Peavy TKO 80

[pbiagi]

Hello,

Could use some guidance in restoring a Peavy bass amp. I could be wrong but believe there are two versions of the TKO 80, and this is the older one, from late 70's.?

When turned on, the amp makes a hum, volume control makes no difference, and does not amplify a source.

My son and I changed out the filter caps, but no apparent change. We did not try connecting the speaker but could observe the semi-saw tooth output on the oscilloscope. Looks like strong ripple to me. It's about 1.5V, see photo. We had to take the picture quickly because we noticed a 0.33 ohm 5 watt resistor started smoking, circled in photo.
The frequency is 100Hz, see photo. (I live in Europe). Both the V and Hz were taken on the output where a 8ohm 25W resistor replaces the speaker.

Could someone point us to a schema and layout diagrams?

We have some experience but obviously are too novice to know next steps. Appreciate your help.

Paul

[soundguruman]

Hello,

Could use some guidance in restoring a Peavy bass amp. I could be wrong but believe there are two versions of the TKO 80, and this is the older one, from late 70's.?

When turned on, the amp makes a hum, volume control makes no difference, and does not amplify a source.

My son and I changed out the filter caps, but no apparent change. We did not try connecting the speaker but could observe the semi-saw tooth output on the oscilloscope. Looks like strong ripple to me. It's about 1.5V, see photo. We had to take the picture quickly because we noticed a 0.33 ohm 5 watt resistor started smoking, circled in photo.
The frequency is 100Hz, see photo. (I live in Europe). Both the V and Hz were taken on the output where a 8ohm 25W resistor replaces the speaker.

Could someone point us to a schema and layout diagrams?

We have some experience but obviously are too novice to know next steps. Appreciate your help.

Paul

"a 0.33 ohm 5 watt resistor started smoking, "
I think that means you have at least 1 shorted output transistor.
I will just pull them all and test them all.

[Jazz P Bass]

Peavey TKO 80 Schematics

It would be best if you did Not hook up a load to the output, until the amp is stable.

[pbiagi]

Thank you Jazz Bass.
It is the '78 design - what you sent is perfect. (Seems Peavey has a '78 and a '90's model).

Stupid question - by Not hooking up a load, you mean a speaker?
I haven't checked but assume the existing speaker is shot. I don't remember the amount, but measured high DC on the output.
Or do you mean not hook up a dummy resistor load (8ohm 25W)?

My limited expertise is with tube amps, and have always connected a load to the output transformer in that world.

[Jazz P Bass]

No load.
Not a Speaker or a resistor.
You are giving the Vdc a path to ground.

I would pull the output transistors.
One or more are shorted.
And check all of the .33 ohm resistors for opens.

[Enzo]

Yes^^^^


I said no LOAD because I didn;t want to narrow it down to speaker OR resistor. I don;t want you to connect ANYTHING to the output until we are certain it no longer makes DC on the output and that it is stable.

[pbiagi]

Thanks for the info.
I ordered a couple MJ15015, which I believe is the replacement transistor. I got the NPN version.
Question - do I need one NPN and one PNP? (If so, I'll need to order PNP too..)

[Enzo]

Look at your schematic, this is a quasi-complementary amp, all the outputs are NPN.


Aha, side note: I could not find a TKO80 in my drawing inventory, other than the newer one. I looked up the owner manual and sure enough it has an 80 under the TKO on the panel. As far as any records I have, this model was called simply TKO. Which I do have in my files. it is moot at this point, but I was getting confused.

[pbiagi]

To close -
Changing out the output transistors worked.
Resistors tested fine, and we checked a sine wave before hooking up a speaker.
Sounds great and thanks for the guidance Enzo and Jazz Bass.
Paul