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 Post subject: Re: Looking for help with 25EC58 Chassis - won't power up
PostPosted: Mar Fri 06, 2020 10:00 pm 
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Joined: Jan Thu 16, 2020 4:13 pm
Posts: 9
The brightness control seems to have little if any effect on anything. this is with bright down and R G B G2 controls all the way down. There's no disappearing of any color going on. Contrast tint and color at mid-range. There are two service switches: normal/setup on the back, and normal/alignment inside. This is setup & alignment, but changing alignment to normal doesn't change anything here.
I really do appreciate the help. Sorry about the window glare in the pic. The black is black, which is encouraging.
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 Post subject: Re: Looking for help with 25EC58 Chassis - won't power up
PostPosted: Mar Fri 06, 2020 10:32 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 1264
Location: Pewaukee, WI
Don't leave that bright line on screen too long you can permanently burn the line into the screen that way.... which is bad.

The brightness control and G2s should always be capable of blacking the screen out completely. That they can't most likely means the CRT grid cathode bias is wrong (I suppose excessive HV might be capable of doing similar but if that were the case you'd probably notice bad hissing and crackling that never stops). You'll need to delve into the brightness circuit/ CRT cathode and G1 voltages......I think this set may have a luminance video output transistor that can affect gun bias if it fails.
I've had the color Video output transistors fail in a hybrid Zenith 3 years older and it had similar effects.


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 Post subject: Re: Looking for help with 25EC58 Chassis - won't power up
PostPosted: Mar Fri 06, 2020 11:58 pm 
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Joined: Jan Thu 16, 2020 4:13 pm
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Reading what I can from the schematics you posted previously, Q201 is in the brightness circuit. I can't make out what's printed on the collector line, maybe 3.3V?? I measured, the collector of Q201 is 4.38VDC. With the Bright Limit pot just above it I can adjust that collector from 4.05 to 4.4VDC. There's more room on the pot after it hits 4.4, but the voltage doesn't go over that. 4.05 is the other limit of the adjustment. If it's supposed to be 3.3V, it's too high for some reason upstream from Q201, or Q201 is bad. I have assorted transistors if there are common replacement part numbers. I'll start to look into that.
I measured a couple other voltages, not sure if they're in the same circuit:
* Q203 Collector is 15VDC,
* Q217 collector is 5.58VDC.
* The 'B+ 125V' signal is 132VDC.
* Both the '24V' signal and the 'AFC B+' are at 23VDC.
That all seems reasonable to me but I don't know how sensitive the tolerances are.

Update: Measured Q201. Don't know what the beta should be, but Vbe is ~ 0.7, so probably OK. Google didn't seem to know anything about the part number - Motorola 695-412.
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 Post subject: Re: Looking for help with 25EC58 Chassis - won't power up
PostPosted: Mar Sat 07, 2020 4:47 pm 
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Joined: Apr Sat 06, 2013 2:18 pm
Posts: 531
Location: New Hampshire
First be sure when you changed the caps you didnt move a part & short it
against another.
You will probably find the cathodes very low. Voltage for them comes from CR221 ( 240 VDC).
Also if the video fails it will do it. Q204 shorts on some older chassii. If in service position
the video is eliminated. If the lines can be turned down you probably have a video problem.
The normal-align SW on top of chassis is just for APC & ACC so ignore it for now.
BTW Zenith transistors are marked 121=### or 121-####. Say you have a hoz output
& it just says 831 on it. Its a 121-831. You can look up those numbers in ECG, TCG, NTE etc.

Zeno


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