Insulation Resistance Meter for repair and troubleshoot uses? - Page 1
Forum section: electronics / testgear

author: vinlove
author_rank: Frequent Contributor
timestamp: February 20, 2020, 08:51:56 am
content: Just wondering, if these meters could be used for repair and troubleshoot work of electronic devices such as tube amps, transmitters and radios etc in where over current symptoms cause parts failures, or checking the mains lead connection and overall safety checks on the handles, dials, knobs and casing etc.Or is it just for checking out dometic mains cabling checking purpises only?Anyone using them? How are you using them?

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 20, 2020, 10:37:56 am
content: Due to their limited current output and adequate voltage range I use mine more often for a quick check of backlight LED strings. Double-check polarity before firing...This use isn't anything to do with their intended purpose of course.They can be used in any case where a high test voltage is no harm such as measuring resistance above your everyday DMM's limit or performing a primary-secondary/ground check on power supplies.

author: Specmaster
author_rank: Super Contributor
timestamp: February 20, 2020, 11:36:00 am
content: Personally, I would not use one of these on electronics, they are primarily meant to be used checking for any potential damage to the cable insulation on mains cables that have been pulled into either electrical conduits or trunking and thus could have been compromised on sharp edges within them. They always recommend that when testing circuits thus, that any electronic items on the circuit either be removed or bypassed in order to prevent any damage to them.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 20, 2020, 11:55:33 am
content: Yes,quite true. I see the worying aspect of using one of these on the tube amps or radio gears.It can be just used as a normal DMM measuring ohms and voltages, and also can be used as testing mains wiring, pumps or motors insulations.But was wondering if anyone is using it for electronics sets repairs or checking out over high current problems due to insulation failure on the wires in the set.

author: shakalnokturn
author_rank: Super Contributor
timestamp: February 20, 2020, 12:10:52 pm
content: Quote from: Specmaster on February 20, 2020, 11:36:00 amWhen you see how much the average electrician knows about electricity, let alone electronics, that's the best advice "they" can give.I'm not much good at just following rules, if I find throwing 1kV at something is not going to harm it and will help me gain time in troubleshooting I'll do it, just not anywhere, anyhow.

author: Specmaster
author_rank: Super Contributor
timestamp: February 20, 2020, 12:31:40 pm
content: Hmm, I think that you are confusing electricians with electrical contractors, the latter of which are not in my experience as highly skilled because they are more concerned about installing new circuits into new buildings under almost ideal conditions because most occasions they are working on installations that are totally dead and pose no threats. Electricians however are more inclined to be working on live circuits, modifying or extending them and therefore use a different skill set. Electrical Engineers tend to the highest skilled as they are required to design schemes in the first instance and also very often to troubleshoot when things go wrong and a more in depth knowledge of how electricity behaves along with knowledge of how various items in a circuit behave in order pinpoint the actual problem with resorting to a total rip out and reinstalling all new items.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 20, 2020, 01:26:03 pm
content: But if you were to use the insulater resistance meter for troubleshooting your sets, then obviuosly you must know about the set throughly such as voltages passing in the circuits, and polarity of the components and their max voltage handling rates etc.You wouldnt just thorw 1kv to anywhere in the set. If max voltage paasing in your set is 300v, then ensure you are pumping in the nearest voltage into the suspicious part of the circuit in your set on repair?Just an idea, I havent actually used one for my sets repair or troubleshooting yet.

author: MasterBuilder
author_rank: Regular Contributor
timestamp: February 20, 2020, 01:35:17 pm
content: As long as you know the circuit and avoid putting the high voltage where it shouldn't be I think it is valid to use this as a test for isolation devices such as optoisolators and transformers.A better test is a DC withstand test but this is considered a destructive test.

author: e0ne199
author_rank: Regular Contributor
timestamp: February 20, 2020, 03:13:28 pm
content: Quote from: shakalnokturn on February 20, 2020, 10:37:56 amanyway how much output current does insulation tester produce? for example when i want to measure continuity between a 20kV-100kVA transformer primary phase windings by injecting voltage around 5kV..

