A Question Regarding a Shocking Power Supply

Discussion in 'Amp Tech Center' started by separateness, Apr 3, 2020.

  1. separateness

    separateness TDPRI Member

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    Not strictly Amp Tech material but I thought I would get a more pertinent response here.

    I got an old Tel-Ray/Fender Echo Reverb unit. Thing sounds great, if a little noisy at the moment. Anyway I've got it open and I've been playing it, replacing some parts etc. Earlier I had it on and I touched the outside of the cap can and received a little shock. Felt like AC, if I had to guess. Had that pulsing feel. Moving along, I decided it might be a good time to take a peek at the power supply circuit, for obvious reasons.

    Measuring to chassis I've got Open Loop from both prongs of the power plug and from both sides of the primary.

    Any ideas on this one? Also what the heck is that 0.047µF cap on the hot of the primary all about? Is this a 'death cap', but without the ground switch? Whats it for? Cleaning up dirty power? Should I remove it?

    I am definitely going to put a 3-prong cord on this joker.

    Any advice is greatly appreciated.
    [​IMG]
     
  2. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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  3. Lowerleftcoast

    Lowerleftcoast Tele-Holic

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    A 3-prong cord would eliminate the use of the death cap.
     
  4. separateness

    separateness TDPRI Member

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    Ah, when in doubt, assume Rob Robinette has a page about it. I thought a death cap implied a ground switch, but this is not the case. I reckon I had it plugged in the 'wrong way' and got the little bite that come along with that. I will replace the cord with a three prong and delete the cap.

    Pretty wild that fellas used to deal with this sort of thing and it was just the way life was. They were made of stronger stuff, I reckon.
     
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  5. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    I suppose the cap can could be leaking voltage when powered up, but show a resistance open when checking for a short. Power it back up, and check voltage at the can, (try it with the plug both ways).
    As for playing with 2 prong power, it was always part of the set up to ensure your mic and amp were in phase. I played drums in those days, and sang a little backup, but didn't care how the plug was for the PA.
     
  6. separateness

    separateness TDPRI Member

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    Well I got 2VAC (?????) with the cap side cold and 125 VAC with the cap side hot. I can only assume that this is a case where the two prong/death cap set up needs to go.

    I just noticed that the picture I attached seemed to not post. Here is the relevant portion of the schematic.
     

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  7. Wally

    Wally Telefied Ad Free Member

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    Stronger stuff? Nah....I was taught to listen for the noise that the amp makes when the polarity is incorrect. If the noise was there, then one was to flip the plug at the wall. This was a part of my guitar lessons....I heard it one time, and the one demo was enough. This was on a little single ended amp that did not have a polarity(ground) switch. When I got my ‘64 Ric B16 Supersonic, it had a polarity switch...along with two ‘pilot’ lamps...one red for standby, one green for play...iirc. fwiw, a three way plug does not guarantee that amp will be seeing proper polarity and thereby be safe and quiet. That is why one should check the wall outlet for proper wiring...with an inexpensive outlet tester.
     
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  8. Peegoo

    Peegoo Friend of Leo's

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    When you install the three-wire power cord, run the hot (black wire) side to the fuse, then to the power switch, then to the power transformer.

    Placing the switch before the fuse defeats an additional level of safety because if it's wired to the switch first, and the switch fails and shorts to ground, the chassis can become live and the fuse won't even pop.

    Remember Rule #3: Everything is a fuse if you use it wrong enough.
     
  9. separateness

    separateness TDPRI Member

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    For whatever reason, this device does not have a fuse. I suppose this might be another wise addition.
     
  10. Peegoo

    Peegoo Friend of Leo's

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    Absolutely it needs one.

    Looking at the voltages on the schematic, a 2A slo-blo would be appropriate.
     
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  11. KC9KEP

    KC9KEP TDPRI Member

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    One additional dinky detail .. when wiring to a fuse socket, it’s probably best to run the black “hot” wire to the tail-end of the fuse socket. This prevents the “collar” contact in the fuse socket from being hot .. which makes the replacement of a fuse a lot safer
     
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  12. LightningPhil

    LightningPhil Tele-Meister

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    This includes concrete. Explodes enthusiastically if hit with enough volts and amps. Much fun.
     
  13. Art VanDelay

    Art VanDelay Tele-Afflicted

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    As Sean Connery 007 would say, shocking.........positively shocking.

     
  14. separateness

    separateness TDPRI Member

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    I rather hate to bump this thread but it is probably better than making a new one.
    Does any one have a line on a good fuse holder for adding in a fuse to this thing? Or a good idea on how to approach this modification?
    I would rather a) not have to drill into this already-pretty-crowded relic and b) be able to access the fuse without taking the whole thing apart.

    This has lead me to these two possibilities:
    1. An inline junction box on the AC cord with a traditional panel-mounted fuse holder mounted into it. This might not be great because I have to buy an small (pedal?) enclosure and also I will have a metal box on the cord which will make stowing it not ideal and some other minor issues. Also it is probably best to have the fuse closer to the circuit it is intended to isolate.
    2. An inline fuse holder tucked between two strain reliefs inside the cabinet of the unit. The problem with this is I cannot seem to find an ideal holder for this. Most of the ones I find are for car fuses or have very thin wires (<18ga).
    Any advice is appreciated.

    PS Also, I posted a thread about a hum issue with this thing over on the diy effects board which has gotten zero responses. I am unsure where best to post about this thing. Anyway, if any one might be able to offer some insight into that issue I would appreciate it.
     
  15. Wally

    Wally Telefied Ad Free Member

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    There are inline fuse holders that could be wired into the hot AC lead inside the chassis. It is not as handy to replace as a chassis mounted fuse, but hey...when a fuse blows I usually want to find out why if I can.
     
  16. separateness

    separateness TDPRI Member

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    Could you post a good source for one of these if you know one? I found a potential contender on parts-express but they are out of stock and I would really like to have a fuse in this thing asap.
     
  17. milocj

    milocj Friend of Leo's Ad Free Member

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    separateness likes this.
  18. separateness

    separateness TDPRI Member

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    Brilliant. An answer so obvious I could not have possibly thought of it.
     
  19. Bill Moore

    Bill Moore Tele-Afflicted Silver Supporter

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    Just make sure you buy one to fit the fuse you need!
     
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