Sansui QRX7001 repair

[SPXER]

Acquired a Sansui QRX7001. I could use some help in getting it working. The online seller stated one channel in front and one in rear did not work, all other good. Opening the unit I found 4 blown fuses. Fuses 2, 4, 5, and 7 on board 2427. I have NOT plugged the unit in. I took some resistance readings at the fuse holders to see if anything looked suspicious. A couple readings seem off compared to the others. A pic of the readings is below. The outgoing side of fuses 2 and 4 are lower than their equivalent circuits. All readings are referenced to chassis ground, and taken after caps are discharged and after recharging from the meter. The C and E of the numbered points refer to the collector and emitter side of the output transistors. Where do I go from here??

 

[par4ken]

I will never understand why people sell equipment such as this without wiping it down first! The first thing that I would do is wipe down the case to remove that cigarette tar! I have a friend who gives me his stuff for repair once in awhile I like to bug him about how his lungs must look! All of his equipment and the walls of his apartment are covered in that sticky mess!

It is a good idea to first power up such equipment slowly via a variac. That being said I like to live dangerously and would simply plug it in to see what happens. You have already opened the unit and found the blown fuses. If you are very lucky simply replacing the fuses will bring it back to life. Before doing that I would look over the boards to see if there are any burnt, charred components. Check the manual to see what those blown fuses power. Most likely they feed power the dead amplifier channels. If you see anything obvious there is no point in trying new fuses, you will have to investigate the blown/burnt board first. Your differing resistance readings would tend to point suspicion at the affected boards.

It should be safe to power it up without replacing the fuses to verify that the other channels do work.

Almost a given that the controls and switches will all need a good cleaning with contact cleaner or DeOxit!

 

[SPXER]

Yes, I intend to look further into the board with the differing resistance readings.
As to the cigarette tar, my concern is cleaning it without removing the labeling underneath. I have seen that happen before.

 

[par4ken]

Yes, I intend to look further into the board with the differing resistance readings.
As to the cigarette tar, my concern is cleaning it without removing the labeling underneath. I have seen that happen before.

I usually just use Windex and have never had a problem with the labeling coming off.

 

[SPXER]

A quick reading through the 2436 board showed no anomalies, so I checked the output transistors. TR702 reads dead short. It seems the 2sc1111 transistor is no longer available. Does that mean I should change them all for parity? What is the preferred sub?

 

[DuncanS]

A quick reading through the 2436 board showed no anomalies, so I checked the output transistors. TR702 reads dead short. It seems the 2sc1111 transistor is no longer available. Does that mean I should change them all for parity? What is the preferred sub?

From TABLE OF 2SC SERIES TRANSISTOR EQUIVALENTS 2SC1211 ⇒ BC337A BC637 BC639

It depends, there is the old adage "if it ain't broke don't fix it".

 

[par4ken]

The output is Quasi-complimentary so is not critical that the output pair be matched. I would replace both outputs even if only one is bad. Check the driver transistors as well, they often blow along with the outputs. I tend to agree with DuncanS and would not change the transistors in the working channels. I would however purchase enough transistors to replace them all just in case.

From experience I would suggest setting the bias low at first, only do the final adjustment after the unit has had time to stabilize thermally.
I've had cases where I've set the bias and found latter that it shot up high as the unit warmed up (thermal runaway). Sometimes burning itself out again in the process! The other pot is adjusted to minimise any offset voltage. I don't see in the manual what the bias should be set to. You might need to check the working channels to determine that if it's not shown in the manual.

 

[SPXER]

Sounds like good advice par4ken. I will check the driver board transistors. Thanks.

Edit- I see no problems with the driver transistors. It looks like the fuses did their job. Hopefully a pair of output transistors will put the unit in working order.

 

[mandrix]

My experience is more with cleaning fairly modern AVR's, and video cards. I hose them down with WD-40 (relatively cheap, and if there is some particle(s) causing a short, could wash it off. Then I use non-oily contact cleaner and give it a good spraying down. Both times over a cardboard box with some newspaper lining (outdoors preferably). The WD-40 won't affect anything in my experience (maybe labels?) and the contact cleaner is just to do a more thorough job and dries relatively quickly.
I've brought a few vid cards back to life this way, assuming there must have been a short caused by some type of debris, since a pc case environment may have a lot of air circulating, thus more debris.
The AVR's...well never killed one yet this way.
YMMV.