author: threephase
author_rank: Regular Contributor
timestamp: February 20, 2020, 03:23:52 pm
content: Devices are unplugged / disconnected when testing mains wiring as they can affect the reading and give the impression of a fault via feedback through the common neutral / earth connection typically found on a mains circuit. You also have to disconnect everything if you are testing insulation between live and neutral as they again cause a short circuit reading.I wouldn't say that electricians are any more skilled than a wireman or an electrical contractor, just that they have a different skill set. Each to their own, an electrical contractor can terminate cables and install trunking way faster and neater than me, but they probably wouldn't be able to fault find on a three phase starter as fast as me etc.An insulation tester can be used as a DC power supply, they are usually designed to deliver just over 1ma at the test voltage and somewhere between 1.25 and 2mA the output voltage will drop off. There are some testers that can deliver more than that, but they are usually in the cable testing domain. You can get some testers that can have the output voltage set anywhere between 10 to 1000V that may be of more use than the standard selected test voltages for your proposal. Naturally they are more expensive. A lot of insulation testers now have a 200mA continuity test for testing earth connections.Yes a DC withstand test is more searching than an insulation test. They are usually only carried out on new apparatus or apparatus that has had significant repairs. After that we just use standard insulation tests. A slight difference to that is for double insulated apparatus that we would routinely carry out a 3kV flash test on.Kind regards

author: threephase
author_rank: Regular Contributor
timestamp: February 20, 2020, 03:30:26 pm
content: Quote from: e0ne199 on February 20, 2020, 03:13:28 pmI would not test winding continuity with a high voltage tester. That would be carried out with a micro-ohm meter for better accuracy, i.e. low voltage-high current, that would be more likely to find a bad connection or inter-turn fault than an insulation tester would.The top brand 5kV testers such as Megger, Sonel, Chauvin Arnoux can supply around 5 to 8mA before the output voltage starts to drop off. Other manufacturer's testers tend to be around the 1.5 to 2mA mark similar to the 1000V insulation testers.Kind regards

author: vinlove
author_rank: Frequent Contributor
timestamp: February 20, 2020, 08:44:42 pm
content: So, as long as one knows enough about the set on the testing= why testing it, and what results expected for which condition, , maximum voltage passing the points of testing, and correct polarities, insulation testing meters could be useful testing / troubleshooting tool for electronics devices.

author: Specmaster
author_rank: Super Contributor
timestamp: February 20, 2020, 09:42:32 pm
content: Quote from: threephase on February 20, 2020, 03:23:52 pmTypically, a contractors course is anything from 1 to 3 years at college but an electrical engineers course is 6 to 8 years as I know because I did the electrical engineers to HNC standards. I have never seen a contractor who was even remotely capable of fault-finding on a single phase starter let alone a 3 phase one. These days it considered standard to use a 3 phase starter of a single phase motor (with a suitable voltage contactor coil fitted), it is also standard now to allow for the "single phasing" protection of an overload relay to wired into the circuit by passing 1 pole of the motor through 2 contacts and overload bimetals in series connection and the other pole through the remaining pole of the contactor and overload. It even mentions the correct wiring method and highlights the need for this connection, but I have been called out to literally 100's of service visits on new installations where the contractor has only used 2 of the 3 poles and consequently the motor runs for a couple of minutes only and trips off because the the third bi-metal is not being heated by the motor current.

