MTX RTP251D wont power up

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Ive got another one of these little amps which will not power up. I removed both power supply fets from the board because my meter is measuring 32ohms across Legs 1&2, only to find the 32ohms is something on the board and the fets are likely OK. There is no voltage on the gates of the fets.

TL494:

Pin 1: 0.001
Pin 2: 4.74
Pin 3: 0.000
Pin 4: 4.98
Pin 5: 1.512
Pin 6: 3.368
Pin 7: 0
Pin 8: 13.24
Pin 9: 0
Pin 10: 0
Pin 11: 13.24
Pin 12: 13.24
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 4.97
Pin 16: 0.000
 
Found a broken trace under a rail cap. Now I've got to figure out why it happened. This trace is directly connected to REM.

IMG_20110617_072849.jpg
 
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The amp now seems to want and try to power up, but the power supply fets are getting hot with power attached and the amp is draining my power supply.

In post one I had found 31ohms across legs 1 to 3 of both fets. I think there is some problem here maybe like a shorted driver or something causing the amp to bridge the gate resistors which are 31ohms.
 
There is no signal coming off pins 9 & 10 of the TL494. The signal from pins 9&10 run through a tiny transformer and then through a pair of ~60ohm resistors in paralel (~30ohms) to the fet gates.

TL494

Pin 1: 0.099
Pin 2: 4.74
Pin 3: 0.067
Pin 4: 0.833
Pin 5: 1.516
Pin 6: 3.337
Pin 7: 0
Pin 8: 12.90
Pin 9: 0
Pin 10: 0
Pin 11: 12.97
Pin 12: 13.17
Pin 13: 4.97
Pin 14: 4.97
Pin 15: 4.97
Pin 16: 0

LM339
Pin 1: 4.74
Pin 2: 0.001
Pin 3: 13.16
Pin 4: 4.76
Pin 5: 3.413
Pin 6: 0.987
Pin 7: 3.411
Pin 8: 4.093
Pin 9: 4.99
Pin 10: 4.99
Pin 11: 8.62
Pin 12: 0
Pin 13: 4.78
Pin 14: 4.78
 
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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:

IMG_20110617_144440.jpg
 
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.
 
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.
 
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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.
 
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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