MTX Thunder 942 drawing hard

This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
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.


Both of these are 6488 transistors

Both of these are 6491 transistors
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.
On the defective fet:

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.

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.
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.
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.
This amp still has a problem with the corssover. When set to anything but full range; the amp produced no sound. This amp is using surface mount TL074C opamps.

I think this is the first time I've had to work with actually fixing blown surface mount components and I've just been using temporary parts for troubleshooting. I've already added the needed parts to my digikey order; I just wanted to make sure the 5v Zener is safe while I wait for parts.

I wish these output fets didnt cost $5 each Ouch.
I've got some strange readings on both the TL074C opamps. Meter measuring from pin 4 to pin 11 shows 16vDC. Referencing amp ground, all pins measure between 5vDC and 6vDC with the exception of Pin 11; which measures 22vDC.

This amp also has TL072C and NE5532 opAmps.
NE5532 here in the middle is showing -15vDC on the outputs referencing amp general ground. I think this opAmp is used for the RCA outputs which this amp has a selectable crossover for.

All other opAmps on the PCB have approximatly -5vDC on the outputs referencing amp ground.

Last edited:
Ive still got a Diode (D91 near D90) which is measuring funny compared to diodes across the board. Its got J3 on its back and compared to other J3's I think is leaking but still checks somewhat OK. Off the board this diode measures 100ohms with reversed dvm leads. This diode's cathode leads to leg 1 of the top-left most 6488 transistor, and anode 'eventually' leads to one of the opAmp's (TL072C) Vcc- pin4.

What is a J3 surface mount diode? I tried google and its definatly a diode.
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.