Kustom 745PA (K200) 60Hz hum

[Delta362]

I've been working on a Kustom 745PA amp (same circuit as the K200 without the vibrato board) that came in with the reverb not working for one side, and bad sound out of the other side. Given the age of the unit, I recapped all electrolytics first, (plus added a 3 prong power cord). Power supply now has new 4700uF 63V caps which are close to the original 4500uF 50V.

I also replaced a few bad transistors and several out of tolerance resistors on the various boards to get the preamps passing signals. I do see a 1V sawtooth on both red wires (positive power supply rails) that connect to the driver board. One is from the reverb board and the other is from the preamp board. The blue signal wires join up at the driver board after they leave a 4k7 resistor from the preamp board or the 4k7 resistor from the reverb board. That signal is clean coming into the base of Q201 of the driver board.

However, with nothing plugged into the amp, I have about 450mVac of 120Hz signal on the output that is unaffected by the controls. No DC on the output. On the output, I can see the input signal being amplified all the way thru max volume where the 120Hz hum is drowned out relative to the much larger output voltage of my amplified signal. If I pull Q1 and Q6, I don't have the hum on the output.

I've attached the schematic and the 120Hz waveform at the output (8 ohm dummy load. Any suggestions on where to dig deeper?

[Helmholtz]

Signal frequency is 120Hz.
(I'm sure your scope can directly display frequency. Consult manual.)
Looks like a power supply filtering issue.

[Delta362]

Thanks, Helmholtz! for some reason every time I counted the spacing, I was getting ~16ms instead of ~8ms, which told me 60Hz. My brain was obviously not thinking clearly last night as I kept going back and forth chasing it.

So, given that it's 120Hz, not 60Hz, would increasing the 4700uF caps to something larger be the preferred solution? The ones that I installed were Kemet ALS40A472DB063 (https://www.mouser.com/ProductDetail...DLSjYLsQ%3D%3D)

Mods, please update the title to 120Hz, not 60Hz as I can't seem to update that field.

[Helmholtz]

4700µ is large enough.
But make sure the caps are well connected and polarity is correct.
C2 has positive to ground!

[Steve L]

Make sure you changed C114 and C128. You may want to use something like 220uf instead of something close to 80uf.
If that does not solve it, you may need to add something like a 470uf/50V cap from Q6 emitter to ground.
That generally cleaned up the hum that many of those had.

Steve

[52 Bill]

The red wires that go from the driver board to the preamp and reverb boards are the +24 volt power supply lines that are feeding those circuits. The voltage regulator is all based on the circuit around Q6. I would think that if there is a problem with the low voltage power supply it may be there.

Removing the driver transistor Q1 will kill all signal to the power output stage. This will also kill the hum to the output.

The grounding scheme at the two main filter caps will effect the hum. Use one cap terminal as the main grounding point and use the small jumper to connect to the second cap grounding point. The power transformer secondary center tap (grey wire I think) and the other main ground wires should all go the the same main ground point.

Check the two 8.2 ohm resistors in the power output section as they will go off value and will reduce power output.

[Delta362]

Thanks for the guidance! I got it fixed finally. No issues with my power supply caps or grounding. It seems during my testing, I had shorted out Q202, Q203 and Q204 on the driver board. With the removal/replacement of the driver board, I also apparently didn't get one of the Q6 mounting screws quite tight enough, which that side of the screw is used to provide the DC+ voltage to Q204 collector. I also replaced Q201 with one that closely matched Q204. All is good now. Thanks again!