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Rookie Build: Champ 5F1

Discussion in 'Shock Brother's DIY Amps' started by TeleSlug, Jul 15, 2020.

  1. tubegeek

    tubegeek Friend of Leo's

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    Agreed re: ground problem. You're getting the usual sterling advice from the Shock Bros. Check the resistance value (use ohms setting) of the cathode resistor that has such a high voltage. Maybe the wrong value was installed, 1.5Meg instead of 1.5K etc.
     
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  2. awasson

    awasson Poster Extraordinaire Gold Supporter

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    One thing that sticks out to me is that you’re reading 2.9 on pin 8 of the rectifier. Pin 8 of the rectifier tube should be the first node of the B+ shouldn’t it? It should be reading somewhere around 360VDC or in this case 336.4VDC.
     
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  3. King Fan

    King Fan Poster Extraordinaire

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    Pin 8 of the rectifier would read 2.5 VAC (nominal) *and* something north of 300 VDC, shouldn’t it? Check your meter settings. Wall voltage, all heaters + lamp, and HT *to* the rectifier are in VAC, but everything else is VDC.
     
    Last edited: Sep 28, 2020
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  4. teek_s

    teek_s TDPRI Member

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    It looks fine to me.

    1) That gap is more than enough.
    2) Preferably yes. It will increase the lifespan of the caps.
    3) You don't have to trim the cloth back, you can just push it back. That's why you see it so much, you don't have to strip the excess off. It's a massive time-saver all around.

    Good luck and remember to be safe!

    And I just made a stupid mistake, I didn't spot the other pages.
     
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  5. TeleSlug

    TeleSlug TDPRI Member Silver Supporter

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    You all are awesome! Thanks all for the replies. Sunday through Tuesday are long work days for me. I will check all my values and report back with a voltage table Wednesday or Thursday.
     
  6. TeleSlug

    TeleSlug TDPRI Member Silver Supporter

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    Okay, back at it!

    Tonight I drained the filter caps and measured the resistance values @andrewRneumann circled in red. The very top left one read 0.L, which according to my meter could mean a failed resistor. That's where the two resistors connect to the lamp. The next two both read at 0. The fourth one, read as 23.99 K ohms. That seems really off. The fifth one, top right read at .1 ohms.

    I did check this and the resistor is the correct value. That would have been too easy, right?

    Here are my tube socket voltages in DVC:

    rectifier

    pin 2 = 336V
    pin 3 = 2.1 mV
    pin 6 = 2.2 mV
    pin 8 = 336V (this matches what @awasson and @King Fan predicted)

    power

    pin 1 = 235
    pin 2 = 294
    pin 3 = 336
    pin 4 = 316
    pin 5 = 193
    pin 7 = 293
    pin 8 = 235

    preamp

    pin 1 = 196
    pin 2 = 2.3 mV
    pin 3 = 1.5
    pin 4 = 295
    pin 5 = 291
    pin 6 = 289
    pin 7 = 17.7 mV
    pin 8 = 217
    pin 9 = 295

    Here are pics of the interior. Please let me know if more or different ones are needed.

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
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  7. andrewRneumann

    andrewRneumann Tele-Holic

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    It looks like you need a jumper between the negative ends of the first 2 filter caps.
    3DDAC45A-BEEC-4DBB-B861-EB6647071EC1.jpeg
    Will have a look tomorrow with clear a mind, but judging by the excellent data you’ve provided someone will solve you issues before I even wake up!
     
    Last edited: Oct 2, 2020
  8. andrewRneumann

    andrewRneumann Tele-Holic

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    Actually 2 jumpers needed!
    5CABE742-857F-4E05-A62B-616A64BD60D6.jpeg
     
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  9. TeleSlug

    TeleSlug TDPRI Member Silver Supporter

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    Thanks a million. I completely missed that. I was so careful on the connections with the jumpers on the back of the board.

    I may be able to wrap up work early today and sort this out.
     
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  10. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    I think this is a typo but I can't verify it with your pictures.
    Rectifier Pin3 is not used. It should be pin4.
     
  11. andrewRneumann

    andrewRneumann Tele-Holic

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    Your non-zero readings make sense if those nodes were not grounded. I don't think you have a failed resistor. If not properly grounded, the top left one at no point makes a circuit with the chassis, even with those artificial center-tap resistors connected.

