1. Win a Broadcaster or one of 3 Teles! The annual Supporting Member Giveaway is on. To enter Click Here. To see all the prizes and full details Click Here. To view the thread about the giveaway Click Here.

Blues Junior | Purchased INOP, No Sound | Diag and Repair

Discussion in 'Amp Tech Center' started by Huddy, Nov 29, 2020.

  1. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    Just wanted to start a quick thread and get some second opinions on my thought process on this diagnosis and repair.

    Background
    I purchased this amp as a project. It was described as powering on and not putting out any sound. I went to check it out. It powered on. No Sound. I figured with the Cannabis Rex (tested) and cabinet it was worth the gamble on being something semi-simple.

    Looking at the ad I figured the no output problem was either the input jack, speaker jack, speaker plugged into the foot switch, bad tube, or needed the tube pins re-flowed to the PCB.

    I asked the guy what happened to try to get a sense and narrow the problem down. He said he bought it with the reverb not working and figured it was an easy fix of replacing the reverb tank. He tried to replace the reverb tank but that didn't solve the problem. He continued to play it and "one day it just stopped working." So I'm initially thinking the reverb is unrelated the no-sound problem.

    I pay the man and drive it home.

    Here's what I found
    So I get home pull the reverb tanks out and they're both reading "open" on the OUT coil. Once the rear cover is off I discover a few things. One is more glaring than the rest.

    BJ1.JPG
    Now I don't know my color codes all that well but I know YELLOW-PURPLE-YELLOW (470 kΩ) is insanely high for any voltage dropping resistor. I tested for continuity between the replaced resistors leads and the filter caps to verify what its intended role was - B+ to X (B+1) voltage dropping resistor. So I check the schematic...
    BJ2.jpg

    The B+ ==> X(B+1) resistor is supposed be a 2.2 kΩ 2w resistor seems normal. But I notice it's marked R32 and I look for another schematic. I find this...

    BJ3.jpg

    Same value... this schematic just matches my board's identifier. The accompanying board layout also matches mine.

    So then I start to wonder what would possess someone to put a 470 kΩ resistor in that place? Did they think R47 mean "Resistor 47Ω" and didn't know about the multiplier? I went back and checked the the original schematic I found and began the hunt for R47.

    BJ4.jpg

    Tadaaaaaah!!!! Found it in the reverb section... 470 kΩ.

    Plan of Action

    So I've got plenty of 22 µF caps. I've got the 47 µF, 2.2 kΩ Resistor, and MOD Reverb Tank on the way.

    I'm going to -
    • replace all of the power supply filter caps (original ICs, if I got the board out it doesn't make sense to not replace them)
    • replace the voltage dropping resistor with the correct value
    • clean up and reflow tube socket PCB connections
    • test all tubes
    • start up amp with out tubes with lightbulb limiter; check voltages on plates, heaters, etc
    • start up amp with just output tubes with lightbulb limiter; check voltages on plates, heaters, etc
    • start up amp with all tubes AND lightbulb limiter; check voltages on plates, heaters, etc
    • Test with guitar and all should be right with the world
    What could I be missing?

    Anything else that I need to check? Other components that could have residual damage? What was the likely culprit? Output Tubes? Or could it have just been a chain reaction with the PCB tube connections? Potentially a failed connection caused the tube to fail that caused the resistor to fail?

    Thanks so much (as always) in advance!
     
  2. Teleguy61

    Teleguy61 Friend of Leo's

    Posts:
    3,498
    Joined:
    May 17, 2010
    Location:
    Eastern Massachusetts
    There are several different versions of the Blues Jr, and the parts are not numbered the same from version to version.
    It is important that you be looking at the correct schematic for your version.
    Good luck.
     
  3. archetype

    archetype Fiend of Leo's

    Posts:
    5,702
    Joined:
    Jun 4, 2005
    Location:
    Williamsville NY
    If you look closely at the board it looks like R47 is a replacement and the previous resistor probably blew up and cooked the board under it. Something made it fail, previously, and the resistor was replaced. Keep that in the back of your mind while you troubleshoot.
     
    String Tree likes this.
  4. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    Yes. That was my assumption. That R47 blew and was replaced with a value found on the wrong schematic. Why it blew is my real question. And what other residual problems could have resulted from that resistor blowing or have resulted from whatever caused the resistor to blow.
     
    archetype likes this.
  5. archetype

    archetype Fiend of Leo's

    Posts:
    5,702
    Joined:
    Jun 4, 2005
    Location:
    Williamsville NY
    Righto. Makes me wonder about the fuse(s), as well. Is the fuze the right value, or did the previous owner increase that?
     