 

[SPXER]

I used windex and and got most of the tar off, The imbedded stain would not budge. So I resorted to a magic eraser. That did the trick. Took a lot of rubbing. The face is looking pretty good.
Now I cannot see how to get to the dial back lights. I have removed the two screws and the plastic holder is loose, but there is no room to get it out.

 

[mandrix]

My experience is more with cleaning fairly modern AVR's, and video cards. I hose them down with WD-40 (relatively cheap, and if there is some particle(s) causing a short, could wash it off. Then I use non-oily contact cleaner and give it a good spraying down. Both times over a cardboard box with some newspaper lining (outdoors preferably). The WD-40 won't affect anything in my experience (maybe labels?) and the contact cleaner is just to do a more thorough job and dries relatively quickly.
I've brought a few vid cards back to life this way, assuming there must have been a short caused by some type of debris, since a pc case environment may have a lot of air circulating, thus more debris.
The AVR's...well never killed one yet this way.
YMMV.

Wanted to clarify I would not spray plastic faceplates, etc with contact cleaner.
Good on @SPXER for coming up with the magic eraser. Be aware those things are abrasive though. (took the clearcoat right off my old truck hood, lol)

 

[par4ken]

Wanted to clarify I would not spray plastic faceplates, etc with contact cleaner.
Good on @SPXER for coming up with the magic eraser. Be aware those things are abrasive though. (took the clearcoat right off my old truck hood, lol)

Yes, I've never had a problem using Windex but I would take great care with Magic Eraser, as you say it is abrasive. Glad to hear that it worked out for SPXER.

I used windex and and got most of the tar off, The imbedded stain would not budge. So I resorted to a magic eraser. That did the trick. Took a lot of rubbing. The face is looking pretty good.
Now I cannot see how to get to the dial back lights. I have removed the two screws and the plastic holder is loose, but there is no room to get it out.

It might still need more disassembly, check the manual for guidance. What type of lights does it use? Some have those tubes that look like fuses. You might be able to replace them with LEDS at the expense of authenticity. Some like the modern look while others would balk at the idea!

 

[mandrix]

Yes, I've never had a problem using Windex but I would take great care with Magic Eraser, as you say it is abrasive. Glad to hear that it worked out for SPXER.

It might still need more disassembly, check the manual for guidance. What type of lights does it use? Some have those tubes that look like fuses. You might be able to replace them with LEDS at the expense of authenticity. Some like the modern look while others would balk at the idea!

Blue LED's! Love the way they look on audio equipment.

 

[SPXER]

I managed to get the lamps out. I used forceps to reach in the small slit that was exposed. Manual was no help. It showed the exploded view, but did not tell how to "explode" it. Yes, glass fuse type. 7v 0.3a

 

[SPXER]

The face is clean with just a slight yellowish color remaining. I don't want to overdo it. Meanwhile waiting on parts.

 

[SPXER]

I ordered a set of 10 Motorola MJ15001 to replace the shorted 2SC1111 trans. Checking the Mj15001's they read as PNP, not NPN as the 2SC1111,s. All the literature on the MJ15001's show them to be NPN. Is there an explanation for this? Mislabeled?
I am disappointed for sure, as I can't put the QRX back together.

 

[mandrix]

https://www.mouser.com/datasheet/2/308/1/MJ15001_D-2316035.pdf
Sure you didn't order the MJ15002? Those show as PNP. I did not cross reference any specs with the 25SC1111.

 

[SPXER]

They are labeled MJ15001

 

[mandrix]

They are labeled MJ15001

Wow. Wonder if they were mislabeled? How did you determine they are PNP?

 

[DuncanS]

https://www.mouser.com/datasheet/2/308/1/MJ15001_D-2316035.pdf
Sure you didn't order the MJ15002? Those show as PNP. I did not cross reference any specs with the 25SC1111.

I looked on Digi-Key and they're definitely NPN