I am working on a Technics SU-50A. The Right channel can be perfectly set to 25mV of bias and close to 0mV of DC offset. The Left channel's DC offset is about 155mV when RV17 is turned all the way. It's bias is ok. I have checked the differential pairs, TR17/19 and TR21/23. They seem to match well.
Can someone educate me how dialing RV17 can affect the DC offset? What should I do to bring the DC offset down?
Can someone educate me how dialing RV17 can affect the DC offset? What should I do to bring the DC offset down?
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RV17 adjusts the amount of tail current through the input transistor pair. Shifting this current slightly can bring the pair in to a current point where they match more closely. Usually if the offset cannot be adjusted then the pair is too far out of match. If they are well matched as you say then something else in the circuit is causing the offset imbalance.
Oh yeah I just noticed that one. Maybe RV15 is a coarse adjust and RV17 a fine adjust. Don't know but his serv manual should say.
The parts list in the service manual says RV15/17 are for DC offset adjustment, but the service manual says nothing about the procedure.
I tried RV15 but it has slight effect on the offset.
What else should I look into? Many thanks.
Do you have access to a curve tracer? How did you test the diff pairs to know that they match?
If you used a battery operated tester, it won't tell you if the transistor is leaking at operating voltage.
I would obtain some new replacement transistors and replace the differential pairs, check the adjustable resistor. Also, check the resistors around the diff pairs for out of tolerance or shorts.
If you used a battery operated tester, it won't tell you if the transistor is leaking at operating voltage.
I would obtain some new replacement transistors and replace the differential pairs, check the adjustable resistor. Also, check the resistors around the diff pairs for out of tolerance or shorts.
You might also unsolder one leg of C70 and C72 (and see if the offset decreases). They're low voltage types, and in spite of having 39K resistors in series, each power cycle probably subjects them to voltage stress.
Cheers
Cheers
I would tweak RV17 until you get the correct bias point (1.1 V on the emitters of the input pair). Then tweak RV15 for minimum DC offset.
I doubt transistor matching is super critical. It would have been incredibly expensive to match the transistors in production.
Tom
I got it nailed. It was still the differential transistors. Thank you very much for your input and help.
@Duke58 was spot on. I don't have a tracer, but just a cheap component tester. Although I got pretty matched reading from the pulled out parts with the tester, they still don't match in reality. Replacing them solves the problem. I also replaced those in the good channel for peace of mind.
@Duke58 was spot on. I don't have a tracer, but just a cheap component tester. Although I got pretty matched reading from the pulled out parts with the tester, they still don't match in reality. Replacing them solves the problem. I also replaced those in the good channel for peace of mind.
I had the same problem with my Marantz 1200. The battery powered transistor checker I bought online showed Hfe matched.
It wasn't until a friend showed me how to use a curve tracer to check the differential pair and showed me one of the transistors broke down (leaked, whatever it's called) before it was at operating voltage.
Glad you got it fixed!
It wasn't until a friend showed me how to use a curve tracer to check the differential pair and showed me one of the transistors broke down (leaked, whatever it's called) before it was at operating voltage.
Glad you got it fixed!