[sdiy] Akai S612 power problems
21pointy at tpg.com.au
Thu Oct 11 00:41:42 CEST 2012
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
So, still not sure what the actual failure mode was - maybe these
transistors copped a significant reverse-voltage when the transformer
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
> Schematic here as PDF...
> 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
> 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!
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