peavey 112 transtube overheating problem

[peter jeurissen]

Hi, I've got an old Peavey transtube 112 on the bench with an overheating problem....
It works fine for about one minute and then it starts falling out and coming back.
On the large heatsink there is a large overheating protector mounted which switches of the current in the power stage.
The output transistors are indeed very hot and so are the big white "sandblock" type resistors.
Visual inspection of the PCB did not give me a clue........

Any suggestions?

[Jazz P Bass]

Disconnect the speaker & tell us if it still gets hot. (most probably not)

If it does not get near as hot, you may have a blown output stage.

[J M Fahey]

Looks like it's heavily overbiased and running so hot that the thermal protectors cut in. Hair rising .

Start disconnecting the speaker or any load, turn all controls to 0, measure voltage across output transistors emitter ballast resistors and postv results.

Such as: "I measure 500mV across 0.33 ohms 5W resistors" or whatever you find.

Also post main rail supply voltages.

With that we can calculate idle power being dissipated in power transistors, which must be very high.

Nothing further unless somebody posts the schematic.

By the way, it's still alive just because it's a Peavey ... many others would be dead by now.

No need to say that as soon as you measure and write those voltages you turn it off.

[mozwell]

schematic attached

Some quick measurements needed
Power supply voltages (+/- 50V ?) across C111 & across C124
Power supply voltages +/-15V, from op amp pin 8 & pin 4 to pcb 0V
Volts DC across each of the 0.33 ohm 10W resistors on the emitters of each of the power transistors
Volts DC between the base of Q28 & Q37

With power off & the power supplies discharged, measure ohms on each of the 0.33 ohm 10W, and from each power transistor collector to emitter, and Q28 & Q37 collector to emitter....

[Enzo]

My first suspects are the bias diodes. Look on the schematic and see CR53,60,64,66,71. Note CR66 is a dual diode inside. Find CR64 and tack a shorting wire across it, or use a small clip wire. Does that make the amp run cooler?

[peter jeurissen]

It's hard to find the right schematic.......
I think there are a lot of variations made by Peavey
This is a sheffield equipped made in china serial G007083
See posted pictures...

I'll soon post the voltage readings......

[Enzo]

OK, the model number is Bandit 112, not transtube 112. That will help find the schematic.

Your board looks like the one in the schematic I have attached.

My previous comments were based on the other schematic. The power amps are similar or the same, but the part numbers are different.

[peter jeurissen]

I've found there is more wrong with the amp so I decided to use this one as a donor-amp......
Thanks for all the help and good advice guy's!

[Roflberry Pwncakes]

I had a Bandit 112 and fried the 22Ω 1W resistor, the component ID is R131. It's directly on the rail between the intermediate and final amp stages. It literally turned to ash, when I touched the component, which was completely charred, it just disintegrated. After getting a schematic to see why it let the magic smoke out I saw that 3 or 4 components on that rail had been reworked by whoever owned it before I did, so it had a history of overwatting. I had a hunch that the transistor in the final stage was in a thermal cascade failure state and would continue to pull too much current for the components in the intermediate stage to handle. So I contacted Peavey to order new transistors for the entire power amp phase just in case the initial and intermediate xsitors had also overheated and were beginning to cascade. Peavey said they didn't sell individual transistor pairs (it's a class AB output so the transistors for each stage have to be matched to each other's individual characteristics) and they only sold complete parts lists, and the price for the entire list was more than I paid for the amp. So I wound up keeping the Sheffield speaker and buying a used 75w Line 6 Spider III digital modeling amp and put the Sheffield speaker in the cab.