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Help with 1/4" Machine - 1 bad channel, newbie...


I'm just about as new to this forum as I am to this machine, an otari MX-5050 BQII 1/4" 4-track, and to analog reel-to-reels in general. My only other experience with this stuff is on an MTR-90 2" 24-track that required minimal calibration to sound amazing.

I picked this guy up a few days ago for only $250 (!!!) complete with the roll-around cart, a bunch of take-up reels, 4 new reels of quantegy GP9, an (albeit ancient and flaky... more or less useless) MRL tape, a demagnitizer, and the original manual. The guy who had it before me barely used it and only owned it for a few months, having bought it on eBay from a quiet connecticut studio. It looks practically new... all-in-all an amazing purchase.

So I started playing around with it, cleaned the heads and stuff with isopropyl, and noticed that it was pretty horribly calibrated... I could slam the inputs but the signal coming off the tape was wimpy. I whipped out the manual and cracked open the case and began twiddling; it wasn't long until I had the first three channels at their proper levels, but channel four seemed pretty flaky. I calibrated the input section and all seemed well, but both the sel-rep and repro heads pull very little signal off tape. I swapped channel strip 4 with 1, and the problems stay on track 4.

So I have some questions... where should I be looking for the problem? It would seem to be in the heads, but I would doubt that both the rec and play heads are equally mis-aligned. If I monitor the sel-rep head while recording (using the onboard test oscillator at 1k), the level is exactly what the input level reads, so the signal is making its way out to the rec head at the proper level. From there... I don't really know what to check... I'm thinking that maybe I should flip the tape over and see if what was not working on channel 4 comes out channel 1 properly...

I'm a touch nervous about playing with the alignment of the heads too much as I don't have the proper test equipment (though it seems that the HP voltmeters and oscillators and stuff the Otari manual recommends are quite cheap on eBay). I just don't know if I should bother getting this stuff, or whether it's necessary in my case.

So if anyone with some analog experience could shed a little light for me, I'd be extremely grateful... thanks in advance!

-Kevin Cox


Member for

51 years 5 months

jibberia Sun, 06/13/2004 - 14:33

So I flipped the tape over, and channel 4 came up almost perfectly (wasn't calibrated right, but I expected that). Channel 1 (which was track 4 when I recorded it) was just as low as it was on 4. So, I guess I have a sel-rep head problem? I'm confused... because when I monitor the sel-rep during record the levels are great.


Member for

51 years 5 months

Cannibal Sun, 06/13/2004 - 19:16

when you are actually recording on a track and monitoring sel/rep you are being switched into input. this is why track four sounds fine at that point.

Simple test.. while the tape is rolling press lightly over the head with your finger (track 4 area).. see if track four gets louder..

oh and for more tech related advice I would suggest

unfortunately this place no longer has a tech forum.

Member for

17 years 5 months

jonyoung Mon, 06/14/2004 - 07:38

From what I remember about adjusting headstacks, the three categories are azimuth, zenith, and wrap. If the wrap were off, all tracks would be equally low, so scratch that. If azimuth is off, top & bottom tracks are typically off from center tracks. Sounds like a zenith tweak. The bottom of the head is probably tilting back from the tape (or top is tilting towards, take your pick). Do you have a calibration tape and oscillator?

Member for

51 years 5 months

jibberia Mon, 06/14/2004 - 10:09

Thanks guys....

No oscillator, and my MRL tape is useless. I haven't seen anything about zenith in the manual... maybe I'm not looking hard enough. I'll check out how to make the adjustments.

Thanks for pointing me to that forum... I'm going to browse around and probably post about this there too.

I'm at work now, but when I get home I'll try pressing the tape onto the head.


Member for

17 years 4 months
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cruisemates Wed, 06/23/2004 - 03:30

Did you notice if the playback head has any wear? It is a simple thing to see looking like a triangle with the base on the bottom. What happened when you went through all the tones on the MRL? if they were down did you calibrate them up? When you do you have to monitor the playback, so are you saying #4 wouldn't come up to snuff no matter what?

You need some kind of oscillator to try recording 100, 1k, 10k and 15kHz? what happened on playback? If the high freqs are low and the bass sounds fuzzy, it may need to be biased. Absent that, a sine wave on a synth will give you a quick reference.

As Mr DIY said, try holding the tape down. Also try gently guiding it higher & lower. The head should have alignment screws to help you adjust the azimuth.

Also - giving technical advice is perfectly acceptable here, no need to solicit people to go elsewhere.