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Author Topic: Getting the VU meters working on a Studer Transport with an EXTERNAL tape pre  (Read 1002 times)

Offline stellavox

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Picked up and “reconditioned” two Studer A812’s (a whole story in itself) and along the way decided to add an “outboard” K/C pre directly to one of them.  Mounted the power supply components inside the back panel and the pre box directly underneath the front “cowling”.  “Wired out“ the PB head to the pre. 

Got everything working fine but then was “bothered” by the fact that the VU meters on the overbridge couldn’t work in playback – which of course is a problem with “wiring out” ANY PB head to an external pre.  Tried a number of “schemes” (like bypassing the repro cards and running the signals directly into the line amplifier cards) but settled for the method described below because it involved the MINIMUM amount of modification and wiring.  Note that this "mod" should also work on the "similar" 807/810/820 series.

To get the meters to work, you (of course) have to run a “hot” lead from each external pre output channel back into the transport.  I elected to bring the leads right up to the VU Meter Amplifier card/assemblies mounted on the overbridge.  If you look at the schematic for the VU card (1.820.730-81) you can feed a signal into the VU driver IC-1 by lifting the minus end of C2 and connecting the signal right there.  HOWEVER, now you’ve lost the ability to monitor the “line” or record signal.  So I added/mounted a DPDT toggle switch on the blank panel just to the left of the Left Channel VU Meter assembly.  In the “up” position of the toggle switch, the signal from the external tape pre is routed to the minus end of C2 (actually replaced it with a higher voltage unit); in the “down” position, the signal from T1 on the VU meter card is routed to C2.  Found that I needed a voltage divider to isolate/attenuate the signal coming from the external pre.  Used a total value of around 50K ohms. You can figure out the divider ratio based on the output of your external pre but do suggest adding a screwdriver adjust pot as the "lower resistor" to allow for meter calibration.  The wires were very easy to run from the overbridge to the external pre.  Found that I didn’t need a separate ground wire.  Everything now “looks” and works fine.  Whew!
 
This is a way to get the meters working if you have your external pre mounted “on” the transport.  It got me thinking that if you did want to use the transport’s meters with a “remote" outboard pre, you could add a (stereo) jack on the back of the meter bridge, and bring the pre’s output signals (and ground?) to a plug that mates with the new jack.  You’d have to put the divider someplace – inside the bridge would probably make the most sense.  Could add the DPDT switch I mentioned, or try finding a jack that may have the switching function built in; like the Neutrik NRJ6HF series.

Happy New Year

Charles
« Last Edit: January 02, 2016, 06:34:25 AM by stellavox »