#61
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How would I measure ripple with an analog meter? I have an analog VTVM and a .1mfd cap. That is to say what would I look for? |
#62
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In that pic of the power supply area that you posted earlier, it doesn't show where the center tap goes to (kinda fades into the twilight zone). Is it possible you could post a pic showing more of that area? |
#63
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I've got some readings on the electrolytics and a better picture of the schematic.
There are 5 multi stage electrolytic capacitors in the set. Towards the top of the picture, just above, and just to the right of the vertical centering control you can see two sections of C116. The negative on this cap reads shorted to ground which according to the schematic is correct. The other four caps are isolated from ground. Coming right off the center tap of the power transformer is the negative leads of C127, C128, and C131. Further along the line you have C132. Rather than give it away, how would I read the schematic and figure out what resistance reading those four caps should have to ground? |
#64
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To check resistance of the -110V source to ground, just look for a DC path to the nearest ground symbol in the schematic (hint: it goes thru the ion trap coils), and add up the values of the resistances in that path. And see if the sum agrees with the ohms reading you get on the meter (+/- 10% or so). ------------------------------------------ *This technique is called "back biasing". You'll find it used in old equipment quite often, particularly in 30s radios. The CT of the power xfmr winding, instead of going directly to ground, goes through a chain of resistors or a large tapped resistor to form a voltage divider, supplying negative voltages for biasing other tubes in the set. A Google under "Back bias voltage" should turn up some good info on it. Last edited by old_coot88; 02-06-2012 at 03:25 PM. |
#65
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I believe that I am checking for ripple, but perhaps I have the test wrong. I have put my meter on an AC 4v scale. I've connected the ground lead to chassis ground and the probe through a .1mfd cap to negative on C128. Is my test correct, and am I seeing ripple on the meter? http://www.youtube.com/watch?v=b5BBxQlqp1o |
Audiokarma |
#66
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Yes your hookup is right. It's showing just a little over 2VAC of ripple. That shouldn't be anywhere near enough to cause the huge hum bar effect you're seeing. This pretty well eliminates the negative supply as the source of the problem.
So check for ripple on the positive supply, first on the 'upstream' side of the spkr field* coming out of the rectifier (there'll be a large ripple here) and then on the 'downstream' side (top of C127A). Then at the top of C116B. The ripple should smooth out to just a volt or two and get progressively smoother the further downstream you go, such as at C116A. ------------------------------------- *The speaker field serves double duty as a filter choke, a common practice in old equipment. |
#67
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I thought outside the box today and considered that there was outside interference so I unplugged my LED work light transformer and every thing else on my bench to see if the problem was external to no avail. |
#68
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Have you considered an H-K short in the CRT?
__________________
Evolution... |
#69
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By golly it looks like power supply ripple has been pretty decisively ruled out. I went back and reread the thread and looked more closely at the pics. Post#43 shows the raster as having nice straight sides (no hourglassing), indicating no appreciable ripple in the B+ feeding the H output stage (good). And post#58 shows the CRT displaying strong video which would rule out a heater-cathode short in the CRT (which would kill the video).
So.. it kinda looks like the hum is arising somewhere in the video chain 'upstream' of the CRT. And since the sync is affected too, the hum must be occuring upstream of the sync take off point. (Just guessing, but the sync may be picked off at the vid. output which was common practice in the old stuff.) It could possibly be nothing more than a H-K short in a tube. Sometimes tapping the tubes will show up an intermittent that won't show up on a tester. If it ain't a tube, maybe you could post a pic showing the video amp and sync areas of the schematic. |
#70
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Occam's Razor: you've been making cap upgrades which means you've been messing with the wiring, find the wiring fault and repair it.
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Evolution... |
Audiokarma |
#71
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Finding the wiring fault can be fun and challenging if you make it so. Also helps a Noob hone his skills. Sorta like in the old days an electronics instructor would deliberately introduce a fault into a piece of gear.
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#72
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Tell me about it. When I went through AE 'A' school in Pensacola, I was forever finding faults with their trainers. Then again, they were as old as most of our sets...
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Evolution... |
#73
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Check your work young student! Good advice for every restoration and I should have done it sooner. I traced every wire on every connection that I had my hands on and three hours later I'm vindicated but sad to report that I have not found a single wire/connection out of place.
I tried a few taps on each tube and no changes there. Maybe I can retest the tubes for HK shorts. Quote:
I could be completely off base on my thoughts above but just trying to think it out as much as I can on my own so I can learn. |
#74
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You can isolate everything from before the video amp by lifting the grid connection (V105 pin 1), then injecting composite video right into the video amp on pin 1. If the problem goes away, the issue is in the tuner/IF. If you remove the sync tube, the picture won't lock at all and I think it would be difficult to tell if there problem were still there or not.
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Evolution... |
#75
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One last thought.
If the problem is coming from the video section would it still manifest itself with no video signal present? |
Audiokarma |
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