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Thread: 388 bias adjustment

  1. #1
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    388 bias adjustment

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    Fellow 388 operators...

    How are you setting your bias?

    This is kind of a new world for me because I've never aligned a 2-head transport, and I've always been directed (by the manuals of my 3-head decks) to record HF tone, monitor off the repro head and adjust the bias trimmer for such-and-such dB's below peak response when increasing the bias amp gain (usually turning the bias gain trimmer clockwise).

    So, okay. Obviously things will be different with only two heads, but the thing that bakes my noodle here is that the bias adjust procedure in the 388 manual doesn't involve HF tone at all...it just says to measure AC voltage between two test points on the amp card (for each track) and adjust the trimmer on the bias card until the meter reads 150mV...:???:

    That seems weird to me...seems like a shortcut as it doesn't take into account different tape formulations. I'm doing it by the book for now just to get to a baseline, but each bias amp was putting out about 70mV...so I dialed them up to about 150mV but my HF repro response is terrible...the manual says to input the tone sweep to check overall record response at -20dB but at that I can't even *hear* the 16khz tone, and the 388 is supposed to be 3dB's down from nominal at 16kHz...I start dropping by 2~4kHz.

    I'm assuming my bias is dialed up too high but I wanted to see if the manual is just bonkers or if something is wackity with my 388; to compare notes with others.

    Oh yeah...I'm using LPR35.

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    Cory, I recall what Dave (A Reel Person) told me once, about setting bias on the 388, something to the effect of "record, playback, adjust, record, playback, adjust, record, playback adjust... (for each channel) Tedious and time consuming but will probably be the best option. I'm not a fan of doing it as they lay it out in the manual (measuring AC voltage between two test points on the amp card). I'd do it the traditional method instead, even tho it will take longer.

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    yeah, I saw that in the manual and thought "what the...?"
    then I shrugged my shoulders and thought I'd wait for you to get there to tell us all how to do it...

    just to be clear: there is no tape running when you're checking those right? (doesn't mention anything at 1-5-4) its just the bias output signal that you are measuring and that is always being produced, right?

    if they just have some standard adjustment for the type of recommended tape (I'm afraid I can't even find where they recommend tape in the manual... anyone know what tape was ORIGINALLY recommended when the 388 first came out?) it would seem strange that it would 'drift' so much on your machine that you'd have to crank the bias pots all the way up like that (I don't know how these bias signal generators work though...) unless it was never actually setup for bias with the method in the manual and was setup for an actual specific tape type in practice, and they never updated the manual or something...

    I'm all ears on this subject - after we hash all this out posterity is gonna have things E-A-S-Y!

    as an afterthought, did you get the tension setup about right? would tension settings bear any influence on bias adjustments? seems to me not much within reason - did you already check your head alignment?
    "Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric. " - Bertrand Russell

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    setting bias on the 388, something to the effect of "record, playback, adjust, record, playback, adjust, record, playback adjust... (for each channel) Tedious and time consuming but will probably be the best option...
    Yup-yup...I hear you Daniel...that's what I was thinking...that I just need to full-on ignore what the bias amps are putting out and do it the traditional way.

    It was tedious also trying to check the repro levels on the stepped sweep I did (30, 60, 100, 200, 400, 1k, 2k, 4k, 8k, 12k, 16k, and 20k tones). Because the manual specifies to record the tones at 0.1VAC (about -10VU) and the VU meters don't have any gradiations between -7, -10 and -20 so there could be significant variances if you tried to eye it by the meter. So I plugged my AC voltmeter into each tape out jack for each tone and just made a mental note along the way how each track was performing, and when it started to tail off across the board starting at about 2k and for sure by 4k I hung up the meter and said "kids, we're just going to go with it". I wasn't going to go through a traditional bias procedure at that point even though I figured that was going to be the ticket.

    So, I'm not whining, but it does really make me appreciate why a 3-head setup was such a de-facto standard for an operation that was running machines all day long.

