1948 Motorola 7" TV Restoration: "Compactron meets Motorola"

Compactron meets Motorola...it sort of sounds like the title of a 1964 Japanese sci-fi flick. All will be explained later but for now, here is a picture of the TV.

This is a 7" "electrostatic" television, made in the 1948/1949 time frame. These days with monstrous flat panel TV's covering entire walls, it is hard to imagine the family gathering around this set to watch Dragnet or Hopalong Cassidy. I bought this set back in 2005 at the HVRA annual convention/auction. The set languished in storage for 9 years until I got motivated to look at it after seeing the identical set sell for $40 at the 2014 VRPS convention. Sadly, I paid about triple what the set sold for at the VRPS auction and mine was missing a knob and the Motorola logo.

A few notes about the set: as mentioned, the picture tube is an electrostatic deflection type, using high voltage applied to plates inside the tube to deflect the electron beam. HV is provided by an HV oscillator (25L6) and boost transformer. The set is "transformerless" and uses a series/parallel filament circuit. Regular B+ for tube operation is provided via a voltage doubler circuit using two selenium rectifiers and electrolytic capacitors. B- is isolated from the chassis. The chassis was completely unmolested, containing all original parts and is a fine example of "point-to-point" wiring. The following are pictures of the top and underside of the chassis:

I began a preliminary assessment. First, I checked the continuity of the 7JP4 filament. It showed continuity which is good but I wouldn't know anything more until I powered up the set as very few picture tube testers will test this tube. Next, I checked out potentially hard-to-find components such as the HV transformer and the transformers in the horizonal and vertical deflection circuits. All were good. Next, I checked out "M5", the ballast resistor assembly. Not good...this unit contains four separate resistors and all were open, and very likely next to impossible to find a good replacement. I didn't consider this a major obstacle though as the ballast unit could be replaced with individual resistors. With Ohm's Law in hand, I calculated the power each resistor would have to dissipate and it was fairly high. For example, each series filament string has a 105 ohm ballast resistor and each string draws approximately 300 mA. The calculated voltage drop is then about 32 volts. Power is 32*.3 or about 10 watts. Today's line voltage is higher than "back in the day" so the actual power dissipated is likely even higher. And then putting in some margin, I would need 25 watt resistors to replace these two 105 ohm resistors. The B+ circuit had two ballast resistors (37 ohm and 200 ohm).

Some might try to install these 4 resistors inside the original ballast can. It would have been tight. Some might install these resistors under the chassis. Some might use a non polarized capacitor to take the voltage drop since AC is being supplied to the filaments. I wanted something more elaborate but what? The Compactron to the rescue! As the sun was setting on vacuum tubes, tube designers/manufacturers were stuffing tubes with multiple functions, i.e. one compactron could do the job of two, three, maybe four tubes. They were primarily made for televisions and as such, a lot of different filament voltages were used in series string circuits. I fine tuned the voltage drop I wanted and came up with 45 volts. I didn't have anything like this on hand so I researched on-line and found the 53HK7, described in the tube literature as a "high perveance diode and beam power pentode". The tube was typically used as damper diode and horizontal deflection amplifier. It had a 53 volt, 300 mA filament which was just what I was looking for. I would need two, one for each series string. A quick search on eBay and I had my tubes in about a week.

But where to put these tubes? One compactron could be located where the old ballast socket used to be but I would have to punch a new hole in the chassis for the other. Fortunately there was enough room beside the ballast socket for the second tube.

A few more design considerations: I think ballast resistors have a higher resistance when cold and drop to their rated resistance at operating temperature. The 53HK7 filaments would provide for the same action but just to be safe, I installed a CL90 current limiter ahead of both series strings. With 600 mA going through the CL90, it's resistance would drop to about 3 ohms. The 53HK7 would serve admirably as a voltage dropper but could it do more? After all, the tube contained a diode and a pentode. I don't mess with selenium rectifiers, I replace them. Could the diodes be used to replace the seleniums? I didn't know what "high perveance" meant and I did know tube diodes typically have a higher voltage drop than a selenium. I decided to wire the diodes up, finding out when the set was first powered up. Here are pictures of the topside and underside of the compactrons.

Now it was time to rebuild the chassis. The first task at hand was replacement the high voltage paper capacitors. There were three .005 uF at 6 kV and two .001 uF at 6 kv, all used in the horizontal and vertical deflection circuits. Capacitors aren't readily available at these high working voltages so I had to come up with something else...wire lower voltage caps in series. The working voltage then becomes additive while the capacitance drops, the same as wiring resistors in parallel. I ended up using six .033 uF at 1000 vdc to replace the .005's and four .0047 at 1600 vdc to replace the .001's. Then there was the issue of installing them. I'm not a paper capacitor "restuffer" but figured this would be a good time to do so. I removed the old caps, gutted them, soldered up the new caps in series, installed longer leads, installed the assemblies in the cardboard tubes and then sealed the ends with black hot glue. Here are some pictures of the effort:

Next up, replacement of the electrolytic capacitors. When needed for space, I will restuff original caps but it is a lot of work. In this case, removal of the two selenium rectifiers made for adequate space to mount the caps underneath. I had a terminal strip that fit perfectly using the two original rectifier bolts. A few capacitors were physically relocated elsewere in the chassis but in the same electrical location. Here's a picture of the electrolytic capacitor installation:

After that, all that was left was replacement of the remaining paper capacitors. I might find a replacement for that yellow cap as it tends to stand out.

The moment of truth...initial power-up! I hooked the antenna up to my home cable system, plugged the tv into my isolation transformer, and positioned the 7JP4 on the chassis. I turned the switch on, anticipating sparks and smoke but there was none! The filaments lit but there was nothing else happening. Darn, it looks like some troubleshooting in my future. I waited a little longer and suddenly a raster developed! I adjusted the various controls and soon I was rewarded with a full raster. Adjusting the fine tuning yielded a very clear picture! Sound was there too but when adjusted for best sound, the picture was no longer good. This is an "IF" sound system so I'll probably need to do some sound adjustments to get a good picture and good sound. I can't remember the last time I worked on a tv that came to life this quickly. Here is a picture of the picture (the actual picture is much better as I took the picture with no flash and a slower shutter speed):

There is still some more work to do...find a knob and logo, clean the topside of the chassis and cabinet and re-install the chassis. I tested another 7JP4 from an Admiral and it is good so the Admiral restoration is in the queue.