12 June 2011

magic - Sansui QRX-5500a, Klipsch Forte's and DC offset

Yep another stereo post.
Got to do something constructive while watchin the kids!

Last year I picked up this old receiver on CL.
A 1973-75 Sansui QRX-5500A.
It's a trippy receiver,
actually a quadrophonic system - 4 channels -
it's the primitive version of surround sound.

Sansui QRX-5500a

This is not to say the insides are primitive, quite the opposite.
There is a knob that controls 5 quad decoders,
basically a 2-channel stereo input can be converted to
4 unique channels - front/rear/left/right.
It's really trippy.

I bought this thing cheap on craigslist.
The ad had pictures with working lights.
It was clean and cheap enough to buy it without hooking it up.
Big mistake.
The clean outside hid years of dust on the inside.
It was like a cat bed.
If there's anything I'm allergic to its cats.

I plugged everything in and it did work,
but it was scratchy, not all the speakers worked, a bad meter...
Ah hell.

After a ton of internet research -
AudioKarma.org Home Audio Stereo Discussion Forums - Powered by vBulletin
Found the tools needed -
Compressed air, a spray contact cleaner,
and a spray switch/pot lubricant.

The suggestion was "de-oxit",
but holy chit that stuff was way expensive.
 A little tiny can was over $10 a pop.
At Fry's they had generic cleaners that were less than $5 for a hugey can.

cheap tools of the trade

Yeah this is not the time to be cheap,
but I'm on a 3-kid budget and $11 vs $35, and triple volume, Cmon.

slip cover

I was green enough to not know even how to take off the cover.
It's not the normal, logical, 3-sided top.
The receiver slips inside a 4-sided box.
I became a pro at getting to the insides of this.
Unsuccessfully I cleaned it 3 different times.
The compressed air blew the dust out,
and I carefully sprayed the potentiometers/pots and twisted them a bunch.
There were some blown fuses to replace as well.

Holy chit there was some violent lightning-like sounds coming out some speakers now!
(there is an A/B and A+B speaker knob - 4 speakers or up to 8 speakers!)

The third time I cleaned it,
it was late, I was over it, I sprayed the heck out of everything,
and used a light brush to actually clean the boards.
I didn't know or care if it was the correct thing to do,
but this $40 deal was turning into a questionable project.

semi-stripped quad receiver

After that time I hooked it up and it sounded so nice.
but only out of certain speakers, front A and rear B,
unhealthy sounds from the others,
so I hooked up the 4 little speakers to just use those outputs.

That lasted for maybe a month and then one night it went berserk.
I ended up pulling it and that's when I put the Kenwood into play.

When you waste time on something,
 sometimes it's good to off it.
Take it out to the shed and pop a cap,
in this case I put it on craigslist to flip it.

Well it didn't work out.
I got tons of interest - too much,
it made me realize it was cool enough to save and maybe get professionally fixed,
especially since it was complete.
So it sat on the shelf for over a month...


OK let's do a prequel really quick.
I had found this interesting thread on that audiokarma forum about DC offset.

Amplifier Distortion, DC-Offset, and You! - AudioKarma.org Home Audio Stereo Discussion Forums

If you get into these old amps/receivers,
this is one of the diagnostics that a hack can at least check.

Basically you put the warmed upreceiver on sleep mode, volume low,
and you hook up a multimeter to the speaker + and -.
The reading should be from 0.00 mv to hopefully 50 millivolts - positive or negative.

Early Sansuis were high-ish end production machines,
so they had adjustable DC offset "pots".
Cheaper machines have a resistor or something that is fixed.
The internet is amazing,
someone had gathered all the adjustment procedures for early common Sansui's.

Sansui Bias Current and DC Offset - AudioKarma.org Home Audio Stereo Discussion Forums

My Sansui 9090 had a weird light buzz in it,
and that's why I had researched a potential cause.

sansui 9090 under the knife

After warming it up for a bit,
I checked the left and right speaker outs.
The left side was great, about 6+ millivolts,
the right side, ouch, over 100 millivolts!
I knew something was off with that side.

adjusting DC offset
Sansui 9090

The guy I got this from had a new driver board made up,
and it was clean enough inside to look like it had been fixed up.
So with absolutely no practice on a lower end machine I tried to adjust it.

not bad

I'm definitely not an electronics guy,
but I think the pot was sticky or something, maybe something was lodged in it,
cause I got it down to 3 mv!
After a couple hours of blasting tunes it's not changed too much,
what a relief.

Now that onkyo tx-8500 is powering the shop and sounds bitchin.
The 9090 is the current overpowered garage system!


With this newfound knowledge,
I had to test out the old Sansui quad receiver.
It had gotten so bad that the overload protector would switch it off.

First I cleaned the heck out of it.
The volume and speaker control pots/dials were the most suspect,
as they were always off and on scratchy.
Instead of spraying and turning it 10 or 20 times,
I sprayed the chit out of it and turned it at least 100 times both ways.
Than I sprayed them again while turning it.
After letting it dry for a bit,
I repeated the same procedure.
It was amazing,
the volume control didn't have the resistance it usually had,
I felt there was a possibility.

bench test

I let it dry while looking up the procedure for adjusting a QRX-5500A.
There was none.
There was a procedure for a QRX-7500 so I hoped for the best.

The strange thing was after I plugged it all in.
I played the radio and a CD...

Gotta love Deadbolt.
What a great test song to blast at 11:30pm on a saturday night!

The thing sounded perfect!
I checked the multimeter on all 4 speakers -
3, 6 , 8 and 13 millivolts!
Holy Chit!

more bench test

I played out the CD, waiting for some poof of smoke.
Since it didn't zap the little test speakers,
I hooked up this pair of Klipsch Forte speakers I grabbed a couple weeks ago.
(don't worry, I'll tell the story for later)

klipsch forte + sideways Sansui 

Holy mother of sound.
First of all,
the Sansui only pushes about 30 watts per channel.
Very meager number, yes,
 but it is the older measuring system so it is deceiving.

Second of all,
these 1985 Klipsch Forte's have a sensitivity of 98db @ 1w/1m,
which means they don't need alot of power to translate to sound.

It's 12 o'clock now and I had to replay the deadbolt clown song just to hear the bass.
So crispy and full sounding.

I let it run an FM station overnight to really test it,
and it is still playing and sounds great.
One of the meters is still soft,
but it doesn't seem to affect the sound.
No problem.

1974 Sansui QRX-5500A Quad Receiver

I consider myself lucky that it only took time and some spray cleaner.
My imagination was running wild when you hear about guys resoldering caps and resistors,
or simply getting into a money pit.
Someone would have gotten a steal if I flipped it!

Now I've got to figure out what to do with it.
My wife's gonna freak at another sound system in the house!



  1. Very helpful, thank you for the time you have put into this post.


  2. Hey thanks for posting!
    I wonder sometimes why people are afraid to type!