author: threephase
author_rank: Regular Contributor
timestamp: February 21, 2020, 12:14:23 pm
content: It depends on the type of industry and experiences, but an HNC is usually regarded as technician level. The traditional route to an electrical engineer in the UK was a bachelor's degree, that was sufficient for entry into the IEE (now the IET), from there you could work within the IEE towards Chartered Electrical Engineer status. This path is no longer available and the IET just offer Chartered Engineer to all applicants. You can also now join as an Associate Member without a degree and move up within the IET membership bands if desired.A very experienced technician wth an HNC could move into an engineer's position, but it is likely that they would be on a specific improvement plan so that they either learn the required theory on the job, or are released to university to obtain their degree as part of their improvement plan.You may get engineers that are more skilled at the same tasks than other engineers, in the same way that you will get electricians that are more skilled than other electricians. But it is fundamentally wrong to say that engineers are more skilled than any electrician or electrical contractor just because they are an engineer. Engineers, electricians and contractors are all different skill sets and each one needs the other other to make it all work.Apologies to the OP for taking the post off track.

author: threephase
author_rank: Regular Contributor
timestamp: February 21, 2020, 12:15:23 pm
content: Quote from: vinlove on February 20, 2020, 08:44:42 pmLooks like a good assessment to me!Kind regards

author: Specmaster
author_rank: Super Contributor
timestamp: February 22, 2020, 12:52:08 pm
content: Quote from: shakalnokturn on February 20, 2020, 12:10:52 pmWe have to remember the different skill levels of people using this forum, it ranges from newbies with almost zero knowledge right through to experts with many years of experience in the industry. This topic has been taken down all kinds of twists and turns comparing the various skills of different sections of the industry etc. The fact still remains that a meter with insulation testing features is NOT a recommended meter for use in the electronics theatre as can be clearly demonstrated by the meter manufactures own literature. They make meters that are suitable for electronics, electrical installation (contracting), Auto electrical etc, with dedicated ranges and degrees of safety to suit each market sector.Yes on all the meters, regardless of which sector they were designed for, there will be ranges that are common to all meters that could be used in all sectors, but there will also ranges that be detrimental in some circumstances and insulation resistance ranges because of the very high voltages given out 600v and upwards is most certainly not one that is used on a meter designed to be for electronic use. I'm NOT saying that in some cases under the right conditions that could be on the odd occasion be useful for measuring high resistances. There are meters have high resistance measuring capability into the Giga Ohms but they do so at a very low and safe voltage that will not damage any of the electronic parts in use. Don't forget that in most modern electronics there are chips that are designed to operate at 3.3 or 5V and many capacitors rated at 10V or lower that would most certainly be damaged if exposed to the voltage of an insulation tester.I therefore stand by my recommendation that such a meter should not be used on electronics.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 22, 2020, 02:57:29 pm
content: Sure, it must be not be used with any modern sets with SMD components in them.But the vintage tube / valve sets with 250 - 300 DCV floating all over in the set, it could be used with 250V - 500V max setting, I was conjecturing.

author: NY2KW
author_rank: Regular Contributor
timestamp: February 22, 2020, 03:19:03 pm
content: For tube based electronics, it depends on what you need to test. These are meant to test dielectric integrity of passive components and wiring. If you want to test the your B+ high voltage connectors, You don't need a lot of current capability, as the leakage current in most of these cases should be < few mA, but you need the right voltage range. The usual rule of thumb is to test with twice the operating voltage + 1000v. I got a Fluke 1587 "Insulation" meter for some reason many years ago and its top voltage is 1000v, so it really doesn't hit the peak voltage for most tube based supplies. Some of my old radios have power transformers with insulated standoffs that are deteriorating over the years and sometimes will arc over... testing with a rated HiPot tester has helped identify (at very low current) those that need new insulation before more expensive damage can occur when they would normally draw large current with a voltage arc.Jerry

author: vinlove
author_rank: Frequent Contributor
timestamp: February 22, 2020, 06:39:22 pm
content: Yes, quite true.Some people seem using insulation testing meters on the solid state device too.

author: Specmaster
author_rank: Super Contributor
timestamp: February 22, 2020, 08:03:49 pm
content: I still fail see the relevance, if you watched this video, Clive said that it should have given a resistance value of around 10M, instead he got a reading .29M, or in other words 290k. Given that every digital multimeter I have ever seen has the capability of reading resistances of up to 20Mand many others up to 40Mand loads more well into several G, just what the advantage is using a 500v or 1,000v insulation tester? Bearing in mind that I have just done a test with a Solartron 7150 which can read up to 20Mand when in resistance mode, it outputs a safe voltage 3.3V (measured on Fluke 8840A) so you can poke about with the meter probes safe in the knowledge that you won't be damaging any circuitry. So why do you want to use a 500V tester on electronics???

