388 bias adjustment

[sweetbeats]

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.

 

[cjacek]

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.

 

[LUNE]

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?

 

[sweetbeats]

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?

 

[sweetbeats]

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

 

[cjacek]

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.

 

[sweetbeats]

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...

 

[cjacek]

I'd not over-complicate / over-think the matter.

What makes you think I'd do that? Heheh...heh.

Heh heh..

 

[LUNE]

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"?

 

[LUNE]

...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...



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.

 

[sweetbeats]

So I just ended up calling Tascam Analog Support in California. Go to the source. Time is valuable and a 90 second phone call is a whole lot faster than me stumbling around making suppositions.

I talked to Jimmy...he's the analog dog there.

First, couple of mentions:

1. He said that 457 and LPR35 have significantly different bias requirments. I cannot verify this but it is worth noting since (I believe) the procedures/specs in the manual were based around 457...might explain why setting the bias amps to output at 150mV seems to be overbiasing my LPR35 (based on diminished HF reproduce response)...

2. He said, FWIW, to be prepared for a 100Hz bias (related response) bump when using the LPR35, and that there's no way around it. Again, I cannot verify this, but I figure it is worth mentioning.

NOW...

Here's the deal:

10kHz tone is suitable for setting the bias, he says they use 12.5kHz there, and then they set the bias by tracking the tone, rewinding, reproducing and adjusting the bias trimmer and repeating the process for each track until 3 percent distortion is achieved. That's it. You all have a distortion meter right? Heheh. He confirmed for me that I can use my analog oscilloscope and using the VOLTS/DIV scale eyeball 3 percent. You all have an analog scope right? Heheh.

Other options (IMO): use your ears and set for minimum distortion on one track, check the bias amp output voltage on that track and then set the others accordingly.

-OR-

You wait for when I get around to setting mine up with my scope and then I'll check what overbias setting on the VU meter corresponds to 3 percent distortion.

BASED ON JIMMY'S COMMENT ABOUT THE DIFFERENCES IN BIAS REQUIREMENT ABOVE, I'M NOT GONNA VOUCH THAT MY RESULTS WILL CORRESPOND TO 457 AS I'M USING LPR35.

Dig?

Comments?

 

[The Great Cobb]

388

Im scared.

 

[miroslav]

... 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.

Mmmmmm...not really...'cuz you still need to have a reel of tape on the deck when you measure across the test points....right?

I don't have a 388...but my multitrack deck's service manual does bias exactly the same way (different voltages though)....as my deck is also a 2-head deck.

I put all sixteen tracks into record mode (using tape of choice)...and then measuring across designated test points on each card, for each channel...I use my 'scope to set the desired peak-to-peak voltage reading via the bias adjustment pot for each channel.

If you use a different tape formulation...you will end up setting that bias pot at a different point to achieve that same voltage...so yeah, the tape formulation plays into it.
It took me quite awhile to get my head around that procedure...but once I started, it made sense.
My deck sounds great now with the tape I used to set it up (911).

My 2-track does bias the traditional way, but it's a 3-head deck.
You can probably do the 2-head in that same way...but you will lose your mind doing record/rewind/playback/adjust/record/rewind/playback/adjust...etc...etc.
The method in the manual makes it much simpler and straightforward...and using an xxx mV p-p measurement it is going to be a pretty accurate bias setting.

I was initially thinking of just doing it the traditional way like my 3-head deck…but then I figured they wrote it in the manual the other way for a reason. It’s the official service manual…so why deviate from that?

 

[sweetbeats]

Nope.

The voltage is measured at the output of the bias amp. There doesn't even need to be tape on the deck...just lift the R tension arm, arm the tracks and put the transport into REC-PLAY mode. This energizes the bias amps...

Betcha the test points on your transports that advise the same methos are also straight off the bias amps.

It is an approximation that does not actually reference the tape like monitoring the distortion level or the reduction in the response of a pre-selected tone (i.e. when you are using a 10kHz tone and over biasing to -XdB's).

 

[miroslav]

I'll have to confirm that with my deck.
But is that what your manual tells you do...hold up the R tension arm while measuring????

