Parasound HCA-1000a Bias Circuit Correct??

[birdyman]

I have Parasound HCA-1000a with cross over distortions on both channel which is prominent when volume is low. So I decide to take Mr. John Curl advice to turn up the bias high. I connect meter across emitters of q119 and q122 and turn TVR1 and TVR2,-resulting no reading change from 0.0 mV ( both ch). I traced circuit which is slightly different than schematic. I found some thing odd about bias circuit..Can some one explain how this circuit works?

Birdyman

PS: How can I insert pdf file or image file to post???

[birdyman]

I am not having any luck to attach HCA-1000a schematics for posting. If anybody who can post pdf for forum discussion, is interested, PM me with email address. I will email you the schematic.

Thank you in advance

[nattawa]

I have the schematic in pdf but it's over the attachment size limit even after compression. Any one knows a way to size down a pdf? Can't send by email due to the IT restriction at work. Will try when get home after work.

A very brief description about the biasing circuit would be like this:

the voltage between the base of Q111 - Q112 is 2*Vbe, or 1.1V, therefore the current over the resisters TVR101 and R116 is

Ib = (2*Vbe)/(R116 +TVR101) --- so trim the TVR101 will change Ib

base current of Q111 and Q112 is negligible compared to Ib, then bias voltage (between the base of Q115 - Q116) would be

Vb = Ib * (R114 + R115) + (2*Vbe)

[birdyman]

Can you change bias voltages on your unit by adjusting TVR1 and TVR2?
I did not see any change of across-emitter-voltage on my unit on both channels. Initially, I thought faulty bias parts on both channel. All bias parts checked out to be normal. Then I look deeper on schematic, and found all trim pot are located between bases of 2 bias complementary transistors. I thought one trim pot should be located between collector to base to skew base voltage when it is turned (I can be wrong on this).

Thank you in advance

[birdyman]

Here is part of schematic and hand drawing of actual on my unit.

[audiomachines]

hi,

could you post the measured voltage across the base of Q116 to base of Q115? Please send me a copy of the schematic. You may send it to jade_808a@yahoo dot com

regards

[birdyman]

It is 0.7mv.
Thanks

[nattawa]

Originally Posted by birdyman

...........Then I look deeper on schematic, and found all trim pot are located between bases of 2 bias complementary transistors. I thought one trim pot should be located between collector to base to skew base voltage when it is turned (I can be wrong on this).

Thank you in advance

I definitely can trim the bias voltage in my unit.

The two pots are where they should be. the 200R pot serves for bias fine tune.

A 0.7mV between the base of Q115 - Q116 doesn't sound right. I'd turn off power and measure resistance between the base of Q115 - Q116, expecting a high resistance on a DMM. If I do get a high resistance then the 0.7mV would indicate the VAS transistors do not turn on at idle, meaning the DC setup with the input stage has problem.

Have you checked all the supply voltages (across all 47uF/63V caps) are good? How about Vbe of Q109 and Q110?

[audiomachines]

could you check C115,C116 for "leaks" thou it is a very very remote possibility, It might be clamping down the voltage across the driver transistors, you could also check the servo opamp that might be messing up the differential input stage.

[birdyman]

Hi Nattawa, Audiomachines

Thank you for your instructions. I will follow your instruction tonight.

Audiomachine, I sent pdf of HCA1000a schematic by email.


Regards