author: vinlove
author_rank: Frequent Contributor
timestamp: February 22, 2020, 09:24:14 pm
content: In my case, current repair project is an old vintage tube transmitter. It has over current problem when set to TX mode in AM. The voltages inside the set is all above 300+ DCV.With normal DMM, it shows no problems on ohms reading at some junctions against the chassis.When you inject 500V, it shows up the insulation problem on a few worn cables and the chassis. The problem was only detachable with 500V Insulation Resistance meter readings.

author: Specmaster
author_rank: Super Contributor
timestamp: February 23, 2020, 01:09:21 am
content: I see, but non the less I still would not personally use it. Have you ever considered that the readings that you are seeing when using are not actually the insulation breaking down of the cables as you seem to be thinking, but the insulation within some capacitors in the circuit? That is where my money would be gambled on.Have you got a capacitor tester? If so, which one and what does it actually measure. When you say that it over loads the current on transmit, that to mind is pointing directly at capacitor issues and if you have one or more capacitors on a supply rail shorting out to ground, which is where they are mainly used on power rails to provide a clean and steady supply to the circuit, then the rail is going to try to supply the power required to transmit PLUS the leakage current to ground via the capacitor(s).Most problems with electronics are capacitors, they dry out with age and if they are installed near to a heat source then this ageing and drying out process is speeded up.Also remember that when you use a ohms tester or a insulation tester at various points in a circuit, without disconnecting the component leg from the circuit (not the ground) that you are going to get false readings because you are trying to test multiple items at once which you cannot do.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2020, 04:03:06 am
content: Quote from: vinlove on February 22, 2020, 09:24:14 pmThis would be a valid place to use such a device if they are similar to the old "meggers".We used them to check PA tubes in large transmitters when we got an arc over in the final, (or modulators & RF driver stages where they also used substantial power tubes).To do this, we would pull the tubes & test them on the bench.The meggers weren't usually capable of the same voltage test levels as the HT supplies, but they were quite sensitive.An internal fault in a tube which caused a serious "zap" at 10kV usually won't give any kind of indication with a normal DMM's low test voltage, but with the 1000V test range on a megger it will probably show up as several Megohms, as compared to infinity on a good unit.If it isn't the tube, you can isolate any other circuitry which might confuse things & test the rest of the circuitry in the PA/mod/driver stage.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 23, 2020, 04:55:48 am
content: Quote from: Specmaster on February 23, 2020, 01:09:21 amA 500v insulation tester is unlikely to damage capacitors used in a tube transmitter, as it cannot produce enough current to do so.Unless there are low voltage solid state devices as well as tubes, I would say the chance of damage is minimal.That said, you are correct, in that if the caps are showing leakage, this may be the problem.The OP hasn't given quite enough information:-"Over current"?Overall high current from the Mains supply or whatever?High current from the HT supply?Excessive plate current in the PA tube?Does the transmitter have a CW mode?If so, when keyed in that mode, does it draw normal current?In AM mode, does it draw excessive current when in transmit with no audio input?Depending on the design, the modulator may not only be out of circuit in the CW mode (if it exists), but powered down completely.In that case, the overall current drawwillincrease in AM mode,--- as an old unit, it probably uses high level modulation.In a perfect world this shouldn't happen, but try re-dipping the PA anode current, just in case.

author: AVGresponding
author_rank: Super Contributor
timestamp: February 23, 2020, 10:07:31 am
content: Given the relatively low cost of (used) high voltage, current limited PSUs, I don't really see the benefit of using an insulation tester for this purpose; it's unlikely to give repeatable results to the same degree as a proper PSU.I'd rather have something I have more control over, in terms of V and A.

