Next on the project list. Back into my least favorite area to troubleshoot. Determine if the HV transformer that was repaired earlier this year has possibly failed again. But not today. Just got the scope situated so I can work on it. It's still very humid here but tomorrow's weather promises to be sunny and dry. The metal cover assembly is not air tight but the HV transformer and the HV rectifiers are totally enclosed in the plastic case that's almost air tight. So I don't want to potentially introduce additional moisture. So tomorrow I'll remove the metal cover and show how to determine if the HV transformer is suffering from leakage. Stay tuned.
Reply from Specmaster:
Well I sincerely hope the HV transformer hasn't failed again, that must be a fairly rare situation that the same fault reappeared in less than a year, especially when it hasn't had a hard life since then
Reply from bitseeker:
Nice mug, too.
Reply from BU508A:
Do you want one?
Set up to check the HV transformer. Reference schematic. Monitor the voltage at V800, pin 8. Should typically be approx 72V to 85V. If the HV transformer suffers from leakage that voltage will increase to the point of HV shutdown at approx 120V. First pix shows monitor point. The unit under the plastic cover is the HV transformer. Initially the voltage was OK. After about a half hour had increased to 84V. After that point the voltage started to increase rapidly. 40 minutes in and by the time I got it shut down it was 104V. Didn't get a picture. So it's confirmed. This HV transformer has failed a 2nd time. I have some things I'm going to try which I'll describe and show later. Watch this space.
Reply from Specmaster:
Is the one that you had rebuilt a few months ago?
Reply from MED6753:
Yes. But it wasn't "rebuilt" I believe it was just heated perhaps in a vacuum chamber. I was warned that there would be the possibility that it could fail again. Unfortunately that has been proven to be true.
Reply from Specmaster:
So humidity has caused it to fail again? I thought that HV transformers were always potted to prevent humidity bothering them?
Reply from MED6753:
The epoxy used to seal these transformers turned out to be hygroscopic over time.
OK, my first attempt to "bake out" the moisture from of the HV transformer I felt probably had little chance of success and was not disappointed. Big fail. Removed the plastic cover and blasted a hair dryer down towards the transformer for about 20 minutes or so. Then I added 2 desiccant bags. Reinstalled the cover and let the assembly cool for about an hour. Result: no real change from today's earlier test. The rate of voltage increase was about the same. When it reached 91V I shut it down as the rate was increasing rapidly. So what's next? I'm going to pull the entire HV assembly and bake in an oven at approx 140-150 degrees F for several hours. I'm also going to replace the 5642 VT rectifiers in favor of SS replacements. This will lighten the load on the transformer by eliminating the filaments. This work will take place next week.
Continuing work on the HV transformer issue.
HV assembly removed. The 5642 HV rectifiers to be replaced with the 2CL75 Rectifiers so graciously sent to me from Mounty. 2nd pix is underside of the assembly. 3rd pix the SS rectifiers installed. All flux residue cleaned up with 100% IPA. I was able to get the table top oven to stabilize at 150 degrees F. Going to leave the assembly in there for 6 hours then place it in a sealed plastic bag overnight with some desiccant bags. Tomorrow install and check results. Fingers crossed.
Reply from mnementh:
Claws, paws, wings and tail crossed for luck. I sincerely hope you win this one, buddy.
We might have gotten lucky. Look at that grid voltage now. 49V! Was starting out at 72V prior. The Fluke is on the HV. -1866V vs -1850V spec. Close enough. Trace on scope is normal.
IS IT FIXED? Stay tuned. Gonna let it cook.
OK, is it fixed? That's the $64 question. Not sure. This is after over 2 hours. It has never gone beyond 45 minutes before and the grid voltage would increase rapidly. In this case it has increased by just 11 or so volts in that 2 hours. But I can't really say it's "stable". Seems to sit at one point for a while then increase a volt or so. So I'm going to try for 3 hours and I'll post the results. But it is definitely much better than it was prior to the "bake out".
