No bias peak on my tape machine?

[Dreams]

Something odd is happening on my tape deck. It's an Otari MX-70 1" 8-track. I play a 10khz tone at +4dbm and adjust the bias, but it never peaks. Well more accurately, it sort of plateaus. It will go as far up as +9dbm and then just sits there, never coming back down until I change the direction of rotation of the bias trimmer. I'm not very experienced with tape machines, but I don't remember this happening before.

What does this mean? Is there a term for this? I've been searching for a while but can't find anything. Help?

 

[madswitcher]

Hi there

Can you give a bit more information.  Does it happen on all channels?  Have you checked the head azimuth?  (the head vertical allignment relative to the tape),  At what level are you checking the bias, this should usually done at -20dB or so?

Cheers, Mike

 

[Dreams]

Azimuth is cool, happens on all channels. I'm using the 15ips speed, but it seems like it works fine at 30ips.

I'm going in at 10khz at +4, as described in the manual under the alignment section. I'll try a lower level and see if anything changes.

 

[pucho812]

check the adjustment pots, those are easy to break on those otari's

 

[ricardo]

What are you trying to do?

Bias is something you adjust on RECORD, not PLAY.  If you are just twiddling for the sake of it, you've probably made the RECORD response and tape overload levels all wonky.

What do you mean by

seems like it works fine at 30ips

 

[SSLtech]

Good god... that machine must be so far out of alignment that it hurt to even THINK about it.

For one thing, you're doing this at FAR too high of a level. -The BBC and most professional UK studios (Abbey Road et al) used to set Bias with at -10VU. Many older consoles had +10dB metering gain switches.

Tape cannot RECORD HF signals all that far above zero VU... you're seriously and significantly into tape saturation and MOL issues.

REDUCE the incoming signal level when you set bias level. ALWAYS. -the meter should NEVER leave the scale.. in fact, it shouldn't even go into the red, specially if you don't have a +10dB metering gain switch.

Studer A827 machines for example have +10dB AND +20dB metering gain, to allow for exactly this sort of correct calibration procedure.

If the tape is saturating, you cannot possibly even HOPE to know where the peak is, and the only way that you can avoid buggering up the calibration is if it's so out of whack to begin with that random chance will leave you better off... all other outcomes leave you worse off.

 

[tubestation]

Hello !! at 30ips you may have a higher Headroom at 10 khz- but +4dB at 10k on a 8trc 1" is too high Level to calibrate,--it is good to check the general Quality of your Oscylator time by time.The Bias Frequency should be as acurate as possible.If you have the right Frequency(like describled in your Manual) good,if not it needs allignment.For example ,my Ampex MM1100 has a adjustible Transformer with a variable core that can be tuned(by a person who knows how to do ,it s dangerous,easy to detune the whole Thing)
Bias Problems can be a real Pain.My Ampex mm1100 once started to make weard Things .Bias arrived at the recordingheadwith the right freq.,but when rec was activated Audio was coming and going like a mad vca.Bias came and went during Seconds;kind of half short.That was at 15 Ips,when 30Ips was selected,everything was fine,defently fine.I thought ,a condenser problem could be the point.I recaped the Transportcard,Servodrivecard and the whole Servodrive for the Engine s 2 Jears before. (Basicly this components have not much conection to my Bias Problem,but you never know)
We recaped all Audioswitching Boards,(needs to demontage all Audioracks,some mechanical fun)cleaned the whole Machine,every single Plug,every conection,every line was controlled from start to beginning with ohnmeters,there was no short nor disconection.Put the wholething together again and.......... noooo
Problem not solved,tried a lot more things etc  In the end it was a rotten bias cable in cannel 16 for 15 ips that created this half short for all chanels.
I think you don t have Problems like this ,just too much level
good Luck ,greatings from Berlin

 

[Dreams]

Adjustment pots=cool. I broke one of those little black ones once, and learned my lesson. Gently now. Anyhow, the bias adjustment on the master card and each channel card uses a 20 turn pot which isn't as fragile.

ricardo. I think I phrased something badly. I meant that I was playing (injecting?) a tone into the machine, recording it, and monitoring it off the repro head. I was not merely playing a tone from a test tape. When I say "seems like it works fine at 30ips", I mean that I can (sort of) clearly find a point where the bias peaks.

