Results 1 to 35 of 35

Thread: Fender Frontman 25R - hum / buzz on clean channel

  1. #1
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0

    Fender Frontman 25R - hum / buzz on clean channel

    Hello!

    A friend of mine has a Fender Frontman 25R guitar amp and the clean channel is producing a low level hum, even when there is nothing plugged into the amp. When switched to the drive channel the buzzing goes away and the amp becomes very quiet. The power supply voltages are all ok. My guess is that the U1-B may be damaged, since there are no diodes to protect it's input. Am I right here or is there anything else I should check before disasemble the board and replace the IC?

    The schematic is here: Frontman+25R+schematic.pdf

    Thanks in advance!

    0 Not allowed! Not allowed!

  2. #2
    Supporting Member Randall's Avatar
    Join Date
    Jan 2013
    Location
    Cape Coral, FL
    Posts
    2,420
    Thumbs Up/Down
    Received: 549/0
    Given: 100/0
    Rep Power
    9
    Assuming the Normal volume effects the hum, I'd say U1 is a good place to start.

    0 Not allowed! Not allowed!
    It's weird, because it WAS working fine.....

  3. #3
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Check voltages change to U5b (TP18, TP19) when switching (sw Gain select) from clean to the drive.
    Check that the voltages at U1, U2, U3 at pin 1 and pin 7 are about 0 VDC.
    If the voltages are OK check Q1, Q2, D5, D6.

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  4. #4
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Hi there!

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

    I guess the small differences I'm reading are ok, right?

    Pins 1 and 7 of U1, U2 and U3 are reading 0 Vdc.
    I removed Q1, Q2, D5 and D6 and tested them and they seem ok.
    I removed U1 and installed a new IC and the buzz is still there.

    The clean channel volume control doesn't affect the buzz. The bass control does amplifies it a bit.

    I tried to put U3b pin 7 to ground and the buzzing stopped. Trying to ground pin 6 doesn't stop the buzz. I changed U3 for a new IC and the buzz is still there.

    Any tips?

    Thanks!!

    0 Not allowed! Not allowed!

  5. #5
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,835
    Thumbs Up/Down
    Received: 1,669/23
    Given: 4,286/11
    Rep Power
    22
    Is the buzz there with the reverb turned all the way down?

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  6. #6
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    ^ Yes. Actualy the buzz is present even when all the controls are on minimum.

    Here is a video showing the buzz:


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

    Thanks!

    0 Not allowed! Not allowed!

  7. #7
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,835
    Thumbs Up/Down
    Received: 1,669/23
    Given: 4,286/11
    Rep Power
    22
    What stood out the most to me about that video was how much the hum was reduced when you turned the treble up. Is that what is happening, the hum gets worse if you turn down the treble?
    Have you looked at the solder joints on the pots and jacks, and looked for any cracks on the pots themselves?

    0 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  8. #8
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 533/3
    Given: 289/0
    Rep Power
    26
    I would remove Q1 jfet and see what the amp does.

    Of coarse the Clean channel will always be On.

    1 Not allowed! Not allowed!

  9. #9
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by g1 View Post
    What stood out the most to me about that video was how much the hum was reduced when you turned the treble up. Is that what is happening, the hum gets worse if you turn down the treble? Have you looked at the solder joints on the pots and jacks, and looked for any cracks on the pots themselves?
    I guess this has more to do with how my camera captures the audio and compresses it when there is other noises around. In the room the amp doesn't change the hum when the treble pot is turned. The bass pot yes, it makes the hum a tiny bit louder, but I guess it happens because it filters the noise when it is turned down and it is not the cause of the problem.

    Quote Originally Posted by Jazz P Bass View Post
    I would remove Q1 jfet and see what the amp does. Of coarse the Clean channel will always be On.
    Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away. I put in new FETs in Q1 and Q2 and nothing changed.

    Any ideas? Thanks!

    0 Not allowed! Not allowed!
    Last edited by Guga Ramone; 11-02-2019 at 05:54 PM.

  10. #10
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 533/3
    Given: 289/0
    Rep Power
    26
    "Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away"

    And this is with Q1 Removed???
    That makes no sense if true.

    1 Not allowed! Not allowed!

  11. #11
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    What should I expect by removing Q1?
    - Clean channel selected = clean sound
    - Drive channel selected = mute?

    Thanks!

    0 Not allowed! Not allowed!

  12. #12
    Don't forget the joker g1's Avatar
    Join Date
    Feb 2010
    Location
    Canada, somewhere north of Fargo
    Posts
    11,835
    Thumbs Up/Down
    Received: 1,669/23
    Given: 4,286/11
    Rep Power
    22
    I think we better make sure we are looking at the right schematic. Seems to me this is the one where there are several versions.
    Does your amp have the TDA1514 output IC, or the LM3876 ?
    The Aux Input is not shown in the schematic in post #1, so that can't be the correct schematic.

    Both versions with Aux.In are in this attachment:
    Attached Files Attached Files

    1 Not allowed! Not allowed!
    Just because they don't have tubes doesn't mean they don't have feelings! - glebert

  13. #13
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    This one has the LM3876 as the output IC.