Success! We have achieved stability after 3 hours. Voltage stays within approx 59 - 61V. I suspect the reason for the extended stability is that V800 (6AU5) is getting weak. That tube takes a beating when the grid rises which drives the tube very hard. I don't have any spares but I'm going to order some. They are a dime a dozen and cheap. The HV is rock solid at -1850V. Trace is the usual Type 547 pin point sharp. The big unanswered question is how long will this repair last? In order to help prevent the transformer from absorbing moisture again I installed 2 desiccant bags in the plastic enclosure. I'm also going to seal the entry points of the wiring with silicone sealant. While it won't be totally air tight it should help in the long run.
That ends today's drama. Going to shut it down and again do another burn in tomorrow before installing the HV case and put the scope back into the covers.
Reply from Kosmic:
Good Job
Reply from mnementh:
Awresome!!! Hope the dessicant packs work out in the long run. Maybe you could try wrapping the whole thing in the big PVC shrink wrap they use for building battery packs.
Reply from MED6753:
Unfortunately while that's a good idea it would be too difficult to implement by the way the bottom half of that plastic case is bolted to the chassis. And also there's the issue of gaining access if needed to get in there again.
Reply from mnementh:
I was thinking zip-ties if nothing else.
Since the only real source of heat is now in the XFMR, how hot does it get in there? Can it be sealed up airtight? There has to be some way...
Final cleanup of this repair.
The plastic cover has been installed over the HV assembly and it fits tight. Silicone caulking has been applied where the wires go through the enclosure. There are two desiccant bags by the transformer. I consider the assembly reasonably air tight and no further actions required. Installed the metal cover.
See pictures. I did some modifications so I can monitor V800 grid voltage ongoing and without having to remove the covers. Coax to pin 8 of V800. On the back of the scope is a BNC for “Single sweep reset”. Not any longer. Other end of coax to that BNC and it's now a “V800 Grid Monitor”. And as shown the Siglent connected and monitoring the grid voltage.
Going to let it burn-in again for several hours and monitor. If results are similar to yesterday I consider this repair a success.
My other two Type 547's are going to get similar modifications/fixes.
Reply from BU508A:
Hello med,
I have pondered a little about this problem and I've come to this theory:
The HV transformer and the HV rectifier tubes are in the same plastic cage.
What if, the heating cycles of the VT diodes have caused some microfractures in the epoxy?
The epoxy surely ages over the years and becomes more brittle. Those microfractures
increases the surface of the epoxy and make it probably more attractive for water molecules.
Therefore it will summing up over time water and then this is causing the problems you're facing.
The remedy for this issue (my suggestion):
- cook the HV transformer as you did in an oven, maybe a longer time than you did.
- apply some water-repellent stuff to the HV coil of the transformer. Maybe PTFE would be a good choice.
What do you think? Any thoughts, other suggestions etc.?
BU508A.....your theory certainly sounds plausible. It's clear that for whatever reason the epoxy potting over the years clearly becomes hygroscopic. And clearly the BEST method for dealing with it is to break down the transformer and rewind it with a different potting material. Lacking that the next best thing IMHO is to bake out the moisture and then seal that plastic case as best as possible to prevent moisture ingress. I think I might have achieved that but only time will tell. But now that I have the ability to monitor to grid voltage ongoing and in real time I'll be able to detect an early warning of impending failure.
I'm not so sure I'd want to seal the transformer and that old epoxy with another material for fear that you might trap moisture permanently and is the material compatible with high voltage.
I have another Type 547 that has never exhibited this HV issue. And it has the original 5642 VT rectifiers. In the near future I am going to check it's grid voltage as currently configured and then convert it SS rectifiers and check the difference. I'll also perform the same case sealing methods as well as install desiccant bags.
And here is the repaired Type 547 after several power cycles and many burn-in hours and the grid voltage on average has dropped even more to around 52V - 55V. Will it stay that way? Only time will tell but I'm encouraged.
