DC voltage on both sides of the presence circuit cap

[gtomax]

Hi all,

I'm giving a 68 JMP50 a checkup and noticed a bit of a hum I'm trying to track down. The mains filter can is original so I'm going to change that out but before I do I was checking the coupling caps.

I noticed I'm reading DC voltage on both sides of the presence cap on the preamp board (the .1uf connected to the turret with the 10k on one end and the turret with a 1M on the other that then goes to the presence pot and v3 respectively). I'm reading 18v on the pot side of the cap and 32v on the tube side of the cap at idle.

Does this mean that cap is leaky and needs to be replaced? As far as I'm familiar a cap should not pass any DC voltage but this cap is hooked into the presence circuit so not sure if that's coming back through and supposed to be there.



Thanks for any insights

[RockinRocket]

Yes that's normal though I get less DC on the presence pot side. Will depend on the Feedback resistor.

A bit of hum is normal though in these old amps.

[gtomax]

OK good to know - didn't want to replace it if I didn't need to. I've also not seen a mustard go bad before - they're not the old yellow Astrons

After chopsticking around I was able to cut quite a bit of hum nudging/re-routing heaters on v2 a bit relative to the signal wires. Cleaning the pots with F5 also helped.

This amp is very low 11,000's serial and clean as a whistle otherwise. Easily the best sounding Plexi I've encountered and apart from electrolytic refreshes all original. It's shared cathode (820r/330uf), 470k mixers/556pf Lemco, 56k/250pf Murata disc tone stack, Murata 100pf disc bright cap, 27k NFB on 8 ohm tap, 32uf+32uf with 10k decoupling resistor preamp filters

Looks like all that's left is to add some screen resistors for safety and this old beauty is in tip top!

[gloeidraad]

That`s normal. If you look at the 1meg and 10k resistors they are in parallel with the cap, so there is always DC on both sides of the cap.