Motorola TS-912 No Raster

[old_tv_nut]

Absolutely no DC coupling for the luma, and a brightness control that appears to swing over way too wide a range (when connected as shown in SAMS). What the bleep were they thinking?

[old_coot88]

(Using the Sams rendition of the circuit) it should be easy enough to customize the brt.range by adding series resistors on both ends of the brt.control.

[DavGoodlin]

There is a factory schematic and even a chapter for this TS-912 in the Motorola Service Manual (TAB books) and authored by Forrest Belt. Ill have a look at that for you.

If I could trade one of my RCA 21's for this TV, it sure would keep me busy. What an interesting one-off design, however "muntzed" it was.

[old_tv_nut]

Here's the schematic from Belt.

You can see how the neon bulb and a resistor were added to provide some DC coupling, along with other production changes.

[6GH8cowboy]

This schematic is GREAT! This will help me with the process. Already I see the 'Alternate" brightness control circuit which at a glance explains the current setup. I'm also going to test the video output tube again seeing it is purpose is going to pull down the plate volts that also get past the coupling cap via the neon bulb on the way to the CRT cathodes. The 270 volts that feed the video out stage winds up at about 100 volts at the tube plate, seems like this is worked pretty hard. Thanks so much. Ill keep y'all posted and include some photos in the future.

[Tom9589]

That is the craziest schematic I think I have even seen. Muntz would have been proud of this baby, full of European tubes and left over B&W tubes. I think The last time I saw parallel HOTs and dampers was in some late 40s B&W sets when they were trying to drive a 14" tube with a 10" chassis.

It's obvious why they changed the brightness control circuitry. In the first runs, they probably had considerable interaction between the contrast and the brightness controls. The version you have should have virtually no interaction as the contrast control no longer affects the DC bias on the cathode of the video output stage.

I was trying to figure out how the HV was regulated. It looks like they have a feedback circuitry going to the HOT grid circuit so when the HV goes down, the HOT grid voltage is changed so that the HOT stage is driven harder. I also noted two focus circuits. One has a separate rectifier and looks fairly conventional. The other one is just a big voltage divider chain with the HV rectifier.

I can't wait to see how this one turns out.

[Yamamaya42]

Originally Posted by Tom9589

That is the craziest schematic I think I have even seen. Muntz would have been proud of this baby, full of European tubes and left over B&W tubes. I think The last time I saw parallel HOTs and dampers was in some late 40s B&W sets when they were trying to drive a 14" tube with a 10" chassis.

It's obvious why they changed the brightness control circuitry. In the first runs, they probably had considerable interaction between the contrast and the brightness controls. The version you have should have virtually no interaction as the contrast control no longer affects the DC bias on the cathode of the video output stage.

I was trying to figure out how the HV was regulated. It looks like they have a feedback circuitry going to the HOT grid circuit so when the HV goes down, the HOT grid voltage is changed so that the HOT stage is driven harder. I also noted two focus circuits. One has a separate rectifier and looks fairly conventional. The other one is just a big voltage divider chain with the HV rectifier.

I can't wait to see how this one turns out.

I'm glad you said something, It was giving me a headache trying to work it out, I gave up, and poured my self a ultra strong James Bond style vodka martini, and watched shango066 videos to relax!

[Electronic M]

Definitely eccentric.

Parallel dampers and outputs were used in the Westinghouses first color TV and RCAs frirst 21" color broadcast monitor... definitely an uncommon touch in color sets though.

[dieseljeep]

Originally Posted by DavGoodlin

There is a factory schematic and even a chapter for this TS-912 in the Motorola Service Manual (TAB books) and authored by Forrest Belt. Ill have a look at that for you.

If I could trade one of my RCA 21's for this TV, it sure would keep me busy. What an interesting one-off design, however "muntzed" it was.

I saw one of those in my time of repairing. A friend got one as a freebee. The two H.O tubes were missing. That model used the 12GC6's. They weren't that common of a tube. After I sourced them and installed them, the set worked reasonably well, but the CRT was a bit weak. He sold the set cheap! That's all I remember after almost 50 years.
There was a Sears, Warwick built set, that was a real oddball. It was a cheapie round CRT, metal table model that I only seen one of. It had a bad CRT and flyback, so it was scrapped. That one is rare also.

[Tom9589]

I'm sure many manufacturers had one very spartan set at a price that got people in the showroom. Once they were in there, the customers would see the nicer TVs (at higher prices) and the salesman has a good opportunity to sell up.

I had a 19" Magnavox table model in a metal cabinet that was given to me in the mid 80s. It was a Japanese manufactured set of the mid 70s. The only thing wrong was a 6J6 in the tuner with a bad filament. All of the other tubes were original and I never had to change any more tubes while I owned it. The only other part I replaced was an electrolytic in the voltage doubler circuit. Over ten years of service with only one tube and one capacitor replaced.

[dtvmcdonald]

That set is not anywhere near Muntzed. Its just very very oddball.

A Muntzed set has far fewer desoupling resistior and caps, and far
fewer parts overall.

The IF amp, however, is very chintzy. It looks like it was designed to avoid
patent royalties. It does have a very strange European look. Europeans were
very strange: thyratrons for horizontal oscillators!

[dieseljeep]

Originally Posted by dtvmcdonald

That set is not anywhere near Muntzed. Its just very very oddball.

A Muntzed set has far fewer desoupling resistior and caps, and far
fewer parts overall.

The IF amp, however, is very chintzy. It looks like it was designed to avoid
patent royalties. It does have a very strange European look. Europeans were
very strange: thyratrons for horizontal oscillators!

The IF strip has two stages of IF, using the frame grid tubes like other US manufacturers. Zenith used three in the better models.
RCA used two stages of IF in several models.
That model Motorola set seems to be the for-runner to the TS914,18,21 chassis.

[DavGoodlin]

Speaking of sets with parallel HOT's, Admiral's first rectangular featuring infamous 23EGP22, used two 6JM6 in the one version and later a single 6KD6.

[dieseljeep]

Originally Posted by DavGoodlin

Speaking of sets with parallel HOT's, Admiral's first rectangular featuring infamous 23EGP22, used two 6JM6 in the one version and later a single 6KD6.

The Motorola ETS908 used two 6JM6's. They lasted the entire ten years I used the set. That chassis also used a 6BK4 regulator and a 1V2 focus rectifier.
That set was really easy on tubes.
BTW, I talked with a technician about flyback transformers in Motorola sets. He said RCA sells more flybacks in a week than Motorola sells in a year.