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Old 10th January 2011, 06:06 PM   #881
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If removing Q3 doesn't work, C1 is my semi educated guess. If its value is too low you will lose bass response, and you may be overdriving the input trying to hear something out of an amp with no response below 1KHz or so. Assuming low to no DC output on your pre you can temporarily short C1 as a test. If that make the amp work, replace C1 with another of at least 1 uf.

Edit: Could you have swapped C1 and C5?

Last edited by BobEllis; 10th January 2011 at 06:16 PM.
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Old 10th January 2011, 06:28 PM   #882
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Anil, please double check by measuring the values of R1 thru R4 and compare with your good board...while at it also check solders/connections on these.
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Old 10th January 2011, 06:33 PM   #883
anilva is offline anilva  India
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Quote:
Originally Posted by Bill Fuss View Post
Check the Vbe of Q4 of both channels.
Posted the values in the earlier post. Exactly same for both channels.

Quote:
Originally Posted by Zen Mod View Post
voltage across R17 seems good ,same as offset, so Q4 probably isn't a problem

remove Q3 and try
Remove Q3.. and replace with another one? At the moment I do not have a replacement ZTX450. I need to remove from a working F5 if needed.

Quote:
Originally Posted by BobEllis View Post
If removing Q3 doesn't work, C1 is my semi educated guess. If its value is too low you will lose bass response, and you may be overdriving the input trying to hear something out of an amp with no response below 1KHz or so. Assuming low to no DC output on your pre you can temporarily short C1 as a test. If that make the amp work, replace C1 with another of at least 1 uf.

Edit: Could you have swapped C1 and C5?
No, I have not swapped C1 and C5. I can check if C1 is faulty. I can short it and check. There is no DC output on my preamp. It is the same preamp which drives my F5.

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Old 10th January 2011, 06:36 PM   #884
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Just pull Q3 - it is protection, not critical for circuit operation. ZM is thinking that it may be turning on too early.
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Old 10th January 2011, 06:58 PM   #885
anilva is offline anilva  India
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Quote:
Originally Posted by Blues View Post
Anil, please double check by measuring the values of R1 thru R4 and compare with your good board...while at it also check solders/connections on these.
Have done that earlier. I will double check once again.

Quote:
Originally Posted by BobEllis View Post
Just pull Q3 - it is protection, not critical for circuit operation. ZM is thinking that it may be turning on too early.
Is it the case with Q4 as well? I will remove and check.

Thanks everyone for chipping with help. I really need.
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Old 10th January 2011, 07:01 PM   #886
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Q4 is part of the active current source. It is needed to control the current in Q5. Removing it will ruin everything.
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Old 10th January 2011, 07:26 PM   #887
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As dc conditions are ok, i would check connecting devices.
For instance , if you use a shielded input cable, a thin wire could partially short the input.
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Old 11th January 2011, 04:43 AM   #888
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Quote:
Originally Posted by bobodioulasso View Post
As dc conditions are ok, i would check connecting devices.
For instance , if you use a shielded input cable, a thin wire could partially short the input.
Good point...one strand of wire bridging signal to ground can wreak havoc...
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Old 11th January 2011, 05:28 AM   #889
anilva is offline anilva  India
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Checked R1-R4 with DMM and all the values are spot-on.

Removed Q3 as suggested. No change in the behavior of the circuit.

Measured preamp output DC offset at 1.2mv. With that confidence shorted C1. Gain jumped and bass came back. Did not listen to enough music. It was just for a few seconds, since amp offset jumped to 140mv when C1 is shorted. I will replace with a different cap and check. There seems to be some hope.

Will post the developments.

Thanks.
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Old 11th January 2011, 06:53 PM   #890
anilva is offline anilva  India
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Default Tragedy...

Quote:
Originally Posted by anilva View Post
Checked R1-R4 with DMM and all the values are spot-on.

Removed Q3 as suggested. No change in the behavior of the circuit.

Measured preamp output DC offset at 1.2mv. With that confidence shorted C1. Gain jumped and bass came back. Did not listen to enough music. It was just for a few seconds, since amp offset jumped to 140mv when C1 is shorted. I will replace with a different cap and check. There seems to be some hope.

Will post the developments.

Thanks.
I bought new 1uF input capacitor, unmounted the PCB from the heatsinks, replaced the old 1uF with the new one. Resoldered the removed Q3 previously and reseated the JFETs, which had gone through some bit of manhandling earlier. Checked for shorts, cleaned up and mounted back on the heatsinks.

Connected this amp board to preamp and speaker. Started playing music for the first time from this board. No problem. Gain was there, frequency response was there. Extremely delighted. Quickly changed the old test speaker to my regular Fostex fullrange, eager to hear the music.

Turned on, nothing happens. Noticed one of the wires of the primary of the transformer has come loose from the connector. Reconnected and switched on. Nothing. Checked fuse. Blown. Replaced fuse. Blown again.

Disconnected my speaker. Connected the light bulb back in series with the power and checked after replacing the fuse. Bulb glows full bright indicating short. Disconnected the amp from the power supply. Bulb comes up and turns off, indicating the problem is with the amp board.

With the amp connected back, the bulb continues to glow. Amp output measures around 220mv. Checked heatsinks for any short with the MOSFETs. No problem.

Need advice, as to how to debug the faulty board. Are the outputs blown? or is the other transistors? I am completely disappointed. I am not back to square one, but back to -10.

TIA.
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