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

[kotofei]

...From the schematic I have, and the colour bars I can read from your pictures, I would say that you can correct the offset if you replace resistors R613 and R614 (actually 470 Ohms) by ~130 Ohms resistors. You can connect another resistor in parallel with the actual resistor, with small jumpers, starting with a 1 kOhms, and progressively reducing this second resistor until the offset is near 0mV. If the parallel resistor is too low, the offset will become negative. The optimum parallel resistor should be close to 180 Ohms. ...

Alain,

Thank you very much for the schematics. It is sort of difficult to understand it on the fly now but I hope I will progress.

I have a couple of questions. The first one is regarding the small jumpers for connection of another resistor in parallel with the existing one. Where I can get them? Radio Snake is first that comes to mind; is it any particular brand that is preferable?

Same question about the source of these other resistors. Should I buy them (in RS or elsewhere) and use new or I can de-solder them from scrapboards? I realized that I have a non-working Technics tape deck, I can use it as a source of components.

Thanks!

 

[ecluser]

See the pictures at the end of post #1 in this thread:

http://audiokarma.org/forums/showthread.php?t=155016

At the other end I've put aligator clips. RS should have them, I think.

You can't imagine how many components I save from scrapboards! My prefered source is old CRT computer monitors. They are not old in fact, most have less than 5 years but peoples replace them for flat screens and there is no after market for them. If you ask for old monitors at your local SA, they will be pleased to give them to you.

 

[kotofei]

Ordered temperature-controlled soldering station from epay, got alligator clips from RS and a vacuum desoldering tool from there. They also have resistors and they are of at least three types: 1/4 watt, 1/2 watt and 1 watt; the 1/4 watt are the smallest. The R613 and R614 resistors in the SA-200 looks like 1/4 watts so I will use 1/4 watt resistors for soldering in parallel. Is this right?

The other question is, since I will use a temperature-controlled soldering station, at what temperature should it be?

Thanks!

 

[ecluser]

The other question is, since I will use a temperature-controlled soldering station, at what temperature should it be?

There is no definite answer to this. You should practice on a scrapboard.

If the temperature is too low, you will need to heat the solder joint for a too long time before the solder will melt. This will damage the component and/or the board itself.

Too much temperature will make damage too, mainly on the board itself.

When you will receive your soldering station, read the notice supplied by the manufacturer, and practice. When the temperature is good, solder melts in 1~2 seconds.

No problem with 1/4 watt resistors for this application.

 

[kotofei]

Here is an update on the project:

The temperature-controlled station (Solder Aid OK) arrived, in working condition. Found resistor color codes and examined the old dead Technics deck -found few at 100 Ohm and a couple at 150 Ohm. De-soldered both 150 Ohms, this was very easy with the desoldering tool.

Connected a meter to right speaker terminals, DC offset is + 180 mV. Connected jumpers with a 150 ohm resistor in parallel to R613- the offset did not change. Then connected jumpers to R614-the offset changed to - 41 mV (minus 41).

Connected the meter to left main speakers, DC offset is again +180 mV. Connected jumpers with a 150 ohm resistor in parallel to R613- the offset changed to - 44 mV. Connection to R614 had no effect on the offset.

When I connected the two 150 Ohm resistors one to another and then connected the resulting 300 Ohm load to R 613, the DC offset in the left channel changed to +25 mV.

I need to find 200- 250 Ohm resistors and try them. Will post the update.

 

[ecluser]

You may try 100+100 Ohms, or 100+150 Ohms, since you have some 100 ohms from your deck

 

[kotofei]

You may try 100+100 Ohms, or 100+150 Ohms, since you have some 100 ohms from your deck

Tried this - soldered out two 100-Ohms from the deck and connected them one to another, then connected the resulting 200 Ohm load to R 614. Right speaker DC offset went from 187 mV to -5.5 mV. Repeated with R613, left speaker DC offset went from 190 mV to -1.9 mV.

Looks like 200 Ohms is what I need to solder in parallel to R613/R614 from the bottom of the Technics. However, wouldn't it be better just to replace R613 and R614 each with 140 Ohms resistors?

 

[ecluser]

Yes, replacement of the original R613 and R614 (470 Ohms) by 140 Ohms would be perfect since an offset of this magnitude is very acceptable.

Usually the difference is not as great as this and the paralleling technique is more precise with standard tolerance resistors. You can also replace with a fixed 100 Ohms in series with a 50 ohms trim pot if you want to readjust this in the future. I don't think it needs to be adjustable. The value you found experimentally is very close to what I predicted (130 Ohms) from calculation...

 

[kotofei]

Look like I fixed it!

