View Full Version : 1968 Magnavox Astronomic Magna color 25" combo tube question


pac.attack76
03-08-2016, 10:34 AM
Back when I piced this up, I noticed it needing the 6JE6C tube. There was also a smaller one to the left of it needing replaced but I can get to the back of it right now although I do have pics taken at the time I got it. Does anyone know what tube that would be? Supposedly, it's what caused the large one to fail.:scratch2:

dieseljeep
03-08-2016, 11:17 AM
Back when I piced this up, I noticed it needing the 6JE6C tube. There was also a smaller one to the left of it needing replaced but I can get to the back of it right now although I do have pics taken at the time I got it. Does anyone know what tube that would be? Supposedly, it's what caused the large one to fail.:scratch2:

From the back view, the one to the left on the PC board should be a 6FQ7, to the right of the 6JE6 is a 6DW4/6CJ3. Also check the 3A3 or one of it's variants, inside the HV cage. A shorted one could cause the 6JE6 to red-plate.
Hopefully, the flyback is alright. :scratch2:

Olorin67
03-08-2016, 11:59 AM
Probably be good to have known good spares for all of those.

Jeffhs
03-08-2016, 01:35 PM
The smaller tube is likely the horizontal oscillator. If this tube fails, it will cause the 6JE6C output tube to fail as well due to loss of grid bias. The output tube's plate current will rise uncontrollably without this bias voltage present on the control grid, destroying the tube in short order. Replace the horizontal oscillator tube first, before anything else. It might not be a bad idea to replace the output tube at the same time; it may have been damaged when the oscillator tube failed.

Note as well if the horizontal oscillator stops for any reason, in addition to damaging or destroying the output tube the power transformer could burn out as well. This is caused by the excessive current drawn by the HO tube without grid bias.

Do not operate the TV if the oscillator tube is defective; there is no point in doing so anyway, since there will be no raster if there is no high voltage. All TVs made after about 1950 derive the CRT's high voltage from the horizontal scanning and therefore depend on the horizontal oscillator and output stages' correct operation; if either malfunctions or fails for any reason, the HV will stop as well and, as I mentioned, the power transformer could be damaged.

Zenith26kc20
03-08-2016, 04:20 PM
T933! Love em! Most likely a shorted HV rectifier (may be a 3CU3, if memory serves me correct). I have never seen loss of drive on a T933 but have had a number of shorted HV rectifiers.
I have seen (and changed) a number of flybacks in them but I find that if the horizontal current is adjusted properly the flybacks don't give a problem. Always discharge the picture tube anode on this chassis. A shorted 3CU3 may give you a bit of a jolt once you remove the cap and try to remove the tube.
It is easy to check drive with the horizontal output removed if you have a VTVM. Just look for high frequency (AC) on the control grid pin of the horizontal output socket. A VTVM is necessary, digital meters get confused when reading this signal. A scope is good but be careful to make sure you are on the right pin. I can bring my schematic tomorrow and give more info.
By the way, a T933 is my avatar and a well used daily driver!
Oh, if the CRT looks like it is weak once you get HV back, the horizontal blanker tube is the main cause of this!

walterbeers
03-08-2016, 06:57 PM
In my years of working on tube type color sets, I have seen many 6JE6s, and other horizontal output tubes, crack along the bottom for no cause at all, other than age. Although I would definitely check the damper, HV rectiifier tube, and horizontal osc tubes for sure as well, as mentioned in the above posts. If the flyback is good, sometimes it's best to change all of those just to be on the safe side, since 6JE6s are are not cheap.

Phototone
03-09-2016, 12:17 PM
I believe this would be an "Astrosonic" not an "Astronomic"

dieseljeep
03-09-2016, 01:55 PM
I believe this would be an "Astrosonic" not an "Astronomic"

I think the term the OP used describes it better. :thmbsp:

pac.attack76
03-09-2016, 04:48 PM
Lol, stupid spell correct. I'm using a pos Samsung galaxy tab 4 and all it ever does is change what I type.:no:
the 6JE6C in this set was cracked along the bottom.

pac.attack76
03-09-2016, 04:54 PM
T933! Love em! Most likely a shorted HV rectifier (may be a 3CU3, if memory serves me correct). I have never seen loss of drive on a T933 but have had a number of shorted HV rectifiers.
I have seen (and changed) a number of flybacks in them but I find that if the horizontal current is adjusted properly the flybacks don't give a problem. Always discharge the picture tube anode on this chassis. A shorted 3CU3 may give you a bit of a jolt once you remove the cap and try to remove the tube.
It is easy to check drive with the horizontal output removed if you have a VTVM. Just look for high frequency (AC) on the control grid pin of the horizontal output socket. A VTVM is necessary, digital meters get confused when reading this signal. A scope is good but be careful to make sure you are on the right pin. I can bring my schematic tomorrow and give more info.
By the way, a T933 is my avatar and a well used daily driver!
Oh, if the CRT looks like it is weak once you get HV back, the horizontal blanker tube is the main cause of this!

Let me know if you get the schematic. These are the only tubes I have atm:

Rca 7558
Rca 6GK6
Rca 6GH8A
Rca 6GM5
Rca 5763
Rca 12AT7/ECC81
Raytheon 6AW8A
GE 6JE6C/6LQ6
GE 6JE6C/6LQ6
GE 6GK6
GE 7061
GE 7061
GE 6BJ6
Sylvania 6BA6
GE 6BJ6
GE 6BJ6
Raytheon 6AW8A
GE 12BE6
GE 6JE6C/6LQ6
GE 6BJ6
GE 6BJ6