    The record quality of what we tracked sounded surprisingly good to my ears though, so...who knows.

    I've got bigger problems that I'll post up in my 388 Story thread. Gonna have to tip it up and pull the bottom off again.

    LUNE...

    just to be clear: there is no tape running when you're checking those right? (doesn't mention anything at 1-5-4) its just the bias output signal that you are measuring and that is always being produced, right?
    Well, I may not be thinking clearly, but I had tape loaded and was basically just recording the bias frequency but it wasn't necessary. I could have armed tracks 1~8, held the right tension arm up with a rubber band (don't we all have one of those in our toolkit???) and put the transport in REC-PLAY. So I believe you are right. The tape isn't necessary. All you are doing is putting it into active record mode so the bias circuit is alive.

    if they just have some standard adjustment for the type of recommended tape (I'm afraid I can't even find where they recommend tape in the manual... anyone know what tape was ORIGINALLY recommended when the 388 first came out?) it would seem strange that it would 'drift' so much on your machine that you'd have to crank the bias pots all the way up like that (I don't know how these bias signal generators work though...) unless it was never actually setup for bias with the method in the manual and was setup for an actual specific tape type in practice, and they never updated the manual or something
    The manual is clear that the 388 was designed for 1mil tape, and a 250nWb/m SOL, and that points to +6 tape. LPR35 or 457.

    I can tell that my 388 was lined up, and there was some drift, but overall it was pretty in line but not for 250nWb/m, so it needed an alignment for my setup...plus the meters had drifted some and some of that may be due to different faders being in there. You'll have level variances coming off those BUSS MASTER faders...They get the job done but they are relatively low-tech. I can go into it in more detail (my observations) if somebody wants to hear that but I'll spare you. Anyway, they're fine, but change a fader and there's bound to be changes to what gets from PGM IN to the meter amp. I changed a fader. So anyway, big point here is that 70mV +/- coming off the bias amp was not drift. Somebody put it there. Based on where the record level was set before I don't think this 388 was setup for 250nWb/m...I'm thinking 185nWb/m, and that may mean they were using tape with a significantly different bias requirement. Fine. But, as far as I can tell, the bias is far from being correct for the LPR35 I'm running.

    BTW, I thought about the manual error too. The 388 manual is indeed frought with errors. It is possible that circuitry changed and the manual was not updated. I don't even think Teac issued anything like a TSB if they made such a change. So it was set at around 70mV which is far from the procedure but also seems to be far from what my LPR35 is needing.

    as an afterthought, did you get the tension setup about right? would tension settings bear any influence on bias adjustments? seems to me not much within reason - did you already check your head alignment?
    I've only done the basic tension setup where you position the tension arms in PLAY mode...I'll have my Tentelometer back on the 26th. I would think that if the tension was so far out of whack that it was causing changes in the bias requirement, you'd have a whole lot more trouble on your hands than that...this is not to say that it didn't cross my mind though. But really, it would mean that your tension was so weak that the tape-to-head contact was bad and there would other issues. So anyway, I did a non-sweetbeats-ish thing and went ahead with the alignment before checking the tensions as the transport operates very nice and I don't expect there will be much needed in the way of changes to the tension settings.

    I decided not to mess with the wrap on the head. Looks like JFR set this block up and the glue is still intact on the fasteners for the rec/repro head so I left that alone. The wear pattern looks very nice. I did adjust azimuth though which was in spec but had clear room for improvement. The rest of the adjustments are fixed on this head, and they would effect bias requirement anyway, but good thoughts.

    So what frequency tone to use for setting the bias? Should I go with 10kHz?

    Anybody try and use the LF modulation mthod for setting bias on a narrow guage deck like this or are the tracks too narrow?

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    Let me unpack the question...it was buried in my last lengthy post...

