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From: "strijw426@... [TekScopes]" <TekScopes@...>
Date: 02/12/2014 13:00 (GMT+01:00)
To: TekScopes@...
Subject: Re: [TekScopes] Zenith 36" problem!
Does changing the brightness also change the focus of the picture? If so the CRT may be conducting too much due to improper bias of the control and screen grids.
Vertical fold-over can also be an indication of power supply problems. So is inadequate horizontal deflection. The first place to look is in the LV power supply. Look for any high wattage resistors that may have fractured solder underneath on the PC board where they are connected. The same can happen on power regulation transistors and ICs. This is a common problem with both tube type and solid state power supplies. The unit could also have electrolytic capacitors that have dried out enough to cause problems too. Many sets produce from the late 1960s onward have special parts that are safety related items and must be replaced with specific part types in order to maintain compliance with fire and other safety issues.
Some sets also had HV regulation circuits that would kick the set out of horizontal frequency lock to keep the HV from rising too high (and consequently cause X-rays from the CRT and HV components). That makes the set unwatchable of course. You did not describe that type problem though.
I would start with the LV power supply. There are still sources for schematics on TV sets, so find the one appropriate for your set first. Working on one without the schematic is a stab in the dark. The schematic will tell you what voltages to expect and what waveforms to expect in each circuit if you get a good manufacturer's schematic or a Sam's schematic.
Joe
KC5LY
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