I'm happy to report that the new doorknob cap did the trick. After replacing an out-of-spec resistor in series with the vertical hold I finally achieved a stable picture!
right away, other issues became apparent. As far as picture quality is concerned, it's pretty...meh...watchable by far, but it's out of focus in appearance (the drawn image, that is. the electron beam itself is pretty crisp) and any text around the size of a nickle or smaller is hard/impossible to read - there's a bit of ghosting/smudging as well. Contrast is not that great, even when its maxed out and brightness is adequate, but very washed out and blooms a bit when turned all the way up.
I guess I should pause here and clarify that yes, I've tested the picture tube. emissions are well into the good. life test is iffy - the needle does fall into the "?" range if you hold the button long enough. I know its never going to be as nice and bright as a NOS aluminized tube, but I'm still pretty convinced its not achieving its full potential (too bad I can't just give it a motivational speech)
there are several parts of this set that appear to be modified, and other parts look original but aren't that way in the SAMs, most of which I mentioned earlier in this thread, but at the time I didn't pay much attention to it as I was too busy trying to get the vertical oscillator to work.
to reiterate: it appears that someone connected the free end of the contrast pot to the 210V bus through a 220K resistor. also connected to that side of the resistor is a long wire that runs up and over to pin 5 of the 6AL5 video det - agc rect tube. the SAMs shows it connected to pin 7 of the 3rd video IF amp 6AU6, which, from there has a 5000pF cap and 150ohm resistor paralleled to ground. The schematic calls for pin 5 of the 6AL5 to be around -.2v... it currently measures anywhere from +2v up to almost +5v with the contrast turned all the way down. The end of the wire connected to the contrast pot appears hacked in, but the other end, as well as the long wire to the 6AL5 and the 220k resistor all look original. turning the contrast knob also affects the sync as well. I tried hooking the long wire to the 6AU6 like the SAMs calls for, but noticeable change was observed.
The brightness control is wired differently as well. the SAMs calls for one end to be connected to the 210v bus. instead, its connected to the higher 260v bus (yet again, appearing to be original). according to Grob, brightness control works via cathode biasing, and the lower the cathode voltage, the brighter the picture...would switching it to the 210v bus brighten it up?
other issues: the picture also appears very stretched horizontally, but unfortunately both the horizontal width and linearity controls have no effect whatsoever. both are coils with screw/ferrite slugs mounted inside the HV cage, and both ohm out to the correct resistance.
lastly, there's an accross-the-board voltage anomaly...all of the voltages, including the one directly off the plates of the 5U4, are a good 30% higher than what they should be...and that's on 117v line voltage...even the HV cup is up into the 14-15KV range when the SAMs only calls for 10.35KV...perhaps with the primary side of the audio output transformer being open (I've have yet to track down a replacement) could it not be drawing enough current to bring the voltage down? or have I discovered free energy?
11-19-2016, 10:30 AM
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