Motorola VT-71 modification question

[bigaudioal]

Seen many posts regarding this set and selenium replacement and ballast replacing. The ballast rebuild seems easy (going with the capacitor/resistor network) as illustrated here back in 2012:

http://www.videokarma.org/attachment...9&d=1290802806

My question is concerning replacing the seleniums with diodes. I know the diodes have less resistance than the seleniums, so need to insert dropping resistors. My questions is where to insert them and do I need one for each diode? Also what are the specs of the dropping resistor(s) needed in addition to where to put them? Attached is a schematic showing each selenium as M1 and M2. Any assistance would be much appreciated.

Thanks.

[bandersen]

I don't bother adding resistors. Were only talking a few volts difference. If you really want to then you can simply increase the 37 ohm resistor to something like 82-100 ohms. Keep in mind it will run hotter.

[bigaudioal]

Originally Posted by bandersen

I don't bother adding resistors. Were only talking a few volts difference. If you really want to then you can simply increase the 37 ohm resistor to something like 82-100 ohms. Keep in mind it will run hotter.

So you just switch the seleniums over to diodes and live with the voltage increase? Some people scream and yell about making sure you add or increase the resistance after replacing the seleniums. Is the VT-71 different than other sets and only has a minor voltage increase when installing diodes? Or have I just been too tuned in to the "SCREAMERS?"

[bandersen]

Yes, the voltage doesn't increase that much. Something like 5-10 volts. Same with most other sets. I'm not saying adding them is a bad idea, just that I personally haven't and my sets work fine.

When you really need to add them is replacing a vacuum tube rectifier like a 5U4. Here were talking more like a 50 volt increase.

[Kamakiri]

Funny, talking about 7" Motorolas. I've got one on the bench ready to go, and it turns out the glass ballast tube I got from a friend in a parts set has cracks in the stupid glass

Well, there goes tonight's viewing

[Eric H]

Originally Posted by Kamakiri

Funny, talking about 7" Motorolas. I've got one on the bench ready to go, and it turns out the glass ballast tube I got from a friend in a parts set has cracks in the stupid glass

Well, there goes tonight's viewing

Won't it work with cracks? It wasn't in a vacuum was it? a

[bigaudioal]

Oh man! I have to catch up Tim and start on mine. I am in Porthole land - but with a light at the end of the tunnel I think.

[bigaudioal]

[Kamakiri]

Originally Posted by bigaudioal

Oh man! I have to catch up Tim and start on mine. I am in Porthole land - but with a light at the end of the tunnel I think.

Well, incidentally I built my first ballast. It ain't pretty, but it does work

Now on to finish the resto

[timmy]

although the ballast will be cooler but dont the capacitors take a mighty beating, regulating and or resisting ? has this idea been proven to outlast simple resistors ?

[dieseljeep]

Originally Posted by timmy

although the ballast will be cooler but dont the capacitors take a mighty beating, regulating and or resisting ? has this idea been proven to outlast simple resistors ?

I found capacitive reactance voltage drops to be very reliable and a better subsitute for ballast drops.
It works better for inrush and surge suppression, very much the same as the ballast tube action.

[Kamakiri]

Well, having mediocre luck with mine so far. Got a video signal, but there's zero sync lock and the audio's totally dead. Horizontal width drifts all over the place and goes right of center. Found a 1 meg resistor off the video detector that looked like it took some heat, but still measured within tolerance. If I turn it off then back on, the picture sharpens up considerably for about a second.

Figured with the weird issues with the sync and loss of audio it HAD to be the 1N34 diode, but that tests good.

I'm used to watching TV on these things after a recap! This one however is going to be a challenge.

[Kamakiri]

Question, was thinking about replacing the selenium rectifiers. I have a couple new 1N5408 diodes here. Will those work as a substitute?

[ChrisW6ATV]

1000V at 3 amps, general purpose. Yes, those should be more than adequate.

[polyphase]

No need to add resistors. The extra voltage is a good thing, as the VT71 chassis is "voltage starved" IMO. The 37ohm resistor I'm pretty sure is just there to protect the selenium from inrush current. With diodes, this is not an issue so one could decrease the value of this and get more voltage, and better performance such as better sync lock (I'm using 20ohm).

As for the capacitor ballast, at 60Hz at less than 1A they are hardly being stressed at all