Quick question regarding replacing pots on a Carlsboro Colt 45

[OwenM]

Hi everyone,

I'm doing up a friends Carlsboro Colt 45 for him, it's all just easy things, pots, contacts and switches. I don't have a schematic but I don't think I need one for this task, but anyway I noticed something I wanted to ask.

The pots have those frames that fit over the body and solder to the pcb in 4 spots each. The master volume pot-frame is explicitly connected to the PCB ground-plane. Channel one and two are setup identically. The frame's/body's of the 3 pots for each section are connected together but isolated from anything else and then they are taken to the ground plane, this time through a 100n cap.

I assume this is to stop the chassis making it to the ground with DC at multiple points thus eliminating loops and so on, but allowing AC to pass for some reason? This front panel does not have a separate chassis ground wire, only the Master volume frame/body going to the PCB ground plane and then over to the power board via a jumper. The front panel is not part of the same piece of metal as back panel which has the mains plug etc.

I'm confused as to what is helped by attaching it in two further places via a 100n cap. Perhaps to stop it picking up noise of some sort in some way?

Anyway, perhaps more importantly, if I were to replace the pots with those that don't have potentiometer frames/baskets can I just omit this 100n connection to ground and just let the pots bodies ground themselves via the front panel (which they do already) or should I solder a wire to the case of any new pots that puts those 100n caps back in circuit? I'll leave the master volume pot and frame so I don't have to worry about making a new chassis ground for the front panel.

Thanks for any info offered!

[Jon Snell]

The design of the original pots were to use a cradle to support the PCB.
You may be restricted to this type of pot; https://uk.rs-online.com/web/p/potentiometers/7293612/
Have a browse you may find something. https://uk.rs-online.com/web/c/passi...133,4294817747
The chassis must be connected to the fixing holes to make a good connection to chassis, so a formed copper wire soldered to the inside washer on the shaft of a new standard pot would suffice ... possibly, if you are neat enough with your soldering.

[OwenM]

Hmm, yes, good point, thanks for making me aware of it. The PCB would only have the three standard connections, soldered only, of each pot to hold it in place if I did without the frames. I'll have a look at the rs-components search you linked and see if I can get something that would offer some extra support, or use the pots you suggested. I suppose if I couldn't find suitable pots I could work out some other way of making sure the PCB was secure... my soldering is pretty good, at least, I have a good iron and the patience to do it well so making a DIY frame with solid wire to the washer could be an answer, cheers!

[g1]

When necessary we used to re-fit those wings onto replacement pots of same body size.

[OwenM]

Thanks, I tried to pull the wings off of the pots but I think in this case they are connected across the diameter of the pot under various other bits, I could probably remove the frame (pulling apart the old pot in the process) and hope I could make them fit the new pots but luckily I found some NOS pots of exactly the same typeand value so with luck I can just slot those in.

For anyone needing the framed pots from 90's ish carlsboro amps see here - https://www.ebay.co.uk/sch/m.html?_o...meter&_sacat=0