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Old 8th Sep 2020, 4:15 am   #21
Radio Wrangler
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Don't have the circuit, but it sounds like the boost converter switch transistor is shorted or stuck on, or the wrong type.

Power MOSFETs, because of the way they're constructed, have an internal diode between source and drain. It's normally reverse biassed when the MOSFET is biassed normally (Drain positive for N-channel, or drain negative for P-channel) So if you get a P-channel and N-channel fet mixed up, they act like a short. This diode isn't shown on the usual symbol, so it takes people by surprise sometimes. It's not a fast diode, so in things like high-frequency SMPS and RF transmitters, you take precautions to be sure you don't turn it on, because you can't affod the time it takes to turn off.

It's times like these where lab type power supplies come into their own. You can set them to give a current limiter action at a low enough current to prevent damage, and when you see the current meter go to the limit you set and the voltage fall back, you have time to think "Oooo - Errrr, Something's taking too much current" and switch things off or even go for a prod around to see where the current is going.

A boost converter like this operates by switching the inductor straight across the incoming power supply! When this happens, the current ramps up at a speed set by the inductance and the supply voltage. Volts = -L times dI/dt where dI/dt is maths shorthand for the slope of the current versus time line.

Obviously, jamming the inductor straight across the supply is eventually going to end in tears (from the smoke) so the SMPS has to decide when the current is enough. Either by monitoring it in a posh design, or just having a time limit. It then switches off the power transistor and the field in the inductor collapses, creating a big transient of back-emf as it does so. This transient is what it's all about in a boost converter, the output rectifier diode catches the spike and routes it to a reservoir capacitor. The SMPS is now back to square one and turns the power transistor on for another go, and some more charge gets dumped into the output reservoir. In the case of this SMPS, the transient is magnified by having that overwind to boost its voltage into the output rectifier.

Eventually, the output capacitor is pumped up to enough volts, and the feedback circuit seeing this, throttles-back the switcher, reducing the on-time of the power transistor, so it turns off sooner and sooner each cycle.

Take some current from the output reservoir, and the feedback brings up the on-time and the inductor gets revved up to more current at the peak on each cycle..

So you're either looking for a hard short, or a fault in the SMPS control arrangement that has locked it into never turning off the power transistor. With a new-build, there could be wrongly loaded parts, fake parts, wrong parts.

If it all just went together and worked, Think how much less you'd be learning!

Sure-fire kits with all the right bits and heathkit-grade instructions are an accolade for the designer and parts purchasing and packing organisations, but all that people learn from them is soldering skills. They assemble something in robot mode, getting soldering experience and meeting the resistor colour code.

They miss out on the how it works bit and even that misses out on the why it was designed like that bit.

Having to reverse engineer the inductor/transformer pulls the whole lot down on top of your head. This is good. An American friend sprinkles humorous slides in his presentations. One says "Experience is what you got when you didn't get what you wanted". My favourite, though, is "If the opposite of pro-something is con-something, then the opposite of progress must be......" Probably not appropriate in their current climate

I've followed Tim Kriegel's lead and have a few of my own:

"Have you tried the famous echo in the reading room of the British Museum?" which is a Gerard Hoffnung original

and

"In Scotland, the gentleman's lavatory is clearly indicated with the silhouette of a man in a kilt" for which jest, I'm the author.

Tim was a specialist SMPS designer at HP, now retired, cruising the US in one of those huge 'motorhomes'

David
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Old 8th Sep 2020, 4:56 am   #22
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

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Originally Posted by Radio Wrangler View Post
Don't have the circuit, but it sounds like the boost converter switch transistor is shorted or stuck on, or the wrong type.
The circuit is nothing special - MC34063 switching voltage circuit with feedback from the secondary of the transformer.

FET is IRF540N.

Feedback path has various switched resistors in a divider network to obtain the various different output voltages.

And if your wondering about the PIC, it is nothing more than a glorified voltmeter cum timer that switches the MC34063 on/off (it has no control over voltage or regulation of the voltage) and varies the discharge rate of the cap being tested.
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Old 8th Sep 2020, 2:42 pm   #23
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Thank you Uncle Bulgaria for starting this thread. it has prompted me to order the pcb and back copies of the epe magazine, so that i can build one of these tester/reformers. I was looking for a solution to an elec cap issue that I am currently having, namely a lot of nos types in my collection. A lot of these caps are testing a bit strangely, such as uf's being upto 100% high. I've got to the point of not trusting these caps due to what the measurements are showing.
Also did you program your own pic for this build job? i ask, as I will need one doing for this build.
Dave
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Old 9th Sep 2020, 2:25 am   #24
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

In this case it was my own cleverness - I'd substituted BC639s for the BC337s as I had some to hand. Of course, the pin-out is different. There I was in autopilot mode, enjoying populating the board according to the silkscreen. A clear example of exactly what David was explaining! Checking around the IRF540N made it pop right out.

