Fender Bantam Bass oscillation coming from a wire

[Duncan]

Howdy.

An acquaintance brought me a 1969 Bantam Bass combo for some work (retube, caps, etc). Pretty standard stuff.

So I went and did the servicing I’ve done a thousand times, fired it up, and was presented with some pretty bad oscillation (helicopter noises).

I narrowed it down to this wire:

https://imgur.com/a/1MGKYY1

I’ve tried running it under the board, over the board, around the board, and all I’ve been met with has been different versions of terrible oscillations. I’ve checked the resistors in the path of this wire, and they’re all fine. I’ve also swapped the wire with a new piece

Any suggestions for this? I have some 600v shielded wire that I might have to try next but I wanted to engage the deep knowledge base here before I potentially waste more time.

Here’s the schem: https://el34world.com/charts/Schematics/files/Fender/Fender_bantambass_cfa7003.pdf

Thanks!!

[tubeswell]

Maybe a non-functioning cathode bypass cap on the shared cathode resistor going to pin 8 of both 12AX7s. (Combining the cathodes there through a shared resistor is a recipe for oscillation if the resistor ‘becomes’ unbypassed). Failing that, add more filter capacitance to that preamp supply node.


Also when you did the recap, did you also change out the filter cap(s) in the bias supply?

[Duncan]

Maybe a non-functioning cathode bypass cap on the shared cathode resistor going to pin 8 of both 12AX7s. (Combining the cathodes there through a shared resistor is a recipe for oscillation if the resistor ‘becomes’ unbypassed). Failing that, add more filter capacitance to that preamp supply node.


Also when you did the recap, did you also change out the filter cap(s) in the bias supply?

Yes, I did. All electrolytics were changed, including in the bias supply.

 So if I made the 470R cathode resistor unshared (by adding an additional one), that would likely help kill the oscillations?

[BrianS]

Leaky .1uf coupling cap?

[SEL49]

So I went and did the servicing I’ve done a thousand times, fired it up, and was presented with some pretty bad oscillation (helicopter noises).

Did you evaluate the amp before you serviced it? If so, was the bad oscillation present before you serviced the amp?

[Duncan]

So I went and did the servicing I’ve done a thousand times, fired it up, and was presented with some pretty bad oscillation (helicopter noises).

Did you evaluate the amp before you serviced it? If so, was the bad oscillation present before you serviced the amp?

I did a basic evaluation before diving in, but it required a full retube, so there was only so much I could observe before it would even get into an operable state. And once I had the thing open and on the bench, I just started working on it.

Leaky .1uf coupling cap?

Yeah, I'm thinking that's a possibility, though I swapped the .1uF film caps out for these (which I've had great luck with in the past): https://www.digikey.ca/en/products/detail/cornell-dubilier-knowles/104MPR630K/5410482

[BrianS]

If you've already replaced the old .1s with new, modern caps it's unlikely they are your issue.  Scrutinize those solder joints.  Lately I've had a couple amps come in that have had extremely excessive solder in the eyelets; not necessarily from the factory, but because someone added more when they swapped components.  Usually this means the actual wire/component/solder joint cannot be inspected or evaluated for quality.

[Duncan]

If you've already replaced the old .1s with new, modern caps it's unlikely they are your issue.  Scrutinize those solder joints.  Lately I've had a couple amps come in that have had extremely excessive solder in the eyelets; not necessarily from the factory, but because someone added more when they swapped components.  Usually this means the actual wire/component/solder joint cannot be inspected or evaluated for quality.

Yeah, I had a look over the eyelets that I swapped components out from, but not in a zoomed-in, critical way. I'll give that a go tomorrow!

[Duncan]

Eyelets look fine.

I tried replacing the .1uF's with some Orange Drops I had on hand, no change.
I replaced the wire, no change.
I turned the bias potentiometer up, the sound went away when I was at the upper 20% of the pot's range (and when I say I turned the pot up, if viewing from the top of the chassis, I turned it CCW). I turned it all the way down, and the sound is gone as well (probably a super dirty bias pot).

The only remaining errant sound is a fairly loud (compared to the signal) hum. I think I've narrowed it down to the heater wiring, because when I move the looooooooooong stretch of twisted heater cables between V2 and the PI, I get a plethora of sounds, including the oscillations.

I'm going to replace that long span and see if anything improves.