Frontman FM25 DSP, low volume

[SeBas]

Hello everyone, while searching for a solution for my low volume problem I googled apon this thread (https://music-electronics-forum.com/...t=45628&page=2) which could potentially be the same solution for my problem. Obviously the models where different, so most likely it isn't, but the idea alone of the IC4 circuit and the Q4.
Heres the quote from the user g1:
"In any case, like Jazz said, it looks like you are losing signal before the output IC, and you said it is weak even using the aux in.That narrows it down quite a bit, IC4 and Q4 areas.What is the gate voltage at Q4 ? "
"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."

This worked for the user and the problem was fixed. Now I did finally find a schematic for the FM25 DSP and it was thanks to that thread. So in my search to see where this IC4 and the possible culprit Q4 is located on my board, and low and behold there is no Q4. So my symptoms are the same to the original FM25R thread. I would like to add that before this low volume issue arose, the amp would start to distort or fuzz ever so slightly with the signal to the output. Then a few more times later the amp started to fuzz out the signal to the point where it removed the amplitude of the sound as if it was being choked. Then it happened. "LOW VOLUME".
I look forward to all your advise or help that you can offer, because from what I have read so far your all talented buckets of knowledge and I really really really want to fix this amp.

Here is the link to the FM25 R schematic from the original thread, https://music-electronics-forum.com/...4&d=1514322135

Here is the link to my board, http://bee.mif.pg.gda.pl/ciasteczkow..._Schematic.pdf

[nosaj]

You want to check the rails
c48 + should have +27vdc
c47 - should have -27vdc
c43 + side should have +12vdc
c42 - side should have -12vdc
U-4051B pin16 should have +5vdc
see if there is at least 5 volts DC. If not, replace it

nosaj

[SeBas]

Thanks for the reply nosaj. All the components check out with their current voltages. The C47/48 had 29.4 volts dc but that should still be within the specs. So it would seem that non of these components are suspect anymore. I'll ask if these components have anything any relation to the FM25 R IC4 mute circuit. Not sure how a U4051b mux works or what it exactly does because I would be at the very bottom of the electronic knowledge ladder.

[nosaj]

Thanks for the reply nosaj. All the components check out with their current voltages. The C47/48 had 29.4 volts dc but that should still be within the specs. So it would seem that non of these components are suspect anymore. I'll ask if these components have anything any relation to the FM25 R IC4 mute circuit. Not sure how a U4051b mux works or what it exactly does because I would be at the very bottom of the electronic knowledge ladder.

What were doing is making sure the power supply works correctly. Cause after the wall outlet is the power supply and if the power supply isn't right then nothing else can be right. I always start this way to define a logical order.

Check U8 pin3 for +5 vdc
Then on all TL072's check pins 4 for -12vdc and pins 8 for +12vdc

Then u7 pin1 for 27vdc
If you want to take it a step further any voltage listed on the schematics check it and then report back on any discrepancies.


Sometimes on schematics you will get IC that don't list all the pins you can sometimes find a separate section that shows power connections on them like this schematic does. Not all schematics do that and in those cases you can pull the datasheet for that IC which will give you operating parameters and pin connections and sometimes a sample circuit

nosaj

[SeBas]

I might be wrong but I think the schematic I have may not be the correct one. I was looking at it to find all the TL072 and on the schematic it makes reference to components that I cant find on the board. I found U4 has two if them U4-A and U4-B. There are a few more like that like U2-A/B and U3-A/B, but on the board there is only one U4 U2 and U3. I will try to attach a pict of the board and you can see what i got component wise.



I contacted Fender via there online request and asked for a service manual / schematics for the FM25 DSP. That was last week and still no reply.
until then I will try to get those readings for you nosaj.

[nosaj]

I might be wrong but I think the schematic I have may not be the correct one. I was looking at it to find all the TL072 and on the schematic it makes reference to components that I cant find on the board. I found U4 has two if them U4-A and U4-B. There are a few more like that like U2-A/B and U3-A/B, but on the board there is only one U4 U2 and U3. I will try to attach a pict of the board and you can see what i got component wise.



I contacted Fender via there online request and asked for a service manual / schematics for the FM25 DSP. That was last week and still no reply.
until then I will try to get those readings for you nosaj.

TL072 has 8 pins It is essentially 2 separate op-amps (containing ~30 transistors each) in an IC package... so U4a and U4b are the same chip The schematic notes the pin numbers .
nosaj

Here is a datasheet for the tl072 package.
http://www.ti.com/lit/ds/symlink/tl072-ep.pdf

[Helmholtz]

TL072 has 8 pins It is essentially 2 transistors in an IC package

Should read 2 separate op-amps (containing ~30 transistors each) in an IC package....

