Front end variable Capacitor Function? - Printable Version
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Front end variable Capacitor Function? (/showthread.php?tid=8105)
+- The PHILCO Phorum (https://philcoradio.com/phorum)
+-- Forum: Philco Radio Discussions (https://philcoradio.com/phorum/forumdisplay.php?fid=5)
+--- Forum: Philco Electronic Restoration (https://philcoradio.com/phorum/forumdisplay.php?fid=8)
+--- Thread: Front end variable Capacitor Function? (/showthread.php?tid=8105)
Front end variable Capacitor Function? - hsusmann - 12-08-2013
Hello,
I just bought a model 86 and had posted a request in the cabinet section a few days ago. I got looking at the schematic for this one and there is a strange circuit at the front end which I've never seen before. The schematic shows some kind of variable cap with a line going into the side of the variable cap (?!?).
When I look at the variable cap, I see that when I turn the knob fully CCW the rotor plates hit a tab of metal that acts as a switch I think.
I have not taken apart that section of the radio yet to have a closer look but was wondering if someone might have some info on this setup.
The schematic is a bit confusing on this.
Have started re-capping the chassis, also tested the tubes, found one weak 226 and a very bad 71A output tube. Rest are good. Par for the course...
I should post a picture of my collection of 5 PT-6 Radios that I've collected over the years in case anyone is interested.
The PT-6 was the first old radio I ever bought back in the mid-70's for $5.00 at a garage sale when I was about 15 yrs old. Ever since I've been snagging them when I see them. Great looking radios!
Herb S., Ithaca NY, new member to the Phorum
RE: Front end variable Capacitor Function? - Bob Andersen - 12-10-2013
What cap are you referring to on this schematic ? http://www.philcoradio.com/tech/images/86.jpg
RE: Front end variable Capacitor Function? - morzh - 12-10-2013
I think he means position #4 cap.
RE: Front end variable Capacitor Function? - hsusmann - 12-10-2013
Bob, that would be #4 on the grid of the first RF amp.
Thanks,
Herb S.
RE: Front end variable Capacitor Function? - Ron Ramirez - 12-10-2013
He appears to be referring to adjustable capacitor (4) on the schematic.
That capacitor serves two functions. In normal use, it is a fine tuning control that helps you tune in stations with the greatest volume.
On very strong local stations, it is also designed to be turned until it clicks, at which point the volume drops appreciably by shorting the grid of the first RF tube to ground, reducing the set's gain considerably. So it is not only a fine tuning control, but also a crude local-distance switch.
RE: Front end variable Capacitor Function? - hsusmann - 12-10-2013
Ron,
OK so it shorts the grid to ground. So I presume the signal then makes it through to the 2nd RF tube via the neutralizing cap?
Herb
RE: Front end variable Capacitor Function? - Ron Ramirez - 12-10-2013
Neutralizing cap (5), yes.
RE: Front end variable Capacitor Function? - hsusmann - 12-10-2013
Ron,
OK, thanks! I still have not inspected the chassis in
that area so now it should make sense to me
when I do so.
One other strange thing I noticed about this radio
is that the RC parts #7,#12, and #17 appear to be
fed the B+ from the opposite end of the string
compared to the schematic!
The schematic shows the B+ coming in at point "D"
which is also the feed to the output transformer
primary.
My chassis was wired so the B+ comes in at the first
RF tube and then daisy chains to 2nd RF, then 3rd RF
then the transformer primary!
Strange. This was all original wiring I'm sure also.
I am wondering when I re-do this that I should make
it like the schematic ?
Herb S.