[sdiy] . . . and now, the power supply (was Akai AX80 Voice 4 DCO2 issue and AX80 Voice 8 PWM over-modulation before that)

james meagher w.james.meagher at gmail.com
Tue Jul 17 05:51:20 CEST 2012


Just when you thought a day might go by where I don't post about my
Akai AX80 restoration . . .

So, with the tact switches, keys, Voice 8 issue and memory back-up all
dealt with, and the Voice 4 DCO 2 problem just awaiting a part, I
decided to actually enjoy the AX80 for a few minutes this afternoon.
I'd just finished swapping IC22 and IC24 to ensure that the Voice 4
problem was indeed IC22 (now the problem appeared on Voice 7 and Voice
4 worked flawlessly), and installing a new memory back-up battery,
when I settled in to create a patch or two. Less than five minutes in
all the LCD displays died (I know they're not really LCD . . . vacuum,
something, something - can't remember the name). Just 100% dark.
Everything else continued to function as it should.

I opened her back up and began probing about. Visually everything on
the LCD PCBs (heretofore referred to as FLD Operations PCBs) looked
A-OK. Eventually I found that the cable harness (P1 on the FLD board
and P4 on the PSB) that runs to the Power Supply PCB, and should
provide the FLD board with  +5V, GND, -40V, was instead metering at
+5V, GND, -5V. What happened next I'm not too proud of.

I decided to start metering around the Power Supply to see if I could
figure out why only -5V and not -40V was going to the FLD board. The
transformer showed to be supplying the correct values to the PSB. D5
metered as I expected it would. Then a fly landed on my face and I
think that in my spastic reaction the probe very briefly connected IC5
Pin 1 (should be 5.6V) and pin 2 (GND). I didn't think much of it
until a second later when R2 began to melt and smoke before my eyes. I
immediately powered off and unplugged the machine. Visual inspection
shows that there is also a black spot on the underside of R3.

My original suspicion was that the problem would reside with C18 or
C23 - but I didn't get that far with my probing before the meltdown.

So, obviously I've not had the board back on since and I'm unsure how
to proceed. Obviously R2 and R3 need to be replaced - but I'm more
than a bit hesitant to replace only them and then turn it back on. I'm
feeling inclined to replace everything on that path to the FLD
Operations board and CPU PCB . . . Is this overkill or not enough?
There are also all the components that lead to the Voice PCB. Nards.

I think I'll go to bed early tonight.

James



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