1949 motorola table top

[timmy]

hi all, another project i got a 1949 motorola model 7vt5r and the chassis has TS18 on it does anyone have a schematic for this set i would feel better having some guidance befor recapping this. the 7jp4 im not sure how im going to check it because my bk440 only shows an 8 inch tube.

[Steve McVoy]

The Sams and factory manual are here:

http://www.earlytelevision.org/tv_sc...s_postwar.html

[timmy]

m5 is the ballast tube, does anyone know how many resistors are in this metal tube and what the values are because the schematic shows m5 at the line in as 37 ohms, or how to test to see if its good. the amount for an original ballast is just nuts at 85.00 i would rather use ceramic resistors to make this up if i had to.

[dieseljeep]

Originally Posted by timmy

m5 is the ballast tube, does anyone know how many resistors are in this metal tube and what the values are because the schematic shows m5 at the line in as 37 ohms, or how to test to see if its good. the amount for an original ballast is just nuts at 85.00 i would rather use ceramic resistors to make this up if i had to.

Capacitance voltage drop is the way to go! I know it isn't authentic, but neither is a cluster of high wattage resistors. Plus, there is a cushioning effect on the inrush current.
The original ballast tube had some kind of an iron alloy wire, that would rust through. That's the reason, the new ballasts are glass, that are filled with, what I understand is nitrogen gas.

[timmy]

ok so i got the resistances and there is a 200 ohm 2- 105 ohms and a 37 ohm so if i redo this resistor network could i use 2 100 ohm 10watt resistors for the 105 ohm resistors, and a 33 ohm 3 watt for the 37 ohm and 2- 100 ohm 10 watt resistors in series for the 200 ohm ? they are alittle low on the 33 to 37 and 100 to the 105 resistors but i dont know what the wattage would be for each one. does anyone know what the hv should be at the crt ?

[bandersen]

I agree - capacitance droppers are the way to go.

Here's a schematic of mine: https://flic.kr/p/mbSWqg

further reading: http://www.videokarma.org/showthread.php?t=249434


If you stick with resistors, I'd go with 120 or 133 ohms rather than 100. They are used to drop the line voltage for the series string tube filaments. Add the filaments up and you'll find that 105 was a little low to begin with and now we have higher line voltages.

You have 0.3 amps running through each so the power is I2 x R or 0.3 * 0.3 * 100 = 9W if you go with 100 ohms. A good rule of thumb is to double the wattage so I would use 20W resistors.

10W is fine for the 200 ohm. The 37 ohm resistor is not critical so 33 will be fine. It's just there to limit the surge current to the selenium voltage doublers on power up.

[timmy]

ok so the 2-100 ohm 10 watt resistors are ok in series for the 200 ohm, and the 33 ohm is ok for the 37 ohm but is 3 watts ok for this one ? is there any way to somehow modify temporaraly my bk440 crt tester to test this 7jp4, there is no listing for the 7jp4 but there is a listing for an 8 inch crt. i have many ceramic resistors and to make up even the 133 ohm would it be possible to use a 100 and a 50 ohm resistors both 10 watts each in series would this stay 10 watts at 150 ohms or would the wattage now be 20 watts.

[bandersen]

You could see if the filament lights up, but a 440 cannot supply the correct voltages to check for emissions.

The 8 inch CRTs listed in the 440 setup chart are conventional magnetic deflection CRTs like the 8DP4 not the electrostatic 8BP4.

150 is a little high and starve the tubes filaments a bit but probably OK at least for testing. The 100 ohm resistor would dissipate twice as much as the 50 ohm so the combination would be a 15W resistor.

[timmy]

ok so i should just try to get 120 or 133 ohms at 20 watts. so if 2 10 watt resistors are in series would this increase the wattage or would it stay the same ? could you tell me what the heater voltage is for the 7jp4 crt, i have never worked on this kind of set, sorry for so many questions,lol...

[dieseljeep]

Originally Posted by timmy

ok so i should just try to get 120 or 133 ohms at 20 watts. so if 2 10 watt resistors are in series would this increase the wattage or would it stay the same ? could you tell me what the heater voltage is for the 7jp4 crt, i have never worked on this kind of set, sorry for so many questions,lol...

Practically all the CRT's of the day have 6.3 volt, 600ma heaters. That's why there's a series-parallel arrangement of the tube heaters. Most of the receiving tubes in the sets are 300ma heaters.

[dieseljeep]

Originally Posted by timmy

ok so i should just try to get 120 or 133 ohms at 20 watts. so if 2 10 watt resistors are in series would this increase the wattage or would it stay the same ? could you tell me what the heater voltage is for the 7jp4 crt, i have never worked on this kind of set, sorry for so many questions,lol...

BTW, two ten watt resistors in series do equal twenty watts. Also two ten watt resistors in parallel equal twenty watts.

[bandersen]

Originally Posted by dieseljeep

BTW, two ten watt resistors in series do equal twenty watts. Also two ten watt resistors in parallel equal twenty watts.

True, but if the resistors are not equal value, the power will not be shared equally. If one is twice the value of the other, it will get twice as hot.

[timmy]

well if i did do the 150 ohms then the value would probably rise even more when they get hot.

[timmy]

ok so i put the ballast together with the 150ohms in place temporaraly and the tubes are brightly lit but is there anything that is common with these sets why there is no hv and how do you check it without an anode button. there is ac going to the rec tube from the flyback but its low maybe 2kv. the flyback looks new its so clean. is there a common cap like the several 6kv caps that are in there, all the caps are original and i decided not to change any yet just the power supply caps i changed.

[Electronic M]

The 6KV wax caps will kill all HV and deflection. Don't expect any wax caps or lytics of this era to be good, 98% are bad from age by now.