2009 Fender Blues Deluxe Reverb Hum and... A poor past repair??

[Steelwitch]

Hey ya'll,

A 2009 Blues Deluxe came to me with a gnarly hum issue. After visual inspection it looks like someone scratched off the trace to C53, a 750pf cap. looks like a coupling cap for one of the preamp stages. additionally theres an 844 ohm resistor soldered between one leg of R9 to negative leg of C36, a 22uf 500v cap.

Schematic can be found here: https://www.thetubestore.com/lib/the...atic-Rev-A.pdf

I'm wondering why someone would do this? If someone could provide some insight, that would help me a lot! I will ultimately end up disconnecting this resistor and reconnecting the trace for this 750pf cap, but curious to know if there is a reason someone might have done this. Thanks!

[xtian]

The 844R resistor, is that connected from the junction of R9/R52 to ground? If so, that attenuate the signal significantly.

Which side of C52 is the trace removed? Show specifically on the schematic which trace was cut. There are too many possibilities to guess.

If it was mine, I'd replace everything to stock and reevaluate.

[drewl]

Some "killer new mod" someone probably read on the net done by someone who shouldn't have.

I've seen several.

[g1]

Aside from the hum, was the amp working properly, both channels? Because with the mods as described, I would not expect it to.
If it is working right aside from the hum, I'd suggest retracing the mod before undoing anything.

[Steelwitch]

Here’s some images of what I’m talking about. I wouldn’t really expect it to work either. Apparently the amp has been working fine for years

https://ibb.co/H77fyd3
https://ibb.co/d0RRsg0

[Steelwitch]

Bringing it back to stock will not be difficult, but it’s kinda crazy how it’s been functioning normally.. but maybe I’m missing something here.

[52 Bill]

The value of R9 has been changed from the schematic value of 270K, and what I see is a 240K resistor which would be the value of R52.

As for the cut trace, I'd like to know what is under the board. If that trace has been cut, there would be no signal path for the clean channel signal to get to through to the output. There may be added wires or components on the back side of the pc board.

[Steelwitch]

I’m pulling the board the check what’s underneath now. Will update

[Steelwitch]

No rewiring or bridged connections under the board.. I’m going to remove this resistor and repair the connection on the top of the board and see where we are

[Steelwitch]

I pulled the 270k resistor and repaired the trace. No effect. I started signal tracing early on in the circuit and a 1k signal looks pure far past the first few preamp stages. I’m thinking it could be the power supply, or output stage.

[Steelwitch]

The value of R9 has been changed from the schematic value of 270K, and what I see is a 240K resistor which would be the value of R52.

As for the cut trace, I'd like to know what is under the board. If that trace has been cut, there would be no signal path for the clean channel signal to get to through to the output. There may be added wires or components on the back side of the pc board.

It’s a 240k for a deville 150k for deluxe, so it’s a 150k in this case

[Steelwitch]

Signal looks good across all controls. So it’s somewhere between the output stage and Output. Voltages on the power supply look normal. From 345-360v across all electrolytics. You can see the hum clearly on P19, primary for the OT, so it’s not a problem across the OT but could it be an open primary winding? Thinking it could be something on the output board too but there’s nothing that visually stands out. Hoping someone who knows these amps really well can nudge me in the right direction! I’ve already tried another pair of output tubes.

[The Dude]

The Illinois electrolytic caps in those are notorious for being bad. Take a known good cap of similar value properly voltage rated and clip it across each cap in the amp, one at a time, to see if the hum goes away.

Also, will the amp bias properly?

[g1]

Those are not mods. There were problems with the circuit board design and that is the way it came out of the factory. I'd suggest you put it back how it was so you do not introduce problems that are not immediately noticeable.
You should also find that the proper spot marked "R52" on the circuit board (far side of K2 relay) has a jumper instead of a resistor.

[Pedro Vecino]

I had never seen it but the cut on the track does not necessarily mean that the 750p capacitor is deactivated because the boost channel would not sound.
It can be to avoid the path of the signal until reaching R52, or to assign another ground point.
It seems to correspond to the original construction but I do not know why because the original design in that same version (the modern one) has no problems.