Selmer Treble and Bass MkIII Echo Loop

[oldcodgeman]

Hi. I'm trying to learn a little about amp repairs and bought an old Selmer T&B MK3 in need of some tlc. I've changed the rectifier diodes, filter caps, bypass caps, cleaned pots, valve sockets and jack sockets and put in an adjustable bias. It now runs and sounds great. If I compare with a modern amp, i.e. AC30C2, there is some very slight hiss but I guess this is probably normal for an amp this age.

The questions I have are regarding the echo in / out circuit. With the chassis out, if I place my hand anywhere near the echo jack sockets from above, i.e. chopstick distance away, there is a very noticeable hum it's pickup up from me, which increases the closer I get. If I touch around this area with the chopstick, for example cap labelled C10 (later version of the layout dgm on https://www.backfromthesixties.co.uk/selmer-treble-n-bass-50-mkiii)  this hum increases still. Is this normal or is there an issue with something in this part of the circuit? Bad ground maybe?

I'm thinking of bypassing the echo circuit altogether from the signal path, as I can't see I'll ever use this and if its picking up noise from me it also may pick up other ambient noise? I'm mindful that this amp's an old lady now and don’t want to do something that can't be undone in the future, but would this be feasible and if so, what would be the best way to do this?

[tubeswell]

Going by the schematic, the echo unit is run from an FX loop, so if you want to ‘disable’ the unit without goosing around with the amp , you merely unplug it from the loop.

[oldcodgeman]

Thanks for the suggestion but there is no physical echo unit. The echo in / out is Selmer's fx loop, but isn't compatible with today's pedals. It's this internal fx circuitry that hums and I'm looking to disable / route the signal to avoid it.

[SEL49]

To bypass the loop, follow instructions on attached image.

[oldcodgeman]

To bypass the loop, follow instructions on attached image.

Fantastic! I can see how that bypasses the loop. It's also easy to undo too. Thank you!

[tubeswell]

Thanks for the suggestion but there is no physical echo unit. The echo in / out is Selmer's fx loop, but isn't compatible with today's pedals. It's this internal fx circuitry that hums and I'm looking to disable / route the signal to avoid it.

Sorry, misunderstood what you meant by ‘echo in-out circuit’.

[SEL49]

After looking at the schematic and layout a bit longer, I decided this is a simpler, better way to bypass the loop.

[oldcodgeman]

After looking at the schematic and layout a bit longer, I decided this is a simpler, better way to bypass the loop.

Time permitting, I'm hoping to do this tomorrow and I'll update how it goes. Thanks.