Peavey KB100 Hum

[misterc57]

I have a KB100 with the 3 inputs with pull gain, 4 EQ, and reverb.

The amp plays and sounds great but there is a background hum which I am pretty sure is 120 hz.

The hum is affected by all 3 volume controls, the pull gains, and the EQ. Hum is present with nothing plugged in but gets more noticeable when you plug into an input. No hum with volume at zero. Becomes noticeable with volume at 5 (without pull gain) or volume at 3 (with pull gain). All three inputs behave the same but I feel like input 1 is a tad louder on the hum.

If I jump the pre-amp out and power amp in, still hum.

If I plug another pre-amp into the power amp IN jack on the back panel there is no hum.

All pots and jacks have been cleaned.

I am going to pull the chassis and inspect, look for bad connections, and search for the proper schematic.

Hoping for some recommendations while I dig in! Thank you!

[Enzo]

Don't search for the schematic, just call Peavey and ask for it.

[blindboybenton]

found this
https://elektrotanya.com/peavey_kb100.pdf/download.html

[Enzo]

There are at least the 1984 and 1988 versions, and perhaps an even later one. PV can help you determine which, or just send you all of them.

[Jazz P Bass]

1988 schematic: Peavey-KB100 amp.zip

[misterc57]

Voltage is High

I have the 1984 version, schematic attached.

I found that my op amps had +/- 25 VDC instead of +/- 15 VDC.

At R108 and R110 I have +/- 40.5 VDC going in and +/- 25 VDC coming out. These are 400 ohm/5 watt resistors. I measured 399 ohms resistance on both in circuit. Wondering why my voltage is not closer to 15 VDC.

Measurements taken with chassis on a variac running at line voltage.

Thank you

[52 Bill]

Wondering why my voltage is not closer to 15 VDC.

There are two Zener diodes that come after the dropping resistors CR32 and CR 33. They are supposed to regulate the voltage to 15 volts. They may be unsoldered or open circuit, etc.

Check the diodes and see what you find.

[52 Bill]

Wait a second, I just read the rest of the thread.

It would be unusual for both Zeners to die or even come unsoldered at the same time, so I would first check to see if the ground connection to the Zeners is intact and is connected to the rest of the circuit. A bad ground could be the source of the hum as well.

[misterc57]

Wait a second, I just read the rest of the thread.

It would be unusual for both Zeners to die or even come unsoldered at the same time, so I would first check to see if the ground connection to the Zeners is intact and is connected to the rest of the circuit. A bad ground could be the source of the hum as well.

I will check tonight. Those Zeners are buried in hot glue (which I will scrape away) and I could barely see them. Thank you

[Enzo]

ANy chance someone else tried to service this first? I too tend to doubt both zeners failed at once. But if someone replaced them with plain diodes - and facing the other way, the we would have less drop across those resistors as you found.

[misterc57]

I think there is a good chance this has been worked on before. The op amps, orange and blue caps, and hot glue do not seem to look original.

Thanks!

[Enzo]

Trick of the trade: Hit the hot glue with some freeze spray. That makes it very brittle and you can shatter the stuff and brush it away. No scraping.

Those gut shots look totally stock to me. The glue goo looks like a zillion 1980s PV amps, and certainly those bluish/silver TI ICs were common.

[misterc57]

Trick of the trade: Hit the hot glue with some freeze spray. That makes it very brittle and you can shatter the stuff and brush it away. No scraping.

Those gut shots look totally stock to me. The glue goo looks like a zillion 1980s PV amps, and certainly those bluish/silver TI ICs were common.

Good to know. Thanks!

[misterc57]

You all nailed it! The grounds of the Zeners were not making contact. Re-flowed all the legs and back to +/- 15 VDC on the op amps.

However I still have the ground noise issue. It is the same sound I hear if I touch the bare volume pots or if I short an instrument cable with my thumb. And it gets amplified through the pre-amp circuit.

I ran out of time last night but I detected that the noise is not present from input to pin 1 of U6. The noise is present on pin 7 of U6 all the way to the end of the pre-amp circuit. I will see tonight if I can isolate further between pin 1 and pin 7 of U6.

Thank you!

[misterc57]

I was wrong, the noise is present at the output of every op amp in the pre-amp circuit (from each input, tested one at a time).

Each input is consistent, no noise until the volume is around 5 (or around 3 with the volume knob in the pulled position).

The amp is very loud around 3 or 4 (volume knob pushed in) with no noise. Playing a keyboard, and it sounds great!

I checked every op-amp pin that had a path to ground and the readings matched the resistance on the schematic.

Not sure what to test next.

I am thinking of starting all over with signal tracing. Actually, I am not injecting a signal, I am just turning the volume pot to 5 (in the pulled position)
and listening along the signal path for hum.

I am also thinking of pulling the board and inspect all solder connections again. I had missed the Zener bad connections on visual inspection, only caught it when checking continuity to ground with my meter.

Thank you.