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Restoring a Harman Kardon A402

Bonjour John,

Voici une explication plausible.

"Selon moi, tu as ce décalage car ta sortie n'est pas à zéro, mais à -0,027V. Mais quand tu mesures, tu te trouves référencé à la masse, qui elle est bien à zéro.
Refais ta mesure en mettant le moins de ton contrôleur au point nodal de tes 2 résistances de 3W."

Capture.jpg

"Donc c’est bien symétrique !
Tu as juste un très léger offset (tension continue) sur la sortie, ce qui n’est pas anormal.
Du coup on peut considérer le problème des amplis de puissance comme réglé alors ?"
 
I figured that the (+) or (-) indicator on your readings could have been probe related and wasn't concerned with that since the actual numbers appeared close. BUT, I thought that the issue was you can't adjust the bias setting using the trimmers VR401, 402. Voltages are off by roughly 1/2V at Q409 (bias Q). Low at the base and high at the emitter. Are you sure that it's installed correctly? Something's wrong if you can't adjust the bias, but my knowledge of circuits and their interactions is limited at best. Logic is telling me there's an issue with Q409 or the immediately surrounding (attached) components, but if you've tested/changed out those, and are sure they're functioning and installed correctly, then you have to look at components further removed (electrically) from Q409. I wish I could help more, but that's the extent of my knowledge.

BTW, Google translate is the bomb, filling in for my lack of French vocabulary. :rflmao:
 
Oui en effet merci Google.
J'arrive donc à obtenir un 0.035V sur les collecteurs, ce que je trouve étrange c'est que je sui presque à mi course du trimpot, et au delà cela redescend.
J'ai remonter Q409 d'origine, il n'est donc pas en cause à première vue, pour le reste changement de D407 (1s2076) par 1N4148, trimpots changer, j'ai branché les trois pattes du trimpot.
Merci Jean.
 
Here's two of your screen grabs of voltages, labelled for which post from. What parts did you change out to cause the incorrect voltages at collectors of Q407 and Q409 and incorrect base voltage at Q415 to become more correct, but in the process, cause the base of Q409 to now become incorrect? The emitter of Q409 is incorrect regardless of what you changed. Also, though I forgot to circle it, the collector of Q405 was off by 5V in the post #39, but no indication of what it's now reading.

A402 earlier work.jpg A-402 amp voltages.jpg
 
Looking at it more, Q409 is not turning on...no voltage out of the emitter which would counter (cancel out) the voltage coming from the Q411 collector, which is why you're seeing Q411s voltage at Q409 emitter. In the early case, it appears that the off voltage at Q409 collector is driving the issue. In the second pic, it looks like the incorrect base voltage is causing the issue. But which part(s) the bad actor???

EDIT: Have you signal traced (continuity checked with power off and unplugged) to make sure there's not a bad trace somewhere around Q409 (like at the base or collector)? Perhaps there's a solder bridge that shouldn't be somewhere, or even one MISSING that should be there. I had that happen when fixing a Sansui 7070. I was too meticulous in my soldering and didn't leave a bridge that was used instead of a trace in the board.

https://audiokarma.org/forums/index...pioneer-sa-8500-ii.710810/page-6#post-9630537
 
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Je vais tester les pistes autour de Q409, puis peut être remettre Q407 et Q411 d'origine, puis que la carte sera démonté.
 
Changement de D407 par l'original, pas de changement sur Q409.
Changement de Q407 et Q411 par original, pas de changement.:idea:
 
If anyone had an A402 on the bench and would be kind enough to take the voltage at the base of Q409 to confirm the service manual.
 
The collectors of Q407,8 should be the same as the collectors of Q409,10. On the voltage chart they are not, first mistake. The collectors of Q411,12 should be the same as the emitters of Q409,10. On the voltage chart they are not, second mistake. The emitters of Q409,10 should be -.63, the bases should be .02 and the collectors should be .61. VR402 controls the conduction of Q409,10, the more Q409,10 conduct the colder the bias and vice versa. In a working amplifier if Q409,10 were completely off the output transistors would fry. I can't figure out what D408 is doing, is it reversed on the schematic, it sure looks to be.
 
So, in the second pic I posted above (p.4 #65), HIS voltages appear mainly correct (red) and the SM listed voltages (in green) are incorrect. So, he SHOULD be able to set the bias. Yet, the trimmer can’t affect it enough to set the proper bias.

mine is opened up to clean the switches again and perhaps change those two resistors back to 470ohm due to the replacement outputs. BUT, I'm away from home til Friday night.
 
Great thank you, it's reassuring, if I understood the translation correctly, the service manual has errors, and my measurements are not that bad, I would like to..
 
I haven't forgotten about getting voltages. Was too tired after getting in from Zurich last night and had a bunch of stuff going on today.
 
Bonjour Jean

No problem Jean, I adjusted the bias and reassembled the amp, it has been running for three days, it's great, just a little scared on Friday, it rose in temperature for some unexplained reason, today it is at a normal temperature is works well.

une réponse de Alain91

IL ne faut pas prendre la masse sur le métal, mais sur le/les fils de masse qui viennent du milieu des condos de filtrage vers la carte audio
Au niveau des condos de filtrage ou au niveau de la carte ampli de puissance.
Tu ne connais pas les résistances parasites au niveau de la tôle !!!

De cette façon, les tensions aux bornes des collecteurs Q417, Q419 sont bonnes 0.02V.
 
Glad to hear it. I thought you weren't able to adjust the bias...that the trimmer had no real effect. I just finished measuring mine after resetting the bias to 15mV each channel. For what it's worth, here's what I measured at Q409,410 (BD139 not original Qs):

Q409
B -0.008mV
C -0.612V
E 0.615V

Q410
B -0.005V
C -0.615V
E 0.618V

Chassis ground for the measurements.
 
Yes, I cleaned them again but will be pulling the amp board to do the 470ohm resistor mod per the service bulletin. I also ordered a bunch of C0G/NPO caps and low value films that I may rework some of those parts too. But, I do enjoy the sound of it.
 
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