Motorola Troubles

[jpdylon]

I''ve been working on this motorola suitcase 7VT5b with the TS-18 chassis. i picked it up for 20 bucks, so I said what the hell. All the caps but the filters have been replaced by whoever owned it last. I had to re-thread the pins on the 7jp4.

Put it on the variac and went up to 50 volts and waited. About 10 seconds after which I heard a crackling noise. I killed the power and looked in the back for any signs of destruction. I also could not smell anything nasty. The filters were not warm or hot. However the "ballast tube" was very hot. I'm guessing this serves as a dropping resistor for the tubes as well as a fuse?

I've got the sams for this set, but I'm not sure where to start. I would like to get this working just for the sake that I could, but I'm not hating the thought of using it for parts either. The case isn't great, its missing knobs, and I've got two others that can use parts.

Any thoughts?

[maxm]

That ballast tube can cause problems, as the resistance elements can break off from their connectors in the base of the tube where they connect with the pins. This can happen even in NOS tubes, so your best bet would be to replace it with seperate high-wattage resistors. I had a set link this, and really learned to hate that ballast tube. It Is normal for it to get hot during operation, and it will make some funny noises and smell funny when the set if first heating up. The ballast serves as a dropping resistor for the tubes in the set, as I recall there are two seperate fillament strings in the set, each adding up to less than 115V, so a dropping resistor int he ballast is needed for both of them.
You can read more about replacing the ballast with resistors here:
http://www.antiqueradio.org/motvt73.htm

I have some information about the exact connections and values of the resistors in the ballast I can find if you are interested.

Good Luck!

[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

[jpdylon]

Maxm: Thanks for the input. I really appreciate you taking the time to find this stuff.

Eric: Thanks to you as well. i can see from your website that you restore thses as well. I'll take the advice and see if the tube lights with another chassis. I 've got 3 chassis, one that works for sure, this one in question, and another that I have not tested yet.

[bgadow]

I've disliked ballasts since my first experience with one in a prewar Motorola radio, many years back. They seem like an archaic way of doing things. Dummy here destroyed the ballast in his VT71 by pulling a tube with the set on. I was able to fit a couple resistors inside the old part & put the others under the chassis. Someday I want to add the filament transformer.

[Jonathan]

I never really cared for AC/DC transformerless radios or TVs. I realize they are lighter, but thats the only benefit I see, and that's not even that much of a benefit. And yeah, a ballast tube is the wrong way to do it, not to mention wasting all that power. There should have been a filament transformer in there in the first place.

Jonathan

[maxm]

Originally Posted by Jonathan

I never really cared for AC/DC transformerless radios or TVs. I realize they are lighter, but thats the only benefit I see, and that's not even that much of a benefit. And yeah, a ballast tube is the wrong way to do it, not to mention wasting all that power. There should have been a filament transformer in there in the first place.

Jonathan

The problem at the time was that Motorola was trying make a cheap TV (The GoldenView was the first TV to sell for under $200), and a transformer would have cost more than a ballast tube, though it would have saved us a lot of troble today, but I don't think that Motorola was thinking that far into the future. It seems that if they had put a little more thought into the tubes they used they could have made a series string set with no ballast, like the later sets, but tubes they needed to do that probably wern't created yet.

I think there were other 7" sets that used a power transformer (Admiral?).

[Jonathan]

Well, true, it did make it cheap. The restoration on antiqueradio.org was a beautiful job, and the resistors to replace the ballast tube was a really good idea. Even though it creates a lot of wasted heat, it still is a really cool set, as electrostatic sets are really cool.

Jonathan

[Chad Hauris]

There is a way to use capacitors such as ceiling fan capacitors to drop AC voltage in filament strings with no resistance heat loss.

A member here, "wa2ise" I think has some experience with this.

[jpdylon]

It woudl definitely be worth looking into. however I definitely have to see if the CRT is good. Otherwise this will become a parts box for two other sets I have.

So good to know their are people around here with this kind of knoledge.

[Jonathan]

Oh yeah, Chad is right. Using the reactance, the AC voltage can be dropped with no wasted heat or energy. It's figured out the same way as it is with resistors, just the reactance is in place of resistance.

Jonathan

[7"estatdef]

>I think there were other 7" sets that used a power transformer (Admiral?).

I know that the Admiral 19A11 and 17T11 both 7" set use a power transformer and the 7JP4 is strapped down to the chaiss not to the cabinet like the VT-71.
Terry