Author Topic: Sanyo JCX 2900KR Power Stage Adjustment  (Read 689 times)

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Offline Nebukad

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Sanyo JCX 2900KR Power Stage Adjustment
« on: November 16, 2016, 09:13:24 pm »
Hi there,

I've been working on an old Amplifier/Receiver for quite a while now. It is the Sanyo JCX 2900KR - the biggest version of this Amplifiert that was available at that time (~1978).
I've already done several things to it like replacing the burnt X-Class Caps on the Mains-Input, changed the Output-Relays and cleaned some of the switches.

But there is one thing I could not figure out yet, whitch is the Adjustment Procedure of the Power-Stages. Each PowerStage (Left and Right Channel) has two pots on it, one says "BIAS" (SVR702) and the other one says "CENT" (SVR701). Please refer to the attached part of the schematic.

Sure enough, the BIAS-Pot controls the Bias-Current for the Power-Transistors. It can be measured by measuring the voltage on  the power-resistors on the Emitters (R724 and so on). It adjusted the current to give me about 7mV on each resistor resulting in ~14-15mA bias-current. This is about the maximum bias-current I can get. Since the Powerstages don't get warm at all I think choosing the highest possible bias-current can't be to bad of an idea.

Strangely though the CENT-Pot does not adjust the DC-offset as one would think. Both channels have almost no DC offset (-1mV and +3mV) so this is not really a problem. But still I would like to know, what this Pot is doing and how to set it.

Sadly the Service-Manual does not deal with the adjustment of these pots. There are several pages about the adjustment of the Tuner-Section but the Pots on the Power-Stage are not mentioned with any word.

So if anyone has any information or experience in the Adjustment procedure of these power stages it would be great to get some information on that.
Maybe someone, who is more into the design of Class AB-Amps, can figure out what the CENT-Pot is doing in that amplifier-stage and how it should be set.

Thanks in advance!


 


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