Oscilloscope drawing lots of current and blowing fuses - Page 1
Forum section: electronics / repair

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 27, 2019, 09:15:14 am
content: I got an old analog oscilloscope that works perfectly well, all functions, rails, voltages are sharp and fine, but it slowly raises current till blows Fuse 501.The power supply can be feed by an external 220v-20v transformer or directly with 12VDC.When feed with DC I can monitor the current and it starts at 1.5amps and climbs all the way to 2.5amps after 20 to 30 min starts a hissing noise then blows fuse F501.If I reduce the crt voltage from 1.5Kv to 1.35Kv it lasts much longer before blowing the fuse. But if I bring the crt voltage to 1.5Kv it blows a fuse in minutes.I am suspecting the second transformer L501 has some winding short or issue but it also provides all voltages perfectly.Is there a way to precisely diagnose if transformer is actually faulty?

author: xavier60
author_rank: Super Contributor
timestamp: June 27, 2019, 11:38:21 am
content: There could be a problem with the buck regulator gradually losing efficiency. If so, there might be a changing in the waveform to see.It would be useful to rule out most of the output rails by looking for change in voltage drop across the filter chokes.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 27, 2019, 07:41:09 pm
content: Thanks for you reply xavier.I toke some measurements on all secondaries and the table bellow shows what I got.My CRT main drops from 1500v to 1350v as the current consumption raises.My 5v rail is a bit above at 6.5. And my 10v rails are uneven. But I guess the big issues is the voltage drop on the CRT main.What you suggest next?

author: L_Euler
author_rank: Regular Contributor
timestamp: June 27, 2019, 08:20:21 pm
content: Try shooting some of those transistors with freeze spray and see if current drops.

author: xavier60
author_rank: Super Contributor
timestamp: June 27, 2019, 08:51:35 pm
content: The time taken for the current rise does seem to relate to something rising in temperature.There is a lot happening with those voltages. Most of the changes correspond with the change in GND voltage.The presence of the GND voltage isn't right. There could be a poor connection between the 2 grounds or excessive current flow that likely should not exist. This needs to be looked into. A current measurement can be taken between the 2 GND points.The CRT Main voltage regulator uses +10V for its reference, so the change of CRT Main voltage could be just because of a change of the +10V.Also check all bypass capacitors for excessive ripple voltage.Is Q510 getting very hot?What is IC 501?

author: xavier60
author_rank: Super Contributor
timestamp: June 27, 2019, 09:19:56 pm
content: CRT Main voltage could be disabled by removing D516 and D517 to see what effect it has.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 27, 2019, 10:08:55 pm
content: I'd follow xavier60's advice on this it may give you a slight clue.The rare cases I've had with similar symptoms were related to the HV winding of the transformer, and with not much measurable effect other than excessive current draw.One definite thing to watch is the transformer's temperature, if the transformer itself is the cause (weak insulation) the extra power consumed is consumed in the transformer itself.I'd bet by the time your fuse blows (D512 and D517 removed or not...) your transformer is too hot to hold.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 27, 2019, 11:47:41 pm
content: Had to build a bunch of jumper cables to be able to flip the power supply and have access to the transformer.And yes I will follow xavier's advice and:- check Q510 temperature- check transformer temperature- check everything else temperature wise around that blockA couple questions:- best way to check what is leaking to ground?In regards to disabling D516/D517 that block goes to wire 2 and 5 that goes to focus. CRT main 1.5k voltage comes from P502.

author: xavier60
author_rank: Super Contributor
timestamp: June 28, 2019, 12:05:25 am
content: Quote from: gkmaia on June 27, 2019, 11:47:41 pmThat could just be a poor connection. Check the resistance between the 2 grounds first with power off.

