Earth Module 440 Bass "Producer" Head (i.e Peavey Musician 400) Problem

[spy]

I'm following the previous posted Peavey 400 schematic, too. It's not exactly the same with Earth's circuit but it's close. Below the two 1K 5W diodes there is a 15 ohm resistor in parallel with a diode. Earth has replaced this diode with a resistor and a trimmer in parallel. Both of these were fried in my amp. I can read on the trimmer 100R but the resistor is totally fried and black. I can see only an orange line at the beginning. You can see them in my photo right next at the left of the totally fried 2W resistor.

I don't know with what I must replace them. What about a diode (1N4001 or 1N4148) and a trimmer? What does this trimmer adjust? The bias at the output transistors?

[spy]

With the covid issue I'm mostly at home so I have some free time now. I'll try to fix this amp but I'll need some help. Can anyone help me out with the trimmer and the fried resistor?

Also, this diode on the chassis (heatsink) is fried in my amp and yes it's a double diode. I can read MZ2361 on it. A lot of parts were fried, let's hope it'll work at the end..

[g1]

I'd think two 1N4148 in series would work for the MZ2361. Spec. is 1.2 to 1.4V forward voltage drop.
Datasheet attached.

[Enzo]

Or you could just order a double diode from Peavey. Just don't mention it is for an Earth unit.

[spy]

Ok, I have replaced all the fried parts and the double diode (stabistor) with two 1N4148 and the amp is on the bench for measurements! I powered it up and no smoke until now but I also have -40Vdc at the output so something is wrong.I followed Enzo's advice to check around that lower left 430 transistor, a TIP31C in my case and nothing is as should.

Here are some measurements,

430 or TIP31C
C: -37.5V
E: -39.5V
B: -39.5V

Between Stabistor plus diode//15R and 1K 5W: -35.3V
Between 1K 5W and 1K 5W: 1.5V
And 39.5V at V+ power supply rail

I suppose between the two 1K 5W resistors I should have around 20V and 1.2V and 0.6V at the bases of the two drivers, 430 and 431 accordingly. These two drivers are new as are the two small ones 3642 and 3638.

Here are some measurements at the input stage transistors,
Left 761 (2N5401)
C: -39.5V
E: -9.7V
B: -1.5V

Right 761 (2N5401)
C: -37.3V
E: -9.7V
B: -10.3V

I haven't replaced those two input transistors as they don't seem to be shorted or open and the diode reading between B-E and B-C is around 600mV in both of them.

I can't figure out what is going on here. I have measured all the resistors and the diodes and they seem to be ok.

Schematic is on page 2 or check the one attached, there are some measurements for reference on version B.

[Enzo]

On that 430, you show the exact same voltage on B and E, is the part shorted?

[spy]

No, I have the same measurements even with a new TIP31 there! The two diodes in series are not shorted too and I measure with my meter around 470R across the resistor.

[spy]

Please help guys! I can replace again that TIP31, you never know. What else to replace? What about the diodes between B-E? Anything else? What about the input transistors (2N5401)? I don't have parts so I need to order, let me know what else to put in the order.

[Enzo]

Fifteen or twenty new TIP31s likely won't make a difference if one new one didn't. You may have a bad part, but what if it isn't a pat at all? WHat if there was a cracked printed circuit trace?If the TIP31 amplifier has no good current path to +V that can be the issue. Perhapps the heat sensing diodes have lost their circuit.