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Restoration Date: Spring 2013
Made in 1948-49, this television was the lowest price national brand TV available. The starting price was around $189.99 (later sold for $159.99 making it the lowest price for a television in the 1940s) and features a 7 inch picture tube. Has a very low tube count of 15 + 7JP4 Picture Tube. Light weight compared to other early TV's (about 20 pounds). This sections will show my restoration of this TV I found at an antique mall in the city of Carson, CA in 2011, that was almost fully intact except the screen back missing. The price I paid $36 was a deal. The picture tube if working is worth a lot more than that.
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1. Picture of the hand wired chassis before I started electronic restoration.
2. Install new electrolytic capacitors into old twist lock cans for the low voltage power supply and the voltage doubler. Some restorers like to reform them but I don't want to do it again and even though reforming works. The 1940's seals that keep the electrolyte from drying out is a chancy thing to depend on.
a. The electrolytics shown are sometimes called twist lock, because there are mounting tabs underneath the base. You can twist these tabs which locks the part to the mounting socket. The cans are made of aluminum which serves as the negative side of the capacitor. Notice that these capacitors are covered with black cardboard which indicates that the can must not touch the chassis.
b. Working with one electrolytic at a time, carefully remove the wire and component connections from each pin and marked them with tape, indicating which pin it was removed from. Then comes the hard part unsoldering the negative polarity, twist lock mounting pins and pulling the capacitor out. These pins are used also for electrical connections. A solder vacuum pickup tool is a great help and BE PATIENT.
c. Once the capacitor can has been removed, it is time to get the old stuff out. Have a vice and a good hacksaw ready. Cut as close to the can base as possible as shown. When that is done, you can pull base to get the old stuff out.
c. Once the capacitor can has been removed, it is time to get the old stuff out. Have a vice and a good hacksaw ready. Cut as close to the can base as possible as shown. When that is done, you can pull base to get the old stuff out.
d. Remove the old capacitor by cutting the leads connected to the pins. Then carefully assemble the new capacitors in such a way that it will fit inside the can. Make sure the new capacitor leads are insulated from from the can and solder the leads to the pins at the base. I had to drill small holes on the base to feed the wires to be soldered to the pins. The capacitor was installed back and all connections were soldered back. The original top of the can was glued back with epoxy at the location that was sawed, and cardboard placed back on. The same thing was repeated for the remaining two.
3. Replaced all paper capacitors with more reliable PTFE capacitors. I decided not to stuff the new capacitors into the old ones. I enjoy seeing how much more room there is under the chassis when modern parts are put in. All high voltage capacitors were paper type sealed with a lot of wax. They were rated at 6000 volts. Replaced them with new plastic type and in other areas with high voltage ceramic capacitors.
4. Check resistors and replace, those that were out of tolerance. About 10% were bad. Carbon resistors tend to absorb moisture and that can change the values. Checked the wiring and replaced the selenium low voltage supply and doubler with silicon rectifiers. I left the selenium rectifier on the chassis for nostalgia.
5. Installed tubes that checked good on my Hickok 600 Tube Tester. Installed a new 1B3GT and made sure the feedback pickoff loop was in the correct spot as stated in the tube data. Electrostatic TV's used a pickup loop to establish the HV RF oscillator for the Horizontal output deflection circuit. Also discovered that the mica capacitor at the RF transformer was open and so it was replaced. Bad mica capacitors are rare, "SO I THOUGHT"!
6. After checking out all the changes I made and making sure I put all the wires I unsoldered back in the right location, I reconnected the original 7JP4 CRT and turned it on. No picture only a audio hiss. Rather than check the high voltage, I have a spare 7JP7 high persistent radar CRT. Tried it again and just got a straight line across the screen. A burning smell came a few minutes later.
a. After checking and testing this was starting to show up, a 1 watt 47K ohm resistor I replaced in the vertical oscillator was scorched.
b. It turned out that the mica capacitor just below it was leaky and provided a dc current path, and pulling the plate voltage down on the 6SL7 tube. Replaced it with a silver mica capacitor. c. I later found several of these shorted mica capacitors (all made by Solar Capacitor Mfg). I replaced those as well. I turned on the TV and I get a raster! Now I am getting somewhere.
Replaced out mica capacitor in high voltage section.
7. Tried to get a picture and could not get anything except raster. The TV video IF was checked and after some deciphering, I found that in some versions there is a detector board mounted inside the final IF transformer. Inside the transformer is a diode (1N34). Solid state diode made in the 1940s and 1950's were housed in a ceramic or molded plastic case. Over time the case cracks and moisture gets in contaminating the PN junction. That is what happened here.
a. Put a modern glass beaded version of the 1N34A. Reinstalled it and tried it out, no change. After checking and checking the circuits, I noticed there was a descrepancy in the schematics I have. The SAMS PHOTOFACT show the diode connected the opposite from the one shown from the Rider manual. I followed the SAMS originally, so I tried reversing. The diode direction is important. If the video signals polarity is opposite of the diode's direction, the video signal is blocked. Kind of like going on a one way street. Result: got a picture, not great but now I get a picture and some sound. THE RIDER MANUAL IS CORRECT!!!
8. After some alignment of each channel oscillator and RF circuits plus peaking the video IF circuits, the TV was working very nicely. This model is very simple to operate. The cabinet was restored and I had to adapt the slightly smaller 7JP7 CRT into the cabinet. Also the original speaker was bad but luckily I found an auto speaker that fit perfectly.
8. After some alignment of each channel oscillator and RF circuits plus peaking the video IF circuits, the TV was working very nicely. This model is very simple to operate. The cabinet was restored and I had to adapt the slightly smaller 7JP7 CRT into the cabinet. Also the original speaker was bad but luckily I found an auto speaker that fit perfectly.
9. FINISHED! Notice that the 7JP7 Radar CRT has a bluish look. The original 7JP4 tube had a good filament but a good chance the cathode is contaminated and non functional.
10. UPDATES
10A. 8/2014 Bought a good 7JP4 Picture Tube (paid $129!!! That's the going price these days). Now the 7JP7 Radar CRT is my spare again. Here is what it looks when installed and operating:
10A. 8/2014 Bought a good 7JP4 Picture Tube (paid $129!!! That's the going price these days). Now the 7JP7 Radar CRT is my spare again. Here is what it looks when installed and operating:
10B. Picture of restored chassis underneath and on top.
10C. I made a protective panel underneath, that suppose come with the set.
FYI:
17A485459 Ballast Tube Replacement
The Motorola VT-71 is a transformerless TV set. There are several tube filament strings. To get each string to add up to 115 VAC they used a special ballast tube. This tube gets really HOT! When it goes bad as they do, one has to be made. A replacement can be made that runs MUCH cooler and more reliable.
From VideoKarma Discussion 11-26-2010 from member "Polyphase". See link below:
The white object on the upper left corner is the Capacitor Reactance Ballast, I did, couldn't duplicate the original.
A restorer came up with a very efficient and cool version using capacitors. Capacitors are frequency dependent devices. They have AC impedance or reactance (Xc). Since the filament string operates at 60 Hz one could design a Xc Ballast. The great thing about this Xc theoretically does not use power.
Photo and circuit diagram of Capacitive Reactance (Xc) Ballast replacement.
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