1949 motorola table top

[timmy]

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++ ??

[bandersen]

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

[timmy]

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.

[timmy]

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.

[timmy]

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.

[bandersen]

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.

[timmy]

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...

[bandersen]

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.

[timmy]

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 ?

[Electronic M]

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.

[timmy]

all the lytics i changed already befor even plugging in i just thought that maybe the seleniums would break down under load.

[Tubejunke]

Originally Posted by timmy

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.

In the past I have read about confusion over the mica or micamold caps found in older equipment. What I read is that there were two kinds that were almost physically identical. One type was actually a true mica capacitor which is known to be very reliable and almost never need replacing. The second type was evidently a wax/paper cap wrapped up in the same type casing as the mica with the horrible color dot identifiers.

Of course the wax/paper would be the ones to replace. It all boils down to simply getting rid of all wax/paper caps as well as electrolytic on a VERY old set an starting from there with any further repair if any is even needed. Also, if I were to do this semi restoration on a very old set (as I have), I would let the set work for several hours over a period of days. I have found that the longer a set works, the more "kinks" just go away more or less.

Finally, the selenium rectifiers will let you know when they fail as a smell like no other will fill your air space.

[Olorin67]

The square caps wwith the color dots are either mica, or paper, but the paper ones are much larger, both in capacitance and physical size, the real small ones are mica. The larger ones are paper.

[timmy]

ok, so to get the proper b+ and b++ voltages the set has to be on and everything in place as if its operating normally as this will have the proper load on the power supply. weather seleniums or silicon recs would this be the proper way to check these voltages ?

[Zenith26kc20]

As a rule, seleniums are trouble waiting to happen. I usually start with about a 22 ohm 5 watt in series with the silicon replacements and once things are running I can change the value slightly up or down to compensate. Not using a dropping resistor in series will raise B+ too high and it can cause instability issues (voice of experience).
Also, make sure the IF tubes are good and have no grid leakage. I have had many problems with the TS-4 and the TS-18 chassis with these tubes.
Those tubular ceramic caps are fine 99% of the time. It's the 6KV ones that will cause problems.
If I remember right you are going to use the capacitive ballast system. If you are going to use the black, iron wire ballst, a variac is a must. At their age they make all kinds of noise when turned on until they stabilize. The glass ballast is fine.
I use a ts-18 as a daily driver for the news and weather. I like the Motorola electrostatics. They are reliable once up and running!