MTX Thunder 942 drawing hard

[Dr Zeus]

Got this amp and found one of the output fets (FET207) shorted so I unsoldered it off the PCB. The amp now accepts power but blows 5A fuses. It will idle with a 10A fuse but appears tobe running hard. On the bad channel which I clipped the bad fet the source resistors are measruring 0.030vDC even with the bias pot fully counter clockwise. Turning it clockwise raises the voltage.

All output fets are measuring identically good on my DVM. I have a question about the drivers which appear to be 6491s and 6488s. Are the drivers supposed to be the same? IE, one channel has two 6488s and the other side has two 6491s. By the traces I thought this would have been a PNP and NPN pair - Leg two to leg three from left to right transistor are connected. Q90/Q1 and Q91/Q4 are whats written on the PCB.

Manual: http://www.mtx.com/caraudio/archive/...42_502_942.pdf

Both of these are 6488 transistors


Both of these are 6491 transistors

[Dr Zeus]

Can somebody tell me what R91 is supposed to be? Its unreadable and just popped off the PCB with no effort at all.

[Perry Babin]

The solder connections appear to be original on the 88s and 91s.

What's the resistance between legs 1 and 2 on the defective FET?

If it's less than about 100 ohms, it may have caused other FETs to become leaky. Measure the DC voltage from leg 1-3 for the remaining FETs in the blown channel. Is it identical for all of them?

I bet you haven't found a better meter for repair work than that Fluke 10.

[Dr Zeus]

On the defective fet:

Legs
1-2: 216 Ohms
1-3: 37 Ohms
2-3: 176 Ohms

All surviving Fets measure in the bank legs 1-2 measure ~5M ohms. All surviving fets on the PCB are measuring either 3.25vDC across legs 1-3 - Even the good channel is 3.2vDC across 1-3. I think the 3 remaining fets may have survived the attack.

Got R91? I say it might be a 1/8w 10k which I have.

This Fluke 10 rocks! My father bought it for me after I broke his larger dial-a-meter a long-long time ago. Its the best asset I've got, followed by the frequency generator, o-scope, then miniamp.

[Dr Zeus]

In the above picture showing R91, I also found D90 to be open and not working at all. On its back is E3Z. Is there a through-hole equivalent I can test with? Also D91 seems to be meauring funny in comparison wo other J3 diodes on the PCB.

[Perry Babin]

E3 is a 6v Zener. I don't know what R91 is.

I always recommend the Fluke 10, 11 and 12. I have a lot of other meters but I rarely use anything other than the 10, 11 or 12.

[Perry Babin]

Look at the attached image. The MPSA42 is an MPSA06 in your amp. Confirm that the connections are the same between the various components. If they are, R91 is likely a 1k.

[Dr Zeus]

I put a 1k in R91 because that schematic seems to be very very close. I also put a 5v Zener into D90 which replaced an E3Z diode - while I clear up any other surprises with this amp.

I get sound out of both channels. This amp has individual gains for each channel; adjusting the Left gain only slightly raises modulation, but also causes some improper woofer movement and popping.

I'm going to go ahead and adjust the bias and see how she sounds.

[Dr Zeus]

I'll call this amp fixed for now. Even with the 5v Zener in place, the guess work on the 1k resistor, and the missing output fet; both channels are sounding pretty excelent. I'll need to order a new IRFP240 output fet.

Do you think the 5v Zener can stay in place of the 6v zener? I measured other E3 diodes on the board and I get 6vDC across them. Of course the 5v zener has 5vDC.

[Perry Babin]

I don't know if the compressor circuit will work properly with the 5vZ but since I've never seen one work properly anyway, I don't think it matters much.

You need to get into the habit of repairing amps with proper replacement parts. Unless the owner needs the amp immediately, order the correct Zener.

You should also replace all of the IRFP240s that were in parallel with the one that failed.