Repair and restoration of my STAX SRA-12S

[Bingturong]

Heya! Sorry for reviving this thread again but I got a little issue with my SRA-12S. I had serviced it (with the help of this thread actually) with all-new electrolytic caps and diodes and it has been working dutifully for quite some time until I moved countries. However I brought it out of storage and well, one of the channels is humming loudly. I tracked the problem down to one of the PB-300 boards as when I switch the boards the humming also switches channel. The sound is still amplified as I can tell but the humming is at a constant volume. I think it might be, what I think is, the 630v filter on the board because...

 

Yeah... I think the 51k 5W resistor is burnt but I could be wrong, that's why I'm asking for help. If its just the resistor would any metal oxide 51k 5W resistor do or does it specifically have to be a TDO RS5B because I cannot find anywhere to purchase them? And what about wire-wounds or something else to replace the resistor?

 

I would post more pics but I'm hitting the 500kB limit~

Thanks

Edited May 23, 2021 by Bingturong
Adding reason for not uploading more pictures

[spritzer]

Normal 51K resistors would work just fine but I'd shoot for a non inductive unit such as the Vishay units Stax use now.  They are 7W but that's even better

[Bingturong]

11 hours ago, spritzer said:

Normal 51K resistors would work just fine but I'd shoot for a non inductive unit such as the Vishay units Stax use now.  They are 7W but that's even better

Thanks! I'll go grab some Vishay ones. Also I got an SRA-3S and the pot has gone to hell. Any recommendations for replacing it?

[Bingturong]

Hello again,

 

So I got the resistors and replaced aaaand still humming. So I started swapping boards and only playing with one board. Now its humming if either one of the boards are in. Before it just hummed on the one board with the resistor issue. What could be the issue now? All electrolytics and diodes have been replaced already. Also would a simple voltage divider from the 640v rail work for a pro bias?

Edited August 2, 2021 by Bingturong
Misspelling

[Bingturong]

Right, I noticed something else. Removing PB-100 and PB-200 does nothing. And there's less audible hum with the board that had the resistors replaced, but still noticeable.

[spritzer]

Voltage divider off the B+ works just fine and hum can be pretty much anything so impossible to tell. 

[Bingturong]

I'm pulling my hair at this. I don't think its the transistors as there is no increase in distortion when playing music, except for when the IS is turned on that just makes everything whack. Are there any other components possibly failing on PB300? Or even the power boards? I did notice the diodes on the rectifiers are still stock.

[spritzer]

I would definitely swap out the rectifiers, they are really a wear item on anything this old. 

[Bingturong]

I'll do it, alongside checking all the electrolytics again for any faults. What confuses me is that the humming is only in one channel... any other components that should be replaced? 

[spritzer]

The PSU's are shared so that shouldn't be the issue but it can be anything, a marginal resistor, marginal transistor etc. etc. 

[federicopol]

Hello everyone, sorry for the necroposting:

my sra 12s is misbehaving so time to grab the soldering iron.

the plan was to get it repaired but... in the end i decided to do it myself.

PLAN:

Strip and Rebuild the AMPLI board completely

Recap, and new diodes/resistances for the psu, recalculate the voltage divider for the normal bias from 200v to 230v

Redo the pro bias circuit i added some years ago.

Maybe strip and rebuild the IS amplifier stage (that i never turn on, so not useful per se but will do just because)

I have a couple of questions:

i remember seeing a service manual for this amp somewhere here but i can't find it anymore, someone has it?

I see the toshiba to3 transistor is difficult to obtain nowadays, is this one suitable?

https://www.mgelectronic.rs/tranzistor-2sc-1167#

Do somone has a reliable sorce for 2SK30-O and 2SK30-GR? (or can point me to a substitute?)

Thank you very much

[Bingturong]

A strange update, so rectifier and a few other diodes replaced and yeap! That strange warbling sound is gone. Except now there's a significant amount of distortion on all channels. After trying some stuff, removing the PB100 board seems to solve it. Since reading the schematic, I assume that the PB-100 is not exactly necessary is it? Since it only does the mag pu?

[Bingturong]

I've been pulling my hair with the damn phono board to the point I might as well rebuild the entire thing leaving only the film caps. Is there anyway to bypass the phono board in the amp? Without the phono board there's considerable hum.

[kevin gilmore]

jumper connector pins 7 and 8 of the phono board connector.  but really it should work fine with the board removed.

[Bingturong]

Another longtime update. Going through the amp I found that pins 7 and 8 on the phono connector were already jumped. After more thinking and pounding my head, I realised that the ground for the for EVERYTHING but the power/bias section is actually coming out of the phono board connector. In the schematic the ground for PB-300, PB-200 and PB-100 were all connected together with the ground wire coming out of the PB-100 board. For some reason PB-500 ground was connected that of PB-200 instead of PB-100 but anyway. I just rewired so the ground connection is at the PB-200 connector instead leaving the PB-300 connector essentially unwired except for the MAG input. The amp is now working flawlessly and is silent than ever!