Shadograph question - Printable Version
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Shadograph question - morzh - 11-27-2016
I have not a whole lot of experience with this, so:
The shadograph I have is the old type from 16 (same used in the 118 ), the one with a rectangular box with coil on top.
It has the original coil, according to Dee the guy from Alabama radio club who repaired vew wound things for me. Mine had some mechanical problems he could not tackle fast so meantime he gave me this one.
It is skewed. The shadow segment is closer to the left when looking at it.
And though reacting to tuning, the reaction is not very strong.
I the beginning the shadow is narrow like it should be. Then a voltage of about 4.5V develops across it indicating a current of 6mA (it is about 850 ohm and 4K res in parallel. Removing the resistor does not change anything much).
When tuned to the local station with me for an antenna (at half volume it can de-cone the speaker, it is so strong) the voltage goes down by 1V to 1.2V.
I changed the detector tube, the result is the same with voltage overall gone up about 0.5V and the shadowmeter behavior remaining largey the same.
So
1) are my currents across the shadowmeter looking correct?
2) could the weak magnet simply not pull the vane strongly enough if the current change is OK (the change is about 1.5mA)
RE: Shadograph question - Ron Ramirez - 11-27-2016
Have you checked alignment of the lamp in back of the shadowmeter? Its placement does have an effect upon the shadow and its action.
RE: Shadograph question - morzh - 11-27-2016
Yes. A bit, not much movement of the shadow has been achieved moving the lamp. Also with the current lamp position the lighted segment is very symmetrical and is dead in the middle of the screen, while if moved it goes left-right.
At this point I am more interested in the sensitivity and the current supplied by the input tubes.
I probably need to look at the AVC voltage from the detector tube 37. Don't know the exact range.
The AVC voltage reaches -4.5V, right where the 100K resistor from the detector, the #50, meets the volume pot and the other resistors.
RE: Shadograph question - morzh - 11-27-2016
http://www.antiqueradios.com/forums/viewtopic.php?f=6&t=225848
http://philcoradio.com/phorum/archive/index.php?thread-10630.html
Found this. Seems to be the same.
Could be this is normal and a large antenna is needed. But then the station is soooo strong, I doubt even with huge antenna I would get anywhere near for non-local stations. Otherwise, what, in a16 the shadowgraph will only work well with superstrong stations and huge antennas?
RE: Shadograph question - Radioroslyn - 12-01-2016
From what I can tell the fixed magnet is just for centering the vane. That's what seems to be doing on the the I just fixed. On the style the magnet is pretty small about 3/16x 3/4"x 1/8"thick. Without it the vane just wobbles around and won't align itself back to center.
Would think that increasing it would make it worst, would take more magnetizum from the coil to move the vane from it's center position.
RE: Shadograph question - Mondial - 12-01-2016
Sounds like your AVC voltage range might be a bit low if it only reaches -4.5V with a strong received signal.
I don't have a 16, but with a 118 with the same type shadow meter I routinely reach -12V on the detector output where the AVC is derived. This level is reached on local stations with a 10 foot wire antenna. At this AVC voltage the shadow reaches a minimum width of about 5 mm.
With only -4.5 V of AVC, the plate current of the IF tubes will not change much. They are of the remote cutoff type and it takes much more negative bias to reduce the plate current to the point where the vane will narrow the shadow.
I seem to remember my meter current changed from about 8 ma no signal to around 2 ma tuned to a strong station.
RE: Shadograph question - morzh - 12-01-2016
Yes it is a bit low but from reading several posts by people who also repaired 16s, this seems to be a popular situation.
Could be that I am not such a good antenna after all
Therdis no electrical reason, as far as I can see, for the AVC voltage to be incorrect, so just maybe a longer antenna will do the trick.