u73 identification and help

[Michael Tibes]

Afaik the U73 and the different verisons were built by TAB. The Reverb listing shows the TAB logo in the bottom right corner, your faceplates look different though, the labeling on the switch looks different and there's no logo. The pcb inside looks like TAB again. I guess that supports the theory that it it might have been a one-offf, maybe a development sample? The 'typical' U73 / U73b I've seen all look like the pic in rock soderstrom's first post. I guess that's how they were built for the radio / TV stations. But there were also versions for mastering / vinyl cutting, going along with a varicom. I've never seen any of those, not even pictures. It might make sense not to have release controls on the U73b for that, since the varicom takes care of that. But the control voltage needs to bw accessible at the pinout for the varicom to work...

It would be interesting to see a schematic, maybe you're in the mood to reverse it?

 

[ben@timberlakeaudio]

Yes, the markings on the faceplate are all stencil painted without engraving.

There are 8 diodes instead of the 3 in the u73 or u73b. 5 of them are somehow involved in the sidechain rectification stage.

I guess I will have to reverse it and create a diagram, at least enough to get it up and running.

 

[Dark1]

>>There are 8 diodes instead of the 3 in the u73 or u73b.

Yes, all the U73a photos I have seen had this (bridge?) diode arrangement. It seems to be rather different from regular U73/U73b. It would be very interesting to see the schematic, please make one! It would be easiest to print-out a U73 schematic and spot/draw only changes, some parts may be identical...

 

[Murdock]

Yes, the markings on the faceplate are all stencil painted without engraving.

There are 8 diodes instead of the 3 in the u73 or u73b. 5 of them are somehow involved in the sidechain rectification stage.

I guess I will have to reverse it and create a diagram, at least enough to get it up and running.

Did you make any progress with this?
I’m very curious how and where the 150B2 regulator was used. The Zener in the standard U73 was (as far as I understand the circuit) just a small 5.1V zener to “pre tension” the rectifier diodes to get the sharp knee in limiter mode. In compression mode the diodes were connected to ground.
I doubt the 150B2 served the same purpose with its 150v stabilized voltage. I would speculate it was to stabilize the screen grids or plates of the two vari mu tubes as was done in a lot of other compressors e.g. U23.

 

[ben@timberlakeaudio]

No, I haven't had a chance to start yet. I've been working on restoring/repairing several v76 modules and needed to get that done before I rewire my connector for the u73. Hopefully I will start on it soon.

 

[Murdock]

No, I haven't had a chance to start yet. I've been working on restoring/repairing several v76 modules and needed to get that done before I rewire my connector for the u73. Hopefully I will start on it soon.

Sorry to bother you again, but I’m really interested in your findings. Did you have time to trace the circuit?