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

[kotofei]

Picked up today a nice and clean Technics SA-200 receiver in the SA store. At home, decided to begin its testing with measuring of the DC offset and I'm glad I did this. Left channel: 168 mV, right channel: 161 mV. Probably enough to cook a pair of speakers.

Anyone has a service manual or perhaps just knows where the pots for offset adjustment are?

Thanks a lot.

 

[ryuuoh]

Doesn't seem to be a readily available free manual for the 200. Here's a hit for a paid copy:
http://www.technics.owner-manuals.com/SA200-service-manual-TECHNICS.html

 

[ecluser]

Can you post a picture of the board?

Without a schematic, at least a picture may help...

 

[kotofei]

Here are pictures of the board.

I found several elements that look like pots; most of them are numbered in one of the files. My understanding of the DC offset pots is that it should be two of them, for left and right channel, looking similar to each other. I identified two such pots that I marked with blue arrowheads, and I also found something looking as a double pot, # 4 in the picture.

 

[ecluser]

No! No! NO! Don't touch those adjustments, they are part of the tuner board and you need spetial adjustment tools (signal generators and things like that) to adjust them properly.

The third picture shows two power packs, probably STK...
Can you report what are the number printed on them?

Usually there is no provosition for adjusting the offset with those power packs. I don't see any adjustment trim pots in the second picture too (close to the power packs), and it makes me believe it will not be easy to adjust.

I repeat, don't touch the adjustments in your pictures. For example, in the first picture one of the adjustment is identified as MPX, this is an adjustment for the stereo FM discriminator. Another is identified as SEP, this one adjust the separation between L to R and R to L in FM reception. You need special measurement tools for this adjustments.

 

[HatchetJack]

Pound for pound, dollar for dollar, I dont think you could find a better receiver.
Mine has been kickin _$$ since 1980.

 

[kotofei]

The two power packs are indeed Darlingtons, STK-0029.
I also add three more pictures, they are not as close-ups as previous and so you can found some additional components(?)

If there is no way to adjust the DC offset, what should I do? I hate the idea of trashing this SA-200; it's very nice and clean, not much outside wear except the power switch. I tested it yesterday with headphones using tuner, aux and tape as inputs (did not test phono yet) - everything works, sound is good. It's a little bit of static when I rotate the balance knob but it will be easy to fix with Deoxit.

 

[ryuuoh]

Your first pic above shows the tuner board. As said earlier, don't touch any adjustments in there. Setting the DC offset and bias will require a service manual and the equipment to measure the settings.

 

[EchoWars]

The offset you are measuring is harmless to speakers, but detrimental to sound quality.

Many low-end units simply do not include provisions to adjust offset. Only fix is to replace differential pair transistors.

 

[ecluser]

You are lucky, there is hope for you. The STK 0029 is an output stage only, there is no built-in differential stage like there is in a STK 080 for example.

This means there is a preamp stage before the STKs, and the third picture shows two dual transistors, identified as Q601 and Q602. They are 5 (or 6) pins devices comprising two similar transistors.

What is printed on them?

It may be dual NPN (if part number start by 2sc or 2 sd), or dual PNP (if part number start by 2sa or 2sb).

 

[kotofei]

Found them. They are 5-pin deivces, the following is writen on each of them:

A798 (or A79B)
BBP ( or 88P or BBFI or 88FI). I cannot tell for sure since I used mirrror to look on writings and they are very small.

 

[ecluser]

They are 2SA798, dual PNP. A complete datasheet is available here (3):

http://www.datasheetarchive.com/search.php?q=2sa798


It is possible that they are mismatched (within each pair), but there may be another cause for your offset since you have approximately the same offset for both channels.

Can you measure transistor's hFE with your meter?

I would like that you take two new pictures. The first one should be centered on C607 and close enough to show the colour bars on resistors. Try to cover an area of ~4 inches by 4 inches. The second picture should cover the same area but on the underside of the board.

Thanks!

 

[ecluser]

I would like to clarify why I asked you to take more pictures. I've found in my files some notes I took many years ago, when I worked on a similar unit. At that time, I've made a very partial schematic (not publishable) and from the schematic I have, I am quite confident that you will not fix this problem if you replace the 2SA798. The two new pictures will help me to revise and complete the schematic I have.

It is my believe that if you turn the dual transistors the other way, the bevel facing the opposite direction, your offset will be approximately the same magnitude and the same polarity. Actually, your offset is certainly positive. If your offset was caused only by a mismatch in those dual transistors, you would have a new offset of the same magnitude but of opposite polarity, by turning the dual transistors the other way.

So, if you turn the dual transistors and you have the same offset polarity, it is a waste of time and money to replace those dual transistors. Your offset, like in any other SA-200, is caused by an error of conception but it can be corrected.

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. Once the optimum bypass resistor is found, you can solder it in parallel with the actual resistor, and your offset will stay very low for years to come.

 

[kotofei]

ecluser,

Thank you for helpful ideas. In fact, I thought about something like changing a pair of the existing resistors to a variable pots in the hope that it will fix the DC offset problem. However, your idea with parallel resistors sounds more elegant.

It took me some time to make supposedly good pictures. Here they are:


I nave almost no soldering skills (just got the wrong Ph.D. back then) but for a long time I wanted to get my hands on some tweaking. Perhaps, it's time! This Technics, which I really don't need, could serve this purpose.

I would be happy to supply other pictures or measure the hFE (BTW, what's this?) and do other things.

 

[kotofei]

Here is the close-up of the R613-R614 area:

I wonder how one can solder a resistor in parallel to the existing one? From the bottom??

 

[ecluser]

Thanks for the pictures. It will help me to check my schematic. I'll be back soon.

It is preferable that you do some soldering practice on scrap boards, not on your SA-200. It is easy to find free CRT computer monitors or TVs. There is many useful components on them and they are great solder gun amusement parks!

hFE is the DC current gain of a transistor. It varies greatly from one transistor to another. If you have a DVM with hFE measurement facilities, you have a special transistor socket for this purpose.

 

[ecluser]

Here is the close-up of the R613-R614 area:

I wonder how one can solder a resistor in parallel to the existing one? From the bottom??

Yes, from the bottom. It's easy.

 

[ecluser]

Your pictures confirm the information I had previously collected on this circuit. The problem is that one side of the dual transistor has a collector current of nearly 1.1 mA, and there is only 0.3 mA on the other side.

Reducing R613 (R614) will bring the collector current closer to each other and it will reduce the offset voltage at the output. You will have less distortion too.

 

[kotofei]

What soldering gun do you recommend? I might have one but it's cheap and I would like to invest in the decent one. Someone here mentioned a solder with the vaccum add-on - is it a good idea to get one? Are any other devices (such as a stand for it) required?

 

[ecluser]

Mine is an old 50W temperature controlled soldering station, without vacum pump. I think I would be embarrassed with this pump most of the time. I am using a free hand suction pump for desoldering. I have no particular recommendation.