Amplifier Distortion, DC-Offset, and You!

Just repaired a couple of output transistors on my KA-6100. Been back in service for a couple of months now
Left channel 9mV
Right channel 11mV
 
I've recently picked up an old Claricon receiver. Checked the dc offset and am getting approx. 147mV on each channel. A service manual is unobtainium. The question is where to adjust? Here's a nudie for reference:

C2DD8D04-A5CF-475B-ACF0-5A3F5B94AB0E_zpsaamgqeap.jpg


There's a pair of trim pots just southeast of the transformer pair. Would that be it?
 
Hi EchoWars, and Everybody!
( Sorry my English)
Please help me.
I have Old Amp, Yamaha A-750a. I am can not find Service manual this model. Please Idling current adjustment value?
I want adjust Idling for my Amp! Please! Now I am adling current adjustment same as Yamaha A-1020 ( look like a-750a): 8mV +/- 2
 
I've recently picked up an old Claricon receiver. Checked the dc offset and am getting approx. 147mV on each channel. A service manual is unobtainium. The question is where to adjust? Here's a nudie for reference:

There's a pair of trim pots just southeast of the transformer pair. Would that be it?

I'm not familiar with that Claricon receiver, but that looks like a cap-coupled output to me. That means that the two 500mfd/25V capacitors near the back, couple the signal from the output transistors to the speaker terminals. They are there to block DC voltage, and therefore the offset should be zero. I think you have to check the offset with a load connected to the speaker terminals on that one, in order for it to mean anything. If you do that, and you still have an offset, it could be due to electrical leakage through those caps. I'm not an expert on this by any means and I would suggest that you post a new thread in the Solid State forum if you need further advice. I can tell you this: Those trim pots are almost certainly not for adjusting DC offset and I would advise you not to touch them at all.
 
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B&K EX4420

R = 99.3 mVDC
L = 27.5 mVDC

No wonder I would hear a lot more distortion from the left speaker. Going in to adjust P1.
 
I noticed something very paculiar while adjusting the DC offset. After adjusting the P1 pot to around 1.0mVDC on the left channel, I thought I was finished. That was until I put back on the cover and the DC offset raised above 200.0mVDC on the left channel. I ended up adjusting the P1 pot without the cover of course, to -135.0mVDC, put the cover back on and now the left channel reads +0.8mVDC.:dunno:
 
What a thread :jump:
I read every inch of it beeing at our cabin by the sea for the easter weekend..like the most exiting book i've ever read. I may have been acused of beeing antisocial, not that I would have noticed
I was counting in my head all of my old amplifier channels, 14 of them, feeling an urgent need to get back home checking for small millivolts of unwanted marching electrons wanting to nuke my precious Klipsches.
Best regards and a big thank you to the starter of this thread
 
Wow, I couldn't read this thread in it's entirety if you paid me! But could be the thread of the century here at AK. Yes, thank you EW for improving the sound and education of many Aker's. You could simply feed us fish but instead we learned how to catch our own!
 
1992 NAD 2700THX with a NAD 917 pre

10 min L 1,1 - R 2,0 mV
20 min L 0,8 - R 2,4 mV
40 min L 0,6 - R 2,7 mV
80 min L 0,4 - R 3,0 mV
160 min L 0,3 - R 3,1 mV

Love this amp
 
Yamaha R1000 bought this unit new in 1983, recaped and balanced in 2010, pots treated to DeOxId and the main vol pot changed to an ALPS (OEM unit) 100K ohm with 150k ohm resistors in paralell with each gang to bring it back to 60k. left amp offset 7.5mv, right amp -7.3mv. Idle or quiescent current 37ma. The idle volatge is 8.1mv / .22 0hm for the current. Just for the heck of it I measured Vee at -73v and Vcc at 73v, supprised me to see it that high. The low was -1 and +1. Lots of heat sinks and a computer cpu fan (slowed down for noise with an 80 ohm resistor) on IC412. That thing would burn you if you touched it before, now it runs just a few degrees above ambient.

Sure like this old unit.
 
1984 NAD 3020B

10 min L 9,2 - R 28,6 mV
20 min L 9,4 - R 23,5 mV
40 min L 9,6 - R 21,8 mV
80 min L 9,9 - R 20,4 mV
160 min L 10,1 - R 20,2 mV
 
1992 NAD 906 6-channel power amplifier

10 min 1: 27,4 2: 31,8 3: 37,4 4: 34,9 5: 36,2 6: 33,1 mV
20 min 1: 26,1 2: 30,4 3: 35,8 4: 33,5 5: 34,8 6: 31,8 mV
40 min 1: 25,8 2: 29,9 3: 35,4 4: 33,2 5: 34,4 6: 31,5 mV
80 min 1: 25,4 2: 29,8 3: 34,6 4: 32,7 5: 33,5 6: 31,0 mV
160 min 1: 25,2 2: 29,5 3: 34,2 4: 32,5 5: 33,0 6: 30,8 mV


I will clean the internals and try to adjust offset a little better, probably not opened since -92.
EDIT:
After cleaning I adjusted the idle current to 5 mV according to the manual, however there seems to be no adjustments for offset, so the DC voltage at the outputs remains the same as before.

Another thing I noticed:
When bridging channel 1+2, 2+3 and 5+6 I got much lower readings of DC
1+2: 2,7mV
3+4:1,7 mV
5+6: 4,7 mV

I dont know why, maybe sombody can explain this :confused:


"Before" pic
18110240_10154461858717551_514274402_o_zpsrbudddzi.jpg


After cleaning
18120462_10154466353732551_579632812_o_zpsiyteeqxz.jpg
 
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ADCOM GFA-545. Checked & adjusted bias pots to 6.5mV (spec), left channel DC offset 35 mV, right channel 57 mV.
 
In posting #4416 I reported the numbers for an ADCOM GFA-545. Today I opened it up and replaced the input differential pair transistors with HFE-matched
Fairchild KSC1845EBU that I have a bagful of. Not knowing what hfe's the original 2sc2362's would have, I removed Q601, Q603 and Q605 (all are 2sc2362's) from the pcb's and measured them all, with the thought in mind that if their hfe's were higher than my KSC1845EBU's I might switch them round for better matches where it counts. The right channel showed Q601 at 495, Q603 at 353. This is the differential pair, and they certainly did not match! Q605 measured 440. Left channel, Q601: 413, Q603: 417, a pretty good match! Q605 measured 469. So I replaced all six transistors: both pairs were matched as precisely as I could: hfe=411. The other two (Q605) were very close in hfe, but they needn't match the pair precisely. I also replaced the two electrolytic caps on each board.

Reset the Bias and checked DC offset. Results: Right channel is now 26.5 mV (was 57). Left channel is 28.5 mV (was 35). I suppose this is as good as it can get unless I start replacing other components that seem to be working fine.
 
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