author: jdragoset
author_rank: Frequent Contributor
timestamp: June 28, 2019, 12:56:37 am
content: If you can disconnect both leads from the L 501 winding feeding the D 516/D 517 HV doubler with all else connected and the current does not increase with time, that would point to a .01 capacitor in the doubler cicuit breaking down, or other HV wiring/terminal leakage, look for carbon burn-through/tracks along all HV wire/terminal insulation.If it were the diodes, I would not expect a gradual change.

author: bob91343
author_rank: Super Contributor
timestamp: June 28, 2019, 02:51:42 am
content: My initial diagnosis is bad electrolytic capacitors in the power supply. You can qualitatively evaluate that by feeling the capacitors to see if they are warming. A power supply capacitor should not generate palpable heat.If they leak, they warm up and that increases leakage. Thermal runaway.

author: bsfeechannel
author_rank: Super Contributor
timestamp: June 28, 2019, 03:36:59 am
content: What is the model of this oscilloscope?

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 28, 2019, 04:20:48 am
content: Ok, some updates.Did measure temperatures with infrared thermometer. The transformer L501 gets hot with time but it is tricky to capture the temperature. It only gives a high temperature if I aim the infrared in the winding gaps. Anywhere else temperature is normal 35-40ºC.The voltage regulator IC 501 gets up to 60ºC and he is the one supplying current to L501 together with Q510. But Q510 does not heat up.Haven't checked the caps. But removed dozens of caps from this scope and they were all Elan and all still in pretty good condition. So not sure I can blame the caps.

author: xavier60
author_rank: Super Contributor
timestamp: June 28, 2019, 05:42:17 am
content: The transformer temperature doesn't look good. Next would be to run it with all loads disconnected if not already. Measure the current draw also.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 28, 2019, 08:09:27 am
content: ok, just to confirm I will disconnect all wires marked in the attached image.It will draw way less current. But I imagine what you want to see is if after a few minutes the current draw start to raise?! Right?

author: xavier60
author_rank: Super Contributor
timestamp: June 28, 2019, 08:15:57 am
content: Quote from: gkmaia on June 28, 2019, 08:09:27 amYes, and the transformer temperature should rise if it has shorted turns. Also check the continuity of the HV winding.What is the current draw?

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 28, 2019, 09:43:30 am
content: I had a bit of a go. No load at all. Temperature of all components including transformer at 35-40ºC to 20min and current at 0.240A all time.First 15 min current was between 0.238 and 0.240.Last 5 min current was fast moving between a 0.225 and 0.255. But temperature was stable.I could try for a bit longer tomorrow. As it is late already.

author: xavier60
author_rank: Super Contributor
timestamp: June 28, 2019, 09:58:23 am
content: As well as checking the resistance of the HV winding, check for leakage to GND, preferably with an insulation tester.Also check for leakage to the winding that connects to P502.

author: duak
author_rank: Super Contributor
timestamp: June 28, 2019, 04:48:34 pm
content: Are the temperatures of the main switching transistors Q511 & Q512 about the same as each other? If one has become leaky at high temperatures or its beta falls off, it may unbalance the transformer primary currents and cause heating. I wonder if these transistors are germanium rather than silicon? If so, they are far more prone to aging. The diodes D510 & D511 could also be leaky or high resistance. I would compare the voltages and waveforms on the collectors and bases of those two transistors.

