This is my latest project, Its a 1937 manufactured 6S-254, unfortunately its not a robot dial, nor does it have a magic eye tube, but it does have a waterfall cabinet and the teardrop shaped dial that was known for the 1938 sales models. The cabinet is in quite good condition, with a few scratches and a bit of grime, so I'll just clean it and wax it. The Chassis has had some work in the past, with someone putting a ECH81 instead of the 6A8 and a 6V6 for the output tube , also a quick check of the speaker field coil and output transformer, both of which are open circuit (darn), so this is going to be a long slow project to complete. I'm going to return the circuit back to the original tube lineup.
I decided to rewind the field coil, luckily these Zeniths have bolts connecting the field coil assembly to the speaker basket. As I don't have a coil winding machine, I made a jig and used my battery drill to slowly unwind the magnet wire onto another former, unfortunately, the breaks (yes 6 of them) were very close to the pole. I ended up loosing about 5 metres of wire. As the coil is not required to be accurate in the amount of turns, I wasn't concerned. Then I rewound he original magnet wire back onto the original former, it took days, at about 10 minutes at a time, my drill got hot, the battery got hot, so even though it worked out ok, so I think my next project will to make myself a coil winder. The now measured DC Resistance is just over 1K Ohms even though stamped on the speaker basket is 1250 Ohms, I doubt it was ever that high.
A wise man would have handed it to Gerry and got it back with new wire perfectly wound and bang-on 1250 ohms... but where's the fun in that? Well done repairing the field - thats the kind of madness that gives a restoration a sense of achievement.
The next coil to rewind, this is the Antenna coil and that's the primary that has been cooked, probably by lightening. It is suppose to read 25 Ohms DC. it actually reads 0.6 Ohms. I've unwound 160 turns of magnet wire, so my next job is to carefully wind a new primary.
The original 6A8 tube and octal socket has been restored to this radio, however because I didn't have an original Zenith Valve shield, I've had to use a 6A8 Metal. Only the top of the chassis has been sprayed after rust removal, I didn't want to loose the stickers and other markings on the back and sides and besides they were in good condition anyway. The dial belt has been replaced with a large "O" ring and the dial pan inside resprayed cream and new dial lamp diffusers made with plastic milk bottle cut out to the appropriate size and shape. The I.F was realigned , the second IF had been "twiddled" and was "off frequency". Someone had replaced the Volume pot with a 500k, which isn't large enough for the ratio between the detector load of 390K, the original Pot being 2M ohm with a loudness tap(not obtainable anymore). I replaced the Detector load with a 330K and the volume with a 1 Meg Ohm with an additional 470K in series to try to address the Tone control circuit (I'll go into this later). The Aerial primary has been rewound and the field coil in the speaker. The radio performs really well.
This is the schematic for the Radio, I've had to make a "mod" for the tone control circuit, as the main part of it used a 2 Meg Ohm "loudness tapped" Potentiometer. I've added a 470K resistor in series with the Vol Pot, so that when "Voice" or "HiFi" is selected with the Tone control, then a 250pF bypasses the 470K resister and effectively boosts the high frequencies. To get bass boost, I had to increase R1 from 47K to 330K , so that the Audio level wasn't loaded down too much and changed C14 from .003 to .001, this gave a nicer bass. I also added a 47K resistor in series with C18 a .004uF, which bypassed (too brutally I think) the high frequencies to ground. Now when "bass" is selected, it works a lot better. I realize this Mod doesn't replace the work of a "tapped" Volume pot, but the tone control now works almost like the original, except for the "loudness" operation at low volume.