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F-2980...too hot transistors

V

vladbond

New Member
Hello everybody!
I'm new here. I am the lucky owner of the Sansui G-7700. I have successfully restored the right channel: replaced the output transistors and 0.33 Ohm resistors. Adjusted the Bias Currents of the left and right channels: 3.3 mV, balancing at the outputs, respectively, +/- 6 mV for both channels (drift a little). Speaker protection works fine. What excites me? Transistors Tr602, Tr603 and Tr604 on the F-2980 board are too hot. I'm not sure if these are factory settings. For example, the temperature on Tr603 is 53-56 degrees. (The middle finger cannot withstand this temperature ... LOL) In this case, the voltages on the board almost correspond to the factory parameters of the VDC: Pin 26: -33.8V, Pin27: + 31.17V, Pin30: + 13.4V, On collector Tr604: + 24V, Pin49: + 46V, Pin50: -48.9V. Can anyone give me advice? Thank you all in advance!
 
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vladbond said:
Hello everybody!
I'm new here. I am the lucky owner of the Sansui G-7700. I have successfully restored the right channel: replaced the output transistors and 0.33 Ohm resistors. Adjusted the Bias Currents of the left and right channels: 3.3 mV, balancing at the outputs, respectively, +/- 6 mV for both channels (drift a little). Speaker protection works fine. What excites me? Transistors Tr602, Tr603 and Tr604 on the F-2980 board are too hot. I'm not sure if these are factory settings. For example, the temperature on Tr603 is 53-56 degrees. (The middle finger cannot withstand this temperature ... LOL) In this case, the voltages on the board almost correspond to the factory parameters of the VDC: Pin 26: -33.8V, Pin27: + 31.17V, Pin30: + 13.4V, On collector Tr604: + 24V, Pin49: + 46V, Pin50: -48.9V. Can anyone give me advice? Thank you all in advance!
Click to expand...
I also interested, please help)
 
Maybe your Bias current setting? This thread would likely be better off in the Sansui Forum.

g7700 - Copy.jpg

Good luck,

Mark K.
 
Mark, thanks for the quick response!
There is a blunder in this instruction from Sansui ... and it has been discussed many times on this forum. For step 3 and 4, the voltage must be within DC 5mV ± 1mV, not 5V! As I indicated my parameters are 3.3 mV.
 
Ok, it did seem unusual and I don't have any experience with this model. You are in the right place now to get excellent help!

Mark K.
 
While it may seem hot, those transistors (2SA939) used in that type of circuit usually do get hot.
But 56C is not bad, as they are rated to run up to 125C
 
vladbond said:
Hello everybody!
I'm new here. I am the lucky owner of the Sansui G-7700. I have successfully restored the right channel: replaced the output transistors and 0.33 Ohm resistors. Adjusted the Bias Currents of the left and right channels: 3.3 mV, balancing at the outputs, respectively, +/- 6 mV for both channels (drift a little). Speaker protection works fine. What excites me? Transistors Tr602, Tr603 and Tr604 on the F-2980 board are too hot. I'm not sure if these are factory settings. For example, the temperature on Tr603 is 53-56 degrees. (The middle finger cannot withstand this temperature ... LOL) In this case, the voltages on the board almost correspond to the factory parameters of the VDC: Pin 26: -33.8V, Pin27: + 31.17V, Pin30: + 13.4V, On collector Tr604: + 24V, Pin49: + 46V, Pin50: -48.9V. Can anyone give me advice? Thank you all in advance!
Click to expand...
They are all Pass Transistors in the regulated power supply, they run completely saturated as pass transistors do and therefore run that hot, this is completely normal.
Those transistors have nothing to do with the Bias settings and their temperature has nothing to do with the bias settings.

The bias setting for the G7700 is 3.3mV.
 
Thank you so much Tom and Kevin! You have confirmed my assumptions ... Yes, you are absolutely correct, my bias currents are 3.3 mV for both channels, as I said earlier. Right now I'm waiting for multi-turn resistors to replace VR01 ÷ VR04 as advised by Kevin in previous articles. Good luck everyone...
 
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If everything is running fine, and you're still worried about the transistor temps, you could always add heatsinks to them. Some clamp on, some bolt on (depending on tranny type).
 
larryderouin said:
If everything is running fine, and you're still worried about the transistor temps, you could always add heatsinks to them. Some clamp on, some bolt on (depending on tranny type).
Click to expand...
Less than 60˚C so not an issue, I've never had heat related issues with the regulated supplies in these amps....
 
Thanks, Larry, for the good advice. And I thought about it too. But, unfortunately, there is actualy no place for this idea to be realized...:(
 
Thanks, Larry, for the good advice. And I thought about it too. But, unfortunately, there is absolutely no place for this idea to be realized.
 
kevzep said:
Less than 60˚C so not an issue, I've never had heat related issues with the regulated supplies in these amps....
Click to expand...
Yea,
The amplifier is working fine now ... I accept your recommendation. I leave it in this state and will observe the temperature from time to time in the future. Thank you!
 
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