But you know...my deck (and it's service procedure) was originally set up for 456 as the factory default tape...so I'm sure the target XXXmV p-p measurements are chosen for that tape.
The 911, which is what I use exclusively (and 468) are an even trade for 456...so I bet the bias is spot-on and no worse than guess-timating with the whole 3% approach…etc.

It's not going to be difficult for me to at least check one or two tracks using the traditional bias method....run a 10k tone and turn the bias till I hit 0VU...rewind, and check playback...etc…drop it by 3dB…etc
But like I said, the deck sounds better than ever now...and I just used the manual's recommended procedure.

Oh…let me know when you locate a distortion meter!

 

[sweetbeats]

I'll have to confirm that with my deck.
But is that what your manual tells you do...hold up the R tension arm while measuring????

But you know...my deck (and it's service procedure) was originally set up for 456 as the factory default tape...so I'm sure the target XXXmV p-p measurements are chosen for that tape.
The 911, which is what I use exclusively (and 468) are an even trade for 456...so I bet the bias is spot-on and no worse than guess-timating with the whole 3% approach…etc.

It's not going to be difficult for me to at least check one or two tracks using the traditional bias method....run a 10k tone and turn the bias till I hit 0VU...rewind, and check playback...etc…drop it by 3dB…etc
But like I said, the deck sounds better than ever now...and I just used the manual's recommended procedure.

Oh…let me know when you locate a distortion meter!

miroslav, my point with the "lift the R tension arm" is just that tape does not need to be loaded. Unless the transport "thinks" tape is loaded (via deflection of the R tension arm) non of the transport functions are active. The manual does say to load tape but my guess is it was written that way to avoid confusion. If you look at the schematic the test points are between ground and the output of the respective bias amp, so even though the manual directs to load tape it is not necessary.

I'm not against using the manual's procedure. This thread is specific to the 388. I'm not trying to suggest everybody needs to get a distortion meter and use method 'X' or y'all are lame, I'm citing what Tascam told me and I'm trying to bias my 388 which isn't working right with the procedure in the manual so that's why I called Tascam to ask for a suggestion/clarification.

I know I gave some indication that using the scope to measure the distortion would be a "guesstimation" but actually it should be quite accurate. "Eyeball it" was the term Jimmy used in response to a question I asked to make sure I understood what he was saying. When I told him I didn't have a distortion meter but that I had a 60mHz analog scope he said "Oh you can use that, sure." I won't need a distortion meter and I don't plan on getting one

 

[LUNE]

could you use a software meter like VA (Visual Analyzer) to measure distortion? this measures THD (Harmonic Distortion) - is this a different thing?
haven't figured out quite all how to use this yet, but it looks like there's lots of good things in there if one could figure out what it all the parameters mean...

 

[sweetbeats]

I suppose, but I'd rather use my scope...no sampling latency issues or resolution issues.

 

[miroslav]

I'll check my schematic....
...but I'm curious, since you actually talked to Jimmy at TASCAM, did he actually say that the tape had no effect on the bias reading when you followed the manual's procedure...and that it was just a "ballpark" setup and not as accurate as doing it the more traditional way in the case of your 388?

I know you have a different deck that what I have...but from what you described initially, the bias procedure in your manual is very similar to the way my manual describes it, so I just want to confirm the above questions before I start chasing ghosts in my machine.

I thought it was an unusual way to do it...but I didn't want to just assume it was not accurate enough. Like I said...I will check a couple of tracks using the HF tone method and see if there is any significant difference in the p-p mV level after I set the bias that way compared to the manual’s procedure.
If not...then I will leave it as I have it.
As I said...I'm thinking that logically, the desired p-p voltage should correspond to 456 tape since that's the default factory recommended tape formulation.

It will be interesting to see how this comes out....

 

[brewbacker]

I'm stuck at the recording bias adjustment. I'm getting no voltage at the test points. Can anyone offer any advice? All record function buttons in and deck is in record mode. I've got 2.3 mv at tp2 and tpg. What am I missing? AC voltage meter ain't picking anything up!!