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C-Audio SR-707 Protect Issue |
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simonp1100 
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Joined: 01 September 2008 Location: Bristol Status: Offline Points: 1150 |
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 Posted: 03 March 2019 at 7:56am |
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Hi, The relays are T9AS1D12-24 & are 24 volt DC relays. Make sure these are working and switching correctly (contact wise). I assume you have no high DC offset on each channel output, if each channel is around 10mv (no signal present), then i would suspect the protection circuit at fault (also check the thermal trips 3 & 4 on the circuit diagram, these should be normally closed) & then start measuring the diodes and resistors on the protection circuit. If you have already removed the soft start PCB from the amplifier, then test each relay (power supply 24 Volts DC and a multi-meter) Let us know what you find.  Edited by simonp1100 - 03 March 2019 at 8:00am |
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qwerp
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Posted: 03 March 2019 at 9:40am |
thank you for your reply i think the soft start circuit would be easy to remove desolder slide out. the protect circuit seems to be on the end of one of the channel circuit boards (A). im sure i could remove it. but it doesn't look easy at all. There are allot of wires in the way I cant see the two thermal components in series. Are the two thermals and protect circuit behind the area highlighted below on chn A? I am not sure what the thermals look like, although i could test them for continuity if i could find them. thanks for your help again.  Edited by qwerp - 03 March 2019 at 9:48am |
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jbl_man
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Posted: 03 March 2019 at 10:05am |
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Wow! i have never seen one so clean, usually the insides of these SR series look like the contents of the hoover bag.
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Be seeing you.
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qwerp
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Posted: 03 March 2019 at 10:16am |
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it is rusty outside though. but yes quite nice inside,
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qwerp
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Posted: 03 March 2019 at 10:17am |
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if the protect circuit and the thermals are behind that orange circle. i am a bit worried.
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simonp1100
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Posted: 03 March 2019 at 10:32am |
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The protection circuit is on both boards, this is to allow easy change of boards (as they then would be identical) BUT only one board is being used / wired for the protection part. The round bit of foam that is glued in place is only there for isolation between the output coil and the lid to stop it from shorting out against the case, just remove this and when amplifier has been repaired just glue back in the same position. Also again check the the 27K 1 Watt resistors, i did have one in for repair where these were measured O.K. BUT the tracking on the underside was cracked and open circuit. Edited by simonp1100 - 03 March 2019 at 10:34am |
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qwerp
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Posted: 03 March 2019 at 10:46am |
hi simonp1100 what do the thermals look like? I may be able to probe behind with long probes to check. thank you again |
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simonp1100
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Posted: 03 March 2019 at 10:54am |
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There are two thermal trips on each channel heatsink / metalwork. They are a 90 degree normally closed trip and a 50 degree normally open (for the fan and has a orange capacitor soldered across it). They are round black plastic disc's with terminals coming out 180 degrees apart. The ones you need to check is the ones without a capacitor fitted across it, they should be closed. |
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qwerp
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Posted: 03 March 2019 at 11:55am |
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hi I tested the thermal trips and the ones without the caps beep when I test for continuity. the ones with caps do not beep but they have a resistance. The 27k resistors are reading correctly. However, on the pads behind them they are a bit golden/brown ( maybe flux?). So I tested from each leg of the resistors to the next pad ( to be sure the solder is connecting to the pad, and got a beep for continuity from each leg>pad >trace> next pad. so the resistors are still connected/ in the circuit. Edited by qwerp - 03 March 2019 at 11:57am |
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simonp1100
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Posted: 03 March 2019 at 12:29pm |
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It sounds like the thermal trips and resistors are o.k. so if you do have a very low DC level on each output (around 10mv on idle) then i would suspect the protection circuit has a fault. You can swap boards (i.e. fit channel A driver board into channel B and visa versa) and i have done this before BUT then the amplifier bias current and DC offset would need to be set up on again on each board.  .You could also check to see if you have the + and - 15 volts on each driver board (this is to supply the op amp on each driver board). Edited by simonp1100 - 03 March 2019 at 12:40pm |
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qwerp
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Posted: 03 March 2019 at 12:43pm |
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oh no, that sounds like a job for an experienced engineer.   i was hoping it was worth putting in new relays. but as the three on soft start seem to be working ( i think they allow power to the protection circuit) and the 2 output relays will only work if the protection led goes out (i think from looking at the diagram) so the problem is likely in the protection circuit area highlighted above ( orange circle) typical, the trickiest place to get too. The other channel board (B) seems to have a couple of parts missing in the protection area. so they are not identical. this amp was working near 20 years ago. no one has been inside since. it was only opened up a couple of weeks ago to see if it was a fuse. I am almost tempted to take out channel A PCB and see if anything needs replacing in the protection circuit as it is so tricky to get too at the moment. Thank you very much for your help much appreciated : )
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simonp1100
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Posted: 03 March 2019 at 12:48pm |
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The only other thing you could try is to put a stereo signal into the amplifier and put the front volumes on low and then with an old scrap speaker (cheap car speaker etc), connect between the back ground speaker connection and the metal cans on one of the MOSFET's on channel A and see if sound comes out, if it does do the same for channel B and again if it does this would then give 100% that the problem is the protection circuit at fault. This above test can ONLY be done if you have 10mv DC or less on each output.
 Edited by simonp1100 - 03 March 2019 at 12:51pm |
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simonp1100 wrote: