Fender FM65 DSP With No Output

[Johnrcurry]

I am working on a Fender FM65 DSP (schematic attached). It powers up but has no output. Also, the lead channel led is illuminated regardless of the position of the channel select button. I've checked the voltages of TP 1-18, 12-15. They all match the schematic except TP15. TP15 is pin 5 of U1 and it shows 5VDC rather than 2.5VDC. The rest of the pins on U1 are:

1= -16VDC
2= 0.01VDC
3= 0.01VDC
4= 0.01VDC
5= 5.0VDC
6= 5.0VDC
7= 5.0VDC
8= 16VDC

I tried to trace the signal through the amp and the signal appears at the input jack, both sides of R4, pin 3 of U1 and stops there.
Am I safe to conclude that U1 is faulty? Would that have anything to do with the fact that the channel switching button does not seem to do anything?
Any other ideas as to what to look at?

Thanks very much.

John

[nevetslab]

I am working on a Fender FM65 DSP (schematic attached). It powers up but has no output. Also, the lead channel led is illuminated regardless of the position of the channel select button. I've checked the voltages of TP 1-18, 12-15. They all match the schematic except TP15. TP15 is pin 5 of U1 and it shows 5VDC rather than 2.5VDC. The rest of the pins on U1 are:

1= -16VDC
2= 0.01VDC
3= 0.01VDC
4= 0.01VDC
5= 5.0VDC
6= 5.0VDC
7= 5.0VDC
8= 16VDC

I tried to trace the signal through the amp and the signal appears at the input jack, both sides of R4, pin 3 of U1 and stops there.
Am I safe to conclude that U1 is faulty? Would that have anything to do with the fact that the channel switching button does not seem to do anything?
Any other ideas as to what to look at?

Thanks very much.

John

I think you've mixed Pin 4 with Pin 1, as Pin 4 is the neg supply terminal. voltages for pins 1, 2 & 3 then make sense. I've no idea where P1 goes, or what VQ2 is at the connector. Must be elsewhere in the documents.

As with most non-working systems, start with the power supplies. Are all the voltages present? As to U1 being faulty, I'd not yet jump to that conclusion until P1's mate and the IQ5 line is found, since that has a 5V potential on it. U1A has gain, while U1B has negative gain from all we can see on the schematic (3.9k input to inverting pin, 91 ohms & 620 ohms feedback, with a tap on that output to Lin.

The channel select LED.....does it work with the foot switch? I've never seen one of these, so these are just random thoughts.

[Johnrcurry]

nevetslab, thanks for your reply. I double checked my work and you were right. . . . I had the pins on the 1-4 side of U1 backwards. You also were right that I was not safe to concluded U1 was bad. As it turns out, U1 is fine.

I redid my signal tracing and found the signal was coming out of pins 1 and 2, through R6, and into P1. P1 is a set of header pins with a ribbon cable that connects to a complimentary set of pins on the separate DSP board. I re-seated that cable at both ends and things started working as they should. The voltage at TP15 came down to 2.5VDC and the channel switching began working, including the led behaving as it should. I've played through it and let it run for a couple hours and it seems stable. Hopefully, the problem was as simple as that and it's fixed. Again, thanks.

John

[nevetslab]

nevetslab, thanks for your reply. I double checked my work and you were right. . . . I had the pins on the 1-4 side of U1 backwards. You also were right that I was not safe to concluded U1 was bad. As it turns out, U1 is fine.

I redid my signal tracing and found the signal was coming out of pins 1 and 2, through R6, and into P1. P1 is a set of header pins with a ribbon cable that connects to a complimentary set of pins on the separate DSP board. I re-seated that cable at both ends and things started working as they should. The voltage at TP15 came down to 2.5VDC and the channel switching began working, including the led behaving as it should. I've played through it and let it run for a couple hours and it seems stable. Hopefully, the problem was as simple as that and it's fixed. Again, thanks.

John

I love it when it's something simple like that.....a little prodding, reseating a cable or two, and all is back to normal! And, I got to add one more document to my Fender database.......thanks!

[alfreema]

I was excited to find this thread. I just bought this exact same amp and have the exact same problem. Unfortunately re-seating the ribbon cable didn't resolve my 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.

I did notice one of the pins on the header was fairly tarnished so I lightly sanded it, but still no luck. I got concerned that the matching pin inside the ribbon cable might also be tarnished and causing issues, so I bought a new ribbon cable, but again no luck.

I'm not the electronics guru you guys are by any stretch, but I know my way around a volt-meter and can probably troubleshoot a little with some guidance. I was going to remove the board from the chassis, but it's really a lot more difficult than you'd think it would be (probably why they say don't repair on this amp?), so hopefully I can test everything I need without getting to the bottom of the board.

Any advice on how to troubleshoot this?

Edit: Oh and really dumb question #1: Where is the most convenient ground? I see on the schematic that one side of R110 is AC ground, so I have been using that. But is there a more convenient location?

Edit: Oh and dumb question #2: Next to the R110 call out, there is a CS1 callout. What is CS1? Is that the chassis or a switch or something?

[alfreema]

Okay, just diving back in, and trying to answer my own dumb questions ...

1) Pin2 on U8 seems like a fairly convenient place to establish DGND -- haven't checked if Pin 2 = Chassis yet though.

2) It's obvious that chassis is at least earth ground. I haven't tried to establish if the chassis is DGND and/or GNDA.

I'm going to start hitting the test points in order ....

Edit: Oooh, I found CS1 under the transformer! So now I have a convenient spot for DGND and GNDA. Yay!

Edit edit: Well nevermind. CS1 (bare wire) to TP1 is 17VAC while chassis to TP1 is 33VAC. So I guess CS1 is not GNDA?

[alfreema]

Oof everything checked out until TP14. My notes ...

TP14 = -1.43VDC (ut oh <--- should be -8.0VDC)
TP14 other leg of R89 = -8VDC -- error in schematic?

Looks like Q15 pin 3 is tied to the -8VDC leg of the resistor, so I checked that pin and it's also -1.43VDC, so I don't think the schematic has an error.

So, when checking ohms, I think R89 is showing 1M instead of 10M.
Also, when I check R88, I am getting 250k instead of 1M.
So I was a bit suspicious about my volt-meter, but when I test R105, I get 1M dead on. Also R87 is 10k dead on.

Just for fun I moved on to TP15, and I get 4.55VDC instead of 2.5VDC.

Thoughts?

[The Dude]

Resistors don't lower in value. If you are measuring in circuit, you are probably also measuring parallel components. For accurate resistor measurements, you'll need to disconnect the resistor from the circuit.

[The Dude]

I'll add: You should start a new thread for your amp. This one is a few years old and, although your symptoms may be similar, the cause may be completely different.