Fender Deluxe 90 DSP Intermittent Problem

[jeffects]

Hi everyone, this is my first post. I'm an electronics tech with about 2 years of experience so there is a lot I haven't seen. I've read the other 2 threads on here regarding this amp and my problem is very similar but with a twist. The amp will play fine for awhile and then go dead. However, if you unplug the guitar cable from the input jack and plug it back in, the amp will come to life again. I will check the solder joints on the components listed in the other threads but I was wondering if anyone had any extra ideas. Thanks.

[Chuck H]

Check ALL connections. Never put coincidence aside. It could be the amp jack, the board connection, the guitar jack, the guitar cable. If you can't make the problem happen intentionally by wiggling one of these connections there's the possibility of an actual problem with the amp (interfacing glitch or some such). It probably wouldn't be an on board amplifier problem.

[Enzo]

Define your problem, then isolate it.

What do you mean the amp "goes dead"? The power light does out and there is no sign of life? or do you mean the sound stops, but the lights stay on? If the thing truly goes dead - no power - then the input jack is not the issue.

Next time it goes dead, ball up your fist and whack the top of it. Does that make it come back on? Or for that matter, when it is running, does whacking it make it go dead? That exposes a loose connection inside. Try a different power cord. With the amp open, use something insulated - like a wooden chopstick - and poke at the fuse. Wiggle the power switch. See on the board near the fuse is a round disc thing that sits at a bit of an angle? Part RT1. Poke that. Any of those things affect the loss of power?

If the amp merely goes silent, try plugging a cord from preamp out to power amp in.

[Chuck H]

ha ha! I just said "never put coincidence aside" and I totally missed the implications of the wording. I assumed a likely jack or connection problem because of the guitar cable plugging and unplugging.

I agree with hitting the amp. I don't know how they're constructed but sometimes plugging and unplugging the guitar from the amp can move the whole board around. Making it appear like a jack problem when it's a fault somewhere else on the board. Sometimes hitting reveals this when wiggling the jack won't.

[jeffects]

Sorry for my lack of detail in saying the amp goes dead. What I meant was the signal stops but the amp remains powered. Right now I have the board outside of the chassis and am resoldering the input jacks. I will also check other solder points listed in the other threads. I'll put the board back in so that the output transistor heat sink is connected to the chassis, turn it on and poke around with my wooden dowel. And I'll definitely plug a patch cable from preamp out to power amp in to see if that makes a difference. One weird detail is that when you plug the guitar cable back in after it has stopped, there's like a split-second delay before the signal starts. This makes me think it's almost like a muting circuit turning on and off.

[g1]

There is a mute circuit and that may be it's normal action.
To verify whether the mute circuit is at fault, monitor voltage at TP26. Voltage when mute is engaged (nothing plugged in) should be approx. 0VDC. Normal operation (mute off) should be -30VDC at TP26.
Check to see what TP26 reads when the signal cuts out, if it is proper -30V, then the mute circuit is not at fault.

[g1]

Oops, looks like a typo on the schematic, for TP26, it says mute condition voltage is "-.39.5V". I said approx. 0V above, but I think it is actually -39.5V at TP26 under mute condition with nothing plugged in.

[jeffects]

Sounds good g-one. Just got a copy of the schematic and will check that first thing in the morning. If the mute circuit is engaged while the cable is plugged in (causing the signal loss), then something is triggering it. I currently have the input jacks out and they look rather normal and functional, although sometimes that's a tough call with only a visual inspection. I would wiggle them with the cable when I lost the signal and couldn't get it to come back on. The only thing that would fix it would be to unplug and plug back in. What else could cause the mute circuit to trigger besides the jacks?

[g1]

Mute can also be triggered by an overheat condition via RT2, or any fault in the mute circuitry itself.
Since they no longer use FET's for mute, I'm less inclined to suspect the mute circuit, I think you may find it is not triggering and your amp fault is a more basic intermittent connection somewhere.
Using the power amp in jack rather than the regular inputs will help isolate the problem. If it is a mute issue, the fault should act exactly the same using the power amp in jack.

[jeffects]

I ended up resoldering the input jacks, U10 and its heatsink, the send/return jacks, and all the concrete resistors. The amp operated fine for over 6 hours without losing signal where it would die in about 15 min. before. Returned to customer. Thank you all!