From: jeh@cmkrnl.com
Newsgroups: rec.audio.car
Subject: Re: bias adjustment
Date: 12 Oct 92 17:13:21 PDT
Organization: Kernel Mode Consulting, San Diego, CA

In article <BvtIt9.J9v@acsu.buffalo.edu>, v523d85n@ubvmsd.cc.buffalo.edu 
(Robert C Maggio) writes:
> i have an older sony 7050 tape player and i am wondering about adjusting the
> bias and the other adjustments that may be inside the unit.

There is no such thing as a "bias adjustment" on a tape player!  

Bias is something that happens during *recording*.  

> there are four small screws for adjustment inside near the tape head that are 
> color coded.  what i would like to do is adjust the bias in order to 
> restore the tape response.  

No.  You want to adjust the head alignment.  

Proper alignment of tape heads requires an azimuth adjustment test tape and a
scope.  You play a test tape with a high-frequency test tone and adjust the
head first for maximum output in both channels, and then for minimum phase 
difference between channels, using a dual-trace display. 

If you don't have a scope, many folks will claim you can do the job with a
meter.  Well, you really need TWO meters, one for each channel.  You also need
to do a correct setup on the deck's other adjustments, particularly playback
level, since you are going to be adjusting the heads for equal playback
level in the two channels, and unless you do the level adjustment first (using
a different test tape, one with a low frequency tone that isn't much affected
by head alignment) you will end up adjusting the head to compensate for the
mismatch in level elsewhere in the deck... which is NOT what you are after. 

The same comments apply to tricks involving connecting a speaker or voltmeter
across the hot outputs of the two channels, or to using a scope in the X-Y
mode... if you aren't sure that the levels are right you will just be shooting
in the dark. 

In fact, while you're at it you really need to tweak the deck's playback EQ
adjustments (if any), so that the highs are EQ'd equally in the two channels.
And how do you do that without first aligning the heads for proper playback
of high freqs?!  Presto, you're back to needing a dual-trace scope.  That's the
only way to get the head azimuth adjustment correct independent of any other
adjustments in the deck.  

However, for "good enough for listening" purposes you can probably get by with
a test tape and *two* AC voltmeters, one on each channel output.  You play the
tape and adjust the screws for maximum output with a high-frequency test tape,
equal in both channels.  (Adjusting for a peak in one channel at a time is
useless.)  Then press "reverse" on the deck and adjust the other two screws. 
You will need the service manual for the deck (easy to get from Sony) to know
which screws are which.  The manual will cost around $10; the
azimuth-adjustment test tape from Teac will be around $80.  There are other
sources for the test tapes, but they are either more expensive, or useless. 

Another poster recommended using a 3 kHz test tape.  No.  The 3 kHz tape is
for flutter and speed checks.  Head alignment is done with a much higher 
frequency, usually in the neighborhood of 10 kHz. 

	--- Jamie Hanrahan, Kernel Mode Consulting, San Diego CA
Internet:  jeh@cmkrnl.com, hanrahan@eisner.decus.org, or jeh@crash.cts.com
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