[ntsc525]

Hi, everyone:

I have a late model 71B with a shadow meter. Before you tell me the 71B didn't have a shadow meter, dig deeper, because the later ones did, in fact, come with a shadow meter, and in fact I've seen two of them, and regret not buying the second one! (Both were cathedrals, though I believe the console versions got the shadow meter earlier than the Baby Grands did.

Anyway, this radio has a 3 position tone control, which I want to recap, but I don't know what the values of the capacitors are. The schematic I have shows only a single position tone control, which I assume was on the earlier units similar to the 70.

I'd like to find an updated schematic, or hopefully someone knows what I'm talking about and can tell me the values for each position. If 0 is off (left), and we count 1, 2, 3 for each step, there are actually 2 wires going to the contact for Position 2, and one each to 1 and 3. Presumably the capacitors are all tied together for the other side to the common wire.

This is a rusty crusty set that should have been junked, but I'm going to try to fix it. When I got it, it was able to tune a handful of stations, but not KFI, who's antenna I can see from my house, so I knew something was wrong, and when I pulled the chassis out, it looked hopeless.

I rewound the shadow meter and got it working, and sprayed plenty of Rustoleum on the rusted chassis, and now I'm going to undo some "shade tree" restoration work done underneath and see how good I can make this junker! (I actually found a replacement tuner cap assembly for it, too, so hopefully it can get KFI. I mean, any coil of wire should get KFI at my house!)


Thanks.

Dan

[Jake Blake]

I believe this was discussed just recently.

See this post : Philco 45C tone control caps? from 4-7-19

[Nathan Slingerland]

The 45C and 71B use different tone control parts. According to the service bulletins Model 45 uses part 30-4178 and Model 71 uses part 04757.

We have the specs for some tone controls in the Philco Library (come from various Philco parts catalogs):
https://philcoradio.com/library/index.ph...-controls/

According to that the 04757 tone control is two-position - either open (0uF) or connected (0.02uF). FWIW that matches what is shown on the schematic.

Since the 71B in question has a three position tone control perhaps there was a change to a different part along the line? Especially likely since yours is the later version (code 125?) with the shadow meter - the service bulletin probably doesn't cover it.
Perhaps it uses the same tone control as in the Model 91 (pure speculation) - but that was a four-position job like you describe, part 04787: 0 / 0.015uF / 0.025uF / 0.035uF.

[ntsc525]

Thank you for pointing out the tone control link, as that was very helpful!

I interpret the tone control chart to give the EFFECTIVE capacitance provided by the switch for each position, since I believe .035 is a pretty high value for a tone control. So, we have to figure out what the switch does and how the caps are wired to see how the switch provides those values in the end.

So, I dug out the tar and tried to measure the values of the caps and figure out how they were connected.

My only good reading was .015 before I dug the tar out. I think it was the first position, which likely turned out to be the first two caps in series before I dug them out of the tar and ruined everything. (Now they are all open.)

Position 0 is off. (Switch is open to ground)

Position 1 is actually 2 caps in series. (The middle lug has 2 leads soldered to it.)

Position 2 is the middle cap direct (series cap is shorted / bypassed)

Position 3 is middle and third cap in parallel, because those two are tied to the common going back to the Audio output.

It is possible, even likely that my first (only good) measurement was been looking at 2 caps in series when I got the .015 (After I pulled them out of the tar to discover how they were wired, they were all open.)

Assuming the capacity ranges given for the tone control represents the effective capacity, then I have to figure out how to get those values based on the first two in series, the second one being .025 (.022 now), and the third one being .01 to add up to .035 when connected in parallel with the middle one.

I think perhaps a lot of people get confused by the capacitances given in the chart, thinking each individual cap has to be the value given, and not realizing what's under the tar. Examining the switch more closely reveals the series / single / parallel arrangement. If you just installed the given values for each, the third position would be a total tone killer (.06 uF).

So, using the series capacitor calculator at Digikey, (algebra, at my age?!!), and plugging in modern cap values, I came up with the following modern capacitors that will get close to the original:

C1: .047
C2: .022
C3: .01

So, the effect is:

Position 1: = 1/(1/C1 + 1/C2) in series, giving .0149 (Target: .015)
Position 2: = C2 with C1 bypassed, giving .022 (Target: .025)
Position 3: = C2 + C3 in parallel giving .032 (Target: .035)

Anyway, this is close enough and what I'm going to try!

It'll be a while before I hear how it sounds, since I started with the tone control so I could mount it after painting the chassis, and there is a lot of kluge work I have to untangle.

Thanks again for the help!

Dan

[ntsc525]

So, on closer inspection, I see a couple more interesting things about the tone control in my 71B.

First, I didn't realize, but discovered that I have another 71 chassis, a Code 221, which probably had the single position tone switch installed, but someone put a pot for a variable tone control, so I still don't have an original two position tone control to compare, but I'm glad I have it for other comparisons, and a possible parts donor if I need it.

On the schematic, I was trying to make sense of the capacitor to the left of the output tube (39), as it relates to the switched tone capacitor to the right of the tube (coming off the plate), also labelled 39. This confused me until I looked at the modern wiring diagram by "Geoff", which I think I got from this very forum. It shows both capacitors inside the original tone control, which explains why they both had the same callout number! So, there is always a .01uF cap from plate to ground, but another one can be switched into the circuit to cut more treble.

Now, on mine, (Code 125), the tone switch did not have a cap always to ground. Instead, it has an additional capacitor block in the open space between the power transformer and the line filter block (item 49), with a .01 uF cap always to ground off the plate of the output tube, and then whatever my 4 way tone control does is in parallel to that capacitor. I believe this block is original, because someone bodged a .025 uF cap (too much) off of it to ground. This seems to suggest this radio did in fact come with the 4 position tone control, and it was still designed to have .01 uF to ground in addition to the tone control.

Barring any Code 125 specific information being available, I'm hoping the rest of the radio is the same as the Code 121, because there is such a rat's nest of bodged floating components, "air joints", and other atrocities, it's going to be a challenge to restuff the blocks (thankfully still there), and wire it like it was originally supposed to be.

(Some people just have no respect for old radios or the voltages exposed on those floating, bare joints!)

I thought readers (or me, if I find myself working on another one years from now) might find this interesting.


Dan

[ntsc525]

Update:

I know you were all wondering:

It works! All recapped and aligned according to the Rider book on Philco radios, and the shadow meter that I rewound works! The tone control is a bit heavy handed once you get past the first .015 uF, but I guess if you're fighting static, maybe you have to be heavy handed with the treble cut.

This was a rusted out, heavily bodged radio with a broken tuner, tuned by a roll of electrical tape around the shaft (I asked about restringing the tuner in an earlier thread), but thanks to Play Things Of The Past, I found another 71 tuner with the bracket for the shadow meter, so now it's as good as only slightly aged!

Oh: And I had to rewind the shadow meter with 40 Ga wire using a sewing machine and hot glue to attach the coil form to a bobbin, and carefully running the wire through the thread tensioner on the sewing machine.

My success with that has encouraged me to maybe try to rewire one of my open 1920s speakers if I can remember which one is open!

Thanks everyone for your help with this radio. I've been thinking about fixing it for a few years, and gathering info and parts all that time. (Yes, this is the same radio.)

Dan