Spent a while looking through my scrap boards for spare 140 or 200 Ohm resistors to fix the offset and found neither. Then decided to replace R613 and R614, each 470 Ohm, with 150 Ohms resistors that I have instead of 140 Ohm in the hope that the offset will be acceptable.

Desoldering was quick and easy; soldering of new resistors took a little more time but went better that I expected. I was afraid that the "legs" of new resistors will be too short but it worked out well. In each case, the leg protruded from a hole in the PCB board and after soldering the resistor was firmly in place and did not move at all if pushed or pulled with forceps. I used Craftsman lead-free very fine electrical Rosin core solder.

I attach a picture of the board; red arrows point on the newly soldered resistor legs. Does it look good? Those were my first soldered electronic components EVER.

Now back to the DC offset. Tested it and... the right channel is now -13 mV, left is -3 mV. The offset was +180/+190 mV before. The receiver and all controls worked. Looks like I fixed it! Later, I will hook the Technics to the main system to figure out how well it sounds.

I would like to thank all of the AK people, especially Ecluser, for the help that was given to me with this Technics. You are wonderful! Now I know I can do it, and hope that it was only a first step.

 

[EchoWars]

I would like to thank all of the AK people, especially Ecluser, for the help that was given to me with this Technics. You are wonderful!

Yes...he is like a ray of sunshine on an otherwise gloomy day.:smlove:

 

[rjcrjc]

Hello all, hoping to resurrect this thread as I have an SA-200 with a similar problem, and I set out to try to fix it today. My DC offset is +80 mV at both left and right speaker output (+/- 5 mV). I wanted to get this closer to 0 before I connected some vintage speakers to it.

Using this as a learning exercise (realizing now I picked the wrong victim), I tried shunting the R613 and R614 resistors in the manner that Ecluser directed Kotofei in this thread, with alligator clips. I started small (added an 80 mV), then I played trial and error with a 50mv and 150mv resistor, observing the changes to the DC offset readings (some swung it higher, some negative).

At one point my measurement on the right side went off the charts. I tried the 80mv resistor, expecting to get the same as the first measurement, and it was not - it was also off the charts.

I looked at my connections and noticed that one of my alligator clips was off the resistor. So I set out to start over. To my horror, while the left was at the original 80 mV, the right was still off the charts. When I adjusted my multimeter, I saw that the right channel now had a DC offset of 20 volts. I also noticed that the the board smelled a little burned, and expecially the R708 (the big white one marked 5W 180 ohm 2836) felt warm even after unplugged.
I let it cool down and tried again, same story, same warmth on R708.

Have I irreparably damaged this? Any help with next steps? Was hoping to fix up to give to my son.

And this is my first post, but I've been reading a ton over the last several weeks as my interest in vintage audio gear has been rekindled (although, when I used to be interested in this gear in the late 70s/early 80s, it was just 'audio gear' then). Thanks to all who have posted such terrific information here over the years.

 

[stereoguy70]

Hello all, hoping to resurrect this thread as I have an SA-200 with a similar problem, and I set out to try to fix it today. My DC offset is +80 mV at both left and right speaker output (+/- 5 mV). I wanted to get this closer to 0 before I connected some vintage speakers to it.

Using this as a learning exercise (realizing now I picked the wrong victim), I tried shunting the R613 and R614 resistors in the manner that Ecluser directed Kotofei in this thread, with alligator clips. I started small (added an 80 mV), then I played trial and error with a 50mv and 150mv resistor, observing the changes to the DC offset readings (some swung it higher, some negative).

At one point my measurement on the right side went off the charts. I tried the 80mv resistor, expecting to get the same as the first measurement, and it was not - it was also off the charts.

I looked at my connections and noticed that one of my alligator clips was off the resistor. So I set out to start over. To my horror, while the left was at the original 80 mV, the right was still off the charts. When I adjusted my multimeter, I saw that the right channel now had a DC offset of 20 volts. I also noticed that the the board smelled a little burned, and expecially the R708 (the big white one marked 5W 180 ohm 2836) felt warm even after unplugged.
I let it cool down and tried again, same story, same warmth on R708.

Have I irreparably damaged this? Any help with next steps? Was hoping to fix up to give to my son.

And this is my first post, but I've been reading a ton over the last several weeks as my interest in vintage audio gear has been rekindled (although, when I used to be interested in this gear in the late 70s/early 80s, it was just 'audio gear' then). Thanks to all who have posted such terrific information here over the years.

If you have volts and not mv now, chances are you have a shorted transistor in the output stage somewhere now. You'll have to trace back which one it is and find a replacement for it. Hopefully you didn't damage one of the "STK" IC power modules as some of those can be difficult to find.

