Blues Jr III Troubles

[munkeyboy]

Hey Guys,

I'm trying to repair a Blues Jr III (cream board).   The PO burnt the tube board and there is still some fall out after I replaced that.   I'm about to give up and do the turret conversion, but want to give it a few more tries first.

First problem is the fat switch.  While de-soldering the broken one, the traces and part of the green lifted off with the wick.  I can't see where the traces were connecting to the top few pins.  Does anyone have the info on what pin goes to each trace?  I feel like i should be able to figure this out myself, but I'm blanking.

Second problem is output itself.  It is super low volume and crappy (distorted) and hums.  I've tried new tubes and swapped known working tubes all around.  Upgraded the filter caps.  I've replaced a few of the resistors and diodes (especially around the power section).  But before ordering the special diodes for D11/12, i started testing voltage.  Taking the meter to D11 and the blue output wire produce a weird result (humming stops and the meter goes wonky-blanks out/screen flashes on off).  Testing D12 (i think that is it, working from memory here) and the brown wire, did not produce the same issue.   Tho the reading was not steady at all.    Is this expected and I should be testing them with some other method?


There are other issues as well with reverb, but i'll get to that later.

[munkeyboy]

Also,  I reviewed the conversion project and saw that Reverb is not included.   Has anyone else successfully fit in a reverb circuit in with the conversion board?

[purpletele]

I had gone down that road with the Blues Junior.  I modded them and made them sound really nice.

When I look back now i wish I would have built a Hoffman Princeton Reverb in lieu of the Blues Junior Point to Point. 

I went down a rabbit hole with that amp and that was fine, but there are other fun amps.  I finally turned mine into a 6 v 6 Plexi after a few months as a Blues junior.

I had my own chassis’ made A a custom pine cab, etc.


In the end I found that I really don’t like that vertical chassis configuration as I moved forward with my projects.

Have fun with it.

You may need  to be much more specific about you current issues to get a comprehensive response from the experts.

[munkeyboy]

Yeah, I don't plan on keeping this amp, just trying to repair and sell.  I do it for the experience not the money.  I have my own projects I'm hand wiring now (trainwreck and tweed deluxe).

I am a bit light on details and in hindsight, it might be better to move this to a fender or some other forum not related to diy amp building.   

Well, forget the switch question... i'm just being lazy. I'll figure that one out once I'm not working 14 hour days and my mind clears up.

Basically,  I'm wondering if I'm testing correctly.   In the attached photo, i've replaced all the items with a blue dot, including the tube board.   But before I randomly start replacing things, I wanted to do some better testing.    I was checking voltages in the circuit until I got to the diodes before the OT blue/brown wires out.   According to fender, test points TP18 and TP17 should be at 29.5VAC.    When I use the meter to test them, i get unexpected results.   TP18 (green arrows), jumps around, nothing steady.   TP17(red arrows) makes the voltmeter freakout (screen flashes on/off like shorting it?).   Am I testing it wrong? 

[shooter]

got a schematic?  cuz the one I have REV D shows TP 17 as grid to V5 and TP 18 is plate of V4.

probing grids with a meter can be problematic

[sluckey]

Basically,  I'm wondering if I'm testing correctly.

All those AC voltage at the various test points require a proper test signal applied to the input jack. Are you injecting a proper test signal?

[munkeyboy]

Basically,  I'm wondering if I'm testing correctly.

All those AC voltage at the various test points require a proper test signal applied to the input jack. Are you injecting a proper test signal?

That would probably be this issue.  I thought that was only needed on the other side of the OT.  Well, i don't have it :(   Off to find one or diy i guess.  Thanks!

FYI schematic is here (page 2)
http://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Blues-Junior-Schematic.pdf

[sluckey]

Look at the note above TP1. These AC voltages are there for you to trace a known signal through the entire amp. I wish every schematic included these.

[munkeyboy]

Look at the note above TP1. These AC voltages are there for you to trace a known signal through the entire amp. I wish every schematic included these.

Ah, that first page is for the previous board version so I missed that.  Missed the hard to read notes on the 2nd page at the bottom too .  Man, I need to step back and take my time.  It's also time to learn about generators and oscilloscopes.   Any affordable recommendations for signal generators, or are those apps that generate signals good enough?

[shooter]

are those apps that generate signals good enough?

software sig-gens work fine, $30 scopes also. 

[munkeyboy]

Sweet.  Thanks for the guidance guys.