Fender Blues Deluxe (1993) -- Very Weak Signal

[thud33]

My first post here, so apologies in advance if I am not using best practices. I am willing to learn / improve!
I have been working on rehabbing a fairly abused original Fender Blues Deluxe (USA, 1993), and this amp has experienced some of the usual problems, and its main board was additionally abused prior to my work on it. I have done quite a bit of testing and work already, but I am experiencing a VERY weak signal from the front end, with barely any sound produced by amp from guitar into input jacks, though it is there. Here's what I have done / tested so far:

--All tubes are good.
--Output section works fine -- guitar plugged into "Power Amp In" is clear and pretty loud.
--I replaced the input jacks.
--I installed new 5w resistors and zener diodes in the 48 to 16volt section of the power supply.
--I re-flowed preamp tube socket connections. All DC voltages (B+, "X", "Y", "Z", tube pins, and test points) are in-spec. AC filament voltages are good.  "C-" is -47.4VDC.
--The channels switch with both panel button and foot switch.
--Because signal is so quiet, it's hard to tell, but I do NOT think that the reverb is working (though noise level changes with movement of Reverb pot).
--All pots test good / in-spec

So some things I wonder about:

--All resistors test in-spec, except for R30 (spec 1M) which tests @ 360k (though admittedly measured in-circuit). Is that enough of a drop that most of the signal could leak to ground through this resistor?

--How common (if it all) is it for the Power Amp In jack contacts to become oxidized / dirty to the point where preamp signal is highly reduced to the phase inverter? It seems that the contacts in this jack are sort of a "gatekeeper".

--Can a problem in the reverb section leak signal to ground?

--If U1 is toast, it appears that the preamp directly to the phase inverter should still be working as it should (?)

--Should I be wondering about coupling capacitors?

So basically, a lot of things seem to be correct, but the amp is not producing (anything near) sufficient volume. The weak sound is not intermittent, and the nature of the tones on the two channels, though very, very quiet, is loud enough to tell that the speaker output sounds as it should on each channel. The nature of the sound is such that it sounds like the amp is highly attenuated. As you might guess, I have done a fair amount of jumpering (I think effectively) to work around destroyed pads and traces. Obviously, I am missing something, but I am not in a hurry to try new things with a trial-and-error approach given that every soldering endeavor with this crappy board is an adventure.

Many thanks -- and with tremendous gratitude -- for any suggestions!

[mresistor]

First thing I'd do is replace V1 with a known good 12AX7.

[thud33]

mresistor, thanks for commenting. All the tubes and tubes connections are good.

[mresistor]

Post up a picture of the input jacks wiring.  Also does the sound change at all when switching the channels?  It's possible to still have a problem with RLY1 or 2. If you have a scope start following the signal from the input jack through to the pre-amp out in both channels and you should be able to see where it's getting lost. If no scope then build a listening amp per instructions on the bottom of this page  https://el34world.com/Hoffman/tools.htm


Have you taken a cable and jumpered preamp out to poweramp in? That would tell if the preamp is working since you've already input a signal into the power amp in and it was loud.

[thud33]

mresistor, good suggestions. For better or worse, I replaced the input jacks with new ones of the original type, so they are pin to pin (on the board) drop-ins. Yes, the sound level changes with switching. With volume controls dimed ("volume" for clean channel, "drive" & "master" for drive channel), the drive channel is significantly louder than the clean. And I was thinking earlier this morning about building a listening amp (as I do not yet have a scope). Thanks for the link!

I have not yet jumpered as you suggest. That makes a lot of sense!

[thud33]

Oh, yes, I actually have jumpered preamp out to power amp in, jack-to-jack — same result. When you mentioned doing that, I was thinking of internally jumpering to bypass the contacts on the Power Amp In jack.

[thud33]

But that answers my question about the jack contacts, too! 🤪
(Yes, a person can become a little stir-crazy on a project like this!)

[mresistor]

Do you have another amplifier so you could take the preamp out to the input of another amplifer?

[thud33]

Yes, I have done that. Same result. And I have measured the voltage of the signal at peak amplitude, strumming a guitar with a passive humbucker. The pickup itself generates well over 100 mVAC signal. Using the same guitar in the Blues Deluxe’s input, peak signal at Preamp Out jack is 0.2 mVAC.

[mresistor]

well I guess it could be the poweramp in jack shorting out the signal with no plug applied have to analyze that   here's the schematic for all interested


https://el34world.com/charts/Schematics/files/Fender/Fender_blues_deluxe_2.pdf

[AlNewman]

I haven't seen on your list where you've cleaned all the jacks and pots. Changed cables, changed instruments.  Start at the simplest point and work from there.

[thud33]

AlNewman, thanks for the suggestions. Those things were done.

[thud33]

So… the resistance from the juncture of R20 & R22 to ground is 19 ohms. This is a problem, right? Note that the R20/R22 juncture is essentially the point of the preamp signal on its way to the phase inverter via the contacts in the “Power Amp In” jack. Any thoughts on if this ground fault might be a result of a bad Q1, bad CR2/CR3, or the power amp in jack? Note that this measurement was taken with master volume pot (R21) in its full 1M position.

[sluckey]

I'm assuming you measured that resistance with the amp turned off?

Remove Q1 and see if the low resistance reading goes away.

[thud33]

Hi sluckey — yes, amp off! I figured that is something that I would need to try. Thanks for confirming the idea. I’m just a little hesitant about trial & error with this amp given that it’s a potential adventure in pad/trace de-lamination. Decent sounding amp, but what an abomination in terms of materials and design!

[sluckey]

"A man's got to know his limitations."  D. H.       

[thud33]

So Q1 was the culprit! I removed it, then measured MUCH higher resistance from R20/R22 to ground. More importantly, the amp sounds brilliant and LOUD on both channels. Interestingly (though not surprising when looking at the schematic), channel switching works fine without the JFET installed.

[thud33]

Regarding Q1, is its purpose simply to buffer the channel switching signals — maybe reducing any “pop” sound when switching channels?

[sluckey]

Looks like it briefly mutes the guitar signal during switching.

[thud33]

Thanks everyone for the suggestions on this issue!