So... I buggered it up...
First thing I did was recheck the DC... this time I got 0.003 on the left and 0.015 on the right, which I thought was suspect because I was sure of what readings I got last night and hadn't done anything to change anything, for some reason I thought I'd move it off the Dim Bulb and put it direct to mains, dunno why I thought that would make a difference but that's what I did.
Measured again and got the same good results, so I thought ok it's fixed itself, dunno how but thats what the measurements was telling me.
I then thought I'd go ahead and do all the checks as directed above so I could report that back and get an opinion on the health as we see it now.
First wobbler is as far as I can see there are no diodes in this amp...
... these are the only components in the circuit between the driver transformers and the outputs on the heatsinks.
There appears to be a resistor and ceramic cap per channel located next to the tranformers on the left side of the image... to the right of those and before the terminations that run out to the outputs there 2 pairs of resistors per channel, the large red ones are 1k Ohm the smaller ones are Brown/Green/Black/Silver which I deciphered as 15 Ohm 10% tol.
I measured the resistance across all of them in both channels, the big ones were all within tolerance of 1k Ohm, we're talking a few decimal points off in any given direction on each. The smaller ones all rang out at circa 11 Ohms, all evenly across the 4, but out of tolerance. I thought not perfect but not an explanation for a hum on one channel alone.
Went a little off script here and thought I'd put the Peak DCA55 on the output transistor terminals, where the wires are attached to the transistor sockets to be exact (with the power button off), went to the upper first and it told me there was a short between E-C... I thought that couldn't be right because it sounded fine (to me) when I put some signal through the amp last night. Decided it must be because I was trying to do it in circuit.
So next I went to check the voltage drop across B-E, connected black probe to chassis ground and with red probe in hand started on the right channel, upper output read 21.4 at the base and 21.2 at the emitter, don't know what I was expecting to see so just wrote it down to report, reached down to the lower output, got 21.4 at the base but when I went to move to the emitter I touched on it but it slipped off and there was a 'pop', followed imediately by a very loud hum out of the right channel... great... I'd blown it up.
First reaction was to get the power off of it, hit the power off button on the unit and reached for the off button on the dim bulb... that's when I realised I hadn't put the thing back onto the dim bulb... dunno if that would have saved me but still felt like a clot.
I dismantled the heatsinks and removed the outputs to test them out of circuit, the Peak still says the upper has a short circuit between C-E, but if that was the case what symptom should I have heard last night that I didn't, surely that's a big issue on it's own??
As for the lower the Peak thinks it's now an LED... a shorted one at that.
These outputs are not the same as the 501 either, instead of the 2SC1030 as in the schematic, what came out were 2SB361 Germainium PNP... wasn't expecting that when all the other transistors in it I was expecting to be Germanium came up as Silicon.
... with no thermal paste on them at all...
... I checked the left channel with a signal (left RCA only), with the right channel circuit breaker held in to isolate that side as well, the left still works fine.
... so... what kind of damage can I expect?... will the shorting have destroyed any other components?... can I just replace the 2 bad ones?... or should I replace all 4?... do I replace with exactly the same that came out (if you can still get them)?... or is there an equivalent that is either better or just available?...
Soooo many questions in my head right now, and I'm kicking myself up and down the house, thought I'd cracked it, my first foray into fixing up some vintage audio... bugger!
... and the other thing is, if I'd just accepted that the new readings I got first thing tonight were good and everything was running fine now and I'd left it alone it'd still be working...
I am sooo p!ssed off!
... mad at myself more than anything else.
Waiting for the outcome.
)
