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
PS: How can I insert pdf file or image file to post???
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I can email Schematics for your review
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
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
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)
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)
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Thanks for reply
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
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
schematic attached.
Here is part of schematic and hand drawing of actual on my unit.
Here is part of schematic and hand drawing of actual on my unit.
Attachments
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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
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
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?
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.
Hi Nattawa, Audiomachines
Thank you for your instructions. I will follow your instruction tonight.
Audiomachine, I sent pdf of HCA1000a schematic by email.
Regards
Thank you for your instructions. I will follow your instruction tonight.
Audiomachine, I sent pdf of HCA1000a schematic by email.
Regards
What is typical voltage of base of pre-driver?
I found both side predriver bases to ground voltage are -120mv(+ SIDE RAIL) and -130mv (- side rail predriver). I just need to find out faulty part on + side rail.
Thank you guys
I found both side predriver bases to ground voltage are -120mv(+ SIDE RAIL) and -130mv (- side rail predriver). I just need to find out faulty part on + side rail.
Thank you guys
that is odd, could you measure the B-E voltage of the predriver transistor, it should be around.5V-.7V if they are "on" good and conducting. Try to measure first the output of the voltage regulator Q113, Q114 Emitter to ground, to establish that we are getting a supply to the front end, the 120mV-130mV your measuring might be a stray "voltages" especially if your using a DVM. If your unit has fusible resistors I would check them also if they are good. Check also for crack solders especially the voltage regulator.
What are 0.05k 100, FULL PSB type Cap?
This amp had dried PS caps which I replaced with 15000uf 63v.
Last night I found shorted c216 and c116 caps and they are the reason for unresponsive bias adjustments. With caps removed, now bias adjustment are working fine. Caps are labeled " 0.05k 100, FULL, PSB". ( two of these are used per channel, paralleled- one per each channel is bad) Can some one kindly explain what this is ?
This amp had dried PS caps which I replaced with 15000uf 63v.
Last night I found shorted c216 and c116 caps and they are the reason for unresponsive bias adjustments. With caps removed, now bias adjustment are working fine. Caps are labeled " 0.05k 100, FULL, PSB". ( two of these are used per channel, paralleled- one per each channel is bad) Can some one kindly explain what this is ?
pics would be nice, probably it would be wise to recap the whole amp. Drying out the main filter caps is a major sign that all the caps on board has reach its max life cycle. hey the remotest possibility becomes possible =)
*audiomachines* is right. In my opinion re-capping with 80V or higher voltage rating caps for this amp is a MUST, because the factory 63V e-caps on the supply rails are in fact operating at about 69V voltage, at least so in my HCA-1000.
Just finished recapping.
Now my HCA-1000a has 15000uf 80v caps and all new Nichicon FW or MZ caps but I am still waiting on four 0.05uf 100v PSB(Polystyren??) cap replacements. I ordered 250v 0.047uf and 0.056uf polypropylen caps from Mouser with 3 Non polar caps. All circuit is functional including bias. I am exciting to see this one working with my rest of stack as soon as all the parts and heat grease are replaced. Thank you very much for your help.
Now my HCA-1000a has 15000uf 80v caps and all new Nichicon FW or MZ caps but I am still waiting on four 0.05uf 100v PSB(Polystyren??) cap replacements. I ordered 250v 0.047uf and 0.056uf polypropylen caps from Mouser with 3 Non polar caps. All circuit is functional including bias. I am exciting to see this one working with my rest of stack as soon as all the parts and heat grease are replaced. Thank you very much for your help.
Birdyman, I am resurrecting this old topic to see if you are still around.
I have a 1000a with erractic bias on the right channel. I just removed the board and will be testing things out this evening.
In the meantime, would you mind posting the specific parts you used? I am aiming to just fix the bias issue but any other replacements that you thought worthwhile are welcome. Thanks!
I have a 1000a with erractic bias on the right channel. I just removed the board and will be testing things out this evening.
In the meantime, would you mind posting the specific parts you used? I am aiming to just fix the bias issue but any other replacements that you thought worthwhile are welcome. Thanks!
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