Type 547 series HV Transformer replacement/rewind


Trevor Carrier
 

Is there anyone currently rewinding HV transformers for the 500 series Oscilloscopes? I have a 547 that has bad HV transformer. Like many other units the HV sags after 20 minutes of being turned on, and the heat causes the HV transformer to loose it's stability. I understand there is alternatives to prolonging the life if an existing unit, but that can only go so far before it ultimately has to be replaced.

Thanks

-Trevor N6KGN


Roy Thistle
 

On Mon, Jan 17, 2022 at 06:53 AM, Trevor Carrier wrote:


Is there anyone currently rewinding HV transformers for the 500 series
Oscilloscopes?
Hi N6KGN:
The subject of "rewinding" these "HV" transformers is a frequent thread on TekScopes (as a search will show)... and on other forums too (EEVblog)
AFAIR, there's been a lot put into the mechanical part of winding the transformer (wire, winding speed, winding machines... ect.) And, this part has been somewhat successful. People have wound them.
Just the same... while the actual winding may be satisfactorily done (at least referring to a home-brew setup)... the remaining... and seemingly difficult problem... is that ... IMO, not as much thought has been put into identifying the materials required for, and the method used for, the successful potting of these HV transformers.
AFAIR there was some investigation into potting materials ( the use of various commercially available industrial resins) ... and some determination that the potting would take place in a vacuum chamber (to release any trapped air). Just the same, getting those resin materials in small quantities (or at all ), and obtaining vacuum systems, and vacuum potting chambers... and obtaining the methodology for their use... that seems difficult.
Anyway, AFAIK .. a successfully rewound 500 series transformer will fail, if the potting is not sufficient.

--
Roy Thistle


Stephen
 

Roy,
I’ve also read that many times. I’ve rewound overall 15 or so transformers. Both PT and OT tube amp transformers. I’ve always potted them in beeswax, and they’ve never failed, even after 15 or 20 years.
To your knowledge, is there anything “special” about transformers used in ages scopes that they have to be potted a certain way for them not to fail? Does anyone know? That seems odd…
Maybe some can answer that…


Stephen
 

Typo. Rephrase:

To your knowledge, is there anything “special” about transformers used in old Tek scopes that they have to be potted a certain way for them not to fail?


Dave Wise
 

For what it’s worth, I rewound my 453’s HV transformer at the end of 2015 using a low-tech approach. I wound it in layers. I insulated each layer with Scotch Magic Tape, which is polypropylene. I did not pot it. I expected it to fail, but it is still going strong in 2022. (Topic and photo album “453 HV Transformer Rewind”.)

Dave Wise

From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Roy Thistle via groups.io
Sent: Monday, January 17, 2022 7:28 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type 547 series HV Transformer replacement/rewind

On Mon, Jan 17, 2022 at 06:53 AM, Trevor Carrier wrote:


Is there anyone currently rewinding HV transformers for the 500 series
Oscilloscopes?
Hi N6KGN:
The subject of "rewinding" these "HV" transformers is a frequent thread on TekScopes (as a search will show)... and on other forums too (EEVblog)
AFAIR, there's been a lot put into the mechanical part of winding the transformer (wire, winding speed, winding machines... ect.) And, this part has been somewhat successful. People have wound them.
Just the same... while the actual winding may be satisfactorily done (at least referring to a home-brew setup)... the remaining... and seemingly difficult problem... is that ... IMO, not as much thought has been put into identifying the materials required for, and the method used for, the successful potting of these HV transformers.
AFAIR there was some investigation into potting materials ( the use of various commercially available industrial resins) ... and some determination that the potting would take place in a vacuum chamber (to release any trapped air). Just the same, getting those resin materials in small quantities (or at all ), and obtaining vacuum systems, and vacuum potting chambers... and obtaining the methodology for their use... that seems difficult.
Anyway, AFAIK .. a successfully rewound 500 series transformer will fail, if the potting is not sufficient.

--
Roy Thistle


Stephen
 

On Mon, Jan 17, 2022 at 04:54 AM, Dave Wise wrote:


For what it’s worth, I rewound my 453’s HV transformer at the end of 2015
using a low-tech approach. I wound it in layers. I insulated each layer with
Scotch Magic Tape, which is polypropylene. I did not pot it. I expected it
to fail, but it is still going strong in 2022. (Topic and photo album “453
HV Transformer Rewind”.)

