oscillating triode vacuum tube circuit with fault - Page 1
Forum section: electronics / repair

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 04, 2018, 04:51:10 am
content: OK, so I have a thing I want to make work as soon as possible. Any hints on how to get this thing working or what kinds of things to look for are very appreciated.It's for a piece of equipment being used by another parent at my kids primary school, who was diagnosed with stage 4 cancer recently. They are using it alongside whatever their doctors are doing (which isn't much...) and it is a good thing for them.Now... from what I understand of what this thing is, the tube circuit should be oscillating around 2-4MHz.It's tuned with a really sweet looking moving vane capacitor in parallel with a huge multi tapped inductor, formed on maybe a 4" PVC pipe. (and feedback to the grid from the bottom of the tapped inductor, past the chassis tap)on top of that, it receives an external modulation signal which modulates the main oscillator (not sure if AM or FM, or something else. even if this input just takes over and drives the frequency of oscillation, turning it into an RF amp...) this signal goes into what looks like a simple NPN transistor amp and from there is coupled into the tube oscillator circuit. I didn't note where it goes last night.the oscillator's output is fed to a gas discharge lamp of some sort which glows when everything is working..Apart from that, it has some kind of moving coil meter wired to the output to show signal strength, and a digital frequency counter board also attached there to show frequency, and a couple of DC fans to move air around for cooling purposes.The core tube oscillator is a replica of a circuit made/sold/used around the 1930s to 1950s.The tube in question is exactly one of these. same logo and all:https://www.tubesandmore.com/products/812a-triode-power-amplifier-chinaSymptom is right now, there's an F0.5A 250V(??) fuse on the +HVDC line that is immediately blowing when power is applied. but there was another issue it had before when this didn't happen.Tube circuits really aren't my thing, and while I have visualisation in my head of how this triode tube circuit should work with the tapped coil, I'm not normally dealing with the kinds of voltages these things run at, so not so up with the kinds of faults that parts in these circuits can develop, or tubes can develop. (and I don't have a meter or meter probes to read over 1100V, which the plate for this seems to be at)some history of the unit:It was bought new recently (dated May 2018?) But I was first asked to have a look at it a few weeks ago when it "wasn't working" (had been working fine, then one day just didn't) The owner had spoken to the overseas manufacturer and been set a bunch of parts to fix it. they had tried swapping out the transistor in the transistor amp, and I think also swapping out the tube, with no luck... All I did last time was have a poke around inside the chassis and buzz out the diodes on the transistor to check the swapped in part was OK (the pulled part seemed OK, too) and then when I tried powering it up to see what was going on inside, it just worked.The owner used it multiple times a day from then on. Then went on holiday without it. when they came back the other day it worked, but when they tried using it later that day it didn't work.I came over to their place last night about 11pm to have a look at what was up, and noticed that now a fuse is blowing in +HVDC feed to the tube plate. I've drawn the tube part of the circuit as I saw it last night (it was late and I'd been working my own stuff all day and through the evening before going to look ...) I'm thinking now that it would have been better if I'd at least noted where the input transistor amp was connected, but eh... I can go visit again and draw it if someone thinks that's a missing bit of data they need to help. I can always disassemble the input amp board and trace that out too, if needed.notes:1) this whole thing is a bit "secret squirrel" - standard parts like Rs and Cs have identifying marks covered or removed, including the resistors and caps on the HVDC power supply board!2) parts of it run hot - the TO220 transistor has a big heatsink and a small fan bolted to it.. some of the resistors around the board have had their secret squirrel ID marking discoloured by heat.3) the components straight off the +HVDC fuse are weird!! They look like large-ish resistors, but they have these 4 bunches of windings of what seems to be a single piece of very thin enamelled wire meticulously wound and glued into place along the body of the part. never seen anything like that before! I think they are resistors, but with a wire wound as 4 separate series inductors built up around them? curious to know why you'd wind something like this rather than just put separate inductors in parallel with a resistor if that's what you wanted...sorry for the ramble - I'm trying to upload as much info as possible. hopefully there's some useful bits of info in here to someone who understands these kinds of things better than me and can suggest what approach would be best to make this work again soon.

