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Author Topic: Diodes on Kneisley Rectifiers Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 04-25-2003 08:49 AM                     Diode readings: Digital ohmmeters are not great for measuring diodes. If that's all you have, you should get a very high (reverse) or infinity reading in one direction, and a fairly low reading the other way (forward). The forward reading is actually not a resistance though - it's a voltage drop - and some variation is normal for all rectifier diodes but it should be about 0.5V. Digital ohmmeters interpret this slight variation poorly, so most have a "diode" setting that reads the actual voltage drop. Also, digital meters can measure a resistance from dirt on the outside of the diode (I know, the inside of the rectifier is always absolutely clean so this isn't a problem... as if) so the reverse reading might be somewhat low. This resistance doesn't affect the rectifier output but can confuse the technician doing the reading. An old needle type ohmmeter gives more consistent readings, they should be infinity one way and a low reading the other way. Whatever, if you get fairly high readings one way and fairly low readings the other way the diode is almost certainly OK. If a diode is "open", not conducting anything, you'll get a lot of ripple and with a single phase rectifier the lamp will flicker so bad you would likely be alarmed. With 3 phase it isn't quite so bad. A diode going "short", conducting both ways, usually blows fuses or causes loud hum & smoke problems from the rectifier... not a ripple or lamp life problem alone. I don't recall what Kneissley uses, but be aware that many rectifiers use both "normal" and "reverse" diodes - these look almost identical but must not be mixed up. There's usually a "R" in the numbers marked on the reverse type, this may be the only visible difference. Make sure the correct ones get mounted in the correct holes! It's fairly impossible to guess which is which with a meter - do you know for sure which lead is positive voltage when checking ohms on your meter?  |  IP: Logged John Pytlak Film God Posts: 9987 From: Rochester, NY 14650-1922 Registered: Jan 2000 posted 04-25-2003 09:31 AM                        Don't forget the Osram technique of letting some of the lamphouse light shine directly onto a loaded (about 500 ohms) silicon photocell, and then look at the voltage output on a scope. Gross ripple caused by an open diode or missing leg should be obvious. http://www.sylvania.com/business/photo/pdfs/ENGR_BLTN11.pdf (See technique on page 18) [ 04-25-2003, 11:52 AM: Message edited by: John Pytlak ]  |  IP: Logged Elvis Navas Film Handler Posts: 13 From: Caracas, Venezuela Registered: Apr 2003 posted 04-25-2003 10:30 AM                     Paul, yes it is a strobing flicker. Dave, all diodes are "normal" positive to the base. i.e. christie's rectifiers use both normal and reverse. John, thanks for the link.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 04-25-2003 11:36 AM                     What is the model number of that power supply? I presume it is a 3 phase power supply? Several things come to mind. * 1 or more leaking or open diodes. It may not show up with a diode tester. * Filter capacitor has lost its poop. * Fried or burned up connections in the power supply. * Loss or partical loss of one AC power phase. * Major ammeter mis-calibration. (Power supply operating outside of parameters) Can you safely check the ac voltages on the plates of the rectifiers? They should all be the same. Can you safely check the voltage drop across the diodes? They should also be the same. There is not a whole lot of things that can go wrong in those power supplies. More than likely, you will either find some weak diodes or a bad filter capacitor. Voltage checks will help determine if you have an internal transformer malfunction. Current draw readings on the transformer primary can also provide very valuable information. If memory serves me correcly, some Kneisley power supplies used selenium stacks. Some were converted to use high-powered diodes. If this is true with your power supply, a possibility does exist a wiring problem could exist - from day 1 of the conversion. This is just a guess...I could be wrong since I don't know exactly what you have for a power supply.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-25-2003 11:40 AM                     Is that a prevost favorite 35/70 machine on the front?  |  IP: Logged German Marin Expert Film Handler Posts: 227 From: Verbania (VB), Italy Registered: Jul 2001 posted 04-25-2003 04:50 PM                     Paul, It's a three phase power suply Kneisley model M135RTX2X. Elvis has tested the diodes and the measurement is little different between they. We had to change the breaker 'cause it failed some weeks ago. The input voltage is 208V for each pair we've tested (it's normal for my country). The capacitor is new, it was change at the same time we change the breaker. We'll change all the diodes, I'm sure they are the problem. Gordon, It's a P60 and it's one of three we keep working. We're rebuilding three more. It's the old lamphouse its used years ago. Thanks for all the support! Regards.  |  IP: Logged Gordon McLeod Film God Posts: 9532 From: Toronto Ontario Canada Registered: Jun 99 posted 04-25-2003 06:02 PM                     It looks a lot like the 35/70 machines they sold  |  IP: Logged Marc Jones Film Handler Posts: 82 From: Elizabethtown, KY Registered: Jun 99 posted 04-25-2003 06:33 PM                     If all diodes test good one other thing to look at is the incoming power. Verify if you have a fused disconnect near the supply that all the fuses are good and also verify that once the contactor is pulled in that you have all 3 phases. Good luck  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 04-25-2003 08:29 PM                     In addition to Mark's comments, I would recommend you check the 3-phase rectifier feed from the power panel. If there is a bad breaker or a corroded bus bar where the fuse or breaker seats, it probably will not show up like you think it should. Even though the load is still connected, one cannot go by a volt meter when the legs are read. The other phases will "backfeed" through the power transformer and give a bogus reading equal to or the same as what you might expect if the circuit is intact. A current circuit tester is highly recommended for tests like that. A simple voltmeter might not do the job either simply because if the breaker, fuse, or bus bar is defective, there could still be an indication that things are OK. But as soon as you put something on that phase that draws a little current, that phase may have a "bad day at the office." I have also seen some fuses and circuit breakers that check good by the use on an ohmmeter. However, in actuality, they were not worth the powder it takes to blow them to Hell. You may not have a diode problem after all! Get one of these devices. They are inexpensive, reliable and accurate. This can be one of the most indespensable tools you could have for troubleshooting 3 phase and single phase problems. You see mine is practically worn out. I have had this thing for 20 years. Follow the instructions on how to use it in 3-phase circuits, and you might have the answer for your problem in a matter of seconds. Here is another tool you should not leave home without. It is also fairly inexpensive. These are old basic tools that are still available, and will do what you want them to do. There is no need to spend a King's Ransom on tools like this, such as digital readouts, memorys, etc. If you spent more than a hundred bucks total for like items or equivilant, it would really surprise me. Both of these will save you many hours in troubleshooting.  |  IP: Logged Marc Jones Film Handler Posts: 82 From: Elizabethtown, KY Registered: Jun 99 posted 04-26-2003 11:46 PM                     One other thing: I am not familiar with that exact kniesley rectifier, but I know previous ones that I have had a selector bar for different taps from the secondary, just curious if moving the bar to the next position up or down helps the problem.  |  IP: Logged Elvis Navas Film Handler Posts: 13 From: Caracas, Venezuela Registered: Apr 2003 posted 04-27-2003 01:56 PM                     Marc, I've checked the contactor and it is ok, the cause of the problem is the diodes. Conclusion: the current diodes are 25 years old and I think is time to change it , thanks for your help and I know you'll help us with another trouble.  |  IP: Logged Paul G. Thompson The Weenie Man Posts: 4718 From: Mount Vernon WA USA Registered: Nov 2000 posted 04-27-2003 03:47 PM                     Elvis, let us know of the results when you replace the whole set.  |  IP: Logged Elvis Navas Film Handler Posts: 13 From: Caracas, Venezuela Registered: Apr 2003 posted 05-13-2003 03:16 PM                     hello my friends, what do you think to place a filter irem with the rectifiers kneisley. [ 05-16-2003, 03:33 PM: Message edited by: Elvis Navas ]  |  IP: Logged

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