    Huddy likes this.
  6. schmee

    schmee Doctor of Teleocity

    Posts:
    14,594
    Joined:
    Jun 2, 2003
    Location:
    northwest
    Buy a $10 PR board and hand wire it!
     
    Huddy likes this.
  7. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    I
    I thought the same thing after I posted this. I checked. They’re properly rated per the board and schematic.
     
    archetype likes this.
  8. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    Once I get the board out I’ll assess the trace around the burnt up area. If it looks suspect I’ve been looking at this sort of option.
     
  9. tubeToaster

    tubeToaster Tele-Meister

    Posts:
    123
    Joined:
    Nov 9, 2007
    Location:
    Brentwood,Tn.
    My Pro Junior became a 5e3 and I’ve never regretted it. Best thing I ever did to that amp.
    upload_2020-11-29_13-13-13.jpeg
     
    ce24, djh22, Huddy and 1 other person like this.
  10. schmee

    schmee Doctor of Teleocity

    Posts:
    14,594
    Joined:
    Jun 2, 2003
    Location:
    northwest
    My Mono 15 became a Princeton non reverb!
     
    hepular likes this.
  11. dogmeat

    dogmeat Friend of Leo's

    Age:
    70
    Posts:
    2,582
    Joined:
    Oct 12, 2017
    Location:
    Alaska
    Last edited: Nov 29, 2020
  12. schmee

    schmee Doctor of Teleocity

    Posts:
    14,594
    Joined:
    Jun 2, 2003
    Location:
    northwest
    I dont know how I flipped to Princeton Rev RI on this thread, :eek: i see it's a BJ! Brain fart.
    Ignore me, carry on.
     
    Huddy likes this.
  13. schmee

    schmee Doctor of Teleocity

    Posts:
    14,594
    Joined:
    Jun 2, 2003
    Location:
    northwest
    delete
     
  14. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    haha! I was about to ask where you got your BJ to PR conversion board cause I couldn't find one :lol:
     
  15. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    Very cool! Did you parallel the two triodes for the first gain stage? Or just use half the tube?
     
    Last edited: Nov 29, 2020
  16. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,219
    Joined:
    Dec 30, 2019
    Location:
    california
    A few tips for the Blues Jr...

    The ribbon connections can break. If it doesn't have it already, run a hot glue bead at the joint of the ribbon to the board. This will help keep the ribbon wires from breaking as movement will flex them. When installing the back panel bend the ribbon wire so it is close to the shielding of the back panel.

    The tube sockets, especially the power tube sockets can crack the solder and/or PCB traces. Check them carefully and reflow the solder.

    For some reason the phase inverter plate resistors can be problematic. I usually just replace them while I have the board out. Check those solder joints and traces carefully as well. On your board it would be R28, R29, and R30.
     
  17. Huddy

    Huddy Tele-Holic

    Age:
    37
    Posts:
    708
    Joined:
    Nov 5, 2016
    Location:
    Newport News, VA
    Great tips! I had seen the right angle trick on BillM years ago as a noise reducer (if I remember correctly) but I bet it also helps with not stressing those joints as well. The last Blues Junior I had needed the output tubes solder joints re-flowed big time. Of course did the other tubes as well.

    This is gold! don't know how I didn't think about that.

    What am I looking for? way out of spec?
     
  18. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    3,219
    Joined:
    Dec 30, 2019
    Location:
    california
    The PI plate resistors in HRD and Blues Jr era amps many times develop microscopic cracks which result in audio snap, crackle, pop. It is more of a preventative when it is not happening yet. When it is making noise, it is rather hard to diagnose if a chopstick doesn't change the symptom. I just change them when the board is out, as it is a known trouble spot. It is best to not have the board out of a Blues Jr any more than you have to. OMMV.

    Another tip:
    PCB construction for the most part does not account for the heat of plate resistors. They set them right on the PCB. A little airspace provides much more cooling which we can provide by simply removing the contact with the board.
    The way the board is mounted in the Blues Jr leaves plenty of space on the solder side of the board. If you do not cut the leads of the resistor short, it can aid in cooling the resistor and solder joint. I leave about 8mm of the resistor leads protruding out the back as a heat sink, if it does not fowl anything in the back of the chassis.
     
    Huddy likes this.
  19. tubeToaster

    tubeToaster Tele-Meister

    Posts:
    123
    Joined:
    Nov 9, 2007
    Location:
    Brentwood,Tn.
    I have a push pull switch on the volume pot to parallel the triodes.
     
    Huddy likes this.
  20. LudwigvonBirk

    LudwigvonBirk Tele-Holic

    Age:
    119
    Posts:
    950
    Joined:
    Aug 26, 2017
    Location:
    Madison
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.