1966 Magnaxox combo flyback

[freakaftr8]

Hey guys, well I checked the emission of all the tubes in this set tonight. The tv is a 8/66 model. The chassis is a U9109 BB Chassis. I had a visual on the Flyback the other night before I powered it up. It looked normal with a small puddle of wax under it. I had it powered on a variac and the set came up. ran it for about an hour. I failed to check the 3A3. Well, it started to act up, the horiz on the left started to shrink and the raster got dimmer, kind of like a HV drop. Well I pulled the cover on the Flyback tonight, there is a build up of wax going up to the flyback from the base of the HV cage, it obviously is getting hot, the 3A3 read into the question mark area, thus it is bad.. Could this bad 3A3 cause the flyback to overheat? Or would this be a capacitor issue? Im stumped, I dont even want to power it back on to check Anode voltage before I can verify what caused the flyback to overheat...

[andy]

You can't reliably check HV rectifiers with a tube tester. It's probably fine. I would check all the caps and resistors in the horizontal output grid drive circuit. Also, check the HOT cathode current. I've found that a bad cap here and a bad resistor there can significantly increase the HOT current. Also, check the HV regulator circuit since it can put excessive load on the HV. It won't hurt to run it for a little while to make tests.

[freakaftr8]

Well im going to recap the whole set first, noticed this is always a safe operation to rule out faulty voltage issues... Then I will go from there.. Im going to have to get a sams for this set, does anyone have one? Oh and do you think the flyback is still okay?

[freakaftr8]

Hey guys, which pin number is it that I unclip and put a meter on to check Horiz Oscillator cathode current? I heard of the efficincy coil and Im sure I can find it on this old set.

[drh4683]

Disconnect the wire from pin 3 to ground of the horizontal output and put your mA meter in series (im assuming its a 6JE6 hot.) Most color sets of that era used this tube. If the cathode current is high (over 210mA, id start to be concerned) check the grid bias on the horiz. output in either case. Should be somewhere around -50 to -60 vdc. Again, if its the 6JE6, you can check this grid voltage at either pin 2 or 6, as they are internally connected to the grid of the tube itself. If the voltage is too low like -20 volts then check the oscillator, thats the problem, its letting the tube over conduct and resulting in the overheated fly. However, I think its common for magnavox flybacks to burn up like the rcas. Moyer has a few still.

Regarding the efficiancy coil, I think the magnavox sets used the coil mounted on the pc board like RCA. If its a high hour set then 9 times out of 10 the slug in the coil is frozen from heat and excessive force with the diddle stick will snap the coil tube in half! the coil tube is probably very brittle. Be very careful! If the cathode current is OK, (around 180mA) then its best not to even touch the coil. Ive learned the hard way..........

[Randy Bassham]

A lot of the Magnavox chassis have the cathode lead soldered to a ground lug right by the tube socket where it's easy to get to you don't have to remove the chassis, just unsolder it and put your MA meter in series.

[andy]

I sometimes add my own test point to the chassis. All you need to do is insert a 1 ohm resistor in parallel with a .1 uF cap between the cathode and chassis. Then just measure the cathode voltage to read the current in A.

[freakaftr8]

Well under the chassis there on the far left, there seems to be some coil switch with contacts. Across it is a 25v 100mfd cap.. Shot, so esr at all as well as a small can cap behind the fbt, and that has no esr reading, acts like it's open. The rest of the 80mfd 450v caps read high, probably melting the flyback. This set is ready for a recap, ill post pics once it's up! Hopefully I dont need a flyback, If I do, does anyone know the model of the flyback, wouldnt hurt to get a spare.

[Chad Hauris]

Andy, did you measure that voltage across the new test resistor with a digital multimeter?
I tried using a digital multimeter in the milliammeter mode to measure current directly in the cathode circuit (without the resistor) and connected the .1 cap across it but the reading would fluctuate all over the place.
The last couple of times I did it, I used a 6 volt light bulb in the cathode and adjusted the efficiency coil for minimum brightness. Would be nice to get a good reading on a digital meter though.

[andy]

My digital meter had no trouble giving a stable reading.