Photo tutorial: how to adjust the bias on a Technics SA-600

[Gnesen]

I recently posted about a nice Technics SA-600 receiver that I picked up and that I had adjusted the bias because it was a bit high. A couple of people reached out asking how to adjust the bias, so this post is about how I went about it.

I started tinkering on amps and receivers as a total novice about 5 years ago and greatly appreciated all the knowledge and help on this board. To be honest, it was all very daunting and I imagine others may feel the same. Here's a minor attempt at paying it forward. Those more knowledgeable are welcome to critique or add to what I have posted below.

DISCLAIMERS: (1) I am not a trained technician, but I've done a lot of reading and have recapped and serviced quite a few units. (2) Be careful. An accidental slip with a probe or screwdriver can and will mess up either you and/or your receiver.

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Items needed:

1) Service manual (download available at HiFi Engine). This is the most important page for setting the bias, but the full manual also tells the best way to open up the unit, etc.



2) Digital multimeter with probes (not clips), set to measure DC. See note in photo about use of clips vs. probes.





3) Flat head screwdriver.



The key points on the receiver -- where to measure and where to adjust.



The arrows on the photo below are an example for DMM probe location for the right channel.



The locations of VR601 (right channel) and VR602 (left channel) to adjust the bias voltage.



The process:
1) Start with the unit turned off. The input should be set to AUX, volume to minimum, tone controls flat, loudness disengaged, no speakers attached.
2) Insert the DMM probes in the locations indicated above and in the photos below. NOTE: my probes didn't reach too far down along the sides of the resistors, but be careful, because it is possible for the probe tips to touch the output transistors located below.
3) Power on the receiver and DMM (either order is fine). Wait at least 5 minutes for everything to stabilize.
4) Measure and adjust the DC voltage to 8 mV by adjusting VR601 for the right channel (you could do the left channel first if you want, doesn't matter).
5) Power down the receiver, move the probes to the resistor on the top of the heat sink of the other channel and repeat steps 3 and 4.



Another look a little zoomed out...



The follow-up:
After adjusting the bias, I checked the DC offset at the speaker terminals. It should read less than ~10mV for each channel. See this massive thread for all about measuring DC offset: http://audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/

Lastly, here she is unopened. I happen to think she's quite pretty.

 

[Gnesen]

Regarding the measurement of DC offset, this era of Technics used twist locking speaker terminals and my probes weren't long enough to make contact. I just inserted speaker wire into the terminals and switched to clips on the bare end. Just be sure to set everything up before turning on the receiver so as to prevent a short.

 

[Harvestor]

Thank you again for your help and time!
Very easy process...
I'll be reporting my findings tomorrow evening...

 

[Goldie99]

"In this case gravity is your friend, and will hold the probes in place" - I don't know this particular amp, but when measuring bias, this is potentially a recipe for disaster.
I would strongly suggest to use mini-grabber leads.... not gravity.

 

[Gnesen]

"In this case gravity is your friend, and will hold the probes in place" - I don't know this particular amp, but when measuring bias, this is potentially a recipe for disaster.
I would strongly suggest to use mini-grabber leads.... not gravity.

Yeah, I would normally agree with you, but in this case the probes fit snugly inside the leads of the resistor, so there is some tension between the leads and the ceramic body. So in effect the approach I used was really more like test sockets on some tube amps. There is really no way for the probes to slip out or short unless I was really trying. A more appropriate wording for what I wrote would be "gravity and tension are your friends".



Still, I agree, I probably would have used mini-grabbers if I could have gotten them to catch hold of the leads. I just couldn't get the correct angle due to the surrounding board and heat sink. I also couldn't get my clips to catch on the short leads on the back side of the resistors. Maybe I should try a different style of clips.

 

[Harvestor]

Okay...

I performed the procedure on my SA600 today and after 5 minutes made the adjustment on bias voltage.....
It tested at first @14.3 millivolts on the Left and 15.1 on the Right , then I set to 8 mv and kept checking after every 15 min for an hour... Beautiful ..
Then I checked the DC Offset Left 2.8mv Right 4.1mv....
All Good...
And Thank you again!

