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#1
06-14-2008, 10:15 PM
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Diode question
Hi all -
I think at least one of the diodes in my 1965 Admiral's rectifier bridge has opened up. I was watching the set the other night and suddenly the picture got small and blurry along with a little hum in the audio. Powered it off as quickly as I could and haven't turned it back on except for a few seconds of attempted troubleshooting. It looks like it's indeed running on 1/2 wave instead of full wave - still have sound, brightness, H and V hold do something (but no sync), etc. but it appears as though the B+ is way low. Anyway, my question is this: The diodes listed in the Sam's are 1N1095 or 1N1696, listed as 500-750PIV, 600-750mA. I've got a whole bunch of 1N4007s laying around (1000PIV,1A). I know the ratings exceed the originals, but are there any caveats to replacing the old diodes with the 1N4007s? I plan on replacing all 4 while I'm in there - if this is indeed the problem. Hopefully will get to crack it open sometime this week... Thanks! -Jim 
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#2
06-14-2008, 10:18 PM
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Since the new ones are probably faster diodes, you should probably add snubber capacitors inbetween them. These caps are usually small in capacitance.
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#3
06-14-2008, 10:58 PM
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You can check the diodes with the diode test function of a DMM. You should read around .6 volts one way and nothing the other way. Usually, these diodes will short, causing the fuse or CB to open. I really don't remember when, or if, I ever saw an open rectifier diode.
The problem could also be the electrolytic caps in the power supply. I've seen those fail and cause a similar problem. Even if they are new caps, I wouldn't put it past one to fail. I see a lot of electrolytics fail in fairly new equipment.
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#4
06-15-2008, 08:41 AM
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Quote:
I have seen diodes open up but think radiotvnut is right, Check all the electrolytic caps. When you get her going again double check the b+ voltage and the horz output tube current. Ed
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#5
06-18-2008, 09:55 PM
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Thanks for all the responses!
I'm not going to be able to tear into this one for a week or so, what with all kinds of other stuff going on. But in the interim, I've been kicking it around in the back of my head now and then. Got a few more questions from a purely academic point of view, if you guys will indulge me.. Capacitor failure was mentioned; how would a capacitor fail to produce these symptoms? I don't have a defined raster pattern, it's more of a blur covering part of the screen. Do they fail 'partially shorted' and could this either drag down the B+ or draw enough current to pop a diode? I would think if any of the power supply caps were only open I'd just have a lot more hum in the audio and some bars in the picture. Don't have a capacitor tester - is there any other way to see if the caps are OK? I've heard of testing ESR - is this a valid test for caps of this vintage? Thanks again for the help - just trying to understand the beast a little better.. -Jim
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#6
06-18-2008, 10:08 PM
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I have seen electrolytics become leaky and drag down the B+. I've also seen them short and take the rectifier tube / diode. In some cases, they will even take out the power transformer. The latter is especially true in tube equipment with no fuse protection. They can fail without warning or they can gradually fail over time.
You may want to find a capacitor tester from the '60's or earlier. Unlike modern instruments, these will test caps at their rated voltage.
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