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Ooops, hate it when that happens....
SR |
Yeah, I've wasted a lot of time over the years from dumb mistakes like this. :drool:
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Eric,
Seeing only positive supply voltages to these stages, I used to wonder how there could be a negative DC voltage on the grid G1 of the HOT or in your case, the oscillator. Normally when you measure an AC sinewave (linear with no DC offset component) with a DC meter, you will get zero because there is an equal amount of voltage above and below zero reference. The negative voltage is the average DC voltage of the non-linear sawtooth. A majority of this voltage is below the zero crossing, a clue that the oscillator is running if you don't have an oscilloscope. With all the parts we replace, its no wonder this happens. Congratulations! |
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A sine wave on the grid will generate it (provided the grid's at high impedance and the signal is capacity-coupled. A superhet local oscillator will make negative voltage on the grid. |
Hi Eric,
The grid voltage on the oscillator comes about by grid rectification. It is known as grid leak bias. The feedback is via the cathode to the grid and when the oscillator is running, grid current with the positive signal excursion forces grid makes the grid negative. When there is no oscillation, there is no negative voltage! This is the same bias on the subsequent stage and because there is no signal, there is no bias. If the capacitor is the correct value and the grid resistances are okay, it will be either the 6K6 or the oscillator coil. Are you sure the coil connections are correct? Terry Quote:
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The explanation is here: http://www.antiqueradios.com/forums/...72016#p2172016 |
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Replaced nearly all the paper caps in this thing, but almost none of the cans yet, still it's starting to work fairly well.
It has a bad Height control that prevents me from getting the optimal vertical adjustment but it's not too far off. I put the 16AP4 back in it to see how it looked, it was really weak originally so I rejuved it with the Beltron, the emissions came up really good but the Cutoff reading is non existent. I wonder if that's why it has the retrace lines, or maybe the vertical misadjustment could be causing them? They are there even with the brightness at a minimum. The High Voltage is good, the speaker wires got to close to the front of the metal tube and it was quite a show. |
retrace lines can show up on portholes easy especialy if it is copyguard but it too could be that the crt is weak and rejuved with no cut off or a combo of the two , are there retrace lines on anything you feed into the set ? you try tapes and dvds?
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The signal is a burned copy (so no Macrovision) of a Japanese version of one of the X-Men films I think, it was in the DVD player when I got it from the Thrift, it's being broadcast with the B-T Agile Modulator.
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does it have a retrace blanking circuit? I have added those to sets that lack them if I have a weak CRT. also how well does the contrast control work.
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The contrast works well, one thing though, only one of the electrolytic cans has been rebuilt, all the others are original.
We'll have to see how it looks after everything is done, there may be some bad tubes as well, I was more focused on getting the basics working first, now I'll have to do the rest. Works pretty well considering. I could also stick a better CRT in and see how it looks, I have a 16GP4 but it won't slide in without loosening the yoke assembly, I think a 12LP4 would be okay for a test, have to see if it can handle the HV but I don't think it's all that much higher than a 12" set. |
Making progress
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I replaced the last three paper caps tonight.
I was having a problem with the brightness, the range was from too way bright and blurry, to burn the phosphors off the screen bright! Turned out to be a leaky Cathode bypass cap (C46), it was a Mica-Mold domino style plastic case cap, replaced it and now it goes from complete black, to very bright. I'm even getting a little bit of sound, it's very low though, possibly the 6V6 is near dead from running it without a speaker but more likely it's somewhat out of alignment. the sync is very twitchy, it jumps vertically at scene changes and flops out of horizontal now and then. It still has the retrace lines (except with the brightness way down) but the CRT is sharp and bright. |
the jumping when scene changes occur could be agc is too high ?
as a test you could put a brightener on it and if you get less retrace lines then you will know it is the crt , those don't remind me of retrace lines from a broadcast but more of the type you see with a weak crt , so add a brightener , if you get enough brighteness when turning the control for less retrace then you will have the answer. mike |
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I'm even getting a little bit of sound, it's very low though, possibly the 6V6 is near dead from running it without a speaker but more likely it's somewhat out of alignment. Could be the dreaded silver mica disease causing that, buried inside the IF cans. |
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