Dave Wise
Now that you mention it, I actually did not pot the first few transformers I rewound, and I used the same approach. And they’re still perfectly functioning today. Albeit they were not scope transformers…. I don’t see why that would be different. After all it’s just a transformer…. But I’m probably wrong…


Trevor Carrier
 

Unfortunately I don't have the tools or hardware to rewind the HV Transformer myself, which is why I was asking if there is someone in the group that has this capability, and has done it before.

I can see that this is a frequent subject on this site.

Thanks

Trevor


Stephen
 

I’d be more than happy to help, but being in Europe, I’m sure someone much closer to your location can.


Roy Thistle
 

Hi Stephen:
A couple of things... if you'll indulge me.
The OP's referring to HV (high voltage) transformers.
Beeswax is like adamantium in that it is unobtainium because in the case of beeswax... the name does not refer to a specific real thing. (Beeswax refers to a large set of substances that are similar.) Maybe the bees know what it is? ... everyone else doesn't know or isn't telling.
I don't think the transformers we are talking about are ''special'' but, they do have AFAIR a universal type winding, using a large number of turns of small diameter wire, all packing into a small volume due to necessary mechanical design constraints.
Whatever the reason, at some point(s) in the volume, containing the winding, the magnitude of the E field exceeds the dielectric breakdown... and ... well, the dielectric breaks down, leading to cascading failures of it. Some of the reasons for this might be the winding geometry...too much heat in to small a space, defective potting et. al. But, a lot has been mentioned about the ingress of water into the 'epoxy' that Tek used to pot... as the cause of the failure, of an otherwise working Tek manufactured transformer.
AFAIK, 'beeswax' won't keep the water out, and a rewound, re-potted, home-brew transformer will fail again... in less time than desired.


--
Roy Thistle


Roy Thistle
 

On Mon, Jan 17, 2022 at 07:54 AM, Dave Wise wrote:


I rewound my 453’s HV transformer at the end of 2015 using a low-tech
approach. I wound it in layers. I insulated each layer with Scotch Magic Tape,
which is polypropylene
Yes. A good fix! (I image you keep the 453 in a low humidity (dry) environment? )
There might be a way to do it as you have done it for a... say 535 545... but, I speculate... the winding geometry, and method of dielectric won't result in a transformer that could drop into the HV supply... the transformer would be too big?

--
Roy Thistle


Dave Wise
 

You have a point there, Roy. My 453 is in a heated basement. Overall house humidity is around 50% year-round.
Like other rewind techniques that have been used in Tek scopes, the one I used reuses the original core. The 535/545* transformer is the same as the 453 except for aspects unimportant to this conversation like turns ratio and the presence or absence of extra primary-side windings. On my 453 I had room for the original 5642 filament loops, and I predict the same for 535/545 and 547, but it doesn’t matter since you can always go solid-state.

\* Which rarely fails since it was wax-impregnated throughout production.

Dave Wise

From: TekScopes@groups.io <TekScopes@groups.io> On Behalf Of Roy Thistle via groups.io
Sent: Monday, January 17, 2022 9:52 AM
To: TekScopes@groups.io
Subject: Re: [TekScopes] Type 547 series HV Transformer replacement/rewind

On Mon, Jan 17, 2022 at 07:54 AM, Dave Wise wrote:


I rewound my 453’s HV transformer at the end of 2015 using a low-tech
approach. I wound it in layers. I insulated each layer with Scotch Magic Tape,
which is polypropylene
Yes. A good fix! (I image you keep the 453 in a low humidity (dry) environment? )
There might be a way to do it as you have done it for a... say 535 545... but, I speculate... the winding geometry, and method of dielectric won't result in a transformer that could drop into the HV supply... the transformer would be too big?

--
Roy Thistle


Stephen
 

I totally understand and agree that the mechanical constraints and the very close proximity of very fine wire and very high voltage can be a problem. You have a very good point. Thanks.

As for beeswax, I get the one I use directly from my father in law’s bee farm. It is real and pure beeswax’s directly from the hives. Don’t ask me how he extracts it.

Thanks for enlightening me (us).