author: bsfeechannel
author_rank: Super Contributor
timestamp: June 28, 2019, 09:11:03 pm
content: Quote from: xavier60 on June 28, 2019, 08:15:57 amA single shorted turn in a transformer always leads to a catastrophic failure. All other windings will be seen by their respective circuits as short circuits.If the power supply is capable of producing the nominal voltages, there are no shorted turns.The overheating of the transformer may indicate some overload.Since the OP didn't deign to declare the model of the scope, we cannot determine its age. Nor have a more comprehensive view of the schematic. But since he said its old and analog, I'll suppose its more than 30 years old. In that case, the usual suspects are capacitors. All of them. Especially the high voltage ones. Not only the electrolytic. This followed by transistors and diodes. Resistors should be checked for drifted values.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 28, 2019, 09:42:12 pm
content: Quote from: bsfeechannel on June 28, 2019, 09:11:03 pmIt is a Trio CS1352. I was trying to find a way to upload the manual I bought as it is not available for free. Just having the model wouldn't be of any help as you would not be able to find an SM to download.SM is here:https://elektrotanya.com/trio_cs1352_sm_en.pdf/download.htmlAll bypass capacitors in the power supply board have been replaced except by a couple I do not have the values here. Otherwise all new. See image.Quote from: duak on June 28, 2019, 04:48:34 pmHave a look at the image attached you will see they are the same. They are silicon. I will try to put them on my curve tracer.Quote from: xavier60 on June 28, 2019, 09:58:23 amI think we can exclude the ground issue. My scope was not properly set to lower voltages. With it set the ground is in the order of 1-5 mV.I will check the resistance of the HV winding.

author: xavier60
author_rank: Super Contributor
timestamp: June 28, 2019, 09:53:44 pm
content: I should be calling it "EHT" winding.If there is a problem with the transformer, it will likely involve the EHT winding. It could be partly open or arcing over to an adjacent winding.This winding was troublesome in oscilloscopes that had the EHT winding on the mains transformer. The DC imposed on it by the doubler would cause accelerated corrosion.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 05:44:54 am
content: There you go.Got the transistors on a curve tracer see images attached. They seem fine.Also tested the resistance of the EHT winding and turned out to be 0.136 Ohms. I believe there is something wrong with it (see image).To get 0.136 ohms on a wire with a cross section of 0.15mm (~35G) would need just 15 cm of wire. That is probably 5 turns. But lets say the wire is 0.25mm thick that would need 40cm in length to build such resistance.

author: xavier60
author_rank: Super Contributor
timestamp: June 29, 2019, 06:13:19 am
content: Quote from: gkmaia on June 29, 2019, 05:44:54 amWith a resistance that low, it would have been producing very little voltage. Are you certain that the correct winding is being checked?Be certain to check for possible leakage to other windings.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 07:52:00 am
content: Pretty sure that is the one. See the attached schematics. I could try measuring it with another multimeter, but the Fluke is precise. And I do agree with you, that length of wire would not be able to generate that voltage. Quite confusing...

author: Chris56000
author_rank: Frequent Contributor
timestamp: June 29, 2019, 08:58:11 am
content: Hi!Thanks for providing the link to that Service Manual – it's a good clear one and I'll be studying it and providing some repair hints later!If you've got doubts about the power supply transformer, Ed Dinning from the UK Vintage Radio Restoration Forum is the person to contact, but before we go down that road there's other things to try first!Chris Williams

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 09:01:56 am
content: Toke the transformer out and checked all leads. No shorts. And resistance values are in the image attached.

author: xavier60
author_rank: Super Contributor
timestamp: June 29, 2019, 09:35:13 am
content: That's the heater winding, EHT is the bottom one.The heater winding will have EHT present on it, so there is the possibility that it could arc to an adjacent winding.An insulation tester is needed to detect this sort of problem.I really hope that the transformer isn't faulty.

author: xavier60
author_rank: Super Contributor
timestamp: June 29, 2019, 09:48:18 am
content: A test run with the heater unplugged might indicate something. Beware that the heater lead is live with EHT voltage.One way of detecting increasing current draw on the other rails is to take measurements of the voltage drop across the filter chokes.

author: xavier60
author_rank: Super Contributor
timestamp: June 29, 2019, 11:05:38 am
content: Quote from: gkmaia on June 27, 2019, 09:15:14 amThe manual calls the 1.5KV "Hi Voltage", so this is what Ill be calling it also.Has the CRO actually been test run with just the Hi Voltage disabled yet?That's with the number 2 connection disconnected.Adjusting the Hi Voltage doesn't actually change the voltage across C521.The voltage at the number 2 connection is C521's voltage less the voltage drop across Q513, set by the regulation loop.

