Minty Motorola 9T1 restoration - Page 3 - Videokarma.org TV - Video

Tom9589 · #31 10-21-2021, 04:27 PM

Why do we use germanium diodes rather that silicon diodes for video detectors?

Notimetolooz · #32 10-21-2021, 04:40 PM

Germanium was developed first.
Germanium has a lower "turn on" voltage, and the diodes worked very well at high frequencies (when new). The early sealing techniques didn't work so well so moisture and air get in.
Silicon withstands higher temperatures better so methods like glass seals could be used.

Tube TV · #33 10-21-2021, 10:11 PM

Interesting that the detector diode would fade out like that. I've got to remember that one for the future.

It sure has a sharp picture. Pretty impressive. Good work Bob.

bandersen · #34 10-22-2021, 03:53 PM

Thanks. I finally tried it out with the 8BP4. It seems to be plenty bright, but doesn't focus as sharply as the green CRT. Maybe that's not such a bad thing though. You don't really want visible scan lines.

Now on to the cabinet. I thought there was a scratch above "Motorola" but it was just surface gunk. Darn near flawless bakelite

compucat · #35 10-23-2021, 07:08 AM

That CRT looks good. Bakelite is beautiful when polished and in good condition. It is great to see what these sets must have been like when new. That set was probably someone’s first experience with television.

bandersen · #36 10-28-2021, 10:30 PM

I've been running the set periodically the past few days with no issues

I'll get it all back together soon.

bandersen · #37 11-15-2021, 12:31 PM

I did a lot of experimenting with various ballast configurations and ended up settling on a diode version.
Remember, the two filament strings are NOT equal in the TS-18 chassis.
81.7 VAC and 75.4 VAC for the two strings.

This is the prototype. More compact version coming up.

I built two capacitor version. The one with AC motor run caps was a bit too big to fit inside the original ballast tube.

This one uses Panasonic caps rated for 150 VAC, but not continuous duty.
They got noticeable warm from self heating after 15 minutes of use.

It also took several minutes for the set to fully warm up when using caps.

The diode version with themistor was a good compromise. Reduced heat, soft power up and set stabilized with in a minute or so. Also compact.

Cap version uses an 8uF and 10uF. Diode version uses difference series resistor values.

Here are the schematics.

Pure resistive. Gets very hot.

Least heat but requires bulky expensive AC rated caps.

Using diodes with resistors to fine tune drop.
Use two diodes in series for protection in case one shorts out.

bandersen · #38 11-15-2021, 12:39 PM

Moving on to final tweaks

I ended up replacing only two of the original Moto tubes. Very happy with the results. Sharp picture, solid sync and excellent sound.

Notimetolooz · #39 11-16-2021, 09:00 AM

Excellent job!

Kevin Kuehn · #40 11-16-2021, 12:07 PM

Very nice! I like the diode dropper, simple and cost effective.

bandersen · #41 11-17-2021, 08:40 PM

Thanks guys. Yeah, after screwing around with a lot of capacitors I concluded it just wasn't worth the money and effort.

The diodes do a fine job and only require a little bit of extra resistance to get the current right on the money.

The CL-90 provides a soft power up. It may look smaller than CL-90s you seen before. The effect is that it warms up and cools off faster because there is less mass. Also makes it easier to fit inside. I put it up top along with the resistors to keep heat away from the diodes.

Here's the compacted version that easily fits inside the metal housing.

Notimetolooz · #42 11-18-2021, 09:34 AM

Yes, the tube filaments don't care if they run off of AC, DC or rectified AC as long as the RMS value is correct.
There is one sort of theoretical aspect however. The rectified AC may contain high frequency harmonics of the 60Hz. High end audio and test equipment sometimes use DC on the filaments to prevent 60Hz hum effecting the operation. In the same way, the harmonics from rectified AC MAY cause a problem. I suppose if it did a small capacitor across the diodes might cut down on the problem. Early solid state (Ge, Si) rectifiers circuits had capacitors across the diodes because of this sort of concern.

Tom9589 · #43 11-18-2021, 10:27 AM

Why did the use ballasts anyway? Why not just mount the resistors on a terminal strip? Did they think that placing the resistors on top of the chassis would provide better ventilation? With ballasts you have to replace all of the parts when one goes bad or way out of tolerance.

bandersen · #44 11-18-2021, 03:45 PM

Good point, adding a small cap across the diodes is a good idea. Also, there are bypass caps and series inductors through the filament string that help suppress noise.

Heat. The ballast kicks out around 30 watts. Hot enough they put a piece of asbestos above it to shield the cabinet.

Also the ballast doesn't act quite the same as a fixed resistor. Especially, if an Amperite regulator type is used. They resist the current surge when a cold set is turned on. Sort of like a modern thermistor.

Kevin Kuehn · #45 11-18-2021, 03:50 PM

Quote:

Originally Posted by Tom9589

Why did the use ballasts anyway? Why not just mount the resistors on a terminal strip? Did they think that placing the resistors on top of the chassis would provide better ventilation? With ballasts you have to replace all of the parts when one goes bad or way out of tolerance.

Besides better ventilation on top side of the chassis, they got hot enough that they were prone to burning open, so I suppose the thinking was to make them user replaceable, like a tube. I suspect a lot of owners would have thought they were just another tube. And everyone knows tubes go bad.