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Old 1st Dec 2019, 1:13 pm   #39
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
Posts: 2,679
Default Re: RCA AR88D Comms Receiver.

Quote:
Originally Posted by allan View Post
None of the coils uses a dust core. Changing the HT voltage from 150 to 300 hardly makes any difference. Clearly another case of nonoscillatoryitis.
Allan G3PIY
Yes, its a little hard to blame the dust iron cores when there are none.

Obviously to sustain oscillations there has to be enough + feedback and enough gain.

One wonders what could have changed over the years. In the absence of dust iron or ferrite to blame then;

Assuming:

1) the valve is good and not gassy (all of our valves are slowly letting in gases you know).

2) the resistors are fine.

3) the capacitors are ok

4)the applied voltages (HT & heater) are normal.

5) the Q of the tank circuit is normal and its not losing excessive energy.

.... then it must either be some losses in the dielectric/insulation materials of the supporting tags/sockets have increased, or some other pathway conducting signal has developed which is phase canceling some of the + feedback or absorbing energy somewhere.

Thinking along the line of dielectric losses it is known to happen with some types of wax impregnated coils.

Of course if the permittivity Ko and permeability Uo of space had altered since the radio was built, we would know by now because the speed of light would have altered since it equal to root (1/KoUo), fouling up everything, so it cannot be that, joking of course !
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