Sansui QRX-5500A - Troubleshooting audio out

[Trompi63]

I just got a QRX-5500A a few weeks back and it was a little dirty, so I cleaned up very well, recapped any cap that looked suspicious, including the main power and protection circuit. Then tested and everything worked perfectly fine. After a couple of days the volume control started with some static when moving to the right or left. So I open the case again and sprayed some deoxit 5 to clean then some deoxit fader. When power up the audio came clear for a few minutes and completely cut off after that. I tried everything on my toolbox already and nothing seems to work. I tried to measure the potentiometer resistance and measured 240 kOhms , then I measured between ground and wiper and I can see the resistance going from around 0 ohms to 43 kOhms on every wipe( there are three on this control) however every one of them at some point showed OL and restored to a right value right away. So my best guess is that this control is bad or needs more cleaning. I disconnected from the board but can’t find a way to remove the shaft I found a very small ring but after removing still can’t disassemble the control… so any tip in how to fix this? Or how to open this potentiometer? I repaired a few not long ago and it was easy to find like a o-ring in the shaft that once removed allowed me to take everything out to clean. Or maybe there is a way to find a compatible dual gang x 4 plus loudness somewhere out there.

 

[swechsler]

I don't think many of us are familiar with the internals of that receiver, but if you post pics of the control and what you've managed do so far, we might be able to advise you on how to proceed from there.

BTW, this would probably be more appropriate in the "DIY" or "Sansui" section rather than "Members' Systems". You can click on "Report" and ask a moderator to recommend which one, and move it there.

 

[Trompi63]

Here some pictures before and after recap. by the way I followed some advice and started looking into power supply instead of volume control, so I found out that the voltage on test point 18 was only 5VDC, the service manual calls for 25 VDC , after replacing some caps I got power up to 11.6 VDC , so I feel like I am on the right track in here, now thinking that the problem is a transistor out on the supply side

 

[Trompi63]

Here the schematic for the test point

 

[Trompi63]

So far I upgraded to LED lights, recap all the audio path with good quality caps and lubricated all switches and pots. Tested all voltage, including offset, adjusted to 0 Volts. All good until I hear some scratchy noise when lowering the volume and then quit the sound completely. I thought it was the volume control, now I believe it’s the power supply board.. so working on that direction after wasting too much time cleaning and reconditioning the volume control.

 

[Richard P]

Take a look at the speaker protection relay . . . .

 

[Trompi63]

Will the protection relay limit my voltage to 11 V from expected 25v?

 

[swechsler]

Take a look at the speaker protection relay . . . .

Even if the contacts are pitted or corroded, that wouldn't explain the inconsistent readings the OP got directly from the volume pot.

 

[Richard P]

Will the protection relay limit my voltage to 11 V from expected 25v?

No. Power supply issue is separate from your original problem of clear audio fading out, although they could be related if that supply is affecting the amplifier circuit. I don't see where the 25V supplies are used on the schematic, though. Points 15 and 16 come from the F1483 board but I can't find them elsewhere . . . let me know if you find them and I'll look further.

 

[Trompi63]

within this service manual is really hard to track voltages throughout the circuit, all I have found is that the Vario-Matrix circuit (F-2048) takes the 25 VDC from the pin 16 of the power supply. This is just a guess as I found the pin 16 reference in here, now how critical is this, I do not know nor even if this statement is correct.

 

[Trompi63]

Also read somewhere that the preamp section is powered from the same 25 VDC source, I am trying to find the related input within the schematic.

 

[Bruno Primas]

You didn't say anything about transistors in your work. Just from the one picture, this unit has 2SC1364s. These are known to go bad and can be replaced with KSC1815s. I had a 771 that had all sorts of issues because of these in the tone board.

There might be other known bad-aging transistors in this unit as well in the different circuits.

 

[kwayde]

within this service manual is really hard to track voltages throughout the circuit, all I have found is that the Vario-Matrix circuit (F-2048) takes the 25 VDC from the pin 16 of the power supply. This is just a guess as I found the pin 16 reference in here, now how critical is this, I do not know nor even if this statement is correct.
View attachment 3685775

That square box you have circled coincides with the F-1488 board above it on the schematic.

Any 25v coming off the power supply will be circled like post #9. (15 & 16)

 

[kwayde]

15 & 16 are here in the upper right corner.

 

[Richard P]

What do the rest of the voltages look like on the F-1483 board? If you are only getting 11V at pin 18 there is definitely something wrong on that board . . . Low voltages will affect many parts of the audio section.

 

[Trompi63]

i m measuring all those now and will post the results
here the results, definitely i am way off the specs.

 

[Trompi63]

I just ordered the transistors to replace TR01, TR02 and TR05 as all of them are either 2SC1384 or 2SC1313 which are basically the same, so I thought is better to replace all at once. I will come back when receive the parts with updates.

 

[Richard P]

Did you test TR03 TR04 and TR05? Results? ZD01 appears to be a 12V, 500mW Zener diode that may be causing your low power supply voltages if it's defective (shorted, open, leaking). TR04 and TR05 could also produce the low voltages you see if they are defective. The 2SD313s have a tendency to fail when used as pass transistors in power supplies for other receivers. I don't have any history on this model, though.

 

[Trompi63]

I tested and appeared to be ok, they were 0.6 V between Emitter and Collector, but the voltage is not correct on that circuit, the base is 44 V which is telling me that the transistor even tested ok but not doing its job. The Zener tested ok reading 0V only in one way. So my plan is to replace those TRs and a couple of caps I detected were over 15 % off one of them is a 100uf coming on the supply line it was reading 120 uf.

 

[Trompi63]

Ok, updates! Received the parts earlier and put couple of hours on it today. Audio is back! Power up to 29.3 VDC used trimmer resistor and backed down to specs 25.0 VDC. As expected TR 05 was bad. I also replaced TR 01 and TR 02 as I had the parts and they tend to fail. Left the others for now. Fully recapped the whole power supply. After all I decided to keep recapping the pre amp board, also bypassed the 6 volt supply into the radio lights board, so now I have all lights on regardless the function activated. Only one that shall stay off is the FM signal meter as bypassing this one will constantly lit the FM signal, so I prefer to have every function independent as factory defaults. Below a few pictures of the entire job. I love the sound, so I am keeping this one because sound is really warm and clear plus phono preamp works great.
Thank you all for the great insides and suggestions and help on rescuing this beauty !