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

[random variate]

VFD

Vacuum Fluorescent Display 

Rarely do the tubes go - built to last - but regulators and supporting display board chips.





On 17 Jul 2012, at 04:53, "james meagher" <w.james.meagher at gmail.com> wrote:

> 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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