Hello,

Has anyone really figured out how to bias with the WM-D6C. The board has 6 bridges per channel to set bias. The service manual states to use a normal type 1 tape.

I am trying to figure out what patter might be the best for TDK SAs. I'd rather not play a guessing game and try the many different permutations possible.

Regards.

Greetings genejazz... check out this page from Tapeheads.net. You are lucky, someone already tried this out for you! He used TDK SA90 (which sound pretty good when recorded in a deck with Dolby HX servo-biasing).

However, read carefully before attempting! What does the manual state? I gather from the other thread that adjusting this messes with ALL of the EQ settings, so it is only correct for TYPE I (for SA90.)

The reason I gather that Sony didn't release any info on calibration of the bias timing is for purely marketing reasons - you buy Sony machine, you buy Sony tapes. Perhaps this is something you face also with other fixed-bias decks of the '80s...

Hi,

Thanks for the heads up. Unfortunately, I am aware of that thread at tapeheads. It does not bias a SA90 correctly in type II position.

The service manual states you set the bias to a type I tape. Let's say a TDK D tape. Then the type II and IV is biased by approximation.

I don't want to keep playing with the bridge pads as it may eventually break from all the solder/desolder.

Hi... well, I'm not an expert (I'm still a student actually :-P) and especially not in biasing, but you have me interested. From studying the circuit layout in the manual I can see that the "complex AC Bias" circuit is pretty much equivalent to a switchmode power supply inverter (steps up voltage.)

As you know the Bias oscillator (timing) is fixed by Q309 transistor, at some high frequency. The EQ switch has two functions, for setting Playback EQ and Bias voltage (Bias circuits are switched by Rec. switch.) You can be rest assured both are completely seperate (Playback EQ is handled by those custom diode packs.)

Now, the EQ switch connects either transistors Q305 or Q306, nicely labelled as "BIAS SWITCHING". By allowing current to flow through either / or none of them, the voltage of the switcher on the primary of the bias transformer is determined by R313 and R312 by whether the current is made to pass through them to give a voltage drop. You are 100% correct, ultimately gives a fixed "stepping" between the bias voltages. Go figure...

So, by using a special calibration tape on "TYPE I", one setting fits all - provided that the tapes are of "Sony grade formulation." Typical, still no better today (perhaps worse... proprietary everything.) So using a TDK D is... not a good idea.

As a sweeping generalization, increasing the capacitance (by adding more capacitors in parallel) should decrease bias voltage. Might this help? Knowing if TDK SAs require more / less bias voltage than say a Sony UX helps...

I guess its best to leave the WM-D6C as it is after considering this. I wonder how other "non-Sony certified" labs calibrated these in the past? You don't need to worry too much about bridging - just make sure you don't overheat the board by applying iron for too long (or the PCB traces would come off.) Run the iron through bridge perpendicularly to disconnect them ;-). I like the idea of fitting DIP switches but where is the room?

After some tweaking, I've come to the conclusion that the bias can be set best by feeding a 12khz tone at -20db below 160nWb/m and setting the playback to be about +2db more.

It might be a bit underbiased but it sounds the best to my ears with TDK SA90s.

The rec level pot sucks in that it tracks unequally quite a bit. Along with the volume fader.

I tweaked the bias of my D6C and many of my clients. The jumpers on the left correspond to the values of capacity for the left channel. The jumpers on the right for the right channel. The jumpers in the upper part the ability to lower up to the jumper that put down the highest capacity. You must register a frequency of 1000 Hz and then a 10000 hz. Displayed on an oscilloscope the amplitude of two frequencies, and open and close the various bridges up to get the right capacity for each channel, which would result in the same amplitude for the two frequencies. It 's easier to do than to say it.

is there a picture from a service manual. so i can see, which jumpers/switch you should use?

Here is a link to someone else who has adjusted their one. Complete with photos.

plop, is this the correct bias procedure?
the last post says

"This adjustment may not be the same for every D6C. Various tolerances in the machine might require different bias for a given sample of tape from unit to unit.

As far as recording an SA at the Type I position- that IS being recorded at 120uS eq, not 70. The results you got are simply a happy accident".

quote:

Originally posted by 19lexicon78:
plop, is this the correct bias procedure?
the last post says

"This adjustment may not be the same for every D6C. Various tolerances in the machine might require different bias for a given sample of tape from unit to unit.

As far as recording an SA at the Type I position- that IS being recorded at 120uS eq, not 70. The results you got are simply a happy accident".

This is the correct process ie soldering over traces to set the bias. As the guy in that post suggested, YMMV with each WM-D6C. Looks like a matter of trial and error to find the correct setting using the correct Bias position.

so, it's for tdk sa90.
is biasing for playback only, not needed?

quote:

Originally posted by dottor walkman:
I tweaked the bias of my D6C and many of my clients. The jumpers on the left correspond to the values of capacity for the left channel. The jumpers on the right for the right channel. The jumpers in the upper part the ability to lower up to the jumper that put down the highest capacity. You must register a frequency of 1000 Hz and then a 10000 hz. Displayed on an oscilloscope the amplitude of two frequencies, and open and close the various bridges up to get the right capacity for each channel, which would result in the same amplitude for the two frequencies. It 's easier to do than to say it.

At what level? -20db below 160nWb/m? I found that this method of 1khz and 10khz never can be made to be exact using the bridge. It is approximate.

quote:

Originally posted by 19lexicon78:
so, it's for tdk sa90.
is biasing for playback only, not needed?

Bias is for recording only.