    The 23.99K reading makes sense if that node wasn't grounded. You are reading the resistance through the cathode resistor + negative feedback resistor + speaker coil. I'd bet you made the reading with a speaker connected? Disconnect it and it would have read 0.L.

    Edit: Correction, even without a speaker connected, you would get a circuit to ground through the output transformer secondary.
     
    Last edited: Oct 2, 2020
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  12. TeleSlug

    TeleSlug TDPRI Member Silver Supporter

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    I swear this thing is going to be a vintage amp by the time I get it working.

    Yes, it's a typo. Thank you for catching that. Pin 3 is open and pin 4 has the red wire from the power transformer.

    This afternoon I added the two jumpers, tested all the AC voltages on the pins without the tubes in. All was good.

    I plugged in the tubes and checked all the DC voltages. Those also looked correct and the connections where I added the jumpers are now grounded. No red plating, no smoke, no other problems so I went ahead and installed the chassis in the cabinet, made sure the speaker was plugged in, put on the back panels, and turned it on. When I turned it on the jewel light turned on and the tubes warmed up.

    Then nothing.

    Not even a hum or crackle from the speakers. I tried both instrument inputs and two different cables. I unplugged, let everything cool down, and then verified that the speaker was in fact plugged in before turning it on again. Still nothing.

    I opened up the back and used my trusty chopstick to see if anything had been knocked loose when I installed the chassis in the cabinet. I also verified the speaker connections were correct (positive and negative).

    [​IMG]

    [​IMG]
     
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  13. Lowerleftcoast

    Lowerleftcoast Friend of Leo's

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    Give us the voltages again now that the grounds have been changed.
     
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  14. andrewRneumann

    andrewRneumann Tele-Holic

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    Do you mean no sound? Or do you mean no power at all?

    Could we see an better photo of the input jacks wiring please?

    Sorry your amp isn’t working! We’ll get to the bottom of it won’t we @D'tar ?
     
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  15. TeleSlug

    TeleSlug TDPRI Member Silver Supporter

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    I am getting power and my tubes are warming up. Just no sound.

    I measured all the circuit board voltages and this is what I got this afternoon:

    B+1 = 350

    B+2 = 306

    B+3 = 267

    Power Tube Cathode/Cathode Bypass = 19

    Between the 22K and 1.5K resistor = 1.2

    100K/Coupling cap/preamp pin 6 connection = 182

    100K/Coupling cap/preamp pin 1 connection = 180​

    All of the ones across the top of the board are at 0 or a value like 2.1 mV.

    I went to check the pin voltages like @Lowerleftcoast asked and plugged in the amp through my bulb limiter. After about 10 seconds the bulb (150 W) started to glow. I know an abrupt flash is bad. Is this a concern?

    [​IMG]

    Here are the pics of the input jacks.

    [​IMG]

    [​IMG]
     
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  16. D'tar

    D'tar Friend of Leo's

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    Are you really getting almost 300vdc on your heater circuit? I honestly dont know why they still use an rca speaker jack with this circuit. I would trade that for a mono jack any day of the week. Do you hear any noise (scratch,pop etc)while probing the tube sockets as you are taking voltage readings on the plates and grids?
     
    Last edited: Oct 3, 2020
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  17. D'tar

    D'tar Friend of Leo's

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    What OT are you using? I dont see a chassis connection, only the yellow tap to the rca tip.
     
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  18. jmp81sc

    jmp81sc Tele-Meister

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    As Dtar said above check the wiring of the RCA jack on the speaker side, then check the output transformer wiring to the RCA tip in the chassis.
     
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  19. andrewRneumann

    andrewRneumann Tele-Holic

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    I believe @TeleSlug fixed this by properly grounding the artificial heater center tap.

    It’s one of those old school OT’s that grounds through its frame to the chassis. @TeleSlug can you measure the DC resistance of the OT secondary? You would do this by turning off the amp, disconnecting the speaker, and having one probe on the RCA plug terminal (where the yellow OT wire and NFB wire connect) and the other probe on the chassis. It should be a low reading, but not 0.

    While you have the speaker disconnected might as well measure the resistance between the tip and sleeve of the RCA plug leading to the speaker.

    EDIT: Until we are happy with OT, RCA speaker plug, and speaker, you might want assume no speaker is connected even if it is.
     
    Last edited: Oct 3, 2020
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  20. andrewRneumann

    andrewRneumann Tele-Holic

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    Yup. Debugging can be frustrating and messy. It’s the sound that counts though!
     
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