1949 motorola table top

[bandersen]

From what I've read brighteners don't really help electrostatic CRTs.

Did you check these voltages on the CRT ?

pin 2 (cathode) - 0 -220 volts (brightness control)
pin 5 (A1) - 1,600 - 2,400
pin 9 (A2) - 5,000 - 6,000

[timmy]

bandersen , the pin 9 would be hv and i checked that right off the 1b3 and its at 5kv and pin 5 is focus and dont know if it makes much difference but the focus works very good on this crt even though its dim. the brightness i did check but an exact voltage i would have to check again but at 0 volts IIRC is full bright and the pot goes to .8 volts. i really hope i missed something here and these crts are infact good. there is one thing i dont understand is at the 6sL7 pin 5 there is a cap to ground, looks like a small dog bone type and its about 1/4 inch long and its got colors on it and at random it will arc between the leads and i dont know if this has anything to do with anything although im sure its not supposed to arc this way. it seems there is hv there and is seeking ground. well if i did miss things here i certainly want to find out, i dont want to condem these crts if they are not bad. ok i just revised the bright voltage 0-220 v , A1 is right about 1600v vary with focus control, A2 just touching 5kv.

[timmy]

ok with all being said above im sure now the crts are bad because i said i applied 9 volts to the crt heater with a separate transformer but i had the wrong tap hooked up i had 7 volts. so now i applied 13 volts @ 300 milliamps which then the voltage came down to 9.5 volts ac and i tried this with the sylvania crt which when plugged in to the chassis the heater was bearly visable at 6.6 volts but after applying the 9.5 v the heater lit well like any other heater i have seen and the picture was bright and watchable in a lighted room. the other crt , the rca did get brighter but not much still watchable in a lighted room. so i know im done with the chassis except for the arcing from pin 5 of the 6sL7 cap to ground, maybe this is caused from bad crts, i dont know. i checked the ohms at 1-14 the heater pins on both crts and they are the same so why would one bearly light but the other seems to light normal. i could see if resistance went up on the crt heater that bearly lights but they are both the same. the sylvania , the gun at the heater looks new no marks or heater signs or discoloration from heat but the rca heater part is black and shows signs as if it has been used alot. so what may cause the heater to not light properly with the proper voltage, but rather more voltage.

[bandersen]

Could be the phosphor is wearing out. I have about a dozen 7JP4s that all test the good emissions well into the green but the brightness varies considerably when used in a TV.

BTW the CRT filament current is 600mA and 6SL7 arcing is a common issue with these sets. The 6SL7 plate voltage is derived from the HV output through that string of megaohm resistors so make sure they are all good and not arcing.

[timmy]

well all i had to try was 300ma but to be so bright with the increase in voltage and the filiment still with that added voltage dont look bright at all like its getting to much voltage as it would get brighter with added voltage. those resistors, there is 2 of them ? increasing the heater voltage would have a positve affect on worn out phosphor ? is it at all possible that this sylvania crt has a defect in the heater like maybe a higher voltage filiment was put in because from several different 7jp4 tubes i looked at all had the heater housing turned black from heat and maybe lots of use yet this one has nothing on the heater housing at all that would suggest lots of use and by putting higher voltage to the heater it did not make the heater much brighter at all where as i thought it would have. the reason i ask this is because i get just bearly a shadow for a picture in a dark room as if there were no hv but adding this voltage was like having a new tube, it went from one extreme to the other.

[bandersen]

There are around eight resistors in the HV divider string. It's all the ones around the centering controls. If any of them are faulty it could allow too much voltage on the 6SL7 plates and they'd arc over. I'd try replacing the 100K right off of the 1B3 HV rectifier too.

Don't recall hearing of much luck with increasing the filament voltage on a 7JP4. Could be why they didn't make brighteners for them.

[timmy]

well i dont know what to think by upping the heater voltage and having a difference of day and night it made, but this one the voltage made a difference but i would still like to get one that works the way it should. the extra voltage on top of 6.6 ac was only 3 volts is this about what a brightener would raise a tube to or more ?

[Electronic M]

You can put a brightener on a strong new tube (of nearly any type CRT or signal) and get a noticeable boost in performance. Cathode emission behavior is proportional to heater voltage, and life span is inversely proportional.

You could operate that 6SN7 at 3.3V or 12.6V if the benefit of either one was more important than the downside.

You need to find someone with a working restored set to hook your tubes up to in order to properly test them. It could still be a cathode-grid voltage issue plaguing you.

[timmy]

all the voltages are where they should be thats why i couldnt understand why the day and night difference with this crt with 3 volts more. the chassis i have here is done except for a few resistors to check and based on the picture quality with the added voltage of that crt its looking really good for a 60 year old tv.

[timmy]

well i got a super bright 7jp4 and it was regunned, dont know when it was done but the neck where it was cut is actually a little bit smaller in diameter then the rest of the neck on the tube itself. the difference in the 2 diameters is so obvious and the line is visable where it was cut. i didnt know this until i unraveled the aluminum and paper from the neck so i can clean it up. so i have a few resistors to change and ill post a picture of the result when i am done. i got 2 of these motorola sets one i recapped and the other was for parts so i went back to this friend of mine that i got these sets from and i did know he had a third set but i had to dig for it and found it with this crt in it, big surprise. i do have a cabinet not the one with the cover if anyone may need a cabinet for maybe the front plastic part or whole. the cabinet is a 7VT1. does anyone know if an admiral b@w set is worth anything, this is one of those 2 tone red and white sets its in very good shape all knobs, cord, antenna, complete im not sure when these sets came out maybe late 50s or early 60 -61 dont know..

[bandersen]

Weird - never heard a 7JP4 being rebuilt. Wouldn't think it'd be worth the effort back then.

If it's an Admiral like this, it's from 1957: https://flic.kr/p/r4DsgR

Usually go for $50-$75 in good, unrestored condition.

[timmy]

here is the pics ,its done for now but kind of got the buzzing in the audio like its not tuned all the way and cant get rid of it but channel 2 is better then channel 3 but its ok. the filiment string on this set calls for 105 ohms x2 , i have 150 ohms x 2 and the crt filiment voltage is 6.06 vac so for it to be right it would show 6.30 on the dvom. wondering if this heater voltage is to low or would it hurt anything along with the other tube heaters are alittle low. i did have 130 ohms x 2 befor and i am debating if i should put it back so as the heaters will be alittle higher, what do you guys think ?

[Electronic M]

Slightly low heater voltage = slightly longer tube life, but slightly reduced tube performance during that life. If you are going to use it a lot keep it that way so it lasts, but if it is going to be a shelf queen that occasionally is powered on to give a Brilliant demonstration of the pinnacle of the performance of that design, then going for 6.3 is fine.

One good idea is to get what IIRC are called TVS diodes. They are essentially AC voltage Zener clamps. Install one across the CRT heater to prevent line voltage spikes from potentially damaging it. There is a good thread either here or on ARF about them in 3" Pilot sets.

[timmy]

here is the 7jp4 that was regunned, notice the difference in the diameter of the glass where the weld took place the top section is clearly smaller then the bottom. and i spoke to someone about this tube and he also said these tubes were not rebuilt because of the type of glass used back then on these tubes would not hold up to certain temperatures during the rebuild process and he was surprised to know that i had one that was redone. early television museum told me they most likely will not be rebuilding the 7jp4, i dont know the exact reason maybe be infact because of the glass i didnt question it further.

[Steve McVoy]

I'm very interested to see that someone successfully rebuilt a 7JP4. As far as I know, Scotty at Hawkeye wasn't able to do it. It might have been because guns weren't available.