Fender Frontman 25R - hum / buzz on clean channel

[vintagekiki]

Q1 and Q2 form switch drive or normal ch, them operated by U5b (half 4560) which receives command from Footswitch (J2) or Gain select (S1)
Depending on the choice J2 or S1, the voltage on pin 7 (TP15) changes polarity.

If the clean ch switch does not work, check (replace) D6 and Q1.
If Q1 short circuit, clean ch does not work.
If Q1 break, clean ch work all the time.

If the drive ch switch does not work, check (replace) D5 and Q2.
If Q2 short circuit, drive ch work all the time.
If Q1 break drive ch does not work.

Clean ch and drive ch are mixed at U3a (pin 2). Amplified signal from U3a (pin 1) goes to tone color, reverb and power amp.

Soft shutdown FM25R is realized through Q3 and Q4

[Jazz P Bass]

Sorry but I messed up a little here. Whe Q1 is removed the amp stays on clean when the switch is set to clean. When the switch is set to the drive channel, the distorted signal from this channel is mixed with the clean signal that is always present because of the absence of Q1.

Yup.
That is how it is supposed to work for this test.
I was trying to determine, by removing Q1, if the issue was in the switching voltage.

[vintagekiki]

Yup.
That is how it is supposed to work for this test.
I was trying to determine, by removing Q1, if the issue was in the switching voltage.

[vintagekiki]

Sorry but I messed up a little here. Whe Q1 is removed the amp stays on clean when the switch is set to clean. When the switch is set to the drive channel, the distorted signal from this channel is mixed with the clean signal that is always present because of the absence of Q1.

What happens when Q2 is removed and the correct Q1 is in place.
When using Gain select (S1) Footswitch (J2) must be disconnected.

[Guga Ramone]

I removed both Q1 and Q2 and the buzz is still there.

I tried to isolate the switching system by removing R54 while Q1 also was out of the circuit, and the problem persists.

Changed U2 for a new IC, just in case, and the buzz is still there.

[vintagekiki]

Same story turns around and we are still at the beginning. I've listened to the youtube video several times , and the amount buzz seems pretty normal to me.
The shortest way is to compare your FM25 with another FM25 (from a colleague) under the same conditions as it says in the text below. The noise in the Drive Ch itself must be significant.

Assuming that the voltages on TP3, TP5, TP5, TP6, U1 (pin4), U1 (pin8) have a value as in schematics, and that Q1, Q2, D5, D6 are correct for this test, you need 5 minutes. The answers to the questions are short only yes/no

Set Treble (R29), Bass (R31) at max, Mid (R30) at min.
Specify in which test (1 ... 4) there is buzz
1. Set Normal Volume (R13), Gain (R20), DriveVolume (R24), Reverb to min (0) and select channels with Gain select (S1)
2. Set Normal Volume (R13) on max and select channels with Gain select (S1)
3. Set Drive Volume (R24) on max and select channels with Gain select (S1)
4. Set DriveVolume (R24), Gain ( R20 ) on max and select channels with Gain select (S1)

If there is still buzz, specify in which test (5 ... 7) there is buzz
5. Disconected reveb pan (P17, P18) (check that the reverb pan is not rotated for 180)
6. Remove C23 (.33) (check soft shutdown, U4a)
7. Remove C33 (.47) (check output)

If there is still a buzz, there may be insufficient filtration of C43 ... C48 or uncontrolled ground loop.

[Guga Ramone]

Hi!

Unfortunately I don't know anyone with the same amp to compare both units. It would help a lot, for sure!

The voltages I'm reading are:
TP3 26,7 Vdc
TP4 -26,8 Vdc
TP5 +15,1 Vdc
TP6 -15,5 Vdc

U1 (pin4) +13,7
U1 (pin8) -13,4

For the tests:
1. Set Normal Volume (R13), Gain (R20), DriveVolume (R24), Reverb to min (0) and select channels with Gain select (S1)
Clean channel buzz / Drive channel dead silent

2. Set Normal Volume (R13) on max and select channels with Gain select (S1)
Clean channel buzz / Drive channel dead silent

3. Set Drive Volume (R24) on max and select channels with Gain select (S1)
Clean channel buzz / Drive channel dead silent

4. Set Drive Volume (R24), Gain ( R20 ) on max and select channels with Gain select (S1)
Clean channel buzz / Drive channel lots of hiss (because of gain selection so high)

5. Disconected reveb pan (P17, P18) (check that the reverb pan is not rotated for 180)
Reverb pan correctly oriented. All tests above were made with and without the reverb pan, with all the same results

6 and 7 weren't tested yet because I couldn't fire up my soldering iron yet. I will try them out this week.

As for the insufficient filtration of C43 ~ C48, can this result in the clean channel buzzing only? Wouldn't the drive channel buzz in the same way if there was a problem in the power supply filtration?