author: e0ne199
author_rank: Regular Contributor
timestamp: February 23, 2020, 10:10:48 am
content: Quote from: threephase on February 20, 2020, 03:30:26 pmanyway is it possible that doing that continuity test will break the transformer itself? although it is a 24kV transformer..

author: vinlove
author_rank: Frequent Contributor
timestamp: February 23, 2020, 07:44:52 pm
content: Quote from: vk6zgo on February 23, 2020, 04:55:48 amIt is the vintage Heathkit DX40-U.When it is powered, and on to TUNE position, all seems fine. All the tubes lit up, and voltages OK from PSU, and the smoothing caps.But the moment that it is set to AM mode, the 5U4G rectifier tube arched and sparked with flashing lights and frying noise.Then when in most speedy way possible, powered the set off, and checked the 5U4G, the tube is totally dead - no continuity on the heater pins.It is same in CW mode too. I have replaced all the eletrolyc caps, and even made up silicon rectifier to save the 5Us.5Us tube are too expensive to blow off every time the set was powered on for testing.The silicon diode rectifier worked OK, but the output rectified dc voltages were a little higher than the 5U4G.But it too was fried when the FUNCTION wafer switch set to AM mode. The heat resistors and diodes were OK, but the plastic base off the dead 5U4G I used to make up the diode rectifier were melting away with toxic smell and fireworks.So I have been working on troubleshooting what is causing the over current killing the 5U4G rectifier, and now melting away the silicon diode rectifier plastic base.I have been looking for some area where its likely causing the shorts in the circuit and the old wires in the set causing possible shorts with the chassis. But still no joy.In the process, I noticed, the sets some wiring had been altered by the previous owner on one area, but it looks not much significant causing the terrible over current. But I might take the extra electrolytic cap out and change the wiring to the original as in the schematic.Maybe I must start a new thread for this topic.So, I was looking at an insulation resistance meter which might point out any insulation problems in the set.Not tried yet, and not sure if this will work.

author: jpb
author_rank: Super Contributor
timestamp: February 23, 2020, 09:25:26 pm
content: The one use I've made of one is to do a sanity check on ESD mats where the resistance is too high to register on a standard meter but does on a 1000V insulation meter.It also is a way of using the 1000V scale on a DVM. Not very scientific - qualatitive rather than quantative though I do compare with some test multi-gigaohm resistors.

author: vk6zgo
author_rank: Super Contributor
timestamp: February 24, 2020, 07:47:23 am
content: Quote from: vinlove on February 23, 2020, 07:44:52 pmHere are a couple of links which may be usefulhttp://www.w4btx.com/DX-40%20Manual.pdfLooking at the manual in the second link, It may be worth checking C33.Note that the video in the first link had a lot of zapping in the mode switch, & the author repaired the switch, so it may be another possibility.One trick is to actually measure the HT current draw, by breaking the HT feed & inserting a multimeter on the amps range.Be careful, because, when things are going OK, the HT is around 750v, which may be in excess of the voltage ratings of some cheaper DMMs.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 24, 2020, 08:20:39 am
content: Thank you for the link. The video is excellent.Will check out the C33. Last night I was checking out a few dodgy looking caps in the set, and found out that one of the cap was supposed to be 4700pF in the schema, but the actual cap was 47pF.Could it be significant for causing the over current? I will look into my junk box, and get some 4700pF ceramic caps out to replace the 47pF one. Not sure which cap in the schema it is, it was too late to look into the schematic last night.Yeah, most of my DMMs (I have a few cheap DMMs) says it can do upto 1000 DCV and 750 ACV. My old Fluke 25 did read one day 1500 ACV. But I get a bit nervous when trying to read HT.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 24, 2020, 10:38:03 am
content: It is C46, 0.005uF = 5000pF 1.6kV in the schematic of the DX40 US version.In my set, the cap was rated at 47pF 400V. That looks like grossly wrong rated cap, the previous owner put in.Would it have contributed the serious over current fault in the set?

