Outboard meter calibration

[Chinagraf]

Hi, I run a fair bit of outboard on hardware inserts and have always just kinda faked it till I maked it as far as levels etc, but I've decided to calibrate everything to -20, so -20 on my daw track is 0vu on my outboard comps meters.
I have a few little discrepancies in level between different units which is no surprise as they are all pretty old. I wanted to check the meter calibration with a DMM, but I'm a bit unsure as to how many volts - .775 or 1.288, loaded with 600ohm or unloaded etc. Is there one standard way I can measure the output voltage of each piece of gear and adjust the meter calibration when the meter reads 0 so that they are all outputting the same level? WIll it be the same from all different comps? Like out of my LA3A's I get .775V, but my 160VU's read 1.228
I have a range of comps, SSL384, DBX160VU's, LA3A's, 1176's 2254's, EMT's, Federals - should they all output the same voltage when the meter reads 0, or should I calibrate them so they should?
I guess what I'm trying to achieve is when a tone is at -20 on a daw track and I insert a hardware comp, the meter reads 0 on each unit, and I get unity gain out and back into the daw with no gain reduction set on the comp. For each compressor.
Any thoughts?
THanks.

[rob]

so you have decided that you'll make -20FSD = 0VU = +4dBu = 1.228 Vrms.
The difficulty you have in calibrating many of those units is that they don't have true unity gain setting. Ie 1176 with compression turned off it is approx unity gain with the input and output controls around 12 o'clock. With LA3 it is somewhere with the level control at 10 o'clock. The 2254 can be calibrated. There are adjustments on the back for metering adjustment. The 1176 and LA3 meters are fairly accurate within 1/4dB or so.

[Chinagraf]

HI Rob, thanks for the reply. Yes I still have trouble finding unity gain on 1176's. What I'm really interested in is
the accuracy of the meters on my old gear, and adjusting them if they are out. So if the meter is reading 0 I should be getting 1.228V at the output. But is that connected to anything, or with the unit just on it's own not connected to anything?
Thanks.

[rob]

there is no meter adjustment for reading level on an 1176 or LA3. I'd have to check the schematic for the dBX. For many older units the accuracy of the meter relied on the intrinsic properties of the meter when used with a fixed resistor, usually 3k6 in series. Making this resistor variable can make the meter trimable within reason. As I said earlier as a rule these meters are pretty accurate, but sure, can be modified to be spot on. One aspect of trying to align metering to super accuracy is that they will drift with temperature and if you get too caught up in it you'll spend your entire life constantly trimming meters!

[Text_Edifice]

Why not save yourself the hassle, assuming your converters are calibrated to -20dBFS = +4dbu, send a -20dbFs 1kHz tone out of your daw, being it back in and tweak your gain stages so you get -20dBFS back in. Quick and dirty I know... (excuse phone induced typos)

[Chinagraf]

Thanks Rob and Dave. All good points.
Cheers.