Fostex G-16S bias adjustment

[Niva]

Hi there!

First post here.

I am in the process of calibrating my G-16S.

I religiously folowed the Service Manual and adjusted the playback levels with my MRL tape at 0 VU (-10dB) (quite a "flat" reponse for a 1/2" 16 track!). So I guess the payback/sel sync head is OK.

Only thing: track 3 and 4 seem to have "drops" at times with 10k and higher. Probably a minor mechanical issue or head wear, but generally, it is quite good!

Now I am in the bias setting...

The Service Manual doesn't say much about it:

1. Rotate Sync crosstalk at the center

2. Put all 24/16 tracks in record

3. Turn the BIAS ADJUST muti-turn pot until it reaches the peak,

4 . And then turn the BIAS LEVEL so that the level drops at 100mv.

I figure this method works well with a brand new or freshly relapped head.

Now, I followed this method (with no signal recorded and no tape on the machine) and I still have some significant differences between the 1K and 10K, both measured at -10dBV.

The manual claims that there should be a +/- 1.5dB difference, but I get 1.8dB (ok), 2.3 dB (not so good) and even 3.3dB (Ouch!) more gain with 10K compared to 1K (with NR OFF)!


So I guess that I should:

1. Record a 10K signal on a tape on each track and then.
2. Find the peak with the BIAS ADJUST pot (CLOCKWISE?) with Playback after record method.
3. Drop -3.5dB or -4.5dB as recommended on the web from the peak level with the BIAS LEVEL pot (COUNTER-CLOCKWISE?).

I'll set the bias for +6 tapes, although I'll be using +9 tapes as well. But I don't want to reset the bias all the time, so my reference tape will be +6.

For that, I guess I should use the PAR technique (playback after record) to see how the changes did affected the signal.


Am I right?

Thanks a lot for your guiding lights!

Guillaume, Québec, Canada

 

[Niva]

Hello!

I am still messing around with calibration issue.

Can anyone help me?

THanks!

Guillaume

 

[miroslav]

It's not "rock solid" or perfect.
I had some similar "fluctuations" with my G-16 last time I did a full calibration...and on some cards I had to turn the pots a lot more to get within or close to spec than others.
Spent a few hours trying to dial everything in *perfect* and then realized that for some things, I had to settle for "close enough".
I hope you haven't been calibrating since last June...

One must remember that these are 20 year old decks, so the electronics will not be perfect across the board.

 

[Niva]

Fortunately, I had had something else to do during those 6 months!

The confusion that I have is that there is two controls for the bias setting on my G-16S: BIAS ADJ, and BIAS LVL...

The Service manual says to find the peak point with BIAS ADJ wich is a multi-turn pot, and then, make the proper setting with BIAS LVL...

But the thing is that, I don't know where to put the BIAS LVL trim pot at start, as it modulates the overall bias level...

Can someone explain it to me?

Many thanks!

Guillaume

 

[miroslav]

I have the G-16 Dolby C version...and my service manual doesn't word it like yours.
There is a Bias Leakage adjustment/check and frequency check that is performed before you get the to the Bias Level. The Biase Leakage is done for both Reproduce and Record modules via two seperate points on the chanel card PCB...the Bias Level is a pot on the edge of the PCB along with other adjustment pots.

The way yours is worded (from how I'm reading it) is that you use the Bias ADJ to first find the peak...meaning on a scope, as you turn that Bias ADJ pot, there would be a point where the peak would reach maximum....that's the "peak point".
Then use the Bias LVL to set the peak point to the correct bias level....whatever your manual says it should be.
Makes sense to you........?

 

[Niva]

Yes, that makes senses!

But I am a little confused about the initial position of the BIAS LVL pot, before searching for the peak bias with the BIAS ADJ multi-turn pot...

Should the BIAS LVL pot be fully clockwise or in the middle (or fully counter-clockwise) before tweaking the BIAS ADJ? Because BIAS LVL would influence on the peak point too...

I just want to make sure I'll be in the right side of the bias...



So this would be the method:

1. Set BIAS LEVEL pots of all tracks fully clockwise (or center position or fully counter-clockwise?).
2. Plug an oscillator (10khz at -10dBV) at INPUT of TRACK 1 and put all tracks in record mode.
3. Find the peak point with the BIAS ADJUST multi-turn pot (turning clockwise?) of TRACK 1, using the Playback After Recording method.
4. Turn the BIAS LEVEL pot Counter-clockwise (?), further increasing the bias level until it drops by 4 dBV (?)

5. Reapat the same procedure for track 2, 3 and so on.

I am not sure about the Clockwise and Counter-clockwise...

I am calibrating it for RMGI 911 tape.


Tell me if that makes sense.


I think this might help a lot of people here.

Guillaume

 

[miroslav]

Yes, that makes senses!

But I am a little confused about the initial position of the BIAS LVL pot, before searching for the peak bias with the BIAS ADJ multi-turn pot...

Should the BIAS LVL pot be fully clockwise or in the middle (or fully counter-clockwise) before tweaking the BIAS ADJ? Because BIAS LVL would influence on the peak point too...

I just want to make sure I'll be in the right side of the bias...

It doesn't matter...just get some level...the Bias ADJ "peak point" will be evident if the BIAS LVL is low/med/high.
You're first looking for that "peak point"...THEN setting actual correct level.

Get it....?