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Stax SRM-T1 Repair


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A friend of mine has asked me to look at his vintage SRM-T1.


The caps are all vintage so I will be replacing them.  I count 10 electrolytic caps in there.

4x 400V 100uF

2x 10V 220uF

2x 35V 47uF

2x 50V 10uF


The left channel has very weak sound.  You can barely hear it when the volume knob is turned up all the way.  The right channel is perfectly fine however.  I'm not sure where to start troubleshooting this one.  Unfortunately, I do not have another pair of electrostatic headphones to try.  The left channel has worked before but it seems to work intermittently.  


I adjusted the offset and balance when he first purchased the amp a few months ago.    


Am I missing anything else that I should be doing with vintage Stax amps?  



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I don't have a spare set of tubes at the moment but I will see if I can get some.  


Before going to bed last night, I swapped the tube positions (on a whim) which got the left channel working again.  I'm going to swap them back later tonight and see if I can replicate the problem.  I turned it on this morning to check the LED's and all 4 were lit.  


I'll measure and reset the offset/balance when I get home from work tonight.  

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I should also add that the last time the left channel was acting up, it started working again after my friend cleaned the dust off the circuit board.  It then stopped working again while listening to some music a couple of months later (which was last week).  

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I swapped the tubes back to their original positions but the Left channel is still working.  I left the amp on for more than 6 hours before I started measuring the offset/balance.


I was reminded of how extremely sensitive the trimpots were.  Are there multiturn replacement pots that I can use instead?


The right channel drifts rapidly.


R+/R- started at -2.2VDC and drifts between -0.5VDC and -1.3VDC.  I adjusted to drift between -0.3VDC and 0.6VDC.

R-/Ground tab on the back drifts between 0.7VDC and 1.6VDC.  

R+/Ground tab drifts between 0.15VDC and 0.7VDC. I adjusted to drift between -0.1VDC and 0.55VDC.  I re-measuerd R-/GND and it drifts between -0.3VDC and 0.95VDC.


The left channel drifts rapidly.


L+/L- drifts between -1.5VDC and 0.25VDC.  I adjusted to drift between -0.5VDC and 0.7VDC.

L-/Ground tab on the back drifts between -1.05VDC and -0.5VDC.  

L+/Ground tab drifts between -2.4VDC and -1.75VDC. I adjusted to drift between -0.4VDC and 0.55VDC.  I re-measuerd R-/GND and it drifts between 0.65VDC and 1.5VDC.


I will be putting in an order for caps soon.  I've read that the large caps are the snap in type and everything else are standard through hole?

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You really need a HV probe th check the bias voltage as the meter is a big drain on the supply.

Normal voltmeter is about 10 megohms load.

Since the bias supply has a 5 megohm resistor in series, this causes a voltage drop of about a third due to the loading of the muiltimeter.

You are better off checking the bias voltage on the other side of the 5 megohm resistor.

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I'm putting in an order for these caps unless you guys have better suggestions.








Dumb question here.  I'm curious as to what the glass components are in the attached picture.  I know the ones on the ends are capacitors based on the silkscreen markings but I've never seen that type before.



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Interesting, thanks!


So I lost the left channel again.  I took the tubes out last night to measure the lead spacing for the new caps.  I put the tubes back to what they were before and the left channel was gone.  I swapped the tube positions and the left channel was still gone.  All four LEDs were lit in each case.


I haven't gotten another set of tubes yet but that's the next thing I will try.  

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Apparently when my friend bought this amp, it came with 2 sets of tubes.  One of the first things he did when the left channel wasn't working was to try the other set of tubes.  According to him, this would sometimes fix the left channel and sometimes not.  


I have new caps on the way.  Is there anything else should I be looking at till then?

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Well, I think I found what was wrong, bad soldering on the wire going from the pot to the PCB.  


I noticed that if I pressed on the pot I would get something louder in the left channel so I figured something was going on with the pot.  I removed the wires to measure the resistance of the pot and in the process, noticed that the left channel wire had come off the PCB entirely even though my iron never went near the solder joint.  I looked at the solder joint and all I saw  was a round dome of solder with nothing sticking out of it.  


I've also replaced the electrolytic caps.  As a result, I've noticed that the drifting while adjusting the bias/offset is not as rapid or large.  


Thanks for your help everyone!

Edited by chiguy
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  • 1 year later...

Thanks chiguy,


Sorry, ADHD got the better of me. Found the links afterwards. FWIW I found the stock caps in an SRM-313 I had to be very average, lacking colour and grainy. Nichicon from memory, but can't remember what type, although being HT ones may be easier to work out.


Anyway, the Jensen ones were way more musical.


Wow I thought it was 2014 this thread, did not look close enough. Used to some other faster moving forums.

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Hi spritzer, do I know you?


You take very much liberty to make fun of me? Where did you gain this close familiarity with me to take such liberties? Usually only my close friends can joke with me this way.


Anyway, if you are nice to me I may be willing to part some of my knowledge to help you improve your audio system.

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Anyway, if you are nice to me I may be willing to part some of my knowledge to help you improve your audio system.


Oh, isn't that lovely.....


Craig, please read your welcome PM.  Then, maybe visit the DIY forums and see who adds a ton of value there.  This is a very different forum, best to get used to it and its inhabitants.


And just to be specific, he didn't make fun of you as a person, but instead commented on your post.  But with responses like the above, that could change.

Edited by skullguise
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No need to get personal but Jensen caps are overpriced garbage.  They are just expensive with gold printing on black which has to mean they are good.  If you want to improve the SRM-323 then regulate the power supply, increase the output stage bias and add a servo.  They are actually real circuit improvements which is what we do here

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