[daveone23]

I need an operation manual for a 715. I think a 561 might be similar. Would like to know how
To set the machine up. Is the needle suppose to be in the middle of the gauge when power
Is on?

[daveone23]

Needle wont center. I checked the movement and its good. 6.3 volts on the 6c4 filament. Shunted cap on movement is not shorted. What can i look at or known problems with these.

[daveone23]

Voltages seem spot on

[daveone23]

Fixed, it was the lamp. Replaced it and the gauge centers now.

[morzh]

Even if it is not a radio, the cap is still bad.
Replace when possible.

[daveone23]

Spoke too soon. Tested some known tubes and most tested bad. A 25z5 tested good. Could it be the switches?

[morzh]

I will be a bit of a nag, but many, many times here on this forum, we try to bring this point across: when dealing with old equipment, don't go half-a**, replace all that should be replaced, clean all that should be cleaned, check all that should be checked. These are 60 to 100 years old things, they had enough time to accumulate all kinds of dirt, dust, grime and oxidation.
Only after this you try for functionality.

Asking questions "why it does not work" before that is a waste of time.

[MrFixr55]

+1. Definitely change the cap. If not shorted, it is open and could lower the Voltage across the meter. There is not many caps and resistors in this set, the big thing are the switches and all the connections, so replace the caps, ohm out the resistors and pots, including that funky wirewound with the lever.

Do you have the instructions, including the dial settings? (Roll built into the unit with the settings?) Most tube testers have what amounts to a gain pot (The Plate pot on your unit?). Not really gain, but using that term, you get the picture. If this pot is not set correctly, tubes that are not power tubes could read "bad" when they are good. Maybe that funky open frame rheostat is a calibration pot. I don't know if this was done on only Transconductance testers or also with emission testers, but there was usually a tube that was "calibrated" that could be used to calibrate the tester.

Note that right at the "bad-good junction" is a line marked "Line". There should be a switch setting and a pot to adjust for line voltage.

[daveone23]

I replaced the 2 caps in the machine and checked all the resistors. I need to clean all the switches which wont be easy. I have a feeling after I do that it still will test bad. And when I mean bad, I mean the meter goes full left.

[morzh]

Use (don flood!) good grade penetrating contact cleaner (many of us use Deoxit D5), spray some inside the switch ()they likely have some sort of openings for it to penetrate, and then work the switch several times. I was able to do that to a close volume control switch yesterday. Do this for every switch.
Then check them for functionality.
Then see if the problem goes away; if not, see if any rewiring was done; if not, start understanding the electrical aspect of it.
Kinda what you did to the 666.

[daveone23]

I did start to do that but I stalled out because I could not figure out how the grid and plate get voltage. In this diagram I created a scenario using a 01a tube, I moved switches L and S into position. 5v filament which I can trace ok. if I know what voltage to look for at the tube socket, that might help me diagnose it.

[RodB]

Usually in an emission tester, the tube under test is measured as if it were a diode. So, some testers connect all the grids to the plate or cathode. This is one reason I don't have much faith in them. All they tell you is that the tube conducts.

[MrFixr55]

Now that you posted the schematic, I don't know why that Sprague electrolytic cap is across the meter, as it is not indicated on the schematic. It does look "1950"s.

I feel for you, as it is a nightmare trying to trace through the switches. I wish that the tester manufacturer drew a simplified diagram as if the set only tested a triode in the 4 tube socket with 5V on the filament.

[daveone23]

Mrfixr55, its working now. I cleaned the switches and sockets with spray cleaner. Thanks for helping on this one.