Re: 7603 P.S. cans, slow warm-up and focus control

Mark Vincent


I already checked the crt using my 752 tube tester with the settings to a TV B/W crt. It comes up normally and tests fine. I tested other Tek. crts that come up faster. The readings were close in emission and time to come up. I know TV repair very well and miss working on them. Before I posted my question, I already checked things. The only original parts not changed yet are the input filter cans. The line on the screen does have intensity modulation moving from right to left with no signal. When the time base is set a line trigger, the modulation movement stops. That is what makes me suspect the original filter cans are going bad. The electrolytics at and beyond the regulators are new with many higher in capacitance. I already checked the high voltage and Z sections. Those voltages are right.

Why the focus pot burned is that 150V across 50,000 ohms is ,45W dissipation with the control being ,5W. That is 90% of the capacity causing a temperature rise of 225C above ambient. I have had other pots in other scopes and plug-ins for 500 series being run this hard or over the power rating of the controls. I added resistors on each side to reduce the power dissipation. These controls are the horizontal and vertical position controls. I still have enough control range after the addition of resistors to reduce power. The Radiotron Designer's Handbook says resistors not be run in power over 1/2 its power dissipation plus ambient temperature in the calculation. Running a resistor at its power rating causes the resistor to raise 250 deg. C above ambient. By what the book says, ambient plus the dissipation should not be over 125 deg. C. I assume 40-50C ambient in operation leaving a rise of 75-85C. This is a third of the 250C so multiplying the power being dissipated by 3 1/3 gives the power rating of the resistor that should be used. Going to 4x larger gives headroom. If the power at 4x is, say 2.2W, then a 2W is used. An example of a underrated power resistor was in an RCA CTC36/7. The 600ohm 18W under the chassis under the high voltage cage was a known problem. It would burn the wire insulation, become open circuit, burn the red fiberglass terminal strip causing that strip to be replaced. A pair of 1200 ohm 25W in parallel is what I used. Now using a transistor or two with resistors and zeners will drop the voltage (series pass) in these types of places. I have done this in a Zenith 14DC15/16. It works fine. I will restuff the cans soon to complete the restoration. The 15NP22 crt was replaced with new and may have five hours on it now.

Zen, thanks for the note on the two connectors to not switch. I will make sure they are put back right before powering on after the work is done. I still think the cans going bad are the problem that soon will be confirmed when replaced. I want to see if anyone had seen cans going bad in this series causing this problem. Usually when the filters go bad, it is obvious.

Thanks for the replies and advice. I appreciate it.


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