Fender Blues Jr IV: crackling noise -> no sound

[jimmywaleseswhale]

Bought a Blues JR IV off somebody. It crackled occasionally when playing and every time you use the volume pot or the "Fat" button. The seller said he hasn't tried changing the power tubes despite buying a new pack (which I bought off him too). This wasn't really true – the 'new' tubes he sold along side them were apparently the stock GT and the new JJ ones were already in the amp.

I tried both sets of tubes and neither was really fixing the problem.

In the morning, no sound is getting through with either set of tubes. Power ones get hot, three preamp preamp ones seem to light up. Any advice of the simple things to check? I'm really not that knowledgable about amps.

[Andy Le Blanc]

spray out the the controls with a good contact or wire harness cleaner, but make sure you re-lubricate the controls with a control/contact /lubricant.
I been in too many of those for the same issue for bad resistors in the signal path. Once you re-establish signal thru you can "chop stick" the components to find the bum parts.
Sometime the "cooling effect" of a spray cleaner can freeze and break components if they've become fragile with age.
I swear they had a bum run of resistors at mfg.

[jimmywaleseswhale]

Need to research the lubrication part.

Some extra pointers. No noise coming through when playing the guitar but just grabbing the guitar chord leads to hum.

In addition, shaking the amp leads to the usual amplified reverb spring noise. Some issue in the preamp section?..

[Stevem]

The humming cable sounds to me like the amps circuit board mounted input jack has taken a good hit and the ground connection solder points from jack terminals to the board have broken loose.
That’s the bet I am placing my money on with this one!

[pdf64]

How about if you plug into the fx return?

[Stevem]

Your thinking of the Blues deluxe, this amp has no effects loop .

Back to the problem, the amp could also have a broken volume pot due to taking a hit, even though these pots do have side supports it is possible for this to happen.

[jimmywaleseswhale]

Is it possible to sidestep the volume pot, shorting it to run at 10'? Just for testing

[Stevem]

Yes. You would have to short the output side of C1 to pin 2 on that first preamp tube.

The output side of C1 will not be that side that does not over 100 volts of DC on it.

[jimmywaleseswhale]

C1 is a capacitor, right? What do you recommend for shorting the two? Some thicker insulated wire?

Sorry, I'm quite basic when it comes to electronics. By the way, remember reading some advice to disconnect one of the preamp tubes for diagnostic purposes but not sure that is true, which one or what to look for?

Sorry for the basic questions!

[Stevem]

Don’t take this as a insult, but with your limited knowledge of these things / workings I would say you need to get the amp to a Tech for repair before you make things worse.

[jimmywaleseswhale]

No offence taken, I'm happy to admit it's not my speciality. I bought this amp off somebody on the assumption it needs a tube change, though that wasn't the case. I'm currently around $70 into this and would love to try out a few easy fixes. If I think it's going to be difficult (which I suspect it might be), I will search for a local tech. Last resort is to list it for a small price to see if somebody wants to try to fix it.

Basically, I don't think I'm capable of any elaborate work on it. If some advice comes what else could be potentially wrong and how to check it, I'd be happy to try to work it out

[sluckey]

I suggest you get some real electronic cleaner and wash out all that WD-40 residue.

[Stevem]

Let me add this also, if it turns out that the volume pot or the fat switch needs to be replaced, they are a real pain in the ass even for a Tech to do!

[jimmywaleseswhale]

https://wd40.co.uk/product/fast-drying-contact-cleaner/
This stuff is a no go?

[rooster]

Stevem is on it, jack has broken loose of its PC board traces.

This requires all the knobs to be carefully pulled off (they are hard to get off and not for the novice). Then remove all the (6?) screws that hold the board to the chassis, remove the nut off the input jack, cut any zip ties that strap the harness together to the point that the PC board is impossible to lift/remove, pull the pilot light LED out of it's socket from the inside of the chassis leaving the wires connected (remember how to but this back together), disconnect the two OT plate connections on the PC board, remove the nuts on the Footswitch and Speaker jack and pull this mini-PC board clear of the chassis. Now gently bend the 3 ribbon cables in an upwards direction while simultaneously pushing the PC board towards the tubes and lifting the side that has the ribbon cables attached - such that the shafts of the pots clear the chassis holes (push and lift carefully, but it takes a fair amount of pressure to have the pot shafts clear the chassis).

OK, when all this is done, lift the front part of the PC board (the edge where the pots are) and with a piece of masking tape, tape the lifted PC board such that you can secure it in it's lifted position to the tube socket side of the chassis (like lifting a lid). Now the $3 plastic input jack BOTTOM CONNECTORS are exposed enough to resolder the five solder points. If the traces are damaged or the jack seems compromised, then buy a new jack and replace it. Note: A clever tech can replace the plastic jack with a typical (shorting) Switchcraft jack, but you have to connect the jack correctly to the right connections on the PC board.

Or you can take the amp to somebody who has done this job more times than he can count. Remember in the future, if you go with the stock plastic jack, that you need to run your guitar cable THROUGH THE HANDLE BEFORE YOU PLUG IT INTO THE AMP - so that next time you trip over the cable you won't have to repeat this work.

P.S. These pots cannot be cleaned or lubricated; this was bad information.