Technics SA-200: How to adjust the DC offset?

Sometimes, to reach non standard values, I just solder a resistor in parallel on the solder side:

This pict is from a Technics SA-400, it solved the DC offset in that unit.

P1140768.jpg
 
Thanks all for the help. I am learning a lot here... two steps forward, one step back.
Here's where I'm at with my SA-200:

Originally b ought from ebay as 'for parts or not working'. Mainly just needed deoxit. Both channels worked, but both had a dc offset of 80 and a big 'thump' through the speakers when I turned the receiveroff, which i thought might be attributable to the dc offset.
Following Ecluser's advice at the beginning of this thread, i became fixated on trying to reduce the dc offset. I began to test by adding resistors in parallel to the 614 and 613 resistors. Long story short, one of my probes wandered and I ended up shorting out the left channel power IC, IC601.

Good news: I was able to get one on eBay that seems to work ok. I replaced the IC and that channel worked again.
I couldn't get the old resistors out, but i found it easy to solder new ones in in parallel. With a 680 ohm resistor in parallel on 614 and (I think) an 1100 ohm in parallel on 613, I got the dc offset down to .005 on the right channel and .015 on the left.
And now it doesn't seem to thump when i turn it off (though I have limited data points).

So here's what's up now:
The right channel, the one that was fine, works fine when I first turn the unit on (cold). But after a few minutes, it deteriorates, getting more and more staticky until you can barely make out the music on that channel. I checked the related IC, and all appears to be fine.

The only thing I've noted is that there is a big boxy cement-ish resistor labeled 5W 180 ohm J 2836 that gets very, very warm during operation (I included a picture). I checked resistance when the unit is off and it's still 180 ohms. I did not measure when warm. I also realize I didn't write down the resistor number from the board. I need to trace it on the schematic but I wouldn't think that is related to an issue in just one channel.

Also, another thing I found very odd - the unit seems to continue to play for about 8 seconds after I turn off the unit. I had my iphone playing through aux on headphones, and after I switched off the receiver, it kept playing. Even more odd, I wanted to check the dc offset when the unit was off - so I plugged in my probes to the speaker jacks after the receiver was turned off - and the iphone picked up volume and started playing through the headphones on the Technics' headphone jack for second!

If anyone can offer help with the deteriorating right channel issue, I would be very grateful. Also, if the staying on for 8 seconds is a concern, please let me know.
 

Attachments

  • IMG_2877.JPG
    IMG_2877.JPG
    90.4 KB · Views: 32
The unit playing for some seconds after power off is normal, Actually it means "healthy capacitors" that hold the charge.

Sound vanishing after power ON sounds like some leaky component, it could be a capacitor, a transistor or even the IC that can't hold the voltage. I can't check the schematic now, but somebody will for sure.

You should monitor + and - voltages while this happens. Also, to identify if the problem comes from the preamp or power amp section.

Don't poke with your probes inside a live unit (you know why). Instead, get alligator clips or mini grabber probes, anr attach them with the unit UNPLUGGED FROM THE WALL, then power on to read the measurements without touching anything. It's a bit tedious to power off and UNPLUG every time, but in the long run it's safer and will save time and money and perhaps your life.
 
Thanks Elnaldo!
I do understand not poking with the probe - I actually was using alligator clips when one of them came off of a resistor and must have shorted something. I am very careful about doing exactly what you say - turn off, unplug, attach, power on. That's when the playing on after shutoff really freaked me out!
I did check the IC voltages on the vanishing side, they seemed in line with the prior measurements and the other channel (which now works great). I will check that again, since i know how to; maybe I shorted it when I was testing it.
SInce that channel WAS working, I assume I must have created the problem somehow.
When you mention checking the voltages - I assume one probe on ground, and the other on the circuit? Can I do that all along the signal path? I assume the two channels should be the same, I can use that as a sanity check, and also to see if the values change when cold/warm on the bad side. Is the expected voltage on the schematic? I'm looking at it right now, but can't see the small writing (will grab my extra strong readers later).
 
I'd start checking (monitoring) + and - voltage on the IC you suspect, or around areas where you see some overheating. What you describe sounds to me that some supply voltage is pulled down by some component failing. An amplifier stage with assymetrycal power supply sounds very distorted since it's amplifying only one half of the signal. I've seen similar problems related to leaky capacitors that started to shunt the voltage to ground after some minutes. But a transistor or an IC could do the same, shunt some voltage to ground... I'm guessing your problem will show a problem on the supply voltages to some circuit, but I'm not 100% sure of course.

Also, monitoring DC offset when failing could tell something

You could also start checking at the preamp, supply voltage to the preamp section, compare channels, and check forward at each amplifier stage until find some anomaly or difference between channels.

Yes, one probe to ground, other probe measuring the voltage against ground.
 
