R/E/P Community
R/E/P => Bruno Putzeys (Designer) - Dave Hecht (Master Tech) => Topic started by: Geoff Doane on March 09, 2011, 01:43:51 PM
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I wasn't sure if this question should go in this forum or Bruno and Dave's, since nothing is actually broken, so here goes.
I was asked to line up a studio's A807-4 MK II for ATR tape. I had previously aligned the same machine for some used GP9 that they had, and everything went well. Using the recommended overbias (-2 dB @ 30 IPS), there is insufficient trim to get the 10 kHz level to match the 1 kHz level. It's about 1 dB high with the adjustment at minimum, and keeps on going up until it's about 2 dB high at 20 kHz.
Most of my analog tape experience is with A810s and A820s, and on those I could tweak the record EQ (as distinct from the record treble) to get the response really flat. The A807 manual has a table of the factory RAM values for EQ (in hex values) in section 2.6.4, but no mention that I can find, of how to change them. For those who have forgotten, pressing the TRANS key and the TREB button on the 810/820 put the machine in EQ adjustment mode. There is no TRANS key on the 807, and the SHIFT key doesn't work that way.
GTD
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a little more bias?? (it's been a few years.. sorry if I'm off base here)
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a little more bias?? (it's been a few years.. sorry if I'm off base here)
That was my first thought. I increased the overbias to 2.5 dB, readjusted the 1 K level, and 10 K was still too high, although 20 K had dropped a bit (IIRC). The figured the first case was the lesser of two evils (evenly rising response), so went back to 2 dB and thought I'd poll the experts.
GTD
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Geoff,
A couple of thoughts. Just out of curiosity, can you try 3db of overbias and post the results? I don't have an 807 manual at the moment (downloading from the Studer site) but I noticed 2 807 manuals - user and service. Any chance the info you need is in the other manual? I also recently ran into a similar issue on an A800 where the record hf adjustment had a very limited range - traced to bad caps on the head preamp card. Any chance the nachine in question is in need of some new caps?
Dave Hecht
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Just finished the service manual download. In section 2 - Startup, Operation, the microswitch id #16 switches the machine into alignment mode for setting audio parameters. Try that and let me know.
Dave Hecht
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Just finished the service manual download. In section 2 - Startup, Operation, the microswitch id #16 switches the machine into alignment mode for setting audio parameters. Try that and let me know.
Dave Hecht
Dave,
I know about the microswitch. That's the one that is accessible through the front panel, and puts the machine in alignment mode.
After re-reading the manual, it looks like the equalization may have to be set using the RS-232 interface (page E2/66). I'll have to find a laptop with an RS-232 port before my next visit, and check that out.
It turns out the client is about to buy an A827, now that his analog appetite has been whetted. It will be interesting to see what kind of condition it's in, and I can get reacquainted with 2" tape after 15 years or so.
Next time I'm at the studio, I'll try 3 dB of overbias on the 807, just to see what happens. It's not like it's difficult to go back to where I was. I'll try to take better notes, and report back.
GTD
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Just a reminder (that often confuses many engineers when playing the bias game)... MORE over bias = less high frequency. Also, I find that the level you record at has as much impact and often more, on the playback as does the bias and at the end of the day. I have found that +3/185 (= 0/250) on ANY tape formula yields the best transients, changes the sound from the console to the tape the least as well as leaves plenty of head room to mitigate print through over long term storage. Take the time to actually print at least a portion of your mix to tape and actually go through the exercise of tweaking the bias until you achieve the best sound. This will change from project to project, but I find is well worth the effort. If you are slamming your meters at ANY level, your bias setup is moot. It is up to your EARS as to which you prefer.
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Ears can help, but for me it's "trust, but verify". Nothing beats a precision alignment done with a world class audio analyzer. I've not found any AE that could tell me whether the tape was set up with THD at .55% or 1%. Use that test rig and you find out quickly where the sweet spots are without all that subjective guesswork. Performing sweep THD vs bias and frequency response plots tell you very quickly if it's got bumps, dips or worse.