VU calibration/ Tape Alignment without outboard oscillator

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tmoneygetpaid
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VU calibration/ Tape Alignment without outboard oscillator

Post by tmoneygetpaid » Sat Feb 01, 2014 1:04 pm

Hey all,

I've been refurbishing an Otari Mx5050 8-track. I finally got all parts installed, got a demag tool, alignment tape, and a new reel (of 1/2" ATR Master tape). I have aligned Studer A820 and ATR-102 decks many times, so I'm comfortable with the alignment process.

I don't have a Loftech, so I figured to calibrate VUs (and to do record alignment) I could generate a sine wave in Pro Tools and send it to the deck. Here's where everything started going amiss and I need someone to tell me where I've gone wrong.

I figured my little desktop interface (Presonus Firebox) must be +4dBm outs. I fired up Pro Tools and put a signal gen on an Aux, it defaulted to sending at -20dBFS peak, which I figured should send at +4dBm. I checked the output's voltage with a multimeter to make sure, and it read .3V. I flipped the signal gen to -20dBFS RMS and it showed .5V. I was confused. +4dBm should be 1.23V RMS.

I thought I could use my scope (BK 2120B) to double check, but it scope shows about 1.75V peak-to-peak, which is around .6V RMS. So I'm assuming my scope is off cal.

To get the multimeter to show 1.2V I have to send a -14.5dBFS signal from PT.

All this info seems to contradict itself. I just want a +4 sine wave to send to the machine. Should I just trust my multimeter and proceed setting +4 on the deck to whatever it sees as sending 1.2V?

Thanks
Thanks

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Nick Sevilla
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Re: VU calibration/ Tape Alignment without outboard oscillat

Post by Nick Sevilla » Sat Feb 01, 2014 10:16 pm

tmoneygetpaid wrote:Should I just trust my multimeter and proceed setting +4 on the deck to whatever it sees as sending 1.2V?

Thanks
Thanks
Yes. 1.23 Volts. If your multimeter hopefully reads into decimals...

That is the correct way. Save the PT preset for the level coming out of that plug in for later.

Cheers
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honkyjonk
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Post by honkyjonk » Sun Feb 23, 2014 5:08 pm

I'm gonna be calibrating one of these soon in the next month or so. MX5050's are pin 3 hot and they're unbalanced. Another issue to add to the equation. I don't really understand all the if's ands and butts of what the voltage should be, and I remember clearly the Otari manual wanting lots of measurments of the voltage with a volt meter during the calibration process. The amount of variables with a balanced connection feeding an unbalanced connection and where to connect the ground lead of the volt meter (pin 2? pin 1? both pin 1 and 2?) confuses the hell out of me. I never know whether I'm getting the right 1.23V.

But, luckily I do have the right equipment (a mixing board with VU's and sine wave tone generator) to mitigate these shenanigans and make the whole process a lot easier.

Basically, what I'm going to do is send a 1K sine wave out of one of the busses at 0VU. It's not exactly 1.23V but who cares. It's close and it's my board's meters! Then loop it back to a channel that is assigned to another buss and adjust that to read 0VU as well. THen I just insert the tape machine in that loop, so I know it's at 0VU on my board, and what I send out is what I get back.
But this I guess this isn't any help to you, since you're reading the signal level with a voltage meter and not a VU meter?

Man that shit is confusing. Oh yeah, how about true RMS? And is your meter capable of reading true RMS at 10k? Man, I'm so glad I have VU's and tones on the board.
Stilgar, we've got wormsign the likes of which God has never seen!

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