Tascam TSR-8 biasing. Hitting a wall! Help!

[pointillist]

Ok, I've posted about this before, but hopefully this is the last and ultimate post. I have a Tascam TSR-8, in nice shape. Came from the Texas schoil for the blind. I'm trying to put out another record using it, but moved cross country and changed tape brands (to ATR) and want to calibrate the whole thing. I have the Tascam shop manual, a Fluke 87 III Meyer, and a little Fostex test tone generator. A disclaimer: the machine records fine. There are no huge problems I'm not disclosing (like bad channels or something). I just can't get it to bias. Here is the issue:

Using the shop manual, I did the Input level check, Meter level check, Reproduce level check, Erase head bias tuning, Record head bias tuning, and Record bias trap. All are spot on. I go to do the bias adjustment and:
Connect the Fluke in A/C mV mode between TP1 and TP7, start recording. The Fluke puts me at about 2.8mV. If I adjust R114 all of the way, I can get 3.5mV. Manual calls for 35mV +/- 2mV. I'm stumped. Any ideas? I've triple checked everything at this point. Could the 35 in the manual be a misprint for 3.5?

The Fluke should be fine with the bias frequency, which is why I bought it. I'm out of ideas! Any help would be appreciated, if we do another Portland Oregon meetup I'll buy you a coffee or beer!

Andy

[themagicmanmdt]

never heard about biasing like that before... you must be looking at the manual. what a wonky manual!

try this....

from your signal generator, choose 40hz.

record it and monitor the playback head as it's live recording to a track.

get on a set of headphones and listen to that track playing back live as you're recording it.

as you adjust bias, start at the very lowest setting. bringing up the bias adjustment, you should hear what sounds like 'rocks' noise, or a car engine idling... this is 'bias modulation noise'.

at bias minimum, the noise isn't there...since we're not biasing the tape yet!

as you increase the bias, the noise increases....then will decrease and get rather quiet....then increase again and get noisier as you check out the range of bias adjustment.

that middle section where the least bias modulation noise exists is the correct bias for the tape.


now, you might have trouble with ATR...or any '+9' tape, with an older or 'prosumer' machine like a tascam...

give it a try.

if you can get a good bias as i described above, then hurrah! go record!

if not... tell me what you found, heard, etc using the method above. maybe the bias osc is off... maybe it can't bias ATR tape... we'll see.

[pointillist]

Thanks, will try this when I get off of work today. I've had a few people go "35mV, what the hell- do it by ear!". I've been trying to be a good boy and stay golden to the manual settings. Will see how the ear routine does today. Keep in mind it's a two head machine, which makes adjusting things off of the tape path a little more challenging..

It's an odd problem. The rest of the machine calibration (and performance) is golden. Just this issue. And I thought about the ATR tape being the culprit, but I threw on some known good Quantegy 456 I had around, and the same problem persists!

[wkrbee]

The meter has to respond to the bias freq. which usally is 100-150KHZ.I don't think the mdl.87 will respond.Most of the "cheaper" DMMs only have a freq. response up to 100HZ.They assume you will use it to measure 120VAC 60 hz.An analog VTVM will get you what you need, and can be much more useful down the road especially if you are setting up bias traps where you need to see peak and/or null points or baising a 3 head deck.If you have a o-scope you can verify if there is even any bias getting to the head.Typical setup for the TRS 8 is 35mv for 456.Hotter tapes dial in 7-9 mv higher.You would also need a sweepable audio oscillator to really get the flattest band width. After the bias is set,you have to go back and print 15-20KHZ tones and tweak the rec. eq to flatten the 15-20 KHZ band.

[pointillist]

My 87 does read a 150khz bias frequency coming off of the test points. Still only hits about 3.5mV. Very odd. I'll look into borrowing an analog meter as well.

[cjmnash]

the TS-8 is a two head deck....and biasing a two head deck is almost impossible outside of the factory. you can't do it in real time since there's no repro head to monitor.

i've aligned a lot of two head decks, and even doing a standard alignment on them is a long and horrible journey because you have to record, rewind, play/look, blindly adjust, record, rewind, play/look, rewind, blindly adjust, record, repeat, repeat, repeat.

and that's just for 1k and 10k record, so when it comes to bias - i usually either leave all the bias pots alone- or if someone has attempted to bias- i set all bias pots at 70% (turn all the way up, then back off a bit) and then adjust high freq record to zero to compensate...basically skipping bias altogether.

then i record some music, listen for distortion and re-adjust bias pots globally and re-tweak 10k record if needed.

good luck! i do alignments via skype if you get into a spot where you need more help, fyi.

-chris mara
www.welcometo1979.com
www.nobraineraudio.com