AMPEX MM1200 / MM1100 Bias and Erasure Adjustments and Tips


 


Bias Tips for Ampex MM 1200 / MM1100 machines

Here are some things I've picked up over 25 years of professional 
audio recording on Ampex machines.....

BIAS FREQUENCY ADJUSTMENT

First you MUST check to see that the Bias Frequency is  150 khz .  

Use a frequency counter, and make sure that it's within  1000 hz 
The Adjustment is in the 39 Volt Power Supply Card, and you 
would adjust the "slug" core of the single Transformer slightly up and 
down until you get the correct frequency.   NOTE THAT THIS 
ADJUSTMENT MUST BE MADE WITH A PLASTIC TRANSFORMER SLUG ADJUSTING TOOL... 
if you use anything containing metal you will get incorrect readings !!!!



BIAS CARD TRANSFORMER ADJUSTMENT:

You cannot correctly bias the machines unless both  Bias 
Transformers are correctly set in EACH Bias card.  Under "Normal" 
conditions, these transformers, once set correctly, will almost never 
have to be re-adjusted, so pay particular attention when you set them 
up.

You'll need an Extender Card for these adjustments, so borrow one if 
you don't have one !!!

1)  Put the Bias card on the extender, and insert it into the Bias slot.  
Turn the machine back ON.  You can "fool" the mahine into thinking 
there's tape on it by using a rubber band and looping it around the 
Counter and the Tape-Break Arm, positioning the arm near a normal 
loaded position.  BEWARE that you CANNOT do this for long periods of time -
audio tape also acts to wick away some of the heat  that builds up in
the heads when the system is in record mode.  You should be using 
tape when you perform Erasure adjustments - almost any tape will do
since this first step is not 'tape-type sensitive'.... of course when
we do the final adjustments and alignment you MUST use the correct 
type of audio tape you are going to be using.

2) Engage Record mode.  Select BIAS on the VU Meter selector.

3) Adjust BIAS CAL on the Bias Card so the VU Meter reads "0 VU"

4) USE ONLY A PLASTIC TRANSFORMER ADJUSTING TOOL.  
if you use anything metal you will be completely changing the action 
of the coil and cannot correctly make these adjustments.   Radio Shack 
should sell these core adjustment tools, so go get 'em.

NOTE: Sometimes the slugs in the transformer cans will stick and won't
      easily turn... do NOT force them - they will break!!
      IF you cannot turn one, leave it as it is and move on to the 
      next can and just adjust that one...
      
5) Adjust the REAR coil first, turning the adjustment "slug" inside the 
transformer (access is thru the hole in the metal cover over the 
transformer).  Turn the slug until the VU meter reads maximum bias.

6) Adjust the FRONT transformer the same way, again until the VU 
meter reads maximum bias.

7) Re-adjust the REAR transformer.  Then re-check the FRONT 
transformer.  If there's a broad area in which the VU meter doesn't 
move try and set the slug into the center of this area.

8) If you have an oscilloscope, monitor the Bias waveform at the Bias 
Cal pot, and adjust the transformers as above, then check that you 
have a good undistorted bias waveform.  Make minor changes to 
obtain the best sine wave you can.

9) Turn the machine OFF, and move on to the next channel.  You 
should make these adjustments will ALL channels in RECORD, even 
though you're making adjustments on only one card.


ERASURE Adjustments

The machine's Erasure is dependent on obtaining maximum current 
flowing through the Erase Head.  To get this happening, you have to 
adjust the ERASE Adjust capacitor on the front of each bias card.  But 
this needs to be set for the particular head stack you're using..... the 24 
track adjustment is different from the 16 track adjustment, so if you 
switch heads, you've got to re-adjust the ERASE Adjust cap.

 For MOST 24 track heads...  
(for 16 track heads see the note at the "5a)"  below...)

1) Thread tape and begin recording or"Fool" the machine into 
thinking tape is on the machine (as above 
in the Bias Transformer adjustment procedure...) - again be aware
that you cannot have the machine in Record mode for long without tape running!!!  
I HIGHLY suggest you run tape!!!

