86' jcm 800 2210 big hum in fx loop

[hotguit]

Hi
i recently traded a 1986 jcm 800 2210 Limited edition (newer circuit).
The amp sounds quite well but there is a problem i can not fix.
When i plug any effect in the fx loop (i usually plug a delay) a loud hum is heard. The hum is very loud and becomes louder turning the master volum pot up.
The hum is present when the delay is off in the loop and it becomes louder when i turn the delay on.
The same delay in a high gain amp (vHT pittbull cl 100) fx loop does not make any noise or hum.
What can i do?
Please help me
thank you
hotguit

[Jazz P Bass]

Plug your guitar straight in to the FX Return jack.
Does that hum?

Run the FX Send to another amp.
Does that hum?

Does the Reverb control have any affect on the hum?

The 2210 FX circuit is as plain & simple as it can get.
It's basically two jacks.

You may want to try changing out V4.

Schematic link: http://www.tremolo.pl/Przedwzmacniac...l-2210prem.gif

[hotguit]

Thank you
my guitar does not hum in other amps' fx loop
I'll try doing what you suggest.
thank you
bye
hot

[Jazz P Bass]

Thank you
my guitar does not hum in other amps' fx loop

That was the point of doing the tests.
You may have been plugged into a much more elaborate FX loop on the other amps.

[52 Bill]

Is the delay ac powered? If it is, you may have a ground loop caused by too many ground connections both audio and power line.

[Jazz P Bass]

Good point.

Try the pedal with a battery for the power to see if there are any changes in the hum.

[drewl]

That primitive FX loop sends a pretty hot signal out to your FX, a delay "should" be able to handle it, but then again the delay pedal is probably used to a guitar level input.

But it probably is the AC power supply.

[hotguit]

yes, the delay is AC powered only (it is a moer reecho)
I'll try to put a G major in the loop or a battery powered pedal
hot

[Aloha_Mark]

Ignore the effect for the moment and just jumper the effect out to the effect in. If it hums, then one of the jacks is making a poor connection to the plug(s).

Have you tried other effect boxes?

[hotguit]

ok
so i have to put a jack cable from fx loop send to fx loop return, right?
hot

[Gtr_tech]

No you don't. when there's nothing plugged into the loop, its the same thing as jumping it with a cable.

You have a gnd loop. Switching pwr supplies often cause this. I'd recommend a transformer based linear regulated supply. Works every time for me...

[hotguit]

Hi
i put a Gmajor (perfectly working) in the loop and a very high gray noise is heard; it is like an old radio out of tune.
The noise is not intermodular with master volume and it always has the same (low) frequency
Put a TS 808 battery powered and no hum or noise is heard.
Maybe V4?
do not know what thinking anymore...
hot
p.s.
put a guitar in the return and NO hum or noise at all