1949 motorola table top
 

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.

The Sams and factory manual are here:

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

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. :yes:

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 ?

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.

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.

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.

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.

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.

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

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.

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.

ok so i recapped the power lytics would it now be a good idea to change the hv caps, the 6kv caps, and see what i get from there. the rest of the caps are aside from micas the same values as the color sets, 200v or 600v. the 6kv caps are stated to be oil caps does this make any difference oil or otherwise ?

JustRadios has the 6KV capacitors. They are bad if they are the black oily ones or the wax type. These were the major failure in these sets.

i went to just radios site and cannot find how to order these hv caps. is there a special way to navagate the site? the caps in there are oil so can just a film or metalized cap be used in place of oil ? i know mica caps in the hv area are sometimes used for oscillator timing, are these known to go bad even as old is 1949, some say they dont go bad but i dont know.

The justradios site has a capacitor shopping cart. Just click it and it will take you to all their capacitors. The original ones were wax, then oil from what I have seen. The ones from just radios are small tubulars. I have used them in all of my Motorola sets and in the high voltage of my Silvertone with total success. Without changing these the sets can act up in so many ways it'll drive you crazy. Also, the 12SN7 tubes can be somewhat critical in vertical and horizontal. Try several if stability or size issues crop up. Also, ohm out the horizontal centering pot. I had one that was 1/2 value and prevented the horizontal centering from working. I had to add resistors on both sides to bring the value to the one listed in the schematic. After that it worked fine.

i found it on just radios and is it ok to use .0047 6kv in place of .005 6 kv ? since i see this as an option but i really dont know.

I used those in both my Admiral and my Philco and they work just fine.

Gregb

Mica capacitors in the deflection stages can cause issues. As far as mica capacitors in any TV go always replace the ones in the deflection stage, but don't change the ones in the IF/tuner unless you have a darn good reason to believe they are bad.

Careful. The mica caps in the horizontal circuit (200pF, 680pF, 900pF) are rated for 2,000 volts. Hard to find mew mica caps rated for more than 1,000o volts and 900pF is an oddball valuee.

Some of the caps in the sweep circuits are tubular types. I've got four Motorola electrostatics running and they all have the tubulars still in them. The ones that look like brown dominos can give problems. In the sweep circuits I try to use 1600 volt replacements. They seem more stable than the 600 volt types (less fiddling with the horizontal and vertical hold).

Yes, there are some tubular ceramic caps. They are very reliable and I'd avoid messing with them.

does anyone know what the hv should be for the crt and is it a fixed voltage when done. also what is the B+ voltage and the B++ voltage , i have no info in the schematics as to what the voltages are.

Approximately

CRT = 5,500 at the highest potential right on the 1B3 rectifier. Not sure what you mean by fixed ?
B+ = 125
B++ = 250

fixed meaning how it was built originally, not having an adjustment. thanks for that info. one thing i noticed is voltages are jumping up and down as if its trying to get somewhere but then hits a block so its probably those .001 and .005 6 kv caps waiting for them from just radios.

to change the selenium rectifiers and put in 1n4007 diodes, if this is the ones to use do you know how the arrangement works with the resistor that goes along with and where it goes with the diode to lower voltage. and the value of the resistor used with silicon diodes for a set like this.

Yes, you can use 1N4007 which are rated for 1 amp. Even better use 1N5408 rated for 3 amps if you have some.

I don't bother using resistors, but if you want use around 47 ohm @ 5 watt in series which each diode. Either end - doesn't matter.

so if i decide to change the seleniums to 1n4007 diodes i wont use resistors and i will keep the ballast as, 130 ohms for the 105 ohms and the 33 ohms for the 37 ohms but should the 200 ohms be changed to a higher ohms because of the voltage difference on the b++ ??

If you want - doubt it will make much difference but 220 are probably easier to find than 200.

ok so there are a few mica caps rated for 2kv that are important in the vert and horiz section if these caps are bad or go bad are there any close substitutes that can be use to get by. i already did a search for these caps and no luck, as i figured. i think there was 3 micas not sure unless i check the schematic , 200, 680 ,900 , mmf . ceramic disc maybe the closest but not as stable but there must be something out there to use in place of an unobtainable part.

there is a 680pf 2kv ceramic disc in the rca or clones, would this even work in place of a 680pf 2kv mica capacitor. i know it wont be as good in some way but not sure, maybe this would create noise within the circuit depending on what circuit it is subbed in. if this were a timing circuit how may this possibly affect that circuit ? im just trying to find possible subs that may work if i find the micas are bad even though they are know to be reliable, but they do go bad.

i am also looking for the front fold down cover for the motorola table top tv model 7VT5 if anyone may have this and is willing to sell it please let me know. if i cant find it i may just make it. thanks.

Unless is it is a NP0 or C0G type, a ceramic caps value will vary with temperature causing the sync to be unstable. Don't worry about it. I've restored a bunch of these and those HV mica caps have all been fine.

the temperature that would cause sync to be unstable, would this heat be from the tubes or the cap in the cicuit. because the cap if were used could be moved to a cooler location, i dont know... :drool:

The ambient temperature under the chassis will rise while the set is playing from resistors, tubes, etc. Don't worry about it - odds are very good your caps are fine.

as for the selenium rectifiers i know if there is no output well then they are bad but what other things will they do when going bad . would they short in such a way where the voltage is there then drops then its back, this is what i have here testing the b+ and b++ and the voltage is jumping up and down and the seleniums are getting warm but not really hot. maybe this is consistant with a shorted cap, i dont know but trying to find out what these rectifiers do when going bad. could this happen where the voltage goes up and drops then goes up and down under a load ?

Just change all lytics and paper style caps, and the seleniums. Trouble shooting it with all those crap parts in it would be like trying to plug the holes in a boat with a chicken wire fence for a bottom. Lytcs usually don't just short, they can leak current at high voltage which you can NOT test for with a DMM, and this leakage may not be constant in magnitude.

60's sets may be worth troubleshooting, since their lytics, and mylar/paper caps are much less failure prone, but everything made before the late 50's (and select sets into the early 60's) with the older style caps just ain't worth trouble shooting with original caps installed. And even if you were to try with proper equipment (a tuning eye type capacitor checker such as a Heathkit C-3) you'd be hard pressed to find 2% of the caps good in most sets of that era.