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Problems in a little PT2 - Printable Version

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PT2 tuning capacitor - elfiself - 01-02-2019

I have always assumed that the plates of the stator and rotor of a tuning capacitor should never be shorted. Now in looking for problems on a current PT2 project, I find the oscillator section of the tuning capacitor is shorted. After careful inspection and not locating plates in contact, I back checked a couple other sets I have on the bench and find that they all have shorted oscillator sections.
I must ask, what's up?

RE: PT2 tuning capacitor - gregb - 01-02-2019

It is in parallel with the oscillator coil so appears like it is shorted. You would have to disconnect the wires from it to confirm for sure if it is shorted. I have come across shorted tuning caps when folks try cleaning with steel wool, never ever use steel wool around them.

Gregb

RE: PT2 tuning capacitor - elfiself - 01-02-2019

Thanks Gregb
In fact when I disconnected from the coil it did not show short and suspected this was the case but I couldn't explain it to myself.
Larry

problems in a little PT2 - elfiself - 01-03-2019

lets start with 56.6k ohm value for secondary of 2nd IF transformer measured between 7C6 pin 5 (d1) and high side of volume control.
next I replaced the 40/80 ohm line resistor with 10w wire wounds. in the circuit the 40 measures good while the 80 measures 3.7 ohms.
thoughts please
Larry

RE: PT2 tuning capacitor - Radioroslyn - 01-03-2019

Wouldn't worry about the IF coil more than likely the 47K resistor has drifted some but is of little consequence. If I had it apart to replace the rubber wire I'd check and replace. But the 4 ohm/80 ohm resistor is an issue. 5w is ok total dissipation is 1.8w, I generally go X3. Dissipation is less for the 40 ohm (abt 1 w), it's job is to stop the lamp from burning out from the ac inrush.

RE: PT2 tuning capacitor - sam - 01-03-2019

threads merged 
http://philcoradio.com/phorum/showthread.php?tid=4586
II. Multiple threads on the same subject not allowed.


sam

Problems in a little PT2 - elfiself - 01-03-2019

lets start with 56.6k ohm value for secondary of 2nd IF transformer measured between 7C6 pin 5 (d1) and high side of volume control.
next I replaced the 40/80 ohm line resistor with 10w wire wounds. in the circuit the 40 measures good while the 80 measures 3.7 ohms.
thoughts please
Larry

RE: Problems in a little PT2 - Ron Ramirez - 01-03-2019

Threads merged (again).

Elfiself, please pay attention to Sam's post (#6 above). Thank you.

RE: Problems in a little PT2 - elfiself - 01-04-2019

Thanks for the reply Terry. I have replaced all resistors and caps including the 470k you mentioned in your reply. My concern is the secondary of the 2nd IF is indicated on schm. as 21ohms but is reading 56k ohms. Do you mean the IF transformer wiring could account for such a dramatic increase in resistance and if so should I consider rewiring it?
Larry

RE: Problems in a little PT2 - Radioroslyn - 01-04-2019

My thought is that are several parts INSIDE the 2nd IF transformer. Two fixed caps (probably 100mmfd ea) and a 47K resistor. The 47K is in series w/secondary of the transformer. So if you measure from the diode to the top of the volume control that 47K is in series it adds to the total resistance measured. Typically the coil would be abt 20 ohms or so but perhaps the 47K has drifted up in value.

RE: Problems in a little PT2 - elfiself - 01-04-2019

got it
thanks Terry
So I finally got some sound from the speaker,( found a miss placed 130 ohm resistor on the 50L6) unfortunately its a loud hum not affected by the volume control.
What would be the fun if it just worked?
Larry