PL 700 II Clair Bros Rising from the Ashes

[MarkWComer]

BTW, all three LED segments above 0 db would flash, I take it this is normal?

Yes, this is normal. This indicates overdriving, the flashing clears when the overdrive condition subsides.

 

[MarkWComer]

Here's the transformer cleaned up.

The Xformer is gonna be the only original part of my rebuild apart from the heat fins. What did you use for cleanup? I used denatured alcohol on the heatsinks since there was a lot of thermal grease from the last repair. Did fine for the aluminum, just wondered what to do to “dress up” the Xformer. Thought about wiping with WD40, is that safe?

 

[Gepetto]

Here's the transformer cleaned up.

Recommend you insulate your bolts Peter.

 

[Peter S]

Hi Mark;
This amp must have spent time in humid locations, so this is what I did;
--removed, bolts and rear cover.
--masked the gap between transformer and the die-cast saddle. Masked the winding on the back side. The transformer was supported to allow a separation from the saddle without disconnection.
--3M abrasive pad was used to remove the rust. Masking of the transformer winding and gap between saddle/chassis is important, as the dust from this operation contained abrasive particles and rust.
--this operation, as well as subsequent cleaning with acetone, removed the paint and oxide finish on the laminations. I had electrical vanish (left over from a torroid transformer dipping project. A small amount of black Tremclad was added to the varnish to return the laminations to their original colour........are you sorry you asked?

Hi "Gepetto";
I guess I have to pull those bolts! I thought I was being meticulous--but I see I have to get up pretty early in the morning to 'out-fanatic' the fanatics! The bolt shanks were quite corroded so they were wire-brushed. Do you think that a generous coating of electrical varnish would be sufficient, along with some washers made from IMAX film (polyester) under the heads and nuts? I believe the laminations are to reduce 'eddy' currents and through-bolts would defeat the purpose; How is this avoided in other transformers, motors etc?---if this was your concern. I don't think there is room for sleeves in the transformer through-holes.

Thanks, Peter

 

[MarkWComer]

Recommend you insulate your bolts Peter.

Insulate the bolts? Can I use heat shrink tubing? I removed my Xformer, bolts were a bit rusty, got new ones from a hardware store. As far as I see, they weren't ever insulated. Nasty little overlooked detail, but is it important or even beneficial to do?

 

[MarkWComer]

......are you sorry you asked?

Any and every hint is helpful, good point about the oxide dust in the windings. On second thought, it doesn't look all that bad...

 

[Peter S]

Heat shrink on the bolt's---great idea! I will check for clearance. I would like to ask; Is it necessary to use insulating washers under the bolt head washers and the nuts inside?----or is the heat shrink sufficient? Are we trying to avoid shorting the laminations together OR isolating the transformer core from the chassis?

 

[MarkWComer]

Heat shrink on the bolt's---great idea! I will check for clearance. I would like to ask; Is it necessary to use insulating washers under the bolt head washers and the nuts inside?----or is the heat shrink sufficient? Are we trying to avoid shorting the laminations together OR isolating the transformer core from the chassis?

I bought new bolts ‘cuz the old ones aren’t shiny...

The big deal for me is that I need to transplant the transformer to a new chassis. The nuts on the old bolts have a self-locking serration (as do the new ones...) so I would expect any kind of insulating washer to defeat the lock.

I don’t recall anyone else insulating those bolts in the past with their rebuilds- did I miss something, or is this detail necessary because you polished your lamination stack?

As far as I know, shorting the layers of the stack together isn’t an issue- there should be no current there. The stack is there to provide something for the expanding and collapsing magnetic field to form.

 

[Bradrock]

I may not remember my phone number or house address......but I remembered this thread: https://forums.phxaudiotape.com/threads/wopl-700b-a-c-hum.8518/

 

[Gepetto]

Insulate the bolts? Can I use heat shrink tubing? I removed my Xformer, bolts were a bit rusty, got new ones from a hardware store. As far as I see, they weren't ever insulated. Nasty little overlooked detail, but is it important or even beneficial to do?

