Fender Frontman 212R DC on output

[booj]

fm212r.pdf Ok I'll try to make this simple. Apparently, I have made some progress. When I dis-abled the mute circuit (quadrant A5), by cutting R73, The DC on the output went away. I changed Q6 and Q7 (which are 2N4403's not 2N4003). Now, when the mute circuit is connected, I get -2.5 volts DC on the output. As before, when R73 is dis-connected, there are 2 - 3 millivolts DC. Anybody?

[g1]

Are you still on the limiter lamp? And no load? Check that the +/- rails remain equal when you have the -2.5VDC on the output.

[vintagekiki]

Apparently, I have made some progress ...

- Check mute line contacts on jacks J1, J2, J4.
FM212R power off. At jacks J1, J2, J4 nothing is connected. At mute point (R73/C46) should be resistance order 10 Ohm (R49).
When plug is inserted into any jacks J1, J2, J4 at mute point (R73/C46) should be resistance order > kOhm.
- Check RT2 (PRC100)

[booj]

vintagekiki
- Check mute line contacts on jacks J1, J2, J4.
They're there
FM212R power off. At jacks J1, J2, J4 nothing is connected.
OK. Nothing plugged in
At mute point (R73/C46) should be resistance order 10 Ohm (R49).
That's there
When plug is inserted into any jacks J1, J2, J4 at mute point (R73/C46) should be resistance order > kOhm.
It is.
- Check RT2 (PRC100)
If that's that thing hidden by all the goop, it reads 200 ohms

g1g1
Are you still on the limiter lamp? And no load? Check that the +/- rails remain equal when you have the -2.5VDC on the output.
They are

[vintagekiki]

- Check RT2 (PRC100)

Correction. Sorry
RT2 (PTC100)
https://www.mouser.co.uk/Circuit-Protection/Thermistors/PTC-Thermistors/_/N-axfwr?P=1z0x8ah
What is its resistance when it is out of circuit.

[vintagekiki]

FM-65R schematic are very similar, only here you have some voltage test points.
Check test point TP23 ¹⁾
https://elektrotanya.com/fender_fm_65r.pdf/download.html

1)
https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/43795-fender-fm-65r-slight-noise-at-turn-on
https://music-electronics-forum.com/filedata/fetch?id=837514

https://music-electronics-forum.com/forum/schematic-requests/29176-fender-frontman-65r

[booj]

vintagekiki replied

08-26-2021, 11:03 PM


FM-65R schematic are very similar, only here you have some voltage test points.
Check test point TP23 ¹⁾

TP23 is supposed to be -31 VDC and its -30.3Volts. The zener diode seems to be working normally. This is with an input plugged in.

[vintagekiki]

TP23 is supposed to be -31 VDC and its -30.3Volts.

The measured voltage at TP23 is OK.
The difference between FM65R (112) and FM212R (212) is in number of built-in speakers, and FM212R has no mute Q5/Q20 at U7-A input.

Question.
1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
2. Have any of semiconductors changed in power amp except Q6 and Q7.

[booj]

vintagekiki replied

Today, 02:31 AM



Originally posted by booj
TP23 is supposed to be -31 VDC and its -30.3Volts.



The measured voltage at TP23 is OK.
The difference between FM65R (112) and FM212R (212) is in number of built-in speakers, and FM212R has no mute Q5/Q20 at U7-A input.

Question.
1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
2. Have any of semiconductors changed in power amp except Q6 and Q7.


Looking at the scat till my eyes are bleeding. What quadrant is TP29 in?

[vintagekiki]

What quadrant is TP29 in?

TP29 is in quadrant B1
PDF zoom to 400

[booj]

TP29 reads 5mv with an empty plug in the input jack. With nothing plugged in it reads about 3 volts.

[booj]

vintagekiki replied
Question.
1. If FM212R has no signal at inputs, and an empty jack plug is installed in J4, what is DC voltage on TP29.
2. Have any of semiconductors changed in power amp except Q6 and Q7.

TP29 reads 5mv with an empty plug in the input jack. With nothing plugged in it reads about 3 volts.
2SD1857 (Q14) and 2SB1236 (Q15) and re-flowing all the connections in the output section. I also changed the drivers Q18 and Q19, the outputs, Q20 and Q21
and Q10 and Q11

[booj]

The amp is done. We sent it out with the mute circuit dis-connected.