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#1
02-24-2017, 05:25 PM
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I do have a vexing issue still with the AGC that I don't understand. Originally I had no AGC voltage at all. After changing the two AGC diodes and both gate and amp transistors I do have voltage of 3-6 volts depending on AGC control setting. BUT there's an 8 volt zener that spills voltage into the tuner AGC line and I never get voltage that high. A high signal input from the test equip. increases voltage from 5.5 to 6 volts, that's it. BUT the schematic calls for around 5.5 normal with signal so I'm not sure what's right. The service manual for similar models says I should be able to get 2-14 volts....
Pulses controlling the AGC gate from the coil on the new flyback are weak on the positive side through the diode but strong on the negative side.... Last edited by Hagstar; 02-24-2017 at 07:49 PM. 
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#2
02-24-2017, 06:20 PM
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It's hard to read the schematic well enough to tell, but two thoughts come to mind: 1.) the zener may be meant to keep the tuner signal under 8 volts in the event of AGC failure to protect the tuner. 2.) they are using the fixed voltage drop off the zener to increase the DC voltage level going to the tuner relative to the IF AGC bus.
Once you start looking at SS gear schematics you start to find all sorts of clever ways to take advantage of the new properties of SS devices.
__________________
Tom C. Zenith: The quality stays in EVEN after the name falls off! What I want. --> http://www.videokarma.org/showpost.p...62&postcount=4
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#3
02-24-2017, 08:03 PM
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No the zener serves a delay function to prevent any real AGC voltage on the tuner until signal strength gets high per the manual.
Another clue- if I add a 9 volt battery to the pulse gate terminal that brings more voltage into the gate transistor collector and tap the connection I can see it shoot up to 10 or more volts. The HOT winding is so simple but for some reason I'm only getting a pulse of a few volts here instead of the 30-35 expected. Last edited by Hagstar; 02-24-2017 at 08:44 PM.
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#4
02-24-2017, 08:08 PM
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my dad bought a philco roundie back in 1964-65.sun tv and spanky mcfarland was there to help promote the products.philco ford with the blue tuning bars.set had a great picture,actually as good as the zeniths and rcas.we had that set for years,with very little trouble.sold it to a neighbor and they had it for years.i hit the crt a few times and it would hold up for about a year before it would fade.never another problem.the chassis ran on original everything for almost 25 years
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