Sansui QRX7001 repair

[SPXER]

I purchased a scrapped QRX6001 for parts, as they are mostly the same as the 7001. There is a 2SC1111 in the power supply, so I used it to replace the shorted one in the output section of my 7001.
I powered it up and did the bias adjust. Hooking up some test speakers, it plays!!

There is a good bit of 60 cycle hum. It is sitting under fluorescent lights, so that may be the source.

 

[par4ken]

I purchased a scrapped QRX6001 for parts, as they are mostly the same as the 7001. There is a 2SC1111 in the power supply, so I used it to replace the shorted one in the output section of my 7001.
I powered it up and did the bias adjust. Hooking up some test speakers, it plays!!

There is a good bit of 60 cycle hum. It is sitting under fluorescent lights, so that may be the source.

It might need new power supply filter capacitors.

 

[SPXER]

I will check it out tomorrow. Thanks for the reminder par4ken.

 

[par4ken]

I will check it out tomorrow. Thanks for the reminder par4ken.

How far gone is the QRX6001? I hate to see vintage stuff scrapped! Maybe you can work on it afterward and get a spare receiver out of it!

I assume that much of it is identical to the QRX7001. Assuming it has plug in boards it would be easy to check out what still works and save those for spares. You could sell some parts on eBay and maybe get your money back. Or just keep everything (like I usually do)!

 

[Owen Smith]

It might need new power supply filter capacitors.

It might need every electrolytic in the entire thing replacing.

 

[SPXER]

How far gone is the QRX6001? I hate to see vintage stuff scrapped! Maybe you can work on it afterward and get a spare receiver out of it!

I assume that much of it is identical to the QRX7001. Assuming it has plug in boards it would be easy to check out what still works and save those for spares. You could sell some parts on eBay and maybe get your money back. Or just keep everything (like I usually do)!

I should have said I bought a box of 6001 parts, not a whole receiver. The seller had already started parting it out. I do have all of the boards, I believe.

 

[SPXER]

It might need every electrolytic in the entire thing replacing.

There are hundreds it looks like. Whew!

 

[Owen Smith]

There are hundreds it looks like. Whew!

Electrolytics don't last 50 years.

 

[SPXER]

It would be nice to have a pre-prepared list. Hint, hint.

 

[par4ken]

It might need every electrolytic in the entire thing replacing.

Not a bad idea to recap the whole unit, however I have a lot of seventies equipment that has not been touched and still works without capacitor replacement! I'm more a believer in if it ain't broke don't fix it. When I do capacitor replacement it is usually to upgrade the quality and thus improve the sound quality. Replacing electrolytic coupling capacitors with a film type where possible.

Electrolytics don't last 50 years.

I remember it being said that the lifetime of electrolytic capacitors is something like 20 years. That might have been true at one time (thinking back to the tube type radio days). Much vintage quad stuff still works fine with the original fifty plus year old caps! I've seen more bad capacitors in late eighties era equipment. Then we had the Chinese made capacitors on the old computer motherboards that only lasted only a couple of years!

There are hundreds it looks like. Whew!

Capacitors like beer are cheaper in larger quantities. I always purchase more than I actually need. With spare boards you might want to do them first. The square shaped Wima polyester (mylar) capacitors are great replacements for electrolytics as they are about the same size. Audio grade electrolytics are available as well.

I recommend the use of a good vacuum desoldering station, it helps to prevent board damage when removing components.

 

[Owen Smith]

Audio grade electrolytics are available as well.

What is it that makes an electrolytic audio grade?

 

[SPXER]

It's not the fluorescent lights that are the source of the hum. (not that I expected it to be, just hoping) I had to run an extension cord as the plug-in is wired to the lights. I never changed it as it provides some safety. The soldering iron goes off when I turn off the lights for instance.

 

[SPXER]

What is the philosophy around upsizing filter caps? Should it be done, or left stock. Those Sansui engineers were pretty smart.

 

[par4ken]

What is it that makes an electrolytic audio grade?

Don't know, check the manufacturers literature about that. Mainly subjective, ratings that are important for other applications may not apply and might even be detrimental to audio. I don't claim that they sound better but use them just because they might. Better dielectrics (non electrolytic) definitely do make a difference. These "audio grade" capacitors are lines available from the major reputable manufacturers. I use Nichicon "Fine Gold". They and other MUSE series capacitors were inexpensive and available from Digikey and Mouser. I don't see them listed anymore but I still have a personal stock of several different values. I suspect that Covid wreaked havoc on the supply lines.

Some UKZ series values are available from Mouser.
https://www.mouser.ca/c/?marcom=196672736
Fine Gold and others are available here.
https://www.audio-high-store.com/nichicon-audio-capacitors/

 

[MidiMagic]

What is the philosophy around upsizing filter caps? Should it be done, or left stock. Those Sansui engineers were pretty smart.

It depends on what the electrolytic is used for.

- Power supply capacitors can be larger, but be careful not to overdraw current from the rectifiers on turn-on.
- Some power supply regulators expect a certain size to work.
- Many capacitors are in filter circuits and must have exact values.
- Changing the values of coupling capacitors might change the overall sound of the set.

Actually i have electrolytic capacitors that have kept going for 50 years. The key is that the equipment is left on continuously.

I have other equipment that was stored for 10 years and electrolytics have failed.

 

[par4ken]

What is the philosophy around upsizing filter caps? Should it be done, or left stock. Those Sansui engineers were pretty smart.

I like to upsize for better filtering. The only thing to worry about is stressing the rectifier diodes at turn on. Just don't go too crazy without using higher current rated diodes. Modern capacitors are smaller than the old ones so you can up the value and the voltage of the replacements.

 

[SPXER]

I like to upsize for better filtering. The only thing to worry about is stressing the rectifier diodes at turn on. Just don't go too crazy without using higher current rated diodes. Modern capacitors are smaller than the old ones so you can up the value and the voltage of the replacements.

Mouser has a 15000uF cap the same dimensions as the original 10K. Is that too large? I can put an inrush current limiter in the unit.

 

[par4ken]

Mouser has a 15000uF cap the same size as the original (10K). Is that too large? I can put an inrush current limiter in the unit.

That should be OK, adding the inrush current limiter wouldn't hurt.

I don't know what they use for rectifiers in your amplifier but in my DIY projects I like to use 25A or even 35A bridges and then I can pour on the capacitance. That is obviously overkill but the rectifiers are unlikely to ever blow even without any inrush current limiting!

 

[SPXER]

I got my Mouser order. The 15K uF caps are a bit of a puzzle, or perhaps I am making it too difficult. They are labeled -NEG, 1POS. I take that to mean what it says, so what are 2, 3, and 4? They do not read common to any other pins. I do not have a meter that will read large caps. The data sheet is of no help.

 

[Owen Smith]

Now that's what I call a capacitor...