Power transformer bolt is stripped.

[Mike_J]

I want to replace a bolt in my power transformer. It is stripped and unfortunately is the bolt that I am going to attach the ground lugs to. The lugs obviously need to be able to be firmly secured. I have never taken apart a power transformer and really don't want to now. All I am hoping to do is remove the 10-24 X 2-1/4" bolt that is stripped and replace it with a new bolt. Can I just remove the bolt and insert a new bolt or is there some disaster which can occur by the simple replacement of the bolt. Appreciate any help anyone could give.


Thanks
Mike

[sluckey]

Just do it. Put it back together exactly as it was before you started. You may find insulating shoulder washers or maybe even a long tube. Reuse. No parts left over. It ain't complicated.

[tubeswell]

Sometimes its not as simple as it looks if the bolts are stuck with varnish. You may need to use a big old wedge/flat tip 200W soldering iron to superheat the bolt in order to get the varnish to unstick


Also, for earth safety grounding, its better to have the stranded ground wire clamped securely to a ring terminal on a single dedicated grounding bolt that is attached to the chassis with a lock-nut. Don't just stick it on a PT mounting bolt along with other ground returns.

[DummyLoad]

#10-24/32 are rare on power transformers, very large PT may use 10-24/32 bolts - typically they are 6-32 or 8-32. not saying yours aren't 10-24.

you can push the bolt through and replace. if there are any insulation bushings, reuse them. mostimes the bolt will be locked in place with varnish, use a propane torch (i have used a butane lighter with success)  on very low setting to heat the bolt and push it out with a pin punch and small hammer - no need to get medieval, work slow and don't bang too hard on anything. it helps if the transformer is not installed in a chassis - you may get lucky and the bolt just pushes through without much effort. 


--pete

[Mike_J]

Sometimes its not as simple as it looks if the bolts are stuck with varnish. You may need to use a big old wedge/flat tip 200W soldering iron to superheat the bolt in order to get the varnish to unstick


Also, for earth safety grounding, its better to have the stranded ground wire clamped securely to a ring terminal on a single dedicated grounding bolt that is attached to the chassis with a lock-nut. Don't just stick it on a PT mounting bolt along with other ground returns.

The earth ground (green wire from the power cord) is currently attached to a different transformer bolt with a ring terminal and lock nut very close to where the power wire enters the chassis. The other ground returns will be attached to a different transformer bolt using grounding lugs. Do you think that is sufficient? It is this bolt that is stripped. Can't get the lugs tight.


Thanks
Mike

[Mike_J]

#10-24/32 are rare on power transformers, very large PT may use 10-24/32 bolts - typically they are 6-32 or 8-32. not saying yours aren't 10-24.

you can push the bolt through and replace. if there are any insulation bushings, reuse them. mostimes the bolt will be locked in place with varnish, use a propane torch (i have used a butane lighter with success)  on very low setting to heat the bolt and push it out with a pin punch and small hammer - no need to get medieval, work slow and don't bang too hard on anything. it helps if the transformer is not installed in a chassis - you may get lucky and the bolt just pushes through without much effort. 


--pete

Good information. Will keep you informed of my progress. This transformer is over twenty years old and has slotted heads on the bolts and they are 10-24. Where would I see the insulation bushings? I will look at it tomorrow and take a picture if necessary.


Thanks
Mike

[tubeswell]

The earth ground (green wire from the power cord) is currently attached to a different transformer bolt with a ring terminal and lock nut very close to where the power wire enters the chassis. The other ground returns will be attached to a different transformer bolt using grounding lugs. Do you think that is sufficient? It is this bolt that is stripped. Can't get the lugs tight.

Sounds good to me

[sluckey]

This thread makes a good argument to use the PT bolts to secure the PT to the chassis AND NOTHING ELSE!

[kagliostro]

If you can, prefer to use stainless steel or brass bolts

they aren't magnetic

Franco

[Mike_J]

If you can, prefer to use stainless steel or brass bolts

they aren't magnetic

Franco

I will look for stainless as a replacement.


Attached are pictures of the offending bolt. Slid right out. Slid it back in to see if there would be any difficulties there. There weren't any. Insulation bushings are very obvious when you look at the transformer. Needed an 8mm to take the nut that is against the bushing off. Didn't check to see how that converts to standard but I will probably replace that nut as well.

[DummyLoad]

if you used an 8mm, then you have a 6-32 bolt & nut - 8mm will not fit 8-32 nut or #10 nut.


--pete

[Mike_J]

#10-24/32 are rare on power transformers, very large PT may use 10-24/32 bolts - typically they are 6-32 or 8-32. not saying yours aren't 10-24.


