Minty Motorola 9T1 restoration
 

A local collector I'd done work for previously dropped a Moto 9T1 a few days ago for restoration. He hinted it was in very good condition.

That turned out to be quite the understatement! I do not believe the back has ever been off this set. The screws holding the masonite back on were pristine

Only cosmetic issues are the knob brights fell out (have two) and a little scratch in the embossed "Motorola"

Every tube and capacitor is Motorola branded.

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Holy crap look at that thing! What a perfect specimen.

Very nice!

I guess very once in a while a example shows up that has been sent here through time from the past!
In this case I would be tempted to re-stuff all the caps instead of making it obvious the caps are new. A lot more work doing that though.
Pretty tight chassis also.

Yep, all Moto tubes. The getter is used up on the 6SL7 and half gone on the 12SN7s, but they all light up.
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I spent several hours tying to slowly reform the caps while using an Amperite ballast tube. They are still warm to the touch but I was able to get some life out of the screen and speaker.
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I am able to pick up 87.7 MeTV FM (AKA NTSC channel 6) radio, but their video subcarrier is so weak now I have trouble getting it on any set.
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With a strong signal source on the antenna terminals. Not a bad start.
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Bob, back at the end of JULY the FCC mandated all NTSC analog cease even ch6 METV FM Chicago. METV FM was able to strike a compromise with the FCC that their NTSC analog video carrier is now replaced with an ATSC carrier, but they are still transmitting analog audio at 87.7.

Well that explains it and marks the final end of an era in my area :tears:

On to more positive thoughts... I carefully snipped out the 5 HV caps and tacked in ASC replacements.
I took a lot of reference photos and am carefully storing all the old caps. I'll be giving the owner two repairs estimates. One with all new caps and one for a full restuffing. Either way the caps will be saved. They have nice Motorola logos under all that wax.

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That got me this. OK reception but unable to get a sharp focus. After a few rounds of troubleshooting, it suddenly started working much better. For a while... then back to a muddy image. Let it cool down and sharp picture gain.

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Still have a few paper caps to replace so I'm not too concerned yet.

I did learn something useful while digging through the HV bleeder resistors. They all have around 700-1,000 volts across them but they appear to only be 1W carbon comps. Those have a max voltage rating of only 500. I've seen other electrostatic sets use 2W which are good to 750. Either way, that's really pushing them.

Now I do have a supply of Vishay VR37 resistors good for 3,500 volts I can pop in, but they are light blue and will run the originality.
Still on the fence - originality vs reliability :dunno: Will consult with the owner.
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I'm using a 7VP1 scope CRT by the way. The 8BP4 tests like new on my Sencore CR70 but I have yet to try it out for real.

Wow, someone definitely took very good care of that all these years. Maybe it was a closet TV. :D

Lol. I got around to testing the tubes and half are bad. The 6AG5 RF amp is full of shorts. The 6SL7 and two 12SN7 and very weak. A few 6AU6 are marginal. Suprising it works as well as it does.

I'm thinking they used it a moderate amount and never had it serviced. It's a shame, but they have to go.
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That ratio detector cap has blown t's guts. Amazing the set works as well as it does on all these bad parts.
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I've never seen one of these sets with every single screw on the HV cover in place.
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Chassis is stamped Dec 14, 1949
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No nonsense recap is done. I saved all the original caps and took loads of reference photos. If the owner wants the restuffed, I will.
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Nuts. Intermittent fading problem persists. Trying to determine if it's a connection or thermal issue.

There are some cursive-logo tubes available on eBay, I might be tempted to replace the bad ones with matching tubes. Such a nice set.

It looks like Motorola used Copper-Oxide Coated chassi in their TVs like Zenith did in their sets, I wonder if that was a Chicago thing... :scratch2: :thmbsp:

I noticed that there are several yellow branded Motorola tubes in that set, it would have originally came with all white logo and apparently it must have been serviced at some time because the yellow logos if I remember right, service or warranty work was denoted by yellow logo tubes. Are the date codes very similar on both tubes?

I suspect you are correct about it having been serviced. There are three 12SN7 tubes and they do not match each other.

I replaced the 12AT7 mixer oscillator and placed a shield on the 6AG5. It seems to be stable now. Will periodically run it a few hours over the next few days to make sure.

Coincidentally, I picked up a cosmetically minty 9T1 of my own in July, 2019. I have not pulled the chassis yet but only one tube appears to be Motorola branded.

The yellow branded Motorola tubes are Sylvania sourced. They're probably original. That must be the first model run where Motorola had the tube supplier print their logo on the OEM tubes. The earlier models were equipped with tube branded with the supplier's name.