I feel a right dolt, but it always seems that having the correct function explained makes the fault more stupid. Something I've scratched my head over (and in this case blown two ferrites on) was put down to dodgy manufacturing, but the wrong kind! I thought I'd wound the ferrites short when perhaps they were fine. Especially silly when David's gone to the trouble of explaining in words of one syllable what's actually going on. Without the time-served experience, I do struggle with seeing the symptoms and relating them to possible faults. It's the wood and the trees again... Sometimes, writing here makes me realise the fault as I'm thinking it out to explain in writing, while looking at the circuit just makes it look more impenetrable.

Without the ferrite in, I now have a reassuring 'click' from the relay and the LED comes on for the duration of the test period. Just another ferrite to wind now...

G6ONEDave - I want one for the same reason! It feels unethical to throw away old components with all their energetic value, so seeing a bit of what's going on will be very handy. SiriusHardware brought a PICKit 2 to my attention that Merlin Maxwell was offering, so I used that to programme the PIC. It's just a case of uploading the hex file into the programme, then writing it with the right pins connected to the PIC. I used some solid core wire as I was just doing one. If you haven't the means to do so, just send me the PIC and I'll do it for you.
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Old 9th Sep 2020, 4:48 am   #25
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

The word 'dolt' is reserved for people who make mistakes and do not learn from them.

People who make mistakes and learn from them are called 'Normal folk'

People who learn something from someone else's mistake are 'Lucky Bas****s'

People who never make mistakes are called 'Liars'

The people who seem to have knowledge acquired it somehow, mistakes were certainly involved - a mixture of normal errors and luck. Next time you see someone really important, some high heejun, politician, emperor, general , admiral, zillionaire, full of pomp and circumstance, just envisage them sitting on the loo. Cuts them right down to size.

It takes a degree of self control and thoroughness to ask yourself the Jeremy Clarkson question "Now what could possibly go wrong?" and to then be honest with your reply to yourself. The really diligent draw up lists, take countermeasures, and still SMPSs blow up. It's a superpower they have... finding any weakness, oversight or assumption.

I've had my share of little mushroom clouds appearing over power supply components.

It's worse when something goes 'crack!' and you can't see what it was. Had those as well.

David
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Old 9th Sep 2020, 9:48 am   #26
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Thanks Uncle Bulgaria, I might take up your offer on the pic programming. I do actually have the hex code zip from the epe site and I also have a PICKit 2 and the free program that runs in Windows. However, I tried my hand at writing my own PIC firmware for another project and got nowhere, just lots of obscure error messages, that had no offered solutions on the internet blog sites. After trying on and off for about 3 weeks I gave up and went back to using logic chips and diodes, which required 8 cmos chips and about 50 diodes to do the job of one PIC program. All somewhat frustrating, a lot of my troubles were down to changes in the basic languages between different PICS, I even tried writing in hex but to no avail.
I will have another go at trying the PICKit 2, since this time I have a supposed working hex file program to try.
Dave
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Old 9th Sep 2020, 3:56 pm   #27
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Wise and reassuring words as ever, David!

I've just had a smoking D5 after attaching a newly wound ferrite on my last core - an FX2241. D5 is supposed to drop the 12V to 11.4V on the input. I measured the current in and the moment the 'test' button is depressed it leapt to 11A so I quickly switched off. No, my Farnell SB 30/10 does not have a helpful limiter for very small currents!

The 2.49V test point checks out when the unit's just 'on' but the addition of the ferrite seems to have tipped it over the edge when it's in 'test' and the boost is engaged. Perhaps rather too much boost in this instance!

The ferrite pot core attachment point is very poorly labelled with S, F and T. This could mean First, Second, Third: First, Tap, Second; Start, Finish, Tap etc. All of which could be right or wrong and cause it to blow up. I recommend checking the continuity according to the diagram before starting up as I almost connected mine to the wrong holes on a couple of occasions.

Better follow it through again...

Dave - I've never done anything with a PIC before and it was a case of three or four clicks after making sure the pins were connected properly. No hassle.
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Old 9th Sep 2020, 4:27 pm   #28
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

I realised this thread doesn't have the circuit diagram in. Here it is.
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Old 10th Sep 2020, 12:27 am   #29
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

OK, I've saved that.

You could run the circuit with both the power FET and the power transformer removed. Then you can look at the waveform from the drivers going to the terminal for the power FET's gate. It should be a train of pulses going from close to 0v and several volts.

