Saturday, January 19, 2013

Where was I? Oh yes, I had polished the legs of all analog ICs in channel 8, reinstalled them, and then observed function. Success!

So naturally at this point I disinter all the patina laden ICs in channels 5 through 7, clean them up, reseat chips, reinstall channel cards and apply power.

Nothing. Well, unless you count the full meter deflection of 5-8; audio certainly isn't passing through any of these.

That includes channel 8, of course, which, by my own observation was just passing the first tier of functional test and literally has not been touched since.

Pull card 5 (again, if I was thinking clearly and weighing out all the evidence this step would have been reduced to one selection, but I was at this point a touch flustered). Power up, 6-8 pulse the VU. Power down.

Leave 5 out, pull 6 & power up; 7 & 8 pulse the VU, power down.

Pull 7 & power up, channel 8 shows a minor bump on the VU and now passes sound. Power down, reinstall 5.

Power up, channel 5 shows a minor bump on the VU whereas channel 8 does not deflect at all, both channels pass audio. Power down and install 6, which completes the pattern as expected.

Channel 7 contains a problem that was pulling down channels 5-8. Having my quarry fixed, I now begin to puzzle this out logically, and based on the power rail observation made previous I begin with measuring the rails for shorts. -15 volts to common measures 0.8 ohms.

Ah, the dogs, they have your scent. All integrated circuits are pulled, the short remains. Of course it wasn't going to be that easy. I pore over the schematic for all points tying directly to the negative rail, and choosing for now to ignore resistors narrow my focus to a dozen or so parts and begin to pull them out, one at a time to determine which part is failed.

Next time, I'm going to start with the caps. The dense two sided PCB has made me out for a bungling idiot, and before I actually pull any traces that are important (this was just an insert) I shift my method to cutting.

Which, while destructive, limits the destruction to parts at hand. I'm still exacting far more destruction than I feel I should on this simple task. This particular component was just fine before I came along, WOOF.

Having ruled out transistors I then focus on capacitors. C4 here is the problem, a Matsushita 1 uf @ 35 volts, it's marked as being polarized. Tantalum? Odd package, I can get those values in a similar dimension in ceramic, which is my plan.

Of course, I threaded up a reel of tape and learned my record levels have been killed (previously 3 & 4 had recorded well), now everything reads high to tape and plays back low, with the exception of previously recorded material. So now I chase a common point in all channel recording. I'm thinking bias as a starting point, have mercy.


krivx said...

Eek. You don't have gear that can test caps in circuit? If you use test signals off only a couple of 100 mV you can avoid turning on any active devices nearby.

crochambeau said...

I do not have gear that will test caps in circuit, I'll put that on my radar. C4 is situated electrically in parallel with four other caps, localized decoupling at the legs of the white supply buss, so I think it would have been impossible to isolate the failed cap itself without removal from circuit.

That aside, testing in circuit would have saved the actives I wrecked. 10 cents poorly spent.