author: Specmaster
author_rank: Super Contributor
timestamp: February 24, 2020, 06:19:59 pm
content: Quote from: vinlove on February 24, 2020, 10:38:03 amI did ask before but don't think I ever got an answer, what are you testing the capacitors with and what measurements are taking? If you are only using a meter that can read the capacity then that is not sufficient, yes it will show up any significant drift in value which may or may not be linked to your problem. You need to do a check on their ESR values using a meter capable of reading ESR and you also need to be able to check the capacitor for leakage and that involves testing with a far higher voltage then and ESR meter or a DMM meter is capable of supplying. You need to use something like a Heathkit C-3U bridge to test for leakage.C46 could well be a smoking gun as with that amount of drift and voltage, it is more than likely to be leaking and could be the cause of your problem. Replace it and see. You could try, after discharging it, reading its resistance using a normal DMM and see if it is reading anything, if so replace it.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 24, 2020, 08:33:52 pm
content: Quote from: Specmaster on February 24, 2020, 06:19:59 pmAhh.. sorry I missed that question I think. I quickly drop by to the forum, and scanned msgs, so tend to miss some posts and questions at times.Yeah, I do have an ESR reading meter which is BK-328, it displays ESR value. The fact is that the C46 is a new capacitor put in by the previous owner, but somehow he put the wrong valued one, because I can see the cap being very recent modern one, only replaced a few months ago?Anyhow, I cannot tell now if it is directly linked to the problem of the over current without powering it on and testing it. I was looking into my junk box, but couldn't find any good matching value of the cap for the replacement. So I had to order some caps from eBay, and will take a few days to arrive, and I will wire the new cap on, and will power on for testing.I am not sure if it is worth paying for the dedicated cap testing device like the vintage Heathkit. The problem is that they tend to be grossly over-priced for collectors value rather than what they do or how well they do it.And it is quite impractical to desolder all the caps from the set, and go through the leakage testing when troubleshooting problems. You just try to identify the problematic areas, and pick out suspicious components and do testing, and if in doubt or problematic for sure, then replace them, that is my strategy. And capacitors are usually very cheap, so it is easy to replace them, if they look old or have usual suspicious tell tale signs for being bad.And I am sure that reason that you are reading the capacitance is that when the value has drifted a lot from the nominal value, that would be the first sign that the cap is not good anymore, so it is vital to get the capacitance readings I thought.All those old paper wax caps, when you take off from the set, and read the capacitance, they are usually well off from the nominal value, which tells a lot.But really, this C46 was a very tricky cap, because it looked shiny new, and I took it for granted as a good cap, which turned out to be well out of value, not because it gone bad, but because it was simply a wrong cap for the junction in the circuit. If I have not tested it just purely by chance trying out my newly arrived DMM, looking at the schematic, it would have never been noticed forever.

author: Specmaster
author_rank: Super Contributor
timestamp: February 24, 2020, 09:12:11 pm
content: The actual capacitance, even if it is very close to the designated value can still be a false indication as I have found many caps that are within spec on their value but fail the leakage test and or the ESR test and don't forget that the ESR reading is also frequency dependant can pass testing at 100Hz but fail at 10KHz or at 1MHz etc. Like wise a cap rated at say 500VDC can pass a leakage test with ease at 16V, but fail as it gets closer to its rated V rating.Clearly from your description it is something that is only switched into a live circuit when the set is switched to AM.I think that if the 5U4GB is pulled from the circuit then you only have 5 pins to check for any signs of leakage to the chassis and with a DMM set to resistance mode then it should be possible to see if anything is leaking to ground.Edit:I take it that you have either tested or had the tubes tested as it could be possible that a tube is faulty and causing your problems as well?

