SANSUI 790 HELP!

[jFF_Bulbo]

Hello!
I recaped a 790 sansui, don't working before recap.
Now relay clicks, DBT show no shortcut, unit sounds Bad and once the volumen reaches 35-40%, distorsion is really Bad and finally relays click and sound go out.
Dc offset is ok, about 3+5 mv, but bias is 45V!!!! On test point 5-6 & 5-7. (Right multímeter)
2sc984 bias transitors checked and ok.

Your help is highly apreciated

Thank You!!!

 

[jFF_Bulbo]

Hello.
In case this could be useful for some one:
Audio inestability and relay going out were caused by a bad pin contact with F2626 board., I resoldered female and male conection and fix the female conector, it had some pieces bent.
New problema: when i turn yo the volumen, lights (LEDS) on the vemeters bright less.

Any idea about how to fix it?
Regards

 

[jFF_Bulbo]

AND
The bias measure is wrong. I ser the VTM on volts, should be miliAmperes.

 

[jFF_Bulbo]

well. unit is working +/- well BUT
FROM TIME TO TIME relay click and sound goes out.
Interesting:
Unit on DBT, playing FM and once removed the termal fuse, the unit is more stable, but from time to time relay goes out but back almost inmediately.
I conected a VTM to check some voltages, and interesting thing is hapening: VTM in Volts DC scale, black to chasis, red to circuit:

when i measure relay coil voltage (unit is sounding), for less than a second VTM shows 24vdc... and then relay clicks, music goes out an voltage raises to 30vdc.... once I remove the probe, relays click again and music sounds.
what it could mean?

 

[CrSens]

The DBT is not to be used for playing music, just as a test are there any short circuits or bigger problems. The lower voltage when on DBT with more load (playing music) will cause unpredictable issues, including protection coming on. Bias should be 3.3 mV, 45 mV will cause overheating, and removing the thermal tracking diodes (they are not thermal fuses!!) is not a good idea. You can quite easily burn outputs on this receiver. Not sure about the relay coil, you are probably not doing the probes correctly.

 

[jFF_Bulbo]

The DBT is not to be used for playing music, just as a test are there any short circuits or bigger problems. The lower voltage when on DBT with more load (playing music) will cause unpredictable issues, including protection coming on. Bias should be 3.3 mV, 45 mV will cause overheating, and removing the thermal tracking diodes (they are not thermal fuses!!) is not a good idea. You can quite easily burn outputs on this receiver. Not sure about the relay coil, you are probably not doing the probes correctly.

problem happens when unit connected to 120v also...
the relays clicks and sound goes out also when unit plugged to 120v.

i think it ould be a ground problem, but i have tested ground line in the circuit and the wires in the unit (the black ones) and nothing is broken.
I noticed the screw with tin in F2625 was loose (3A location on service manual) i thought this ground was the culprit, was tightened an resoldered, but problem persist

i Replaced all the transistors in F2625, all the capacitors and fuse resistors in all the boards. Many many reoldering job hass been done...
DC offset is about 3-5mV both channels.

On F2624, tr05 (2sd315) is running hot, about 35-37°c.
is it normal?
any ideas??
thank you in advance.

 

[CrSens]

What is the thermal fuse you are mentioning, is that two heat tracking diodes for bias?
I am not aware of any thermal fuses in that receiver.
37 Celsius is not very hot for transistor at all.
Dimming LED lights when turning volume up indicate problem with bias, not enough bias because there are no heat sensing diodes to increase it when playing louder. Or in general you have a problem with bias circuit or solder points between the drivers and outputs. Are output transistors original or changed, and if changed, with what? Fake NOS output transistors from ebay could cause problems like you have.

 

[jFF_Bulbo]

Changed D05&D06 an f2625. (10d1 diodes, uf4004 installed) problem persist, unit is pluged to 120v.
i timed the faliure for the first time, relay goes out ofter 15 sec of unit working...

On F2624 i changed:
TR01-04--- 2SC1708 with ksc1845 matched
tr05&06 -- 2SA899 with ksa1220
tr15&16 --2sc945 with ksc1845
tr17&18 -- 2sa733 with ksa992
new bias and dc offset adjustment pots.

2SC984 bias transitors tested ok, both.

problem with relay has been present before and after the transistor change.

as i write this post, the unit is playing without problemes on relay for 9 minuts sfter a faliure with 15 secs playing:
unit on, relay click, sound for 15 secs, relay goes out, relay come back and music has beeb sounding for 12 minuts without problems.....)

it is making me crazy.

 

[CrSens]

So no more distortion when playing louder and dimming LED display?
You did not answer about heat tracking diodes.

 

[jFF_Bulbo]

What is the thermal fuse you are mentioning, is that two heat tracking diodes for bias?
I am not aware of any thermal fuses in that receiver.
37 Celsius is not very hot for transistor at all.
Dimming LED lights when turning volume up indicate problem with bias, not enough bias because there are no heat sensing diodes to increase it when playing louder. Or in general you have a problem with bias circuit or solder points between the drivers and outputs. Are output transistors original or changed, and if changed, with what? Fake NOS output transistors from ebay could cause problems like you have.

termal fuse, sr. it is marked on SM as "S705" on terminal 62&63 in f2625 board. unit has the same problems i describe with and without this termal fuse.

*********yes. i changed the output transistors with mj15023&24 (generic, not fakes) i forgot to mention.
could seems the transistors are the culprit, but i´ve used them in many units. not suspicious for me

BUT i´ll try to install the old ones. I changed them because one of them has rust on the metal case, hope the rust doesn´t affect the unit.

relay goes out again after 19 & 24 minuts.

 

[jFF_Bulbo]

So no more distortion when playing louder and dimming LED display?
You did not answer about heat tracking diodes.

i don´t undestard well about tracking diode, i´m not tracking diodes, i only mentioned the tr05 on f2624 is a little hot, and the fuse resistor S705, problem persist with and without it.

as you recomended me, unit sounds only plugged to 120v, on FM (no left chanel in stereo , but both sounding in mono), and i has been linsten it for about 30 minuts, with 4 relay failures. no distortion listened when volume is high.

regards

 

[CrSens]

Maybe the problem is in the protection circuit then, false tripping. Relay tripping when you measure the relay coils would also point to that. Possibly some intermittent solder joint around the relay. Check carefully with the magnifier and light for tiny cracks.

 

[jFF_Bulbo]

Maybe the problem is in the protection circuit then, false tripping. Relay tripping when you measure the relay coils would also point to that. Possibly some intermittent solder joint around the relay. Check carefully with the magnifier and light for tiny cracks.

Hello. I resoldered relay and some other pieces, no more problema with relay going out.
BUT the unit sounds Bad and when i turn yo teh volumen to 50% the relay goes out. Sound horrible just before....
Bias is in 0.97 mA no matter the adjustmet i do o VR203 and 204....

 

[CrSens]

Check that you oriented the new VR 203 and 204 trimmers correctly (center pin).