VT-71 Making me crazy!

[7"estatdef]

On the bench I have a TS-4J chaiss that I can't seem to get the low B+ above 60V. As you can see from the Motorola diagram V4,6,&10 are the tubes that feed the low B+ circuit. With 250v on the plate the cathode is use in a voltage divider to derive 125v for V1 rf amp, V12 audio output, and V10A half of the clipper.

V4,6,&10 have 235-250v on the plates. Have checked all of the 1K resistors the coupling the HV. Replaced C78B filter for 125v buss. Have lifted the cathode resistors on these tube to check voltage all have around 200v. Replaced output tube V12 thinking in might be loading the circuit.Grid voltage is -4 on the V10 with 60v on the plate. All paper caps have been replaced. Filters seem ok C78 was replaced years ago. All ouput voltage of the power supply seem good and replaced E2&3 w/silicone jobs.

V4&6 have 5.7v on the heater V10 13.2v. R43 tested at 7200 ohms should be 6800. All 1k's are within 15% as are the 68 ohm cathode resistors. Control grid to cathode voltages seem normal as there are no high - voltages on the grid.

Throw a bucket of water on my head my hair is smoking!!!!
Tnx for any help
Terry

[Adam]

It's good to see I'm not the only one working on sets in the middle of the night. I was also working on a TS-4J so I have the schematic infront of me already. So I feel like I should write something, even though I probably have no idea what I'm talking about, but could this be caused by a bad audio output transformer?

[7"estatdef]

Snipped the red wire on it Voltage went up to about 80v. Don't think it's the calprit.
Terry

[7"estatdef]

So did you get your's up and running?
Terry

[Adam]

No, I haven't gotten a hold of a replacement for those hard to find 6kv capacitors yet. For the last one I fixed I managed to find some 35kv caps, but I was hoping to find something a bit smaller this time. And I have also yet to pick up some high wattage resistors to replace the bad ballast tube.

I have 4 of these in various states of completeness.
I have 1 working chassis, 1 complete (but not working) chassis, 1 chassis missing the whole HV section.
1 nice wooden cabinet, 1 nice suitcase cabinet, and 2 wooden cabinets in need of refinishing.
1 good 7JP4, 1 really weak 7JP4, and 1 7JP4 (not weak) but it seems to have some kind of short in it (something inside it pops, then the picture shrinks, then it goes back to normal. It does this usually about 3 times in the first 5 minutes it is on, and works fine from then on. It also has a dot burned in the center, it came out of the set with the whole HV section removed, I wonder if someone used it like that for a while?
At the moment I have that crt in the good wooden cabinet with the working chassis, and plan on putting the chassis I'm working on with the good crt in the suitcase cabinet.

[vintagecollect]

Those TVs can produce an awesome picture, surprising considering the wafer switch tuner. Check the ballast that has nichrome like toaster wire --- I think this was for the filament voltage circuit from memory. maybe double check?? definetely worth fixing.

[Phil Nelson]

Originally Posted by Adam

I haven't gotten a hold of a replacement for those hard to find 6kv capacitors yet.

I got mine at Allied Electronics, http://www.alliedelec.com/ .

They may also have the high wattage resistors you need to make a ballast replacement. I used Dale type resistors on a little copper board. You can read more about that project at:

http://antiqueradio.org/motvt73.htm

[7"estatdef]

You can get them here also.http://www.justradios.com/orderform.html
Terry

[7"estatdef]

Hey Phil,
Tnx for documenting all that VT-71/73 info on your site. Nice set and a great picture It's helped me in trying to get this one going.

It's funny that the folks I've talked to with these little Motorolas most have more that one. I've got three, two VT-73's one has a TS-4J the other is unmarked but it has the square HV cage and no fine tuning. Both of these don't have the copper plated chaiss. Maybe earlier models than the copper ones? The set on the bench is a VT-71 with a copper plated TS-4J chaiss. All three are not working and I need one more 7JP4 to complete all of the sets.

I'm at a little bit of a loss even on how to troubleshoot this B+ problem because it's a matter of balance between the top tube with 250v and the bottom tubewith 125v. If the top tube isn't drawing enough current then the botton tube dosen't get enough or if the bottom tube is drawing too much then it loads the circuit and causes the voltage to drop. Pardon, it's my hair . It's smoldering again.
I guess I could measure the current from the top to the bottom tubes and see if it comes out equal.
Terry

[Adam]

Thanks for the information on finding those capacitors, for a long while I couldn't order anything because the mail delivery here is awful, and stuff would always go in wrong peoples boxes, get lost, and anything that wouldn't fit in the box just got returned to the sender. But I finally went and got a PO box, so I don't have to deal with that mail delivery awfulness anymore, so now I can order something and actually expect to get it.

"They may also have the high wattage resistors you need to make a ballast replacement. I used Dale type resistors on a little copper board. You can read more about that project at:http://antiqueradio.org/motvt73.htm"

I enjoyed reading that, I had been debating how to place the ballast resistors, and I liked that piece of metal ontop of the chassis, and then plug them into the ballast tube socket idea you had. The last one I fixed was unsuccessfully converted to filament transformers by a previous owner, rather than stripping the rather extensive modifications out, I managed to finally get them working as I think they were intended to, but it was a real mess to sort out and I don't want to go through that again. Also that arcing crt socket made me think that that 1 crt I have might not be arcing inside the tube, but maybe inside the base, I know it's not the socket, because it doesn't do it with the other 7JP4s.

All 3 of my TS-4J chassis are 1948, the one I fixed previously with the filament trans. is not copper plated, the other 2 are copper plated. I think there must be alot more of these 7" motorolas out there than there are good 7JP4s left.

[Phil Nelson]

You guys probably know this, but there were something like eight chassis variations for this TV. Different ballasts, you name it. I haven't looked at it in a year or two, but I believe the Riders manual runs to 80 - 100 pages. Make sure you're looking at the right schematic for whatever chassis you're working on.

The devil's in the details :-)

Have fun.

[7"estatdef]

The manual I got from the radiio guy which is a copy of the original Motorola manual is great. It's about 130 pages. But it doesn't have much info on the J just the diagram. Sam's has the info for the J but the diagram is harder to read because it doesn't show the split power supply very well. Both use different numbers for the componets so you have to keep all the numbers staight.
Maybe us VT-71/3 owners should write up a supplement of service notes and burn it to a cd with the manuals?
Terry

[7"estatdef]

Well after about five days of troubleshooting It turned out to be L-43 the limiter grid coil was open!! Was causing the grid to be at -9v when it should be +95v (split power supply) The limiter tube was hauling down 40ma of plate current ouch!
Terry

Phil you are so right

The devil's in the details

[Adam]

It's good to hear you got it going. This was actually a very informative thread for me, I never understood exactly what was the source of the low B+ in these sets until I read up above.

"As you can see from the Motorola diagram V4,6,&10 are the tubes that feed the low B+ circuit. With 250v on the plate the cathode is use in a voltage divider to derive 125v for V1 rf amp, V12 audio output, and V10A half of the clipper."

With all the different chassis variations the only one I've ever come across is the J, they must have really stepped up production of these towards the end.