Question about Bias - Otari MX5050

[thehammondman]

Hi all, speaking of my Otari MX5050 mkiii 8 track:

I haven't messed with the bias yet, but was able to obtain a 0 VU reading at both 1K and 10K while in record mode, listening off the repro heads. The Otari 8 track has a built-in oscillator, and I've depressed the SRL switches, so everything is consistent at VU now, from input levels to repro levels.

However, I haven't adjusted the bias yet. Should I have done this first, and then reset the record/repro levels to 0 on the VU meters once the bias was complete? I've found so many threads addressing calibration/bias, but am still confused. When all is said and done, should the VU meters read zero?

I even have the original manual, but even it is confusing at times.

Help is much appreciated. Thanks

 

[miroslav]

First....are you using a MRL calibration tape (or similar) to set up your machine...or just doing basic input/output level settings using the built-in oscillator...?
Also...you should have/use and external oscillator for setups. The internal one is OK when dialing in the levels using the front panel Min/Line knobs, but not really good for actual calibration, IMO.
If you are messing with the actual calibration pots and not just the I/O knobs...then read below, because there's more to it than what you are doing, and I wouldn't mess with the bias of you are doing a complete calibration process.

In general...
Start with the Reproduce/Playback head calibration & alignment...and for that you need the MRL calibration tape.
You set the levels...and then the equalization for the Reproduce head.
Setting levels using the calibration pots without a known reference (MRL calibration tape)...you may see "0"...but it may not really be "0".

Then you move to the Record head.
There you start by setting the bias for the type of tape you plan on using to record with.
From there you move to the Record level setup and then finally the equalization.

Technically, if you were also going to do azimuth alignment for each of the heads...that would come before the level and equalization steps on both the Reproduce and Record heads....but it's not always necessary, assuming you know that the azimuth is already as best as it can be.
If you are just guessing or assuming...then all subsequent settings are affected by that.

I don't have the 5050 8-track....but I do have the 5050 BIII 2-track, and also an Otari MX-80...and those are the basic steps, but each machine may have some intermediate step, and each machine will have different adjustment pot labels, so I can't tell you specifically how that goes for your deck....but you have the manual, and while it may seem a bit confusing, the Otari manuals are actually very well written and should be easy to follow...just take it one step at a time.

So...what have you actually done from the start AFA calibration and alignment? You are asking about the bias, but (and no disrespect) I get the feeling like you may not be doing the complete step-by-step process, or you are just focusing on the basics by looking at the available adjustment pots.

Go to the manual, and start at the first page where calibration is detailed....you can skip most of the involved mechanical alignment, though again, the azimuth should at least be checked, and you need an scope for that, because if you just try using a test signal, it's not going to be accurate.
There are some ways to get around the scope thing...though most are just makeshift solutions.