    If I'm going to use the traditional "input HF tone and overbias to X dB's", any suggestions on which tone to use? I'm assuming 10kHz since that seems to be the norm on other decks with this formulation family of tape (i.e. 456/457/SM911/LPR35)

    Also, will the LF modulation procedure work for setting bias on this format (that's where you record a LF tone like something in the 20Hz~40Hz range or something and listen for the "rocks"; the modulation distortion

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    Cory, not sure about this so take what I say with a grain of salt but, for the first part of your question, my instinct tells me to go with the traditional 10khz. I say go with the traditional bread & butter all the way through. Decks are similar enough to warrant that procedure, me thinks. I'd not over-complicate / over-think the matter.

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    Quote Originally Posted by cjacek View Post
    Cory, not sure about this so take what I say with a grain of salt but, for the first part of your question, my instinct tells me to go with the traditional 10khz. I say go with the traditional bread & butter all the way through. Decks are similar enough to warrant that procedure, me thinks. I'd not over-complicate / over-think the matter.
    What makes you think I'd do that? Heheh...heh.

    There will be a little bit of a challenge setting teh bias for track 1 because of the issue where the tape is lifted from the head by the head groove...HF response is much diminished over the other tracks so I'll likely get 2~7 set using bread and butter...er...the bread and butter method, and then measure what the bias amps are producing for those tracks and then take an average of those readings (in mV AC) and the just set the amp for track 1 to that average. Then someday I need to lap that head...

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    Quote Originally Posted by cjacek View Post
    I'd not over-complicate / over-think the matter.
    Quote Originally Posted by sweetbeats View Post
    What makes you think I'd do that? Heheh...heh.
    Heh heh..

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    Quote Originally Posted by sweetbeats View Post

    So what frequency tone to use for setting the bias? Should I go with 10kHz?

    well, I'm not the one to answer this, but to add to the conversation, I've been studying (or rather trying to...) the Analogrules bias adjustment procedure recommendations, and, well a bit confused when it comes to 7 1/2 ips.

    http://www.analogrules.com/basicalign.html#bias

    at one point it states:
    " Bias should be done at the proper audio frequency for the speed
    you're using. Note that there IS a relationship between gap distance
    and the frequency you should use to set the bias, but as a general rule
    you everyone uses 10 khz for all speeds, except 7 1/2 ips and slower speeds.

    The "Proper" frequencies are 10 khz for 15 ips; 20 khz for 30 ips
    and 5 khz for 7 1/2 ips. Almost everyone uses 10 khz, and changes
    their "Overbias" level to compensate for the different speeds. "


    then later:
    " If you are using 7 1/2 use 1 khz instead of 10 k, and only go 1/2 db
    past the peak reading. You may use this as a rule of thumb: 1/2 past
    the peak (clockwise= more bias) for any speed and for any tape, at 1 Khz

    Using 1 khz is not as accurate as using 10 khz, but always puts
    you in the general range you need to be in. "



    and:
    "Note that you cannot BIAS the recorder at operating level either, if you
    use 10 khz. It is much easier to use 1 khz for bias (1/2 db overbias at
    any speed), since there is a almost no EQ Record boost at 1 khz,
    you CAN record 1 khz (and frequencies below that) at Operating Level. "


    but what would be the OVERBIAS POINT for 7 1/2ips 456/457 type tape on the 388? isn't it usually the manual that gives you the clues for where to overbias? and how would one use 5 khz if one wanted to be "proper"?
    "Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric. " - Bertrand Russell

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    Quote Originally Posted by sweetbeats View Post
    ...with a rubber band (don't we all have one of those in our toolkit???) [/COLOR][/B]
    yes, I like to have a few on hand myself...

    rubber-bands-2-jpg

    about using the one as a belt - it was rather less tight than the new belt, but rather more than the stretched old one, so I figure it would be fine as long as it lasted, which from reading accounts of rubber bands used on turntables could be from 4 months to a couple years.
    "Do not fear to be eccentric in opinion, for every opinion now accepted was once eccentric. " - Bertrand Russell

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