Tektronix 465M repair

[andy3055]

Trace and 95 Volt supply (as steady as it can be!) and 13 kV DC on my old DC EHT probe. I am glad I had this probe which I had forgotten about all this time. It has 2 ranges 30 and 45 kV DC.

Now the issue I have is that how to package this setup considering that only this section of the U 550 HV block is bad and I am still using the old block that has the rest of the HV supply to the cathode, focus etc. I could pack these components separate and mount it separate from that U 550 block but that means I will have 2 "boxes" in there. The other thing to do is to recreate the rest of this also and package all together as in the original U 550.

My thought is to try using SMD type caps since they have 6kV caps that are very small compared to the disc ceramics. I already have the diodes that are quite small. If any members can throw some ideas, it will be much appreciated.

Thanks in advance.

[tautech]

I think the 2 box solution is acceptable.
jdragoset showed how he did it earlier in this thread and Gapsus used 2 boxes too in the thread I linked earlier:
https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083

By using a plastic jiffy box and potting it you have options on where to place it ....within reason.

[andy3055]

I think the 2 box solution is acceptable.
jdragoset showed how he did it earlier in this thread and Gapsus used 2 boxes too in the thread I linked earlier:
https://www.eevblog.com/forum/repair/tektronix-465/msg672083/#msg672083

By using a plastic jiffy box and potting it you have options on where to place it ....within reason.

Thanks tautech, that is true. Sometimes I think I should stay away from trying to do everything as "original" and make it easy on myself. But the challenge is hard to resist 

Certainly, physical sizes of parts have shrunk so much today. So, how did Tek make such a compact unit with all those components some 30+ or more years back, when there were no parts like today? Apart from the EHT tripler section, it contains a bunch of caps, diodes and resistors for the rest of the HV circuitry. I am amazed!

Another note: I did not have to use too many diodes to get to 13 kV. Only 4 diodes and 4 caps even though I hooked up 6 of each to begin with that gives about 18 kV. The schematic does not indicate what the EHT is even though it clearly says it is a x3 multiplier. I decided to stay at just 13 kV based on a 465 schematic where it indicates the anode voltage as 14-16 kV. Considering the age of this 465M, 13 kV should be a safe bet I think where I still have good intensity and focus.

Sadly, the replacement U 550 unit I got also has the same identical problem as the unit that was in the scope, which was hard to believe. It cost me more than US$ 100.00 by the time I had it in my hands. That was all down the drain to say the least.

[james_s]

It's vacuum potted in epoxy so the parts are all crammed in there in a way that would flash over instantly if operated in air. The potted HeNe laser power supplies are constructed similarly, notice the HV capacitors are practically touching with the diodes zig-zagging between them. https://www.repairfaq.org/sam/lgn60xry.jpg

[andy3055]

Thanks, James. So, as I understand, by potting the parts provides the option to bring them close together as the potting compound acts as insulation strong enough to prevent arching. That makes sense as some of the potting compounds have dielectric strengths as high as several kV per mm!

I was thinking of using some SMD caps that are rated at 4-6kV. Do you think that is a viable option? I know I still have to pot the whole thing anyway. I am also looking for some one part potting compound so that I can avoid air bubbles that happens when mixing 2 part compounds. There are also those that come in pouches where one can simply break the seal between the compounds and mix so that air bubbles will not form. It is nice if I can find something that does not harden like rock so that it leaves the option to dissect it later if necessary. Any recommendations on such stuff?

Thanks for any input from any member.

[james_s]

I don't know, I've never tried the surface mount parts. Seems like it ought to work fine, I'm not sure if it would offer advantages over through through hole.

Vacuum potting need not be anything fancy, you don't need to pull a hard vacuum, you just need enough to draw the air out. One of those hand pumps meant for bleeding brake systems and testing automotive vacuum actuators and valves should do the job. You do have to be careful pulling even a modest vacuum on containers as the force of the atmospheric pressure gets to be immense however I've used glass jars without imploding one yet, I place them inside a plastic bucket for safety.

[andy3055]

Thanks, James. Good points.

[2N3055]

How about Corona Dope?

