Re-coating flybacks, what do you use?

[jpdylon]

I'm working on a porthole that has a crusty flyback. The set is in perfect working shape, I just think the fly should be re-coated so that the windings are not exposed.

So what do you use? and where is a good place to grab it? Is there a good way to apply it?

Thanks in advance.

[andy]

...

[Dave A]

I use some liquid stuff called Star Brite Liquid Electrical Tape. Found it at Home Depot. Just brush on and it dries in about 5 minutes. It claims to form a "dielectric coating". Seems to do the trick.

Dave A

[Eric H]

If you use Silicone get the low acid type, (doesn't smell like vinegar)
I used it on my CTC7 and it worked fine.

[Tubejunke]

Tonight I coated one that had oveheated and dewaxed itself with" Rubberize IT". That stuff really looks good. I just hope a capacitor was the problem and not the fly.....

[jshorva65]

Silicone RTV typically won't require as much thickness as the original wax due to the much greater dielectric strength of TV Service grade silicone RTV specifically intended to be used as HV insulation. About 1/16" thickness should withstand 10kV. After all, that stuff is intended for use with modern 27" to 35" color tubes, projection sets, computer monitors, etc. and their anode supplies of up to 30kV or more.

Now that I have a set of transparencies and an instruction manual (an eBay find) on the way for the B&K 1076 that I picked up at a garage sale a few years back, I've removed the old, cracked and partially-missing wax from its still-working flyback and a thin coat of HV-rated silicone will be replacing the thick wax, with extra thickness around where the 1X2 plate cap lead exits to maintain mechanical strength.

[hellhound94]

So.....for people like me who don't know what some of these terms mean....what is a "porthole" and what is "crusty flyback?"

[jpdylon]

A porthole was a line of round screen zeniths from the late 40s to early 50s that had a mask around the picture tube that looked like the porthole of a ship. They are getting more rare by the day and are desirable if working properly.

The crusty flyback refers to what happens to the high voltage flyback transformer after years and years. The insulation coating covering the exposed windings has dried up and flaked off, leaving a crusty appearance. Leaving the windings exposed can lead to high voltage arcing and corona discharge, so its best to cover them.

[hellhound94]

Thanks, jpdylon, for taking the time to explain the terminology. And, in such a manner, that even I can understand it!

[Tubejunke]

My coating with "Rubberize-It" worked very well although I feel that I may have coated a bad flyback. Basically it is getting very hot. I guess it could be a capacitor issue like most things, but so far I have not found one that is shorted.

[jpdylon]

check voltages on the damper and Ho circuits. Incorrect bias voltage on the Ho tube will also cook flys in a hurry. If this is a color set, HO cathode current is critical to the life of the fly back.

Pull the HO and damper tube and check voltages compared to a sams or schematic. A sams or service manual would also tell you the resistance measurements of the fly windings

[Tubejunke]

The set is about a 1955 21" black and white. Thanks a lot for the advice! Do you mean to pull both tubes and check the voltages, or one at a time? My knowledge base usually leads me to simply looking for components that are out of tollerance. Mostly resistors and capacitors. I sure would like for your suggestion to be the trouble and maybe the fly is ok, just overheating due to some other fault.

This set is one of those vertical chassis jobs where ALL of the circuitry is kind of blocked from reach due to the CRT. The whole crt and chassis comes out as one unit. The chassis is mounted around the crt neck. Control knobs on top.

[jpdylon]

pull both tubes and check voltages. Compare them to a sams or schematic. Just use small probes, you wouldn't want to short the voltage running through those components

If the coupling caps in this set have not been replaced, do so now. It will save you headaches trying to troubleshoot problems like this. i like to bring a set up to snuff before trying to troubleshoot. then at least I have many less problems like dried up caps, to worry about.

unfortunately you need to pull the whole unit from the cabinet to do these tests. Yeah, it sucks, but its a big help when trying to troubleshoot the wire side of a chassis.

[jshorva65]

When I restore a B/W set, I typically add a 1/4A (250mA) fast-acting fuse to the B+ Boost output of the damper circuit. I typically consult a tube manual to determine which pins on the damper socket may be used as tie points and move all connections from the tube's cathode pin to some unused "tie point" pin on the tube socket, and connect the fuse between the tube's cathode pin and the new location of all the leads that had previously been connected there. Since the B+ Boost is fed back to the Horizontal Output plate circuit (using the RCA 721TS as an example, notice that B+ Boost voltage is fed into pin 1 of the flyback transformer, the B+ terminal of the HO plate winding), any excessive current demand by the HO stage will cause a proportional increase in damper cathode current and will normally blow the fuse and stop the HO circuit from operating before the overload can destroy the flyback. I use a "pigtail" fuse (insulated against accidental contact with nearby components by a short piece of clear PVC shrink tubing slipped over the fuse) in most sets, or a PicoFuse unit (about the size of a 1/4 watt resistor) in ultra-compact chassis sets such as the GE 800C (aka "The Locomotive" due to the unique profile of its Bakelite cabinet which resembles the shape of the "coal car" of an old steam train) where space is critical.