author: vinlove
author_rank: Frequent Contributor
timestamp: February 24, 2020, 09:38:15 pm
content: I found that capacitors testing is tricky issue, with all the multiple factors involved, as you pointed out.I still will read capacitance first then ESR, when testing caps, but if it looks old or physically not sound, then just replace them with the same value and voltage rating. That is the way I do it. For that, I would still keep the capacitance meter and ESR meter which were cheap, but I aint gonna pay over hundreds for the vintage Heathkit capacitor tester.Yes, I made sure the 5U tubes were all sound. In fact the set has destroyed 3x new 5Us already in the middle of test powering on session. The moment it was switched to AM mode, the 5U tubes got fried instantly. And then I made up a few diode rectifiers to work as 5Us. They worked well, but the over current problem is still there. Hopefully the new cap replacement will make some difference to the situation.

author: Specmaster
author_rank: Super Contributor
timestamp: February 24, 2020, 10:03:05 pm
content: I agree, keep all the capacitor testing you have but where in blue blazes did you get the idea that a Heathkit C-3U would cost hundreds from?? You don't state your country in your profile, which actually makes it harder for other members to assist you especially when giving you links to refer to which are often going to be country specific, so maybe you might like to revisit your profile page and rectify that issue for your own benefit.I say Heathkit C-3U because that is the UK variant of that tester, if you are in the USA for instance than that reference becomes just a C-3.Anyway, I can assure that your guess at the cost of that particular tester is way off, I got mine on eBay just a few weeks ago for only £34.95.That Video in the thread is worth watching. It really is a simple piece of kit and it wouldn't cost a fortune to replace all the caps and resistors in and that would certainly eliminate those from the equation.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2020, 01:19:40 am
content: Vinlove, have you restored vintage valve gear before?I recommend you start with a schematic, and testing transformers for insulation resistance, winding resistance and winding turns ratio (to confirm no internal shorts), then rewire all insulated wires where the vintage insulation is suspect. Then test subsections by themselves, and use a variac and AC current meter, staring with just the AC mains side wiring and only primary winding connected to confirm acceptable idle current. Any HV secondary side valve rectifiers should have redundant ss diodes inserted in series, and secondary windings should have appropriate fusing added where practical. Then it is common to test with B+ but without valves, using a variac, to confirm no secondary side circuitry faults, and to allow coupling capacitor leakage to be tested, as well as testing B+ filter capacitors up to their max rated operating levels.If you dive in to testing by powering up straightaway then you run the risk of damaging parts (as you have experienced), and sometimes those damaged parts can be irreplaceable, and you may end up being frustrated and still not able to localise faults.

author: Specmaster
author_rank: Super Contributor
timestamp: February 25, 2020, 01:43:23 am
content: Quote from: trobbins on February 25, 2020, 01:19:40 amIn this instance, I think that we can ignore the testing of transformers seeing as the problem apparently only manifests itself when the unit is in AM mode.

author: trobbins
author_rank: Frequent Contributor
timestamp: February 25, 2020, 04:13:01 am
content: I suggest that nothing can be ignored as the equipment is vintage valve gear, and as the OP has not identified the likely vintage or provided photos then it could be significantly old and had a tough life, or been left in damp storage for decades, or ... the list goes on. Imho it would take some confidence building to see it as just a likely single fault within an otherwise clean, looked after, recently serviced piece of valve gear.

author: Specmaster
author_rank: Super Contributor
timestamp: February 25, 2020, 07:42:21 am
content: The reason why I think the transformers can be skipped is that these are in circuit all the time and the OP has already stated the fault does not show until he switches the set into AM mode. The transformers IIRC are switched on via 1 switch only so as long as the set is on, the secondary are connected, may not all be under load though but if there was shorted turns, dampness etc then I'd have expected any such problems to exhibit it self regardless of what mode it was in.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 25, 2020, 08:59:42 am
content: specmaster, I am in UK, and I was bidding for a Heathkit Capacitor tester recently, and my max bid was 38£, but the final bidding was well over 125 or something like that. If you bought it for around 35 then you have been lucky, and got the bargain. Good for you. I think Heathkit devices work, but there is definitely collectors value gets added to them when they are being sold on auctions.But yeah, transformers are OK. If the mains transformer is bad, then you wouldn't get good HV, and really the set is only good for parting out I would say. The set had been powered under the lamp current limiter and also via Variac, so it was well protected for the over current. And indeed all looks fine until it is switched onto the AM mode, and bypassing the lamp current limiter and under full mains power, then it goes into fireworks. So, I was suspecting some wire or component in the set and chassis shorts somewhere?But then also found out the C46 cap is wrong value, so will replace it and power on test again. I was thinking of injecting 500V from the Insulation Resistance meter, and see how good the insulation on the set is with the old wires, but it is still just an idea. But it stands as the last option for the testing.

