Hi Dear members,
I am currently repairing a stubborn Yamaha M-45 amplifier.
The left channel was blown, so few components down the chain needed to be replaced: output transistors, emitter resistors & few small signal TO-92.
Once this was completed I also replaced all electrolytics with Nichicon UFW series, leaving the original Black Gate capacitors in place.
I have installed multi-turn trimmers for accurate BIAS.
The amplifier turns-on & goes out of protection without hesitation.
BIAS adjustment is working fine in Class A & AB.
The concern I have is the following: (please refer to attached PDF schematic), DC offset at speakers terminal is about 860 mv both channels very close.
Transistor Q109 & Q Q110 (2SC2705) are quickly getting very hot upon powering-up, to give you an idea I measured 75 degrees Celsius after 2 minutes, then I shut off so that I would not risk destroying them. These were replaced with Digikey KSC2383Y to see if there was a difference but unfortunately the problem is the same.
The voltages on the transistors are close to the service manual but not in all transistors.
All transistors have been checked for basic functionality (not leakage).
The differential pair Q101 & Q103 have been closely matched (HFE) and replaced.
The mystery has not been solved & honestly, every time I ask for help all of you have been fantastic and I ended-up resolving the problem.
I am currently repairing a stubborn Yamaha M-45 amplifier.
The left channel was blown, so few components down the chain needed to be replaced: output transistors, emitter resistors & few small signal TO-92.
Once this was completed I also replaced all electrolytics with Nichicon UFW series, leaving the original Black Gate capacitors in place.
I have installed multi-turn trimmers for accurate BIAS.
The amplifier turns-on & goes out of protection without hesitation.
BIAS adjustment is working fine in Class A & AB.
The concern I have is the following: (please refer to attached PDF schematic), DC offset at speakers terminal is about 860 mv both channels very close.
Transistor Q109 & Q Q110 (2SC2705) are quickly getting very hot upon powering-up, to give you an idea I measured 75 degrees Celsius after 2 minutes, then I shut off so that I would not risk destroying them. These were replaced with Digikey KSC2383Y to see if there was a difference but unfortunately the problem is the same.
The voltages on the transistors are close to the service manual but not in all transistors.
All transistors have been checked for basic functionality (not leakage).
The differential pair Q101 & Q103 have been closely matched (HFE) and replaced.
The mystery has not been solved & honestly, every time I ask for help all of you have been fantastic and I ended-up resolving the problem.
If both channels have the same symptom, then the first thing to check is power supply voltages; then common components like R168, C150, R143, C127.
You have to give more info, like voltages on Q101, Q102, Q103, Q104, Q109, Q110, C150, R119, R120, R115, R116,... which transistors do not have voltages like in the service manual and actual voltage values...
If the transistor voltages are ok, but they are too hot, then their current must be too high, or they are oscillating.
I had the same issue (DC on speaker out put) )with Yamaha M4 DC AMP, what I did was removed all the transistors on pre driver+OP one by one and tested. Found just one PNP shorted transistor on pre drive cct is causing the issue,
Anyway I have replaced all the small signal transistors did the repair in 90s the amp still works.😊
PNP small signal transistors are prone to fail and generate 'cracking noise' IMO
Anyway I have replaced all the small signal transistors did the repair in 90s the amp still works.😊
PNP small signal transistors are prone to fail and generate 'cracking noise' IMO
Thanks Floriant, I will check this evening & report. I pulled the Black Gate large caps (22,000 uf) they measured within specs with ESR of 0.9 ohm and 3 ohm. The voltage on the transformer secondary center to side is very close (symmetrical).
Thanks Prasanna, I will go over one more time. Unfortunately I do not have a leakeage tester so the TO-92 can only be checked for basic functionality & HFE value.
With 5750 ohms in the collector on 55 volt rails that transistor will dissipate about half a watt. A small transistor with no heat sink will easily get to 75C. It’s one of those giant TO-92’s, right? Find a way to add some heat sinking. Epoxy a metal fin to it if you have to. They broke the collector load up into parallel units - obviously they were having heat issues with those too. I’ll bet they‘re at least “quite warm”.
Hi wg-sky, that transistor is a TO-92L (0.9 Watt dissipation) original 2SC2705, substituted with Digikey KSC2383 (ONSEMI).
I am having a problem digesting the fact that those transistors are getting too hot too quickly as I was explaining earlier, I bet if I leave the amplifier on for 15 minutes those units will be destroyed. The second issue is the 860 mv DC Offset. So I am still investigating.
I am having a problem digesting the fact that those transistors are getting too hot too quickly as I was explaining earlier, I bet if I leave the amplifier on for 15 minutes those units will be destroyed. The second issue is the 860 mv DC Offset. So I am still investigating.
Do Q111 and R137 test good? That’s the only other thing that could be messing up normal operation in that part of the circuit.
Voltage across R119? I=V/R will give you the collector current. If you get around 10mA, the heating you’re experiencing is normal, because all of the currents will have been accounted for. If somehow you’re getting more, then something is amiss.
One specific factor I did not mention before is that on the LH channel (the one that was originally blown) the output transistors have been replaced with Digikey Sanken 2SA1386 & 2SC3519
The collector voltages taken from speaker negative are the following:
Right channel (original Toshiba devices): Q128-61.9.......Q130-61.9.......Q124-62.....Q126-62
Left channel (Sanken devices): Q127-60.45.....Q129-60.5......Q123-60.3.....Q125-60.3
Sometimes recognizing to stop at the right moment can be a fine line.
After replacing the two hot transistors with my 2SC3423, i go check the DC offset and great it drops down to 30 mV on both channels. Then I check the BIAS, this one seems to be fluctuating all over the place but eventually settles down to 5 mV. Now here comes the fun: the heat sinks are getting nice and warm (40-50 degrees) then another minute .......boom the 100 ohm resistor on the RH Channel (the original one) goes up in flames and after checking kaboom....the four output stage transistors are shorted. This is when it is time to call it a night my friends.
Time to think over and get new transistors, this time they will be all the same on both channels.
After replacing the two hot transistors with my 2SC3423, i go check the DC offset and great it drops down to 30 mV on both channels. Then I check the BIAS, this one seems to be fluctuating all over the place but eventually settles down to 5 mV. Now here comes the fun: the heat sinks are getting nice and warm (40-50 degrees) then another minute .......boom the 100 ohm resistor on the RH Channel (the original one) goes up in flames and after checking kaboom....the four output stage transistors are shorted. This is when it is time to call it a night my friends.
Time to think over and get new transistors, this time they will be all the same on both channels.