Back to the VT-71H
 

Juggling multiple TVs this weekend that I have right on the edge of working. I'm still trying to get this 7" Motorola to produce a picture. I replaced the 1N34 germanium detector diode that seems to be the culprit in most of these TVs with a diode kindly sent to me by Don S. who has used them successfully in his VT-71s. I thank him so much for sending me 10 of these for no charge. Unfortunately, that didn't solve the issue.

I have a nice and bright raster on the screen. I'm trying to see if I can get a crosshatch pattern from my Sencore VA62 to go through the antenna with a 1KHz audio tone. I get sound OK but still no picture, just the raster. I'm not all that familiar with the 62 functions yet but I have used it as a pattern generator on other sets and it works fine. Any suggestions as to the next step should be? I suppose I should start with injecting a video signal through the video amplifier 6AU6 but I'm not exactly what I should try to put through it from the VA62. I've been told this is a pretty good tool and I suppose I should start to learn how to use it. Thanks all.

C

The camera can't pick it up but there's a bright 4-3 raster on the screen. I'm feeding a crosshatch pattern with a 1KHz audio signal through the antenna leads. Sound is fine...no picture. I've tried a bunch of different RF levels with no change. Should I start by feeding a video signal through the video amp tube and work my way back? Is that a good strategy? All the caps and any out of tolerance resistors have been replaced and all of the tubes test strong or are NOS. Replaced the 1N34 diode with the recommended substitute. It's a shame I can't get an image on this. Very strong 7JP4...an RCA replacement that's almost NOS as well. If I can get this producing a picture, this will be a really nice piece.

Have you verified that the horizontal and vertical sweep frequencies are correct, or at least close to being correct?

jr

No I haven’t. Should that be the first step before I dive into the IF series?

I would check the frequencies when the screen display is messed up...Looks to me like the horizontal frequency is way off. :scratch2:

jr

+1 , looks like horizontal is off.

Greg b

I know with those vt71 sets if the horizontal hold is turned all the way to one side you get lines and looks like a horizontal problem until you turn it the other way try that maybe that’s it. There is also a few mica caps in the horizontal section I had to change because of really bad horizontal issues so look into those. these mica caps look like dominoes. There is a 680,200,900,mmf

Thanks Timmy I’ll try that. I didn’t replace any of the dominoes because it’s pointless for most restorations I’ve been told but I’ve never been comfortable leaving old components in these things. Such a shame this is stuck where it is. Very strong CRT and a beautiful cabinet. Sometimes I just think I’ve got a black cloud over my bench. I guess I’ve seen too many videos where people change a couple of electrolytics and the thing fires right up!

Yes a lot say that the old dominoes mica caps don’t go bad but there’s more that go bad then not. I have 6 of those little Motorolas and iirc 3 of them needed the horizontal micas

I'll give it a shot...not hopeful but I'll try. Thanks.

Careful - those might be 1,000 volt mica caps. I'm assuming by the thread title this is a VT-71 with a TS-4H chassis?

That's correct. Most of what I've come across are 500v maximum but I have seen 1 or 2 1000v caps listed in the Sams.

You can check the micas with a cap checker as I did with mine and the few 1 kv micas I did use 500v and they are in there for years and never had a problem.

It's not like ya can't find 1kv ones if you really need to! :D

https://www.mouser.com/ProductDetail...vQNQEczw%3D%3D

but, they are very tiny next to the original! :O

Generally speaking don't touch micas in the tuner or IF system, but change sweep circuit micas with maximum prejudice...
Typically RF/IF micas have very little DC on them (worst case 200V) and maybe a volt of AC signal... They'll live forever like that.
Sweep stage micas can have 350-1000VDC on them while being whacked by 200-1000VAC pulses.
Logically you can see what is more stressed and thus more prone to failure.

The nice part about this is the sweep can be eyeballed back into adjustment (so no reason not to change sweep micas), where as the tuner and IF take special equipment and a lot of skill and effort.

Another thing to watch out for is I have encountered tin whiskers in Motorola pots in these sets.

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.

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.

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.

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.

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.

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.

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

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. :)

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!

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

The damage

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

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.

It's a bit more tricky than just resistors, :yes:

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

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

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?

I'm sure that if you follow his examples in the video, it should be OK, he may even see this thread sooner or later and offer some sagely advice, having done this so many times before. :)

This is what I used on my sets that have this ballast config. The 200 ohm is 20 watt.

Wow thank you so much! I'll probably make both the resistor and capacitor version and see how they work.

BTW, I didn't check video signal with the scope but I had continuity between the 6AU6 plate and the CRT so there's no mechanical issue. The coils are good and I changed the capacitor just for good measure. Still no image getting through...just sound and the bright and full screen raster.

What checking with the scope would tell is if the video is getting to the cathode of the CRT at the needed amplitude for a picture, w/o any distortion, if it is, the sync may be SO FAR out of wack that it MIGHT obscure video display ( unlikely, but who knows.) , or the voltages at the CRT are so far off, it just can't let it show video, assuming that it's good enough to in the first place.

Yeah...way to go on this one. On these electrostatic focus tubes, I don't have anything to go on to judge quality other than the brightness of the picture and the fact that it's not the original, it's an RCA 7JP4 and bright. N way to test on my Sencore CRT tester. No matter at this point. I've got a ballast to build. I can check the CRT voltages once that's done...as long as I know what pins are HV.

Did you check the position of the contrast control ? I didn’t give the wattage for the others all the resistors listed that have the = mean that’s the ohms that I used for that one. The 2-150 ohm 2watt. 22-or 25 ohm 20 watt 37or 50 ohm 10 watt all these are what is in my sets. How about the position of the horizontal hold pot?

A few things. You had a very rare after market glass Amperite ballast :tears: That's only the second 303 glass ballast I've seen in many years of looking.

They do not have a shield around them. They are prized because they actually regulate the current to the tubes. They're filled with hydrogen gas and have finely twisted iron wires. The hydrogen seeps into the iron and affects it's resistance. When the wire get's hot, some of the hydrogen is forced out and the resistance drops. That causes the wire to cool and it absorbs more gas.

That perforated metal tube is the stock ballast. It's just some resistance wire on a mica wafer like a toaster. That metal shield is to keep fingers away.


Here are four types. Left to right.
Clarostat aftermarket replacement.
Stock Motorola ballast.
Amperite glass ballast (this is a more common '459 type)
Homebrew using Capacitors, resistors and thermistor.

https://live.staticflickr.com/3771/9...7ebc2b92_c.jpg

This is my current design using diodes. Compact enough to fit in the original metal tube.

https://live.staticflickr.com/65535/...5e067c42_z.jpgAndersen[/url], on Flickr

https://live.staticflickr.com/65535/...69d3dee1_c.jpg

Note this is for the 17A435459 ballast. The '303 is very similar but you will need to move one wire. I believe it's moving the connection from pin 6 to pin 5.

http://suzaku.live-evil.org/0222231655.jpg
I knew i had one!

SYLVANIA 40a1 ballast tube.

Not sure what it can be used in. :O