2 to 3 prong conversion

Discussion in 'Amp Tech Center' started by AlbertaGriff, Feb 9, 2020.

  1. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Just want to double check that I've done this okay...

    Just serviced my new to me Vibrolux Reverb. All electrolytics replaced, I moved on to removing the death cap and 2 prong cord.

    In putting the 3 prong cord in, I took the ground switch out of the circuit. So my neutral goes to the AC adapter, meeting one side of the PT primary. My hot goes to the other side of the AC adapter, jumps to the fuse. Other end of fuse jumps to power switch. Other end of PT primary to other side of power switch.

    Is this correct?

    Also - any tips on grouding the cord to chassis? Been very frustrating so far, and the last thing I need to do, I think.
     
  2. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Pics of the recap.

    20200208_122015.jpg 20200208_133803.jpg 20200208_133812.jpg 20200208_133807.jpg 20200208_133817.jpg
     
  3. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    The ground wire terminal should bolt to the chassis. Do not solder the ground to the chassis. Nothing else can be on that connection. (The ground wire should not be bolted to the transformer mount.)

    If your power switch has dual poles the hot and neutral wires would connect to each so the hot and neutral would be disconnected in the "off" position.
     
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  4. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    I notice your filter caps are rated for 475V. Be careful with the voltages across the caps. At startup an amplifier has higher voltages than at idle.

    The AA763 schematic shows 465V. With todays wall voltages, our vintage amplifiers will run higher than the 465V. Are you using a different rectifier to drop the voltages to safe levels?
     
  5. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    No I'm using a GZ34. It's an AA964. Just using stock values.
     
  6. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    Your work looks good.:D
     
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  7. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    The schematic voltages for the aa964 are 410V. Sorry if I alarmed you with the A763 numbers.
     
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  8. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    I noticed the "brown turd" cap. I have seen a similar aged amp with the same "brown turd". I assumed it was an old repair when I saw it in the other amp. Is the "brown turd" stock?
     
  9. schmee

    schmee Poster Extraordinaire Silver Supporter

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    The ground wire from the factory on some of mine are on the transformer lug. One I worked on today (SF) was on that lug AND soldered to the chassis at that same location from the factory. I've never had a problem using the transformer lug. IF I make sure the transf isn't isloated by plastic washers etc. In fact from the factory if anything they may be only an inch or less away. Fender never bolted the ground to the chassis that I'm aware of on the older grounded amps.

    There is a convention for changing to 3 prong. Black vs white locations matter. I never knew that years ago and occasionally would get just a mild shock on a mic when playing. Since following this I have not had that issue. Here it is:
    [​IMG]
     
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  10. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Thanks!

    Yes I basically used that, just without the ground switch. It's still there, but unwired.
     
  11. Lowerleftcoast

    Lowerleftcoast Tele-Meister

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    AFAIK. Attaching the ground wire other than to a dedicated chassis point does not meet IEC standards.
     
  12. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    Forgot to answer this. Believe it is stock, as the amp was untouched besides 2 filter caps which also looked very old. I think I've seen others from this era with a single brown putty cap as well.
     
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  13. schmee

    schmee Poster Extraordinaire Silver Supporter

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    Yeah , I saw one recently with one brown turd also.
     
    Last edited: Feb 10, 2020
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  14. King Fan

    King Fan Friend of Leo's

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    Nice work, sir.

    Did you ever resolve your AC ground wire? I will admit my first amp still has it tied to a PT bolt, and you'll see quite a few amps doing that. Amp repair guys here say they've seen a fair number of old amps with loose PT bolts, though, so if you resort to that at least use a really good crimp-on terminal and stack it between a star washer and a keps nut -- with a dab of loctite.

    OTOH, once I knew better I found it easy to drill a hole for a #8 bolt in the side wall of the chassis to meet IEC/UL standards. I drilled this while the chassis was empty, but you could locate the hole so the bit won't run into stuff on the inside. You could also use a depth stop, and I might stick a magnet inside the chassis during drilling to catch metal shavings (and add a compressed air mop-up).

    upload_2020-2-10_9-28-8.jpeg
     
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  15. AlbertaGriff

    AlbertaGriff Tele-Afflicted

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    For the time being, I attached it to a transformer post. I believe I'll buy a ring terminal to attach it more securely this week. Thanks @King Fan
     
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