Yes you can use heat shrink to insulate them Mark. insulating from the stack cuts down on eddy current losses and also putting eddy currents into the chassis shell which you want to be a benign ground shell.

 

[Gepetto]

Heat shrink on the bolt's---great idea! I will check for clearance. I would like to ask; Is it necessary to use insulating washers under the bolt head washers and the nuts inside?----or is the heat shrink sufficient? Are we trying to avoid shorting the laminations together OR isolating the transformer core from the chassis?

You wish to insulate the bolts from the chassis. The transformer lams are insulated at the each lamination level. Unless there has been damage to the laminations, the lam stack will be insulated from the chassis and each other lam.

The part of each lam that is not insulated are the edges since those are punched, exposing raw electrical steel edge. The varnish dip takes care of the outside but it is hit or miss inside the holes. Also the bolts themselves are subject to eddy current buildup and the associated losses.

The reason they make transformers laminated like they do is that a solid electrical steel core would have quite high eddy current losses. They make electrical steel high electrical resistance as well to further reduce eddy currents. The bolts are NOT high resistance steel (very low resistance), thus high internal eddy currents.

 

[Gepetto]

I bought new bolts ‘cuz the old ones aren’t shiny...

The big deal for me is that I need to transplant the transformer to a new chassis. The nuts on the old bolts have a self-locking serration (as do the new ones...) so I would expect any kind of insulating washer to defeat the lock.

I don’t recall anyone else insulating those bolts in the past with their rebuilds- did I miss something, or is this detail necessary because you polished your lamination stack?

As far as I know, shorting the layers of the stack together isn’t an issue- there should be no current there. The stack is there to provide something for the expanding and collapsing magnetic field to form.

Shorting the layers (laminations) of the stack IS a big deal Mark. It leads to a very lossy core stack.

 

[MarkWComer]

Shorting the layers (laminations) of the stack IS a big deal Mark. It leads to a very lossy core stack.

Something new each day. Stuffing my brain with all sorts of new stuff! Thanks!

 

[Gepetto]

The ideal magnetic core is electrically very high resistance in nature but very low resistance magnetically speaking.

The bolts are very low resistance magnetically AND electrically. The best thing to do is to ditch the star washers used on the transformer bolts, insulate the bolt shanks and use nylon washers or equivalent under the bolt head end and the nut at the other end. Use a flat washer under the bolt head and nut to spread the load on the nylon washers.

Stackup starting at outside is bolt head>flat washer>nylon washer>bolt shank insulation>rear transformer cover>transformer stack>die cast capacitor holder>chassis>nylon washer>flat washer>nut.

 

[MarkWComer]

So then...

The transformer buzz on my 400...

Hmmm...

 

[FredR]

Joe, if we wanted to replace bolts do you have any specific part number(s) you could recommend? Thanks. Fred.

 

[MarkWComer]

Joe, if we wanted to replace bolts do you have any specific part number(s) you could recommend? Thanks. Fred.

4” x 1/4” carriage bolts, 1/4-20 thread. Common hardware, I bought mine at Ace.

 

[Peter S]

Here is my bolt-isolating "kit"; large drinking straws, IMAX film for washers (3 layers= .012 inch thick) hole punch for ID, sharpened tubing for OD.

 

[Peter S]

Has anyone actually done this? Quit a 'PITA' ! But the final test was a success; the bolts were isolated from the transformer laminations and the chassis.

 

[Gepetto]

Nice work Peter. That is how it is done. I ditch any toothed type locking hardware because it tends to dig its way through any insulation you use. Straws are a common choice for the shank as you did. You really want the bolts isolated from the chassis and the transformer shell itself. If the bolts contact the chassis, then you can set up currents flowing through the aluminum chassis due to the induced voltage in the bolt that you do not want. You can think of the bolt as a partial winding turn passing through the magnetic field flux that exists in the transformer core. If you want the absolute lowest noise floor, keep your bolts insulated.