65 maggie

[timmy]

ok so i have this 65 maggie console it has a very sharp nice picture but it is not bright and to get it where it is now the crt bias has to be all the way up. the boost voltage i checked from the R G B drives turned clockwise and it should be around 700 volts but instead its at 1075 vots seems to high and i dont know if this has to do with a dim picture. i thought with votage this high i would have drive lines across the screen and it would be very bright but it has none of this. resistors at the drives are ok as well as the tubes, checked and subbed. any ideas what or why this is.

[DavGoodlin]

It seems like every mid-60s mag I ever worked on had a similar issue, leading in the direction you have taken. It may be worth scrutinizing the video output circuit and its effect on CRT bias.
How are the cathode, G1, G2 voltages at the CRT? This is a series 45 chassis, right?

[timmy]

i am not sure what chassis it is as i dont even have a sams for it. the other voltages are close to what i think they should be but are alittle higher then they should.

[DaveWM]

brightness issues:

voltages at the pins around 200 for the G1, 300-350 a the cathode,700-1000 at the G2

anode around 23-25kv

focus around 5kv

if still a problem then prob a weak CRT, or sunglasses still on.

[timmy]

actually that about what the voltages were yesterday but this is a second crt and still the same. but i dont own sunglasses.

[DaveWM]

then its a weak CRT. Note the G1 and cathode need to be close to those numbers. double check the cathode. most likely issue is a weak video out causing the K to be high cutting off the tube. Or a weak CRT tube, Or you are expecting too much out of it. do you have a set that uses a round tube CRT and is a lot brighter that you are comparing to?

[timmy]

yes i have 2 more roundies and alot brighter. are you talking about the last video tube or the video output tube? i tried another video out tube no change. but it is dull, dark .

[timmy]

as always the green makes the screen much brighter but the colors have to be balanced to look right.

[DaveWM]

the video out tube is the 12XXX prob a 12BH7 or 12GNY etc...

when it gets weak the current drop thru the tube declines, the causes a lower voltage drop across the load resistors in the plate, this lower drop means the plate voltage goes high, this plate voltage is directly coupled to the Ks of the CRT, this higher than normal (195-200v) results is a bias in the CRT that cuts off the electron guns, this reduction it emissions from the electron guns reduces beam current and overall intensity of the CRT.

Now back to the orig question what is the K voltage EXACTLY. Also note if it varies with the brightness control (it should).

Remember if the voltages are correct then the ONLY reason the CRT is not as bright as the others is the CRT is weaker than the others (assuming all the voltage are the same on the others).

[timmy]

well i should have wrote down the voltages but i didnt but they are close to what you posted befor. i tried another video output tube but the same result. and the crt that was in the set was replaced in 82 but i took that one out and put it in my other set because i my have been getting rid of the maggie but i still have it with the crt from the other set and the same result with both crts.

[DaveWM]

try this

get the good bright set turned on and the Magnavox turned on.


write down all the voltages side by side

3 cathodes
3 G1
3 G2
HV
focus

if they are the same then and one crt is brighter then the dim crt is weaker.

[DaveWM]

when it comes to bias "close" does not cut it, need to know exactly, a 20v swing high on the K and low on the G1 will cut off the CRT completely.

by bet is the video out either a weak tube or a defect in the circuits around it. But that is a guess with out voltage checks you are just blindly subbing and replacing parts.

once you narrow down by voltage checks of the CRT pins you can address the fault area.

[timmy]

i worked on my 63 silvertone yesterday and checked voltages and i will tell you they were alot lower then this maggie and this set is bright, because i had a moment of thought that the boost diode took a dump but was fine and the picture would have been dim as well had the diode was bad. the highest on that set was around 650 volts and 190 volts and if memory serves from yesterday was 205 volts

[timmy]

i will be on the way out here but i will get these voltages posted ASAP.

[DaveWM]

and the G1? if they are all low is possible the blanker coupling cap could be leaky but will wait to hear back on all the voltages.