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Sansui 7070

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Dusan Bozic

New Member
Hi!
I have a Sansui 7070. I changed allmost all condensators, transistors, 4 Power transistors, Bias and DC is ok.
It works superb. Suddenly it works bad: I have to turn the volume for maximum that it plays. All inputs, FM, works Ok, but only on full volume. The relay turns on normal.
Can somone help?
Regards Dusan.
 
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Make sure you have not inadvertently turned on a tape monitor or the 4 channel / Dolby adaptor. With a monitor or the adaptor turned on you will get some audio (due to bleed through) but it will be very weak. Be sure to turn the volume down before turning the offending switch off, or you will be in for a LOUD surprise (and possibly damage).
 
Check headphone output, if OK check F02 (7A) and F03 (7A) on F-2625, if all good, check ALL PSU voltages.

If you do find a blown fuse don't replace it and power up again before checking for shorted output transistors.
 
Headphone output works, but also only on full volume. Fuses are ok. Voltages on the R diodes is +-48V, on D6 is 26V.
What do you mean PSU voltages?
 
Dusan Bozic said:
What do you mean PSU voltages?
Click to expand...
Refer to the schematic (of F-2625) and check that all the Power Supply Unit (= PSU) voltages are correct.
Dusan Bozic said:
Voltages on the R diodes is +-48V,
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There are 2 types of 'R' diodes in this receiver, the 'R' just means they are reverse polarity compared to the non-R diodes, I assume you mean D01 & D02 the SS-5 and SS-5R diodes for the voltage reading you have given.

What I suspect is that you have lost a power supply voltage somewhere, hence the request to check voltages - which is the very first thing you should do.

Good luck.
 
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There is a muting circuit connected to the Phono 1 / Phono 2 switch. An intermittent connection in that switch can mute EVERYTHING!! with TR603 and TR604. Ask me how I know o_O It drove me nuts until I figured out what was going on. I would clean that switch and try again. Or just switch between phono inputs a few times to see if it is the issue.
 
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Hi. I ment the SS-5 and SS-5R diodes, they are good the voltage is 48V.
I think it must be a lost of power supply voltage somewhere.
The phono 1/2 switch is ok.
 
Dusan Bozic said:
Hi. I ment the SS-5 and SS-5R diodes, they are good the voltage is 48V.
I think it must be a lost of power supply voltage somewhere.
The phono 1/2 switch is ok.
Click to expand...
Check R27/28 on the main board. They should be a 4.7 Ohm fusible resistor and if they fail, you won't have input power to the driver board. It's separate from the main rail voltage for the output stage. The schematic shows them at 1 Ohm, but were later changed to the 1/2w 4.7 Ohm Fusible type resistors. Or check for voltage at pins 28 and 05 at the driver board socket. They are the outermost pins, one on each end and should have +44 (or so) volts. Or alternately if the F-2624 board is in, probe the 180 Ohm resistors (R33/34) for about 43v. Then check pins 15/18 for -53v. If that checks out alright, then the problem is elsewhere and not in the amp section. Also make sure the -20db button is out instead of in.
Also, as @AE Bob mentions, recheck the muting portion of the selector switch. It's operation is explained in the service manual and it will mute all signals with every turn of the selector so there is no "pop" when selecting inputs. If something is wrong in the muting circuit, ALL signals will be muted no matter what. On the F-2620 board, the muting transistors are TR603/604 (2SC1636, rarely go bad) and are turned on by TR602 which is a 2SA726 (known to go noisy/fail) and if it went noisy (or failed) it could be keeping the muting transistors turned on cutting off all signals.
 
Thanks Sky. I am a few days not at home, so I will check it later. But I have measured the voltage on the Relay. On the pin with a brown wire ia 12V. Peraps the relay is too week?
Thanks.
 
Dusan Bozic said:
Thanks Sky. I am a few days not at home, so I will check it later. But I have measured the voltage on the Relay. On the pin with a brown wire ia 12V. Peraps the relay is too week?
Thanks.
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Have you made any progress? If the relay clicks, then it is not the problem. Probably in the muting circuit like @AE Bob suggests. Something is triggering it.
 
Dusan Bozic said:
Hi! I will change the transistor 602. I have one. The relay clicks normally after 4 - 5 seconds.
Thanks Sky. I will be back on friday.
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That board is a real PITA to get out. So while it is out, you might as well plan on doing the rest of the work on it if you haven't already. Caps, transistors, etc and be done with it. And clean that Phono switch good while it is in the open too. That switch alone can cause problems.
 
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