Two prong to three prong/amp repair-Solved!!

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Canuck
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Two prong to three prong/amp repair-Solved!!

Post by Canuck » Sat Jul 11, 2020 10:19 pm

The hot wire joins in at the power switch. Then to an unused tab on the rectifier that’s leads to a black wire off the power tranny and the pilot light. There is a 0.047uf cap to ground at the switch.
The neutral wire is going to the fuse then on to another power tranny black wire and the other side of the pilot light. As well a 0.05uf (haven’t changed that one yet) to ground.
I’ve seen a few diagrams, though nothing exactly like this.
So my best guess is..

Hot wire -> fuse -> on/off switch -> black tranny wire/pilot light.

Neutral wire -> pilot light/other black tranny wire

green of course to ground lug on tranny

am I close?

If this is right/close, I’m not sure what to do with those two caps. Are they even death caps. This thing has an actual standby switch not one of those fender ground switches.

Again any ideas?
Last edited by Canuck on Mon Jul 13, 2020 8:32 pm, edited 3 times in total.

Canuck
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Re: Two prong to three prong

Post by Canuck » Sat Jul 11, 2020 10:25 pm

Not sure what happened but the top of my message got cut off.
I know this subject has been covered, but I’ve got an early to mid 60s Canadian made amp. It’s a Kent a-160. Most of what I can find is fender or other bigger name brand info. For some reason I’m unable to attach any pictures, so until I can here’s a quick description.
And now refer to the first post.

Canuck
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Re: Two prong to three prong

Post by Canuck » Sun Jul 12, 2020 7:07 am

Is there a trick to attaching a photo? All I get when I try to use attachments is “sorry the board quota has been reached”

glpg80
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Re: Two prong to three prong

Post by glpg80 » Sun Jul 12, 2020 3:18 pm

No need for a picture, I can follow what’s going on. You are correct - fuse the hot side (black) immediately before the switch. The pilot hot and black of the PT then go to the other side of the switch on the same side for hot (black).

Neutral goes to switch which controls the other throw of the switch for the pilot light and the PT.

The cap on the hot side is a death cap and if the ground is isolated and the cap goes bad, it becomes resistive to wall voltage and therefore will float the chassis to wall voltage.

The neutral cap is dangerous but for different reasons - if you ever have a gig place that has the hot and neutral out of phase which is common, it too can make the chassis hot.

Remove both of the caps and don’t replace them with anything. Then you’ll be good. I don’t see a need for the pilot light to need a cap across it on the primary.

Canuck
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Re: Two prong to three prong

Post by Canuck » Sun Jul 12, 2020 4:45 pm

Appreciate the reply. Gave it a go. It fired and is breathing again.
New caps, tubes and now an appropriate three prong cord. No death caps anymore.
Unfortunately my new problem. Although. Think this is an easy fix.
The volume has about 1/3 volume and any setting. The only spot is at the end around 8-10(max) where it actually drops very slightly and the treble, which wasn’t doing anything starts to work slightly.
I personally haven't touched the Tone stack at all. A previous owner may have or maybe a corroded ground.
Was thinking just a bad pot. Maybe two.
Unfortunately none of the pots are labelled. I’m hoping to get a good enough reading off of it.

glpg80
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Re: Two prong to three prong

Post by glpg80 » Sun Jul 12, 2020 5:08 pm

Sounds like a bad ground. Assuming Marshall tonestack layout, the treble, mid, and bass are all relative to a single ground on the other side of the bass.

Use Deoxit cleaner to clean the pots and then deoxit gold to lubricate them. Pots of old are no comparison to the garbage made in China these days - keep them unless physical damage is present.

Canuck
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Re: Two prong to three prong

Post by Canuck » Sun Jul 12, 2020 5:28 pm

That’s good advise. I’ll hit it with the multimeter and see if anything comes up. I used a contact cleaner on it, but not the deoxit. I’ll see if I can find some and I’ll give it a try

glpg80
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Re: Two prong to three prong

Post by glpg80 » Sun Jul 12, 2020 8:26 pm

Sounds good! Potentiometers are easy to ohm out. If you have the ability to measure resistance with a high count recordable meter, it will catch any dropout regions.

We haven’t been seeing good results with CTS pots over on another forum. Quality is abysmal and we think CTS is outsourcing.

Canuck
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Re: Two prong to three prong

Post by Canuck » Sun Jul 12, 2020 10:30 pm

You were right. Pretty sure anyhow. Pulled the volume pot and it tests fine 1M. The ground seemed fine. There’s a cap across lugs 2 and 3. It has old colour coding from top down.
Purple green black black white. I’m trying to look it up but if anyone knows could save a bit of time. Not sure if it being bad could cause my problem though.

glpg80
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Re: Two prong to three prong

Post by glpg80 » Sun Jul 12, 2020 11:35 pm

Blue green black black white is 90pF 630V 0.5%? Although I doubt they used 0.5% tolerance back in the day. I want to say white tolerance is as much as 80% positive 20% negative.

Canuck
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Re: Two prong to three prong

Post by Canuck » Mon Jul 13, 2020 8:31 pm

Absolute success!!
A bad bright cap on the volume. Changed out working perfect. It sounds fantastic. Not a loud amp. Perfect for home/jam use. A great clean machine with a touch of crunch at the top of the volume. I’ve got a jtm45 and a tweed deluxe (kits of course) so a clean amp is a perfect addition.
Thanks all for the help again!
I’ve got one more amp to redo, so I’ll be back I’m sure.

glpg80
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Re: Two prong to three prong/amp repair-Solved!!

Post by glpg80 » Tue Jul 14, 2020 11:08 am

Awesome congrats!! I always love a success story. Glad I could help! I enjoy the challenging builds moreso than anything else especially when you get them running!

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