SC501 questions

James Brines

I have an SC501 I looked into awhile back but life got busy and I didn't get to resolve its issues. I've got some free time so I'm back at it.

The CRT shows no trace whatsoever. The unit burned out the 10 ohm resistors in its HV OSC section. A lot of this seems similar to what jerry massengale posted about on 11/22/15. I found C378, C380, C382 and C420 were all out of tolerance so I replaced them as well as the burned 10 ohm resistors. I turned it back on and in under 30 seconds one of the resistors was starting to smoke so I turned the unit back off. While these resistors are still within spec they will get replaced just to play it safe as one of them has a chunk burned clean through to the conductor inside.

I did a thorough visual inspection and saw an odd residue on the rows of ceramic disc caps near T380 as well as an odd coating on CR389. I figured I could pull the caps, test them out of circuit with the LCR and get in closer to look at CR389. A shorted cap while unusual for a ceramic cap could explain the excessive draw. I did so and found that most of the caps in that area have an off-white waxy\greasy coating on them. I can't determine a source of the residue. CR389 turns out to be black on its lower half and orange\clear on its top half. All the caps and the CR check out with the LCR with a few exceptions.

C394, C395 and C397 all have a similar part description in the manual. not exactly the same part but I'm hung up on one point with all three when determining their status. One thing I have noticed about working on equipment older than I am is the subtle differences in documentation over time. As such I wasn't 100% sure how to process this and thought I'd ask for a more experienced interpretation.

C397 283-0013-00 CAP.,FXD,CER DI:0.01UF,+100-0%,1000V

Now most of it reads just fine what is odd for me is the "+100-0% portion". Does it mean that the tolerance is anywhere from 0% below spec to +100% or an I totally wrong here (ie never lower than 0.01UF but can be up to 0.02UF)?

I believe the base of my problem is Q360 and Q365. Time ran short yesterday so I was not able to pull them out of circuit for proper testing though I did test them in circuit as if they were diodes with a multimeter. There may be an in circuit test situation muddying my results but neither acts as they should. The testing was done without power applied to the unit and Q360 conducts in both directions which is very unlike an unpowered SCR. Q365 had similarly odd behavior under the same situation conducting form any pin to any other pin in either direction except for base to emitter. I noticed the previously mentioned post where they discussed similar concerns. My plan is to pull both and test them out of circuit before I call them bad though.

I checked the windings on T380 and they seemed fine.

I am also not the first person to do rework in this unit which is always a box of surprises. The last person was rougher with the AUX board wires than I would have preferred and I was looking to replace them with fresh wire. Any recommendations for appropriate gauge\insulation wire?

So the long and short of this is;

1. Any ideas on what the off-white waxy\greasy coating on my ceramic caps is?
2. Any ideas as to what the half and half coloration on CR389 is? My guess is that waxy stuff got on it's lower half and then it got hot?
3. Am I understanding the nomenclature on the part listing correctly?
4. Is there anything else I should check out to cover my bases?
5. Anybody know a good replacement wire type for the AUX wires?