I see you have a 2SA798 on that picture, that double transistor with 5 pins. I'd replace it with 2 single transistors as described in other threads. We can't trust that part, those transistors always fail.
 
Last edited:
Elnaldo - thank you - can you please point out where on the picture that 2SA798 is? I don't know which one it is, and can't count pins from the picture.

I re-tested the IC, and it checked out OK.

your problem will show a problem on the supply voltages to some circuit
So if i check the supply voltage along the circuit, and compare side to side, I should find an anomaly and that may indicate the source of my problem?
Since the sound gets bad after a couple of minutes, will the voltage appear ok at first and then change? I would think that would be the case?

Also, monitoring DC offset when failing could tell something
Curious - what am I looking for? should I test DC offset when I first turn the unit on, then continue to test, am I looking for a change? What would that indicate?

I will have a better multimeter this weekend to work with.
Also, I have an MCS3338 receiver that I am also debugging, going to spend a bit more time with that as my first victim and be a little more cautious with this one.
 
Elnaldo - thank you - can you please point out where on the picture that 2SA798 is? I don't know which one it is, and can't count pins from the picture.

Just google 2sa798 images, it's a little rectangle with 5 leads.

2sa798_sch.png


http://audiokarma.org/forums/index....nsistor-2sa798-is-there-a-replacement.281614/

So if i check the supply voltage along the circuit, and compare side to side, I should find an anomaly and that may indicate the source of my problem?
Since the sound gets bad after a couple of minutes, will the voltage appear ok at first and then change? I would think that would be the case?

I suspect something pulling down the voltage, so yes, voltage should be OK at power ON, and vanish or change when the unit starts to fail.

Curious - what am I looking for? should I test DC offset when I first turn the unit on, then continue to test, am I looking for a change? What would that indicate?

The same, perhaps monitoring the DC at the output, some change could happen when failing.

I'll download the schematic and try to point you to some test points to monitor. I'd monitor supply to the preamp, and then to the power amp.
 
PLEASE check with the AUX input only, not FM, not Phono, to isolate the power amp section. I don't see any active components before the power amp section if you use the AUX input, so if the problem appears, the problem must be at the power amp.

I see pins 2 and 9 carry the + and - V to the power IC. I'd monitor the voltage there while the problem happens.

If OK, and the problem persists, it could be the DC offset going off. Monitor the DC at PIN 1 (signal input, to see if it stays at near zero volt), and at the output, to see if the input is OK, but the output not. (that would indicate a problem at the IC)

Also, it could be the power amp being OK, and the signal arriving distorted. In that case, I'd check the voltages around Q601 Q602 Q603 Q604. Anyway, I think a shift there should be seen at PIN 1 of the power IC.

If every voltage is OK, but the sound is not, you'll need a signal tracer, a probe to send the signal to a 2nd amp. But that would be a next step.
 
Time to resurrect this thread a little.

I have a Technics SA-200 I got from my grandparents. When I first got it everything sounded real scratchy whenever I touched any of the knobs, the left channel was almost dead, but all the lights worked so I just left it on the shelf for a couple years. I pulled it out last week and gave it a good deoxit bath (it needed quite a bit) and it came back to life. I set it up in my living room and I'm feeding the aux from the pre-out on my Denon avr-3803, this way I can run everything through the Denon's digital inputs. I gotta say, playing through my Boston Acoustics A150s, I like the sound on this better then the Denon, it's warmer and the bass hits a little better and I hear slightly better imaging front to back. But the left channel still sounds just a wee bit off, maybe some noise at times and maybe just a tiny bit lower volume. So I checked the DC offset and I found the right channel at 74mv and the left at about 84mv. With the help of this thread, I hope to get it back down close to 0 over this weekend, I'm hoping that'll improve the sound, I'll post back after. Otherwise, this thing cleaned up really nicely, aside from some of the vinyl veneer peeling on top of the left end.
 
I just want to say, big thanks to everyone on this thread, this info really helped me tonight on repairing the offset on my sa-200. I got it down to 0.8 mv (from 74) on the right channel using a 680 & 39 ohm resistor in series, and those in parallel with r614 on the underside of the board. And I got the left channel down to about 2.7 mv (from 84) using a 470 & 150 ohm resistor in series, and those are parallel to r613 on the underside of the board. Because I used 2 resistors in series, it wouldn't fit neatly between the 2 points on the board, so I wrapped them heavily in electrical tape leaving just enough exposed on the ends to solder, and routed them around and between the other components solder joints. It might not look pretty but it worked. My 11 year old son enjoyed helping on the project. Now I can sit back and enjoy this receiver.
 
Do you remember the value of the original resisors R613 and R614, and the value once "modified"? Perhaps this can help somebody else.
 
