mr.blisset
New member
My 388 sounds great and it's always been reliable. However, I bought it straight out of a storage locker and I have no idea if the last person to use it was using a different tape with a higher bias. It came with a misc tape on a maxell reel and a box of bad 456 so no clues there. I'm also interested in doing this as I'm hoping to learn something and get better at fixing my own gear in the process.
As I understand it, the manual's method of biasing the 388 is very different from the usual methods when calibrating a three head machine. For instance, from looking at the manual, I've noticed I would only need a MRL test tape if I was attempting to calibrate the reproduction levels and bias. The only checks ive done so far is to record at 0db and note that everything recorded at 0db appears to play back at 0db. It would be nice if I didn't have to buy an MRL tape just yet. If this is not the case then that's fine, I want to make sure I'm doing this right otherwise I think it's best to leave things as they are. Perhaps it would be nice to know of a simple way to check if it's relatively calibrated for 0db tape if this is the case and I'll leave it at that.
I do however have a true RMS multi-meter I could use to adjust the record levels and bias. I've found I like using non-backcoated Maxell 35-90, which is apparently the original tape the design engineers were using with the 388. In using the original tape, I'm hoping adjusting the bias pcb for 150mV as stated in the manual will work best. I don't see anything in the manual regarding using this particular tape stock so not sure why I've seen it mentioned a few times that they used 35-90, does anyone have any more info on this?
I'm having hard time understanding why the biasing involves measuring ac voltage at 150 mV and how this could relate to 0db bias tape.
the 35-90 bias specs are
Reference output level 200 nWb/m
0 dB bias Specified Bias of UD Tape
Playback EQ 3180 µs + 50 µs
So for example, say I wanted to adjust for -3b or +3db, would there be a formula for adjusting the ac voltage to compensate?
I think I'm mostly trying to understand the theory of how this is accurately biasing the machine before I start messing with it more than anything. I understand the procedure for a three head machine but this seems very different. Is the 1.5.6 Overall Frequency Response procedure where most of the calibration is actually happening depending on the tape being used, and the 150mV ac voltage is just a ballpark voltage for tape with a 0db bias?
I've searched these forums quite extensively but only see references from forum members wanting to bias for +3 and using the methods one would go with on a three head machine. If I've missed something or if there's some reading I could be doing elsewhere that would help simplify this for me please let me know.
Any help or advice from those who have navigated these waters before is greatly appreciated!
Thanks
As I understand it, the manual's method of biasing the 388 is very different from the usual methods when calibrating a three head machine. For instance, from looking at the manual, I've noticed I would only need a MRL test tape if I was attempting to calibrate the reproduction levels and bias. The only checks ive done so far is to record at 0db and note that everything recorded at 0db appears to play back at 0db. It would be nice if I didn't have to buy an MRL tape just yet. If this is not the case then that's fine, I want to make sure I'm doing this right otherwise I think it's best to leave things as they are. Perhaps it would be nice to know of a simple way to check if it's relatively calibrated for 0db tape if this is the case and I'll leave it at that.
I do however have a true RMS multi-meter I could use to adjust the record levels and bias. I've found I like using non-backcoated Maxell 35-90, which is apparently the original tape the design engineers were using with the 388. In using the original tape, I'm hoping adjusting the bias pcb for 150mV as stated in the manual will work best. I don't see anything in the manual regarding using this particular tape stock so not sure why I've seen it mentioned a few times that they used 35-90, does anyone have any more info on this?
I'm having hard time understanding why the biasing involves measuring ac voltage at 150 mV and how this could relate to 0db bias tape.
the 35-90 bias specs are
Reference output level 200 nWb/m
0 dB bias Specified Bias of UD Tape
Playback EQ 3180 µs + 50 µs
So for example, say I wanted to adjust for -3b or +3db, would there be a formula for adjusting the ac voltage to compensate?
I think I'm mostly trying to understand the theory of how this is accurately biasing the machine before I start messing with it more than anything. I understand the procedure for a three head machine but this seems very different. Is the 1.5.6 Overall Frequency Response procedure where most of the calibration is actually happening depending on the tape being used, and the 150mV ac voltage is just a ballpark voltage for tape with a 0db bias?
I've searched these forums quite extensively but only see references from forum members wanting to bias for +3 and using the methods one would go with on a three head machine. If I've missed something or if there's some reading I could be doing elsewhere that would help simplify this for me please let me know.
Any help or advice from those who have navigated these waters before is greatly appreciated!
Thanks
Last edited: