sajjad amer
Active Member
I have swapped left channel (good side) output transistors to right side (bad side) and still not able to adjust bias.
KA-7100 No Bias voltage on Right Channel
sajjad amer
Active Member
Your switch reading is spot on. That's how I've wired it.
Q19,Q21 left channel
Q-22, Q20 right channel
Once again thank you for taking the time to help me. I appreciate it.
Goldie99
Super Member
In that case, for the time being, you should leave the speaker select switch in the 'A' position say, and not change it. You'll need to rearrange the speaker switch wiring at some point - you really don't want the scenario where you can physically turn the switch into a position that shorts both channel +ve outputs to ground...
Is there any chance you were turning the speaker select switch while the amp was playing on both channels ? esp. to the A+B position ?
I'd refrain from swapping components between the 2 channels - it's a quick way to potentially spot a bad component, but it's an equally quick way to potentially destroy a working component, and lose your working reference channel.
Probably a good time to start checking the transistors in the Right channel, working backwards from the drivers, so Qe22, 20, 18,...... etc.
Is there any chance you were turning the speaker select switch while the amp was playing on both channels ? esp. to the A+B position ?
I'd refrain from swapping components between the 2 channels - it's a quick way to potentially spot a bad component, but it's an equally quick way to potentially destroy a working component, and lose your working reference channel.
Probably a good time to start checking the transistors in the Right channel, working backwards from the drivers, so Qe22, 20, 18,...... etc.
Who recommended that you replace 2SC945 with KSC3503DSTU ? - not a recognised substitution.
Last edited:
sajjad amer
Active Member
John you are right, I made a grave error.Who recomended that you replace 2SC945 with KSC3503DSTU ? - not a recognised substitution.
KSC3503DSTU is a substitute for 2SC1451 FROM SA-9900, which I am also repairing side by side. I will now also check SA9900 also for parts substituted across.
I will replace it back. Thank God, I did not blow anything. Having said that Bias non adjustment was an issue I was trying to solve by substituting this part.
sajjad amer
Active Member
I have noticed that if I leave the Right side speakers connected, nothing connected at AUX, volume turned to zero, then I can measure few mV at the CP for right channel Bias. Otherwise the reading remains zero. Does this give you any clue to my issue?
Goldie99
Super Member
It doesn't say anything new to me - the problem you have is that there are various possible faults which might cause the issue(s) you're experiencing (loss of signal, and no bias current).
Without using a scope or signal tracer, all you can realistically do is target the most likely area(s), and work from there - which for me would be the various semiconductors in or associated with the bias circuit, that's why working systematically back from the outputs was suggested.
Without using a scope or signal tracer, all you can realistically do is target the most likely area(s), and work from there - which for me would be the various semiconductors in or associated with the bias circuit, that's why working systematically back from the outputs was suggested.
sajjad amer
Active Member
I am sorry to be a pain but I do not understand what working systemically backwards means.
I do have an oscilloscope what I am not sure is what points specifically to check for signal. Being a medical doctor, I do not have any in depth understanding of different components, only very basic idea of what they are. I bought all these instruments in the hope that I will be able to learn on the job with guidance from experienced people like you on this forum.
Is it possible to get idiot proof instructions e.g. check signal on E, B or C of Q 17 etc. Then I can work it out. I haven't the foggiest otherwise where the signal is going .
Apologies for being "thick"
sajjad amer
Active Member
looking at schematic, picture do I need to trace the signal across Qi 1,3,5 and so on. ?
Goldie99
Super Member
Nobody suggested you were "thick", and I certainly had no intention of implying that.
If you look at the schematic for the amp, on page 19 of the service manual, you'll see the electronic 'map' of the amp - there's one thicker, darker, black line which goes from a Phono input all the way through the amp, to the Left 'A' speaker output. That line represents the audio 'signal path', that an audio signal takes through the various stages of the amplifier, from the input all the way to the speaker output. It's obviously not as simple as that, but it's a useful visual representation - typically you'll see the inputs drawn on the left, and the signal passing to the right as it goes through the amp, so "working back" from your outputs simply means checking the transistors etc. starting at the final output transistors (Qe28 & Qe30 for the problematic R channel), and going backwards, e.g., to the left, so the next 2 would be the drivers (Qe24 & Qe26), and so on, back at least as far as Qe14 where your bias current is actually being adjusted.
The exact sequence you check them in is not critical, but your original description of the problem, e.g., you had sound and it was working, but now it's not, and it has no bias, suggests to me that the problem is likely to be in the main amp stage (so between Qe2 & Qe30), and probably closer to the Qe14 - Qe30 parts). I'm assuming you've checked / replaced the 2SC945 that were changed there before. You said you'd already checked various transistors - how are you checking them ?
Personally, I can't give you the 'idiot proof instructions' (your words) that you're looking for - my skills are not at that level yet. I'm a doctor too, but in chemistry not electronics, although I am (self) studying electronics now I'm basically retired. The various questions people are asking, are all essentially trying to identify additional symptoms, to pin down where the problem is. The questions are not as 'random' as they may seem.
You could certainly 'trace' an audio signal through the amp, all the way from the input to the speakers, e.g., follow the black line when probing... it'd be an interesting exercise in itself, but I don't think it's necessary to go that far back in the circuit for this, not yet anyway.