author: jdragoset
author_rank: Frequent Contributor
timestamp: June 29, 2019, 11:52:22 am
content: I believe the EHT winding is the one just below the one you highlight in red.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 04:23:56 pm
content: Quote from: xavier60 on June 29, 2019, 09:35:13 amThe transformer is of the PCB so I havent run tests with Heater or Hi voltage disconnected. But would be a good idea to remove these and also the rails to see which one is triggering the current draw.If that is what you mean.If the transformer is faulty that is not much of a big deal I have what is needed to rewind it.The problem is I do not have an insulation tester, would have to buy one. Is there another way to test what you suggest before I put it back on the PCB to run the other tests?

author: duak
author_rank: Super Contributor
timestamp: June 29, 2019, 05:04:15 pm
content: Gkmaia,- do you still have Q511 & Q512 out? If so, could you hit them with some heat whilst on the curve tracer?- you could try testing for interwinding leakage with a HV DC supply - the closer to 1 or 2 KV the better. Even the 320 VDC from rectified mains will give you an idea of the leakage. Put a few 100K - 500K resistors in series with each lead to limit current and measure the currents when connected to the various windings and to the core. I'd expect microamps. Also try heating the transformer up while making the tests.- I wonder if there's an EHT leak somewhere. Did you smell any ozone while the funny noise occured - are the 0.01 uF caps in the EHT circuit clean without any surface damge? ie., cracks or marks?

author: bsfeechannel
author_rank: Super Contributor
timestamp: June 29, 2019, 09:49:48 pm
content: WhileChris56000studies the schematic, I'd like to give my two cents about the secondary HV circuit. The picture below shows a simplified diagram.Without losing sight of a possible arc-over between windings, as suggested byxavier60, I'd check every component that is subject to high voltage and that, if leaky, could represent a return path or a path to ground for the HV winding. Namely:D516D517C519C520C521Q513C429C424C441C425D108D407D404C422C423in that order.Below we have a redrawn and even more simplified diagram of the HV circuitry. It's a simple voltage doubler followed by a voltage regulator with a feedback loop and reference, and then a negative voltage generator on top (or better saying below) the -1500V applied to the control grid so as to provide intensity control and blanking during retrace.

author: xavier60
author_rank: Super Contributor
timestamp: June 29, 2019, 10:29:31 pm
content: Many things are pointing to leakage current overloading the Hi Voltage, to be confirmed.It's puzzling as to what could gradually break down and then recover afterwards. Capacitors maybe?A problem I have seen in computer monitor back-light inverters is the paper PCB gradually becoming conductive in high voltage areas.But there was always a burning smell.Whatever it is should be getting very warm as we as the overloaded power supply.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 11:46:45 pm
content: Hey bsfeechannel that was nice. Thanks for spending time on it. Also Xavier and everyone else I appreciate. I would not be able to fix it alone.Some feedback on the components you suggested testing.D516 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingD517 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingC519 - 0.01 HV are 2 months oldC520 - 0.01 HV are 2 months oldC521 - 0.01 HV are 2 months oldQ513 - tested on curve tracer. Display in one of my previous posts.C429 - 0.01 HV are 2 months oldC424 - 0.01 HV are 2 months oldC441 - 0.1 not tested yetC425 - 0.001 not testedD108 - not tested yetD407 - tested with multimeter. could test on a curve tracerD404 - tested with multimeter. could test on a curve tracerC422 - 0.01 HV are 2 months oldC423 - 0.01 HV are 2 months old

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 29, 2019, 11:50:36 pm
content: Quote from: xavier60 on June 29, 2019, 10:29:31 pmIs there a way to investigate the Hi Voltage winding further without an insulation tester? Something like what Duak suggested? I could buy one but that would take days to arrive.Also sending some photos of the transformer. Red wires are the Hi voltage and blue are the heater. The insulation between the Hi voltage and everything else seems pretty thick.

