WEM Custom Copicat valve echo help

Discussion in 'Shock Brother's DIY Amps' started by Acobberson1, Aug 11, 2019.

  1. Acobberson1

    Acobberson1 TDPRI Member

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    Wasn’t sure if this should go in effects or amps, but since it’s all valve I figured I’d post it here.

    I’m working on bringing a Watkins Copicat back to life and could use some guidance from anyone who is familiar with the circuit. I couldn’t find a schematic specifically for the custom, but the valve mk3 looks to be identical.

    My problem is that I’m getting no power to the tubes, or at least that’s the first issue I’ve found.
    Obviously I get no signal from either input.
    It looks as though someone has replaced the old diode rectifier with a 1n4007 diode. Is this a sufficient replacement?

    Visual checks of all parts don’t show any glaring issues (bulging caps, fried resistors, etc).

    After reading a few posts from others working in old valve Copicats, it seems the bias oscillator, original diode and smoothing caps are likely culprits.

    Unfortunately I’ve never worked anything with a bias oscillator, so I’m totally in the dark on how to test that. As mentioned, the diode is already replaced, but I’m not sure if it is an adequate one.

    So, my basic question is if anyone has an idea where to start? I’ll post voltage readings on the tubes and smoothing caps next.

    Cheers!
    Link to schematic http://www.vintagehofner.co.uk/britamps/watkins/schematics/copicatmk3.html
     
  2. PC_Hater

    PC_Hater Tele-Meister

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    The 1N4007 has a reverse voltage rating of 1000Volts so that part of the spec is OK.
    The forward current is 1Ampere, if you are pulling more than 1Amp through it for a Watkins Copicat you have big problems!
    It'll be fine.
    The diode might have died of course but replacing it with another 1N4007 will work and be safe.

    I haven't seen the insides of a Copicat in 40 years so I'm no help with the other questions.
     
  3. sds1

    sds1 Tele-Afflicted

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    Yeah do that first. You mention the tubes aren't getting power -- but on which pin(s)? Voltage chart will help move you along to next steps.
     
  4. Wally

    Wally Telefied Ad Free Member

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    Imho, one needs to start at the beginning with such a situation. Determine if the PT is putting voltage out to the rectifier. If so, then replace all Electrolytics. At that point, one can begin to sort through the rest of the circuit.
     
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  5. Acobberson1

    Acobberson1 TDPRI Member

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    @PC_Hater thanks for the info! I knew the voltage on the diode wouldn’t be the problem. I went ahead and put a new 1n4007 on it as i have no idea when or why this one was added.

    I should probably point out that the motor is turning, so voltage seems to be getting there.

    AC voltage on the positive side (from PT) of the diode is hovering around 230V.

    Before and after soldering in the new 1n4007 the DC readings on the smoothing caps are in the mV range.

    ALL tube pins are also only reading in the mV range, including the heaters. It’s not even worth adding them here because they’re so low. Super weird. I’m wondering if my multimeter is faulty. I triple checked my range AND set it to auto and got the same readings.

    I also don’t know if the unit was worked on before. After checking the schematic it looks like there are no mods. My unit does not include the transistor output impedance circuit. That shouldn’t effect the voltage obviously, but I thought it’s important to note.

    Here are some photos.

    Tubes look to be all original, Mullard and Brimar, so I’m hoping they aren’t the issue. Unfortunately I don’t have a way to test the Brimar. The Mullards I can throw in my amp to see if they work.

    Thanks for the help!
     

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  6. Wally

    Wally Telefied Ad Free Member

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    Lift the connection from the rectifier to the filter section...is there voltage at the B+ side of the diode? If so, replace all electrolytics and proceed. There is no need to wonder about those old electrolytics. If the unit is to be brought back to life, those e-caps need to be replaced.
    The motor’s power supply comes off the primary side of the PT and is therefore not related to the rest of the power supply.
     
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  7. Snfoilhat

    Snfoilhat Tele-Afflicted

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    Tubes in or out? I would begin your troubleshooting with tubes out and only add them once you are confident the power supply is solid. Good luck
     
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  8. Acobberson1

    Acobberson1 TDPRI Member

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    @Wally thanks for weighing in. I’ll do that and report back.
     
  9. Acobberson1

    Acobberson1 TDPRI Member

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    @Snfoilhat that was with tubes in. I’ll pull them and see if there’s a difference in readings. Thanks!
     
  10. Acobberson1

    Acobberson1 TDPRI Member

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    @Wally I desoldered the diode from the cap and its only reading mV. Sort of pulsing between 10 and 30 mV. I’m right in assuming that should be DC following the diode, correct?

    I then checked the AC coming from the PT. 1-2V on both the primary and secondary. That sucks.

    Moving back from there, I get 230V (I’m in Germany) on the voltage selector, so the power seems like it isn’t getting through the transformer.

    I checked the fuse (should have done that long ago) and it’s a 0.63A
    Schematic shows a 1A, but if anything mine would just blow earlier. Fuse is in tact.

    So, does this point to a bad PT?

    One other thing, I found this little guy hanging out. There’s no center taps on the schematic, should this be grounded?
     

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  11. Acobberson1

    Acobberson1 TDPRI Member

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    @Snfoilhat readings with tubes out are the same. Virtually nothing at all.
     
  12. Wally

    Wally Telefied Ad Free Member

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    Acobberson1, your post #5 and post #10 conflict with each other on the voltage reading coming off of the PT secondary feeding the rectifier. In post #5 you say there is 230. In post #10 you have no voltage. And...in post #10 you say that there is no voltage on the PRIMARY side of the PT. Confused, I am.
    So, you have 230vac coming off of the voltage selector switch, correct? Is there not a power switch in between the voltage selector and the PT primary? Where does that 230vac no longer ‘exist’? If there is a switch, is there voltage coming in to and going out of the switch?
     
  13. Acobberson1

    Acobberson1 TDPRI Member

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    @Wally yes, that is confusing. I apologize. I got my terms mixed up.

    I meant to say there’s no voltage on either the secondary or on the heaters.

    The initial measurement (post#5) either changed or I misread it somehow. There is definitely no voltage being measured coming from the PT.

    I traced the power coming in. There is 230V going first to the switch, then 230V going to and from the fuse, then 230V going to and from the voltage selector. I unfortunately can’t measure whether the voltage is getting to the PT without removing it. I’ll have to try that tomorrow.
     
  14. Wally

    Wally Telefied Ad Free Member

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    I have to admit that this is a situation that would better be dealt with in person.....and I for one might be confused even in that situation.
    That said....I took a look ate your pics. I am not sure that the diode is correctly 8nstalled. I. The schematic, the diode takes its voltage feed dir3ctly fr9m the high tension secondary and thus provides the rectified DC voltage to the first filter cap. In your pics, I see that diode taking a feed fro: the HT secondary and then going to the board where it makes contact with not just a filter cap through that red wire but with everything else that is connected to that trace in the board. Am I seeing that as it is? Lift that diode from the board...what do have for voltage there? If you have voltage there, then that diode needs to go directly to a section of that filter cap.
    Or is that diode connected to that red wire and nothing else...trick of the camera???? At any rate, I do not like how that diode is installed. I prefer to prevent accidental shorts...and there should be voltage on both of those bare leads there.
     
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