With color sets, I typically disconnect all leads from the HO cathode and insert a 3/10A (300mA) fuse between HO cathode and its original return point. Color sets typically draw slightly more HO cathode current than B/W sets and use output tubes which are rated for slightly higher plate dissipation. Again, excessive HO cathode current will blow the fuse and stop the circuit from operating before the flyback is destroyed.

If the set hasn't been recapped, thoroughly inspect the two B+ Boost filter capacitors, which are normally 0.03 to 0.1uF at 1kV (0.05uF at 1,000V is a frequently used value) paper capacitors connected at opposite ends of the horizontal linearity coil which bypass the 15.75kHz ripple component of B+ Boost voltage to some "AC virtual ground" chosen by the set's design engineers, which is usually the set's B- bus. The term "virtual AC ground" accounts for the fact that some engineers chose to use a DC supply bus (a point in the circuit where only a filtered DC voltage exists with no superimposed AC component) as the return for the B+ Boost filter. This point is often a negative voltage supply bus and usually the same point to which the HO cathode circuit returns. In the RCA 721TS, for example, the B+ Boost filter capacitors are C71 and C72 (0.035uF at 600V and 0.1uF at 1,000V respectively) and the filter circuit's return is the -85VDC source bus. If these capacitors are badly discolored or the wax shows signs of having partially melted, replace them as the paper dielectric has absorbed an excessive amount of moisture and they are passing excessive DC leakage current and thus acting more like resistors shunted across the B+ Boost supply than as filter capacitors. Such a condition will overload the HO stage and likely destroy the flyback transformer if the trouble is not corrected. When replacing these capacitors, I normally use SBE "Orange Drop" units matching the original capacitance and rated for 1600VDC. Many sets will also have a cathode bias resistor shunted by a bypass capacitor in series with the HO cathode return path. Using the RCA 721TS as an example, R85 and C69 are 82 ohms 1 watt and 0.25uF at 200V respectively. If the bypass capacitor is passing excessive DC leakage current, this will reduce the HO tube's cathode bias. Reduced cathode bias will increase HO tube plate current and overload the circuit, eventually damaging the HO tube and flyback.

Also, NEVER attempt to directly measure voltage or observe waveforms at the HO tube plate. That would be a sure way to damage your meter or scope and to overload the set's circuitry. There is about 6kV of pulsating DC present at the plate of the HO tube, more than sufficient to fry expensive test gear. You'll notice the plate terminals of the HO and HV Rectifier tubes are marked "DO NOT MEASURE" on virtually all television set schematics, and the extremely high-amplitude waveforms found there are the reason for that. When measuring HO tube cathode, control grid, and screen grid voltages or measuring voltages in the damper tube circuit, always use a VTVM or similar high-impedance (10 megohms input impedance) electronic voltmeter since a standard 20,000 ohms-per-volt meter tends to load these circuits and can cause erroneous readings or disrupt critical circuit waveforms. When observing waveforms in vintage tube gear, I typically use a scope with a vacuum tube input stage (a carefully-restored and frequently-inspected Tektronix Type 547 with a Type 1A1 or Type CA input module is my scope of choice, usually with a frequency compensated 10:1 probe for minimal circuit loading). For the newbies, the 547 is NOT a portable scope like the Tek 465 and other transistorized instruments. The 547 is about a 90 pound tube-transistor hybrid instrument of late-1950s vintage which uses plug-in input modules such as the Type 1A1 (DC to 50 MHz dual trace), Type 1A4 (DC to 50[?] MHz quad trace), Type CA (DC to 24 MHz dual trace), Type 1L20 (spectrum analyzer) and various other "53/54-series compatible" modules, are extremely versatile and, when properly maintained, have an exceptionally long useful life expectancy. When restoring vintage electronics, ALWAYS have a copy of the set's service literature (Sams Photofact or manufacturer's service manual) handy and check the tube electrode voltage and resistance charts. These statements are "obvious" to the experienced members here, but might not be so obvious to newbies.

[Chad Hauris]

Just replace ALL the paper and electrolytic caps. If it has original caps they will get worse! and especially if the chassis is hard to service you only want to do this once.
The fuses are a very good idea in the horiz. circuit...also make sure there is a fuse on the 110 volt AC line input.