Motorola Troubles

[maxm]

Here is some specific info on the resistors in the ballast in case anybody is interested. Somebody sent this to me a while back when I was working on this set, sorry but I don't remember who it was, or I would give them credit.

"On the real Motorola VT71 schematic in Riders TV vol 2, they give a breakdown of the ballast, which they call R80. There are 5 sections to it. The AC input goes to pin 5, there are two 105 ohm sections from 5 to 2 and 5 to 7, there is a 37 ohm section from 5 to 1, a 200 ohm section between 3 and 4, and a 22 ohm section between 6 and 8. You will need to calculate the wattage and overrate by about 50% for a safety margin. The filament strings are each 300ma which gives you about 9 watts, so you need something in the 15 watt+ range for safety. I used two 50 ohm 10 watt resistors in series with a 10 ohm 5 watt resistor for each one of the 105 ohm sections. The 200 ohm section is the one that needs to be about 25 watts. For the 37 ohm section I used a 50 ohm 10 watt because I replaced the selenium rectifiers with silicon and needed the additional voltage drop there. The 22 ohm is a shunt across one of the tube filaments and you could get by there with a 20 ohm 5 watt. Because of the strange series/parallel filament strings in this set NEVER under any circumstances pull ANY of the tubes while the set is turned on. Many restorers choose to remove the CRT filament from the series string and add a small 6 volt filament transformer to power it, thus protecting the expensive CRT against accidental burnout due to failure in the series string."

[Eric H]

Hi Jordan,

First thing I'd do is plug the 7JP4 into your other set and see if it lights.
If it's dead then it's probably best to call it a day on this set and use it for parts, too many Motos and too few 7JP4s to spend too much time on a set with a not so great case and other problems.

Eric