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 12:14:20 am
content: You could apply mains voltage that has been passed through back to back transformers.That transformer looks to be really well made. I would refit it. The 2 suspect wingdings can easily be checked later.Also, test running the CRO with the heater plug disconnected will be useful."C429 - 0.01 HV are 2 months old" was the overload fault present then?

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 30, 2019, 12:19:33 am
content: Quote from: xavier60 on June 30, 2019, 12:14:20 amYes, the previous owner sold it because of the overload. I have replaced these caps on a hope to fix the overload issue.I will put the transformer and components I removed back then and we can test it more.

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 12:26:36 am
content: I was mixing up the 2 versions. If a capacitor is responsible, it would have to be one directly between Hi Voltage and GND.C441, C424.

author: bsfeechannel
author_rank: Super Contributor
timestamp: June 30, 2019, 02:16:43 am
content: Quote from: gkmaia on June 29, 2019, 11:46:45 pmNo worries.QuoteD108 should be D408. I didn't recognize my own writing.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 30, 2019, 04:37:00 am
content: I got the transformer back on the PCB but I am missing a few caps. I will have to order them and that will take a few days. So we will have to wait to do more tests with the scope running. But I will still finish testing the list from bsfee.One of the caps I want to replace is C441.I am planning to replace it with one of these. All good?!https://www.digikey.co.nz/product-detail/en/kemet/PHE845VF6100MR30L2/399-12794-ND/5731285

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 05:12:41 am
content: Quote from: gkmaia on June 30, 2019, 04:37:00 amThat original capacitor(C441) is an oil filled type. I can't find much about failure modes. Was it getting warm?That Kemet capacitor is a self healing type and can be trouble. They can loose capacitance in AC applications. I have no idea if the same applies to this application.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 30, 2019, 05:29:48 am
content: Quote from: xavier60 on June 30, 2019, 05:12:41 amI could never diagnose this cap. But since I am changing all of them. I could go ceramic as per BOOM and try to following.https://www.digikey.co.nz/product-detail/en/vishay-cera-mite/565R20GAP10/565R20GAP10-ND/5058750Or could go for this onehttps://www.digikey.co.nz/product-detail/en/illinois-capacitor/104PPA302K/1572-1266-ND/5410501I could get some data on it. As you said oil, also 3000V 0.1Uf, series Cp72

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 06:38:34 am
content: I was about to mention the high price of the film capacitor, then saw that the ceramic wasn't much less.I'm certain that either would be fine, I do like the film capacitor.Some ceramic dielectric types decrease capacitance with increasing applied voltage, Odd that I find no mention of this in the data PDF.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 30, 2019, 07:36:06 am
content: Yes they are quite expensive.As a feedback on components test list.I got D407/8 on the curve tracer. They are fine.C441 tested on capacitance meter and returned 0.12 but don't know ESR.D516 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingD517 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingC519 - 0.01 HV are 2 months oldC520 - 0.01 HV are 2 months oldC521 - 0.01 HV are 2 months oldQ513 - tested on curve tracer. Display in one of my previous posts.C429 - 0.01 HV are 2 months oldC424 - 0.01 HV are 2 months oldC441 - 0.1 not tested yetC425 - 0.001 not testedD408 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingD407 - tested on curve tracer and forward is fine. Held up to 100v reverse without leakingD404 - 2 months old - replaced with BAV21C422 - 0.01 HV are 2 months oldC423 - 0.01 HV are 2 months old

author: xavier60
author_rank: Super Contributor
timestamp: June 30, 2019, 08:53:46 am
content: The capacitors need to be tested for leakage current at high voltage. Use any safe voltage for a start.Measure the voltage drop across a series 10K resistor and calculate the leakage current.

author: shakalnokturn
author_rank: Super Contributor
timestamp: June 30, 2019, 10:25:37 am
content: The Xfrmr looks like it's behaving unloaded.What is trace intensity like on a cold start? Any chance excessive cathode current is overloading the HV winding?I'd be inserting low value resistors on each secondary to monitor current draw evolution.

