Calibrating a Tascam 22-2

C

citizenkeith

New member
Yup, another calibration thread. :D

Last night I worked on calibrating my Tascam 22-2. I tried to follow the directions here:

http://www.recordingwebsite.com/articles/tapecal.php

I have an MLR tape (unfortunately, it's 250 Nw/m instead of the 185 that I need for Quantegy 407, but I'm compensating as instructed). For test tones, I used Sound Forge's built-in tone generator.

The playback calibration went fine. The problems started when I attempted adjusting the bias. I wasn't sure which screw to adjust. I have three pairs of adjustments that use the word bias: Output Bias Trap, Rec Amp Bias Trap, and Bias Trimmer. I'd be lying if I said I didn't try all of them. And now I realize that mistake may have serious consequences. After I gave up on calibrating last night, I searched out an older issue of Tape Op with an article on calibrating. They warned not to adjust anything labelled Bias Trap, but didn't explain what it was or what to do if you DO adjust it.

Can I assume that I want to adjust "Bias Trimmer?"

I continued onwards and adjusted the Rec EQ. If I remember correctly, the REC EQ Hi bias was not problem with 10k. However, when I tried to adjust the Lo EQ at 100 Hz, nothing happened. I turned those screws as far as they could go in either direction, and the meters didn't move one bit. Looking through the manual right now, I THINK they recommend 400 Hz.... but not sure if that's the problem.

And lastly... there are some other adjustments on the 22-2. There are a pair of adjustments called "METER CAL" (left and right), "MON GAIN" (left and right), as well as a a single adjustment called "DUMMY COIL." What do these do? I'm guessing that MON GAIN is for the headphone output. I did a slight adjustment of METER CAL and noticed the meters moved (when I had the REPRO head on), but couldn't figure out the purpose of it, so I moved it back to where it was. I have no idea what that DUMMY COIL is.

To summarize: 1) What does OUTPUT and REC AMP bias trap do? 2) Is bias adjusted using BIAS TRIMMER? 3) Why didn't the meters move during LO REC EQ? And lastly, 4) What are the METER CAL, MON GAIN and DUMMY COIL adjusters?

Thanks for your help! From reading the archives, I know that Beck uses a 22-2, and a few others do too. I hope you guys can give me a hand today, so I can proceed with my mixing session tonight. :)

Keith
 
citizenkeith said:
To summarize: 1) What does OUTPUT and REC AMP bias trap do? 2) Is bias adjusted using BIAS TRIMMER? 3) Why didn't the meters move during LO REC EQ? And lastly, 4) What are the METER CAL, MON GAIN and DUMMY COIL adjusters?

Keith
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Just starting to have my mornings coffee before going sailing for the weekend so who knows if this is coherient. Also I don't have a 22-2 (Do have a 32)

First off, get the service manual. I know it won't help you today but...

1) Bias traps are notch filters that capture and ground the bias frequency while letting the audio you want to pass. The adjustments tune the notch frequency and they must be tuned to the same frequency as your bias oscillator.

2) Bias trimmer could be the bias level (which is what you would want to adjust) and would be a capacitor or the bias frequency (an inductor) which you do not want to adjust unless you have a frequency meter.

3) On my decks EQ high and EQ low adjustments are EQ adjustments at high speed (15 ips) and low speed (7.5 ips). The EQ low would not do anything at 15 ips. (Other decks do have a low frequency EQ adjustment YMMV)

4) Meter cal is used to set the meters to 0 VU after all the playback electronics are adjusted. (see the other thread for a brief discription). Not sure about monitor cal, the name suggests that the headphone levels. The Dummy coil is where the bias energy goes when not in record. It presents an equilivent load to the bias osc as a recording head does. It keeps the bias osc stable.

Good luck to you as that it sounds like you have your work cut out for you.

regards
 
Thanks for the reply.

Update: I did find the service manual, so I have a better idea of what I'm doing on this machine.

1) Bias traps are notch filters that capture and ground the bias frequency while letting the audio you want to pass. The adjustments tune the notch frequency and they must be tuned to the same frequency as your bias oscillator.
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I tried the directions in the manual (using a voltmeter) but I never got a static reading. I'll give it a shot again today. I have a feeling one of the leads on my meter is damaged, so I may pick up a new pair.

I've done some Bias Trap research via Google and saw that some people recommend using an oscilliscope. I may be able to borrow one... will that be more accurate?

2) Bias trimmer could be the bias level (which is what you would want to adjust) and would be a capacitor or the bias frequency (an inductor) which you do not want to adjust unless you have a frequency meter.
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Yup, you're right on that one. :) (It's amazing what you can learn when you RTFM :D Thankfully I found it buried in the basement).

3) On my decks EQ high and EQ low adjustments are EQ adjustments at high speed (15 ips) and low speed (7.5 ips). The EQ low would not do anything at 15 ips. (Other decks do have a low frequency EQ adjustment YMMV)
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Ok, I forgot about the EQ low at 15 ips. However, I recalibrated last night and the EQ high wasn't doing anything either. :confused:

4) Meter cal is used to set the meters to 0 VU after all the playback electronics are adjusted. (see the other thread for a brief discription).
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Cool, thank you.

Not sure about monitor cal, the name suggests that the headphone levels.
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I checked it... you are correct.

Thanks for your help... I'll keep working at it. :)
 
evm1024 said:
Bias traps are notch filters that capture and ground the bias frequency while letting the audio you want to pass. The adjustments tune the notch frequency and they must be tuned to the same frequency as your bias oscillator.
Click to expand...

Sorry if I'm coming off stupid here, but I just re-read your reply. So if I understand correctly, I calibrate the bias trap to the same frequency that I set the "bias trimmer" to?
 
yes eh well no...now I'm confused

citizenkeith said:
Sorry if I'm coming off stupid here, but I just re-read your reply. So if I understand correctly, I calibrate the bias trap to the same frequency that I set the "bias trimmer" to?
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Sorry about the confusion. Does this machine have a bias level adjustment. Or is the bias trimmer used to set the bias level?
 
citizenkeith said:
Bias trimmer is used to set the bias level. :)
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OK we are on the same page. I'll just ramble along, I'm sure that you know this.

The bias trimmer is an adjustable capacitor that sets the bias level. Fully counter clockwise is minimum level on most systems. The bias level is set by setting the bias level to minimum. Apply a 1kHz signal to line in and set the machine to record. Monitor the signal with the playback head. Adjust the record level to -10 VU. Then increase the bias level (turn clockwise) and observe the playback level. This level should increase and reach a peak. (If the peak is too high for your meters reduce the record signal level). Then continue to increase the bias level as the playback level decreases. Your bias level is correctly set when the drop in playback level is 3 or 4 dB less than the peak level.

Thus the bias level does not change the bias frequency (which is set by the bias oscillator).

The bias traps block the bias signal and prevent it from going where it should not be. (more rambling)

There are typically 2 bias traps. The record trap blocks the bias signal generated by the bias oscillator from entering the record electronics. Thus the bias only goes to the heads.

The playback trap blocks the bias signal that the playback head pickups off of tape. This trap passes the audio from the tape and passes it to the playback amplifiers while blocking the recorded bias signal.

Traps are notch filters which pass all signals except for the filter frequency (could be a low pass filter in some decks).

So it is important to have the bias frequency set to the standard frequency for that deck so that tapes may be interchanged without having to adjust the bias frequency for eqach tape.

Once the bias frequency is set then the bias traps can be adjusted so that they block the maximum bias signal that they can (adjust for minimum bias level). No audio signal in this adjustment just bias.

PM me for a phone call if that will help.

Regards
 
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