Question about biasing a Tascam 38

Discussion in 'TASCAM Analog Forum' started by Olaf Ben, Nov 24, 2019.

  1. Olaf Ben

    Olaf Ben New Member

    Joined:
    Oct 2019
    Messages:
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    From:
    Cork, Ireland
    Gear:
    Tascam 38 8
    I have managed to play and record on my Tascam 38, but it works only on Quantegy GP9 tape I received with the machine, somehow it plays the other tapes I have, but when I try to record on them it does not work.

    I thought that the Tascam 38 would play and record on any 1/2" tape, am I missing something ? One tape I have was new Quantegy 499 type, and another a 900 not used at all. I am thinking of buying some pancakes so, now I am unsure what tape will actually work. I you have some advice guys ?

    I think it's the biasing, the owner told me he had it calibrated and optimised to use Quantegy GP9 only, I was able to successfully play and record on that tape without any issues.

    So second question, Oscilloscope ? Which one to choose ? There are a lot on eBay, many might not be suitable for biasing the audio on a reel to reel tape recorders.

    I tried to record on RTM SM 900, a brand new tape that came with the Tascam 38, and used AMPEX 456 which did play, but when recording over to test I get nothing but the hum and hiss.

    All of the 8 VU meters work on INPUT and SYNC when recording and when I am playing a tape.

    Some biasing applications seems to be available, even for free, I would put my hand in the pocket for a real oscilloscope thought as it will always come in handy to have around, just like a head demagnetiser and a bulk eraser.

    If you might have some advices as you guys would have done this when restoring your Tascam 38 or other tape decks, any help would be welcome.

    Cheers and OM :)
  2. Mark Richards

    Mark Richards Well-Known Member

    Joined:
    Dec 2017
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    From:
    Southern USA
    Gear:
    DP-24, vintage 40-4
    Very odd. Optimizing the Bias Current should not prevent the machine from recording and playback of other brands of tape.

    There are several circuits that interact with the Bias Current: The Erase Current; the Master Bias Oscillator; and the Bias Trap. These have to be adjusted in a specific order and can get tricky. It's not for the faint of heart - in fact, it's best done by a professional technician who will have access to all the necessary test gear. The good news is the Erase Current, Master Bias Oscillator, and the Bias Trap don't drift much over time, and only need be looked at if major repairs have been done.

    If you've performed all the routine service maintenance steps, and if you know specifically how many dBVU to reduce the peak for your specific model multitrack recorder, you can do the bias adjustment using the VU meter reading to optimize the Bias Current. (The general service sequence, as I recall, is: adjust head alignment; adjust input electronics; adjust the playback electronics; adjust the bias current; adjust the record electronics.)

    You use an o'scope to do the head alignment. The o'scope must have a vertical and horizontal input and an X/Y mode to do the head alignment.

    You don't need to use an o'scope to optimize the Bias Current.
    Last edited: Nov 25, 2019
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  3. SkywaveTDR

    SkywaveTDR Active Member

    Joined:
    Feb 2014
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    From:
    Chicago area
    Gear:
    Teac, Tascam, Sony
    I would concur that no adjustment will stop the unit from recording but just sound bad due to all the bias that is applied to that tape. It is silly to use a +9 tape on a deck with those track widths as the previous owner just listens to sales media and doe snot use common sense.
    When a track width is that small the higher level just plays a part to the detriment of the audio rather than a plus. Clean heads, make sure the deck erases- that tells you the master oscillator is on then it would be the slave bias amps but they usually do not fail 8 at a time. Maybe they have been over driven so much that they all blew up. I have had to fix some of them.
    Caveman, -mjk- and Mark Richards like this.
  4. SkywaveTDR

    SkywaveTDR Active Member

    Joined:
    Feb 2014
    Messages:
    504
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    From:
    Chicago area
    Gear:
    Teac, Tascam, Sony
    The other day a post was read that a person could not record on his Tascam 48 at all. When he posted the pictures of the heads the gaps were entirely covered up with junk so that the tape was never touching the heads properly. Head have to be cleaned well to be in contact with tape otherwise all bets are off. The 48 and 38 use a similar head if not the same. I have had slave bias amp block go out now days due to shorted caps on the output side. I did repair the one I caught. This was a cap on the inside of the can. Scopes are used for alignment but I rarely use X-Y mode, I for much of 45 years use dthe dual trace display. The scope is used in bias trap setting and doing all this is not tricky or difficult- I have done it thousands of times. The over bias method was abandoned at Teac Chicago back in the early 80's and we used the 1K and 10K comparison and then fine tunes from there. We had Recording Engineer on staff at the time and we also all agreed that the over bias method was inaccurate as could be seen if you did frequency sweep tests plus it was only a ball park process and needs lots of modification. I think it is in the manual for the assembly line.