Fender Blues Deluxe troubleshooting repair tips lost power

[toobnoob]

Good morning,
After my success getting the Vox tremolo working
I've decided to tackle the pile of broken amps I have here.
Can anybody help me figure this out, I have a Blues Deluxe here that lost power while I was playing, it faded out and now it's dead. I'm not really sure where to start.
I measured plate voltage, tested the tubes, looked for anything burnt and haven't found anything obvious.
What kinds of things would cause it to lose power, fade out and go silent?
Thanks

[sluckey]

Could be almost anything. Start by subbing known good tubes. If no joy, open it up and measure voltages for each B+ node and all tube pins. Compare to schematic.

[toobnoob]

Could be anything right?
I checked all t he tubes, they're good.
Pulled the board, no burnt marks or anything obviously suspect.
The way it faded out made me think something in the power supply...?

[shooter]

Like Sluckey said, volts, good or bad?  want odd, OT, speaker.  Imagine you are the "sound", you go in here, you come out there, enjoy the ride, take a schematic, meter, listening amp n ALWAYS a towel!

[toobnoob]

Here are some more details I can think of.

My friends says his amp lost power and quit working and also had a broken input jack so I say bring it over let me take a look.
I get the amp plug in and it seems to work fine so I replaced the input jack and notice two ceramic 5w resistors on the board look burnt so I replace them.
I bench test it and it seems fine, I put it back together and discover that at a loud volume it has a crackling distortion that is triggered by the vibration of the combo cab.
At this point I start guessing and I replace all the filter caps, put it back together and the crackle is still there with vibration only, but now I notice it's more of a crackle/cut-out and I see the Drive LED flicker with the noise/effect.
I prod the power supply caps with a chopstick and the power/sound fades away to silent (like the original complaint) and now I have no signal passing.

Anybody?

[shooter]

do you have a meter or just a soldering pencil?  I've learned here crackling can be a lot of things but bad solder connections, especially grounds, are a good bet.  Your really need a meter, a schematic n your chopstick.  Start at power, end at speaker, fixing things as you go.

[thermion]

Were the resistors you replaced R78 and 79? These are in the solid state part of the amp and notorious for generating a lot of heat in their vicinity causing intermittent/ weak/ bad solder joints and manifesting in switching issues, reverb loss, etc. Then also you mentioned poking around the b+ filters and losing signal...can you measure b+ plate voltages anywhere at this point, now that it's dead?
 

[jojokeo]

These amps are notorious for crappy pots, jacks, wave soldering, etc...look closely at soldering contacts like Thermion says. BUT first make sure that the speaker is plugged into the correct output jack and look at the jack's soldered posts. Next, get a 1/4" jack plug and plug it into and out of the Send/Return jacks maybe along with contact cleaner. It wouldn't be the first time that this could be your culprit.

[BrianS]

I'm hesitant to respond, as you've already gotten some good advice, but maybe I could add some "reinforcement" to some ideas already talked about.  FWIW, I've worked on 3 different Hot Rod Deluxes over the past couple months and they all had similar issues.

I found it's very easy to lift the traces when soldering parts in on these amps.  I think it's possible that you may have done this (or close to it) when replacing the filter caps.

I also found numerous cold and cracked solder joints in the 3 amps I repaired.  I used my magnifying light to carefully look over the PCBs and carefully reflowed any suspect joints.

If you haven't yet, I'd suggest building a "listening amp".  There are instructions on the tech tips board.  This will allow you trace your input signal to see where it is disappearing.

Good luck.

[Willabe]

I'm hesitant to respond, as you've already gotten some good advice, but maybe I could add some "reinforcement" to some ideas already talked about. 

Why?           Good to hear from more guys in the same thread who have worked on and fixed the subject amp.


                Brad     

[toobnoob]

Oh boy third time trying to post this.

I have the time now to fix this amp and after sitting a while I plug in and it seemed to work fine at first then started crackling and then cutting out with the Drive LED flickering, then it slowly faed out as if it were being choked off, then silent, no signal. Still measured plate voltage etc but no sound

I have replaced the filter caps, input jack, two power resistors that were burnt looking R85 and R86' cleaned the FX loop jacks, speaker jack, checked all the ribbon cables with the meter for breaks, tested the tubes. I'm  at a loss at this point.

Also how do you check and adjust the bias on these?
There's a 27k resistor R87 do I have to change that? How can I put a trimmer in there?
The bias voltage is 44.8v a little off from what the schematic calls for -46.7


Any help would be GREATLY appreciated :)

[toobnoob]

Weird thing, I tried another amp's preamp out to the Fender's power amp in and it worked! But then it slowly got more distorted, with hard picking it was distorted then after a minute or so it faded out and no signal.
I measure all the plate voltages of the power amp and preamp and the all seemed normal.

Another strange thing is no sound when I probed the plates in the power amp and the preamp except the PI which you could hear a click.
The reverb was mute as well, before I could rattle the tank and hear the springs but now it's dead UNTIL I switched off the standby switch and turned it back on, then I could hear rever again but still no signal otherwise.

I have no idea what's going on here.

[BrianS]

This is what I'd do:

-Clean all pots, jacks, switches & sockets. (I know you said you've done at least some of this)

-CAREFULLY look over all the PCBs for bad solder joints, i.e., cracked, cold, suspicious looking joints.  Use a good light and magnifier.  Repair as needed.

-Check the low voltage power supply for the solid state devices in the amp.  You should have a +16v rail and a -16v rail.  There are numerous solid state devices throughout the amp's circuit that could cause problems if they are not working properly.

-Inject a signal into the input and start tracing it through the amp with either an o-scope or listening amp.

I don't believe there is any shortcut to troubleshooting the problems you are describing.

Good luck!!

[shooter]

You measured bias volts, was that while it was working or after it quit working?  Do the same for plate n cathode on power tubes, while working, and after it fades to nothing.
Use the other amp to drive your problem unit.  pull all the tubes from the problem unit that are NOT needed with the other amp providing signal. Any changes?

[thermion]

Bias is derived through R84 and 87 and accompanying circuitry. No pot, you must change static resistor values.
Perhaps the channel switching circuitry has turned off both channels somehow? Does the preamp out work if you run it to another device? Considering what you said about the preamp in, I vote for a failing TL072 at this point.