Otari MTR-90 maintenance

[Matt C]

Recently got an Otari MTR-90 mkIII that I'm getting up and running and I have the feeling I'll keep coming back with more questions so I figured I'd just make a catch-all thread.

I just finished going through all the transport calibration checks as outlined in the manual. For the most parts things looked good but I ran into a couple odd things that I'm not sure about.  A little background - the heads were just relapped and are in good shape. The rollers were replaced about 5 years ago and seem to be in good shape (as far as I can tell). I haven't confirmed for sure that the swing arms are not bent, but haven't noticed any really obvious signs that they are bent.  Any ideas about the following?

1. Tape is sometimes rubbing against the reel flanges. I can't tell if it's because the reel table height needs adjustment or if the flanges themselves are slightly warped (they are new reels of tape).  The rubbing is not constant, the tape will just scrape against the flange one or two times per revolution. I first noticed tape rubbing against the bottom flange on the supply side. I adjusted the reel table height and got a strange result - the tape rubbed the bottom flange when tape was playing, but rubbed the top flange when rewinding. Seems very odd.

2. One adjustment involved disconnecting the capstan motor, turning the capstan by hand while in fast wind mode, and adjusting the speed at which the tape moved while monitoring the capstan tach output.  I found that the frequency was bouncing back and forth a bit, at a constant speed. Just as a rough estimate, is was bouncing between maybe 600Hz to 900Hz, about once every 1-2 seconds.

3. Weirdly similar to #2 - I tried adjusting azimuth while monitoring tracks 2 and 23 of an MRL tape (at 8kHz and 16kHz).  When looking at both sine waves together on a scope, one was always moving back and forth horizontally, so the relative phase was never consistent.  Not sure if this could just be caused by a problem with how my scope was setup, or if it reflects a real problem with the machine.

 

[radardoug]

Like all tape recorders, accuracy of the tape path is the most important thing. Your experience with reel rubbing indicates your tape path is not perfect.
Normally bent arms will show as lower tension at the top edge of the tape. You can normally judge this by holding the top or bottom edge of the tape and see if the tensions are different.
Your azimuth problems are typical of a MTR90. Errors in the mechanical setup will show up in azimuth errors. However, when checking azimuth, work with adjacent tracks. Sometimes heads can have a bow in the gap. I would only go 4 or so tracks apart when checking azimuth. If the azimuth shows a regular wobble, see if you can tie it to a roller rotation. Worn rollers will cause this.

 

[Tubetec]

We had a bent swing arm on the otari mtr 90 in the place I used work in years ago ,fluttering of the arms on fast winding was a sign there was something a miss. Very slight rippling of the tape in the vicinity of the rollers on playback might also indicate a geometry problem with the arms.

Ocassional tape scrape does occur ,sometimes it can be slight missallignment of the reel plate ,once on a session I noticed it was happening badly ,I rewound the tape, took off the spool  ,and saw that one of the screws holding the plate to the hub was cross threaded ,this was a brand new roll of Ampex ,anyway just make sure all the screws are in fully . Checking with the eye that the spool is true by looking horizontally at the edge while playing back might reveal the problem , its most likely not the height adjustment but the spool itself thats causing the issue.

You may have a slipping capstan ,pay special attention to cleaning , residue from the back of the tape tends to build up here,and can be stubborn to remove properly ,the capstan rubber should be matt finish ,if its worn smooth it will probably lack grip on the tape . I did on one or two ocassions use very very fine grit <1000 emery paper to clean off the capstan and help give it more grip,
I used a metal block covered in a layer of grit paper ,I positioned this against the capstan ,while rotating it by hand ,one or two turns with very light preasure on the block should do it . Using the tape counter is a fairly good way to test capstan slip ,load up the tape ,fast foward in about a minute ,stop, reset tape counter to 0 , make a chinagraph mark on the back of the tape at the record head position, now fast forward almost to the end of the tape ,rewind again to the start ,or return to zero if you have an autolocater, your chinagraph mark and 0 on the tape counter should co-incide resonably well ,if not your cap is slipping

 

[Matt C]

Normally bent arms will show as lower tension at the top edge of the tape. You can normally judge this by holding the top or bottom edge of the tape and see if the tensions are different.

I did try this and didn't notice any difference in tension, but it seemed like it would be hard to tell unless it was really severe.

We had a bent swing arm on the otari mtr 90 in the place I used work in years ago ,fluttering of the arms on fast winding was a sign there was something a miss.

