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.

Help with troubleshooting Bassman Micro

Discussion in 'Shock Brother's DIY Amps' started by subgenius_one, Jan 9, 2021.

  1. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    I finally got around to finishing my Bassman Micro - had to install a new speaker. I had some inconsistent results with the sound quality, but at it's best it sounded great - it is what I had hoped for.

    I had to change the input jack and re-solder it. I also took the tone controls and tone control bypass switch out to inspect and try to find any issue. After re-installing and testing the amp, I still had some inconsistent sound issues.

    I found out during testing that there was voltage on the pot shafts and the metal case of the guitar cable, as well as the strings on the guitar. It is on the ground buss on the turret board. After testing with meter, you can see it charge somewhat slowly up to 307V DC.

    I have looked and looked and I cannot find any component or connection that I can see is putting voltage on the on the preamp ground buss. Could it be a failed capacitor or ?

    Is there anyway to find this without beginning to disassemble the board?

    Also, have I missed something on this - like should the preamp ground buss be grounded to the chassis in some way? I do not see such on the schematic or layout, but perhaps I missed it. Also, would grounding the preamp ground buss to the chassis stop the signal from working?

    I will probably rebuild the whole thing if needed, but I would sure prefer to salvage what I have if possible.

    Also - my wall voltage is about 130V, so of course my voltages are all high. B+ at 307V. What is the best way to lower the voltage in the amp chassis?

    Thanks for your help...
     
  2. Mark the Moose

    Mark the Moose Tele-Afflicted

    Age:
    45
    Posts:
    1,258
    Joined:
    Sep 4, 2018
    Location:
    Erie, CO
    Can you post some images of your build? Maybe someone here will spot the issue.
     
    King Fan likes this.
  3. Tom Kamphuys

    Tom Kamphuys Tele-Holic

    Age:
    41
    Posts:
    668
    Joined:
    Sep 18, 2018
    Location:
    The Netherlands
    Clearly safety ground is not working.
    For your own health: If you are not sure how to fix that, I would advice you to bring the amp to a guitar amp guy and explain the problem before anyone plugs it in again.
     
    King Fan and D'tar like this.
  4. jmp81sc

    jmp81sc Tele-Meister

    Posts:
    382
    Joined:
    Feb 6, 2015
    Location:
    Orange County
    I think you have some wires crossed up or your high voltage DC is touching something it should not be. Don't plug it in until you figure out how your getting voltage on the chassis.
     
    King Fan likes this.
  5. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,141
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Agree: you shouldn't plug it in again until this is solved. And there's no shame in taking a homebuilt to a good tech for safety reasons.

    You could and should use an outlet tester on the outlet you use -- all your outlets, in fact -- to make sure they're proper especially for ground but also hot / neutral. That's not your problem, but it's important. Then you can and should do Rob's safety ground test with a continuity tester / DMM on the unpowered amp.

    "...verify the safety ground connection. Plug a guitar cable into an input jack and check that you have continuity (meter "beep") between the guitar cable's sleeve (the part that isn't the tip) and the amp's power cord ground prong.
    Plug a cable into the main speaker jack and test again for continuity to the ground prong.
    You must not power up the amp until you have safety ground continuity between the power cord ground prong, guitar cable sleeve and speaker cable sleeve because the amp is a deadly shock hazard without it. If you don't have continuity use your meter's continuity function to verify your power cord wiring: Hot (small prong) to fuse, Neutral (large prong) to the power transformer primary and the ground prong wire bolted directly to the chassis.”

    Presumably, you'll fail the safety ground test. °But° even if you get the safety ground fixed, you still need to find and fix the error that's putting 300V on the chassis. The fact it's 300VDC tells us it's beyond the rectifier, not just household AC. Good pics here often help.
     
  6. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
  7. liddown

    liddown TDPRI Member Silver Supporter

    Posts:
    8
    Joined:
    Nov 15, 2015
    Location:
    Phoenix, AZ
    Is the cathode bypass cap solder joint touching the mounting standoff for the board?
     
    King Fan likes this.
  8. King Fan

    King Fan Poster Extraordinaire Ad Free Member

    Posts:
    6,141
    Joined:
    Jan 1, 2013
    Location:
    Salt Lake City
    Great, photos really help. It may help even more to put the photos inline in the post. You can drag / drop a jpg file right into the compose window, or (one extra step, but easier to control) use the 'upload a file' button, then mount them full-size. Make sure they're big enough (like 1600x1200?) so they mostly fill the post L-R.

