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

[Bob Weigel]

On 7/17/2012 1:21 AM, random variate wrote:
> 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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>>      
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>    
btw everyone should read my repair notes on my website to avoid damaging 
the mux chips...was just talking to someone else about this same thing..

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