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Member Since 04 Nov 2013
Offline Last Active Dec 06 2013 10:19 AM

Topics I've Started

D150- One Channel Down!

04 November 2013 - 03:57 PM

Hello dear people. First let me say: awww... y'all are great.

But further:
I've had 2 lovely D150 amps since approximately the dawn of time. Try to take them away from me: suffer my wrath. Now then: I'm not an engineer, but I've burned my way through innumerable carpets with dropped soldering irons (stinky!), so I fancy myself an expert of sorts.

Here's the prob:
I lent an amp out to a touring band (how long will I keep making the same old mistakes?), and when I got it back, one channel was effed. So I've been poking around. While doing that, I kept a log of sorts, and here's a copy/paste:

Turned on, stereo signal (music), 8 Ohm speakers (load).
ch 1 OK
ch 2 Low output, very distorted.

Pulled AC plug, and as the power died out, ch. 2 suddenly un-distorted itself and sounded fine for the last half-second or so before fading out altogether (bad cap somewhere?)

I had a few transistors lying around that matched: swapped both ch2 output transistors, tried the amp, no change. Swapped one driver, no change. Swapped the new driver over to the location of the other (old) driver, replaced first driver with old second driver: no change. Replaced all transistors with originals.

Bought a ua739c IC, swapped it for the old one, oops! Now ch2 doesn't show any output at all.

previously: drivers, output transistors may have been changed at some point, but many many miles on them w/o problems. Something burned on the board, many miles ago, there is a hardwired bridge along the bottom of the pc board between q108 (?) and q208 (?) (it's hard to read the pc layout scan from Crown!!), but it's all ancient history and appears to be ok- no shorts, no burns…

visual inspection: yes I found a couple of bad solder joints, fixed 'em, hard to say whether they caused the problem, but fixing them didn't fix the problem...

Turned on, no signal, no load:
No DC voltage present at output...
Fuse doesn't blow. Can't find -7.5 to 9.5V at pin 7 or +7.5 to9.5V at pin 14 of IC, but where should I be taking the - pole measurement from?…

checked bias resistors:
across R127 shows .294VDC, across R227 shows .039VDC.
across R120 shows .258 (variable), across R220 shows .039VDC

checked R115, R215, without removing them from the circuit:
hmmm, not really sure… it's SO HARD to read the Crown scan of the PC layout…  but it looks like 115, 215, 116 and 216 are all both neither shorted nor open. They all need a minute or two to settle down, but read 5.8- 6.8 KOhm after a while. Likewise R117 and R217, but it's so hard to tell! The scan! AAAGH!

I give up. It looks like no resistors are screwed, but…

Poked around checking diodes, too, nothing conclusive...

Oh man, what I'd give for a legible scan of that 7891A PC layout! Or maybe y'all got a better idea?

Any gentle pushes in the right direction will be most, most graciously appreciated.