Using the tape counter is a fairly good way to test capstan slip ,load up the tape ,fast foward in about a minute ,stop, reset tape counter to 0 , make a chinagraph mark on the back of the tape at the record head position, now fast forward almost to the end of the tape ,rewind again to the start ,or return to zero if you have an autolocater, your chinagraph mark and 0 on the tape counter should co-incide resonably well ,if not your cap is slipping

When the machine is in fast wind at full speed the swing arms are stable, but they do flutter a little during speed up and slow down. I wouldn't call it severe but definitely noticeable. I remember seeing somewhere that this is a bad sign.

I did the trick with the china marker to check for capstan slippage, and it actually performed well. after fast winding back and forth through  a whole tape, the mark was only 1 or 2 inches off from where it started.

 

[Tubetec]

That all sounds good then ,
I'd be inclined to say that a certain amount of arm movement during speed up and slow down of the reels is normal ,
I cant remember the exact alignment procedures ,as far as I remember there are gain pots on the transport card in relation to arm control /damping, maybe some very small tweeks on these might help it handle the tape better .Visual inpection of the tape passing the heads while in replay should tell a lot about how well its performing ,there should be no visible movement of the tape vertically on the heads. Another test that can be done is using a special marking pen ,you'll have to google this , you basically put a marking on the head surface in contact with the tape ,then run the tape in playback for a minute or two ,now examine the marking you made. the contact mark should be worn off equally from the portion of the head in contact with the tape ,it should be the same width along its lenght  and  be located more or less centrally on the face of the head. You could also try shining a strong light on the tape in the vicinity of the rollers ,this will make it easier to see any missalignment or drag,which will cause the aformentioned rippling ,visible as a flickering of the reflection of the light off the tape .

 

[Matt C]

after staring at the tape path for a while I do see the tape moving up and down on the heads during playback.  It's slight, but definitely there. Having trouble picturing what would cause that.  I didn't see the edges of the tape scraping on the guides anywhere.

 

[pucho812]

what tape manufacture is it?  At one of the studios I used to do tech for, when it came down to the two tape brands left ATR and RMG,  the tech staff was given a reel of RMG to make a new house calibration reel to leave in the rooms for alignments. The MRL's live locked away in the tech shop. But that is another story. Anyway we took the RMG and started to make a tones tape. Right away the tones on there studer were far from perfect.  what should have been needles on the VU fixed at the correct level they were moving all over the place.  A back and forth a few times with the MRL and the tones tape and we found the problem. The tape was  not cut to the correct width. It was slightly less then in should be. To further complicate the issue, after about a min in the tape  was the correct width.  so not sure how that happened, but we had to cut out the first minute of the reel. The rest of it was fine, and in the end it worked o.k. but for some reason the tape was not right for the first minute.

 

[Matt C]

I've been using RMG 911.  I do have one reel of Quantegy 499 that I could try out and see if it behaves any better.

 

[radardoug]

You need to observe whether the movement up and down is related to other mechanical parts. Is it periodic? Does that rate relate to other turning parts? Bad tape slitting is always a possibility. You need to find a reel of tape that you know is good. Does the output at 10K also follow the movement? Otaris were always a bit dodgy for this test.

 

[Matt C]

Since I sort of intended this to be a catch-all MTR-90 thread, I'm just adding a link to a recent thread about replacing the MDA transistors:

https://groupdiy.com/index.php?topic=68001.0

Also wanted to note that I asked Otari directly about whether they ever put out any technical/service bulletins for the mk3 units, they said no.

I haven't had time to fine tune the tape path on my unit yet, but I did get some reels of GP9 and they did not exhibit the weird flange rubbing that was going on with my RMGI tape.

 

[mrclunk]

I'll post the useful info here as well.
MJ21193 and MJ21194 work for the Toshiba 2SD552, 2SB552 drive transistors in the Reel MDA and Capstan drive.
If one goes replace them all.

If you have an MTR90, worth flipping down the back plate with the fan fitted and vacuum the dust from the heat sinks.

 

[digitall2000]

yeah
on the one that i am working we turned the fan around to blow cool air onto
the heat sink for the mda's instead of sucking he warm air from inside the
machine across the mda's.... this helped to stabilize the swing arm jitters

 

[Matt C]

Does anyone know if the audio channel cards are interchangeable between the MKII and MKIII machines?  I have a MKIII and I'd like to get a couple spare cards, which would be much easier if I could find some from a MKII since those seem much more common.

 

[radardoug]

Theres still an Otari site in USA and they list the MTR90 Mk3 in their discontinued items. Maybe see if you can get a phone number and try and talk to a human who remembers tape? Otherwise find manuals for the 2 and 3 and study the cards with regard to the edge connector details and general appearance. I would say if the cards look similar, then there is a good chance they would be compatible.