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2018, 06:07:42 am
content: Back in the day when tubes were king, it was extremely common to wind chokes on a high-ohm resistor body as a former. The sectional winding and fine wire are typical of a high Q RF choke.If its blowing the HT fuse, there's probably a DC short to ground on the HT line. Most likely its the capacitor between the anode and the top end of the coil that's failed shorted (or breaks down when HT is applied) as an anode-cathode short in a top-cap valve that hasn't gone gassy*is highly unlikely. A replacement capacitor *MUST* have a voltage rating of at least double the max HT voltage with plenty of safety margin A 2.5KV rated cap is probably a bit on the low side with a 1.1KV HT rail. %DEITY% only knows what the value is. Other (remote) possibilities are shorts to chassis due to tracking inside the insulated standoffs supporting the chokes in the HT feed.Unfortunately the use of such RF oscillators is medically rather dubious - if the output is strong enough to be effective for diathermy, it should only be used under the direct supervision of a qualified radiologist, and if its weak enough for injury due to incorrect use to be unlikely, its probably not going to have a therapeutic effect. 'Secret Squirrel' obscuration of component values + poor reliability is *NOT* a good indication - most likely its primary purpose is to separate truly desperate people from a significant chunk of their money. If the manufacturer actually believed in their product, they'd have engineered it to be reliable and wouldn't be obliterating component marking in the PSU. However in this case the money has already been spent, and if it helps the patient maintain a positive attitude via placebo effect its continued use may be justifiable.* Vacuum is still O.K. as indicated by the getter patch on the envelope still being shiny & metallic)

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 04, 2018, 12:17:30 pm
content: thanks Ian!that's all very helpful. thanks so much for taking some time to share your experience.I'll see what I can see with that cap and give it a good solid going over. Though from what you said it seems the only way to really check it is replace it and see if the problem goes away, and to do that I would need to know the value.The getter patch inside the glass envelope is still nice and shiny, and the filament lights up warm and bright, so yeah I think it's probably still good.. is there anything I could do with a standard multimeter that might show anything wrong with the valve?I'm waiting till I have more fuses before I can try anything else, so I'm planning to have a look at power on tomorrow night, darken the room and see if I can see any signs of a spark anywhere when it turns on and blows the fuse. Just an idea - would pulling the tube out before power up be OK here? - I assume not having any power draw will result in higher voltage on +HVDC, which may cause secondary issues, but removing the tube will also mean its not illuminating everything, so sparks will be easier to spot.. (I already checked for signs of arcing around the place. didn't see any)If worst comes to worst and I can't find anything obviously faulty, or work out part values for a known bad part, then it can always go back to the manufacturer under warranty, it's only a few months old! I'm really just trying to avoid an unknown number of weeks in transport around the world and repair time for something that's used every day.As for the science around the treatment - my belief in the treatment is unimportant. This funny circuit and its gas discharge tube lamp helps someone in a really difficult situation keep on going. That seems worth the effort to fix it alone.

author: Ian.M
author_rank: Super Contributor
timestamp: August 04, 2018, 01:08:26 pm
content: With the unit disconnected from the wall, and the H.T rail discharged, remove the fuse and check resistance across the capacitor. If you get anything other than open circuit, desolder one end and check again to confirm its bad. Open circuit doesn't confirm its good as it may still be flashing over internally, but it does give you a chance to check the value with a cap meter.Pulling the tube would be OK if it was properly designed and anyway the tube doesn't pass current until its heater's warmed up, but who knows how close to the edge the PSU is? If you do try that, don't keep it powered up for more than a few seconds. Unfortunately no arcing doesn't mean the cap's good as there will be far less stress on it without any oscillation.Try googling: shunt fed valve OR tube hartley oscillatorand you may find some similar enough circuits to get a feel for the likely component values, as they are quite popular with hams as single tube CW transmitters.N.B. even with a meter or probe you *think* is good for the H.T voltage expected, *DO* *NOT* try to check the anode voltage with the valve connected as with it oscillating the peaks will be at least double the supply voltage and with >2KV Pk-Pk >1MHz RF on top of the >1KV DC level just about any meter or probe is likely to break down and fail.

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 05, 2018, 07:21:13 pm
content: Please be careful with medical devices (even fake ones, especially with HV), as there is a person on the end of it.Besides the decoupling cap, is that tuning capacitor arcing over or something? The tube could also be arcing, it's a 1250V tube which on a 1.1KV oscillator is running a bit high.

