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Voltage regulator SA 7700

nsun

nsun

New Member
Hi!

Im new here, need some help with an SA 706 (SA 7700)
This European version is on 240 V (or 220V)
The protection relay doesn,t engage, and when I measure pin 1 its only 5,3 V...
So I suspect something in the 13 V regulator, or before that.
The output from trafo is 68,7 VAC (TP 12 - TP 13) or half of it (34,2 VAC) compared to ground.

Btw i bridge C-E on Q22 (activated relay) and it engaged and sound came out, so there is no major issue with amplifier circuit I guess.

Output from rectifier bridge is 46,2 (D1 cathode) and -46,2 (D4 anode). A bit low?
I tested zenerdiode D8 in an own circuit (with 220Ohm and 20V) and it took 14V, the spec says 13V.
But in circuit the zener gets only 5,9V?

D5 gets 34 VAC via fuse 500mA, and puts out 17,6 VDC, seems fine?

Help me think here. What can be wrong? R614 is tested 10,1kOhm

Thank for a great forum, so much knowledge!

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Maybe a leaking cap? C614? C604?
You could try lifting one leg of R614 then measure Vdc at lifted leg, expect better than 13Vdc
 
D5 should have better than 40 volts at the cathode. Check the value of C610

Also check the value of the two 75 ohm 2 watt resistors.

And as @mbz mentioned. Check the voltage on both sides of R614. One end should have 50 volts and the other end 13.6

7700ps.jpg
 
I recapped C602, C603, C604-C607, C608-610, C613, C614
Now the PA3004 gets 13V! But doesnt click...
The readings on the PA3004 is as follows:
1 13VDC
2-
3 50mV
4 0,94V
5 0V
6 -1,27V
7 0,53 VAC
8 0,26V
Should pin 6 be -13?
I measured also anod side of D18 (zener) to - 10,8, should be -13,5. Could it be a problem here?

I also read somewhere that you can lift one leg off D17, which i did, but now relay clicking..
 
nsun said:
I recapped C602, C603, C604-C607, C608-610, C613, C614
Now the PA3004 gets 13V! But doesnt click...
The readings on the PA3004 is as follows:
1 13VDC
2-
3 50mV
4 0,94V
5 0V
6 -1,27V
7 0,53 VAC
8 0,26V
Should pin 6 be -13?
I measured also anod side of D18 (zener) to - 10,8, should be -13,5. Could it be a problem here?

I also read somewhere that you can lift one leg off D17, which i did, but now relay clicking..
Click to expand...
Yes pin 6 should be near -13vdc. Check the -13 vdc supply voltage.

7700ps2.jpg
 
If -13.5v load is drawing to much current it will bring it down to the point the zener D18 is not regulating anymore. It could also be that R604 1.1k is out of spec so check it with ohmmeter with power off of course. C13 could be out of spec, not hold up the peak supply voltage or it could be installed backwards which would draw more current too.
 
@rcs16 Did you mean C613?
I changed all electrolytes on the AF Assembly (thanks @xlbee )
Still not working, now its -10.5 V where it should be -13.5 and on pin 6 only -1.2 V... So the current through r412 (750 Ohm) is 12.4 mA. The resistans is measured correct to 750.
So the question is why. the current is so much higher? Should be 0,67 mA? (13.5 V - 13 V (pin6) divided by 750 ohm)
R614 is also ok.
 
nsun said:
@rcs16 Did you mean C613?
I changed all electrolytes on the AF Assembly (thanks @xlbee )
Still not working, now its -10.5 V where it should be -13.5 and on pin 6 only -1.2 V... So the current through r412 (750 Ohm) is 12.4 mA. The resistans is measured correct to 750.
So the question is why. the current is so much higher? Should be 0,67 mA? (13.5 V - 13 V (pin6) divided by 750 ohm)
R614 is also ok.
Click to expand...
The only load on that -13.5 volt supply is the PA3004 chip, it is probably bad. Pull it out and see if the supply comes up. Or if you don't wan to unsolder all the pins just lift one end of R412.
 
Thanks, will try when I get back home..
A thought, if I just bypass the pa3004, by bridging Q22 the worst thing that could happen is what? Damaged speaker?
 
Thanks :-) Just had to ask... Curiosity and shortcuts is something I like in general...

So lifted out the PA3004 and now voila -14,5 V on R412...
Theoretically, could somethging AFTER the PA 3004 be broken/shorted? Like Q21, Q22?
@xlbee Otherwise I need to construct that little IC on my own :-) Thanks for the link
 
I put in a new 2SA720 , was lucky to get the original. Thanks anyway @xlbee . For future reference, where can I find replacement info, is the a data base or anything?

The realay still dont turn on, only -1,2 V on pin 6 and 0,2 on pin 8, so I guess soething is wrong, could it be anything other than the PA3004?
 
Is there someone in EU that sells this DIY PA 3004? The shipping from US is a bit expensive.. Or is there someone that wants to share? (Im in sweden)
 
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