08-10-2014, 05:28 PM
Morzh has inspired me to revisit an old nemesis while I wait for Gary to reopen at Playthings so I can start getting serious about that 77. I put it aside as beyond my very limited skills a while back, but I've been reading my "Elements" and lurking here and at that other board. I had recapped, checked coils, etc. no luck. Voltages were never right, and rather than fix it until I killed it, I put it away (the cabinet turned out beautifully after some minor veneer fixes). I just ran voltages again, and my voltages are still off, but more informative to me, now (I think). The 80 was filaments=5v, plates=344 (all heater voltages were around 5.3v for all the rest of the tubes)-
42: plate=209, screen=215, grid=-6, cathode=1
6A7:plate=211, screen=75, anode grid=135, control grid=1.8 (positive voltage)
78: plate=210, screen=75, suppressor and cathode (pins 4 & 5 are connected)=1.6, grid=0
75: triode plate=149, diode plate=-80.6, cathode=.1, grid=-.5
http://www.nostalgiaair.org/PagesByModel...013827.pdf
The antenna coil and oscillator coils have good primaries and secondaries, and both IF towers measure with valid primary and secondary numbers. Tubes all test OK on my Triplett 2314. Seems to me all voltages are low to begin with, plates are 50v or so low. Grid voltages are preventing oscillation? So, I have a shorting/grounding issue that is sapping voltage, or some of those new capacitors need to be replaced again. The wave trap was shorted, but I jumpered around it. No open trimmers. I get resistance without shorts across the dial on the tuning cap.
I guess maybe doing some resistance math and point-to-point checks might be in order. Checking all grounded connections (I have some concerns about how C27, R47, R45, C46, R43, & C42 ground)?
I get a loud buzz touching the 75 grid cap--actually a kind of squeal, a bit less loud touching the cap of the 6A7, and a soft buzz touching the volume pot if that helps.
It's still more than my limited knowledge and skill can solve right now, but if anything jumps out at you, I'd appreciate the input.
42: plate=209, screen=215, grid=-6, cathode=1
6A7:plate=211, screen=75, anode grid=135, control grid=1.8 (positive voltage)
78: plate=210, screen=75, suppressor and cathode (pins 4 & 5 are connected)=1.6, grid=0
75: triode plate=149, diode plate=-80.6, cathode=.1, grid=-.5
http://www.nostalgiaair.org/PagesByModel...013827.pdf
The antenna coil and oscillator coils have good primaries and secondaries, and both IF towers measure with valid primary and secondary numbers. Tubes all test OK on my Triplett 2314. Seems to me all voltages are low to begin with, plates are 50v or so low. Grid voltages are preventing oscillation? So, I have a shorting/grounding issue that is sapping voltage, or some of those new capacitors need to be replaced again. The wave trap was shorted, but I jumpered around it. No open trimmers. I get resistance without shorts across the dial on the tuning cap.
I guess maybe doing some resistance math and point-to-point checks might be in order. Checking all grounded connections (I have some concerns about how C27, R47, R45, C46, R43, & C42 ground)?
I get a loud buzz touching the 75 grid cap--actually a kind of squeal, a bit less loud touching the cap of the 6A7, and a soft buzz touching the volume pot if that helps.
It's still more than my limited knowledge and skill can solve right now, but if anything jumps out at you, I'd appreciate the input.
