Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Colas LeGrippa on February 20, 2022, 05:02:34 pm

Title: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 20, 2022, 05:02:34 pm

Hi buddies. Just came in a Premier 90 reverberation unit from long time ago...
Very clean inside, my client told me his father had bought it new. I replaced both tubes , I get the reverb going far away but witj a huge hum.
What could I check first to get rid of the issue ?


Thanks

Title: Re: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 20, 2022, 05:08:18 pm

The schem migjt be this one

Title: Re: Huge hum from Premier reveberation unit
Post by: pdf64 on February 20, 2022, 06:20:37 pm

Maybe one or more HT ecaps are bad? There’s only 3. Try clipping a good one across each in turn.

The ‘ground strip’ circuit should stop mains earth loop type hum, provided repairs / upgrades don’t bypass it, eg non isolated jacks.

Title: Re: Huge hum from Premier reveberation unit
Post by: shooter on February 20, 2022, 06:28:02 pm

^
what he said, plus broken/frayed shields on the cables, cracked solder connection on the RCA jacks center pin

Title: Re: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 20, 2022, 07:17:08 pm

Thanks i ll get back to you

Title: Re: Huge hum from Premier reveberation unit
Post by: tubeswell on February 20, 2022, 08:01:33 pm

What the others said about filter caps, grounding and cable shields, plus, check your reverb pan orientation w.r.t. the PT. (The pan's hum-sensitive output transducer wants to be located away from the PT-end of the chassis to minimise unwanted EMF coupling into the transducer).

Title: Re: Huge hum from Premier reveberation unit
Post by: PRR on February 20, 2022, 08:20:25 pm

Quote from: Colas LeGrippa on February 20, 2022, 05:02:34 pm

....from long time ago... witj a huge hum.

That's filter caps. That's what they are for.

Yes, it could be other things, and probably will be other things, AFTER you fix the rotten useless main filter caps.

Title: Re: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 22, 2022, 04:46:47 pm

Hola amigos, yep, filter caps. There were 4 in a can but the ratings were tpo close to the real voltages.  Check this out:


100 uF @ 150V
100 uF @ 25 V
   80 uF @ 150 V
   40 uF @ 150 V


The voltages are 130V but  16V at the 100 uF 25 V.


I put singles ones rated from 100 to 200V.
Hum disappeared at 95 percent.
Both pots are scratchy so I ll give them a bath in WD40


Merci beaucoup guys.


Colas

Title: Re: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 22, 2022, 04:49:27 pm

More pics

Title: Re: Huge hum from Premier reveberation unit
Post by: Colas LeGrippa on February 22, 2022, 04:50:56 pm

Job done