    0 Not allowed! Not allowed!

  14. #14
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 533/3
    Given: 289/0
    Rep Power
    26
    Quote Originally Posted by Guga Ramone View Post
    What should I expect by removing Q1?
    - Clean channel selected = clean sound
    - Drive channel selected = mute?

    Thanks!
    Q1 mutes the clean channel when On.
    If you remove it, the switch position should not influence that channel.
    There should no longer be any connection to the switch voltage.
    So how can it make the clean channel hum?

    0 Not allowed! Not allowed!

  15. #15
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by Guga Ramone View Post
    Removing the Q1 the amp stays on clean, regardless the position of the switch. When the switch is out (clean selected) the hum is present, as showed in the video. But when the switch is in (drive selected, but clean sound), the hum goes away. I put in new FETs in Q1 and Q2 and nothing changed.
    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.

    0 Not allowed! Not allowed!

  16. #16
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    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

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  17. #17
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 533/3
    Given: 289/0
    Rep Power
    26
    Quote Originally Posted by Guga Ramone View Post
    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.

    0 Not allowed! Not allowed!

  18. #18
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Jazz P Bass View Post
    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.
    Click image for larger version. 

Name:	switching voltage.jpg 
Views:	8 
Size:	25.6 KB 
ID:	55857

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  19. #19
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Guga Ramone View Post
    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.

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  20. #20
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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.

    0 Not allowed! Not allowed!

  21. #21
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    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.

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  22. #22
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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!

    0 Not allowed! Not allowed!

  23. #23
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Guga Ramone View Post
    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?

    1 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  24. #24
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by vintagekiki View Post
    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?

    0 Not allowed! Not allowed!
    Last edited by Guga Ramone; 11-12-2019 at 10:56 PM.

  25. #25
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Guga Ramone View Post
    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)

    Click image for larger version. 

Name:	FM25R buzzing.JPG 
Views:	10 
Size:	102.1 KB 
ID:	55932

    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.

    Quote Originally Posted by Guga Ramone View Post
    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.

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  26. #26
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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.

    0 Not allowed! Not allowed!

  27. #27
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    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?

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  28. #28
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by vintagekiki View Post
    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.

    0 Not allowed! Not allowed!
    Last edited by Guga Ramone; 11-19-2019 at 08:57 PM.

  29. #29
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    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.
    Click image for larger version. 

Name:	R74.jpg 
Views:	7 
Size:	126.9 KB 
ID:	56003
    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.

    1 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  30. #30
    Supporting Member Jazz P Bass's Avatar
    Join Date
    Dec 2008
    Location
    Wernersville, PA
    Posts
    12,940
    Thumbs Up/Down
    Received: 533/3
    Given: 289/0
    Rep Power
    26
    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.

    0 Not allowed! Not allowed!

  31. #31
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    Quote Originally Posted by vintagekiki View Post
    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?

    Quote Originally Posted by Jazz P Bass View Post
    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.

    0 Not allowed! Not allowed!

  32. #32
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Jazz P Bass View Post
    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

    0 Not allowed! Not allowed!
    Last edited by vintagekiki; 11-20-2019 at 11:54 PM. Reason: EDIT
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  33. #33
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    Quote Originally Posted by Guga Ramone View Post
    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

    Click image for larger version. 

Name:	DSC07133.jpg 
Views:	11 
Size:	94.9 KB 
ID:	56007

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

  34. #34
    Junior Member
    Join Date
    Nov 2012
    Location
    Curitiba - Brazil
    Posts
    14
    Thumbs Up/Down
    Received: 0/0
    Given: 0/0
    Rep Power
    0
    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.

    0 Not allowed! Not allowed!

  35. #35
    Senior Member vintagekiki's Avatar
    Join Date
    Jul 2006
    Posts
    1,640
    Thumbs Up/Down
    Received: 292/1
    Given: 241/0
    Rep Power
    16
    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.

    0 Not allowed! Not allowed!
    Who does not know and knows that he does not know - teach him Confucius)
    Who knows and does not know that he knows - wake him Confucius)

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Similar Threads

  1. Replies: 24
    Last Post: 12-01-2019, 03:35 AM
  2. Fender Frontman 25r clean channel problem...
    By hongaku in forum Maintenance, Troubleshooting & Repair
    Replies: 6
    Last Post: 12-18-2010, 03:27 AM
  3. TSL Clean Channel Buzz
    By PaoloJM in forum Maintenance, Troubleshooting & Repair
    Replies: 13
    Last Post: 07-02-2010, 08:21 AM
  4. Fender Frontman 25R Channel Problem
    By RogerM in forum Maintenance, Troubleshooting & Repair
    Replies: 2
    Last Post: 03-30-2010, 03:28 AM
  5. Fender Twin Amp - Gain channel sucks, clean channel has limited Bass
    By TerryCline in forum Maintenance, Troubleshooting & Repair
    Replies: 5
    Last Post: 07-16-2009, 07:45 PM

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •