Motorola VT-71 K Chassis Restoration Issue - Videokarma.org TV - Video

Chris K · #1 12-31-2022, 01:20 PM

Hi gang...yes Old TV Destructo here! Actually, I'm in a good place with this one. I was conservative and took my time with the HV capacitor replacement and I have a nice bright picture on a fairly new RCA replacement CRT. A raster is present. All of the wax caps have been replaced. I have replaced several out of spec resistors but honestly, I haven't tested all of them. I plugged in my pattern generator (it is working...tried it on another TV) and there's no signal getting through. The tuner is clean as are all of the control pots. All the tubes have tested good. There's a slight buzz/whistle on the speaker that disappears when I go from channel 4 on the TV to another channel even though my generator does not produce an audio signal. This makes me think something might be getting through or at least that my channel setup and the coaxial connection is OK.

I want to be very conservative and not mess this up like I did my RCA 721 when I started moving coil slugs and the like without understanding what I was doing. What would be the most basic steps to begin to troubleshoot this issue?

Thanks all,
Chris

Electronic M · #2 12-31-2022, 01:57 PM

Honestly this sounds like a known issue with some of these Motos where the video detector diode fails...When that happens you get no video and weak sound.

If you have a line isolation transformer to power the set with, you could try injecting video at the output of the detector. If you have a pattern generator that can output on the old IF frequencies (like a B&K 1077 Analyst) you can try injecting IF at the ant terminals or 1st IF and see if it gets through.

timmy · #3 12-31-2022, 04:48 PM

Yup I have 6 of those Motorola sets and 2 of them needed detector diodes had no video or sound, new germanium’s and all was history.

kvflyer · #4 12-31-2022, 05:15 PM

Let me also add that I have been down that road. That 1N34 is often bad. The good guys here steered me to that many years ago. The arrow points to the can which has the diode inside. The other picture is what that diode looks like.

Chris K · #5 12-31-2022, 09:02 PM

Quote:

Originally Posted by kvflyer

Let me also add that I have been down that road. That 1N34 is often bad. The good guys here steered me to that many years ago. The arrow points to the can which has the diode inside. The other picture is what that diode looks like.

Thank you so much everyone for your generous answers! What should be the replacement diode for this swap?

Chris K · #6 12-31-2022, 09:30 PM

...and I'm sorry it's the TS-4H chassis.

old_coot88 · #7 01-01-2023, 02:39 AM

We ran into a certain line of GE 19" BW portables that were chronically popping the vid det diode. The fix was to put a 680 ohm resistor in series with the diode. Totally cured the problem. Why they were popping remains unknown. Same fix would probably work for your Motos.

(Edit.) Coulda been 560 ohm, hard to remember for sure.

kvflyer · #8 01-01-2023, 08:11 AM

Quote:

Originally Posted by Chris K

Thank you so much everyone for your generous answers! What should be the replacement diode for this swap?

First thing to NOT use is a 1N4007 diode. 1N34, 1N60, 1N4148... just about any signal diode will work. eBay may not be the best place to find parts because of fake/counterfeit parts. But these diodes are easy to come by. I would take a chance on "The Auction Site" if that is the direction you want to go.

"But wait, there's more!" If you PM me with your address, I will send you a few in an envelope, no charge, no strings attached. Happy New Year!

Chris K · #9 01-01-2023, 12:28 PM

Quote:

Originally Posted by kvflyer

First thing to NOT use is a 1N4007 diode. 1N34, 1N60, 1N4148... just about any signal diode will work. eBay may not be the best place to find parts because of fake/counterfeit parts. But these diodes are easy to come by. I would take a chance on "The Auction Site" if that is the direction you want to go.

"But wait, there's more!" If you PM me with your address, I will send you a few in an envelope, no charge, no strings attached. Happy New Year!

Amazon has 1N34A diodes. Will that work?

kvflyer · #10 01-01-2023, 12:35 PM

Quote:

Originally Posted by Chris K

Amazon has 1N34A diodes. Will that work?

Yes.

timmy · #11 01-01-2023, 12:38 PM

1n34 or 1n34a will work just take note of where the band goes.

Chris K · #12 01-01-2023, 01:30 PM

I have a parts chassis and removed and tested the diode and it tested good. The diode in the working chassis was a General Electric replacement and tested bad. I replaced the bad diode with the old but tested good diode and tried to send a crosshatch signal through. No change. I did order 1N34A diodes from Amazon that’ll be delivered Tuesday. I’ll try one of those and see if there’s any change.

Chris K · #13 01-01-2023, 02:11 PM

Quote:

Originally Posted by kvflyer

First thing to NOT use is a 1N4007 diode. 1N34, 1N60, 1N4148... just about any signal diode will work. eBay may not be the best place to find parts because of fake/counterfeit parts. But these diodes are easy to come by. I would take a chance on "The Auction Site" if that is the direction you want to go.

"But wait, there's more!" If you PM me with your address, I will send you a few in an envelope, no charge, no strings attached. Happy New Year!

Oh my…thank you so much! I’ll try the new ones I ordered and if I have no luck I will contact you! Thanks again

jr_tech · #14 01-01-2023, 02:32 PM

Quote:

Originally Posted by Chris K

Amazon has 1N34A diodes. Will that work?

Read the reviews... some people complain that they actually got silicon rather than germanium diodes.

jr

Chris K · #15 01-01-2023, 03:20 PM

Quote:

Originally Posted by jr_tech

Read the reviews... some people complain that they actually got silicon rather than germanium diodes.

jr

I think you are correct and I looked at Digikey and Mouser and they're listed as obsolete components and not in stock.