author: jdragoset
author_rank: Frequent Contributor
timestamp: June 30, 2019, 02:43:25 pm
content: Try adjusting the "HV adjust" pot down to 1200 volts and see if timing to over-current draw changes.

author: gkmaia
author_rank: Frequent Contributor
timestamp: June 30, 2019, 11:31:10 pm
content: Quote from: xavier60 on June 29, 2019, 10:29:31 pmBought an insulation tester. I wish it was a fluke but too expensive. It will arrive on a few days and we can test the Hi voltage insulation.So I went for this one:https://www.aliexpress.com/item/32344557665.htmlRegarding the caps. I replaced almost all of the Hi voltage ones plus all electrolytic in every board a couple months ago in hope to fix the issue.The two missing ones are C441 and C425, but I wont bother testing those, will just replace them. They must arrive in a few days time.Quote from: jdragoset on June 30, 2019, 02:43:25 pmAt 1.3kv it would draw lets say 1.4 amps on cold start. At 1.5kv is would draw 1.6 amps on cold start. So yes, higher the voltage calibration, higher the current draw. Same happens with the intensity controls.I have not done this exact test with a chronometer but after calibrating it at 1.5kv I felt it went to 2.5amps pretty fast, so I moved it back to 1.35kv and seems to me it is lasting longer like that. When it is back in one piece I can runs this test more scientifically.

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 01, 2019, 02:41:21 am
content: Quote from: xavier60 on June 30, 2019, 08:53:46 amI was about to say the same thing. Although the OP opted for replacing the caps, which is a good call, by the way, not everybody understands how leaking works for capacitors, since measuring capacitance and ESR won't detect leakage.For those who want to go down the rabbit hole there's an interesting explanation by Paul Mr.-Carlson's-lab in one of his recent videos.The relevant part is from 33:08 to 41:46. Or up to 47:14 for a more comprehensive explanation on how dielectrics behave under high voltage.Quote from: shakalnokturn on June 30, 2019, 10:25:37 amExcellent idea. That way he could pinpoint what circuit is exactly drifting in terms of consumption. He could even do that right now if he wants to monitor the cathode current and/or leakage in C424 and C 441 by measuring the voltage across R459 over time.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 01, 2019, 02:54:34 am
content: I will do the leakage test just to experiment with it. But as I never did it I am not sure if I trust myself to do get it right and then that will just confuse you guys.Could any capacitor tests be done on a curve tracer?

author: jdragoset
author_rank: Frequent Contributor
timestamp: July 01, 2019, 03:21:58 am
content: Concluding from that test indicates the leakage current is downstream of the EHT winding and HV doubler but could still be in the heater winding.Disconnecting one wire to the CRT heater with all else connected and HV set back to 1.5 kv without current increase would clear the heater winding along with the rest of the HV caps and point to an excessive CRT current draw (mentioned earlier) being a possible cause.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 01:35:54 am
content: Quote from: xavier60 on June 29, 2019, 09:35:13 amThe insulation tester finally arrived. Added a link to it's manual bellow.https://toolboom.com/nfs/product/830018/file/830018_UT511_Insulation_Tester_User_Manual.pdfProblem is I am not sure how to use it for the Hi Voltage and Heater tests you guys suggested. I will need some help to set it up.

author: xavier60
author_rank: Super Contributor
timestamp: July 05, 2019, 03:57:54 am
content: Quote from: gkmaia on July 05, 2019, 01:35:54 amBe careful what you test with that, it can cause a lot of damage especially to semiconductor circuitry.Test the capacitor first. You can use the full 1000 volts, start with 100V first.Be sure that the capacitor is left in the discharged state if it tests good.I would expect the insulation tester to discharge the capacitor when it's switched off while connected to the capacitor. Confirm with your multimeter.If uncertain, report back with the resistance readings and voltage setting.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 05:19:55 am
content: Quote from: xavier60 on July 05, 2019, 03:57:54 amThanks Xavier. But I thought we would use the insulation tester to test the transformer Hi Voltage and Heater windings. is it still the plan?

