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Sansui 7070 Left and Right Output Issue

coastman49

May 1, 2022
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I have a Sansui 7070 receiver that I originally purchased in 1976. Used it for about 10 years before putting it away in leu of a AV receiver. I recently brought it out of storage. It still works, but the right channel has low output. I replaced caps on the F2624 driver circuit board. When I attempt the driver board adjustments according to the service manual, Voltage a the speaker terminals with an 8 ohm load, I get .017 volts or 17 mv both sides. Service manual says + or - 10 mv. I can't adjust any farther.
When I do the current bias or output idle current, both channels are very high. On my Fluke 73 Series meter the reading is 24.45 on the right channel and O.L on the left channel I don't have a milliamp input, so the reading is 24.24 . I am assuming my meter auto scales the current readings The unit still works except right channel has low output.
 

Martaine2005

May 12, 2015
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Welcome @coastman49 ,
You seem to know what you are doing.
17mV is not excessive but may show signs of oxidation on the trim pots. Were they extremely sensitive to adjust?. Try cleaning them.

I personally would have brought the mains voltage up slowly over 20 minutes or so as it’s an old unit (with a bulb current limiter). Not essential of course.
Then, with different channel volumes observed, cleaned the speaker switch, balance and volume pots to start with.
You re-capped the driver board and nothing else?.
Check the power supply caps too.

Martin
 

coastman49

May 1, 2022
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I thought I knew what I was doing too until I resoldered some connections and trimmed some that look like they were touching on the power supply board. Emitter and collector solder joints for TR01, TR02 were touching and TR03 was close but not touching. Now I got no relay kicking on. Could I have messed up on the current bias or those solder connections? Where do I go back and start?
 

coastman49

May 1, 2022
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To measure the rail voltages, I have to measure pins 5, 6, 27, and 28 of the driver board, (F-2624). Its difficult to measure these with the board installed. I am trying to figure out where else to measure the +44 v and the -44 v. Kind of disappointed in myself and seems like a whole day wasted working on this thing. The Sansui schematic in the service manual I got from HiFi Engine is not very good. I will try to make these measurements later today or tomorrow and let you know. Thanks. I really appreciate you reaching out to help me.
 

coastman49

May 1, 2022
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I checked the supply voltage (+44.8 @ output NPN transistor collectors) and -44.8 at PNP collectors output transistors.. They both were ok. I then followed the +44 back on to the F-2624
driver board . I then checked (the + side of C07 &
C08) and it read Neg. 15.2v on C07 instead of +44. C08 read 37.7 vdc
R33 was 204.6 ohms & R34 was 50.4 kohms . Both should be 180 ohms. Looks like I need to replace these transistors. Can you see anything else?
 

Martaine2005

May 12, 2015
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You are telling us about ‘C07 & R33 etc.
We don’t have the schematic or board in front of us.
If you can, lift a leg of the resistor out of circuit to measure it. The measurements can be influenced by other components. Are C07 and C08 in series?.
Did you clean the pots and switches before pulling it a part?.

Martin
 

coastman49

May 1, 2022
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Replaced R33 and R34. Both 180 ohms. Rechecked the operation with the new resistors. Comes out of protect mode and speaker relays kick in. Both channels work great. I just need to find new pots so I can set the proper current bias for the output transistors. Thanks for your help.
 

coastman49

May 1, 2022
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Tried readjusting the DC current bias per the service manual. Still could not get 30 mv. 24.6 mv on both channels. ).0 volts at the speaker inputs on both channels were dead on. Should I worry about the 24.6 mv bias instead of the 30 mv? If yes, then I should try to find the new 4.7 k ohm and 1 k ohm variable pots, hopefully?
 

Martaine2005

May 12, 2015
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I personally wouldn’t worry too much. The 30mV is probably the maximum. You are well below that with 24mV.


Martin
 

coastman49

May 1, 2022
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Ok. Thanks for your help. Now both channel are outputting the same. Before the left channel was always low compared to the right. Sounds like a vintage 70's receiver should sound. Thanks again.
 

coastman49

May 1, 2022
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The original issue was the output on the right channel was always low. The watt meter movement was always less on the right channel. If you isolated the right you could tell. When I first brought the unit out of storage, I replaced caps on the output board. Still had the problem until I replaced the R33 in the right channel. I checked the idle current in both channels and both at 24 6 milliamperes. Turned the unit on through the load bulb. Bulb dimmed as it should. No shorts. Then I plugged it to full power. Both channels have equal output. I think there is still an issue in the right channel. Sometimes until the unit warms up the right power meter has spurious movements. But the protect circuit never kicks in. Once warmed up every thing is fine.
 
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