That makes sense. I've worked on a lot of the earlier 7" sets and don't recall seeing any Moto branded tubes.

Rats! After running about 10 minutes, reception dropped off considerably. I hate chasing down intermitent issues. No tapping or wiggling helps. Only leaving it off for a while helps.

I bought the 7" portable model complete with the antenna and base. The date stamped on the chassis is January 1949. Most of the tubes are original and branded with the manufacturers name, RCA, Sylvania etc.
One of the tubes that aren't original is the ballast tubes. It's an Amperite glass type. :thmbsp:

Wow is that ever a minty little Motorola.
As for intermitents, have you tested these two resistors warm? 6 megs and looks like they toasty.

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No, but I did measure all the resistors in the HV divider cold. They're within spec, but all have between 600 and 1,000 volts across them. They appear to be 1 watt resistors which have a max spec of 500 volts so I suspect all of them.

OK, time to stop screwing around and get out a scope!
I monitored the output of the detector and waited for the image to fade out.
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Sure enough after about 10 minutes the video signal dropped to almost nothing.

DC voltages and waveforms in the IF seemed OK so I went after the detector assembly. I took the covered off and hit it with a little compressed computer duster can turned upside down to cool it. Bam! image was restored :D

Now, I just need to determine if it's one of the resistors or the 1N34 diode. My bet is on the diode. I'm not wild about having to perform microsurgery to replace it, but I'l give it a try if I have to.

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Also started building up a new ballast using small AC motor run capacitors. Works fine, but need to add bleeder resistors. Got a nice jolt when I pulled it out of the socket!
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My bet is on the detector diode too. I've had a bad one in a VT-71 before and I've seen others have it go on that model too.

I'd just snip the leads fish it out then do the soldering to reduce the chance of overheating or physically damaging the other parts there.

The nice part about the video detector is that unless someone decides to open the can it's in for some reason nobody else will ever see the modern replacement.

I have had the same issue with that diode. Remember it is an ancient germanium diode. I replaced mine and the picture was immediately restored.

Yes, it was the diode. It measure open in both directions. Replacing it with this germanium diode did the trick.
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Nice example of the 9T1 Bob, and nice work, as always. That's one of my fav early compact TV's. I love how the CRT seems to be bulging out of the set. Interestingly, one can't call this one a '40's set because it came on the market in 1950. One of the last of the 3 year run of small electrostatic sets. The only issues with these 8" models is that the CRT's are often found weak and are very difficult to locate. I had to buy a parts set for mine for the CRT and to complete the knob set with brass inserts. The red knobs are very unstable Tenite that want to shrink and lose their medallions. The other issue is that the plastic-weave grille cloth wants to wrinkle and pull from the speaker board. I fixed mine and then it wrinkled again. It's very comfortable to watch compared to the 7" models. Good luck with the resto!

I have the wood cabinet version of that set and it is very watchable from across the room. These electrostatic sets are quite reliable once properly restored.

Nice. You can call that one the "top-of-the-line" 8" in '50. The key with these Motorola'a, like you wrote, is a PROPERLY restored chassis. A minimalist resto on these chassis's will often leave you with "pie-crusting", intermittent arcing, and/or jittery picture.
The best early electro chassis's proved to be Sentinel (also often used by Crosley and Silvertone) and Emerson. The worst are the Philco's and Hallicrafters due to HV fade.

When I restored mine in addition to a full recap, I replaced all the resistors in the sweep and high voltage circuits. I wanted to have a set I could use regularly and have on and off over the years. Mine was made in 1949 and had the picture tube replaced in 1954. These Motorolas may have been economy sets but they are well made. I always wanted one of the Bakelite Admirals but the twice I bought one it got smashed in shipping. I’ve accepted the fact that no one knows how to properly ship a Bakelite TV.

Don't feel bad, I had 2 7" Philco Bakelite shipped with damage. The only way to do it is to have the chassis and cabinet sent separate. Yes, the 8" Motorola's can be marked as '49's, but you'll see them in ads from 1950.

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

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.

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.

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

I've been running the set periodically the past few days with no issues :) I'll get it all back together soon.
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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.

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This is the prototype. More compact version coming up.
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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.
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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.
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Least heat but requires bulky expensive AC rated caps.
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Using diodes with resistors to fine tune drop.
Use two diodes in series for protection in case one shorts out.
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Moving on to final tweaks
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I ended up replacing only two of the original Moto tubes. Very happy with the results. Sharp picture, solid sync and excellent sound.

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Excellent job!

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