MTX Thunder 6500D burnt

[ifrythings]

I got this amp blown from a friend that gave it to me. It has previous work (notibly resolding) and missing output fets (FET101- Fet104 are missing). The power supply seems to work but I have some burnt resistors that I have no idea what value they are and what the output fets should be. If anyone can help me with this it would be highly apperiated.

An externally hosted image should be here but it was not working when we last tested it.

 

[Perry Babin]

140 and 142 = 475 ohms

139 = 221 ohms

132 = 1k

CAREFULLY check Q115, 1116, 118, 122, 121 and 125. Lift two legs to isolate them from the board when checking them. Sometimes one of the transistors operating in parallel with another transistor will fail and if you don't catch it, the amp will fail prematurely when the other parallel transistor fails.

With a high power lighted magnifying glass, carefully examine all of the resistors near the LM311.

Desolder and resolder the connections on all of the PZTA transistors that have been running hot.

 

[ifrythings]

Thanks for the super quick reply Perry Babin.

R139 doesn't make sense to me, should that be a 220 ohm resistor, it's markings are 221?

I had already resoldered all PZTA transistors cause they looked hot.

Also found so far Q116 shorted (B-E), checking other asoon as I have some dinner (at 10:50 pm at night )

 

[Perry Babin]

2210 is what it should be. Most of the resistors are 1% tolerance and have 4 digits on them.

 

[ifrythings]

Are the 475 ohm resistor (R140, R142) 1% or 5%?
I'm guessing the 1K (R132) is 1%, all the other 1K are 1% so it makes sense.

Just checking, are Q116, Q119 (Both say 2L) - MMBT5401's?

And what are the output mosfets (they were missing when I got the amp)?

Thanks a lot for your time and help Perry

 

[Perry Babin]

In general, all of the small resistors in this amp are 1% components. The 475 ohm parts should have been marked 4750.

Yes. MMBT5401. MMBT5401LT1G for the on-semi part.

The 6500Ds used SSP45N20A outputs. They are now obsolete. I've been using IRF3415s with no problems.

 

[ifrythings]

OK, I've changed all the burnt resistors with new 1% ones. I have to order the MMBT5401LT1G and the IRF3415. Is there anything else I should check? Around the LM311?

Q115, 1116, 118, 122, 121 and 125 are all good, pulled them out of the board and checked them.

Thanks Dave.

Edit: P.S gate/source/drain resistors all test good.

 

[Perry Babin]

If the 0.22 ohm resistors survived, the components around the LM311 have probably survived.

After replacing all of the components, clamp the transistors to the heatsink before applying power.

 

[ifrythings]

Can I fire this thing up with out the output fets and check it's waveforms? or can I sub some cheap IRF640 in it for basic testing? I'd rather not use the expensive fets for testing, even though I do have a current limited (10A) power supply?

 

[Perry Babin]

There will be no nigh frequency drive waveform on the outputs without the transistors in place.

These amps can be very sensitive to the outputs used. If the amp doesn't work properly with the 640s, it may not mean that there's a problem.

If you clamp the FETs to the sink, there's very little chance that you'll damage them with a 10 amp supply.

 

[ifrythings]

Well, I tried the IRF640's (2 of them only, not 4) and it seems to work, it drove a 8" speaker to it's maximum excersion. Everything seems ok. Now I'll have to order the FET's you mentioned above.

Thanks a lot Perry for helping me fix this amp, saves another amp from the junk yard.