It's a little confusing but I think I understand fundamentally what is happening in most of the sections of the amp. The whole + and - circuit thing is kinda hard to wrap my head around but I think I'll manage. The hard part at this point is trying to reproduce the problem. It's been playing for eight hours today without a pop. I tried getting it hot and that didn't help.
Intermittent faults are always difficult. If are 100% sure it is not just a poor joint then it might be worth just replacing any transistors that obviously run very hot. These would be ones that run so hot they discolour the board.
Its not a very scientific method but if the problem is so intermittent then it might help. You could also look at swapping over the left and right channel inputs to the power amp section to see if the problem swaps channels or not. That would prove if the problem is before or after the volume control. Also see if the level of popping is affected by the volume setting. If it is then it seems to be occurring before the control.
Its not a very scientific method but if the problem is so intermittent then it might help. You could also look at swapping over the left and right channel inputs to the power amp section to see if the problem swaps channels or not. That would prove if the problem is before or after the volume control. Also see if the level of popping is affected by the volume setting. If it is then it seems to be occurring before the control.
Thanks. I can't be 100% sure it's not a poor joint still but poking and tapping everything on the main board didn't seem to change the behaviour although I can't really tap the tone board when the front cover is on. Swapping is a great idea, it's fairly easy for me to use jumpers to swap the L and R when the signal comes back from the volume board which would isolate it to the power amp if nothing changed. Just a quick question as I am looking at the diagram here. Is what they're calling "buffer" the op amp? I'm trying to educate myself more on how amps work. After this, the signal heads to volume board -> tone board -> volume board -> main board and into the power amp circuit.
IC 402 = LC7822 is your input selector swithc.
I have more tahn once seen these fail, when users had put something in the input plugs, that had too much
static and the amp was on.
It usually fails completely on one channel...... or both, with some static instead of sognal.
Also check for cracked solder on all the pins.
I have more tahn once seen these fail, when users had put something in the input plugs, that had too much
static and the amp was on.
It usually fails completely on one channel...... or both, with some static instead of sognal.
Also check for cracked solder on all the pins.
Thanks. Might be a week or so before the scope gets here so now I am trying to come up with a way to reproduce the problem reliably and failing haha. In the meantime, more education. I used to be an auto tech so I am familiar with this type of troubleshooting, just happy it's not on-the-clock time 😉
oh right, hm... the problem is I feel like it has changed the way it has presented itself over time. Possibly because there were multiple bad joints in conjunction with the speaker relay not being clean (it did exhibit the problem where after power on, the left channel actually didn't reach the volume of the right until things were turned up a bit). Next time it does it I'll crank up the volume but I do know the volume can be all the way down and it still does it. I have it on these dinky little speakers now since I don't want to risk damaging my good ones.
You can always link out the speaker relay as a test. If you do I would recommend disconnecting the speakers at power on and then connect them after a couple of seconds. Same at power off, disconnect them and then switch off.
Hi. I would like to add my few cents. It's a stereo unit, so a few questions, to be more clear/ exact - those pops appears only on one channel, or on both? Pop's volume is volume control position dependent? If problem is in power amplifier part, then it would not depend on source selected or volume control position. If just one channel disappears after some time playing, you can try to swap channels , and notice if problem is moved. Also you can try to link channels together with a temporary switch, wire it and place it outside of amplifier. And when you hear one channel is missing, switch that switch - if sound will restore, then problem is in preamplfier part, or volume control potentiometer itself. If that will not return missing channel to play, problem is in power amplifier part of that channel. Fading style reminds me of bad relay contacts, bad soldering, broken pcb track, volume potentiometer, or bad resistor, and some circuitry works only until some capacitor discharges. So , do more tests to narrow down source of problem. Also input switch can be bqd, but in your case there's input selector ic, so no mechanical switch in signal path probably ( maybe tone defeat is present? ) .
When I received the unit, the right channel was barely working/severely distorted. After fixing every bad solder joint I saw, the distortion was gone but then after some time, the right channel would start to have episodes of loud popping and immediately dying with like a fading static sound, only to return to normal one or two seconds later. This would continue steadily sometimes after another couple seconds or a bit longer. I didn't want to hurt my speakers so I never let it persist for long. At this time I remember thinking the volume affected the loudness of the pop but even at zero volume, there would still be an audible pop. I also feel like the left channel was quieter than the right but may have gotten even after the volume was increased from zero.
I repeatedly would re-flow more solder joints trying this or that and after one of these sessions, the popping behavior became much less frequent. During one of the more recent re-flow sessions, I pulled the speaker relay and cleaned the contacts and left the cap off in order to be able to tap on if if I heard the popping which now when it happens seems less "severe" where the audio would drop out for a couple seconds and instead is just a more frequent and smaller pop sound but not in any way rhythmically repetetive. This may simply be due to the tiny speakers I am using to test. I couldn't get it to change where anything I tapped seemed to make it happen so I put the cap back on the relay. As of right now I haven't been able to get it to do it even after 16+ hours even trying to get the unit hot by covering the top cover vents and playing it loud for a while. I feel like if I go take it upstairs and plug it into my largers speakers, after a while it will start happening again but I won't do that until I have my oscope so I can start tracing. When it does this I will also note with certainty if the volume level affects the loudness of the popping.
