Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: keithpop on April 11, 2019, 12:39:11 pm

Title: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: keithpop on April 11, 2019, 12:39:11 pm

Hi y'all,
A friend of mine asked me to look at a Blues Junior.  I don't know how to tell which version it is.  The schematic at the bottom of this document seems close
(Fender_blues_junior_service_manual.pdf (https://el34world.com/charts/Schematics/files/Fender/Fender_blues_junior_service_manual.pdf)

Initial complaint:  Plugging a guitar chord in and no output..... but I hear some noise out of the speaker). 

I have:
- Checked most voltages (my meter has a problem with millivolts....)
- Continuity along circuit traces are ok.
- Resistor values ok
- Capacitors not shorted
- Swapped 12AX7 with known good tube

I put a clip lead between R2/Input jack to R7 (bypassing the 1st half of V1).  With this,  the amp seems to work fine but this is not the fix.


I attached a picture of where I attached the clip lead.


Why does the amp work like this?  What am I missing with the components or circuit board?


Any and all ideas of what the real problem may be would be appreciated. 


I thank you all in advance.


Keith P.

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: shooter on April 11, 2019, 12:49:00 pm

I've worked on a couple, the main problems have seen;
1. the yellow board with all the parts, sucks
2. the gray with red stipe cables, suck
3. the green board where the tubes plug in, sucks
beside that, not a bad sounding amp  :icon_biggrin:

re-flow solder connections till you can't stand it and it'll probably work, well, till it gets bumped again

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: Papa Jim on April 11, 2019, 01:00:13 pm

Shooter is right. I recently fixed a friends. The green board had bad solder joints on the 2 output tube sockets from getting too hot and melting.

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: keithpop on April 11, 2019, 01:28:24 pm

Quote from: Papa Jim on April 11, 2019, 01:00:13 pm

Shooter is right. I recently fixed a friends. The green board had bad solder joints on the 2 output tube sockets from getting too hot and melting.

I failed to mention that I re-flowed the solder for the complete preamp section and it didn't fix it.  I will look at it again.  Thanks for your help!

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: Papa Jim on April 11, 2019, 01:57:52 pm

Lot's of BJr. repair info on this http://billmaudio.com/wp/

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: keithpop on April 12, 2019, 04:15:35 pm

Thanks for the responses on this posting.   I finally sat down and re-soldered every connection in the preamp section (both boards) and as y'all said....it's working. 


Thanks again for the valuable input.


Keith




by the way....this amp is not my favorite...   
- I hate PCB
- I hate ribbon cable

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: Tbone55 on April 14, 2019, 07:27:51 pm

Sounds like you need to convince your friend he needs Doug's Blues Jr. conversion kit.  :icon_biggrin:

Title: Re: Blues Junior - no sound - tubes light -possible problem with circuit around V1?
Post by: keithpop on April 14, 2019, 08:53:37 pm

Quote from: Tbone55 on April 14, 2019, 07:27:51 pm

Sounds like you need to convince your friend he needs Doug's Blues Jr. conversion kit.  :icon_biggrin:

Sounds like a fun thing to do but he's doing the smart thing and selling it. 


thanks
kp