Back to the VT-71H - Page 2 - Videokarma.org TV - Video

Chris K · #16 02-20-2023, 11:58 AM

I changed the sweep mica caps and checked them. They were all open. I replaced them with the correct size and voltage new mica replacements. The control pots on the back were all cleaned out with CRC contact cleaner but I did it again for good measure. Tubes checked again...all good and I cleaned the tube pin sockets with a mini-wire brush I got. Rechecked the antenna connections and went through all the solder joints and connections in the sweep circuits and IF. No change on the TV. Still no picture getting through. Audio yes but no video. Might be time to put this back up on the shelf for a while.

timmy · #17 02-20-2023, 12:00 PM

Did you trace down the cathode on the crt ? What about the contrast knob to much one way picture gone but sound will stay. Is the b+ up where it should be and take an extra look at the video amp.

Yamamaya42 · #18 02-20-2023, 12:49 PM

According to the Motorola vt71 svc_manual, you should be able to see a healthy 1.8v ptp video signal with an oscilloscope on the grid of the video amp U6 (P1 6AU6), if, all is working OK, and the signal is getting to and past the detector, at point D.

Electronic M · #19 02-20-2023, 01:01 PM

I feel like this was discussed on your last thread on this set, but I don't have time to dig for the info (BTW it's kinda moderation policy here to not allow multiple topics on the same TV to avoid this situation) but if you have anything that can generate a test pattern IF signal on the frequency this uses (such as a B&K 1075-1077 family analyst, modified Sencore VA-62, another working old IF band TV tapped for signal just before the detector) signal injection at the IF grids in your Motorola may yield some guidance on where the problem is.

Yamamaya42 · #20 02-20-2023, 01:11 PM

Looking at things closer, V6 has to be working for there to be sound, as the sound pickup is through C35, but perhaps problems with coils L32/L33 or C72, it should be easy enough to trace with a scope, cause you KNOW, it has to be getting as far as the video amp for sound, just need to see where it may be getting lost from pin 5 to CRT point C.

Chris K · #21 02-22-2023, 08:58 AM

Modest disaster. Ballast tube popped and shattered. I knew this was the weak link. I supposed it got tapped while manipulating the chassis upside down when it was blazing hot. My chassis was missing the steel thermal shield around the tube. All the elements are intact but I can't use it as is. I guess it's rebuild a replacement or try to find a best fit replacement for the glass. In my chassis, the ballast is the rarer 17A4...3030 and almost all of the rebuild plans I've found are for other designs. Part of the adventurous learning curve in this hobby.

Yamamaya42 · #22 02-22-2023, 09:24 AM

I actually have an old Ballast Tube in my collection of weird stuff someplace, no clue as the make or value of it.

Yamamaya42 · #23 02-22-2023, 09:47 AM

Motorola 17A470303 R80 R-80 are quite unobtainium, however... Bandersen has been down this path a few times, and I'm sure can offer guidance if needed.

Chris K · #24 02-22-2023, 09:49 AM

Interesting! This is the first TV I've bought that had one. I did a lot of research looking for replacements in case something like this happened and came to the conclusion it's the rarer of the 2 ballast tubes Motorola used in this TV. Phil has a simple resistor build on his website for the more common one and I suppose I could just modify the values to match the one I'd be substituting for. I have seen the alternate capacitor based, soft start ballast replacement but it's also a replacement for the more common ballast tube so, I don't think I want to mess with trying to adjust that design for my needs. If you come across the ballast, let me know and I can provide the pin values I would need but don't go digging just for me!

timmy · #25 02-22-2023, 09:57 AM

Yup had to make several ballasts for my sets just a few resistors and that’s it works fine.

Chris K · #26 02-22-2023, 11:32 AM

The damage

timmy · #27 02-22-2023, 12:15 PM

Oh yeah well check the schematic and the pins of the ballast and just use resistors just make sure the wattage is high enough.

Electronic M · #28 02-22-2023, 12:30 PM

Resistors of sufficient wattage are good.

Capacitance isn't hard. If you know the ballast resistance just Google search capacitance impedance calculator and look for one that lets you plug in the impedance and frequency and get the capacitance...Or plug in the frequency and guess and check the capacitance a few times (8.2uF is a good starting point).
IIRC the heater portion of both Motorola ballasts used in that chassis family is the same so that math shouldn't need to be redone...The non-heater resistors need to be resistors anyway so there isn't much to dealing with them.

If you're concerned about messing it up start it on a variac and measure heater voltage as you crqnk up the line voltage.

Yamamaya42 · #29 02-22-2023, 12:34 PM

It's a bit more tricky than just resistors,

https://www.youtube.com/watch?v=VEbozz-pVok

As mentioned, He has dealt with this b4! :P

Chris K · #30 02-22-2023, 01:15 PM

Best I can figure from the schematic pin out below. I think Phil said he used 25 watt resistors:
Pins 3 and 4 200 ohms
Pins 1 and 5 37 ohms
Pins 2 and 5 105 ohms
Pins 5 and 7 105 ohms
Pins 6 and 8 Can't find on schematic. Measures 21 ohms on what's left of the ballast.

So my plan would be to use a piece of aluminum as a heat sink mounted on the chassis and screw these to it with thermal paste. Then I'll make the appropriate connections to either a spare pin socket or directly to the socket contacts on the chassis. I will also look into the capacitor calculations and if that's viable, I'll post the plans here first. Any comments or suggestions?