I've just seen that the lower driver transistor is a PNP emitter follower, so it could leave a bit of voltage on the power FET gate, maybe more depending on the output from the controller chip.... is this low enough to be sure the FET is off? I wonder.

This is a time when a scope really earns its keep.

With no HT being generated, the controller chip should be trying its hardest, so the output pulse width will be at its greatest.

If all is well, then the power FET can be put back in , no transformer, but if you can find one, a 2W resistor of a few hundred ohms subbing for the transformer between the power rail and the drain of the power FET, so you can hopefully watch the drain waveform showing it switching on and off.

Small steps, double checks as you go. The aim is to prevent confusion. Getting trapped in circles is a big waste of time and is very discouraging.

David
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Old 11th Sep 2020, 10:49 pm   #30
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Thank you, David. Unfortunately (see Bat Detector thread) my remaining Gould OS255 appears to have blown its mains transformer like the previous one, so I'm scopeless and very frustrated, having been looking forward to a weekend of getting these projects sorted out. I think I'll be looking for recommendations for oscilloscopes with rather more reliable power supplies! I did get these two for free, so I can't really complain but it's so annoying when something breaks irreparably just when it's most needed, and at the weekend too!
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Old 12th Sep 2020, 3:54 am   #31
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

The lab I did my final year project in got given a new OS3000 by Gould-Advance. I wound up with it and I rather liked it, but in later years I learned that they are very prone to eating EHT transformers etc. Other gould models don't seem much more reliable. I'd give them a miss.

Philips, I don't know my way around at all. They didn't have enough performance for the things I was doing, and by then I was working for a firm which preferred to equip its labs with its own make of test gear, which does tend to distort your spread of experience.

Tektronix, you can't discuss scopes without them. Somewhere in the range is virtually everything you could need. I have a pair of 465B 100MHz portables. They've been my fallback for years. Plenty of them and the earlier 465 around. Good scopes. The complexity of stuff on their panel means great versatility, but can discourage people. But once you learn which controls to park and ignore, you can drive them as basic scopes so if you think one looks fearsome, so do other people, but the fear can be conquered.

Tek's plug in scopes were the wonderful 7000 series. OK if you find a good one, but getting a bit old by now.

I also have a Tek 2220. This is their later series and seem generally OK. It's on the dining room table with its probes in an Icom IC765 that I really must finish!

Hameg - much liked by many on this group. Really a front for Rohde and Schwarz, so you'd expect them to be good. Quite solid dependable instruments.

HP have a reputation in scopes. Not a good reputation. A great many people believe that no HP scope EVER triggered on any waveform. This dates to when HP (and Tek) used tunnel diodes in fast trigger circuits. The HP circuit in their 180 series scope plug-ins was a devil to service and not many people learned to adjust them correctly. So the reputation grew. The 180 series was a rather nice scope with a gorgeous CRT. So long as the internal trimmers round the tunnel diode had been set right, they were great scopes. Things got a lot easier with the 1740A this uses logic for trigger control, and leaves the tunnel diode far behind. It is the competitor to the Tek 765. It's not quite as comprehensive in what it can do, but this relates to things very very rarely needed. However, it doesn't have (or need) a fan so it's silent. Inside, it's an awful lot easier to get to things than the Tek. I have three 1740s. Usual problem is dirty switches needing a shot of cleaner. Second hand HP scopes are a fraction of Tek prices because everyone and their father knows they don't trigger.

Digital scopes.... early ones were awful. They used undersampling, and only reached their full bandwidth on repetitive signals. Controls via a menu structure were painfully unergonomic for something you drive as hands-on as a scope. Modern ones have pushed the sampling rate up a lot and they now oversample. You lose a lot of the quirks of sampling scopes. Also around the same time, there was a trend to fit lots of knobs, for all the frequent-use controls. There are some lowish priced new ones which are pretty good.

So for me, I've got a couple of 1740s to fix. The two 765B give me back up. But if the whole lot packed in, I'd go buy something new.

Some scopes of the 765/1740 sort of era marked the start of custom fast analogue ICs getting used. These are irreplaceable (but so is the CRT) however, they aren't often trouble, and scopes for spares aren't expensive, so I don't worry over them.

So the best guide to buying another scope is to see what turns up.

CRT scopes DO NOT travel well in the hands of couriers. There is a very high risk of damage to the CRT . So it's a case of going and fetching or FCS.

David
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Old 14th Sep 2020, 10:00 pm   #32
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Quote:
Originally Posted by Radio Wrangler View Post
OK, I've saved that.

