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.

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.

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.

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.

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.

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.

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.

Um, OK. :scratch2:

of course I will try a new sync sep tube 1st.

Hey Dave post some pix of your roundie let us see the beauty ... Timothy

Talking to ones's self is a sign of madness...

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.

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

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.

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.

Is this the 17MT80 chassis? If so, all the pots on mine were dirty and needed a good scrubbing. Seems it was a pretty cheap chassis, the controls are not up to RCA standards.

yep that's it. Very cheap pots used on the back. every single pot in the front is also very dirty. The IF section is all SS, my other one has a good pic, but a bad tube socket (I think) in the blue output tube (likes to flicker more or less blue) socket. I checked the PCB very closely, triple checked all the ground stakes, prodded the parts around it. if you move the tube it would flicker blue. a real PITA. I don't like the use of large PCBs for tube sets, just seems to be asking for trouble.

If trying a new tube ,cleaning the tube pins and socket, and reflowing the traces going to that tube don't help then you should swap the socket out for a new one.

yea I know I should replace the tube socket, just got tired of fooling with it.

Heres a trick, if you can find a garden snail, squeeze the juice out of the snail on the socket. It will cause the socket to swell and the pins will tighten up.

You really need to get laid.

cleaned the pots on the front, that cleared up all the sync issues. Tuner needs cleaning. the brightness pot needs to be turned up pretty high. I will be checking the 12GN7 video out soon, if its weak it can cause a CRT bias issue cutting off the crt. I tested the CRT and it tested very good so the high screen settings tell me something is a not right.

this set controls the brightness by biasing that 12GN7 thru the cathode with a 500 ohm ww pot and a 82 ohm resistor. On my other set that 82ohm had drifted up, which would increase the bias, causing the higher plate voltage and again upsetting the CRT bias. The Vert seems to shrink as it warms up, the vert out bypass cap sits right next to a big power resistor so it gets pretty hot from that. I will replace that one just because I dont like the high heat its getting. its in a single can, the PCB has holes for axial and a can in for that cap.

I think I can get to that 82 ohm resistor without pulling the chassis, its mounted right at the pot.

One last issue is the focus, varies from about 3.3 to 3.7 kv.

I plan to check the 66meg resistor, and maybe try a new focus rectifier.

I made up a 120pf cap to replace the 130pf 6kv that couples the output from the focus coil to the cathode of the focus rectifier. I have had issues with focus coils before when not using the EXACT replacement. Hope that is not the case here.

I would still like to try a HV silicon rectifier some day instead of one of the Se sticks.

Mine had a tube rectifier in it, I prefer OEM style.

yea but those stick types are hard to find unless you know of a source. I am down to 1 NOS GE CR-1 and a couple salvaged ones. I came up with a unscientific approach to test them. Used a VTVM on a low volt scale and a few 9v batteries in series for 45v and the stick in a circuit to see how much voltage I get thru the stick then reverse the stick and try again. I used the NOS GE and when compared to a known weak rectifier the diff was large. Crude but it is a quick way to compare. Not sure how much difference is ok, so its still a process being worked out.

Use that leakage test on the cap tester. They get sensitive to the temp and work better as they warm up. SI would be the way to go. Take apart a trippler and harvest the diodes

Yeah, rapidly fading from scene is the old craft of circuit diagnosis/troubleshooting by old techies who did this fulltime for a living back in the day, and the many little hints, tips and quirks that were gleaned only from experience.

it was the focus coil. I subbed in another (this time with some jumper leads so as to save the time if it was no better) and voila I get very sharp focus with plenty of range on either side. that was easy...

I will install the new coil and button it up later.

operating on a hunch that the phasing of the 1st replacement focus coil was not adding but only subtracting to the focus voltage, I reversed the leads in the yoke circuit and now get a much sharper pic, but I still like the second focus coil as it seems the adj range is broader, and easier to nail the right setting.

ok back to the 1st replacement. the 2nd one broke right at the mount enough fo a chunk off the form to make it unreliable. so the 1st replacement was used. Don't know why but the range is fine with it now. focus is right in the middle and easy to adjust. Of course I managed to break the center coil wire, argh... but was able to graft on a hair thin piece and reconnect to the term lug. thats what I get for working on it out in a dimmly lite garage.

Anyway its looking VERY good. it has all the orig parts and a couple new tubes.

the vert is a bit bouncy, so I will end up pulling the chassis and going thru those cap in the feed back circuit/the cathode bypass cap and coupling caps at the video and the color amp tubes. I see some brown drop low volt caps around there.

I am kind of tempted to just set it up on the bench and monitor it to see if that vert settles down.

then I need to decide if I should just stash it away for spares or do something with the cabinet. the ply base is starting to delaminate. It looks like it screws on to the wood frame rails. It would not be hard to make a new base. the sides are cheap fake looking wood that is all bunged up.

I have been wanting to try to build a new cabinet, this looks like a good candidate. I am thinking of using the frame to hand on some nice lauan ply.

I have some nice wood legs that have a danish modern look. the performance of these philco hybrids is very good. I have a nice metal cabinet model, but a pretty wood one would be nice too.

Very helpful post. It's given me a few things to test. I am working on a Philco 50-T1404 and had no HV (i've only tested tubes including crt and replaced the electrolytics). I pulled out my Sel-Sun test pic tube/yoke (removed the existing yoke plug which also disconnects the focus coil) and voila - had HV and a raster on the test tube. I'll ring out the focus coil and deflection coils directly tonight and see what I come up with. I'll also check my HOT cathode current.

Dave, Is your Philco 17MT80 one with the tuning eye tube? We had one for practice in HS Vo-tech that did. I do remember the cheapo PC-mounted gang pots with blue 1/4" hex shafts. My 17KT50 (19" heavywieght) has them also.

Somebody posted this set was $299 new as it was one of the last 21" color sets (1967)

yep thats the one, with the cheap pots. I remember seeing an add, touting how big the screen was, had some guy holding the set up.

Those PC mounted gang pots on Philco roundies were always a service issue. Not unusual to have to replace them late in their lives as daily watchers back into the early 1990's. The last one I restored had to have every one of them replaced around 4 years ago.

This thread sure helped with my Philco's blackened focus coil- thanks, I have one coming. Could have taken a long time to figure out.