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Hello all, I've got this fabulous 565 that works pretty well, other than losing the traces after about 40 minutes or so. The lower beam goes first, starts dimming at around the 40 minute mark, then is completely gone around 50 minutes, with the upper beam following just a few minutes later.

My main concern is that I may have one of the fabled epoxy potted HV transformers that like to fail. Serial number of the scope is 1723.

Let it cool off for around 30 minutes and it's fine for another 40 or so minutes.

I got the serial number wrong, it's 1782, not 1723. I do have the original manual for it as well, which matches the serial number.

Eric,

What you have is the sign of the high voltage transformer going bad. I suspect your beam blooms when it is going out and turning the intensity up lasts briefly. The high voltage rectifier filament will dim to zero when it is going down. The cooling period making it work again is a sure sign the high voltage transformer is leaking. The 500 series is known for this problem. Maybe someone will be able to rewind or make new ones that do not have the leakage problem.

Mark

Thanks for the reply Mark, that's what it's doing. I was afraid of that.

Hi Eric , I dont have a 565 myself mores the pity but I think the situation is a little worse than that -- if memory serves me correctly that scope has 2 HV transformers that are failing in the usual way .
regardsBrian (UK)

The 565 has only one HV transformer. Production started in 1962 so it may not be late enough to be one of the problem transformers. Failure of a circuit after a period of time can also be caused by a leaky capacitor (the leak causes heating of the capacitor) or a bad tube. Suggest you check the primary side of the HV circuit, especially the 0.1 uf bypass capacitor C802 and the two tubes, V800 and V814. If the problem continues, purchase some modern HV caps and replace the caps on the secondary side of the HV transformer before you give up on the transformer. Good luck!

Bob Krassa AC?JL

I have found that if you remove the filter the working time doubles. Since it's in the house I'm tempted to leave it off.

Also, there's a nice hot 6080 right below the transformer, slowly cooking it. I'm tempted to make a small heat shield right below the transformer to separate it from the power chassis below. This should divide the air from the fan a little better and keep the heat off the transformer some.

On a side note, if this doesn't work well, who's still rewinding these transformers these days?

And Brian, there are two transformers on the HV supply chassis, one on the right side is a 120-0288-00, which is the HV transformer. It's enclosed in a metal case. This is T801.
See

And here's the schematic, the smaller transformer on the left side is T841:

Hi Eric , I think I must have been thinking of something like the 556 - HV supply for each gun . I was thinking this would have been very similar but very apparently not . I wish you luck with this masterpiece . I am working on a 661 with a 547 and a 549 in the queue - then a 7104 and a 7834 .
Brian (UK)

If you want to do your own and have patience and hand-eye coordination, I documented a low-tech procedure in topic and photo album “453 HV Transformer Rewind”.

FWIW,
Dave Wise

From: [email protected] <[email protected]> On Behalf Of Eric Boyle via groups.io
Sent: Saturday, January 15, 2022 7:27 AM
To: [email protected]
Subject: Re: [TekScopes] Tektronix 565 loses trace after 40 minutes

I have found that if you remove the filter the working time doubles. Since it's in the house I'm tempted to leave it off.

Also, there's a nice hot 6080 right below the transformer, slowly cooking it. I'm tempted to make a small heat shield right below the transformer to separate it from the power chassis below. This should divide the air from the fan a little better and keep the heat off the transformer some.

On a side note, if this doesn't work well, who's still rewinding these transformers these days?

And Brian, there are two transformers on the HV supply chassis, one on the right side is a 120-0288-00, which is the HV transformer. It's enclosed in a metal case. This is T801.
See <>

And here's the schematic, the smaller transformer on the left side is T841:

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That's good news Bob, I was hoping that it was early enough, but being serial number 1782, I didn't know if it was an early production or a later one. When did they start using the epoxy potting in the HV transformers?

I did already order a new 1X2 as well as a new 6CZ5, before I made the op, just in case those could be the problem. I don't have spares of those tubes, but should have some 6DJ8's around here. I will check the capacitor as well. Hopefully it's something simple. The scope works excellent otherwise. I love the thing, it's an amazing piece of engineering.

Dave, rewinding the transformer would be an absolute last resort. I could do it, but that's not really something that I want to do. I do appreciate your suggestion though, and will check out your procedure.

There is one more capacitor that I would check. There is a second secondary on the HV transformer which is related to the intensity/unblanking circuits but is connected to the HV system. Connected to that secondary is C828, 5uf 25 volts electrolytic. If it were shorted it could drag things down enough to possibly cause this problem. While I am not a fan of shotgunning parts, I do recall a number of other Tek scopes were I have found bad 5uf electrolytics so replacing this 50 plus year old cap might not be a bad idea.

Bob Krassa AC?JL

I went ahead and replaced C828, as well as two more 10uf electrolytics in the time bases that looked like they had been leaking for quite a while. Still no change, but I'm around two hours time before it breaks down and loses the trace. I have new HV caps coming to replace all the discs in the HV section. Pretty much every one of them are cracked around the leads. Usually this isn't much of a concern, but when there's 4kv involved I'm inclined to change them out. As well as the 1X2 and the two other tubes, the 6DJ8 and 6CZ5 when those tubes arrive.

Interestingly, the HV transformer itself is completely cool after two hours, barely warm to the touch. I'm starting to think it's not the problem at all.

Hi Eric, just in case you didn't think of it: a rising screen grid voltage of V800 is a simple indicator for rising losses in the HV circuits, especially for the epoxy problem. In the Tekwiki schematic I saw no clamp diode to prevent excessive voltage there (like in the later 547) so perhaps running the scope too long while the traces are dimming or gone might be bad for V800.
Albert

Thanks for the reply Albert. When the traces start dimming I usually shut it off and record the time. I do have two new 6CZ5's on the way.

But I'm wondering, wouldn't the HV transformer get really hot if it was the problem?

Eric, I would expect the transformer to get at least quite warm. The explanations say eventually up to the Curie-point of the core material, but I remember still touchable outside temperatures of my failing 491. That's my only own experience with failing epoxy.
When the screen grid voltage doesn't rise dramatically it's a sign that no excessive power is lost in the -4 kV HV circuits. (But it's still no guarantee that the -4 kV itself is correct since resistance ratio in the feedback circuit might drift when resistor temperatures rise).
Albert

Yeah, it's not warm at all. It's basically right in front of the fan so it gets most of the air from it. The main problem is the 6080 directly below it which cooks it. After I added my little heat shield and pulled the filter out the run time doubled.

I still think that the old capacitors in the HV section might be part of the blame, as well as the 1X2 tube in there. There's NO way any cooling air can get in and out of that section, so I'm thinking that the 1X2 is cooking the capacitors as well as the resistors in there, compounding the problem.

Which brings me back to my previous question, when did they start using the epoxy potting in the HV transformers? So far the only date I've found is on the fan, which is Feb 14 1966. But of course it could have been replaced at any time in the scopes history.