5150 III 50w died. Amp gurus please come on in

[Stramm8]

so I turned the amp on, flipped standby off and heard a very faint "pop" then nothing. No signal. Swapped out power tubes, nothing. Rolled through all the pre amp tubes (which I'm going to do one last time), nothing. SLO blow fuse tests good. Checked all the internal fuses ( the schematic shows 5 fuses total) all have continuity. Checked the bias resistors and they measured right at spec, nothing. I'm at a loss and I hate taking stuff to the shop LOL! Any other ideas??

 

[kurtsstuff2003]

I'd start looking for burnt resistors/Capacitors. Did you notice any kind of "smell"??

 

[Stramm8]

I'd start looking for burnt resistors/Capacitors. Did you notice any kind of "smell"??

Not really. I've tested just about every resistor so far but will take a look at the caps. Thanks!

 

[glpg80]

The golden rule of troubleshooting is thall shall check all source voltages and ground connections. If you replaced the tubes with known working ones, and still have no sound, I'd be suspect of the muting circuit they use in them. The JFETS are common troubleshooting points because they fail drain to source short, which fender usually tie the source to ground, so it pulls the signal to ground.

5150 III's are a pain to troubleshoot because of the complexity - you can't bring it up slowly on a variac because of the digital muting circuit mucking it up. IMHO I'd start with signal injection and start scoping the signal.

 

[Stramm8]

The golden rule of troubleshooting is thall shall check all source voltages and ground connections. If you replaced the tubes with known working ones, and still have no sound, I'd be suspect of the muting circuit they use in them. The JFETS are common troubleshooting points because they fail drain to source short, which fender usually tie the source to ground, so it pulls the signal to ground.

5150 III's are a pain to troubleshoot because of the complexity - you can't bring it up slowly on a variac because of the digital muting circuit mucking it up. IMHO I'd start with signal injection and start scoping the signal.

How would the JFETS be notated on the schematic and can they be tested with a DMM?

 

[mystixboi]

Message sent

 

[glpg80]

The golden rule of troubleshooting is thall shall check all source voltages and ground connections. If you replaced the tubes with known working ones, and still have no sound, I'd be suspect of the muting circuit they use in them. The JFETS are common troubleshooting points because they fail drain to source short, which fender usually tie the source to ground, so it pulls the signal to ground.

5150 III's are a pain to troubleshoot because of the complexity - you can't bring it up slowly on a variac because of the digital muting circuit mucking it up. IMHO I'd start with signal injection and start scoping the signal.

How would the JFETS be notated on the schematic and can they be tested with a DMM?

Easy to google and get a schematic symbol of a JFET - they're very common. Yes they can be tested with a DMM. You have gate, drain, and source. Using the diode measuring function, remove the JFET from circuit, and treat gate to drain and gate to source as a PN diode junction. P is anode (positive), N is cathode (negative). JFETs only operate in depletion mode unlike MOSFETs, but you can have N channel or P channel like MOSFETs. Assuming N channel JFET, P would be the gate and N would be either the drain or the source. See that you have a forward bias voltage one way, and open circuit the other. Do this for each PN junction - gate to drain and also gate to source. The open circuit direction and forward biasing direction should be the same for gate to drain as it is for gate to source FWIW. The forward bias voltages might be different due to residual charge buildup.

Again, I would signal inject and follow it through the amplifier to determine where it's dropping out. Wherever that location is, would determine the area you would then want to inspect.

 

[Stramm8]

Message sent

Thanks Just emailed him.

 

[Stramm8]

The golden rule of troubleshooting is thall shall check all source voltages and ground connections. If you replaced the tubes with known working ones, and still have no sound, I'd be suspect of the muting circuit they use in them. The JFETS are common troubleshooting points because they fail drain to source short, which fender usually tie the source to ground, so it pulls the signal to ground.

5150 III's are a pain to troubleshoot because of the complexity - you can't bring it up slowly on a variac because of the digital muting circuit mucking it up. IMHO I'd start with signal injection and start scoping the signal.

How would the JFETS be notated on the schematic and can they be tested with a DMM?

Easy to google and get a schematic symbol of a JFET - they're very common. Yes they can be tested with a DMM. You have gate, drain, and source. Using the diode measuring function, remove the JFET from circuit, and treat gate to drain and gate to source as a PN diode junction. P is anode (positive), N is cathode (negative). JFETs only operate in depletion mode unlike MOSFETs, but you can have N channel or P channel like MOSFETs. Assuming N channel JFET, P would be the gate and N would be either the drain or the source. See that you have a forward bias voltage one way, and open circuit the other. Do this for each PN junction - gate to drain and also gate to source. The open circuit direction and forward biasing direction should be the same for gate to drain as it is for gate to source FWIW. The forward bias voltages might be different due to residual charge buildup.

Again, I would signal inject and follow it through the amplifier to determine where it's dropping out. Wherever that location is, would determine the area you would then want to inspect.

Thanks!! I've located most of them. I read a similar thread about a Peavey 5150 with the same suspected issue. The owner removed the JFETs one at a time until he found the bad one. To test, he played through the amp after removing each one. Do you think I could try the same method? Sounds like they are used to momentarily mute the signal during channel changes to reduce the "pop"...

 

[glpg80]

It's not for certain they are the cause - as I stated earlier you should be checking all source voltages and grounds first and foremost. Start with the basics - don't overcomplicate the repair by shotgunning parts at random - let the measurements guide you. You could end up removing all of them and the problem be located somewhere else. However, if you plan on replacing all of them anyway while you're in there, just clip the drain leg of each one to see if signal comes back. Start logically from the ones closest to the input jack and then working your way forward.

 

[scottosan]

Even if it were muting you would get faint white noise through the cab.

 

[strungout82]

You can try to plug your guitar directly into the effect loop return and see if the power amp section is fine. Or you can slave preamp to another amp poweramp section (connect effect send of your 5153 to another amp effect return). This way you can isolate the problem to preamp/poweramp.

 

[Stramm8]

Even if it were muting you would get faint white noise through the cab.

I am getting faint white noise. I’ve reached out to Howard Kaplan and he gave me some things to try along with some of ya’lls suggestions. If the simple things don’t work it’s off to the shop. Thanks!