--pete

Is it that I am getting older and have lost a little snap? Yes somewhat. Was it the time I had an abdominal aortic aneurysm rupture and lost a lot of blood and blood pressure so I didn't get a lot of oxygen to my brain for a little too long? Yes somewhat. Was it the six weeks in a drug induced coma, on a respirator while I caught everything the hospital had to offer while recovering from the AAA rupture? Yes somewhat. Will it keep me from doing things I enjoy like trying to build a decent amplifier? No!!!


The first paragraph is a small list of excuses I am relying on to cover for a mistake on my part which you figured out a lot quicker than I did. Thank you for this post because I had it in the back of my mind when I went into the hardware store. The hardware store has these displays you can use to check your bolts or nuts for sizing. I used a 10-24 keps nut from my bin to check for size but I only had one of them and needed two. So I took the 10-24 to the store and found out it was 10-24.


Well today I took the bolt in and put it in the display space for 10-24 and noticed there was quite a bit of slop that shouldn't have been there. Remembering your post I thought to myself why don't I check the 8-32 nut to see if it fits. Well probably not to your surprise it did. I checked the 6-32 and it didn't fit. Turns out you can get a 10-24 nut on an 8-32 bolt, you just can't make it tight.


Went to the stainless section and found a 2-1/2" phillips head screw, the old screw was slotted, reason enough for replacing it in my opinion, don't really want a slot headed screw in any of my amps personal thing. Found the old screw was a little longer than 2-1/2" and already a little difficult to work with when trying to put a keps nut on it so I went with a 3" bolt to stop this frustration. Didn't have a flat washer between the head of the screw or the nut holding it to the insulating washer. Decided it would be better for the insulating washers if they had washers protecting them so I bought them. If anyone knows why anyone would manufacture these transformers without washers to protect the insulating washers, other than to save a nickel per unit, please let me know.


Thanks
Mike

[Mike_J]

One more thing I needed to ask. How tight do you make the screw and nut against the insulating washers? Wouldn't think you would want to make them too tight. Those insulating washers look like they are made out of plastic.

[Mike_J]

Pictures of new bolts in the power transformer for the 5f6a build. Should work easier than the bolts that were there especially after I boggered them by using the wrong sized kep nuts to use sluckey's vernacular. If you look close you will see where I smashed the bell cover with a hammer to keep it from rubbing against one of the top speakers. Don't remember doing it but know why I would have done it. Thank you to everyone for the help and comments.


Thanks
Mike 

[DummyLoad]

looks like a drill bit went for ride across that end bell.


--pete

[Mike_J]

looks like a drill bit went for ride across that end bell.


--pete

I made the amp 22 years ago. The chassis has been out of the cabinet many many times and placed on a piece of plywood where it gets moved around and scratched. Try to use towels now to keep the transformers from looking like they have been through a war zone.


Thanks
Mike

[sluckey]

Now that you can take the bolts out of the PT you could paint the bells.

[DummyLoad]

looks like a drill bit went for ride across that end bell.


--pete

I made the amp 22 years ago. The chassis has been out of the cabinet many many times and placed on a piece of plywood where it gets moved around and scratched. Try to use towels now to keep the transformers from looking like they have been through a war zone.


Thanks
Mike

i use painters masking tape. comes off easily and doesn't slide around.


--pete

[PRR]

If you can, prefer to use stainless steel or brass bolts
they aren't magnetic

I could be wrong, but I *think* the point here is electric, not magnetic.

Look how we "sliced solid iron" into thin laminations. If you picture little circles of eddy-current, thin lams make small circles and less eddy-loss. But any electric conductor put where the bolts go makes eddy-circles many times larger.

We want bolts with electric conductivity much less than Iron. But ceramic and plastic won't take the strain. Spider web? Historically any metal bolt, plus paper tube and washers, does the job. Today you may see Nylon or other good plastic.

[kagliostro]

If you can, prefer to use stainless steel or brass bolts
they aren't magnetic

I read about that some time ago and made it my own, but I don't remember where I read it or the related explanations, as far as I can remember it was still a source that at the time I found to be authoritative on the subject

Franco

[sluckey]

I don't think I've ever seen a PT with brass or stainless steel bolts.

[kagliostro]

I've seen brass bolt used on old transformers


Franco

[Mike_J]

I don't think I've ever seen a PT with brass or stainless steel bolts.

They are too cheap to even use two cent washers to protect the plastic washers. The extra cost of stainless would be even more. My PT has stainless bolts washers and keps nuts. Went all out. Think I have a can of black spray paint to give the bell a nice new coat of paint. Would really be an upgrade.


Thanks
Mike