LA 2A compressor passing signal but no gain reduction

[Ampzone]

So my LA 2A passes signal but will not kick into gain reduction. The meter drops and stays on -20 on the VU. When you turn the unit on it will sit at 0 VU and then drop off. I replaced the photo-cell with a Drip electronics product and re-seated the tubes. No luck.
Any ideas?

[km6xz]

Why did you replace the T4? They do not fail often. If they do it is almost always the electrolumenecent panel that drives the 3 CS photocells. Check with a scope for drive to the panel. It should be a pretty good voltage swing at the plate of V4. If you do not get any signal or very much at the plate of the 6AQ5 V4, look further back in the signal path, say at V5 pin 6.
If you get a good signal swing at the plate of V4 check pin 3 of the T4 attenuator. It ought to be a good level there also. If there is, figure out what is wrong with your T4 modules.

[Ampzone]

Why did you replace the T4? They do not fail often. If they do it is almost always the electrolumenecent panel that drives the 3 CS photocells. Check with a scope for drive to the panel. It should be a pretty good voltage swing at the plate of V4. If you do not get any signal or very much at the plate of the 6AQ5 V4, look further back in the signal path, say at V5 pin 6.
If you get a good signal swing at the plate of V4 check pin 3 of the T4 attenuator. It ought to be a good level there also. If there is, figure out what is wrong with your T4 modules.

Thanks for the suggestions, I read they loose some of their properties, and this unit is older. At least I have a backup now right. I will check the plates and post the results. I did order another NOS 6AQ5 for good measure as reading I found a couple threads with users having the same exact issue and a 6AQ5 replacement was needed in every case.

[km6xz]

A NOS 6AQ5 in that position would be a waste of money since it is not in the signal path. The T4 modules a few people sell will have different characteristic so be sure to use the compensation resistors supplied with it to get the meter to track the gain reduction. When one is in normal, reasonable condition, the LA-2 is one of the sweetest compressors every made. Also the simplest to build and use. It is probably on more iconic albums than any other single device, except maybe a TT patch cable. They are very forgiving in settings and you have to work hard to get it to sound bad. 1176's were also used on most great records.
Good luck