I repeatedly would re-flow more solder joints trying this or that and after one of these sessions, the popping behavior became much less frequent. During one of the more recent re-flow sessions, I pulled the speaker relay and cleaned the contacts and left the cap off in order to be able to tap on if if I heard the popping which now when it happens seems less "severe" where the audio would drop out for a couple seconds and instead is just a more frequent and smaller pop sound but not in any way rhythmically repetetive. This may simply be due to the tiny speakers I am using to test. I couldn't get it to change where anything I tapped seemed to make it happen so I put the cap back on the relay. As of right now I haven't been able to get it to do it even after 16+ hours even trying to get the unit hot by covering the top cover vents and playing it loud for a while. I feel like if I go take it upstairs and plug it into my largers speakers, after a while it will start happening again but I won't do that until I have my oscope so I can start tracing. When it does this I will also note with certainty if the volume level affects the loudness of the popping.
Ok , so problem is with Right channel. Also , poping sound can mean some dc offset is present at output of affected channel, and that dc offset is changing by lets say broken track or bad soldering . If yoyyare saying, it happens even with zero volume position , then problem is in power amplifier. I have not looked at schematic yet, but thinking about bad dc balance trimpot , in power amp input stage , if such pot is present. Sometimes when problem dissapears , its very hard to diagnose it further.
Yes I suspect it's in the power amp too. I don't believe there are any pots anywhere in the circuit besides the volume, tone and balance knobs. Here's the power amp, excuse my highlighting but do correct me if I haven't drawn the correct (simplified) signal path.
There isn't really a simplified path with an amp like this, it all operates as a whole and the audio signal within the amp can look unrecognisable at some points. The path to the right of the input stage is really the negative feedback path. The voltage gain is the value R762 divided by R761 plus 1 so it comes in at (30000/220)+1 which is 137. So if you apply 100mv peak input you would see 13.7 vol;ts peak at the output.
It seems indecently high does that voltage gain figure but if those are the values then that is what it is.
Those dual PNP transistors used at the input stage can in some amps give trouble. Toshiba amps come to mind. Just a thought.
It seems indecently high does that voltage gain figure but if those are the values then that is what it is.
Those dual PNP transistors used at the input stage can in some amps give trouble. Toshiba amps come to mind. Just a thought.
Ok... What i see from this schematic - output relay is common for both channels , so if protection would trip (overload or dc offset) in any channel, both channels would be muted ,you will hear no audio at all, not another channel playing ok . So still bad relay contacts ( weared , burned, etc) left as possible .
I'm thinking loud - what if current source Q752 or R754 would fail (bad soldering or defective ) ,we would have all positive rail at speaker output ,because Q753 would pull-up output to positive . But what if R752 will fail open, bad soldered , or zener D751 shorts sometimes ,begins to fail ? Q752 then would not conduct too , because D751 would have no voltage drop on it. But - the same voltage drop through R755 opening Q755 too , so no zener current or zener shorted - just no sound, and no protections tripped ,no dc on speaker output. So i would check those parts . Also resistor R752 should be high power ,dissipated about 0.48W all the time ,so it may have just failed or bad soldering . Check those carefully .
I'm thinking loud - what if current source Q752 or R754 would fail (bad soldering or defective ) ,we would have all positive rail at speaker output ,because Q753 would pull-up output to positive . But what if R752 will fail open, bad soldered , or zener D751 shorts sometimes ,begins to fail ? Q752 then would not conduct too , because D751 would have no voltage drop on it. But - the same voltage drop through R755 opening Q755 too , so no zener current or zener shorted - just no sound, and no protections tripped ,no dc on speaker output. So i would check those parts . Also resistor R752 should be high power ,dissipated about 0.48W all the time ,so it may have just failed or bad soldering . Check those carefully .
😎
Just waiting for you to get your scope, to hear if you have signal "OUT" to both channels from IC402 (LC7822) 😉
Just waiting for you to get your scope, to hear if you have signal "OUT" to both channels from IC402 (LC7822) 😉
I don't think it will come for another week. I'll go throw it up on my main system to see if it acts up.
It did start acting up this morning while leaving it on the fm tuner and tuned to a station. I went upstairs and it was sitting in protect mode so I brought it back down and plugged it in to crappy speakers and it started popping. Volume knob does not affect the popping loudness. I took out the connector board between the volume board and main board and switched the L and R out from volume board and the popping was still in the right channel so it must be in the power amp. Can't wait for that scope to come.
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