You could run the circuit with both the power FET and the power transformer removed. Then you can look at the waveform from the drivers going to the terminal for the power FET's gate. It should be a train of pulses going from close to 0v and several volts.
Could I do this with a DMM's frequency function while I remain bereft of oscilloscope, or would it just blow it up? I ought to check the PNP/NPN driving circuit again as that's where I erred previously... Is the BC639 man enough to replace the BC337 in this instance?
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Old 18th Sep 2020, 2:36 pm   #33
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

The BC639 is rated at 1 amp against the BC337 at 800ma, so should be OK but make sure that you use the same voltage rating or higher. This is denoted by the suffix on the device which also denotes the pinout. Books like the 'Towers international transistor selector' are very useful for sorting out the specs on transistors and for looking up possible replacements.
I am currently awaiting the ferrite pot core for my own build, coming from Bulgaria believe or not. I thought about testing the completed transformer on my sig gen by sweeping the frequency and checking the output on my oscilloscope. That way proving that the tx will actually work at the required frequency in the cap reformer. Maybe overkill but pot core type transformers are not a component that I have played with before and it appears that the type of ferrite can have a big effect in whether or not the tx will work as required.
I will try the PIC programming once the pcb is fully populated and I am in a situation where it can be tested in action.
The 6 volt relay has been a challenge and I ended up with a 5 volt coil type, so will either use some extra resistance or possibly a diode in series with the coil.
The controls will be mounted on the enclosure wall and wired to the pcb inside. I say enclosure but it will be more like a two sided 'L' panel, which will be mounted inside a case of some sort. My idea is to stick the control overlay onto a thin sheet of steel and then cover it with a thin sheet of clear plastic sheet. The controls will be mounted through this 'sandwich', hopefully the overlay will stay clean and not get rubbed away with this method.
Dave
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Old 18th Sep 2020, 5:40 pm   #34
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

As an update here are some pics of where I'm currently at. I have mostly kept to the 1% resistor parts listing but in some cases have substituted with 2% or 5% types, as well as 0.5 watt to get the higher voltage rating, such as when across the HT supplies. All the 2% and 5% resistors along with the capacitors have been tested to get as close to the circuit design values as possible.
Dave
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Old 18th Sep 2020, 7:32 pm   #35
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

Thanks Dave - that looks good! Diabolical Artificer's oscilloscope arrived today and checks out on the calibration so immediate panic and frustration is over. I hope to have David's tests done over the weekend. I got the 6V relay from ESR in the end (I think I posted the link somewhere in the thread). As I wasn't keen trying to get the exact pot core at vast expense from somewhere else, in a previous thread about the pot cores I was given a generosity of information by the hive mind on which alternatives might be suitable. I've got a few from kind members of various sorts. Currently an FX2241 is in there and I cracked two FX3440s before realising the misconfigured transistors were to blame.

I enjoy my TITS - got on a recommendation from here when starting my first foray into transistors repairing a Uher cassette deck. I'm using some labelled "F708 BC639" and I don't know what that implies for voltage rating.
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Old 18th Sep 2020, 9:44 pm   #36
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

You really have to look at typefaces carefully, that F could be a Fairchild logo and 708 a date code.

David
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Old 19th Sep 2020, 1:06 am   #37
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

FET out; transformer out: at Gate of IRF540N pulses approx. 9V with an on time of 17µs. 'Off' voltage is very close to zero and has a period of about 3µs, so the wave is about 50kHz.
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Old 19th Sep 2020, 1:09 am   #38
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

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You really have to look at typefaces carefully, that F could be a Fairchild logo and 708 a date code.

David
Thanks for the tip, David. It doesn't look like anything so interesting with this one.
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Old 19th Sep 2020, 1:43 am   #39
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

That looks like one of the nameless ones.

I always have a bit more confidence in a part when someone actually owns up to having made it!

But as a general thing, it's worth familiarising yourself with the logos and markings of the main manufacturers... mind you, they get faked along with the parts...

David
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Old 19th Sep 2020, 3:00 am   #40
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Default Re: EPE Electrolytic Capacitor Tester/Reformer Construction

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If all is well, then the power FET can be put back in , no transformer, but if you can find one, a 2W resistor of a few hundred ohms subbing for the transformer between the power rail and the drain of the power FET, so you can hopefully watch the drain waveform showing it switching on and off.
I've put a 200 ohm 2W resistor in place of the primary of the transformer. Scoping the drain shows the attached waveform. 5µs and 0.5V/div (on x10), DC coupled with the centre X axis as 0V. This is on a second IRF540N as the previous was showing a strange signal, and a continuity test showed all the legs were shorted. Either it was from the transistors being inserted back to front, or the transformer is borked and will blow the new MOSFET too.

If that looks all right as a waveform, I'll hope for the best, put the transformer back in and see...
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