Frontman 212R Hum

[Jazz P Bass]

I am not convinced that you have the 'same problem'. as described by the OP.

You need to find out why Q21 is being turned on (that is why it's getting warm).

My concerns with any combo amp is that they usually require basic reflowing of the main components (vibration from the speaker tears up solder joints).

Also, while the board is out, I will look for 'assembly gaffs'.
My all time favorite is resistor leads bent over into the next copper trace, which will eventually cause a problem.

[g1]

Plugging into either of the "in" jacks does more than lift the ground tab. It also signals the mute circuit, look at the very top left of the schematic where it says "mute", which connects to the point below Q7.
I would guess the mute circuit is somehow putting a DC offset in the power amp, which you can probably measure at the speaker.

[J M Fahey]

Personal opinions follow:
1) muting an amp by killing the differential input stage is horrible

2) the amp is not "an amp" any more, has no amplification, no NFB, no output DC balancing, no line ripple rejection. (hint hint )
I compared it to shooting a bullet through its brain, it certainly becomes as dead.

3) I can accept it if triggered by a thermal switch, I much prefer a silent, slightly humming alive amp, which will work again as soon as it cools off, to a strong humming, 40VDC out speaker killing one so in that context it's fine

4) Fender seems to have found it a good idea to *also* mute the power amp with no guitar plugged ..... think again.

Marshall does so by directly grounding the preamp with the jack contact itself ... works like a charm and obviously, no hum or hiss.

5) so I suggest to just remove the input jack connection for this mute, leaving it functional for its true purpose: thermal shutdown which IS a good idea.

6) I guess you can still mute the preamp by using the now free jack contact to ground some convenient point.
If you do it, please post it here, it will help others, the problem is inherent to the design itself.

[Dave H]

2) the amp is not "an amp" any more, has no amplification, no NFB, no output DC balancing, no line ripple rejection.

My personal opinion.

The mute signal turns off the current through the LTP which will turn off Q11, Q12 and Q13 so the output DC offset depends on the matching of the two halves of the output stage and also for there to be no ripple on the output the +42V and -42V supplies would have to have equal and opposite ripple, good luck with that with the tolerance of big electrolytics.

[drewl]

Removing R75 , the feed to mute circuit fixes the problem but we have to see where the initial hum at turn on comes from.
I don't think grounding the input at the jack will stop it.
Maybe further down the line.
Other amps like the newer Vox AC 50/100 CP models ground the input around the phase inverter, but if the initial hum is coming from the power amp there may be no place to cure it.

Anyway, so far everyone is fine with that initial noise as it goes away after a second or so.

[loupy31]

Hi Jazz p bass,
I have read the OP again to be sure I haven't posted in correctly, But It is exactly the same issue ,
Obviously there is a reason you made that comment, can you tell me what that is, any info will help

Peter

[Dave H]

Removing R75 , the feed to mute circuit fixes the problem but we have to see where the initial hum at turn on comes from.

I think it could be the time it takes C45 to charge up through R71 to turn on Q7 and the tail current of the LTP. I'd guess the time constant is there to mute switch on thump. Try removing C45. If that replaces the initial hum with switch on thump then you can decide which you prefer

[J M Fahey]

FWIW most of my own SS amps thump at turn on and off and a few also hum for a brief second at turn on, until some 100uF electrolytics charge and output stabilizes.
I'm a minimalistic designer and don't waste time in such niceties.
90% of my customers don't care, the few who do mainly get worried that it will damage the speakers or something.

About 0.1% DO have a REAL problem with that, in that they can't tolerate the least distracting noise: Hare Krishna / Guru Maharaj Ji / Yoga / you_name_it Meditation centers / Art Galleries with soft background music / dark and silent Camera Club slide projection rooms / Classical Music venues / Buenos Aires Planetarium / etc.
Yes, I have such Customers.

For such, I add a "standby switch" which actually opens the Hot line going to the output jacks, with the instructions: "turn amp ON, let it warm up 10 seconds, turn standby ON ; reverse procedure when turning off"
Works like a charm.

[Drewline]

It is interesting that this thread lives on. I'd still love to know the cause of the hum. Enzo's suggestion worked for me. My customer was pleased when he got the amp back as it had the hum the entire time he owned it.

[g1]

Try removing C45. If that replaces the initial hum with switch on thump then you can decide which you prefer

This won't do anything for the "hum" that is there when nothing is plugged into either jack, correct? I guess this is after R75 has been removed?
And I said "hum" because it seems the real issue is not typical hum, but DC offset. From Loupy's description, when you can hear the hum, Q21 gets warm but Q20 does not. This indicates only Q21 is conducting, so there would be negative DC on the output at this time.

[loupy31]

g1, You said, "when you can hear the hum, Q21 gets warm but Q20 does not. This indicates only Q21 is conducting" Firstly thanx for understanding my question, I have a real problem describing things.

Ok, Following on from that, When I then plug in a lead, after a few seconds the hum stops, Q21 drops in temp to the same as Q20, and all seems good.............I would love to know why this is happening so the prob. can be fixed properly instead of just a band aid fix.

Peter

[g1]

Sorry loupy, but it appears to be a design problem, as JM described in post #33.

[Jazz P Bass]

So why does the FM65R Not exibit this issue?

Just a thought.

Edit: Oh, that's why.
They used a different circuit!
They moved it forward. (Mr Fahey may approve)

[Enzo]

So ask the other question: why don't ALL the FM212Rs do this? That is what design flaw implies.

[drewl]

Well they don't come this way new.
Something is failing. Improper design causing stress on parts ?
Or intermittent connection somewhere.

We're bound to find it.......someday.