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Accuphase P-266 bias adjustment

F

fox

New Member
Hi everyone!

I have changed all of the capacitors in this power amplifier, including the filter capacitors, as the old Elna's were very swollen. In addition, I've replaced all relays as well. I am very happy with the results! Now, I need to adjust the bias in the "Normal mode" (class AB) and class A. Unfortunately, I seem to have hit a dead end.

According to the bias adjustment instructions in the service manual, as I understood them, a voltmeter should be connected between + and - at the test point on the left and right channels, respectively.

In class AB mode, I am able to adjust the bias to "88mV". However, in class A mode, I cannot get the "-300mV" as shown in the service manual; the most I am able to get is positive 250mV on both channels.

Which makes me question two things:
1. Am I measuring at the correct place?
2. Perhaps the schematic contains an error?

In case it's important, I am powering the amplifier through a dim bulb tester with a 300-watt light bulb. In addition, the transformer's primary is configured for 220V, as the mains voltage here is 230V.

I've attached the service manual in question and some photos.
I appreciate any suggestions on how to adjust and test this circuit properly.
 

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fox said:
powering the amplifier through a dim bulb tester
Click to expand...

This is the reason you can't achieve the class A bias. It also distorts your class A/B adjustment.

I would suggest keeping it powered on the tester while you set both bias adjustments to, oh, let's say, 1/2 of the target values. Then power the amp normally and re-do the adjustments to the target values.
 
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I know I shouldn't, but I couldn't help pondering the class A output power specs for this amp, since the bias current is given in the schematic. Sure enough, 1.36A bias current works out to 30WPC RMS into 8 ohms in stereo mode, as specified. Good for Accuphase!

But the specs also say 110W for class A operation in mono. I assume mono mode is a conventional bridged output arrangement. Now, if the amp can produce nearly the same output voltages in bridge mode, then the total voltage across the load is nearly doubled, and power will be nearly four times higher compared to stereo mode. So far, so good. But output current demand will be doubled in bridge mode, so I don't think the amp can be said to be operating in class A at this output level. I think the true "pure class A" output in bridge mode would still be about 30W. It's too early on a Saturday morning, so my logic might be faulty. Am I missing anything?
 
chazix said:
This is the reason you can't achieve the class A bias. It also distorts your class A/B adjustment.

I would suggest keeping it powered on the tester while you set both bias adjustments to, oh, let's say, 1/2 of the target values. Then power the amp normally and re-do the adjustments to the target values.
Click to expand...
That solved it, thank you so much! After plugging the amplifier directly into the mains I was able to adjust the bias. Could you please explain why the dim bulb tester distorted the adjustment?

One thing that still bugs me about this circuit adjustment is that the schematic says that the bias in class A mode should be adjusted to "-300mV", whereas I adjusted it to "300mV". Am I missing something here?
 
chazix said:
I know I shouldn't, but I couldn't help pondering the class A output power specs for this amp, since the bias current is given in the schematic. Sure enough, 1.36A bias current works out to 30WPC RMS into 8 ohms in stereo mode, as specified. Good for Accuphase!

But the specs also say 110W for class A operation in mono. I assume mono mode is a conventional bridged output arrangement. Now, if the amp can produce nearly the same output voltages in bridge mode, then the total voltage across the load is nearly doubled, and power will be nearly four times higher compared to stereo mode. So far, so good. But output current demand will be doubled in bridge mode, so I don't think the amp can be said to be operating in class A at this output level. I think the true "pure class A" output in bridge mode would still be about 30W. It's too early on a Saturday morning, so my logic might be faulty. Am I missing anything?
Click to expand...
I am also interested in learning more about how this amp works in bridge mode.
 
fox said:
why the dim bulb tester distorted the adjustment?
Click to expand...

The DBT is in series with the amplifier power input, so the AC line voltage reaching the amp is reduced by the voltage that develops across the bulb. In turn, the amp's internal DC power supplies run at lower voltages than they're designed for. With these lower DC voltages, the bias regulation circuitry produces a lower bias current than expected. (A lot of "hand-waving" over details there. Hope it suffices.)

It might also be worth highlighting that when the amp is in class A mode it draws more current from the AC line than in class A/B. The higher current causes even more AC voltage to be lost across the light bulb, so even less AC voltage for the amp, so the above effect is more pronounced.

If the polarity of the measurement for setting the bias doesn't agree with the service manual, then I'd have to say that the manual has an error, which is not terribly unusual. Don't worry about it - it's not possible for the polarity to be wrong, so just pay attention to the magnitude.
 
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Thank you for the warm welcome and the knowledge you shared with me! This was my first big restoration project, and it was A LOT of work (and money). However, seeing the result makes it all worth it!
 
Just wanted to say, I recently picked up one of these - and it's amazing in every regard. Build quality, serviceability, sound quality. Accuphase really knew what they were doing. And to think, I haven't even activated it's class A mode, yet.

Unfortunately, was in the process of converting it to 120v properly, get it off the borderline undersized step-down, until the female spade terminal snapped on one end. Such a shame. Appears it fractured long ago, as the crack was highly oxidized under the microscope.

Needless to say, the Kenwood 700M has some serious competition. It's upcoming restoration better have some audible improvements, which I'm quite certain it will, least the TFM-25 did. If not...
 
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