 

Roy Thistle wrote:

I don't think the transformers we are talking about are ''special'' but, they do have AFAIR
a universal type winding, using a large number of turns of small diameter wire, all packing
into a small volume due to necessary mechanical design constraints.
I know next to nothing about making transformers (aside from what anybody with three semesters of undergraduate physics would know) and I'm trying to get conversant enough in the topic to rewind a couple of these HV transformers (for a 545B and a 546), so please excuse my ignorance, but I have a couple of questions:

1. when you say "large number of turns" how many are we talking aobut? I am trying to understand the transformer spec sheet for the 545B transformer (https://w140.com/tekwiki/images/3/30/Tek_120-0308-00.pdf), and my (possibly incorrect) reading is that there are 60 turns on the primary winding, and 733 & 525 turns on the secondary windings. Is this what you mean by a "large number of turns," or have I misunderstood the spec sheet?

2. am I correct in thinking that a "universal winding" is specific winding pattern? I'm having trouble looking that up on the internet, are there other names for this pattern?

--Jeff Dutky


Jean-Paul
 

Bonjour, We designed and manufactured HV transformers in the 1970s..1990s. Built many 1000s.

universal winding uses a back and forth wire guide at an angle on each layer.

Result is much lower self C and used in most flyback and high freq HV transformers.

Not needed in mains freq designs.

Old Stevens winding machines had this capability.

Often they windings are in pies to further reduce self C and stress on the insulation.

Potting has been discussed before. Long discussion. We ised a proprietary 100% solids 2 part mix with vacuum oven cure and annealing. Took a YEAR to design and debug the potting and gethigh yield.


Suggest to check all of the fine work from our old friend Chuck HARRIS.

But he is expelled for unknow reason from the forum years back.

Bon courage,.

Jon


 

Chuck Harris can be found on Tekscope2

David


Morris Odell
 

There has been a lot of discussion and videos of this subject on the facebook old tek scopes group recently. I'm surprised nobody here has mentioned it. A mate of mine here in Australia has recently successfully wound Tek HV transformers and potted them using very simple home made equipment. I made a computerised winding machine for doing these but have been out of action for a while due to other issues, fortunately now resolved.

A few points:

1. Tek potted their early transformers in beeswax . Many of them still work perfectly 60 years later. The problems only arose when they started using epoxy as an "improvement". They would have had no way of knowing that the material would deteriorate with time or that the scopes they made would still be prized many years later.
2. Vacuum impregnation is not hard to do - see my friend's posts on FB. Beeswax is commonly available and he improvised using a home bottling jar and solder sucker pump.
3. The biggest difficulty is getting the winding to stay structured as winding proceeds. Various sticky substances have been tried to "serve" the wire as it's wound but I certainly haven't found the ideal one. Any advice would be welcome especially if anyone knows of a volatile solvent for wax. I'm currently trying warmed up soft wax sold for coating cheese!
4. Universal (basket weave) winding is desirable but not essential. My 547 and 549 transformers were "scramble" wound 20 years ago with insulating tape between layers and still work well.
5. Chuck Harris made transformers very close to the Tek specification and I believe they worked "out of the box". Experience with home rewinds has shown that minor tweaks may be required to the oscillator circuit if the transformer has altered capacitances or coupling from different winding arrangements.
6. Replacing the vacuum rectifiers in older scopes with silicon diodes makes life a lot easier as there will be more window room available for the rewind and the oscillator runs at a lower power level. The screen resistor for the oscillator tube may need to be increased to keep the regulation working properly .

Morris


Trevor Carrier
 

David,

I went ahead and posted in the second Tektronix group. Hopefully Chuck Harris will see it.


Roy Thistle
 

On Mon, Jan 17, 2022 at 02:28 PM, Morris Odell wrote:


Universal (basket weave) winding is desirable but not essential. My 547 and
549 transformers were "scramble" wound 20 years ago with insulating tape
between layers and still work well.
Well then... problem solved!
IMO... given the original core... rewinding and potting these transformers isn't rocket science. It's not even science. But, it is a technology.

--
Roy Thistle


 

Roy,

I consider transformer construction both a science and a craft, and I know that I have no relevant experience in the craft, which is why I'm asking these questions. The craftsmanship of functional objects can often be as important as the scientific principles, and I'd like to go into this with at least one eye open.

-- Jeff Dutky


 

David,

I will post the entirety of my ignorance (I have extensive notes) to Tekscopes2 in the hope that Chuck will enlighten me.

-- Jeff Dutky