Reply from bd139:
If this stays working I'd keep it. But the potting isn't going to get better. At best if you get all the cracks and moisture out of it, it's a ticking clock again.
Thus at this point I'd rewind one from scratch. The transformer plans are available so it wouldn't be terribly difficult. It's a ferrite E-core with a few well defined windings. The main issue would be preventing flashover between windings so you'd have to tape it properly. Once it's done bake it then dunk the bobbin in transformer varnish.
If I ever get a 547 I will attempt to rewind one and test it.
Reply from tautech:
Bit more complex that it initially appears as there are several factors involved in building a HV transformer properly.
First the wire enamel needs be of the correct type to withstand inter layer voltage differentials and correct for the heat range, where for a power transformer or motor winding a higher temp range enamel is used.
Varnish is a similar matter in that it must infiltrate the windings, provide mechanical bonding and have an appropriate temp rating to suit the application.
Much harder is getting sufficient varnish penetration to properly pot it where heat and vacuum are beneficial to doing a permanent rewind with modern materials of the like Tek didn’t have decades ago.
Reply from Zenith:
On Thu, Sep 22, 2022 at 03:57 PM, bd139 wrote:
Once it's done bake it then dunk the bobbin in transformer varnish.
I have an idea a vacuum would be useful to remove the air bubbles from the varnish and ensure proper infiltration. It wouldn't need a pump capable of exhausting a CRT, but it would need to get down to 2 or 3 cm of mercury. There are probably designs for home made pumps or ebay could provide something, but it's another stage and another complication.
Then there's the question of baking it at the right temperature. Could this be done in a domestic electric oven?
Having a suitable oven and vacuum pump wouldn't be the slightest problem, were this being done regularly, but for a one-off something of a PITA.
Reply from mnementh:
My thought... for a "repair" solution... would be to do both as well... in a vacuum... then drop the whole mess in a hot pot of corona dope. Maybe devise a mechanism where one can do that inside the vacuum chamber without releasing the vacuum until after the dip...? I wonder how long the stuff will stay liquid under a vacuum and heat... or maybe a two-stage process; first long-duration vacuum/heat, then dip vacuum/heat with corona dope...?
Reply from bd139;
Actually reading further, a new transformer probably doesn't need the moisture removing or vacuum impregnation with modern synthetic tapes. They certainly don't do that for Chinese HV transformers. The problem was the original ones were usually paper or cellulose based. Another consideration is that the inter-winding capacitance is likely higher if the thing is packed with any dielectric substance.
Reply from BU508A:
On Fri, Sep 23, 2022 at 03:22 PM, bd139 wrote:
Actually reading further, a new transformer probably doesn't need the moisture removing or vacuum impregnation with modern synthetic tapes. They certainly don't do that for Chinese HV transformers. The problem was the original ones were usually paper or cellulose based. Another consideration is that the inter-winding capacitance is likely higher if the thing is packed with any dielectric substance.
Maybe using PTFE tape as an insulation between the layers may help.
Air has an epsilon-r of around 1, PTFE has an epsilon-r of around 2.
Reply from vk6zgo:
Such things must be possible as TV set manufacturers produced & used EHT transformers of higher voltages than those common in Oscillosopes, with relatively low failure rates, considering the quite severe use case for domestic TVs.
Reply from Zenith:
They were also produced in volume until about 15 years ago for large colour CRT monitors. ISTR both they and colour TVs had something like a 25kV EHT. A few companies, such as Topward, still make CRT oscilloscopes, so obviously the EHT transformers can still be made.
I suspect there may be a few tricks of the trade involved in this, such as you do need special measures to dry components out sufficiently, you do need vacuum impregnation, and you do need particular grades of material. Nothing very demanding if you are doing it regularly, but a tall order for a one-off.
