Fender Frontman 25R Very low Volume problem

[CoopDaKill]

Gate voltage on Q4 is +0.572 vdc

[g1]

At this point, if it were me, I'd clip Q4 off as close to the board as possible and see if the sound comes back. Hopefully the legs are long enough that it can be re-used if it does not solve the problem.
It could be something else in the Q4 circuit, but my experience has been that it's more often the fet than the circuit.
Then if the sound comes back you can measure the spot on the board where the gate was. If the +.5VDC is still there, there is some other problem in the Q4 circuitry.

[CoopDaKill]

g1 ,,, I clipped off Q4, and the amp works great. So, the amp doesn’t need Q4? I measured the voltage at the Gate spot, and it’s +10.1 dc. Can I live without doing anything else now? Thanks

[Enzo]

Q4 is a power up/down mute. it helps prevent thumps and noises at those times. If you amp is not making objectionable noises at power up or down, then leave it out.

[Mark Hammer]

I have a Champion 110, which is essentially the identical amp in all respects, but has an external speaker jack on the front panel, instead of the dual phono inputs found on the 25R.

What seems to have not been asked so far is whether the low volume problem is sopecific to the Normal channel or occurs on the Drive channel as well. If common to both then whatever the problem is will likely be found in their common input path or common output path.

I will note that the Volume control on the Normal channel is annoying, and attempts, quite poorly, to adjust the gain of the stage and degree of post-gain attenuation simultaneously. If the problem is specific to the Normal channel, I'd consider examining the volume pot.

[g1]

I measured the voltage at the Gate spot, and it’s +10.1 dc. Can I live without doing anything else now? Thanks

The -10V there sounds like the correct voltage. The bad fet was making it collapse. So that -10 indicates that the rest of that mute circuit is ok.
Like Enzo said, it is not necessary, just if the turn on or off thump bothers you.

[J M Fahey]

Looks like the problem was a bad muting FET muting all the time, hence common to both channels.

That active volume control has been used for ages, it was actually invented by Philips for use in their cassette decks (think early 70´s), I still remember the Electronics World magazine where they introduced it and showed all design equations.
If well made it´s a quite acceptable Log pot approximation , and has been a staple of Peavey amplifiers, specially in the PA ones., also some Ampeg ones.

The main point is to make gain "on 5" 10/15% of "gain on 10" and then it mimics a 10/15% Log pot.
It also allows, as shown, an active Bright control with 0 effect on 10 ... as any decent Bright control does.

[CoopDaKill]

No loud thumps or pops during power up and power down. The amp sounds good. Both channels. Thanks again guys for your expert help and guidance. I’ll be donating a few dollars to this fantastic forum.

[Bass 300]

Hello all. New member here. I actually joined specifically to comment on this thread.

I had the same problem with the same amp. Made in in 2010 just like the OP. Low volume across both channels. I found this thread in search of the solution.

I pulled the j111 jfet at the Q4 position as suggested and it resolved the problem. No loud noises on power up or down and the amp plays normally now.

Thought I post here for anyone else with this amp that runs into this problem. Thank you to all that contributed to this fix!

[glebert]

Welcome to the forum!

[Bass 300]

Thanks for the welcome

I've since replaced the transistor at Q4 and the amp is back to the same problem. I understand leaving this part out appears to cause no harm but it's not mine and the owner would like it functional with all parts in place.

Any ideas?

[Enzo]

Hi welcome.

You should start a new thread for your amp.

Q4 is a mute. A bad Q4 can silence and amp. but then the circuit that controls Q4 can go bad and also silence the amp. If you pulled it and the amp works, and replacing it returns the problem, your issue is the control circuit, not the part itself.

[SeBas]

Like Bass300, I registered just to respond to this post, but I have a feeling I will be visiting this forum more and more. I have a Frontman FM25 Dsp and the same low volume problem. I have used the fender website to request a schematic (board has a 2005 label on it while the dsp board has a 2006) which I have never found online. Still no response on their part. I now feel that I have a chance on fixing this amp. Now to find the Q4. Great site fellas.

[nosaj]

Like Bass300, I registered just to respond to this post, but I have a feeling I will be visiting this forum more and more. I have a Frontman FM25 Dsp and the same low volume problem. I have used the fender website to request a schematic (board has a 2005 label on it while the dsp board has a 2006) which I have never found online. Still no response on their part. I now feel that I have a chance on fixing this amp. Now to find the Q4. Great site fellas.

Please start a new thread. Welcome to the forum . As a general rule although symtoms can be similar they may not always be the same failure. Here is the schematic http://www.electronicstudio.net/sche...ematic_653.pdf
nosaj

[SeBas]

Thanks for the heads up nosaj. Unfortunately your link is not the board I have but because of the link you gave me I figured to search the board type of PR650 (if thats what that number is) and I found this http://bee.mif.pg.gda.pl/ciasteczkow..._Schematic.pdf. YESSSSSS... but like you mentioned I should start a new thread because the fix may not be the same. I was not able to find a Q4 on my board anywhere so that means that you are absolutely correct. Thanks to the thread I now have place to start and learned about the so called IC4 circuit and now must determine if my board has something similar. Now to also figure out how to start a thread to refer back to this one in case someone whats to see what the heck I am referring to .