Fender FM65 DSP w/power but no sound

[alfreema]

I have a Fender FM65 DSP that powers on (get a red power light), but no sound. When I press/depress the channel selector the light stays on regardless.

I used the schematic and circuit board component layout found here:

https://schematichell.com/fender/FM65_DSP_sch.pdf

Note: I was continuing an old thread with an identical set of symptoms here, but sadly the resolution there didn't work. I tried replacing the ribbon cable discussed in that thread, cleaning some header pins, etc, but nothing I did with the ribbon cable fixed the issue. While probably very dumb, I tried removing the cable with the amp powered on to see if possibly bad contacts were causing any issues, and while removing the cable I did get a nice loud output on the speaker for a brief moment (scared the heck out of me too because I had the volume up louder than I realized). I'm too nervous to keep doing that though, as I don't want to damage the DSP.

Since the resolution about didn't work, it was recommended to me that I start a new thread for my amp issue, so here goes ..

---

My first major concern is TP14. A minor concern was TP9 was off a little more than I would expect, but doesn't seem too big of a deal.

For ground I am using the chassis.

Chassis to ...

TP14 = -1.43VDC (UT OH <--- should be -8.0VDC)
TP14 other leg of R89 = -8VDC -- Hmm, error in schematic? Verify by checking Ping 3 of Q15 -- ut oh -1.43 -- should be -8VDC so not an error in the schematic.
TP13 = 0.97VDC
TP12 = 3.98VDC
TP11 CH1 = 3.15VDC
TP11 CH2 = 3.82VDC
TP11 CH1 effects off = Can't test
TP11 CH2 effects off = Can't test
TP10 CH1 = 1.8VDC
TP10 CH2 = 2.48VDC
TP10 CH1 effects off = Can't test
TP10 CH2 effects off = Can't test
TP9 w/ plug = -33.4VDC (is -1.4V too much? -- get slight static when testing, cap to side)
TP9 w/o plug = -42.8VDC
TP8 = 4.9VDC
TP7 = 12VDC
TP6 = -17.3VDC
TP5 = 17VDC
TP4 = -43.7VDC
TP3 = 43VDC
TP2 = 33VAC
TP1 = 33VAC

I'm not sure how to dive into the issue with TP14. Out of curiosity I moved on to TP15, and I get 4.55VDC instead of 2.5VDC, similar to the OP in the other thread. But he didn't complain about TP14, so I probably have something else going on upstream.

I get 1M across R89 in circuit (instead of 10M) and 250k across R88 in circuit (instead of 1M) -- seems okay (thanks for reminders regarding testing resistors The Dude ).

Not sure what to try going forward? Any thoughts are appreciated.

[g1]

Fet gate at TP14 can be difficult to get a measurement on with some meters.
First thing, try with headphones. If you get sound, you may just have a problem with the headphones jack.

[alfreema]

Fet gate at TP14 can be difficult to get a measurement on with some meters.
First thing, try with headphones. If you get sound, you may just have a problem with the headphones jack.

Yes, I tried with headphones, and still no sound.

When you push the channel selector switch, the channel doesn't switch -- the LED stays lit no matter what, so that's an indicator to me that there's a circuit issue. Also with TP15 being 5V instead of 2.5V and that voltage reading I am getting on TP14, I'm concerned about a component issue somewhere, or the DSP.

[g1]

Sounds like you may be dealing with 2 separate issues. For the 'no sound' and TP14 issue you can try removing Q15.
That is just a mute during turn on, so you might get a pop or thump, but see if you get sound with it removed.

[alfreema]

For the 'no sound' and TP14 issue you can try removing Q15.

Interesting. I'll try this early next week.

[alfreema]

Sounds like you may be dealing with 2 separate issues. For the 'no sound' and TP14 issue you can try removing Q15.
That is just a mute during turn on, so you might get a pop or thump, but see if you get sound with it removed.

Getting the board out of the head unit is a major PIA. I was hoping to work off the top of the board but that would mean clipping the JFET (Q15) instead of desoldering to remove it and test without it in the circuit. So I spent today fighting just getting the circuit board out where I can get behind it. In doing so I noticed an area where the board is getting particularly hot. The diodes in this area seem okay and the ceramic resistors do too, but just thought I would bring it up. I'll post a picture when I log in from my phone.

[g1]

If it's the zeners and power resistors in the low voltage supply circuit, it's normal for them to run hot and the board usually gets browned in those areas.
Clipping Q15 is a good option, especially if the leads are long enough that it can be re-used later. Then you can pull the board to install a replacement, or deal with some other problem if it is not at fault.

[alfreema]

If it's the zeners and power resistors in the low voltage supply circuit, it's normal for them to run hot and the board usually gets browned in those areas.

Yes, they are the power resistors and diodes. Good to know this is normal.

Clipping Q15 is a good option, especially if the leads are long enough that it can be re-used later. Then you can pull the board to install a replacement, or deal with some other problem if it is not at fault.

I went ahead and desoldered it to remove it. I do get a little pop from the speaker now when I turn the amp on so I guess the JFET was working.

Based on the other thread where the OP had a similar issue, and fixed it by re-seating a ribbon cable, my guess is the problem is on that end of the board. Like him (before fixing it by re-seating the cable), I am getting +5VDC at test point 15, instead of +2.5VDC. I think I need to focus over there, but kind of clueless on what to try.

[alfreema]

Okay my transistor tester came in last night, and the JFET seems to be bad. I'll get a replacement, and see if TP14 is corrected before I move on to TP15.