P.S......I used 3 Fluke Test Meters to verify that the voltage was tested as accurately as possible with these tests instruments

 

[Harvestor]

Well after a few days I noticed that the Left Channel was slightly Quieter than the Right....especially at low volume....
So I opened the Receiver up and checked the Bias again.......... and sure enough the right amp was @ 9.8mv and the Left @ 7.1mv.....so OVER the course of 2 HOURS I continued to make adjustments to both allowing it to settle between EACH Adjustment !.... Because it would drift up or Down for several minutes afterwards..... So after 2 HOURS of this.....I got them to stay at 8.3mv Each Side.... but they both would FLUCTUATE between 8.1 & 8.4 mv.......

So..... Now the Left and Right Channels are playing at Equal volume now.....

 

[Harvestor]

I wonder how many other people here have rechecked the voltage after a few days or weeks after to see if anything drifted or changed?
There was noticeable difference in loudness especially at low volume that was the reason I rechecked....

 

[awillia6]

I wonder how many other people here have rechecked the voltage after a few days or weeks after to see if anything drifted or changed?
There was noticeable difference in loudness especially at low volume that was the reason I rechecked....

I know a guy who goes into ALL his gear every January, cleans it, and adjusts it to factory specs. For stuff that he finds way off (like yours), he'll do a mid-year check as well. If the symptom persists, he'll track down the problem and fix it. Lotta work, but his stuff always sounds great (in February!).

 

[Gnesen]

I wonder how many other people here have rechecked the voltage after a few days or weeks after to see if anything drifted or changed?
There was noticeable difference in loudness especially at low volume that was the reason I rechecked....

I agree with @awillia6, in cases like this it may be good to do some follow-up service.

 

[SamuelArthur]

I realize this thread is a little dated now, and I really appreciate everything you've shared so far @Gnesen, but I was wondering if it's possible to actually adjust the DC offset on this receiver. The idle current adjustment seems pretty straightforward, but I haven't seen anything about pots to adjust the offset if it's reading high at the speaker terminals. Thanks in advance!

 

[pieterv1]

Resurrecting this old thread.. Could it be that the procedure as described in the service manual is a bit misleading?

If I connect my probes correctly, and adjust the trimmers, I'm getting a negative voltage reading which goes up to 0mv with the trimmers all the way clockwise. So I can only set the bias to -8 mV and that's with the trimmers pretty much nicely in the middle.

If the DC voltages are way off, you should probably replace the matched transistor pairs Q601 & Q602 (2SA798A). These are notorious for drifting due to age. They can be replaced with manually 2 matched SA992's or with their SMD counterpart (Toshiba 2SA1873) on a small pcb.

 

[bzenic]

@pieterv1
I'm resurrecting this thread because I have exactly the same issue trying to adjust the bias. On both left and right channel if I connect the DVM leads as in the service manual I can only go to (negative) -8mV with voltage going more negative by turning vrs anti clockwise.
Btw, one thing to mention is that I did not have connected the lighting bulbs while doing this, not sure if that matters at all.

 

[painttoad]

if that is the case then i solder little wire loops on the pcb that i can grab with minigrabbers.

don't do this with probes.

 

[Warren-Oates]

@pieterv1
I'm resurrecting this thread because I have exactly the same issue trying to adjust the bias. On both left and right channel if I connect the DVM leads as in the service manual I can only go to (negative) -8mV with voltage going more negative by turning vrs anti clockwise.
Btw, one thing to mention is that I did not have connected the lighting bulbs while doing this, not sure if that matters at all.

-8 or positive 8, you are still measuring the voltage drop across that resistor. It just depends on what meter probe you connect to each end of the resistor. If you switch them you will see your number will go positive instead of negative. The key is 8. And as everyone mentions; be real sure you don't short to something.