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  #1  
Old 02-28-2012, 07:06 PM
DaveWM DaveWM is offline
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Philco roundie

I have a roundie philco that has HV issues. B+ is good, H drive is good, B+ is at the plate of the HOT, cathode current goes to 400ma pretty darn quick but the fly is stone cold. Looking around I noticed the focus coil looks like toast. So does it follow that a shorted focus coil could account for the excessive current?

I suppose if the FLY primary was really badly shorted then it would not get hot, but that should show up with a primary resistance check...which I have not done yet.
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Old 02-28-2012, 07:10 PM
DaveWM DaveWM is offline
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hmmm the bias voltage at the HOT is only -30 should be -60 per sams, but the drive form looks right (shape and PP). there is a horz bias pot, adjusting it did nothing.
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Old 02-28-2012, 09:25 PM
DaveWM DaveWM is offline
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wow that focus coil was really smoked, burned to a crisp. I had a new one so in the process of replacing. I wonder if overloading the HOT could drive it into grid emissions, accounting for the -30 vs -60? There is no DC leaking as I checked the grid voltage with the HOT pulled, right around 0-1 VDC, per the DMM. If there was a leaking coupling cap I would expect to see B+ there, esp before the osc starts up.
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Old 02-28-2012, 10:21 PM
DaveWM DaveWM is offline
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well the new focus coil did the trick HV came right up and got a good raster.

tried a weak signal but could not get a horz lock, and could not seem to flop the horz lines. the pots are very very dirty, so not sure the horz pot was even working. I will pull the chassis and to a good cleaning of those and see if I can clear that up. There is a .47 cathode bypass cap on the horz AFC that is connected across the horz hold pot. If its one of those 200v jobs, then I would suspect it. Before I dig too deep I will scope the output from the sync sep to make sure I am getting a clean pulse as well. could also bee an AGC issue, I have had flaky pots do odd things to the noise invert and really goof things up, so that's why I like to look at the sync pulse before assuming anything.

Brightness and vert kept winking out as well do to dirty pots.
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Old 02-28-2012, 10:24 PM
DaveWM DaveWM is offline
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the set had its fly cage pulled off, and of course the fly looks like crap (but the cathode current is right where it should be). I wonder how often were flys replaced when the problem was somewhere else.

clearly there was no reason to pull the cage, the focus coil was very obviously the problem and it was not inside the cage. I did not even have to pull the chassis, just lay the set on its side, remove the two bolts from the bottom, and pull the chassis out a few inches to get at the coil. total replacement time was maybe 5 min.
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Old 02-28-2012, 11:08 PM
DaveWM DaveWM is offline
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fooled about with it some more, got a horz lock and vert lock. the vert is pretty stable, the horz is very touchy but I can indeed flop it, so my guess is the sync pulse is good (vert good) and will prob just replace the horz AFC diodes. they are easy to get at with the way the chassis is still pulled out from the focus coil fix. convergence is terrible, as is the purity, but I can see the color is demodulating. contrast seemed a bit weak, will bridge the video output cathode bypass cap (200uf) and see if that is the problem.
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Old 02-29-2012, 12:21 AM
ctc17 ctc17 is offline
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The flyback is a lot of wire turns in a coil filled in a doughnut like was thing. Sometimes if it rotates to fast the hot will make the wax come all over the bottom. Then they will think its bad but its really the other thing with lots of coils around and Allen screw with a plastic thing like the one in the furnature at Ikea. Those ones come wiht the Allen driver in the box but this one has a plastic insert for your fingers not to shock your self. If the flyback is domesticated and tame you can remove the cage and let it out to play. The extra x-rays will help vasectomate like the TSA scanner at the airport. In the long run that will help save you money on pleasure babys that will require more tv sets in the house.
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Old 02-29-2012, 12:44 AM
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Eric H Eric H is offline
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Um, OK.
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Old 02-29-2012, 12:53 AM
DaveWM DaveWM is offline
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of course I will try a new sync sep tube 1st.
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Old 02-29-2012, 06:22 AM
tvtimeisfun tvtimeisfun is offline
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Hey Dave post some pix of your roundie let us see the beauty ... Timothy
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Old 02-29-2012, 07:47 AM
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Talking to ones's self is a sign of madness...
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Evolution...
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Old 02-29-2012, 07:57 AM
DaveWM DaveWM is offline
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well I am hoping some of my observations may be found useful to those that view. Something I did not know before was the effect of the excessive load on the HOT on the grid bias voltage, I still need to go back and see where it is now, that is if its closer to the -60 it should be now. It may be that the kind of HV regulation that the Philco uses could also be the reason, as it uses a feedback type regulation from a winding on the fly. Since the HV was low (due to the short in the focus coil) then maybe the feedback regulator was just trying to increase the drive to the HOT.
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Old 02-29-2012, 11:12 AM
ctc17 ctc17 is offline
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Very few have true diagnostic skills like Dave, most are just parts changers or have pattern/known failure knowledge.

The value of these posts are they get the cap changers to think outside the box. Its not the caps and its not the flyback even though it looks burnt to a crisp

It has been said over and over ARF is where you go for real tech problems and VK is where everyone brags about their collections. Maybe we can change that
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Old 02-29-2012, 12:01 PM
DaveWM DaveWM is offline
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did a front to back ratio check, in circuit, on the AFC diodes using my simpson 260 (like it since it can quickly change polarity), looks ok. They are modern so did not really think they were the problem. Starting to think its going to be the pot, its REAL touchy. there was a 6GH8 in place of the called for 6GM8 in the sync sep circuit. I put in the correct tube but its still touchy, guess the 6GH8 was an acceptable sub. Since its easy to get at, will scope the cathode junction of the horz AFC diodes next just to confirm it getting the correct pulse. I think this set is just going to need a good electrical contact cleaning. all the controls are very noisy and most of the tubes are frozen in the sockets.

Think the only other thing I will check will be the voltages at the CRT, the G2's needed to be maxed out, want to make sure the full boost is avaliable to the adj pots.
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Old 02-29-2012, 12:44 PM
DaveWM DaveWM is offline
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horz sync pulse was clean and right at the correct PP of 10v. the flyback pulse was clean but a little under PP at 13v vs 20v. If the cleaning of the horz hold pot does not resolve it I will check another working philco chassis just to see if the PP on it is also lower than the SAMs. It sure helps sometimes to have a working set that you can confirm thins on. will also check the triode section voltages of the horz AFC tube.
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