 

[rjcrjc]

Thank you Stereoguy. I did see those STK power modules on ebay the other day before I had cleaned the pots and thought that might have been a problem as I had a week channel before Deoxit. So maybe that has become a self-fulfilling prophecy. But before I go parts swapping, I i will search this forum for how to trace back and debug. Any other direction/next steps would be appreciated as well. Trying to learn...

 

[stereoguy70]

Thank you Stereoguy. I did see those STK power modules on ebay the other day before I had cleaned the pots and thought that might have been a problem as I had a week channel before Deoxit. So maybe that has become a self-fulfilling prophecy. But before I go parts swapping, I i will search this forum for how to trace back and debug. Any other direction/next steps would be appreciated as well. Trying to learn...

The best thing and first thing you should do is get a copy of the factory service manual. Here are a couple of links to sites with free versions.....including the AK Database.
http://akdatabase.org/AKview/thumbnails.php?album=39
https://www.hifiengine.com/manual_library/technics/sa-200.shtml

On HFE, you'll have to join as a member, but it's free to join. Once you join, you can download it (you need to be logged in).

As a side thing about those STK modules......be careful about ones from ebay. I've heard that most are counterfeit. If one of yours is fried, try to find genuine ones if you can. But....we are kind of putting the cart before the horse here. Best thing is to study the schematic/manual and test, test, test. Then diagnose and fix.

Any effort is worth it with one of these, they were nice sounding receivers in their day. I had an SA-101 recently, its younger lower powered brother and it had a nice sound to it. Years ago my dad had an SA-202 (the next incarnation of the SA-200) that sounded really nice as well. Good sounding receivers when they are working properly.

 

[rjcrjc]

Again, thanks Stereoguy. This is kind of surreal for me - I've been lurking for awhile, reading a lot of older threads here, and now I'm interacting in the forum with real forum members! Kind of like watching a TV show for years and then suddenly being on the set. Kinda.

When I picked up this unit, it had a few issues and I thought it would be my learning vehicle. I didn't realize the upside of the unit at the time, I should have gotten more of a junker first to learn on, because most of the issues went away after a Deoxit bath. The only other issues were the 80mv dc offset in both channels, and a 'thump' when it turned off. I thought the thump might be related to the offset and that's why I was messing with it in the first place.

I do have the schematic, I've been poring over it the last few days and today. But itt's hard to read, my eyes aren't what they used to be.

Thanks for the heads up on the ebay STK modules. The ebay item I was looking at is here, from a US seller with high ratings. Is there any way to verify which are good? I figure a US seller with 'electronics' in their name and 99.9% approval might be ok? Or is there a better source for parts?
http://www.ebay.com/itm/STK0029-Hea...385881?hash=item1c56cd6619:g:aMwAAOSwa-dWs1KR

There is a post by Elnaldo in this thread (http://audiokarma.org/forums/index....-help-for-technic-sa-200.516151/#post-6715608) that says:
But try to figure if the power is reliable, check Voltage at PIN 2 and PIN 9 of the STK0029, against the amp chassis. IT SHOULD MEASURE + AND - 31.4V APPROX. MEASURE AC TOO. If your multimeter measures AC, it means the current is not being rectified. It should be DC there, not AC.
I see in the schematic the note that pins 2 and 9 should be +31.4V and -31.4V. So if I test that, and voltage is correct, and same for pin 9, does that mean the IC is good? If it's not 31.4V, does that mean it's bad? EIther way, I assume I'll have to keep moving down the path to find what all components may be toast?
To test this - alligator clip on #2, alligator clip on ground, then clips connected to probes on multimeter set to DC voltage? Is that correct? Looking for validation before I fry more things (including myself).
And lesson learned - be really careful with the alligator clips, because they can slip off...

 

[stereoguy70]

rjcrjc, it is best to check what all the voltage values are on all the pins according to the schematic. If any of them are significantly off, then chances are the STK module has a problem. If they are spot on, the STK may not be the problem and may still be good. The problem may be further up the line in the power stage. I haven't actually looked at the schematic, but if there are no discrete transistors before the STK modules in the power amp circuit, then chances are one of the STK modules is bad. For example, if its only the preamp before the STK modules and nothing discreet in between, then I'd say most likely the issue is with one of the STK modules, but checking the voltages on the pins would also confirm this.

To answer your other question about the multimeter, yes, DC voltage is what you are checking. Set it to DC voltage, most likely the 200v scale, as the 20v scale will be too low for proper readings. Also, use the probes and avoid the alligator clips! Take your negative probe and put it to a good chassis ground and leave it there. Probe the STK module one pin at a time with the positive probe lead ONLY. Most of all, be careful not to cross the pins by touching more than one at a time! Much electronics frying can ensue if you touch more than one pin at a time by accident. You're not in much danger because it is relatively low voltage, but always test with one hand behind your back. This way you eliminate yourself as a possible ground source. (Something I learned when working on tube equipment years ago, which uses much higher voltage than solid state devices do. Electricity is your friend when treated with respect )

 

[rjcrjc]

OK, thanks Stereoguy. Just spent some time running tests and have a lot of great info here.

