Mooly, last night it happened once and then started playing again a few seconds later, and then again about an hour later, twice within a couple of minutes, but each time it resumed playing. I wasn't able to check measurements so I will continue to play it and check them the next time it cuts out.
I also did not hear the relay click when it stopped/started each time, but then I was not close to the amp when it happened.
I also did not hear the relay click when it stopped/started each time, but then I was not close to the amp when it happened.
It does sound the relay activating tbh. If it were not then the 'good' channel would keep playing, so both cutting out together (which I understand is what is happening) points to the relay tripping.
So I ordered the relay for my SA-8500 but never installed it. Should I give that a shot, or are you thinking other circuitry in the speaker protection relay circuit that I should check?
Definitely other circuitry at this stage. You need to catch it in the act and see if anything unusual shows both in the inputs to the relay driver stage or perhaps an issue with the relay driver itself. If one channel develops an offset then that will trip the relay as will excess current across any of the 0.5 ohms (so that would show as excess bias).
Hi Mooly, going to have another look this evening. In the service manual, pages 8-9 discuss the Protection circuit. If this is a DC offset issue, then section 2. Detection of DC Voltage on page 9 indicates that Q20 and Q21 "listen" for excess voltage and trip the relay. Maybe I'm missing it, but I cannot find Q20 and Q21 anywhere on the diagrams and boards found at the end of the service manual. I would think these were on the same board as the relay itself, but I'm not seeing them.
I'd like to check voltage on these two transistors at V/B/E points as well as Vbe to see if they might be failing.
I'd like to check voltage on these two transistors at V/B/E points as well as Vbe to see if they might be failing.
Sounds like the manual is wrong. Its Q7 and Q8. The first check has to be measuring the voltage at the junction of the 0.5 ohms when it goes faulty. If the voltage there is zero (and the bias current is correct as well because that is monitored) then the fault is going to be in the protection circuit.
Common problems could be as simple as a dry on the relay driver transistors and/or relay coil.
Common problems could be as simple as a dry on the relay driver transistors and/or relay coil.
Certainly 😉
So that means that Q11 (and R45 which looks like it could run hot) would be favourites for poor soldering. Also the soldering on the relay coil pins themselves are worth checking.
If the relay is dropping out then it is either being told to do so, or its a problem with the actual relay and its associated circuitry. So initially we have to pin it down to either being one of the two channels having an issue or the protection circuit itself.
So that means that Q11 (and R45 which looks like it could run hot) would be favourites for poor soldering. Also the soldering on the relay coil pins themselves are worth checking.
If the relay is dropping out then it is either being told to do so, or its a problem with the actual relay and its associated circuitry. So initially we have to pin it down to either being one of the two channels having an issue or the protection circuit itself.
To confirm, you are talking about Q7, Q8 and Q11 on the Switch Circuit Board (same board as the relay). I see all of those on the schematic diagram in close proximity to the relay (physically located on the other side of the board from the relay).
As for the Power Amp board, given I have low fluctuating voltage when trying to set bias to 0 and .6 mV at the .5 ohms, unless those numbers peak higher when it shuts off, focus should be directed at the protection circuit. Right?
As for the Power Amp board, given I have low fluctuating voltage when trying to set bias to 0 and .6 mV at the .5 ohms, unless those numbers peak higher when it shuts off, focus should be directed at the protection circuit. Right?
Yes, all that circuitry around those transistors. If possible you need to catch it in the act to be sure, otherwise its pure guesswork as to which (if any) channel is faulty, and what the problem is (excess bias current or excess DC offset), or whether both channels are in fact OK and the problem is elsewhere.
If the amp really wont co-operate then there are various tricks we can use to get an idea but its easier if possible to 'measure' the problem.
If the amp really wont co-operate then there are various tricks we can use to get an idea but its easier if possible to 'measure' the problem.
Ok. First thing I did this afternoon was reset offset and bias. By the way, which is which. Is page 17 offset (0v) and page 18 bias (30mV)? I got close to 0v fluctuating a few mV on each channel. Got 30 mV steady on both channels.
Took some Q7, Q8, Q11 measurements. Q7 & 8 appeared ok but Q11 had 8+ volts on All 3 pins. Don't know if that is normal. Diagram did not indicate voltages that should be seen on Q11.. Vbe was .667.