2) go into RECORD on all tracks

3) Select BIAS on the VU meter selector

4) Turn the ERASE ADJ cap COUNTER-CLOCKWISE until the VU 
meter drops a few db from the maximum level

5) Now start turning the ERASE ADJ cap CLOCKWISE until you get 
to the maximum VU meter reading.   Now turn CLOCKWISE until 
the VU meter falls by about 1.5 db from the maximum reading.   In 
most machines the fall will be a rapid drop, usually more than you 
want, so you'll have to slowly turn the adjustment cap back until you 
get a 1.5 db reading lower than the max reading.

5a) FOR 16 TRACK HEADS YOU ADJUST THE ERASE 
ADJUSTMENT FOR MAXIMUM VU METER READING.... not the 
1.5 db drop!!!

6) adjust the other channels.....   there is some inter-play between 
channels.... this is nothing to worry about UNLESS you see that a 
large change happens on another track as you're adjusting any one 
card.  If this happens you'll have to go back to that particular card and 
re-adjust the ERASE ADJ again.

7)  Once you have adjusted the entire machine this way, you MUST 
check the machine for MAXIMUM ERASURE>>>>>>>



MAXIMUM ERASURE TESTS

Demag the machine, clean the heads and guides, and use good quality 
NEW tape.  I'm assuming that the machine is somewhat correctly biased for 
the type of tape you're using... if not, you should bias the machine 
BEFORE making these final Erasure Tests !!!


1) Record a 1 khz tone for a few minutes on all tracks

2) Rewind to the head of this section

3) Monitor REPRO

4) Remove any signal going to the recorder

5) Begin recording 

6) monitor each track one at a time, and listen for any residual tone... 
you should hear only a very very small amount of tone, if any at all, 
and this should be buried in the hiss of the tape itself... if not, make 
small adjustments to the ERASE ADJUST cap until you get the 
LOWEST amount of tone as you're erasing it.  Also note that the 
ERASE ADJUSTMENT also controls the "quality" of the erased noise, 
and you want to adjust the ERASE ADJUST for both Maximum 
Erasure AND Minimum tape "gravel" noise.



BIASING for Specific instruments

Here's a real trick that few know about.

One of the best ways to bias a machine is to know that the "normal"  
bias is an "overall" good point, but not the best for certain 
instruments... in some cases you are not looking for great frequency 
response, but a specific great "sound".  Kick drum and bass guitar are 
examples of instruments that really do want to be "incorrectly biased" 
rather than set up at the bias for everything else...

So if you can, get the drummer to hit the kick drum, record this onto 
the track you're going to use for the kick drum, and Monitor REPRO, 
and now slowly turn the Bias adjustment way down (counter-
clockwise) then begin turning the bias back up..... listen for changes in 
the sound, and adjust the bias for the BEST SOUND....   You'll often 
find that you'll get to a bias point way beyond "normal" for low end 
things like kick and bass.... say 5 or 6  db over at 10khz rather than 
the "normal 3 or 4 db overbias.  Try this, it works really great!!!

But do this ONLY with low frequency instruments, for anything else 
use the "normal" bias of between 3 and 4 db at 10 khz (15 ips).


USING THE BIAS CAL

Bias Cal is just a small amount of the actual bias signal that's sent to 
the VU meter when you select BIAS on the VU meter selector.  It has 
NOTHING to do with actual bias, it's just a convienent way to check 
bias levels.  And it's something that you should set when you're biasing 
machines.  That way, if something goes wrong, you can quickly look 
at the Bias Cal reading and know if that track is putting out bias to the 
heads.... 


NOISE ADJUST pot...

on the later Bias Cards there is a small trimpot in the middle of the 
bias cards.  This is to adjust the amount of bias noise that's printed 
onto tape.  To adjust this, monitor REpro while recording a black 
signal onto tape, and adjust for minimum noise.  If you've got the 
proper test equipment, you can adjust this for minimum second 
harmonic distortion, but if you've got that type of test gear around you 
already know what the heck to do don't you?


PURC BIAS  CARDS...

completely different, and beyond the scope of this discussion folks...



MM 1000 and AG 440's

Basically the same as above.  Erasure adjust is set for Maximum VU 
readings, and the maximum erasure tests should be performed.




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