Arrived today. Spare 6AU5's. Used of course but supposedly tested "strong" or whatever that means. Both for $10 USD including shipping.
Next week I'll be pulling out the other Type 547 that does NOT suffer from the HV transformer issue and converting it over to SS rectifiers and sealing up the HV enclosure to make sure it never suffers from moisture and leakage issues.
Reply from Zenith:
On Fri, Sep 30, 2022 at 07:48 PM, MED6753 wrote:
Arrived today. Spare 6AU5's. Used of course but supposedly tested "strong" or whatever that means. Both for $10 USD including shipping.
Usually it means they have been bunged onto a Hickok or AVO valve tester and the seller couldn't be bothered to, or more likely didn't know how to, determine the characteristics of the valve properly, so on a crude use of the instrument they are OK. They may even have used a Mullard or drug store valve tester, which were notoriously pessimistic. They are almost certainly are OK and at a decent price.
I have an AVO valve tester and it's produced a few surprises. Grotty looking octals bought for 25p, with the envelope taped to the base, test better then new. No problems in use. Quite nice looking valves have thrown the trip consistently.
Reply from bd139:
On Fri, Sep 30, 2022 at 10:15 PM, Zenith wrote:
I have an AVO valve tester and it's produced a few surprises. Grotty looking octals bought for 25p, with the envelope taped to the base, test better then new. No problems in use. Quite nice looking valves have thrown the trip consistently.
Always wish I had a valve tester. The old kit i used to play with when I was a kid was tested by looking for white getters, checking the heater with an ohmmeter and furiously swapping any old thing out and hoping it worked. Had a big crate of valves filched out of dumped TVs and radios. Went in a skip one day when I got banned from high voltages by my parents
Reply from Zenith:
My parents, especially my mother, never liked my interest in valves and other dangerous old rubbish. After my father died my mother stayed here and strongly objected to sharing a bedroom with an AR88. "It would be a nice little house if you just got rid of that nasty old wireless". It was the only major item of junk I had at the time, but it was the start of a slippery slope, so she may have been onto something.
The AVO MkIV came in peculiar circumstances about 20 years ago. I'd chased up a classified for a boatanchor I'd been looking for for a couple of years and didn't deal with the seller, but necessarily his agent. After some argybargy about the price of the boatanchor, he thought was worth a ton but I pointed out was advertised for £40, I looked at the numerous other things in the room. He said were all up for sale. He knew what they were worth and so there was no point arguing. I asked about the valve tester and he said £125. They were going for £250 on fleabay no probs at the time. There was indeed no point in arguing with such a knowledgeable seller.
I'm glad I've got it, and I'd hate to part with it, but I don't use it much.
More HV work. This time on the Type 547 that does NOT suffer from HV shutdown. The intention is to just convert from the 5642 rectifiers to the SS 2CL75 rectifiers. Decided to monitor the grid voltage on V800 anyway to see what's up. I got some very interesting results which surprised me.
Upon initial power up the grid voltage was +87V. After a few minutes it decreased to +82V. So far, so good. But as time went on the grid voltage started to slowly increase. Within a half hour it was +105.5V. This is not so good. I was about to shut it down when all of sudden the voltage started to DECREASE. Within approx 10 minutes it was down and appeared to stabilize at around +88V to +90V. But then it appeared that the grid voltage was rising again so I decided to shut it down. My conclusion to all this shenanigans is that the HV transformer does suffer from leakage issues but as it warms up it's characteristics change and the HV Oscillator settles down. But eventually it's going to fail completely so I pulled the HV assembly and it's going to get a 6 hour bake.
Here's the HV assembly on the bench after I pulled the 5642's and installed the 2CL75's. Later today it will go in for a 150 degree F bake for 6 hours then a sealed bag with desiccant bags overnight.
Reply from Zenith:
You're probably right that it's the HV transformer which is acting up, but just a thought - I have sometimes seen old resistors which have done strange things as they warmed up, such as the resistance going down then going up as they warmed. HV resistors in old scopes are always suspect as years ago they weren't great at making them. Sometimes I've seen them replaced with resistors that definitely were operating above their rated voltage.