Another useful approach would be to compare between the channels, e.g., with the amp on (preferably using DBT), no signal input, vol on zero, and no speakers connected (but the switch set to 'A' still) - measure what dc voltage you have at say the Base of Qe23 (neg. meter clip on the amp chassis, and pos. meter clip on the base of Qe23). The schematic shows it as 1.15V, you should find something similar, although it's unlikely to be exactly the same. That value will be basically 'correct' because Qe23 is part of the Left channel that's working fine. Then you just repeat the same measurement for the Base of Qe24, which is the analogous transistor in the Right channel that's not working. Both measurements should be more or less the same, if they're not it potentially helps identify the problem - it's analogous to checking pulses in both ankles... If you do that for the various indicated voltages on the schematic, in the area we're interested in, you may be able to start localizing the problem. If nothing else, it'd be good practice in a useful technique !
One word of caution - any time you're measuring voltages like that, be extremely careful not to short anything with the meter probes or clips, e.g., insulate / tape off any probes so that only the very last 1mm say is accessible.
If you look at the schematic for the amp, on page 19 of the service manual, you'll see the electronic 'map' of the amp - there's one thicker, darker, black line which goes from a Phono input all the way through the amp, to the Left 'A' speaker output. That line represents the audio 'signal path', that an audio signal takes through the various stages of the amplifier, from the input all the way to the speaker output. It's obviously not as simple as that, but it's a useful visual representation - typically you'll see the inputs drawn on the left, and the signal passing to the right as it goes through the amp, so "working back" from your outputs simply means checking the transistors etc. starting at the final output transistors (Qe28 & Qe30 for the problematic R channel), and going backwards, e.g., to the left, so the next 2 would be the drivers (Qe24 & Qe26), and so on, back at least as far as Qe14 where your bias current is actually being adjusted.
The exact sequence you check them in is not critical, but your original description of the problem, e.g., you had sound and it was working, but now it's not, and it has no bias, suggests to me that the problem is likely to be in the main amp stage (so between Qe2 & Qe30), and probably closer to the Qe14 - Qe30 parts). I'm assuming you've checked / replaced the 2SC945 that were changed there before. You said you'd already checked various transistors - how are you checking them ?
Personally, I can't give you the 'idiot proof instructions' (your words) that you're looking for - my skills are not at that level yet. I'm a doctor too, but in chemistry not electronics, although I am (self) studying electronics now I'm basically retired. The various questions people are asking, are all essentially trying to identify additional symptoms, to pin down where the problem is. The questions are not as 'random' as they may seem.
You could certainly 'trace' an audio signal through the amp, all the way from the input to the speakers, e.g., follow the black line when probing... it'd be an interesting exercise in itself, but I don't think it's necessary to go that far back in the circuit for this, not yet anyway.
Another useful approach would be to compare between the channels, e.g., with the amp on (preferably using DBT), no signal input, vol on zero, and no speakers connected (but the switch set to 'A' still) - measure what dc voltage you have at say the Base of Qe23 (neg. meter clip on the amp chassis, and pos. meter clip on the base of Qe23). The schematic shows it as 1.15V, you should find something similar, although it's unlikely to be exactly the same. That value will be basically 'correct' because Qe23 is part of the Left channel that's working fine. Then you just repeat the same measurement for the Base of Qe24, which is the analogous transistor in the Right channel that's not working. Both measurements should be more or less the same, if they're not it potentially helps identify the problem - it's analogous to checking pulses in both ankles... If you do that for the various indicated voltages on the schematic, in the area we're interested in, you may be able to start localizing the problem. If nothing else, it'd be good practice in a useful technique !
One word of caution - any time you're measuring voltages like that, be extremely careful not to short anything with the meter probes or clips, e.g., insulate / tape off any probes so that only the very last 1mm say is accessible.
sajjad amer
Active Member
TBH, I was embarrassed asking such a basic question so used the words "thick and idiot" as a joke for myself. I did not imply sarcasm, apologies for any offence caused.
Thank you so much for detailed instructions. I have looked at the schematic and found the dark black line. I will test all the components as you suggested and will report back.
I am grateful for all the help
Thank you so much for detailed instructions. I have looked at the schematic and found the dark black line. I will test all the components as you suggested and will report back.
I am grateful for all the help
Sajjad, i use a scope and signal generator all the time to check the signal path. set your signal generator to 1K and 2v pp with your scope. connect the signal to the AUX port. turn the volume down, and apply power. select the speaker to A. look at the schematic to see where the signal goes and start looking for issues. signal before the volume pot, turn up volume pot, look for signal at the in of the power amp section, ... continue until you find the issue.
i recently had a bad protection board (signal good into the protection board, but relays not opening).
I never have the volume on full for fear of killing the outputs. You are looking for a clean signal, not max power.
i recently had a bad protection board (signal good into the protection board, but relays not opening).
I never have the volume on full for fear of killing the outputs. You are looking for a clean signal, not max power.
Goldie99
Super Member
Just a random thought Sajjad - I see you changed the bias adjustment potentiometers, it might be worth just double checking that the pot in the R channel is installed properly, and working OK.
The Bourns pots are extremely reliable, but it only takes one bad joint.... Also, if it's a 25-turn pot, it can take a lot of turning if it's nowhere near the setting you need (ask me how I know that one !). I normally measure the old pot as it's removed, and preset the replacement to already be close to what's needed, before I install it.
The Bourns pots are extremely reliable, but it only takes one bad joint.... Also, if it's a 25-turn pot, it can take a lot of turning if it's nowhere near the setting you need (ask me how I know that one !). I normally measure the old pot as it's removed, and preset the replacement to already be close to what's needed, before I install it.