bias adjustment - variable capacitor?

[FALKEN]

so I was working on the bias on my fostex and it seems as though the adjustment is made via a variable capacitor. I came to this conclusion because that spot on the channel card was labaled cap. The adjustment is not going as described in the manual - you cannot rotate it all the way ccw...it just keeps going around and around and the meter keeps rotating up and down. there is no FULLY ccw... so if you turn cw, you can hit a peak, and then descent, but it might start coming back up before you hit the -4db decrease that the manual recommends. eventually I am able to find a position that is -4db down from *a* peak but I still don't know if this is correct. Has anyone experienced this?

Also, I was having trouble with the rec level after doing this...I could not get the recorded level hot enough to match the input, so I took the cards out and cleaned the pots with de-oxit (although I feel it might have been related to the bias issue). I also cleaned these Bias adjustment caps with de-oxit. after inserting them back in the machine channel two now gets no recorded level, and the bias adjustment no longer works. arrghh!! so I removed them, cleaned them again, and have now left them out to dry overnight and will try again tonight. anyone with experience with this???

 

[Beck]

Is this your E-22? Those trimmer capacitors are “non-stoppers” and one of the cons of Fostex models. You can get lost, and be on the wrong side of the peak. You have to get a visual on where those trimmers should be and keep them within a certain range. See the pic below.

If you have a trimmer that you haven’t touched yet, use it for a visual reference to get the other back in the ballpark. You’re tweaking for Quantegy 456 or similar, right? 3 dB down seems more like it for 456 @ 15 ips, and only 1.5 to 2 dB down for 30 ips.

Deoxit is very strong, and can damage some older components that have thin layers of carbon, like pots and such. I don’t know… are you sure you have the cards all the way in? Try switching the cards around. The good news is that those trim caps are cheap and easy to find if they need replacing.

You took the cards out with the machine off, right? Just checking…

Tim

 

[FALKEN]

Yes this is the E-22.

I put the cards back in today and everything is fine. I suspect there was some wet deoxit in there yesterday. The only channel I haven't touched is the center "timecode" channel. The manual says to overbias 4db but at that point I can't get one of the rec levels hot enough. Overbiasing 3 db lets me get my rec levels all the way up to 0 (at 1.23 V). Still, this doesn't seem right....I will take another look at those bias trimmers. Thanks for the info!!

Unfortunately I don't have the service manual but I am thinking of ordering it. The E-22 has this funky "meter cal" mode where the meters suddenly change positions....which might be the cause of my issues. I'm not sure what that mode is really.

 

[Beck]

What are you using as your tone oscillator?

 

[FALKEN]

I am using test tones generated in sound forge playing out of my PC at the specified voltage level, checked with a meter.

 

[Beck]

Yup, that will work.

 

[FALKEN]

so no idea on what the "meter cal" mode is then ?

 

[Beck]

We could take a guess at it, but considering the level you’re into the machine at this point, I recommend you snag a service manual as a next step.

 

[A Reel Person]

"Meter Cal"...

is a function that should set (or reference) your meters to "0vu".

I can't remember the exact verbage of the description,... but I'll look it up again if I have a chance./DA

 

[FALKEN]

here's the deal:

This is what my manual says:

CHECKING REPRODUCE ALIGNMENT
-METER selector set to CAL, thread the alignment tape and play the tape.
-All meters should indicate 0VU levels during reproproduce of the 1khz reference tone. If not, adjust the reproduce amplifier level for a 0VU reading.
-repeat for sync, repeat for low speed

BIAS LEVEL AND RECORD LEVEL ADJUSTMENTS
-switch tape speed to low, thread blank tape
-apply a 10khz signal of approximately -20dBV (0.1V) to the INPUT XLR connector and connect an AC voltmeter to the OUTPUT RCA phono jack corresponding to the input.
-switch to repro and hit record, bias to 4 db drop based on voltmeter
-check frequency response

CHECKING RECORD ALIGNMENT
-switch meters to CAL
-send +4 dbm 1khz tone to inputs
-match input to 0db on meters by use of input level knob (not a calibration!)
-adjust record amp to match 0db on output.
-switch to repro and do the same for rec level.