Any chance that resistors are causing or contributing to this?
Reply from MED6753:
I certainly wouldn't rule out resistors contributing to this. The proof will be after I bake out the transformer.
Reply from BU508A:
@med6753
I'd suggest to put a little note besides those diodes that they are 2CL75. Would make things easier if somebody after you must troubleshoot that scope.
Besides that: good luck and success with with your HV transformer journey.
Reply from MED6753:
It's all documented in the repair records that I keep for all my TE. If I ever sell a unit the repair record goes with it.
The HV assembly is currently into 2 hours of it's 6 hour bake.
HV assembly installed with 2 silica desiccant bags. Initial grid voltage good. Going to let it cook. More later.
Reply from bd139:
Always looks sad and empty in there once the tubes are gone. I am tempted by an early 453 on eBay at the moment so I can suffer the same ?
Approximately after one hour the grid voltage has increased from +57V to +64V. I wouldn't call it stable just yet but at least the grid voltage is not going wild. Will continue to monitor.
The polarity LED on the Fluke 8000A has gone tits up. WTF. Turns out I have a scrap 8600A that uses the same part so should be an easy fix.
Success. More than 2 hours into this run and the grid voltage peaked at +65V then backed off and stabilized at +62V to +63V. HV is in spec. Trace is typical Type 547 pin point sharp. Going to continue the run at least another hour. Then seal the cover where the wires enter with silicone and installed the coax for the grid monitor as was done on the other Type 547.
The boat anchors have taken over the lab. In the rear is the Type 547 previously repaired with the grid voltage monitored by the HP 3456A. In the foreground is the Type 547 repaired yesterday. Grid voltage monitored with a Fluke 8600A. It's on a scope cart normally occupied by the Type 535A which spends most of it's time sleeping in the TEA closet. This Type 547 will remain on the scope cart and get numerous burn-in cycles until I'm convinced the HV Oscillator doesn't go bananas.
I have one more Type 547 which several months ago I had to trim and repaint the covers because the chassis is tweaked. It's HV assembly is currently in Germany for repair and I don't expect it back until the end of the year. That assembly already has the 5642 rectifiers removed with SS rectifiers installed. Once it get it back I am going to subject it to the same bake cycle before installing. And once that's done then proceed with the balance of the restoration. (Re-cap the PSU, etc).
Some final notes before closing out this project.
Both Type 547's have had many hours of burn-in so far and the results have been outstanding. One scope stabilizes at approx +51V to +53V grid at V800 and the other at approx +60V to +63V grid at V800. I assume the difference is due to the unique characteristics of the HV transformer. But in each case no matter the grid voltage the HV as measured at the CRT cathode remains a stable -1850V.
TekWiki mentions the conversion of the 5642 vacuum tube rectifiers over to SS rectifiers but states that due to the lack of sufficient load from the tube filaments the HV Oscillator may not regulate properly. The recommendation is to change R803 from 56K/2W to 82K/2W. (See schematic) I did not perform this change and so far have not seen an issue. However, when I perform this work on the 3rd Type 547 I will apply this change and note the results. If positive I'll go back and apply it to all of them.
TekWiki also talks about the moisture issue with the HV transformer but other than performing a break down and rewind really provides no other solutions. I'm sure others have tried baking the moisture out of the transformer but I took it a step further. Prevention of the moisture getting back in. Seems logical to me. If the moisture can't get back in the transformer shouldn't fail. So I sealed up the HV enclosure as best as possible and inserted 2 desiccant bags on each side of the transformer.
Finally, I make no claim that this a permanent repair. Only time will tell. But if it goes say 6 months with no degradation in performance then as a preventative measure I'll open up the HV enclosure and change the desiccant bags. So this will be an ongoing process and I'll update as required