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 05, 2018, 11:45:11 pm
content: You know, it's possible plate voltage isn't 1100V.. I was looking at the tube spec which mentioned 1250V operating for the plate, and the manufacturers website mentioned something about 1160V or so driving the gas disharge tube that it makes glow...But like I said before, the whole thing is very secret squirrel.. it has a huge bank of electrolytic caps on the plate supply that have had their printed plastic removed, and even the resistors on the board which I assume are for bleeding the HV away on power off are blacked out. there's no docs on anything that I've received yet (though may get some manufacturer support - waiting for the US to wake up to monday morning) and very little in the way of PCB markings that could be helpful.All I suppose I really know about the voltage present is I should have a 1160V AC component at the output tap of the coil.That leaves the rest of the nodes still unknown unless I can get a simulation of this thing to run, but really I don't care that much about what it's meant to do right now - I just need to fix it.Anyway - yes - the fact that even somewhere in it, there's more volts than my multimeter (or I) can handle has kept me very conservative with my testing... I give it a full minute to run down after power off, and really just inspect the circuit when it's totally powered down.

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: August 06, 2018, 03:28:24 am
content: Sounds like a Rife Machine.https://www.healthline.com/health/rife-machine-cancer#claims

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 06, 2018, 04:51:18 am
content: yep it's one of those.But I'd really prefer to keep the discussion to repairing the circuit and the overall construction of such a thing.

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 06, 2018, 01:54:04 pm
content: Well, good news. A supply of fresh fuses let me try out power on and watch from a few locations for arcing. It became pretty obvious that the arcing was inside the tube.There was a spare tube left over from the first time it stopped running, which had been swapped out. I put the first tube back in, and everything is running as it was meant to.The bad tube doesn't have white powdery getter (though it's looking dark, rather than nice and shiny...)There's still a bit of a mystery around this.. hopefully the bad tube was just no good, and was just an early failure on the bathtub curve... but even then, the original problem this thing had where a working tube and a working transistor got swapped out, then after a poke about it started working again, is still a bit mysterious..Anyway, fingers crossed the new(old) tube is everything it needs, and it keeps behaving itself!

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 08, 2018, 12:09:44 am
content: Quote from: julianhigginson on August 06, 2018, 01:54:04 pmSmells of terrible engineering or possibly another fault. It is overvolting that tube as I thought. Might even be due to poor power regulation.EDIT:https://www.w8ji.com/demonstation.htmAlso:Quote

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2018, 08:16:43 pm
content: This sounds like one of those quack devices from the early days of electricity.If you try to build an amplifier instead it will probably oscillate perfectly.

author: julianhigginson
author_rank: Frequent Contributor
timestamp: August 13, 2018, 11:53:39 am
content: Quote from: Cyberdragon on August 08, 2018, 12:09:44 amquite possibly there's fundamental issues.. my understanding is the tube part of the circuit is a clone of an original design - hence the funny hand wound inductors... (though the caps in use sure aren't original style! that's probably for the best..) if it's just a faithful clone, it *should* be fine. But yeah, who knows?Now it's working again, I don't have any need to poke in it, but if it goes down again with no manufacturer support (manufacturer never got back to the owner about their failure report... my opinion of them is pretty low right now) I'll be doing a 100% schematic capture on it, doing a full teardown and rebuild, with LCR meter analysis of all the parts it has, and will publish that (probably here, I guess) alongside the actual manufacturers name and model identification, with a report on failure mode and symptoms and repair approach that fixed it... just so anyone else in the same predicament can get support.I've been thinking a bit about the HV supply. I think it would be OK To disconnect the tube (just remove the fuse, like when it was blown the other day) and then I'll have the pure DC output of the HV power rail with no crazy super high voltage RF mixed in... then I would just need a probe to measure a few thousand V DC and safely divide it down to something my multimeter won't burn when exposed to... like this:http://au.element14.com/tenma/72-3040/probe-high-voltage-28kv-40kv/dp/4908351Knowing the DC voltage applied should make it a lot easier to guess/estimate the other operating stresses and part voltage requirements.Quotereading up on this link - It's possible it could be run sometime without the load, but the load is connected pretty solidly (banana jacks) and everything is still when it runs. so I don't suspect intermittent connection. And it's not normally moved anywhere.. from what I can tell this thing is always connected... when sitting there unpowered, and when powered up. Maybe it was once or twice with the load disconnected, which may have weakened parts of it that then went on to fail? There's certainly no warning about no-load operation on the panel of the thing.

author: Cyberdragon
author_rank: Super Contributor
timestamp: August 13, 2018, 04:47:35 pm
content: There could be no problem other than bad (almost nonexistant) engineering. I mean just look at that fan, what is it blowing on, the chokes?EDIT: if the fan is sucking, it's probably okAlso, again...QuoteYou have a 1250V tube, so the anode voltage would ideally be around 300V for fault tolerance, 600V for normal operation. Now of course, people (homebrewers, and even companies) don't always follow that rule (hence don't disconnect load warnings), but if you are running it close to the 1100V it says it is, there's bound to be problems even with the load.