[sdiy] Akai S612 power problems

[Adam Inglis]

OK, just for the record:

I rigged up an external +/- 5 volt supply and used that alongside the  
working 15 and 12 volt rails from the PSU, to prove that the problem  
was not on the load side.
I re-checked all the voltages and diode-drops around the transistor  
regulator sub-circuit again - all seemed pretty close to those quoted  
in the service manual, at least within a few tenths of a volt.  
Certainly there was no result that pointed to a single component.
Replacing the 4 transistors and the zener in this circuit fixed the  
problem and restored a robust +/- 5 volt supply, although the rails  
now measure about 0.3 - 0.4 volts higher than before (when the load is  
unconnected).

So, still not sure what the actual failure mode was - maybe these  
transistors copped a significant reverse-voltage when the transformer  
opened?

Adam
On 04/10/2012, at 1:56 PM, Adam Inglis wrote:

> The power supply board for this old thing (circa 1985) provides the  
> usual +/- 15 volts, and + 5 volts for the digital side..
> As well, a separate zener/pass transistor circuit  provides a + 12  
> volt rail, and  similar circuits provide +/- 5 volts for analogue  
> purposes.
> Schematic here as PDF...
> http://www.adambaby.com/tempdownloads
>
> First, the transformer blew.
> I had it re-wound, and before hooking up the load, checked the  
> voltages around the power PCB and all was well. Hooking it up, the  
> +/- 5 v rails "sagged" giving about half a volt and -1.7 volts. Then  
> as I was looking into this, I noticed the 12 volt rail had gone AWOL.
> Unhooking it, I find the 12 volt zener has shorted. Replacing that,  
> then the associated pass transistor doesn't work. Replacing that, I  
> get my 12 volts back, and checking all voltages again around the PCB  
> is all good... until I hook it up, and again, the +/- 5 v rails are  
> sagging.
>
> Now, I'm thinking, maybe there is some component on the main board  
> that is shorting, but why both rails affected? Is this a diagnostic  
> failure mode? The power PCB was heat-affected, I found a lot of  
> cracked joints, so ended up refreshing every one.
>
> My investigations are hampered - although I purchased a good quality  
> copy of the service manual, Akai didn't provide a schematic of the  
> main board, just a "connection diagram", probably to prevent copycats.
>
> Any suggestions greatly appreciated as always!
> TIA
> Adam
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