Tape Machine Alignment Problems... need help!

[AnalogBrain]

Hey All,

Got a question.

Recently purchased an Otari MX-70 1" 16 track machine. I have done countless alignments on 2" machines, Otari and Studer, but this has never happened.

On channels 14 & 15 the record trim pots are maxed out, and the meters are only reading about -3db VU. What do I do???? I am unable to set those channels up.

Any thoughts???

Thanks,
-Neil

[adamcal]

Firstly, does the problem follow the channel cards as you swap them about?

[Goreski]

Neil....

You need to read your MX70 manual and look through the schematics! This will show you the adjustments available in the cards, and give you a guideline to go thru first.

You may have to bring in a good tech to fix her if she's broken... !

Try swapping good cards into the 'bad' tracks and see if problems follow the cards or stay with the track.

regards, Goreski

[loujudson]

Totally intuitive response, but then my business name is Intuitive Audio...

Might be a power supply problem? Like a cold solder joint on the cards, or such? Or dirty edge connectors? Have you thoroughly cleaned ALL connectors?

Just a sort of guess.

Lou

[AnalogBrain]

Hey Goreski!

I was waiting for you to show up. Love your website by the way!

Anyway, I have the manual, but not the schematics. In the alignment section of the manual it talks about "Erase Transformer Peaking Adjustment." This involves a volt meter, and adjusting the erase bias transformer for a peak reading. Is this what i am looking for? If so, it seems easy enough.

Also, please tell me if this "way of thought" is fine... "screw using the master bias, and just adjust each individual channel bias pot."

Thanks guys,
-Neil

[Goreski]

Neil....

First off see if you can follow those instructions in the manual.... often when I don't have the right extender card, I just solder wires to the test points and extend the wires out so I can make measurements.... this you can easily do. Note you will need a voltmeter that can measure 200khz signals!

You'll want to do a measurement, WRITE down the value, turn the trimpot or coil 1 turn clockwise, then make the measurement again and WRITE down the value.... you'll have to do this a number of times to find out in which direction you need to turn the trimpot or coil (clockwise or counterclockwise) to get the readings the manual requires. Of course you have to turn OFF the machine each time you pull out your audio card - yep it's a hassle, but once you've gone thru and done the erase peaking, you won't have to do it for years....

Also you didn't tell me if the problem follows the card or stays with the track... this is essential troubleshooting... so this is almost always the very first thing you want to track down....

Since all the other tracks are erasing well, we can rule out the Master bias card.

So.... first off swap known good cards into the 'bad' tracks. Record a 1k tone at say '0vu', then kill the tone and record over the 1k tone... and listen in playback. The 1k tone should be way way down in the noise... hearable but way way down.... compair this to other 'good' tracks. And also listen to the 'bad' cards in 'good' tracks slots... are they erasing now or still having the problem.

If the erasure problem follows the 'bad' cards, then it's likely an adjustment or bad components. If the erasure problem stays with the track even with known 'good' cards, then you've got a connection problem or bad erase head on those tracks, or depending on which tracks they are - possible poor 'tape to head' contact..... which you can test by placing your fingers on the tape at the Erase head.

Odds are you just have a couple 'bad' cards, and the erase just needs adjustment. I think there is an erase relay in the MX70 circuit, and this can easily cause poor erasure....

I can put up the schematic for the Audio card on my site if you and others need it.... let me know.

regards,

Goreski

[Goreski]

Neil...

Thee will always be a bit of noise when you are monitoring the heads... not much but it's there. If it's way down, say -70 or lower (no tape, head shield closed), likely it's just normal noise if you are hearing hiss. If it's much higher, then I'd first off re-do your alignment just to check everything is right.

Troubleshooting in any situation starts with the question....

1) is it 'global' or just a card/track?

If it's global, that is on all tracks, you're looking for something beyond a audio card problem, like a power supply fault, master bias/control card, master muting system, head wires run close to a power supply, and that sort of thing.

Often folks who are working in digital freak out by the 'noise' that an analog tape recorder has... until they actually start recording music and realize the noise is not a concern (if you're recording correctly) and way way way down.... so if you haven't worked with a tape recorder recently, this could be happening to you.

So re-do your alignment, then record something and see if the noise you're hearing is a kind of 'fool-yourself' thing or if it's really much higher than normal. If it is much higher than normal, you should measure the standing noise and let me know what level it is. And when you say 'wind' noise I assume you mean a nice hiss, not a fluctuating noise that changes level by itself.... correct?

regards,

Goreski

[MsM]

How do you adjust the 1 kHz repro and sel/sync levels? Do you use an MRL tape? What's the reference flux on that, and what reference flux do you adjust 0VU for? One source of error may be the level adjustment pot's turned too high up. The MX-70 isn't that quiet a machine, anyway, IMO.

M.

[AnalogBrain]

Hey guys, thanks for the replies!

Here is the most recent update. First off, did a complete new alignment using a brand new MRL tape. Here are answers to y'all's questions...

1. Noise level with no tape, head shield closed is between -52 & -58dbu as measured by my minilyzer.

2. The noise is a hiss, but when turned up, there is some faint crackling and considerable low end energy. I guess it sounds closest to white noise.

3. I have worked with 2" analog for quite some time and am used to the noise, but this is my first time with the Otari MX-70.

4. Problem is global.

5. MRL is a 355nW/m test tape. I amusing ATR tape, and set for 0VU.

6. The noise increases/ decreases with the turning of the Repro and Sync gain pots. When aligned at 0VU, the noise is about -55dbu, but gets as high as -35dbu when gain pots are turned to max.

7. I disconnected heads, and noise still present.

8. Machine is DEAD quiet when monitoring input.

9. All relays in channel cards have been replaced in past year.

10. I did notice something peculiar. On the master bias/ control card there is a missing resistor and missing capacitor, R60 & C18. Could this contribute to noise?

Thanks guys so much for trying to help!!!
-Neil

[Goreski]

Neil....

Do this:

put a 100 ohm resistor in where the say, track 5 head would be in repro or sync, just put the resistor leads into the head connector (you'll need to look at the schematic to find the correct pins). Then measure the noise on that same track.... see if it drops a lot or stays about the same.

While the MX70 is a good machine, it is a bit noisy.

And since you increase the noise level with both the Sync and Repro pots it thus is something before those pots that is causing the noise..... so re-do a noise test with a resistor, and see what happens.

A -58 dbm noise floor gets you about a 70 db signal-to-noise at max level on modern tape, and that's not bad at all.... But then I haven't measured the noise level on an MX70 for some time now.

Anyone else who has a MX70 can check their standing noise levels for you.... so see if you can dig someone up.

Specs on the MX70 16 track are:

Signal to noise: 58 db (30 ips)
Signal to noise: 57 db (15 ips)

So you are already beating those specs! Though they are using not a distortion level (which would be the correct way to measure this), but a simple ref of 320 nW/m... Unweighted, bandpass limited at 18khz.

Again, try recording some music on the machine to check 'reality' levels!

regards,

Goreski

[Goreski]

Neil and others...

the Otari MX-70 audio card schematic is now up at my site:

Free Pro Audio Schematics and Manuals - from AnalogRules.com and AudioSchematics.com

go to the Otari Manuals link.

regards,

Goreski

[stellar]

I thought I would chime in with a couple of points that haven't been made yet.

My first thought is that it is either a tension or head alignment problem, or the guides are bent. It seems like the tape might not be making even contact with the heads, which would cause a level problem on the edge tracks as you described.

Second thought is that the MX70 wasn't made to bias up to ATR tape levels. You should probably use SM911 or 456.