Shadow Meter Repair & Adjustments

Shadow Meter Repair & Adjustments

Philco used the novel tuning indicator, the Shadow Meter, starting in 1932-33 on a few models and continued to use it through 1937 and finally dropped the Shadow meter after 1938. This listing shows which models carried the Shadow Meter Tuning feature.

Older Style Meters
 

Newer Style Meter

There were two types of Shadow Meters; an early model used prior to 1935 or so, and a later type used up through 1938 models. The early types meters used a screw base lamp and the later used a bayonet base lamp. The screw type can be replaced with a type 46 lamp and the bayonet type replaced by a type 44. Both are rated at 6.3 volts drawing 250 milliamps. To learn more about how this unique tuning indicator works, see this article from “How It Works“, Special Section of Vol. IX, Rider’s Manual.

Here is a breakdown by model year, of the Philco sets that used the Shadow Meter Tuning:

1932-33 14  15  23  19  91
1933-34 16  17  18  29
1935 49  97  118  144  200  201
1936 116  630  635  640  641  645
650  651  655  660  665  680
1937 37-116, Code 121  37-630  37-640
37-641  37-643  37-650  37-660  37-665
37-670  37-675, Code 121  37-2650  37-2670
1938 38-5  38-8  38-2630K  38-2650  38-2670

 

Newer Style Shadow Meter Adjustments

WARNING – SHOCK HAZARD! Shadow Meters are part of the high voltage B+ circuitry to the RF section of the radio. Before touching the Shadow Meter coil assembly, be sure that there is no high voltage leakage or short to exposed parts. Use a voltmeter to check exposed metal parts and look for cracked, frayed or damaged coil windings or covering.

  1. Remove aerial, volume at minimum and allow tubes to warm up – 5 to 7 minutes.
  2. Move the Shadow Meter coil backwards and forwards, until the shadow is within 1/8 inch of each side of the screen.
  3. While power is on, remove the rectifier tube (BE CAREFUL! It’s very hot!) from its socket and rotate the Shadow Meter coil for minimum shadow width.
  4. Replace rectifier tube. The shadow should then return to maximum width or within 1/8 inch of each side of the screen. If the shadow does not return to maximum width, re-do steps 2 and 3 above until it does.
  5. If the above steps do not give satisfactory results, try using another pilot lamp with a straighter filament which will tend to concentrate the light in one direction or a lamp with a less distorted glass bulb.

C. Schwark – Information from Philco service sheets and John F. Rider.

Newer Style Shadow Meter Coil Repair

  1. 1000 feet of #40 gauge enameled copper wire will be needed to rewind the coil in the Shadow Meter assembly. It just so happens that #40 gauge wire has a resistance of about 1.1 ohms per foot. So in a typical radio, a current draw of 10ma. will give the 10 volt drop across the coil that it takes to operate the shadow vane correctly.
  2. Use a drill motor to turn the bobbin, take your time – wind the coil as smoothly as possible. Use a 1/4-20 screw about 3 or 4 inches long; a couple of cones of rubber, or wood shaped to hold the coil’s form (bobbin); chuck the “assembly” in a 1/4″ or 3/8″ variable speed drill (or use a variable transformer to control speed); and TAKE YOUR TIME!
  3. Antique Electronic Supply carries a 750 ft. spool of #40 gauge for $4.95 (Cat. No. S-W320) – you need to use approximately 1-1/4 spools so order two spools. Ocean State Electronics also has #40 gauge wire. Cat. No. MW40 for $15.25. It’s not that critical to be exact, but if you want to be close, you can set up a “turns” counter and transfer the wire from one spool to another, counting the turns, or you can use an “odometer” type counter like one you might salvage from a tape recorder.
  4. Determine the total turns of all of the wire; rewind it back and transfer it again. Only this time using 2/3 of the turns. Many of those tape counters have a pulley on them. Wrap one turn of wire around that pulley and let it “count” the wire. Hint: Use bee’s wax as a tension element. It provides just enough drag to keep the wire from being “wild” yet not enough to seriously stretch or damage it.
  5. So here is the setup: Supply spool, some sort of “brake” to keep it from spinning too free (piece of cloth dragging the side of the spool a bit); the turns counter; a guide post (ice cream stick with a notch in it with a dab of bee’s wax to maintain a little tension on the wire as it wraps around the counter’s pulley; then the drill with the take-up spool (the coil’s bobbin).
  6. Be sure you have some sort of hand guide to move the wire back and forth as it goes on the bobbin to ensure the windings are in even layers. It’s easier to set it up so the wire naturally goes to one side then use your hand guide (large diameter round screwdriver shaft, for instance) to move the wire across the bobbin as it winds. That way you only have to push it across one way and then let it come back across on it’s own.

