Autotek Mean Machine mx3000 protection

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I have a Autotek MX3000 that instantly protects on power up.

It uses a TL494 for PWM for the power supply and pin three is up around 4.78 volts and pin 1 is much higher than pin two. 11.3 volts on pin 1 and 3.14 volts on pin 2.

It looks like they are using a njm2904l as an error amp next to the TL 494.

The voltages on the pins of the 2904l are as follows:

1 0.00v A out
2 8.99v A input
3 3.22v A-input
4 0.00v gnd
5 11.95v input B+
6 5.00v input B-
7 12.01 B out
8 13.50 v vcc

I figured out that input A goes to the temperature sensor and it operated properly by heating and cooling the temperature probe.

I haven’t been able to figure out what could be driving input B high. Best I can tell output B is driving Pin 1 high on the 494.

There is no obvious damage and all of the power supply mosfets and output mosfets test good and the low voltage regulators operate briefly when the amp tries to power up.

I have included a photo of the area I’m concentrating on.

David
 

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I pulled the rectifiers from this and the amp powered up fine. So my problem is definitely in the output section. All of the output mosfets (IRF 9640 and IRF 640) check fine.

I turned my attention to the driver board and see what looks like optocouplers at first blush but on closer inspection reveals that they are IRFD110 power mosfets. There are two of them so I compared them. One appears to leaky between drain and source so I’m thinking this may the issue triggering the protection circuit.

David
 

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I don't remember what ti was but I remember strange readings on one of those FETs. Yours may be defective but do more troubleshooting before ordering them. You may need other parts. The Zeners on that board fail fairly often when the outputs fail (so may not apply in your amp).

That otoscope is the best thing I've found for reading resistor colors and markings on parts. Be sure that you don't clean it with alcohol or acetone. It will destroy the lens. Keep it clear of the bench so it doesn't get dirty.
 
I pulled the output section driver card and D11 and D12 on the back side of the card test shorted(5H marked). D11 and D12, with a diode check, test 0.019 forward voltage and the other 5H diodes test at 0.608.

I looked up the 5H marking and I get two possible results.
MMBD4148 standard diode
MMBD701 schottky diode

Perry, are these the diode you mentioned earlier? If so, should I replace them with 4148s?

David
 

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I ordered some of the daughter boards that Jason reverse engineered for this driver board and installed it in this amp and low and behold, the amp is no longer in protection, relays kicks in and I have good oscillation in the output section. I have to run a few more checks on the amp but I don’t anticipate any problems.

Thanks Jason for producing the daughter boards and thank you Perry for giving me the link to that thread where this is discussed. Time and money well spent.

David
 

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