[sdiy] Akai S612 power problems

Adam Inglis 21pointy at tpg.com.au
Thu Oct 4 05:56:58 CEST 2012


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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