Quote:
Originally Posted by Electronic M
Resistors are usually among the more temperature sensitive parts. Usually the best way to find a thermal sensitive part is to run the set till symptoms manifest and hit sections with freeze spray (or something similar like canned air) to see what makes it clear up. Start with general coverage of suspicious areas and once you get a good reaction to the spray, target specific components in the area of sensitivity to narrow it down.
Looking at the schematic you attached earlier I'm not seeing the -1.8 volts grid or 60V plate specs on either section of the 6AN8 sound IF/sync separator tube...Are you referencing a different listing of voltages? Also on multi section tubes like that one it is good to specify which section's plate and which sections grid when you say one is not right so we know which stage to look at.
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Sorry about that, I meant V7B. Attaching a
close up of that part of the
schematic. I picked up a fresh can of duster and will try cooling off some components to see what might be heat affected.
A few discoveries I've found:
- By fiddling with all of the controls I can pretty much dial in most of the DC voltage measurements called out in the schematic (thought it only produces pretty much a bright white raster). The exceptions are the grid of V7B that measures -1.8V and no matter what I do the range is about -0.1 to -0.5. V11 (6CS6 SYNC SEP) pin 7 shows -10V and closest I can ever get it to is about -2V.
The scope trace shown for the grid is somewhat confusing to but I assume I'm looking for some kind of beat on a 60Hz pulse there? (See attached)
- By feeding in an incredibly strong signal (from the Blonder Tongue) I can get the plate voltage on V7B up to 60V, however at that point the signal is incredibly overloaded and has a negative image and poor sync (both horiz and vert)
- The scope traces on the AGC keying (V8A) look consistent with the traces on the schematic. I cannot measure the plate on it since the voltage is beyond the range of my scope and can't measure DC since those huge pulses confuse my DVM, however visually it seems to be correct. G1 (pin 8) measures pretty much the same as the plate of V7B as the schematic suggests (see attached)
- Adding -9V bias to the wiper of the AGC control has what would be the effect of the AGC control, and at that point the AGC control actually affects the picture. However, the control has no effect without it.
- Sound, contrast and sync continue to degrade as set warms (will try freezing components tonight)
At this point, is it possible that some of this could be alignment-related? The multiburst bandwidth test from the VA-62 looks pretty close to the trace from the manual (
see attached) and there are distinct vertical lines on screen up through 3.0MHz, so would that imply alignment is healthy?
Thx!!
FK