Fosgate 1000a2 with blown output

[81300sd]

I had a couple of shorts in the PCB and even one on the Mehsa board. Got them fixed.

Now when I power it up I have +.7v on the output
Both the MPSA56 are getting hot

I am getting 15.4v on one side of C314(10uf cap) and .9v on the other side I have changed q326 and q328 chasing the drain with no change.

Something is switching q326 to ground.

Still searching

 

[81300sd]

I am getting equal voltage on all 3 legs of q323

I have desoldered and checked resistors r380 r382 r383 r384 r385 r387, all good.

 

[Perry Babin]

If D305 and D307 are out of the circuit, Q326, Q327 and Q328 are out of the circuit and can't be part of the problem.

Have you removed all of the jumpers?

 

[81300sd]

All the jumpers are removed. I had reinstalled D305 and D307.
I just removed them again. Now I have +.20v on the outputs.

Gate voltage now on the irf9640's is ~ -2.30v and bouncing around. irf640's is 3.03v and steady.

 

[Perry Babin]

Does the amp produce clean audio?

Do not play it for more than a few seconds.

Are the 56s still overheating with the diodes out of the circuit?

 

[81300sd]

Yes it is producing clean sound.

The 56's are producing some heat but not as much. They were nearly unbearable to hold, now they are just good and warm.

 

[Perry Babin]

When you measured the DC on the output, did you measure it across the speaker terminals?

 

[81300sd]

No, Black on chassis ground.

Across the terminals it is .16v

 

[81300sd]

I just found that I have 2.25v on the gate leg of one of my irf640's.

Could that be what is making my 56's run hot?

I adjusted R324 to get the 9640's up to the level of the 640's, seems to be more stable since I hooked it up to the speakers.

 

[Perry Babin]

If the voltage from leg 1 to leg 3 for any output FET is greater than on the others in the group, you may have a leaking output. If you pull the center leg out of the board for the one with the greater voltage, and the DC offset is closer to 0.000v, that FET is very likely defective.

 

[81300sd]

It was the resistor.

It all seems pretty stable right now. Both the 56's and 06's are running warm, but the other side is about the same.

Just need to fix the problem that removing D305 and D307 fixed.

 

[Perry Babin]

Reinstall one at a time to see which one is causing the problem.

 

[81300sd]

Its fine with them reinstalled. Must have been other problems affecting it.

Going to reinstall the Mesha board and test it out.

I really appreciate your help. No way I would have been able to get this working without you.

I have learned a lot about how these work and how not to solder and desolder.

Still have to get the Thermistor from Rockford. They do have it, I just need to find some more things to get to get over their $25 minimum.

 

[81300sd]

Well I hooked it up and ran it for a bit.

I burned up one of the power supply transistors Q8, MTP75NO5HD

When I screwed the Mehsa board down to the chassis It popped off the mehsa board. I should have reattached it.

I was pushing the amp pretty hard and it sounded good though, until the transistor cought fire

I cought it and removed power immediately, it doesn't appear to be any other damage. Is there a chance that everything else survived?

Do I need to change all those or can I just change the one. Of course I don't have any and I just made an order from Mouser yesterday. Maybe Rockford has them and I can get them with the thermistor.

 

[Perry Babin]

If the transistor detached from the strip, it's likely that they are not soldered down to the strip properly. Did you tin the backs of the transistors before soldering them to the strips?

If you cut the defective FET out of the board and the amp plays, everything else likely survived. You should confirm that the gate resistor for the blown FET is within tolerance.

 

[81300sd]

The first couple that I did(the power supply ones) I didn't add any extra solder to them and the one that blew definitely wasn't soldered down properly. I am going to redo a couple of them that I suspect may not be down good enough.

This happened purely because I was being a bone head. Lesson learned.


You did catch that it was a power supply FET right?

 

[Perry Babin]

Yes. I saw that it was the PS FET.

The transistors with the shiny/mirror-like finish don't accept solder very well. If that's what you have, scuff the back of the transistor with ~360 grit sandpaper and use flux to help promote better solder flow when you tin them. This will make it quicker and therefore less likely to cause damage to the transistors.

 

[81300sd]

It doesn't work on that side.

I have full voltage(12v) to the gates on the PS FET's with the bad one removed. The resistors are fine, I get ~7.5v on one side of the resistor and 12v on the other so I gather that I have at least one other FET that is shorting to the gates.

I am not finding any MTP75N05HD FET's but I did cross reference it and found a NTP75N06G that might work.

 

[Perry Babin]

It appears that you have shorted FETs on both banks if you have 12v on all of the PS FET gates. You'll have to replace all 4 of the FETs in that supply.

I'd use IRF3205s as subs. If you do, you'll have to replace the gate resistors. I'd use 47 ohm resistors but if you have anything from 39-51 ohms, those will work as well.

 

[81300sd]

I can't seem to find any of those IRF3205s FET's
Mouser lists a NTE2991 as a cross reference for a IRF3205.
The specs seem to be about the same.