General info on biasing

[jpmorris]

I've just discovered that my A807 isn't doing quite as good a job as I had hoped. Basically, when listening to recorded test tones playing back, I realised that they were accompanied by a variety of rather unpleasant rumbling-type sounds.

I'm fairly certain that this means the thing is under-biased, though I'd welcome any info on the subject. If need be, I can probably record a sample of what it has been doing, but that will probably have to wait until I feel better about it all. (I've laid down an entire side with the machine in this condition, which I guess will have to be remixed now).

Thing is, I haven't been able to follow the steps in the manual for setting the record bias properly. It recommends an input level of -20db, but that seems to be too low for any of the meters I have. Hence, I used a higher level tone for the adjustment, and that seems to have screwed things up.

As an experiment I adjusted the biasing by ear. It is possible to set it such that the noises go away, but then it seems to be overbiased to the point at which the 10khz tone won't even show on the meter.

Any suggestions? Would a 'scope work instead of a meter?

[RRuskin]

1. Make certain your repro electronics are properly physically and electronically aligned. Otherwise, you are wasting your time. (You will need a scope and test tape for this.)

2. Find a Teac MB-20 meter bridge. It's accurate throughout the frequency range and has a sensitivity switch that goes down to -20.

3. Set azimuth on the record head before doing anything else with respect to record alignment. Look at 10khz, 1khz, and 100hz to make sure alignment holds from top to bottom. Deal with any level variations after azimuth is confirmed and bias is set.

4. Set the record eq pots at midpoint before before attempting to set the bias.

[jpmorris]

Originally Posted by RRuskin

1. Make certain your repro electronics are properly physically and electronically aligned. Otherwise, you are wasting your time. (You will need a scope and test tape for this.)

That will not be a problem.

2. Find a Teac MB-20 meter bridge. It's accurate throughout the frequency range and has a sensitivity switch that goes down to -20.

Thanks. I'll look out for one. They don't seem to be too common, though.

4. Set the record eq pots at midpoint before before attempting to set the bias.

I'll bear that in mind. One of the nice things about this machine is that it is software-controlled, so the machine can be reset to its earlier state (unless like me, you lose the scrap of paper with the settings on).

One thing which irks me about the procedure is that it has a table of bias drop levels (delta-U) for each tape type, and of course, the Zonal 700 I'm trying to calibrate for isn't on the list. I'll probably use the values for 456 since the stuff is supposedly compatible.