Technics SU-V7 bias problem

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triplexy

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Joined 2018
#1
Hello,

Had this one in my office for a while. I did a full recapping job, calibrated most of the things, per service manual, then ran into a bias problem and left it there.
6 months after picking it up where I stopped.

What I did:
  1. all electrolytic capacitors changed​
  2. both Thyristor replaced (were damaged)​
  3. Supply voltage adjustment & check done and working
  4. Adjustment of load impedance detection circuit done and working
  5. DC balance calibrated and working
  6. Protection circuit calibrated and working
  7. Overload detection circuit calibrated and working
  8. replaced output relay
  9. cleaned controls and potentiometers

The problem, adjustment of Ico (bias).
The right channel is nicely biased @ 2mV, as the manual suggests. Everything works smoothly with it.
The left channel, oh boy oh boy... Looking at the schematic, my Q317 collector measures 0.5V, Q319 collector measures -3.3V, with respect to 0V. Everything after that is not even close to the values in service manual...
My thought, I have one channel working, and I don't have these transistors in stock, let's change transistors and diodes from one channel to the other and see.
I exchanged (left channel with right channel): Q317, Q319, D305, D307, D311, D309, Q321, Q331, Q333, Q327, Q329. Of course, I didn't replace everything just like that.
I check the bias adjustment after each exchange. The thing is the same... I also checked resistors, all good. I resoldered most of the left channel. Still the same. I measured all ceramic capacitors on the left channel, all good.

The amplifier works, it passed the signal, but after few watts of power, as expected, the left channel waveform (the bottom sine wave) starts to distort.

Any ideas? Any help will be appreciated. Maybe I'm missing something, or everything 😀

Ciao,
John

EDIT: VR301 is also replaced with new trimmer, Z301 power resistor has been replaced also. Schematic is attached, so you don't have to search through manual 😉 The voltages before bias circuit (Q301, Q303, Q309, Q313, Q315, Q311) are correct.
 

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#4
Hi,
Have had a similar issue with SU-V7x and finally found an open resistor next to the output transistor of the faulty channel. If I recall correctly it was between collector and rail of the output transisor in the faulty channel. Note this resistor was not in the schematics.
 
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#5
triplexy said:
Hello,

Had this one in my office for a while. I did a full recapping job, calibrated most of the things, per service manual, then ran into a bias problem and left it there.
6 months after picking it up where I stopped.

What I did:
  1. all electrolytic capacitors changed​
  2. both Thyristor replaced (were damaged)​
  3. Supply voltage adjustment & check done and working
  4. Adjustment of load impedance detection circuit done and working
  5. DC balance calibrated and working
  6. Protection circuit calibrated and working
  7. Overload detection circuit calibrated and working
  8. replaced output relay
  9. cleaned controls and potentiometers

The problem, adjustment of Ico (bias).
The right channel is nicely biased @ 2mV, as the manual suggests. Everything works smoothly with it.
The left channel, oh boy oh boy... Looking at the schematic, my Q317 collector measures 0.5V, Q319 collector measures -3.3V, with respect to 0V. Everything after that is not even close to the values in service manual acft calculator...
My thought, I have one channel working, and I don't have these transistors in stock, let's change transistors and diodes from one channel to the other and see.
I exchanged (left channel with right channel): Q317, Q319, D305, D307, D311, D309, Q321, Q331, Q333, Q327, Q329. Of course, I didn't replace everything just like that.
I check the bias adjustment after each exchange. The thing is the same... I also checked resistors, all good. I resoldered most of the left channel. Still the same. I measured all ceramic capacitors on the left channel, all good.

The amplifier works, it passed the signal, but after few watts of power, as expected, the left channel waveform (the bottom sine wave) starts to distort.

Any ideas? Any help will be appreciated. Maybe I'm missing something, or everything 😀

Ciao,
John

EDIT: VR301 is also replaced with new trimmer, Z301 power resistor has been replaced also. Schematic is attached, so you don't have to search through manual 😉 The voltages before bias circuit (Q301, Q303, Q309, Q313, Q315, Q311) are correct.
Click to expand...
The issue could be with the biasing circuit or something related to the power supply or output stage. Given that you've swapped the major components, check for any possible incorrect resistor values, unstable power supply voltages, or heat-induced problems. Ensure the voltages before the bias circuit match the right channel and compare behavior carefully. Also, recheck the output stage components for any issues, as the distortion happens after power is applied. 1/2
 
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#6
Yes, collector voltages at Q317 and Q319 aren't correct--- spread is too small, and not centered re the output.

Assuming supply voltages look OK, would you report voltages re ground at the following points? Set VR301 for max resistance (CCW) and no load at amp output.

Base Q323
Emitter Q323
Base Q327
Base Q331
Output (TP301)
Base Q333
Base Q329
Emitter Q325
Base Q325
Base Q321

IC301, pin 7

Thanks.
 
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#7
Hello,

Thank you all for the great ideas. I will get to it in a few days, I'm currently out of office in the field job...

I'll check one more time, all components around that channel, and do more tests. Sometimes you really need a slight push from you lovely people!
Cheers
 
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