I had a beef with this process, which is that you are matching the alignment tape to 0 db on the meters, with disregard to what the output level is. so "0 dbu" might be anything on output. then you are to match your record levels at nominal level to 0 dbu so in the end you are balanced, but your amps might not be at the same levels (although the record amp and playback amp in conjuction are even, separately they might be way off). Then when you take your machine out of CAL mode your levels REALLY might be anywhere.

so what I ACTUALLY did is ignore the "CAL" mode. I played the tape and adjusted the repro amps so that I read +7 dbu on the volt meter (cal tape is +6 and I am biasing for 456). I then adjusted the meters on the Fostex to equal +3. Then I adjusted my inputs and record amps to equal 0 at +4 dbu, at a STANDARD input level on the input knobs. This is a fairly typical alignment, IMO.

The only issue is that I ran out of room on one of the record amps, and had to overbias by only 3db instead of 4db, in order to get the 0db level reading. In addition, the "difference" seen on the meters between the "cal" mode and normal mode has now increased. the readings in "CAL" mode are now 2-4 db off from regular mode (was 1-2 db before). and I'm not exactly sure how to reconcile that. I guess I gotta get the other manual. cuz its not in this one. In fact, the meter adjustment isn't even in this manual, I just happened to find it.

 

[FALKEN]

and another thing...

how come the manual always says, in the biasing section, to feed a .1V (-20db) tone into the machine? every machine I've ever had the meter stops at -20 db, so at that level you can't even get a reading. ?? ?? I always end up going with -6 or something.



do you think its normal for the record eq to have no effect whatsoever?

 

[FALKEN]

here is a picture while recording a mixdown....thought you might enjoy.

 

[FALKEN]

Beck,

I have been toying with this. The only way to get the capacitor on the right channel to look like the one in the photo is to bias to a 2 or 2.5 db drop. is this a bad thing? I went around it a few times and it never got there within that range. I can get it to a 3 db drop but then it is out of range. what do you think?

 

[Tim Gillett]

how come the manual always says, in the biasing section, to feed a .1V (-20db) tone into the machine? every machine I've ever had the meter stops at -20 db, so at that level you can't even get a reading. ?? ?? I always end up going with -6 or something.



do you think its normal for the record eq to have no effect whatsoever?

It's normal practice to check high frequ's at well below 0VU to avoid risk of saturating. The highs are probably already boosted in record well above the lows and mids, especially with slower speed recording.

Yes, the meters read poorly at those low levels. That's why it's also recommended using an external meter or if it's available on your machine, racking up the play back gain just for this measurement,( user front panel control only) so you get a decent reading on the meter while still recording at well below 0VU. The book is usually right.

Cheers Tim.

 

[cjacek]

here is a picture while recording a mixdown....thought you might enjoy.

I did enjoy! I'm always a fan of picts of this sort. I like the vintage B&W Btw.

 

[FALKEN]

Beck,

I have been toying with this. The only way to get the capacitor on the right channel to look like the one in the photo is to bias to a 2 or 2.5 db drop. is this a bad thing? I went around it a few times and it never got there within that range. I can get it to a 3 db drop but then it is out of range. what do you think?

nobody can comment on this? is a 2.5 db drop bad for 456? The manual recommends 4 but to get there the adjustment is out of the range pictured in the manual which beck posted..........

 

[FALKEN]

this might be an interesting topic...

I was re-calibrating and I found the best frequency response to be at a 1.5 db drop past zero. ??

 

[Beck]

1.5 dB down from peak sounds right for 30 ips, but kinda weird for 15 ips.

I lost track of this thread, so in answer to some older questions:

The trimmer doesn't have to be kept exactly within the range of the picture. The pic is just to show in general what side you should be on because you can be completely reversed with a non-stopping capacitor.

-20 dB input is just so you don’t run out of meter during bias cal. The meter doesn’t function as an absolute while biasing, but only a relative measurement from the peak, which could be anywhere on the meter.

 

[FALKEN]

Thanks, Beck. I thought 1.5 down sounded weird, too. but any more overbiased and I get too much bass and not enough highs....