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Bias problem on a KA-7100, new trimmer does nothing

tusntuk2 said:
I notice the s/m makes a distinction between IS2076 and IS2076A. Do you know what that is?
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The part ID should start with a 1 not an I. 1S2076 and 1S2076A are different in voltage. 1S2076 is VRM 35, VR 30 and 1S2076A is VRM 70, VR 60.
 
Busy days, sorry. Here's some voltages on transistors involved. Measurements were taken with the main outputs removed on the L, and with 1K resistors in place of Re57+59, per the dr's prescription. I have a full set of measurements, but will only include the most telling ones on the L side.

(S/m voltage)
Qe7 E (44.5v) 49v C (5.3v) 5.8v B (44v) 48.3v KSA1220

Qe9 E (44.5v) 48.9v C (1.15v) 350mv B (44v) 48.4v KSA1220

Qe11 E (-45.5v) - 50.4v C (-1.15v) -378mv B (-45v) -49.9v KSC2690

Qe13 E (1.15v) -350mv C (-1.15v) -380mv B (.6v) -300mv KSA992

Qe15 E (1.15v) 351mv C (0v) -7.2mv B (.4v) 1.83v KSA733c

Qe17 E (0v) -7.7mv C (-.4v) -2.57v B (0v) -6mv KSA733c

Qe19 E (-1.15v) -378mv C (0) -6.9mv B (-.4v) -2.56v KSC945c

Qe21 E (0v) -8mv C (.4v) 2.16v B (0v) -8.6mv KSC945c

and VR3 351mv / -346mv (Qe13)

Thanks for any new ideas!
 
Here's a better picture. Some of these voltages were measured over several minutes, so they seemed to change a tiny bit when I compared to tother readings. Overall they are very similar.7100 Trans Picture.JPG

There is a mistake at Qe11 E and Qe11 B - both of these values should be negative.
 
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Please measure the voltages on D5 and D7 A(anode) C (Cathode.) Also please check R41 value and then check continuity between pin3 on the board and the chassis, and power supply ground.
 
D5 anode= - 49.8v D7 cath= -50.4v R41 = 14.96 k Pin 3 (and 13, 16 through 21, 5) all "beep" with a continuity check to ground (chassis)

I see an error in the photo - the D5 anode and D7 cathode should be negative as reported here

Right side, D6 anode= -49.2v and D8 cat was -50.5v for comparison.
 
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tusntuk2 said:
D5 anode= - 49.8v D7 cath= -50.4v R41 = 14.96 k Pin 3 (and 13, 16 through 21, 5) all "beep" with a continuity check to ground (chassis)

I see an error in the photo - the D5 anode and D7 cathode should be negative as reported here

Right side, D6 anode= -49.2v and D8 cat was -50.5v for comparison.
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D5 or D7 is shorted.
 
Took out both diodes, checked with DMM and cheap trans. tester. Both checked OK, with normal readings (.563v, anode to cathode, 0L cathode to anode)

Replaced both diodes with new 1N4149, triple checking orientation. Passed DBT.

Full AC gave the same measurements as before, with D5 anode at -49.7v and D7 cathode at -51v. These are very similar to D6 and D8, the working side, as well as the voltages on the s/m

Sure appreciate you working on this with me, dr.! It gets a little "fusstratin'" sometimes.
 
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This is now normal. Before you got: D5 anode= - 49.8v D7 cath= -50.4v, which is only one diode drop, not 2. put the original resistors back in instead of the 1K's and put the outputs back in. See if it biases up now.
 
Unfortunately, no changes. Still have many odd voltages and 00.0v bias. After several minutes, relay clicked off when I tested a transistor, so I shut down again.

Have I made the right choices for transistor substituitons? Here's what I put in:

Qe 3, 5 (2SC 1775) NPN KSC 1845 f

Qe 7, 9 (2SA 915) PNP KSA 1220 ays

Qe 11 (2SC 1940) NPN KSC 2690 ays

Qe 13 (2SA 721) PNP KSA 992 f

Qe 15,17 (2SA 733) PNP KSA 733 c

Qe 19, 21 (2SC 945) NPN KSC 945 c

Qe 23,24 (2SC 1567) NPN KSC 2690 ays

Qe25,26 (2SA 794) PNP KSA 1220 ays

Qe 31 (2SC 1419) NPN MJE 15032 g

Qe 32 (2SA 755) PNP MJE 15033 g

Qe 33 (2SC 1222) NPN KSC 1845 f

Qe 34 (2SC 1213) NPN KSC 2383
 
Finally - success! I found the problem, fixed it, and the amp rocks. Simple but awkward, I want to share what I found so we can all keep learning

I went back to the bias pot itself to test it again, and found out it was NOT 500 ohm as prescribed, it was 500K ohm. Well that solved it. I don't know how I got a 500K ohm trimmer mixed into the Mouser order, or how I missed that little "k" on the side of my DMM whenever I tested it, but there it was. Once I changed to a 500 ohm everything worked just fine. Set the 0 and the bias and it sounds great!

Thank you so much to all who got involved! I am grateful for your time and the opportunity to learn. Thanks for sticking with me Warren, you're a good dr!
 
This was driving me crazy. I kept thinking I knew I could fix it if it was in front of me. Glad you got it done.
 
Glad you found the issue. :thumbsup:

A lesson to be learned here. Always check the incoming parts number . Long ago I put a driver transistor into an amp coming straight from a sealed Mouser bag that had the correct part number. My mistake was not actually double checking the part itself. Of 20 parts it was the only incorrect part in the bag. After some smoke and a whole lot more work/cursing I never trust a thing until I closely spy the part under magnification.
 
I once had a TO-92 transistor I ordered from MCM. It was supposed to have ecb basing but it was ebc.
 
Drove me crazy too, dr.! Been staring at it for two weeks, often with a 10x jeweler's loupe, and the frustrating thing was every part with new solder I looked at seemed OK. Until I started taking parts out one at a time, then testing, and finally found the wrong part. Good lessons for me too.

At least next time I see a 0.00v bias problem I'll know where to start looking. And the good news is...the amp sounds awesome. So clean. I'm happy!
(but not so good for The wife... "YOU'RE MAKING ME DEAF IN HERE!")

Thank you one and all! Next project is a KA-3500. I found one with a wood case in lovely shape. Never saw wood on a 3500 before. Maybe I'll wind up with a matching pair! I'll be sure to post a picture.
 
dr*audio said:
Have a look on YouTube for how to use an oscilloscope videos. There are lots of them. Bad small value (100pF or less) caps can cause the amp to oscillate. They are put in there to roll off the frequency response at some point above audio frequencies but below rf frequencies. The reason is complicated to explain and would require more electronic theory knowledge on your part.
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Thank you, this comment touched a nerve. I need to replace all the low value (nf) caps in my KA-7002. What type should I use? Polystyrene or mica? (I have not tyet checked this site for my question but will now.) Any comment is appreciated.
I ask because my amp works fine at high levels into dummy loads but trips at low music loads.

Thanks again.
 
dstariha said:
Thank you, this comment touched a nerve. I need to replace all the low value (nf) caps in my KA-7002. What type should I use? Polystyrene or mica? (I have not tyet checked this site for my question but will now.) Any comment is appreciated.
I ask because my amp works fine at high levels into dummy loads but trips at low music loads.

Thanks again.
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You need to figure out what is going on rather than just replace caps. Look at the output with an oscilloscope.
 
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