Thanks!

[vintagekiki]

For the tests:
1. Set Normal Volume (R13), Gain (R20), DriveVolume (R24), Reverb to min (0) and select channels with Gain select (S1)
Clean channel buzz / Drive channel dead silent

This test indicates that the cause of the buzz is not in the preamp but already occurs after IC3b (pin 7)
Clean ch has a buzz because it has a "large" C6 capacitor (22u), the drive ch has a "small" capacitor C49 (u1) that does not transfer bass.
Buzz may be coming cause Aux In (J3) which is constantly open.

Before test 6 Remove C23 (.33)
Put into Aux In (J3 L / R) 2 x male RCA jack shorted with gnd.
Is there still a buzz?

[Guga Ramone]

Put into Aux In (J3 L / R) 2 x male RCA jack shorted with gnd.
Is there still a buzz?

Yes, the buzz is still present under this condition.

Test 7. Remove C33 (.47) (check output)
- Clean channel still buzzes when selected. Drive channel is very quiet, just like the initial tests and as shown in the video.

Test 6. Remove C23 (.33) (check soft shutdown, U4a)
Removing C23 stops the buzz, but gets me no sound on the output. The amp turns on/off normally with no noises or pops.
Anything else I should check on the soft shutdown?

[vintagekiki]

1. Set Normal Volume (R13), Gain (R20), DriveVolume (R24), Reverb to min (0) and select channels with Gain select (S1)
Clean channel buzz / Drive channel dead silent

https://www.youtube.com/watch?v=n3y4S3lCdc0 ¹⁾



On initial tests shown in your video¹⁾ (0:05) all pots are at min (0) and you have a small buzz on (both ch) the amplifier.
You're somewhere wrong in the procedure or explanation.

Test 6. Remove C23 (.33) (check soft shutdown, U4a)
Removing C23 stops the buzz, but gets me no sound on the output. The amp turns on/off normally with no noises or pops.

Test 7. Remove C33 (.47) (check output)
- Clean channel still buzzes when selected. Drive channel is very quiet, just like the initial tests and as shown in the video.

https://music-electronics-forum.com/attachment.php?attachmentid=55693

How do you explain that clean channel still buzzes when remove C23 (.33) or C33 (.47).
In both tests the preamp is physical disconnected from power amplifier and no any sound on the output.

Question
Do you have a ground on power cord cable, respectively is it cable with two-wire or three-wire.

[Guga Ramone]

I'm sorry for the confusion. I ended up removing the wrong (C35) capacitor on test 7. Removing the correct capacitor (C33) on this test stops the buzz when the clean channel is selected.

I tried using a two-wire (ungrounded) and three-wire (lead/neutral/ground) and the buzz always occurs, no matter the type of power cord.

[vintagekiki]

We have to go back to the beginning again. Maybe we missed something. Slowly without a hurry.
What have we found so far?

https://www.youtube.com/watch?v=n3y4S3lCdc0
- When all the pots are at min (your video1 at 0:05) the amplifier has a small buzz on both (clean and drive) ch.

https://music-electronics-forum.com/attachment.php?attachmentid=55693
- Removing C23 (.33) or C33 (.47) on both (clean and drive) channels stops the buzz and no sound on the output. That's OK.

Since everything is right, and there is always a small buzz when is the preamp connected to a power amplifier, I would ask a few question.
1. Is it in your free estimation after looking at "Under the Hood" before you, some repairman did some intervention on the FM25R.
2. How derived (- point) speaker connection to the chassis: directly or via R62 (.33)
3. Is there a quality connection between PCB ground and chassis ground.
4. Is there a buzz on any (clean and drive) ch, when you remove R74 (470).
5. Do you have correct ground in the electrical installation?

[Guga Ramone]

the amplifier has a small buzz on both (clean and drive) ch.

The camera mic on my cel phone is very sensitive and compresses the audio A LOT, maybe this is misleading you. I'm sorry for this. The buzz happens only in the clean channel, doesn't matter the position on the controls.

Answering the questions:
1. The amplifier looked as it was factory stock when it came to me. All the solder joints looked the same, with no signal of intervention.
2. Speaker derived (- point) connects to the chassis via R62, which is reading ok (0.33 Ohms)
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.
4. Removing R74 doesn't stop the buzz from happening when the clean channel is selected.
5. Yes, correct ground in the electrical instalation. I also tried to power the amp without the ground and the same buzz happens.

[vintagekiki]

4. Removing R74 doesn't stop the buzz from happening when the clean channel is selected.

When R74 (470) is removed, signal path for both (clean and drive) channels from mixer (U3a pin 1) is interrupt and no any sound on the output.

The same thing happens when you remove C23 (.33) and when you remove C33 (.47)
In all three cases must not heard buzz and any sound when change Drive/Clean mode on the Gain Select switch.

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.

[Jazz P Bass]

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.