[Eric]

I have a Zenith 7-S-240  http://www.nostalgiaair.org/pagesbymodel...025342.pdf 
 that is done and plays well, my problem is the eye tube it had a 6T5 which only the outer ring lights pretty weakly. I have changed R15 1 meg resistor the original was open. I have put in a 6U5 in place of the 6T5 which is impossible to get at a reasonable price if you can find one. The problem is I'm not seeing the sensitivity on the stronger stations. Is there a resistor that I can change the value to increase the sensitivity like R-14 22k

Thanks 
Eric

[Eliot Ness]

I think the 6U5/6G5's have about the same sensitivity as the 6T5, requiring ~ -22 volts to close.  I'd check all the resistors in the AVC chain to make sure they haven't drifted.  You might also want to measure the voltage going to the eye tube to see what it is reading.

http://www.magiceyetubes.com/substitutions.htm

[morzh]

6U5 is the electrical equivalent of 6T5. If it is misbehaving and the tube's good then what John said.

[Radioroslyn]

Also rf/if alignment will affect it. You may want to do an alignment with a meter connected to the avc line and adj for max - volts.

Terry

[Eric]

Did a voltage check using page 2 (don't know I didn't see this earlier) this is what I got
Pin actual Should be
1 X
2 55vdc 16
3 -3.1vdc -2
4 216 vdc 255
5 0 -2
6 6.4ac 6.4
117 volts at the plug
R-14 24k (22k)
R-5 1200 ohms (1000)
Is pin 5 the cathode? so it should be zero?

[Radioroslyn]

Check R6 it's in the avc line. Also weak tubes can be an issue too.

Terry

[Eric]

    Rechecked the following Resistors
R6 1.1megs  (1meg)
R7 442k   (330k) replaced now 330k
R8  216k   (220k)
R9  494k   (470k)
Replaced the tubes one at a time no difference.
Attached is the voltage check 6f6 pin 5 stands out unless misprint and Pin2 of the 6e5

[Radioroslyn]

Well seem like your triode plate in the 6E5 is too low. Target is good which means that it with light up good if the tube is good but with low voltage on the plate will affect the gain of the tube. It takes -8v on the grid to close the eye as per the RCA tube manual. Perhaps the triode is weak. Try adding another 1M resistor in parallel with the existing one to make it 500K this will raise the plate voltage on the triode. Plate current is low like .2ma.
  Terry

[Eliot Ness]

Well seem like your triode plate in the 6E5 is too low........ It takes -8v on the grid to close the eye as per the RCA tube manual.......

Terry, he is using a 6U5, that takes ~ -22 volts to close.

https://frank.pocnet.net/sheets/049/6/6U5.pdf

I don't quite understand that voltage chart.  The target grid is pin 3 and you show nothing measured there.

[Eric]

Sorry about the Pin 3 now using a 6e5 (brighter) it is -2.8 but when I clip a external ant it goes to -4.6 When you said add 500k you were referring to R 15?

[Eric]

Swapped out the 1 meg for a 680k and saw no change in the eye. going to see about doing an alignment.
eric

[Eliot Ness]

The 6T5 and 6E5 only need ~ -8 volts to close, while your set is supposed to provide ~ -22 volts for a 6T5.  That means you're pretty far off.  I'd try the alignment while monitoring pin 3 like Terry suggested previously. 

If you get the voltage back to where it should be the 6E5 might even be too sensitive on stronger stations, but let's see what an alignment does.

[Eric]

Ok this thing is running me ragged did an alignment several times have no problem picking up the signal at 456, 600 and a little weak on the 1500, while playing with with the Ifs A,B,C &D I get decent response H&G don't seem to do much and J is very sensitive, Right now I'm at 620 no station have about 6.5v on pin 3 and the eye is almost closed, if I move to 680 a very strong station the eye is half open and about 3.4v on pin 3.
Should I put the 1meg resistor back or leave the 680k in?
Just to make sure I'm doing this right the first step calls for .5uf in series SG with the grid cap of the 6A8?
and Rec. Ant. Post is the ant post in the back?
Also should Z & G be shorted together?
Thanks

[Eliot Ness]

It's working kind of backwards isn't it?  It should have an open eye with no signal and a closed eye on a strong station.  I'm not sure which 1 meg you changed out, but I would put it back in and compare results.

For some reason your AVC is way too low.  Did you actually peak the IF's?  J is the padder that you adjust the low end of the dial to.  How about your 6H6 tube?  I'd re-verifiy R6 & 7 and swap in a different 6H6 and double check the 1M resistor in the eye socket between pins 2 & 4. 

After all of that, I think I'd stick the 6U5 or 6T5 back in and see what voltages you read on pin 3 with a strong station vs no station with about a 10' antenna.

[Eric]

ok went back to square one, Changed 6H6 put the 1meg resistor back in the eye socket and did a socket voltage test.
verified
R6 1meg (1meg)
R7 326k (390k)
R8 240k (220k)
Three thing stand out on the voltage check 
6T5 pin2  3X target
6A8  pin3 3X target
6f6 pin5 10x  target
Eye tubes all Pin5 0 should be -2?
What should I check mica caps?