Ok. So. 

For alignment, I'm working with a full, original copy of the manual for this machine, an almost perfectly calibrated HP 8903b, a Tektronix 212, a new MRL test tape, and a plastic screwdriver and hex key. When I got the machine, I had someone who used to be an authorized service tech for Otari who was familiar with MX-70s to check it out and do a full mechanical and electrical calibration and alignment, and we put on a new pinch roller from Athan. Since then, I have been the only person doing any sort of alignment. I have never needed to turn any of the adjustment pots very far from where they were, and most of the time, I don't need to turn them at all. The only exception being the bias controls on the master audio control card and on the channel cards. The bias coils have never, to my knowledge, been adjusted.

The bias procedure in the manual, which I follow very precisely every time (since I am an amateur as far as multitrack machines go) clearly states 'inject a 10khz sine wave at +4dbm'. Maybe this is where my problems started. Maybe every time I adjusted the bias I put it a little further away from where it should be. Playbacks have gotten progressively noisier- could this be a reason for increased tape hiss?

I tried injecting a lower level signal (-4dbm) and this seemed to help a bit bias peaks were slightly easier to see (on the VU), and the 8903 would peak at a certain voltage at the same time.

I guess I'm still a little unclear on a few things.

Say I start out biasing and the meter reads -20 on the VU, WAY overbiased. I turn the channel bias pot counter-clockwise until it peaks. I continue to turn CCW, and at a certain point, the needle stops and the voltage stays the same, no matter how much more I turn the pot. What is happening HERE. What is that point, and what does that mean? It seems like it happens awfully close to the bias peak to me, which may mean nothing. 

I can't seem to clearly state my other question right now, which is probably fine, considering how much I just gave you to read.

Thanks everyone.

 

[wkrbee]

Verify that the 'bias traps' are adjusted correctly.If your bias freq.has drifted,or if the traps have been misadjusted,(due to fingeritis,i.e. turning the wrong control), it will suck off bias to ground and you will never get enough bias to set up correctly.Are you using the blank tape that the manual says to use?If you are using a Hi bias tape,(Ampex 499,Scotch 996 etc.), the machine may not be designed to use that type of tape.

 

[RuudNL]

In general I use two methods to adjust the bias:

1 - The traditional way, with a 10 KHz tone, bias x dB 'over the top'

2 - With a 20-40 Hz tone recorded at a high level while monitoring the modulation noise.

With a bit of luck the optimum bias point as advised in the alignment procedure from the manufacturer will match with the point for minimum modulation noise.
Be aware that the LF modulation noise will not be significantly reduced by any Dolby, DBX or other noise reduction system!
(Think about bass guitars etc.)

 

[ricardo]

1 - The traditional way, with a 10 KHz tone, bias x dB 'over the top'

Dreams, what happens is that at HF (10kHz) as the bias increases, the HF drops while the LF distortion & modulation noise get better until some optimum point.  As distortion and modulation noise are more difficult to specify and measure, the HF overdrop method is used to set bias.

Most bias procedures will specify you increase the bias voltage until the 10kHz (recorded at some specified level) drops by eg 4dB, the overdrop.

What does the manual say?  Does it say you look for the overdrop while turning the pot CW?  ie the plateau is your peak.

This will give "flat" response with the recommended tape.  With different tape, the MOLs and distortion may be good but the response may be wonky.  The better recorders, eg Studers have separate tweaks for response after you get bias correct.

BTW, any Noise Reduction eg Dolby, should be switched off

 

[sodderboy]

Seems like it is working correctly.  You should start with bias fully CCW, and increase level to the peak, and continue CW for the manufacturer's spec'ed drop for the speed and tape used.
You never start with full level and go CCW.  This activity indicates that you need some alignment coaching.

Is this what the service manual states to do?

Have you messed with all channels?  What happens if you do a sine sweep?  Did you even need to start mucking with it?
Mike

 

[pstamler]

What kind of tape are you using, and have you ever used that tape before? Some tapes require such a high bias level that many machines can never reach the 10kHz peak.

Also: You should start with the minimum amount of bias signal and increase it. On most machines minimum is with the bias pot all the way counterclockwise.

Peace,
Paul