Randy Gutery – Information from the rec.antiques.radio+phono
Usenet Newsgroup, 12/08/99 (Wire source updated 01-15-02 c.a.s.)

Older Style Shadow Meter Repair
by Gary Tempest

The procedures below pertain to the older “boxy” type Shadow Meter style. If you haven’t already done so, it would be best to read the “How It Works” section above first to familiarize yourself with the inner works of the Shadow Meter. Possible faults are one, the coil is open circuit or two, the meter does not mechanically operate smoothly.

Once you have the meter out of the chassis then bend back the tabs on the bottom plate and remove it. Before doing anything else check the movement of the aluminium shadow vane. Using a toothpick, push it across and then release it. It should re-center due to the flux from the horseshoe magnet, underneath the coil, passing through the tiny soft iron bar beneath the vane. If it doesn’t then the fault may be mechanical. The pivot points may need cleaning or the bar may be loose. If this is so, it can be fixed by applying tiny beads of Superglue with the toothpick. If it is neither of these then the magnet may not be strong enough. It will be necessary to strip the meter to correct this.

If the coil is open circuit then still do the vane centering check. Make a mental note of how smartly it re-centers and how many oscillations of overshoot it goes through. This gives a rough guide to how powerful the magnet is.

To remove the coil, bend back the tabs and remove its cover box. Now take out the single screw, passing through the vane pivot bracket. Once the coil is removed the horseshoe magnet can be seen. If there is no problem with this then remove it and apply a ‘keeper’; a dressing-making pin will do or a small nail.

If the coil is open circuit then rewind it as given in “Shadow Meter Coil Repair” above by Randy Gutery. With luck the vane and its pivot bracket will stay in place held by tabs passing through the case. Before refitting the coil it is worth holding the magnet in place and rechecking the vane re-centering.

If the magnet is weak, then in use, the shadow action will not be smooth. It will jump across and not return. Fortunately there is a simple way to re-magnetize it that worked for me. Another small powerful magnet did the job. I used one on the end of those telescopic pickup aids; the magnet is about 1/4in. by 1/4in. First, remove the keeper pin or nail. Then, using a circular motion, rub the magnet around the horseshoe, on one pole and off the other, in the same direction, a number of times. Being right handed, my motion was clockwise, on the underside of the horseshoe.

I suspect that this is not critical and if north and south poles are reversed (by doing it the other way for example), then the vane will deflect in the opposite direction. However, the device is symmetrical and either way will produce the same shadow action. Check the magnet’s new strength by seeing how it attracts a dressmaking pin or small nail. Laid flat, on a smooth surface, it should attract it at about 3/4in. +/- 1/8in. It is not that critical but if necessary the re-magnetizing process can be repeated. The final test is to put the horseshoe magnet in place and check vane re-centering.

To test the reassembled meter, out of the radio, a variable 10 volt DC supply can be used with a source of 6 volts AC to light the bulb. Once hooked up, and DC voltage is varied, the vane should move smoothly and the shadow open and close. The vane can be carefully adjusted to give a central shadow with the best rectangular shape. This needs to be done in conjunction with adjusting the light source as described in this Service Tip. The vane should be almost fully across with 10 volts and this is approximately 10 ma. In the radio chassis it will be more like 5-6 ma. maximum reducing to around 2 ma. ‘on tune.’ This will give typical shadow lengths of approximately 1/2in. ‘off tune’ and about 3/16in. ‘on tune.’ If the magnet strength is high both will be narrower and conversely wider if its strength is on the low side.

The covers can now be refitted but, if you are cautious, it can be hooked up to the radio first for a final test. The cover tabs are brittle and some may well break off if bent open a second time so be careful.

Gary Tempest