[SeBas]

Been real busy this week. Finally got some time and all the testing I can do. Rather than write up all the test spots I just put it on the schematic. Hopefully you can decipher it.



I then tested the P1 connection that goes to the dsp board.

[g1]

Here is the link to my board, http://bee.mif.pg.gda.pl/ciasteczkow..._Schematic.pdf

Notice in the lower right of the schematic it says 'sheet 1 of 2'. The missing page will be the schematic for the uDSP board. Not much can be done without it, aside from swapping out the DSP board.
Any luck with getting the schematic from Fender? Maybe their version would have the other page.

[DrGonz78]

It's really easy to just call them and ask for it.

https://shop.fender.com/en-US/contact-us

[SeBas]

Notice in the lower right of the schematic it says 'sheet 1 of 2'. The missing page will be the schematic for the uDSP board. Not much can be done without it, aside from swapping out the DSP board.
Any luck with getting the schematic from Fender? Maybe their version would have the other page.

I spoke with a guy at fender via telephone and low and behold they don't have sheet 2 of 2. No trace of this uDSP board schematic anywhere. I decided for some strange reason to plug the speaker back into the unit as it sits on my workbench and I fed a signal from my laptop (music) to see if the sound is still very low.

IT WORKED !! I was surprised and thought what did I do? The answer was I did nothing. So I played with the channel 1 and channel 2 (dsp) and then it just went quiet. I was playing with volume on channel 2 at that moment, so I checked channel 1 and it was the same. Then the volume came back up then low again seconds after. Then on again but this time it was fuzzy and got lower and lower is volume. Then my dog wanted to see what i was doing and stood up on the bench and knocked the amp and disconnected the speaker. So I reconnected it up and tried again. Sound is back and it stayed on for almost a minute while I wiggled each pot and played with the volumes. I left it playing and without touching anything I can heat the volume decrease ever so slightly, then a little more and start to get fuzzy then the volume came up then down with more fuzz and then lower still until I can hear nothing. Sorry for the long description. It sounds like the dsp is doing its thing as the effects and the different amp types all worked. I mentioned in an earlier post it sounds like the amp or signal is starving for power or being choked out slowly( fuzzy until nothing)

Is there not a mute circuit on this amp like the FM 25r and its q4 issue? Going to test these diodes that are on this PCB. I dont really want to remove each one so i hope can I test them reliably on the board. With the power disconnected and shorting out the u7 pin 1 and 4 to remove any power to the D5 and D6 (because i found 2.2v on these silicone diodes) I went ahead and tested all the diodes. Here is the result of my finding with the diodes still on board and my mutlimeter set to diode. First reading will be with negative lead of DMM to anode and positive lead to cathode. The second reading is in reverse.
D1 : 1.668v / 0.642v
D2 : led
D3 : 1.668v / 0.634v
D4 : 1.666v / 0.632v
D5 : 1.5v >>2.1v / 0.563v Im using this >> symbol to show that the voltage climbs
D6 : 1.5v >>2.2v / 0.571v
D7 : 0.64v / 0.64v I think this might be a bad one
D8 : 0.64v / 0.64v or these two diode might be some thing else in line that is making the readings the same
D9 : 2.774v / 0.64v
D10: 0L / 0.642v oh oh is it dead
D11: 2.76 >> 2.81v / 0.635v
D12: 1.74 >> 3.0v then 0L / 0.734v
D13 : >>> 3.0 > 0L /0.594v
D14 : >>> 3.0 > 0L / 0.634v
D15: >>> 3.0 > 0L / 0.634v
D16: >>> 3.0 > 0L / 0.593v
D17: 0.5 >>> 1.997v / 0.637v
D18 : 0.5 >> 1.752v / 0.671v
D19 led
D20: 0.5 >> 2.075v / 0.555v
D21: 1.5 >>2.077v / 0.552v
D22: 0.5 >> 1.904v / 0.555v
D23: 0.5 >> 1.904v / 0.554v

I might have just wasted my time doing all this while they are still in a circuit but I hope this might help.