    BTW, another DMM test in the unpowered amp might be to measure resistance from the positive end of each filter cap to the chassis. Here's Rob's layout as a jpg with those spots circled in orange:

    LTP_Layout copy.jpg
     
  9. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    the bypass cathode is not touching the standoff. It is just close. I was reusing a chassis with existing holes and had some trouble getting good placement for the standoffs that did not interfere with the components on the board or land in existing holes on the chassis.

    0109211152.jpg
     
  10. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,259
    Joined:
    Dec 29, 2012
    Location:
    United States
    subgenius, the preamp bus ground is at the input jack--see the layout above. That input needs a star washer to make firm chassis contact. If you are using an insulated input jack then your voltage symptoms make perfect sense.

    Your chassis safety ground is also suspect. Do you have the power cord safety ground connected to the chassis (lower left in layout)? If this is connected properly you should not be able to get any chassis voltage.

    Disconnect the power cord, verify the filter caps are drained and verify continuity from each power cord prong to the proper place in the chassis--verify all three cord prongs.
     
    Last edited: Jan 9, 2021
    Nickfl likes this.
  11. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    The test results as follows...

    ground tip of power cord to:

    speaker jack sleeve has continuity

    guitar jack sleeve does not have continuity​

    Each of the filter caps show open (1) between the positive terminal and the chassis.

    I believe the sleeve grounding issue is because I switched to a cliff style jack when I replaced it and I did not run a ground wire from the sleeve to chassis ground. Certainly an easy fix.

    Are there any other suggestions for locating the DC voltage source?

    Thanks again for all of the suggestions...
     
  12. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    Rob,

    Thanks for your awesome work and contributions! I am excited about this amp!

    The power cord prongs each has continuity (~0 ohm) to their connection in the amp chassis.


    Ground prong to chassis ground, hot to fuse, neutral to PT.
     
    robrob likes this.
  13. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    I have grounded the input jack ground to chassis and the cable jack sleeve now has continuity with the chassis.

    The sleeve no longer has dc voltage, and the positive side of the filter caps seems stable at 318v dc. So is that the resolution?

    More specifically does the circuit bleed that dc voltage to the chassis ground in normal operation such that the absence of a chassis ground on the preamp ground bus would cause the build up of DC voltage on the preamp ground bus?
     
  14. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,259
    Joined:
    Dec 29, 2012
    Location:
    United States
    The preamp ground bus was not grounded so it was floating. A floating bus will have random voltage compared to grounded parts of the circuit so that should be the fix you needed.
     
    D'tar likes this.
  15. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    At this point the previous issues seem to be resolved. I do not have sound now.

    I have checked and I have DC voltage on the positive side of the coupling caps, but none on the negative side. What voltage should be at the connection of the coupling caps and grid stop resistors?
     
  16. dan40

    dan40 Friend of Leo's

    Posts:
    2,098
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    The coupling caps on the board are designed to block DC voltage but allow AC signal voltage to pass through to the next stage. You will see a DC voltage on one side but the other side should read zero DC voltage.
     
  17. dan40

    dan40 Friend of Leo's

    Posts:
    2,098
    Joined:
    Aug 19, 2015
    Location:
    Richmond Va
    Do all of your tubes light up and get warm after 30 seconds or so? When taking your voltage readings, do you hear a "pop" sound through the speaker?
     
  18. robrob

    robrob Poster Extraordinaire Ad Free Member

    Posts:
    8,259
    Joined:
    Dec 29, 2012
    Location:
    United States
  19. subgenius_one

    subgenius_one TDPRI Member

    Age:
    48
    Posts:
    44
    Joined:
    Sep 25, 2020
    Location:
    Texas
    I do hear pops when probing, but only when checking the continuity of the signal path to the grids, not when measuring for voltage on grids.

    All of the signal path through the pots pop when measuring continuity to ground. I also hear pops on all except preamp vol and master vol when probing on voltage setting or off.

    I have 5.8v AC between heater pins on all tubes. The rectifier and 12BH7A are lit and hot, but the preamp tubes are only marginally hotter than room temp and I cannot see a glow.


    B+ Measurements at tubes:

    V1 252v
    V2A 206v
    V2B 290v
    V3A,B 276v
    V4A,B 305v


    I have tried at least 6 different tubes, and the original tubes were new. I guess I will try to validate continuity along each part of the signal path next. Let me know if there are any other questions or suggestions.
     
  20. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

    Posts:
    2,898
    Joined:
    Dec 30, 2019
    Location:
    california
    I don't see the heater wiring for V1 and maybe V2. Do you measure 6.3VAC with meter probes on pins 4/5 and pin9?

    Double check all the wires to V1 and V2.

    EDIT: I don't see pin9 connected on the other tubes either. Check heater voltage on each tube.
     
IMPORTANT: Treat everyone here with respect, no matter how difficult!
No sex, drug, political, religion or hate discussion permitted here.