance, frequency, diode. Soon, however, I will get myself a Fluke 87.
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