author: Specmaster
author_rank: Super Contributor
timestamp: February 25, 2020, 10:07:59 am
content: Totally agree with the transformers and with the dim-bulb / variac approach but still disagree with the 500V insulation tester. If you watched that video, there was nothing in that video that would not have shown up just the same using a normal multimeter connected between chassis ground and the various points that you wanted to use the 500V tester on. If you do your ohms law calculations then 1 megohm leakage to ground is only going to result in a current flow of 0.00075A and resistance of 1 kilohm will result in a fault current of 0.75A and both values are well with the scope of even the cheapest of multimeters without risking possible damage. Incidently, referring to a comment you made earlier about de soldering capacitors, you only need de solder on end of them, not both the measure their capacity and ESR. As a general rule of thumb, never measure the value of any part with it still connected to the circuit because you will be measuring the values of anything else that has a current path through it at the same time and that might be possibly why the capacitor you measured 47pf. Did you test it while it was still connected at both ends to the rest of the circuit?As to the Heathkit capacitor tester, I got mine in an eBay auction, here in the UK, you just have to be lucky and if you're patient enough, you'll pick one up in the end for a good price, just set up a standard search and save it on eBay and every time one is listed, you'll get an email informing you. I got mine on the 19th Jan this year using that method.

author: vinlove
author_rank: Frequent Contributor
timestamp: February 25, 2020, 10:41:10 am
content: Yeah, quite see your point about the 500V DC input insulation test. I was really being suspicious about the old wiring in the set, and some of them have deep burn scars with hot soldering iron.use the insulation resistance meter for usually checking long cable or long wire for its continuity and insulation. Sometimes I test heater and demonic appliances for their insulation, but wondered if it could be used for troubleshooting vintage tube electronics sets too. But maybe it is not good idea from this thread opinionsYeah, I took the C46 cap off from the set, and tested it. I know that you never test caps while they are in the circuit for the reason you explained. It is a bit of hassle desoldering it back after the testing, but thats just life.I also have a LSR tester device called BK-328, and it can test caps, coils and transistors and resistors. On cap testing it reads capacitance but also ESR and % of leak. I find it very consistent and accurate. But not sure what ESR value and Leakage value is regarded as good or bad.But yeah, if I can see the Heathkit cap tester for cheap, then I would buy it. But definitely will never pay over £30. Because I would never use it that often. Usually I find new caps are almost always fine. It is the old wax and paper caps are problem, and you can tell them by their looks, don't have to test them

author: Specmaster
author_rank: Super Contributor
timestamp: February 25, 2020, 10:59:16 am
content: When say new caps are generally fine, are you referring to electrolytic caps? If yes, then again, I disagree, many can be easily spotted by their appearance, but equally I have had to replace caps that looked to perfect externally because their readings had changed massively, often caused by using low temperature ones near to heat sources such as heat syncs, transformers etc, or just generally where the equipment has been used in a high ambient temperature etc.Generally you can spot ones that you want to check first and these are ones that are directly connected to the power rails and ground, if these are leaky they will cause (depending on the amount of leakage) cause over current to flow and in severe cases will cause the power rail to fall significantly below its voltage or collapse completely in the case of a short circuit in a cap.Paper and wax caps really should be replaced on sight, regardless of their appearance, they cannot be trusted, ceramic, micra and polyester caps are generally OK.