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Sansui 7070 Power Amp Question

I recently acquired a non-working Sansui 7070 Receiver. The main symptom is the speaker protection circuit is activated all the time. This appears to be due to the presence of a high DC voltage at the power amp output. Actually the neg. rail voltage (-42v) is present at the output of BOTH channels. I checked the output transistors expecting to find a couple shorted but they all check OK so I'm thinking the problem is up the chain somewhere in the driver stage. It seems odd though that the DC is present on both channels suggesting the same part failed on both channels. Is it reasonable to think that there may be a power supply problem since it is common to both channels?

Any help/advice is greatly appreciated.

Reply to
2Czech
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Check pins 5, 6, 27, and 28 of the driver board, (F-2624). They should be

+44 volts. The source of this voltage is TR01 in the power supply (F-2625).

Good luck.

-- David Farber David Farber's Service Center L.A., CA

Reply to
David Farber

Look for bad 1 ohm resistors in the regulated power supply - the 8080 and

9090 models suffered from this.

Mark Z.

Reply to
Mark D. Zacharias

Hi David, Thanks for your help. I checked this voltage and it appears to be OK reading +43v. The neg. side also reads -43v. (Both occasionally measure around 42v.) Am I correct in my assumption that with the speaker protection relay de-energized the voltage at pins 10,11 and 22,23 of the F-2624 board should be zero and not -42?

Thanks, Joe

David Farber wrote:

Reply to
2Czech

Hello Mark, Thanks for responding to my question. The only 1 ohm resistors I can find associated with the regulated PS are in series with the power transformer output leads and the rectifiers. Are these the ones you are referring to?

Thanks, Joe

Reply to
2Czech

the

be

(F-2625).

Reply to
David Farber

the

be

(F-2625).

Hi Joe,

Yes, you need 0V at pins 10, 11 and pins 23, 23 on the F-2624 board for the relay to energize. The working state of the amplifier is for the relay to be energized. I believe you stated the opposite.

Check the collectors of the NPN output transistors. Do you have about +44 Volts there? The source of this voltage is from the bridge diode in the power supply (F-2625), pins 20 26, 33, and 54. This is not the same source as the regulated +44 V that you already checked.

Good luck.

-- David Farber David Farber's Service Center L.A., CA

Reply to
David Farber

Thanks again for the guidance. Now that I read my initial question I see I should have worded it differently but the jest of it is with the amp. powered on, the relay does not energize. You also confirmed for me that there must be 0v on the output of the F2624 board BEFORE the relay will energize.

I will double check the other +44 source on the output transistors tonight. I'm pretty sure I already did and it was OK.

I really appreciate your help. This receiver is in great shape and according to the previous owner was working a few months ago. It sat unused for several months and when he tried it again it came up in its current state. I just keep thinking I'm missing something obvious.

Thanks again, Joe

Reply to
2Czech

Hi David, Yahooooooo!! - I fixed it! I checked the supply voltage you mentioned (+44 @ output npn transistor collectors) and it was OK. I then followed the regulated +44 that you initially had me check back on to the F-2624 driver board and HELLO the second place I checked (the + side of C07 & C08) and it read Neg. 15.4v instead of +43. As it turns out both R33 & R34 had changed value from 180 ohms to around .5 Meg ohms. I replaced them both and the receiver sings like bird.

Once again, thanks so much for your patience and help.

As it turns out I didn't need luck - you were helping me :) .

Cheers, Joe

Reply to
2Czech

Hi Joe,

I think the lucky part was that the receiver seemed to indicate a problem that was common to both channels. In fact, there were problems in each of the amplifiers. I guess it's not like winning the lottery but your Yahooooooo!! sounded every bit as loud. (-:

-- David Farber David Farber's Service Center L.A., CA

Reply to
David Farber

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