Sansui G-7700 Safely Operate Light Not On

J

jugulator

New Member
First, let me say that this website and community look awesome.

Sansui G-770 receiver purchased in 1979 or so has failed.

Unit was working fine, stored in 1999, but does not work now.

Powers up, but Safely operate light does not turn on - neither red nor green.

Tuner seems to work, locks, display light come on, heatsink gets warm after a while, power level leds flash at powerup. 4 internal fuses check out. No obvious smoke leaks or blown or leaking caps. No low level outputs either (tape mons)

I suspect it is in a protect mode, but....

A simple fix or a "doh" on my part?

All I have to work with are multimeters - no scopes, signal gens, etc.

Thanks for any help.
 
jugulator said:
First, let me say that this website and community look awesome.

Sansui G-770 receiver purchased in 1979 or so has failed.

Unit was working fine, stored in 1999, but does not work now.

Powers up, but Safely operate light does not turn on - neither red nor green.

Tuner seems to work, locks, display light come on, heatsink gets warm after a while, power level leds flash at powerup. 4 internal fuses check out. No obvious smoke leaks or blown or leaking caps. No low level outputs either (tape mons)

I suspect it is in a protect mode, but....

A simple fix or a "doh" on my part?

All I have to work with are multimeters - no scopes, signal gens, etc.

Thanks for any help.
Click to expand...

If it has no blown fuses it might be a simple case of adjusting the DC offset....I would give that a go before going in too deep.....

The light doesnt go red, it just comes on when the amp turns on and stabilizes....

Oh, and WELCOME to the Sansui forum!!
 
kevzep said:
If it has no blown fuses it might be a simple case of adjusting the DC offset....I would give that a go before going in too deep.....

The light doesnt go red, it just comes on when the amp turns on and stabilizes....

Oh, and WELCOME to the Sansui forum!!
Click to expand...

Thanks for the welcome.

To confirm, safely operate light does nothing.

Before I dig in, could you expound a bit on your suggestion above? I will search the forum for the DC offset procedure (I saw it while looking earlier), but I'm curious as to WHY that would be indicated as it was fine before being stored.

Why would having been stored for 14 years affect that? Cap drift? Something else? Or for whatever reason does it just often turn out that DC offset wanders and is a frequent cause of no-worky?

Just curious.
 
i would clean all fuses and fuse holders first .i have seen lots of amps out of storage with corroded fuses
 
petehall347 said:
i would clean all fuses and fuse holders first .i have seen lots of amps out of storage with corroded fuses
Click to expand...

Checked fuses, checked continuity from fuse ends to outside of fuse holder clips with one end of fuse pulled up, and cleaned fuses and holders even though there was no visible corrosion. No change.

Can someone point out test points where I should check for DC Offset and approx values I should see? (I am having trouble finding specifics on AK)
 

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14 years is a hell of a long time for a piece of electronic equipment to be in storage, I wouldn't be at all surprised if some switch and control related functions are intermittant, noisy or inoperative due to oxidisation, from your description of what works, it sounds like you were lucky.

I seem to remember people mentioning a particular resistor feeding the LED which fails open, which may be what you are seeing with this unit.

Regarding DC offset measuring, referring to your pic - I think you need to measure (with multimeter set to DC volts range) from the blue wire to ground, and from the grey wire to ground - OR if the speaker relay is actually operating just measure at the speaker terminals - the ideal voltage will be 0.00v - but don't be surprised if you see up to a few hundred mV at those points.
 
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Speaker relay is not operating - no output at speaker terminals, and no clicks or clunks when power button is pushed.

Assuming the blue and gray wires are the correct points for the DC offset measurements (see annotated photo), I found the following measuring between chassis and the 2 points:

Had to start on on 20V scale at powerup as voltages start out offscale.

Gray: -3.42V dropped to 0 in 30sec
then switched to 200mV scale and saw -24mV increasing to -36 mV by 2 minute mark and seemed stableish. (Note voltages are negative)

Blue: -1.34V changed to +0.03V in 30 sec.
then switched to 200 mV scale and saw 30mV dropping to 27mV by 1 minute mark and seemed relatively stable.

I took the bottom steel base cover off and noted that it looks like the big caps on the power supply might have leaked. It's hard to see from the photo, but the outboard cap (closest to left side of unit) shows more possible leakage.

Would one or both of those caps having failed cause the neagative voltage I noted on the gray wire (left channel?), the lack of "safely operate led" action, and speaker relay inaction?
 

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don't worry about the negative reading it doesn't matter .
its looking like you have a power supply fault to the protection circuit or a fault on the protection circuit .its likely a capacitor near the relay . i don't have the schematic here at the moment .
i am sure someone else will chime in soon .
 
OP here.

Has anybody got some further input on this?

Were the readings I took in the correct locations for the G-7700?

If it is a bad cap are there any suggestions on where to start?

Does anyone have the schematic for the G-7700?

Thanks.
 
Yes its between the test points, 88 and 87 I think they are called, and the blue and gray wires. At least thats the case on the G-6700, and they look to be the same in the service manual. The correct voltage should be 3,3 mV as I understand, though it was 5 mV in the first service manuals?

Edit: dont have the service manual with me now but the test points could have the numbers 89 and 90 to. Dont remember. But its the ones sticking up from the board.
 
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petehall347 said:
check voltages at board pin 09 +44v
Click to expand...

sansui_g7700_board_46_testpoint9.jpg


If test point 9 is the white wire in front of the red wire and between the diode legs at the front of the F-2980 board, then I measured point9 to chassis gnd: 43.7 Vdc at poweron which dropped to 43.0 Vdc after about 10 sec.
 
Jonas_Skaegg said:
Yes its between the test points, 88 and 87 I think they are called, and the blue and gray wires. At least thats the case on the G-6700, and they look to be the same in the service manual. The correct voltage should be 3,3 mV as I understand, though it was 5 mV in the first service manuals?

Edit: dont have the service manual with me now but the test points could have the numbers 89 and 90 to. Dont remember. But its the ones sticking up from the board.
Click to expand...

sansui_g7700_board_45_testpoints.jpg


I measured at the blue and gray wires (points 43 and 45).

Points 89 and 90 are what I assume are probe points (look ideal for holding microclips) Points 88 and 87 are empty holes.
 
Jonas_Skaegg said:
Yes its between the test points, 88 and 87 I think they are called, and the blue and gray wires. At least thats the case on the G-6700, and they look to be the same in the service manual. The correct voltage should be 3,3 mV as I understand, though it was 5 mV in the first service manuals?

Edit: dont have the service manual with me now but the test points could have the numbers 89 and 90 to. Dont remember. But its the ones sticking up from the board.
Click to expand...

I believe you are referring to voltage readings which are associated with setting the bias - the problem being addressed relates to the DC offset - 2 entirely different adjustments.

Carry on gentlemen.
 
Ah, absolutely. My bad.

But maybe its good to check the bias before you look at the offset as it will change the offset. Am I right?
 
Jonas_Skaegg said:
Ah, absolutely. My bad.

But maybe its good to check the bias before you look at the offset as it will change the offset. Am I right?
Click to expand...

Change the offset? possibly a little. You are correct to say check the bias :yes:

Then work on the main problem = speaker relay not closing, which in this case is looking like a protection circuit failure.
 
Hyperion said:
Change the offset? possibly a little. You are correct to say check the bias :yes:

Then work on the main problem = speaker relay not closing, which in this case is looking like a protection circuit failure.
Click to expand...

How do I address the Protection Circuit Failure?
 
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