[SeBas]

I will try to keep my posts short. I feel maybe the lack of responses may be attributed to amount of time it will take to read my post. \\

So the question is can you tell if a diode is bad while still on the PCB. From my last long winded post here are my numbers.
First reading will be with negative lead of DMM to anode and positive lead to cathode. The second reading is in reverse.
D1 : 1.668v / 0.642v
D2 : led
D3 : 1.668v / 0.634v
D4 : 1.666v / 0.632v
D5 : 1.5v >>2.1v / 0.563v Im using this >> symbol to show that the voltage climbs
D6 : 1.5v >>2.2v / 0.571v
D7 : 0.64v / 0.64v I think this might be a bad one
D8 : 0.64v / 0.64v or these two diode might be some thing else in line that is making the readings the same
D9 : 2.774v / 0.64v
D10: 0L / 0.642v oh oh is it dead
D11: 2.76 >> 2.81v / 0.635v
D12: 1.74 >> 3.0v then 0L / 0.734v
D13 : >>> 3.0 > 0L /0.594v
D14 : >>> 3.0 > 0L / 0.634v
D15: >>> 3.0 > 0L / 0.634v
D16: >>> 3.0 > 0L / 0.593v
D17: 0.5 >>> 1.997v / 0.637v
D18 : 0.5 >> 1.752v / 0.671v
D19 led
D20: 0.5 >> 2.075v / 0.555v
D21: 1.5 >>2.077v / 0.552v
D22: 0.5 >> 1.904v / 0.555v
D23: 0.5 >> 1.904v / 0.554v

Bear with me and my lack of knowledge on this matter.

[Enzo]

Stop testing every part at random.

Page 2 is simply a VERY short parts list for the model. PArts like a few knobs, transformer, handle, etc.

Components do not fix themselves. Your problem is the amp works, and then some of the time works poorly, then works again. That indicates a connection somewhere.

My first suspect is the phones jack. The cutout switches inside get dirty, and exactly what you describe can happen. Get some cleaner in there. In fact just poking a plug in and out of that jack might wake it up, and if so that agrees with my diagnosis. For that matter, does this same condition exist listening on phones?

[SeBas]

Stop testing every part at random.

I started testing components only after the suggestions made on this thread. "nosaj" explained to check the power with all the power points listed on the schematic. Only after it all checked out, did I decide to check only the diodes, doesn't seem to random to me but hey I'm the new guy to this so I guess random is what I am.

Page 2 is simply a VERY short parts list for the model. PArts like a few knobs, transformer, handle, etc.

the pdf of the schematics that I post actually has the parts list as the first page. So I believe that there is another page to the schematics itself like "g1' suggested. But its not about the second sheet now is it. I'm looking for the culprit behind my issue.

Components do not fix themselves. Your problem is the amp works, and then some of the time works poorly, then works again. That indicates a connection somewhere.

I realized that components aren't self healing, I just wanted to see if I got any output to the speaker as it hasn't done that the last dozen times I tried it. I started this teardown and inspection of the board with cleaning all the pots and jacks, and re-soldering all the contacts to the pots and jack after that initial cleaning the amp still had very low volume.

My first suspect is the phones jack. The cutout switches inside get dirty, and exactly what you describe can happen. Get some cleaner in there. In fact just poking a plug in and out of that jack might wake it up, and if so that agrees with my diagnosis. For that matter, does this same condition exist listening on phones?

The problem of low volume was also present on the headphones as one of the symptoms when this first happened. For good measure I cleaned the jacks again and the same problem occurred, it was playing and then just dropped in volume, plugged in headphones, same thing. So I cleaned them for a third time now with some more cleaner and while typing this post I turned the unit on again and its been working since. Crossing my fingers "enzo" that you are right. I'll update the group when I get back from walking the dog....then I'll test them with a guitar.
Thanks,,,

[SeBas]

So I cleaned them for a third time now with some more cleaner and while typing this post I turned the unit on again and its been working since. Crossing my fingers "enzo" that you are right. I'll update the group when I get back from walking the dog....then I'll test them with a guitar.
Thanks,,,

(well darn to that)

I have been enjoying myself practicing on the newly revived amp that has been working since my last post. Well so I thought. I was about 30 mins into a session when it stopped throwing out those sweet loud tones. The low volume has returned. Crap , I had my fingers crossed that this nightmare was over. So I shut it off and gave all the jacks another small squirt and cleaning. I made sure I gave it some extra plug action to work the cleaner in good. I turned it back on and the sound was back and bright. Hmmmm could it be the jacks..or was it the reboot... I continued to play for another 1.5 maybe 2 hours to work the amp when it gave up again. This time I was on channel 1. There was no sound but a faint buzz with volume at full. I tested for low volume on channel 2 and with the knob at full only a quiet guitar sound and I tested all effects or amp types. The headphones could also hear the low guitar sound. I tested the rca out and there was no sound at all. ....so
channel 1 buzzing,
channel 2 very low. on speaker and head phones but not rca out.

I did not clean the jacks but this time just shut the unit off while I wrote this post. I wanted to see if that down time, the amp had, would help and when I tried the power, it worked again. The volume was back. well that's the update... I'll wait before I open up the unit again..