I tested every pin on both ICs, IC601 and IC602. I am using the values on the schematic here to validate. At least with the online schematic I can enlarge it, but I still can't really make out all the numbers.
http://akdatabase.org/AKview/albums/userpics/10006/Technics SA-200_200K SM.pdf

From the schematic, it looks like the values should be

0 = 1.36v??? (can't read it)
1 = -1.2v???? (can't read it)
2 = -31.4v
3 = 0v
5 = ??? marked but no value
8 = 0v
9 = 31.4v


I did note that pins 4,6, and 7 are not soldered to anything. So I would expect 0v for all those and that explains why they are not marked. And that was the case when I measured both ICs, so I will omit here.

When I measured IC602 (right channel):
0 = between 1v and 2v
1 = about -.1v
2 = -31v
3 = 0v
5 = 31v
8 = 0v
9 = 31v

When I measured IC601 (left channel):
0 = 11v
1 = 7.5v
2 = -31v
3 = 18v
5 = -30v
8 = 18v
9 = 31v

So it looks like IC602 is pretty much on the mark. I can't confirm pin #5 is correct because I don't know what is the expectation, but given the rest are ok, and the DC offset is not completely 'whack' on the right channel, I think that IC is ok.

The left one, IC601, is clearly whack. And that channel is the one that now has a standing DC offset of 20V when on (and, interestingly, 5V when off). So it looks for certain that I fried it.

I ordered a new one from this seller on ebay. Hope this one is legit.
http://www.ebay.com/itm/STK0029-Hea...385881?hash=item1c56cd6619:g:aMwAAOSwa-dWs1KR
If anyone has any better places to go to get good electronic parts, please let me know. I assume Radio Shack is useless at this point.

I know I need to continue following the signal path to see if anything else might be fried, but I'm not real sure how to test each component. If anyone has a reference to point me to, please do.


...And then since I was getting more comfortable with the probe, I decided to try to see if I could get the DC Offset on the right channel, which was still the original 80mv, back to zero.
So I experimented with a bunch of different resistors shunted to R614 (again, following Ecluser's advice at the beginning of the thread).
Here's what I found:
No shunt (orig) ==> 80mv dc offset
51ohm shunt ==> -.22 mv dc offset
150ohm ==> -13 mv
200ohm ==> -.11 mv
300ohm ==> .-07
430ohm ==> .-035mv
560ohm ==> -.015mv
620ohm ==> .005mv
680ohm ==> 0 mv
750ohm ==> +1mv

The R614 was a 471ohm resistor to begin with. So does this imply that I can either add a 680ohm in parallel, or if this equation is correct: 1/R = 1/R1+1/R2, then I could replace the 471ohm resistor with a 280ohm one to get the offset pretty darn close to zero? Or could there be a different reason / root cause for the 80mv DC offset (which sounds really good right now compared to the 20V on the left channel).

While I've created my own problems today, I've learned a lot, and I am eager to learn more here. I tend to get obsessive about new hobbies, and I am looking to soak up knowledge from other members and would appreciate good references to learn as well. I was an engineering student in the day, and still have my Electrical Engineering text book from 30 years ago on my bookshelf 2 feet away from where I type (of course I think it's been sitting there untouched for the last 20 years). I believe I have the background and capacity to learn and certainly have the interest.

That being said, I would certainly appreciate any guidance related to my issues above and/or explanation for why things happen.

 

[stereoguy70]

Yes, it looks like IC601 is junk. Not much else you can do but try and search the internet for replacements. Radio Shack is pretty useless these days for components. They are generic parts usually that don't correspond to anything that was made in the last century, I've found. Best places I've found were Mouser, DigiKey, and to a lesser degree, Parts Express.....although they are a great source for Solen brand film capacitors for speaker crossovers and such. Searching for STK modules through them is kind of tough though, as they don't carry them. eCompass sometimes still can get the STK modules, depending on the number of the STK.

 

[rjcrjc]

Can anyone validate my resistor math above? Id like to pursue fixing the DC Offset on the good channel while waiting to receive my new IC for the bad channel.

 

[jasoncauthron]

This is a very old thread, but I'll verify that your resistor math is correct, with one small concern: 471 is an odd value, so I'll assume you mean 470. 470 Ohms in parallel with 680 Ohms would be (470 x 680) / 470+680, or 319,600 / 1150 = 277.9
So yes, it looks like you could replace R614 with a 280 Ohm.