Hooked up amp and played CDs. Towards the end of second CD, speakers cutout, in, crackling noise. Had unit upside down to take quick measurements of Q7/8/10 and Q7 collector was up at 32.5v. Was at 21v before. Started seeing fluctuating voltage across base as well as on Q8/10.
Flipped unit over to check current on power amp board and instead of 0v, I was seeing wide fluctuation as high as 30v. Relay clicking on and off frequently along w speaker static. Turned amp off at that point.
So it would seem I still have issues on the power amp board. Right?
Took some Q7, Q8, Q11 measurements. Q7 & 8 appeared ok but Q11 had 8+ volts on All 3 pins. Don't know if that is normal. Diagram did not indicate voltages that should be seen on Q11.. Vbe was .667.
Hooked up amp and played CDs. Towards the end of second CD, speakers cutout, in, crackling noise. Had unit upside down to take quick measurements of Q7/8/10 and Q7 collector was up at 32.5v. Was at 21v before. Started seeing fluctuating voltage across base as well as on Q8/10.
Flipped unit over to check current on power amp board and instead of 0v, I was seeing wide fluctuation as high as 30v. Relay clicking on and off frequently along w speaker static. Turned amp off at that point.
So it would seem I still have issues on the power amp board. Right?
It would appear so. The crackling noise is a giveaway too. Were it just the protection circuit cutting in and out then the audio would simply be cleanly switched. The crackling is the DC conditions rapidly changing.
Assuming its not a bad joint somewhere then the favourites would be Q1/Q3 and Q5. If it were me I would go for Q5 first.
(the DC offset is the very small DC voltage present at the speaker terminal and that we trim to as low a value as possible. Anything below 100mv isn't going to cause a problem. Bias is the quiescent current that flows in the output transistors under no signal conditions. We set and deduce the value by measuring the voltage across the 0.5 ohms and setting to the recommended value)
So to clarify. Crackling noise came from right speaker. So Q2/4/6. I've looks over both the amp and switch boards really closely and don't see any bad/cold solder. Should I reflow these transistors?
As for DC offset and bias, I was trying to confirm what pot adjusts what. The service manual simply says adjustment on page 17 w VR1 and 2 to get 0v. And then idle current adjustment on page 18 with VR3 and 4 to get 30mV.
As for DC offset and bias, I was trying to confirm what pot adjusts what. The service manual simply says adjustment on page 17 w VR1 and 2 to get 0v. And then idle current adjustment on page 18 with VR3 and 4 to get 30mV.
Mooly, Chris. Back on page 8 of this thread when we were troubleshooting my amp, Chris said:
I would suggest replacing the diff pair (Q2, Q4) with 2N5401 and the Vas with 2N5551 transistors. Tie Q2 and Q4 together so that the cases touch.
I ordered 4 2N5401 so I have them to swap out Q2/4. Also those 5401s are about 1/2 the size of the originals. Is that ok?
As for the 2N5551 Vas, is that for Q6?
I would suggest replacing the diff pair (Q2, Q4) with 2N5401 and the Vas with 2N5551 transistors. Tie Q2 and Q4 together so that the cases touch.
I ordered 4 2N5401 so I have them to swap out Q2/4. Also those 5401s are about 1/2 the size of the originals. Is that ok?
As for the 2N5551 Vas, is that for Q6?
If the transistor soldering looks OK then don't reflow them. The heat of the iron could temporarily 'fix' them. The 5401's are fine for the diff pair but watch the pinouts. Same applies to the 5551 for Q6, the VAS or voltage amplifier stage.
Might be an idea to revert to the bulb tester when initially powering up.
Might be an idea to revert to the bulb tester when initially powering up.
Mooly, I'm not familiar with a bulb tester. I can google how to make one, but how/where would I test with it when powering up?
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OK, having read how a dim-bulb tester works and how to make one, I'll need an outlet and light bulb socket, along with a bulb. But the Service Manual indicates that this amp draws 240 watts. Not sure I'm going to find a bulb to match that.
So I have ordered the 2N5551 transistors. In the meantime, would it be worth running one more test, playing the amp, and the next time it cuts out, freeze spray Q6 first to see if music resumes, then spray Q2 then Q4, to see which one is failing?
Should I turn the amp off before spraying each tranny?
Should I turn the amp off before spraying each tranny?
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