[jdragoset]

Andy,
You could try hot glue and test the results.
It is easy to melt at 275-300 degrees and would be easy to remove with a heat gun.
I would not expect operating temperature to be high enough to melt in service.
I'm not sure of the dielectric strength, but would expect it to be quite high.
I would also think a carefully laid out 3-D architecture could eliminate high adjacent voltage differences.

[james_s]

Hot glue is not great for HV stuff, I've tried it in the past.

I would just use epoxy. Even if you don't want to use a vacuum chamber you can get pretty good results by using a thin epoxy like the stuff meant for laminating fiberglass. I believe many epoxy resins will also thin if you heat them, although they will also cure faster.

[andy3055]

Thanks for all the suggestions. I am still looking around but not decided yet on any one thing. Meanwhile, I am away from home baby sitting my 4 year old grand daughter which is paramount now after retirement! So the scope has to wait. The good thing is, it gives me more time to think

Will post back once I have a concrete plan.

Thanks and have a great Thanksgiving!

[tautech]

You don't want to consider anything else but a good potting compound, one with low viscosity, low shrinkage and good dielectric strength. Consult the data sheets for working time after mixing and temperature sensitivity before it cures as you may be able to warm it so to lower viscosity to help prevent air voids.
It ain't rocket science if you take care with product selection.

[andy3055]

Thanks, tautech. I do want to get something that gives the most time for sure.

[andy3055]

Update:

I have tested with 20kV diodes and 6kV caps and it seems to be working well with just 4 diodes and 4 caps at 13kV on the anode. One question remains still about the resistor at the output. I have tried various values like 820R, 330k, 470k, etc. with no difference. I have not tried 1M yet as I don't seem to have a good quality piece on hand. Is there any way to find out the best value since there seems no info available on this particular tripler from anyone who has reverse-engineered one yet. Since the rest of the HV stuff in the old unit is still in circuit and in use, I do not want to destroy it trying to drill/cut/heat to find what is there on the EHT output lead.

Any response will be much appreciated.

Thanks.

[james_s]

The resistor may just be there to protect the diodes in the event of a short circuit. If the value makes no difference I wouldn't worry about it too much. If your new multiplier does ever fail or you find other shortcomings, you know how to make a new one.

[andy3055]

Thanks, James. Would you use a lower value such as 820R or go with a higher value like 1M or so?

[tautech]

Would you use a lower value such as 820R or go with a higher value like 1M or so?

Do the sums with the voltages involved to limit the current to less than I_max of the diodes.

[james_s]

Also use several lower value resistors in series instead of one high value resistor so as to limit the voltage across each one.

[andy3055]

Good advise. Thanks to you both.

[m3vuv]

could you not use hv diodes from a microwave oven?

[m3vuv]

for potting compound ive used stuff called copon hycoat 60,used to use it doing repairs on hp gas pipelines for the cathodic protection,was 2 pack about as runny as thick cream,as regards insulation it was good to 4kv per mm

[andy3055]

could you not use hv diodes from a microwave oven?

Those are too big in physical size. There are very much smaller (about 3 mm diameter x  8 mm L) 20 kV diodes that are pretty cheap on eBay.

Something like these: https://www.ebay.com/itm/8-pcs-2CLG-20KV-20mA-High-Voltage-Diode-HV-Rectifier-Tesla-Ham/273101370267?hash=item3f961c339b:g:ApMAAOxyE-dRzJsp

[andy3055]

for potting compound ive used stuff called copon hycoat 60,used to use it doing repairs on hp gas pipelines for the cathodic protection,was 2 pack about as runny as thick cream,as regards insulation it was good to 4kv per mm

Thanks for the info on the potting compound.

[tautech]

As mentioned before, you don't want to venture outside what is the appropriate potting compound for this job.
Not too hard or the chance of shrinkage could ruin the circuit albeit unlikely with passives daisy chained or too high viscosity so to minimize the chance of voids and with good dielectric strength.

Pay the bucks for a good product and sleep at night.
A quick hunt in Mouser for Encapsulating and potting compounds finds this one that should be fine for your needs:
https://www.mouser.com/ProductDetail/MG-Chemicals/832B-375ML?qs=sGAEpiMZZMvJqaFk9BIiv6jhjM0Pk6Jz0ZBekZO8Zms%3D

[andy3055]

Thanks, tautech.