Do you remember the value of the original resisors R613 and R614, and the value once "modified"? Perhaps this can help somebody else.

Sure. The service manual says both R613 and R614 are 470 ohm resistors, I did not desolder them and check their actual resistance. If you do the math with the ones I added in parallel, it comes to about 269 for R613 and 284 for R614. Both of those are non standard values, or at least not in the set I got, and although I probably could have just gotten away with replacing both original resistors with 270 ohms, I decided to do it the way I did to try to get it as close to zero as possible and because I knew it worked when I tested it with alligator clip wire leads.
 
Looking at my own previous post, I remember I've used 680 ohm resistors... in parallel to 470 gives 277 ohm. It seems that around 270 ohm instead of 470 solves this "issue", I hope without secondary effects...
 
Looking at my own previous post, I remember I've used 680 ohm resistors... in parallel to 470 gives 277 ohm. It seems that around 270 ohm instead of 470 solves this "issue", I hope without secondary effects...

Yup, your post actually helped me a lot, it gave me a starting point, I started with 680. Thanks! Let's hope nothing gets damaged, but it's seems to be working fine. I wonder if it was just a fault in the design, because it was one of the lower end units in the "SA" lineup and they just left it that way, both our numbers were pretty close. Or maybe the same component is failing in all these units which causes it to be out of spec by about the same amount? Who knows....
 
I guess it's just the original design accuracy or tolerance. Perhaps in paper the circuit performed a bit better than the real thing. Many low end amplifiers consider "within specs" up to 100mV DC offset.
 
Hello guys, another person here posting about their experience trying to lower the dc offset on this particular machine. From what Ive gathered from this awesome thread is that basically, I need to replace two transistors in the board with new ones. What Im confused about is how I go about finding out exactly which transistors Im looking for, and what ohms they need to be. Ive measured the dc off set, and its about 90 across the board, so Im not sure what I need. Im also uncertain about what parallel or inseries transistors mean, Ive done some googling to try and find out but its quite confusing when you dont really know anything about electrical diagrams. If anyone can help me out here or throw some points my way before I try to repair this and end up ruining something, Id really appreciate it.
 
Just google 2sa798 images, it's a little rectangle with 5 leads.

2sa798_sch.png


http://audiokarma.org/forums/index....nsistor-2sa798-is-there-a-replacement.281614/



I suspect something pulling down the voltage, so yes, voltage should be OK at power ON, and vanish or change when the unit starts to fail.



The same, perhaps monitoring the DC at the output, some change could happen when failing.

I'll download the schematic and try to point you to some test points to monitor. I'd monitor supply to the preamp, and then to the power amp.

Hello. Welcome to AK.

Please check the quoted post. The transistor in question is a particular one , 2 transistors in one case. But if you still don't understand very well the terms "series" or "parallel", my best advice , for your safety, is to stay away of the circuit board by now, until you study a bit more and feel more confident. As you said, you can ruin a working amplifier, and more important, you can get electrocuted.
 
Hello. Welcome to AK.

Please check the quoted post. The transistor in question is a particular one , 2 transistors in one case. But if you still don't understand very well the terms "series" or "parallel", my best advice , for your safety, is to stay away of the circuit board by now, until you study a bit more and feel more confident. As you said, you can ruin a working amplifier, and more important, you can get electrocuted.

Thank you so much! I realized that I meant resistor, not transistors, the R614 and R613. I figured that it was as simple as soldering those out, and soldering new ones in, or am I mistaken? I have soldering experience, and limited experience working inside electronics so I know to be careful.

Sometimes, to reach non standard values, I just solder a resistor in parallel on the solder side:

This pict is from a Technics SA-400, it solved the DC offset in that unit.

View attachment 891555

Excuse me if Im wrong, but I couldnt just do this, solder two resistors on the underside of the board to hopefully try and lower that offset?

I might just end up running the unit as is, as I hooked it up to my headphones, and my speakers and to my ear it sounds pretty good, and reading some other threads on DC Offset, it should actually damage anything, just possibly degrade the sound quality, but Im not sure the offset on this lower end unit is that big of a deal. Do you have any opinions? Thank you so much for your time, I really appreciate it.
 
Reading the old posts, I see a previous poster and I arrived to the same solution, I added 680ohm in parallel with those 470ohm on the solder side, lowering the resistance to 277 ohm. So in both cases replacing 470 ohm with approx 270 ohm lowered the DC offset. You can add a resistor on the solder side as I did. The effect is the same than replacing the original resistors.

A small offset won't damage your speakers, headphones or amp. Some amps are working with 100mV DC at the output. The ideal value is zero, so getting closer to zero would be better. But if you'll be risking the amp, a working amp with 90mV at the output is way better than a blown amp.
 
Back
Top Bottom