Yorkville 100watt Mixer/Amp - Replaced Transistors, distorted volume.

[jgrilo]

Hey all, new here and hope for some direction and assistance. Here is the backstory.

This amp has a bass/treble pot (50k linear) and the shaft was broken. I wanted to replace for cosmetics, amp functioned just fine.

Bought 2 pots from digikey and replaced. They were 50k linear.

Fired amp up and it didn't quite sound the same, played with bass/treble and turned amp up and then POP. Blew the 5amp fuse.

I replaced the fuse and every time i powered on the fuse pops again. Took it to a buddy and we decided I probably fried the 2 output transistors (MJH11018 and MJH11017). Now when i was testing/playing I never put the transistors back on the metal frame so we assumed they overheated and fried.

For added measure we replaced the BC550C transistor as well.

Order 2 orig 50k pots from yorkville and replaced them as well.

The leads on the BC550C were too long and was shorting out, we figured this out as BD139 was getting really hot. Fixed the leads and now the amp runs and sounds good at low volume. No fuse issues.

As soon as we push the amp more than 30% volume the sound is distorted and sounds terrible. No matter what input/channel.

The headphone jack works just fine and i can drive the amp loud and the headphones don't sound distorted.

However as normal the amp is not right, i don't know what i'm missing and what to do next? We're thinking of replacing BD139 as maybe the heat ruined the chip.

Here are the 2 transistors i replaced from DigiKey, slightly different spec but we were told they should be just fine.

https://www.digikey.ca/product-detail/en/on-semiconductor/MJH11022G/MJH11022GOS-ND/919545
https://www.digikey.ca/product-detail/en/on-semiconductor/MJH11021G/MJH11021GOS-ND/919544

I've also attached a schematic.

Could we somehow have switched up the NPN PNP transistors? if so would the amp work at all?

any tips would be super helpful! thank you!!

[Armadillo]

Can you show photos TOP and BOTTOM of the pots you have done the soldering.

Please tell us what measuring instruments do you have at hand.?

[Andy Watson]

Could we somehow have switched up the NPN PNP transistors? if so would the amp work at all?

Unlikely - I think it would have smoked again if the transistors were swapped.

Do you have the facility to measure the voltages at TP1 and TP2? Also, do you have another amplifier to check the loud-speaker?

[anachrocomputer]

The fact that the headphone output is OK must be a clue. Can you trace that part of the circuit and see whether the signal to it passes through the parts you've replaced?

[Ray Gianelli]

Did you check the output transistor emitter resistors?