Motorola VT-71M-A
 

I got this Motorola VT-71M-A at our local flee market a few years ago. I think the chassis is a TS-4C based on the schematics from the ETF. Does anyone know if there are variations of the TS-4C chassis? If there are, can someone recommend where I can get a high resolution CD copy of the second Riders television manual. The Sam's folder 55-16 I have doesn't have a picture of the vertical blocking transformer.
My chassis production number is 5923. The chassis uses a 12AT7 osc/mixer and 6SQ7 for the first audio stage.
The cabinet is in rough shape and the 7JP4 crt has a loose socket.
Thanks,
Ed

The TS-4 had many variants...At least up to the TS-4H in my VT-71. The VT-71/VT-73(TS-4 family) were the highest production 7" sets (and perhaps the highest production 40's TV set). There were so many production changes that not all are well documented and some chassis letter suffixes don't fully match the schematics/documentation. Unless a section appears to have been worked on in the past by a hack assume that is how it is "supposed" to be wired even if different from the schematic...Yours looks pretty original though so it should be a fairly cut and dry resto.

Well I have three of them and the ts18 chassis looks the same but have many differences between the ts4s line. For example the 6sq7 is not in the 18 chassis there is a 12sn7 and used for both the verticle and audio and also the fixed trimmer for the horizontal on the ts4s is a 50mmf mica where the trimmer on the 18 is a 20mmf mica so becareful there are little things that are different.

I finally reached the point of being able to plug in my Motorola TS-4C or earlier:scratch2:, today. I still have more to do, there is hum in the picture. the focus doesn,t seem to work. This set was repaired/ modified before, parts were added most likely because someone was working from the wrong schematic.
Other than recapping, I had to repair the HV oscillator secondary winding, make up a ballast substitute, repair the broken HV pot extension I was looking for, and make a HV oscillator cover.
After adjusting the tuning slugs to channel 4 I got an image from a VCR on the screen, I was suprised to see how stable the horizontal and vertical were. I had more trouble with my Garod 1244G's horizontal sync when I connected it to the same VCR.
This TS-4C has channel 1, horizontal and vertical blocking oscillators, and no AFC.
ED

More Pictures.

Does the set have strong audio ? Maybe change the 1n34 diode it may help in the overall tuning if the diode is leaky. Done several of these sets, they are fun to work on and to watch once all is done.

The audio is very strong depending on the station setting, almost no audio buzz. Focus control just needed cleaning, works fine now. The hum bars were from not having the HV oscillator cover on.
I did a little experimenting with the HV oscillator feedback loop that goes over the 1B3 rectifier. I could set it at just about any position and the screen still lit up brightly.

The voltage dropping resistor approach is OK, but I prefer the Capacitive-Reactance voltage drop method. The old-fashioned ballast tube, cushioned the warm-up of the tubes. The cap method does the same, without having to us inrush limiters.

Perhaps I am missing something here, but wouldn't the peak inrush current be higher when using a capacitor vs a resistor as an ac dropper? :scratch2:

jr

No the cap actually acts like a cushion itself. I was going to use that method but I thought the caps may not last as long as a resistor or inrush thermistor.

Are you referring to the method of using a capacitor and lower ohm/ wattage resistor in series for the filament strings? I figured I'd try capacitive-reactance after I made sure the set was working. Anyone have experience with the resistor-diode in series with the filament strings? I purchased parts to try all three ways. If the capacitors ever short, couldn't that burn out the filaments in some tubes?
Best thing about all those power resistors is the heat they give off to keep me warm in a cold basement, it's air conditioned to keep the humidity down.:yes:
What I don't like about this set is running the entire filament load current through the 7JP4. I'd hate to wreck a brand new 7JP4 that I bought in 1974.:no:
Ed

Use a clamp diode across the crt filiment. And you could also put a thermistor in series with the ac.

The recommended diode to protect the heater is called a "transient voltage supression diode", 1.5KE8.2CA :

http://www.mouser.com/ProductDetail/...P4OSHz1w%3D%3D

jr

Yes, I've tried all three methods including the diode. Here's a video I made while experimenting: https://youtu.be/4IgEYXcJ2xY?t=17m36s

The diode works well, but does not offer a soft power up so I recommend adding a thermistor in series.

Thanks for all the responses, guys. I'll check out your video Bob.
Ed