author: shakalnokturn
author_rank: Super Contributor
timestamp: July 05, 2019, 06:42:27 am
content: Well... You can test inter-winding insulation and winding to core or winding to screen insulation with your new toy.That won't tell you if you if you have leakage between turns on a same winding though. For now we'll have to suppose this isn't the case as the Xfrmr seemed to do well unloaded.

author: xavier60
author_rank: Super Contributor
timestamp: July 05, 2019, 07:29:53 am
content: Quote from: gkmaia on July 05, 2019, 05:19:55 amYes, it will be useful for many things. I even use mine as a bias current source for testing zener and TVS diodes. Don't try this until I explain the necessary precautions.Just use it like a multimeter on resistance range but don't test things indiscriminately.Take a reading on the 1000V setting between either pin on the heater winding and GND.It would be reassuring to prove that the capacitor has a problem.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 08:57:25 am
content: Quote from: xavier60 on July 05, 2019, 07:29:53 amCool, I got it setup and ran a test on my multimeter up to 500v.For the transformer test if I understood right I would have to short the Hi Voltage winding leads and connect those to the tester line (big red connector) and short the Heater windings and connect to tester's earth (small black connector) and then see what resistance it returns between both windings at 1000v (which the maximum this tester do).Is that a fair assumption?

author: xavier60
author_rank: Super Contributor
timestamp: July 05, 2019, 10:23:05 am
content: yes, there is no need to short together the winding ends.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 08:15:27 pm
content: Cool! There we go. Got the main combinations tested. Images are attached. All at 10GOhms range @ 1000v.Is there any other combination you would like to see?

author: xavier60
author_rank: Super Contributor
timestamp: July 05, 2019, 09:16:25 pm
content: The heater winding was the main suspect. To be thorough, make certain that it's tested to all other windings.Test the capacitor also. It has become the main suspect.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 05, 2019, 09:38:18 pm
content: Quote from: xavier60 on July 05, 2019, 09:16:25 pmOk, will make sure about that and do the capacitor. i toke some time to do some calculations based on the wire gauge/resistance used to wind the primary and the hi-voltage. To then try to calculate the number of turns and ratio to see if the hi voltage winding calculations would return something near 1500v. And it does. See attached chart. I guess I am a bit more confident the Hi voltage winding has no short. Does that makes sense?

author: bob91343
author_rank: Super Contributor
timestamp: July 06, 2019, 12:35:40 am
content: If you can use the high voltage winding to test for step up ratio, it isn't shorted. Or at least there are no shorted turns. Otherwise the magnetic field can't build and it won't act as an iron core transformer.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 12:43:29 am
content: There you go. Capacitor tested.1018v before the resistor. 1016v after the resistor.By my calculations it is offering 2.35MOhm resistance allowing 500uA to flow. I could not find a datasheet for it. Is that in spec for a 3kv 0.1uf cap?https://youtu.be/MOZtnhTLG9g

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 12:45:13 am
content: Quote from: bob91343 on July 06, 2019, 12:35:40 amYou mean, apply 25v on the primary and see if I reach the 1.5k at the Hi voltage winding?

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 06, 2019, 02:10:19 am
content: Quote from: gkmaia on July 06, 2019, 12:43:29 amYou're doing it wrong. You should place your meter in parallel with the 10k resistor. By connecting it in parallel with the capacitor, the internal resistance of your meter (around 10MΩ) will be in parallel with the capacitor and you will obtain a false reading.By the way, you have a megohmmeter now. Why don't you use it directly to test the capacitor (leakage current and resistance)?

