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Old 17th June 2011, 06:19 PM   #11
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There won't be anything on 9 and 10. This amp drives the PZTA56s from pins 8 and 11.

Do you see drive pulses on the gates of the FETs?
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Old 17th June 2011, 06:47 PM   #12
Dr Zeus is offline Dr Zeus  United States
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Pins 8 & 11 have a saw-blade waveform +10v output. The fet gates have the same looking waveform but a +-10v. I should have measured this on the scope as my dvm was just reading 0v. Here is whats at the gates of the fets with 10v/Div and the scope set to DC triggering:

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Old 17th June 2011, 06:57 PM   #13
Dr Zeus is offline Dr Zeus  United States
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I think the power supply of this amp is fine: I lifted the center legs of the main rectifiers and the amp is steady with +-62vDC on on the rectifier center legs. There may be something inthe audio section dragging the ps down but it is going to be kind of tough for me to find out what exactly with the recifiers out.
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Old 17th June 2011, 07:12 PM   #14
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With the rectifiers in the circuit, how much current is it drawing?
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Old 18th June 2011, 01:47 AM   #15
Dr Zeus is offline Dr Zeus  United States
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While testing this amp I just burned out one of the fets. With the single surviving fet and the damaged fet now removed, the amp will draw more than my 10a power supply can provide; pulling the input voltage down to about 10v from 13.4v.
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Old 18th June 2011, 02:01 AM   #16
Dr Zeus is offline Dr Zeus  United States
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OK I think I'm an IDIOT! But at least I think I've fixed this little guy. The cap which I had previously removed; I had re-installed it backward! Not because I didnt try to, but because the can/casing of the cap I now notice is able to spin right around!

Also, I've been noticing this amp running about +-60 rail voltage. I'm not sure that is right, but the rail caps of this amp are 4700uF at 50v. I have some 4700uF 80v caps which are about to be installed.


Replaced the 4700uF 50v caps and now the amp powers up just fine. Also, rail voltage is +-40v not 60v.

Last edited by Dr Zeus; 18th June 2011 at 02:12 AM.
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Old 18th June 2011, 02:22 AM   #17
Dr Zeus is offline Dr Zeus  United States
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Well im further along but this amp still has problems. For a brief moment the amp played some clean signals on the scope, but the moment I attached the test speaker something happened and the signal completely degraded and distorted. Now its a bad signal all the time; almost as if something half blown like a driver just blew out under the slight speaker load.
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Old 18th June 2011, 06:38 PM   #18
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Do you still have all rail and regulated voltages (with the distorted output)?
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Old 19th June 2011, 12:53 PM   #19
Dr Zeus is offline Dr Zeus  United States
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With no input signal and no test speaker attached I've still got +-40 rail voltage. Regulated voltage at the 0740 opamp is +-17.5 with less than 0.100v on the output pins referencing amp's general ground.

Im not 100% sure whats going on here, because today everything appears to be working correctly with this amplifier: Speaker plays clean audio and it looks great on the oscope.


The only thing I can find wrong now with this amp is the power LED, which has been blown I believe ever since I started working on it, and also has ~41vDC across its leads. Its not worked since I had this amp on the bench.
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