Hi Ki-
Your were confused because, from what I can see, the different Studer manuals vary considerably in what they explain and what they assume you already know! My A80 VU MKII manual does not explain the difference between PPM and VU meters. There is one example of a hypothetical calibration where they tell you to set repro to -6 before you set line level and zero, but they don't say why you do this! (More on this at the end of this post). My A810 manual does not explain the voltages associated with different operating levels (0dBm, +4dBm, etc.), so I had to read, read, read... (more reading), and learn. In the end I recalibrated my machine to the +4dBm (1.228V) line level because I couldn't read and say "Ah! Oh now I get it"- I wasn't getting it! I normally use 0dBm (0.775V) as my line level. The only way I finally got it was by doing it! And I set the meters to PPM from VU, which I had been using. Sometimes the only real test of what a person knows is to actually do it, and
then you can say "Ah, now I get it"! The A80 Vu MKII manual has a long table which lists the voltages that correspond to different line levels. The voltage of 1.228 is correct for a line/studio/operating level of +4dBm. These 3 terms are used interchangeably. (Notice I did not use the words "reference level", which is another thing entirely and always pertain strictly to the magnetic flux level that's going on the tape). There's so many "levels" I thought this might be worth mentioning again! The manual states this voltage is to be measured across a 600 Ohm load for NAB eq. For CCIR/IEC1 eq the resistance is 200 Ohm. If you don't have a 600 Ohm resistor, don't worry too much. It doesn't appear to have a large effect. I use a very normal Wavetek multi-meter and tried it both ways. Not using a resistor causes the machine's meters to read 1/2db higher than they read when the 600 Ohm load is connected across the positive and negative wires of the output, and, of course, the voltage remains the same in both cases, so you would still adjust to 1.228V.
So, to answer the last 2 questions of your original post:
The voltage output of the AF generator is irrelevant, because you are concerned with setting the deck's meters properly for the line level of 1.228V. So, input a 1kHz tone, usually from your MRL tape. It is possible to set line level with the AF generator and then adjust meter amps if you do not need a complete calibration of the machine. You do not need a test tape for that. All you need is a multimeter to confirm 1.228V at tape outputs, and then you adjust the meter amps. The info presented below will take us through a basic level calibration, using your MRL test tape, and at the end you will know why I have been using the word "adjust" instead of saying"zero".
There is a 6db difference in the way the Studer PPM and VU meters react, due to the dynamics of the different types of meters and how they respond to a sine wave test signal. Here's the procedure for calibrating which is straight from the A80VU MKII manual:
Step 1:
"Mode selector at SAFE; Meter selector at REPRO; mount reference tape, select preferred tape speed; Line level of this example: +4dBm/600ohms"Step 2:
"Reference tape, 1kHz reference tone section; Depress calibrate button;" (Yeah, that's right partner! They are, in fact, instructing you to engage your repro amp at this point! You'll see why shortly!).
"Adjust playback level pot of the corresponding speed to obtain the following meter reading: -6db". (The manuals are vague here. I was able to determine from looking at A80 and A810 manuals that they are assuming you have meters set to PPM indication. If you are using VU metering you need to set playback level pot to 0db).
Step 3:
"Reference tape, 1kHz reference tone section; Adjust line level with control REPRO" (this is referring to your main output knob)
"to obtain the following reading on the external voltmeter: +4dBm". (The table in this book defines this value as 1.23V, but they don't carry out the decimal point past 1/100th of a volt).
Step 4:
"Reference tape, 1kHz reference tone section; Adjust meter calibration control REPRO/SYNC to obtain the following reading: 0db". (This is where you are "zeroing" your meters, but as you can see, a correct calibration depends on playback of the reference tone to be set at -6db for PPM meters)!!
Here's a test I'm going to reccomend to you so that you can see and compare the action of PPM and VU meters:
Perform the above calibration. Now, don't touch anything, except to switch one channel to VU metering. The VU meter will read precisely +6db higher than the PPM meter when displaying the 1kHz sine wave at reference level, as it should. But instead of seeing -6db on the PPM and 0db on the VU both readings will be changed by 0.5db since you no longer have the 600 Ohm resistor inserted. My suggestion is to now play a tape and observe how the different kinds of meters respond to the music. For me it was a real learning experience because the PPM meters move quite slowly compared to VU meters. But this is only because the PPM has a slow "fall" time in order to give a better indication of the actual RMS output of the music. You will notice as you continue to watch, that the PPM has an extremely fast rise time and will reveal peaks that the VU meter can't catch. And you will also see the VU meter falsely over-shooting, giving false peak readings where none exist. Here's a way cool link to a page that tells about PPM and VU meters:
http://www.mediacollege.com/audio/monitoring/ppm.htmlHaving a properly calibrated machine is a great feeling, isn't it Ki? Happy reeling!