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Philco 90 RF Grid Reception? - Printable Version

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Philco 90 RF Grid Reception? - Joe - 02-02-2012

I'm probably going to annoy a lot of purists, and I try to be one to the best of my limited radio knowledge, so here goes...

I have a Philco 90 (version with one #47 tube). I restored the cabinet and chassis and they look great. With the radio itself, I replaced the filter caps eliminating the 60 cycle hum. (At least, it is much lower than it was). Used 10 microfarad 450v caps, disconnecting the original cans, but leaving them in place.

The problem is this.....When I put an antenna wire on the ANTENNA lug, I hear some scratching when I connect the wire, and that's it. No reception. Looking at the schematic, I went down the line as best as I could from the ANTENNA lug through all points on the coils and components, touching them with a screwdriver and got scratches and crackles as I went along.

With that, I assumed there were no opens, or I would not hear the scratching from the speaker? When I got to the RF GRID, the radio played crystal clear. So I clipped the antenna wire to the RF GRID and checked the reception. The volume control worked smoothly, and I picked up many stations at the right places on the dial (e.g., station 1550 tuned in at 1550 on the dial and station 600 tuned in at 600 on the dial). Also, I did not have to adjust any pots, trimmers, etc.

With that, I wound up cutting the wire that went from the ANTENNA lug to the first coil in the circuit, and re-wired the ANTENNA lug to the RF GRID, bypassing the coil circuitry before the RF GRID

Question is, how important is this circuitry? Again, volume control knob works great and dial tuning tracks. If I did not know this was done (say someone else did and I did not know about it), I would not suspect any modification of this kind was done, as the radio performs well, or as well as can be expected from an 80 year old radio with original components (except for the filter caps). Is it ok to leave the radio like this?

Thanks guys!

Joe

Re: Philco 90 RF Grid Reception? - codefox1 - 02-03-2012

I think you antenna coil is open. Measure the windings, should be only a few ohms. In fact there is a chart of what you should expect to see here. Coils seem to be same for all variations of the 90.

You should get much better results if this is set right.

http://www.nostalgiaair.org/PagesByModel/881/M0013881.pdf

Re: Philco 90 RF Grid Reception? - Joe - 02-03-2012

Hello Codefox. Checked for opens and touched up the lugs with a pencil type soldering iron. Now I have continuity all the way through! However, I tuned to a very weak station with the antenna wire now in the right place, and hardly heard the station. Without touching the volume control, I moved the antenna wire through the circuitry working my way to the RF grid itself. As I did the weak station got louder, and when I got to the RF grid itself, the reception was almost 5X what I got at the proper point in the circuit for the antenna wire. I know it's not the right place to be, but the radio seems to pull in many more weak stations on or near the RF grid, and the volume control still works smoothly and evenly with no big "jumps" or distortion. The purist in me says put the antenna wire in the right place, but the actual reception aspect tells me put it on or nearer to the RF grid. Any further thoughts? Have you come across this yourself?

THANKS!!! Joe

Re: Philco 90 RF Grid Reception? - codefox1 - 02-04-2012

Please see the schematic and by serial number or landmarks see which chassis you have. There seem to be some differences in the antenna to RF arrangements. I assume you have done an alignment, because the symptoms you have could be caused by mistuning of the first stage, or a bunch of crap caught in the compensating capacitors. There also seems to be some information on resistances, etc.

Why not treat yourself to some better quality schematics on co moderator Chuck' Schwark's site:

http://www.philcorepairbench.com/schematics.htm

Meanwhile I'm sure others will have some thoughts.

Re: Philco 90 RF Grid Reception? - Ron Ramirez - 02-05-2012

Discussion continues here:
http://www.philcoradio.com/phorum/showthread.php?tid=4255

Since both threads are on the same subject, this one is now closed. Please post to the other (open) thread, thanks.