chiguy Posted July 25, 2013 Report Posted July 25, 2013 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? Thanks!
spritzer Posted July 25, 2013 Report Posted July 25, 2013 A few things to try, are all the LED's in the circuit lit and have you swapped the tubes or replaced them? How is the offset/balance?
chiguy Posted July 25, 2013 Author Report Posted July 25, 2013 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.
chiguy Posted July 25, 2013 Author Report Posted July 25, 2013 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).
spritzer Posted July 25, 2013 Report Posted July 25, 2013 Dust is a problem at 700VDC but I bet this is just a tube issue. I would change the caps though...
chiguy Posted July 26, 2013 Author Report Posted July 26, 2013 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?
chiguy Posted July 26, 2013 Author Report Posted July 26, 2013 One more thing, the bias voltage was around 420VDC using the ground tab on the back. I thought pro bias was 580VDC? I measured the normal bias using the ground tab at 185VDC.
wink Posted July 26, 2013 Report Posted July 26, 2013 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.
spritzer Posted July 26, 2013 Report Posted July 26, 2013 Yeah and even so you may load the voltage divider and/or the doubler so no way to get a good result. An offset/balance drift of less than 5V is nothing to worry about.
chiguy Posted July 28, 2013 Author Report Posted July 28, 2013 I'm putting in an order for these caps unless you guys have better suggestions. http://www.mouser.com/Search/ProductDetail.aspx?R=UKL1V470KPDANAvirtualkey64700000virtualkey647-UKL1V470KPDANA http://www.mouser.com/Search/ProductDetail.aspx?R=UBT1A221MPDvirtualkey64700000virtualkey647-UBT1A221MPD http://www.mouser.com/Search/ProductDetail.aspx?R=ECA-1HM100Bvirtualkey66720000virtualkey667-ECA-1HM100B http://www.mouser.com/Search/ProductDetail.aspx?R=EET-ED2G101CAvirtualkey66720000virtualkey667-EET-ED2G101CA 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.
chiguy Posted July 28, 2013 Author Report Posted July 28, 2013 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.
chiguy Posted July 30, 2013 Author Report Posted July 30, 2013 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?
spritzer Posted July 30, 2013 Report Posted July 30, 2013 It can be pretty much anything as it's an intermittent fault.
chiguy Posted August 3, 2013 Author Report Posted August 3, 2013 (edited) 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 August 3, 2013 by chiguy
CraigBuckingham Posted August 30, 2014 Report Posted August 30, 2014 Bit late on this one. What was the grid leakage current and what tube types, sorry don't have a schematic for that amp? I'll go looking in a minute. What caps did you use. Low dissipation factor ones, yes?
chiguy Posted August 31, 2014 Author Report Posted August 31, 2014 I didn't measure the grid leakage current and the tubes are the default types the amp takes (sorry, I don't remember as this was a friend's amp). The caps I used are linked above.
CraigBuckingham Posted September 1, 2014 Report Posted September 1, 2014 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.
spritzer Posted September 1, 2014 Report Posted September 1, 2014 There is only one thing to be said about this post and that is....
CraigBuckingham Posted September 2, 2014 Report Posted September 2, 2014 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.
skullguise Posted September 2, 2014 Report Posted September 2, 2014 (edited) 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 September 2, 2014 by skullguise
spritzer Posted September 2, 2014 Report Posted September 2, 2014 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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