Diagnosing a flyback transformer from the waveforms - Page 1
Forum section: electronics / projects

author: Stonent
author_rank: Super Contributor
timestamp: October 02, 2013, 04:11:33 am
content: I've got two identical Geiger counters. One works perfectly the other does not. The one with the problem when I adjust the oscillator input, it seems I can either get it to work for about 15 seconds and then it shuts down (needle drops to zero) or on the other end it works for a few second and the needle pegs hard right and I hear a high pitched whining from the transformer.I have two pictures here of the bad flyback input and the good flyback input. I've set them both to 1ms/Div and there's 4.5 Divs per wave rise and 20V per Div in voltage.5X magnification is turned on to see the detail.Here's the bad sample from the Geiger counter with the issue:To me the bad sample looks very clean.Here's the good sample from the Geiger counter that does not have an issue:It looks very dirty.What can be inferred from these two waveforms?There was a 12V 400uF capacitor (C1) with a split in it that was replaced with a 16V 470uF Rubycon cap, but that is just the cap that charges up from the ticks and makes the needle climb.The one that is very dirty has the original selenium diode (CR3) but the one that's not working has a modern silicon diode. (Verified it is installed correctly and rated for the voltage)V1 is the Geiger tube and the same tube was transferred between units.V2 is a 900V Victoreen Corotron GV3A tube regulator.The original owner suggested that maybe the germanium transistor that drives the oscillator (I'm assuming Q2) could be replaced. He said a 2N404A would be an equivalent replacement.Also from this 50 year old schematic I noticed the +6/10 and +3.0/10. I'm guessing those are test points?

author: Paul Price
author_rank: Super Contributor
timestamp: October 02, 2013, 06:58:04 pm
content: Transformer pulse looks good enough, leads me to think that the pnp Ge transistor is malfunctioning.

author: Neilm
author_rank: Super Contributor
timestamp: October 02, 2013, 07:01:01 pm
content: Are there any other symptoms? I have seen something similar where the ferrite of the transformer had cracked. The thing worked until the load increased and then there were some loud clicks and it stopped. The clips held the ferrite together but the crack was an extra air gap that changed the inductance of the transformer and prevented it working.

author: Stonent
author_rank: Super Contributor
timestamp: October 02, 2013, 08:36:09 pm
content: Here's a quick video. Sorry about the low audio, I shot it real quick before getting ready for work.The voltage is a bit tough to measure, the two meters I have are 1M Ohm impedance and 10M Ohm impedance, but you're supposed to test with 20M impedance. I tried running it through my x10 probe but got nothing at all.The point between immediately squealing and running for a few seconds and shutting off I can get to 735V. If I turn in the direction that lowers the frequency and it starts warbling, there were a few spots where my meter beeped several times and flashed O.L. on the screen on the 1000V DC scale.The tube is supposed to run at 900V DC.The way the tubes work is they ionize the gas inside and when a beta or gamma particle passes through, I think it gives the voltage a little stepping stone and it causes a jump from ground to + and that registers a tick.I could see if the voltage went too high that the particle strikes an arc inside and keeps it going, but I'm not 100% sure about that. But my voltage testing showed otherwise because the voltage was lower than 735 when it would do that.

author: SeanB
author_rank: Super Contributor
timestamp: October 03, 2013, 04:55:20 pm
content: Temporarily replacve the transistor with a 2N2905 and try it again. The transistor should not fry with this loading on it. Likely the original device has diffused through almost all the way, so the gain is high, but the breakdown voltage has dropped signifigantly so it no longer will oscillate well.

author: Paul Price
author_rank: Super Contributor
timestamp: October 04, 2013, 02:13:40 am
content: could see if the voltage went too high that the particle strikes an arc inside and keeps it going, but I'm not 100% sure about that. But my voltage testing showed otherwise because the voltage was lower than 735 when it would do that.----------------------------------The Gieger tube is biased just below the continuous breakdown point for normal operation..setting the V too high will cause it to arc over, a single alpha particle causes the Gieger tube to go into avalanche breakdown and deliver a pulse to the transformer. The voltage across the tube is then reduced to near zero by transformer inductance and the Gieger tube recovers to the HV bias voltage, ready for the next Fukushima.

author: Stonent
author_rank: Super Contributor
timestamp: October 04, 2013, 04:14:50 am
content: Quote from: SeanB on October 03, 2013, 04:55:20 pmSpecs look very close to the 2n404a but made of silicon instead. Interesting.

author: Stonent
author_rank: Super Contributor
timestamp: October 06, 2013, 08:05:24 am
content: Yeah I might skip on the 2n404A and go for a silicon variant. Best price I can find on ebay is 1 for about $6.50 including shipping from China no less. On AliExpress there's not a way to get just one. Best deal was 81 cents each with 53 cents each shipping in lots of 10, so $13+ to get started.However there are 2 qty HP Branded 2N404As on ebay for $8.50 including shipping. That might be cool just for the wank factor of having HP brand transistors in my Geiger counter.Apparently 2N404As and other Germanium transistors are frequently sought after by musicians for use in repairing and home brewing 1960s style guitar effects pedals. The silicon equivalents aren't as warm sounding as the germaniums.2N2905A parts are in the range of 1 unknown brand for $2 shipped or 5 Motorola brand for $5 shipped.

author: CarlG
author_rank: Regular Contributor
timestamp: October 06, 2013, 12:13:26 pm
content: Hi all,I don't know much about selen rectifiers, but I take a guess at this one..."modern silicon diode" doesn't necessarily mean "adequate"; and 1N4007 isn't the same as BYV26E...so what do you have? What's the leakage current at whatever the voltage is? Is there a risk for thermal runaway?"...at the bad flyback input"; exactly at what point is that? To me, the "input" would be at V1, but obviously that's not the measuring point.Are the test point voltages OK? (And yes, as the legend tells, the +3.1/10 etc are test points)So, to the point: it may not be significant, but (referring to the two pictures) the peak voltage is higher on the "nice looking", not working one, and you also have a higher secondary pulse than on the working one. Could this mean that the "modern" diode has lower losses, leading to higher leakage energy and more reverse current, etc etc?I would try to put the selenium diode in the not working unit, and see what happens. (Not the other way round of course, since Q2 may have been damaged due to the silicon diode. If so, just replacing CR3 probably won't do any good.)BTW, what says you need 20Mohm input impedance on the voltmeter? If you need 1000V range for any testpoint yes, but there's no test point at the flyback voltage output (which I just guess you're referring to). With R9=1Mohm you probably need even higher input impedance to measure that voltage.Carl