Fender Frontman 25R - hum / buzz on clean channel

[Guga Ramone]

Since Gain Select is powered by a VAC voltage (20 VAC), for each case check that are correct D7, D8, D9, C31 (.033).

Does change voltage on TP19 (U5b pin 6) as it says on schematics (+0.07 VDC Drive ch. sel.) or (+1.22 VDC Clean ch. sel.) when change Drive/Clean mode on the Gain Select switch.

Removed D7, D8, D9 and C31 from the circuit and tested them. They are all good.

The voltage changes I'm reading on U5b are:
TP18: +13,8 Vdc (drive channel) / -14,1 Vdc (clean channel)
TP19: +0,03 Vdc (drive channel) / +1,29 Vdc (clean channel)

There is a small difference from the schematic reference values. Is this small difference enough to make this buzz?

Can the U5b (4560 Opamp) be busted and somehow may be inducing hum?

I have worked on these amps with the PCB unattached to the chassis, sitting on the work bench.
I do not recall having that issue.

I tried again and there is a lot of noise without this ground connection.

[vintagekiki]

Quote:"3. The connection between the pcb and the chassis is made by 2 screws in the power amp heat sink. If I try to power the circuit without this connection the hum is unbearable in both channels."

This may be THE problem.

I have worked on these amps with the PCB unattached to the chassis, sitting on the work bench.
I do not recall having that issue.

I didn't have any Frontman on a work bench, so I would ask is there heatsink insulation pad under TDA1514A (pin 4 is - 27 VDC not gnd) since it works with symmetrical power supply.

EDIT

LM3876 datasheet
http://www.farnell.com/datasheets/2001459.pdf

Should it be necessary to isolate V− from the heat sink, an insulating washer is required.

TDA1514A datasheet
https://www.alldatasheet.com/datasheet-pdf/pdf/19196/PHILIPS/TDA1514.html

Heatsink insulation pad under TDA1514A
http://image.noelshack.com/fichiers/2012/38/1348427740-dsc-0031.jpg

[vintagekiki]

Is this small difference enough to make this buzz?

Can the U5b (4560 Opamp) be busted and somehow may be inducing hum?

The voltages are correct.
There is little chance that the U5b (4560 Opamp) produces hum.

We checked everything, everything is correct and the FM25 still produces hum.
This indicates that there is an uncontrolled ground loop between the signal gnd and chassis gnd.

Chassis gnd
https://www.tdpri.com/threads/frontman-25r-club.209672/page-9

[Guga Ramone]

Yes, there is an insulator pad between the power amp IC and the heatsink and it is providing the correct insulation.

I checked the topic on the TDPRI Forum and tried the cable twist trick and nothing changed. I am giving up on this little amp...

I'd like to thank very much everyone that somehow helped on this quest.

[vintagekiki]

The amplifiers is like a guitar. Goes from series to series.
The schematic concept is always same, but it rarely happens some problematic component (resistor, capacitor, semiconductor, dead solder). And there begins walk in trouble.
Diagnosing noise problems them is difficult, especially over the internet.

If it's a consolation you are not the only one who is not happy with FM25. Scroll through links ...

https://music-electronics-forum.com/showthread.php?t=22904

https://www.tdpri.com/threads/frontman-25r-not-silent-enough.702393/

https://www.reddit.com/r/Guitar/comments/adjiyn/gearquestion_drive_channel_has_less_hisshum_than/

https://www.acousticguitarforum.com/forums/showthread.php?t=489128

http://www.squier-talk.com/threads/how-about-them-frontman-amplification-devices.84/

https://www.ultimate-guitar.com/forum/showthread.php?t=1613510

Good luck.