5F2A build help

Discussion in 'Shock Brother's DIY Amps' started by nrogers, Feb 28, 2020.

  1. nrogers

    nrogers TDPRI Member

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    Hi all,

    First time poster here. I just built my first kit, a 5F2A from Mojotone, and got a lot of help from @theprofessor's build thread.

    I've got the whole thing wired up, and am now checking voltages etc. I have wall AC at the fuse, and the right AC voltage at the tube heater wires. I also have high voltage at 4&6 on the rectifier (around 325V measured to ground for each).

    But:
    - The rectifier tube isn't heating up. I did test it (2 to 8) and it does have continuity.
    - I'm not getting any voltage at the filter caps.

    What's my next step to troubleshoot this?

    Thanks so much!
     
  2. King Fan

    King Fan Friend of Leo's

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    Welcome. Great amp. Hang in there.

    Do you have a light bulb limiter? Easy to build and a really good simple tool right now.

    Is your rectifier known to be good? Can we see some nice clear pics of the PT/rectifier/filter cap wiring?

    What do you measure at rectifier pin 8 *in both VAC and VDC?*
     
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  3. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    As your rectifier tube is not heating up, it can not conduct the HT. That is why you measure nothing at the filter caps.

    You have thus localized your problem: The rectifier tube isn't heating up.

    You said you have the correct voltage, but can you provide us with measurement results, clearly stating how you measured. Just writing a clear log of what I did/measured alone has helped me several times (kinda like rubber duck debugging).

    Also an image might help.
     
  4. Tom Kamphuys

    Tom Kamphuys Tele-Meister

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    Great minds think alike...

    ... though fools seldom differ.
     
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  5. nrogers

    nrogers TDPRI Member

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    Sure, thanks guys. On the rectifier pin 8 has around 400 mv DC and 4.7VAC, both measured to ground.

    I did test the rectifier using my multimeter, and got continuity between pins 2 and 8.

    Here are the earlier readings:

    At the fuse: 120 VAC
    Power tube, 6.7 VAC (pins 2-7)
    Preamp: 6.7 VAC (pins 4-9 and 5-9)

    Putting a photo in my next post. Ignore the nasty glob of solder on the pilot - taking that off soon.
     
    Last edited: Feb 28, 2020
  6. nrogers

    nrogers TDPRI Member

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    IMG_1160.jpg
     
    Last edited: Feb 28, 2020
  7. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    Pin contact is always a suspect with tubes. Make sure you have clean pins and socket. Check the socket tension for each pin. Try moving/wiggling the tube about. Sometimes the pins will make contact and the filament will start to glow.

    Try another tube if you have one.
     
  8. D'tar

    D'tar Friend of Leo's

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    Make sure you're connected to the proper pins/terminals and your tube is then inserted properly as well.
     
  9. D'tar

    D'tar Friend of Leo's

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    Looks like belton socket heres a comparison as to my earlier post...

    Yours

    Screenshot_20200228-114921_Chrome.jpg

    Another

    Screenshot_20200228-114902_Chrome.jpg
     
  10. nrogers

    nrogers TDPRI Member

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    Thanks! Can you explain why it is relevant that it is a Belton socket?
     
  11. elpico

    elpico Tele-Holic

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    On the rectifier, whats the AC voltage between pin 8 and pin 2? Not the voltage from those to ground, the voltage with the probes on those two pins.
     
  12. nrogers

    nrogers TDPRI Member

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    Around 5V AC between 2 and 8 on the rectifier
     
  13. D'tar

    D'tar Friend of Leo's

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    Relevant that the orientation should be the same and it looks to me that you may be connected to the wrong terminals perhaps. Like pins 7 and 1 instead of 8 and 2. Also 3 and 5 instead of 4 and 6.

    Easy enough to confirm right.
     
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  14. nrogers

    nrogers TDPRI Member

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    Winner winner! Can't believe I messed that up after checking it like twelve times. Thank you! I'll resolder and report back.
     
  15. nrogers

    nrogers TDPRI Member

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    OK, I moved the various wires and got the rectifier working. Following @robrob's startup procedure, I measured the voltage at the filter caps and the preamp. Around 470 in each spot. However, I'm not getting any reading on the power tube where the OT wires link in (pin 3).

    Any advice what to try next?
     
  16. D'tar

    D'tar Friend of Leo's

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    A full set of pictures may help all to see the entire circuit and spot any obvious issues.

    I see one yellow wire coming off your rectifier toward the board connected to first filter. Is that connected under the board to the next filter?
     
  17. nrogers

    nrogers TDPRI Member

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    Sure, here you go. The yellow wire goes from the rectifier to the first filter cap, and the OT blue line goes to the next cap. I did jumper those together underneath the board. Also the pink/yellow wire from V2 to V1 is a heater wire--ran out of green cloth wire.

    Thanks guys!


    IMG_1163.jpg IMG_1162.jpg
     
  18. D'tar

    D'tar Friend of Leo's

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    upload_2020-2-28_14-36-22.png

    I would reflow any joint that looks like this one. fill that void full of solder. If you still do not have B+ after that you may want to Ohm the primary (blue to red) and see whats what.
     
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  19. D'tar

    D'tar Friend of Leo's

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    Preparing for Hum? Where is your heater center tap. You will need an artificial one with that PT. I do not see one.
     
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  20. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    The power cord colors. Blk = hot. White = common. Green = ground. The black should go to the switch and then the fuse. When the fuse blows = no power. When the switch is off = no power. The way you have it the common is disconnected when the switch is off so there is still power to the amp.
     
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