Opto Attenuator Unit

[hikowinter]

Does anyone know about how to test T4B or T4C unit?

 

[abbey road d enfer]

Does anyone know about how to test T4B or T4C unit?

Put it in an LA2 and measure it! The test jig is almost as complicated than building an LA2...

 

[Bo Deadly]

The electroluminescent panel inside needs like 100V of AC (not DC) to illuminate. So you would need a function generator and a 70V line distribution transformer (also called an autoformer) used in reverse (this is what the Urei LA3A does). Run some tone or maybe something with a little wider content like a low-frequency square wave into the secondary of the transformer and then measure the AC on the primary. You'll have to use specific taps to get the target 100VAC.

For example, the LA3A uses a 5W autoformer and puts the signal between the 2.5W and 5W taps (not to be mistaken with COM). Once you see 100VAC-ish on the meter, hook it up to the T4B and measure resistance. As you turn up the fn gen, you should see the resistance drop. Turn it down and the resistance should go back up.