Amp buzz until cable connected

[AJStrummer]

Hey Gents.

I have a Fender Frontman 212 which buzzes until a cable is plugged in. Dead silent once a cable is plugged in. Tried replacing the filter caps but that didn't solve anything. I removed the input jacks but nothing improved there either. Any ideas as to what might cause this issue?

Cheers
AJS

[glebert]

There is a mute function tied to the input jacks, so that it should be quiet when nothing is plugged in. Sounds like that isn't working. I think with no cable plugged in the MUTE is grounded, which then prevents Q7 from turning on, which prevents the base of Q8 from coming high enough above the -42V rail to turn on Q8, which is the current source for the diffamp that feeds the rest of the power amp. Or something.

Looks like the MUTE gets its ground through the sheild contact of the POWERAMP IN switchjack. I would try cleaning that first.

[Enzo]

Not only is there a mute function, but the basic jack also grounds off the tip when no plug is inserted. Removing the jacks naturally then means the input remains ungrounded. SO in addition to exploring the mute as glebert suggests, you could also just have a grounding contact not working in the jack.

[AJStrummer]

Thanks gents. I'll be looking into both these suggestions today and get back to you.

Cheers

AJS

[AJStrummer]

I cleaned the contacts on the tips of input jacks and ring of the power amp in, still humming until jack inserted. I have continuity form jacks to ground, so I'm assuming there is a good contact on jacks. I followed the mute circuit. Tested D19, R70,71 & 73, and replaced C46, -16V is present as well. Still a bit stumped

[g1]

Continuity means different things in different contexts. Continuity beepers are not all that accurate. Make sure you find 0 ohms instead, and always report readings in ohms rather than 'continuity'..

As for the mute circuit, there is lots more to it than -16V and the jack switches. What is Q7 collector DC voltage under mute and unmute conditions?
I seem to recall a similar thread about hum in FM212 when in 'mute' state. If memory serves, that one ended up just getting the 'mute' disabled.

[g1]

Ok, this is was the fix from the long thread I was thinking of: https://music-electronics-forum.com/...305#post588305

[drewl]

Yeah, that's amazing.
that guy figured out the problem, posted a fix (that I have implemented) and disappeared....

WTF happened to that guy?
Where did he go?

It's a mystery!

[AJStrummer]

Cheers G1. Will dive into that tomorrow. I'll report back with accurate Ohm measurements

[catalin gramada]

Yes, pretty sure you input jack didn't close. Phisical and electrical which is not same thing by modern standards. I saw so many poor galvanized plugs as I can tell you it is possible even the contacts are phisical closed to have still some high resistance in between. So better check you input with a multimeter to get confirmation the input is shunted when nothing pluged.

[AJStrummer]

I measured the resistance to ground on the jacks as suggested. There was a reading of 0.7ohms. So I guess that contributed to the hum. The collector voltages for Q7 were -41.9 in mute and -10.2 when cable connected. I followed the solution from the post you highlighted G1, thanks for that. I can confirm a 470K resistor from collector of Q7 to the cathode of D18 works. totally silent. Thanks for pointing me in the right direction.
AJS