Peavey Classic 50 effects loop hum

[glebert]

Just got this amp working again and prepping it for sale. One issue is that there is a bit of a hum, present on both channels, only control that changes it is MV. Hum gets noticeable when MV gets to 4, gets louder all the way until about 11, then quiets down some as the MV is dimed (or 12'ved, in the case of this amp). Found that a shorting cable in the effects loop jack pretty much gets rid of the problem. Cleaning the jacks didn't really help. Trying to figure out if this is just a switching jack problem or something with the MV (since the hum gets quieter when MV is essentially 0 ohm). Hope I don't have to replace the MV pot since this amp is a PITA to get the preamp board out.

https://www.thetubestore.com/lib/the...-Schematic.pdf

Anyone else ran into this?

TIA,
Greg

[lowell]

Your Classic 50 has an FX loop?! Working on one now with a loud 60hz hum. No FX loop.

[glebert]

Your Classic 50 has an FX loop?! Working on one now with a loud 60hz hum. No FX loop.

On the one I had the effects loop was kind of hidden on the bottom edge of the chassis, near the footswitch jack. I don't know if there were other implementations or ones without a loop. Mine was one of the Chinese made ones with the big oval Peavey logo on the front. I did find that I had a lot of ripple on the 27V DC used for the effect loop buffer, but I ended up moving it along as a project for someone else to fix.

[Enzo]

The first Classic 50s had no loop. SOOOOO many people wanted one, PV came out with a kit to add one. The amp came out 30 years ago. A year later they came out with the FX loop kit.



No, they no longer have them. Hey, after 30 years...

PV Classic 50 FX loop mod.pdf

[g1]

The first Classic 50s had no loop. SOOOOO many people wanted one, PV came out with a kit to add one. The amp came out 30 years ago. A year later they came out with the FX loop kit.



No, they no longer have them. Hey, after 30 years...

[ATTACH]n944507[/ATTACH]

That attachment link is not working.

[Enzo]

I give up, I have no idea what I am doing wrong.

[nosaj]

..

[Enzo]

That is the schematic, but the whole file includes layouts and other data.

[g1]

Does this attachment work?

[nosaj]

yep it does for me.
nosaj

[Mick Bailey]

If the shorting cable gets rid of the problem, I'd be thinking there's something mechanical going on - either the switch contacts still have some resistance, the contact pressure is weak, or when the plugs are inserted there's a change in force that affects something - maybe a solder joint.

[glebert]

Hey guys, just an FYI I no longer have the amp from my original post. Lowell has a non-effects loop amp with a hum.

Lowell, I would suggest starting a new thread and maybe we can help you fix it.