author: xavier60
author_rank: Super Contributor
timestamp: July 06, 2019, 03:31:35 am
content: Quote from: gkmaia on July 06, 2019, 12:43:29 amThat 2V difference indicates 200µA, 100µA of witch would be the DMM's 10MΩ loading. I hope that you repeated the measurement?100µA leakage is high but not enough to overload the supply. Maybe the leakage increases with time and temperature.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 03:48:25 am
content: thanks bsfeechannel & xavier60I've done another video. Hopefully I did it right this time. If not let me know I do it again.Also please let me know if I did my model right. it is attached as an image. The resistor in parallel to the cap is there just to allow some current to flow and get the 700mv across the 10k resistor. This means cap would need to leak 70uA..I am really keen to do what you say "By the way, you have a megohmmeter now. Why don't you use it directly to test the capacitor (leakage current and resistance)?"How would you suggest I hook the cap to the megohmmeter? Same thing but without the cap?

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 03:59:40 am
content: Quote from: xavier60 on July 06, 2019, 03:31:35 amThat makes sense now. Nothing was flowing thought the cap when I got the DMM in paralell, but through the DMM. The drawing bellow explains the 100ua you suggested.

author: xavier60
author_rank: Super Contributor
timestamp: July 06, 2019, 04:33:24 am
content: Quote from: gkmaia on July 06, 2019, 03:59:40 amThe readings do indicate some capacitor leakage. I should also have told you to just measure the voltage drop across the 10K resistor.Try a longer test and warm the capacitor with a hair dryer.If the capacitor can't be proven to be a problem then we might have a permanent mystery.Current measurements of the power supply's output will need to be done.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 06:17:20 am
content: There you go. heat has made some difference.The voltage across the resistor was 700mv initially. The video shows it at 1.4 but that is because I had heated the cap a few seconds before. Adding heat pushed it all the way up to 2.8v. The resistance of the whole circuit went from 8Mohm to 4 Mohm.Also, if I understood it right the voltage drop you want if you look at the graphic attached. It is the is the voltage I measure on the right schematic minus the voltage I measure on the left schematic?

author: xavier60
author_rank: Super Contributor
timestamp: July 06, 2019, 07:13:35 am
content: Quote from: gkmaia on July 06, 2019, 06:17:20 amThe important information is the leakage current calculated from the voltage drop across the 10K resistor.The diagram on the right just shows what voltage is being applied across the capacitor.Modern insulation testers tend to maintain a constant voltage up to about 1mA of load.It is now fairly safe to assume that with the full 1500V applied, the initial leakage current would be much higher, causing the capacitor to self heat, causing the leakage current to rapidly increase further and so on.This is called "thermal runaway".

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 06, 2019, 08:27:14 am
content: Nice, so sounds like we found someone to blame. The replacement cap will arrive next week.I ended up testing every transformer lead against the Hi Voltage winding and all the same high resistance result we got before. Nothing out of the ordinary. I assume we do not need to test it further and it can go back to the PCB?By the way your next post is the 1000th :^)

author: xavier60
author_rank: Super Contributor
timestamp: July 06, 2019, 10:21:57 am
content: Quote from: gkmaia on July 06, 2019, 08:27:14 amIll have to slow down a bit.Yes, might as well put it together for now.

author: gkmaia
author_rank: Frequent Contributor
timestamp: July 13, 2019, 06:17:29 pm
content: Caps finally arrived! And you were right, the cap was the problem. Current draw is very stable and is also lower than it was before.Thanks for all the help guys!It is now running my BK curve tracer.

author: jdragoset
author_rank: Frequent Contributor
timestamp: July 13, 2019, 09:05:17 pm
content: Congrats! your perseverance is admirable.Most would have relegated to the scrap pile, but verifying the transformer winding's and knowing the problem would re-correct on cool-down made it key to keep going, not to mention acquiring a very valuable troubleshooting tool added to the arsenal.

author: bsfeechannel
author_rank: Super Contributor
timestamp: July 14, 2019, 04:14:04 pm
content: Quote from: gkmaia on July 13, 2019, 06:17:29 pmThat's good news. Thank you for sharing all that information with us. I learned a megaton. And kudos for giving a new life to an old warrior.