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VolvoHeretic 1st March 2013 03:57 PM

Help with Stax SRA12S Electostatic amp/preamp
Greetings all, I am looking for some ideas of how to get my dead Stax SRA-12S electrostatic head phone class A power amp / preamp fixed. I use it only as a preamp to drive a Crown DC300A pushing a pair of Altec A7-500 speakers.

My technician says that the dual voltage transformer is shot and an E-mail to Accutech, the authorized Stax importer/service center says they no longer carry parts for it.
He tried to just install a low voltage transformer but it still did not function and said that I am SOL. It keeps blowing the main fuse. I do not need the electrostatic side of the unit, is someway of a workaround to the transformer issue?

I am electrically illiterate so anything said will have to be in baby talk. Thanks for any help. James
Here is a link for the service manual and schematic.

Cal Weldon 1st March 2013 04:23 PM

Hi James,

Welcome to diyAudio. I am glad you joined us. For some reason I thought you were already a member here but maybe that's just overlap with the Altec site. Let me know if there's something you need help with here. Other than the post above that is. :D

I am sending you a PM.

VolvoHeretic 1st March 2013 05:52 PM

Thanks Cal, I will try to behave. I hope I shouldn't have posted this under the power supply threads. :) This is all new to me. Kind of like joining Mopar, Chevy, and Jeep forums to find out how to modify my Volvo.

VolvoHeretic 4th March 2013 08:59 PM

Friendly bump. No one? Could I use two transformers mounted externally to get the needed 26 and 260 volts output (? I can't open the PDF schematic on this computer) ?

VolvoHeretic 17th August 2016 11:22 PM

Stax SRA-12S Service Manual
2 Attachment(s)
Stax SRA-12S Service Manual

xrk971 18th August 2016 02:06 PM

Can you take the transformer in question out and take a photo of it showing clearly the wires and take a digital multimeter ohm meter function and measure the resistances between the various leads? There will be input side (primary) and output side (secondaries). We are trying to determine if any are shorted or broken (open).

When an amp blows the main fuse it means something is drawing a lot of current. Typical culprits are dead (fried) output transistors and those will give basically close to zero ohms when you check with ohm meter between collector and emitter pin. Should be in tens of kiloohms to hundreds of kiloohms.

Next thing to check for are bad electrolytic caps. They should not be shorted.

Start there and we can help you further.

VolvoHeretic 19th August 2016 02:47 AM

Thanks a million xrk971, I have long given up trying to fix this thing, I was just posting the user service manual for a fellow diyaudio member that was asking for a service manual since the above link has died. I am posting it for further prosperity also along with AudoKarma, I forgot to post an explanation. :) Thanks for the suggestion, I will try it.

My buddy the electrical engineer went over the schematic with me and showed me how it works and also showed me the individual boards in the amp and which ones to remove and disconnect of the high voltage headphone boards to remove them from the issue, I will never use this as a headphone amp.

I will do as you suggest and see what I can find out, thanks again. :)

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