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 Fusing outputs in radios and TV's

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Join date : 2010-11-22

Fusing outputs in radios and TV's Empty
PostSubject: Fusing outputs in radios and TV's   Fusing outputs in radios and TV's Icon_minitimeFebruary 17th 2019, 9:55 pm

A recent post reminded me of this fact. 
There is merit in this fusing, as it can save other damage from occurring.

As one fuse mfgr. stated in their advertising:
 "A Fuse is Electricity's Safety Valve".
 (I can't remember it it was Buss or Littlefuse that advertised this.)

 Many of the TV mfgr's did that very thing in the cathode of the HO tube. (Horizontal Output) When the tube or the osc. failed, the fuse would blow, saving further damage, especially to a FBT.

 I remember some tubes would just fail in normal usage, almost immediately.
 (Remembering the CASES of Japanese made Raytheon 6JE6's that the wholesaler had returned back in the early 70's. Holes literally melted thru the glass on the side).

In TV sets, it recommended that when you had a HO tube failure, the oscillator, damper and HO tube be replaced as a set.

I remember a very short lived job at one shop where the owner never adjusted the Horiz efficiency coil. He'd replace a CRT in a set, then when I asked why he didn't do the adjustment, he told me "It would guarantee another repair."  And it did. The HO tube and the FBT would fail within a month.  After finding out his greedy and unethical repair policy, I was  out of there. Often the poor owner of the set had more into a new CRT, and FBT repair than a new set cost! Twisted Evil Twisted Evil Twisted Evil

The Horizontal efficiency coil adjusted the cathode current thru the HO tube, so adjustment was critical. It was not unusual to have one of the coils burn up when the HO tube failed, or was really misadjusted. 

Pomona made test sockets to plug into the HO tube socket to measure cathode current, using an external meter, (often on your HV probe), and SECO actually made a tester unit that had the meter built into it. Dipping the cathode was as important as dipping the cathode current in the final of your Ham rig. I remember that many early color TV's had fuses in the cathode circuit as well. They were of the leaded solder in type, slow blow, IIRC. A snap in type fuse holder could develop a poor connection and wreak havoc with the HO section.

I saw similar testers available for audio output tubes. BIG BUX!

 The old SECO units are dirt cheap at the hamfairs. and I see no reason one of them could not be modified and used for this sort of audio output testing.
 Originally when I started out I made my own test socket for specific types of Octal tubes, by using an old 8 pin tube base and socket, then running straight thru wires for all pins except the cathode, which I brought out through a hole in the side of the socket. I could clip a meter on them and easily measure the current. Best of all, it cost me nothing but a few minutes to make.
 Later on the commercially made ones were better to go with, as finding Noval, Novar, and Compactron tube bases was almost impossible.

So by fusing the cathode circuit you can save yourself some problems,  be it in a radio or TV set. 
One word of warning: Check any fuse you intend to use for resistance value. Even a few ohms in a cathode circuit can drastically change the bias current. I have seen brand new slo-blow types and even standard fast blow types have high resistances.
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