PRECAUTION 1 - Insert a 1/2A fuse in series with the B+ feed from the 5U4 filament in sets using a single 5U4. Use a 1A fuse for applications where a pair of 5U4 tubes are operating in parallel. This will help protect the set's power transformer and other costly parts in the event of an overload at some time after the set has been restored.

PRECAUTION 2 - If the manufacturer did not fuse the set's B+ Boost as part of the original design, insert a 1/4A fuse between the damper cathode and the junction of the Horiz Lin coil and first B+ Boost filter capacitor. GE models 800C, 810 and other similar models did not include a B+ Boost fuse. RCA, Fada, Dumont and many others included a B+ Boost fuse in nearly all models. Fusing the B+ Boost will help prevent a blown flyback in the event that the Horiz Output, Vert Output, or Damper tube becomes "gassy" or in the event that severe HV arcing or some other such condition should occur.

PRECAUTION 3 - A slow-blow fuse rated the next standard rating above the normal line current specified in the set's Sams folder should be added in series with the set's AC line circuit. For example, GE model 810 draws 2.2A and therefore should have its AC line circuit protected by a 2.5A slow-blow fuse.

Clip-syle fuse holders for 1/4x1-1/4 standard glass fuses can be concealed under the chassis on most larger chassis. If you're drilling a new mounting hole to attach a fuse clip via a screw or rivet, verify that the area on both sides of the hole is free of parts that might be at risk to be inadvertently drilled into. Use the old "piece of wood on the drill bit" trick to limit the drilling depth to about 1/4" just to be doubly safe.

Where space is critical (compact sets like GE 800C and similar), subminiature PicoFuse devices (about the size of a 1/4W carbon film resistor) can be used. I always keep a supply of AES #P-0100H single-lug, no-common terminals on hand in case no existing unused tie point is available under the chassis. Most sets' damper and rectifier tube sockets do have one or more unused pins available for use as a tie point, however. When in doubt as to whether an unused pin is actually a non-connected pin inside the tube, consult a tube manual. RCA and Sylvania tube manuals generally specify which unused pins should and should not be used as tie points in the specifications for each tube in the full spec sheets. Condensed tube data tables often provided for older types may not include such detailed data. Most often, I find it necessary to add a tie point only for PRECAUTION 3 in sets where no unused tie points exist near the on/off switch or interlock. Carefully drilling out one of the interlock mounting rivets and adding a P-0100 positioned so that the lug is away from the interlock is a good technique. Re-secure the interlock and P-0100 to the chassis with a Pop rivet of the proper size, move one of the interlock wires to the new lug, and join the two points with the PicoFuse or attach lengths of hook-up wire to connect to the terminal lugs of a fuse holder.

RCA 721TS, for example, has PRECAUTION 2 factory-installed. Always verify that this fuse is present and has not been subbed with one larger than 1/4A during some prior repair. There is a metal support under the chassis between the power transformer and rectifier socket. The support is secured by 2 screws at the top and 2 more along the side. A pair of fuse clips for PRECAUTION 1 and PRECAUTION 3 can be easily mounted to this support if it is removed from the chassis and then re-installed with the fuse holders attached to it. Mounting the fuses under the chassis is a good reminder to troubleshoot the cause of the blown fuse if it becomes necessary to replace one of the fuses. Fuses don't just blow for no reason, so it's a good idea to place the fuse in a location where the chassis must be removed in order to reach it.

QUICKIE QUIZ - An RCA 721TS has been restored, B+ Boost fuse F2 is confirmed present and of the correct rating. Fuses F1 and F3 are added per PRECAUTION 1 and PRECAUTION 3 instructions. After several years of being operated at least once per week, the set is left unused for about a year and produces no raster or sound at next power-up, although the tube heaters light when power is applied. After pulling the chassis, a blown fuse is found. Which fuse was blown? What is the most likely trouble?

So very right, and very much needed. I could not agree more.


See you at the convention.

precaution 1 and a bad lytic.
or a shorted 5u4

I agree, these fuse suggestions are very good. They can prevent a lot of costly damage.

I always add a fuse in the powerline of any old device that did not come with one: TV's, radios, stereos, reel to reels, phonos, etc. Many of the consumer devices especially from the 40's and 50's did not have one. The fuse has saved a lot of damage in cases where a power supply diode has shorted.

Also I use a 300 ma fuse or a 6.3 volt 250 ma lamp bulb in the cathode circuit of the color set horizontal output. The lamp allows you to adjust the current just by watching the brightness.

Also I use a 300 ma fuse or a 6.3 volt 250 ma lamp bulb in the cathode circuit of the color set horizontal output. The lamp allows you to adjust the current just by watching the brightness. I think we might have studied the same TAB Books color tv service book from 1968 at one point. I use a plug-in version of the "#44 lamp as plate-current indicator" to find the dip that marks the proper setting of the Horiz. Efficiency coil also. My version uses a mini bayonet lamp socket insulated with shrink tubing. One lead has a plate cap socket salvaged from a dud flyback, the other has a plate cap taken from a dud 6CB5 or similar output tube. If I'm reasonably sure I've fixed all problems in a Horiz Sweep circuit, I insert this device in series with the plate cap for the first post-service power-up just in case I might have missed something and leave it there during the Horiz. Efficiency coil adjustment. One of these days, I hope to find some spare time to make up a "bias probe" device to insert between 6BK4 or similar HV Reg. tube and socket to provide easily-accessible metering connections during regulator current adjustment. I was 8 years old when I did my first paid radio repair, so I've been at this 30 years now between working for tv, vcr and audio shops, hobby work, and "serial temp jobs" in contract engineering and industrial maintenance. My high school social life, a collection of LP's and CD's, audio gear, and the guitars I played in several "nameless garage bands" were all financed by repairing tv sets independently and/or for local shops. My interest in vintage electronics started around age 12 when my grandfather found an antique radio in the attic of a house he was restoring, although I'd been working with tubes in tv and via an interest in guitar for a couple of years by then.