Otari MX5050 8 track repro popping/crackling

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999imw

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I've been looking for an 8 track recorder for a while now and finally came across a great deal on an MX5050 mkiii in great shape. I also bought a soundcraft 200b 16 ch mixer to go with it an I'm pretty exited to get everything up and running. I got a new pinch roller last week and a bunch of new RMGI 911 tape. I decided to run some record/repro tests for fun before I got the mrl tape..... I recorded 1k and 10k tones on all tracks using the internal oscillator. The levels looked good on playback but I noticed when listening to the 1k playback, there's a popping/crackling sound in the background. The same thing is present on each channel. It's wierd because I don't hear it when listening to the 10k tone. I thought it was maybe bad tape and decided to wait and double check with the MRL tape. My test tape arrived today so I cleaned and demaged the heads and the rest of the tape path and loaded the tape. I was surprised to hear that the same popping/crackling while listening to the test tape. It's mostly noticeable on the 1k and not at all in the higher frequencies. Has anyone else heard this or know what i'm talking about? It's a very subtle sound but noticeable.
 
deoxit on all switches, connectors, trimmers potentiometers and relays if any. will be a start. open the machine and move connectors while monitoring to try to identify intermittent connections.
 
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Isn't that the "rocks" (low frequency rumble) they say to listen for when biasing?... meaning if you hear it the machine is not correctly biased? Some of the more in-depth technical guys may stop by and comment further. Also do research on biasing tape machines, you might find the reference I'm mentioning.
:spank::eek:;)
 
A Reel Person said:
Isn't that the "rocks" (low frequency rumble) they say to listen for when biasing?... meaning if you hear it the machine is not correctly biased? Some of the more in-depth technical guys may stop by and comment further. Also do research on biasing tape machines, you might find the reference I'm mentioning.
:spank::eek:;)
Click to expand...

Biasing would only affect recording, so assuming this mp3 is the new MRL tape played directly to the digital recorder to make this mp3 it does not sound like biasing. That is noise, however, and not part of what you should expect from a good Otari 8-track. Is that isolated to a few tracks or is it the same on all eight tracks? That may help identify the source of the problem. I have encountered similar noise with dirty tape paths, bad or sticky tape, dirty or worn heads, and suspect electronics. Assuming your MRL is new and clean and you have cleaned the tape path extensively, I would suspect dirty or failing electronics first, perhaps dirty contact points as suggested above? Or maybe old electrolytic capacitors in the audio path of the playback and/or output stages? You could further isolate the problem with a signal generator and oscilloscope if you or your local repairman have access to them.
 
yup... it's straight from the MRL tape to my computer. I cleaned everything really well too. It's the same on all tracks (except track 2 which doesn't seem to be reproducing bass frequencies at all right now but I'll worry about that later...) The heads look good... I think. When I bought it from the original owner he said he'd only done 2 projects on the machine.
I just opened up the back and none of the capacitors seem to be bulging at all (which is what i've been told bad caps look like).

IMG_4621.webpIMG_4622.webp
 
999imw said:
yup... it's straight from the MRL tape to my computer. I cleaned everything really well too. It's the same on all tracks (except track 2 which doesn't seem to be reproducing bass frequencies at all right now but I'll worry about that later...) The heads look good... I think. When I bought it from the original owner he said he'd only done 2 projects on the machine.
I just opened up the back and none of the capacitors seem to be bulging at all (which is what i've been told bad caps look like).

View attachment 78349View attachment 78350
Click to expand...

Not all head problems are going to visible or obvious to the naked eye, which is why JRF Magnetic Sciences does head reports. Nor do leaking or failing capacitors always bulge, as that is only one visible symptom of only one type of failure. How about these additional trouble-shooting methods: If you input a signal (try a CD of music) and set the Otari's output to monitor the source instead of the tape, do you still have the same problem with the audio? When playing the MRL tape, does the audio change if you monitor off the record head versus the repro head? How about if you play a completely new and virgin RMGI tape with nothing on it... are there still noisy pops and crackles in the hopefully-silent audio output?
 
Could it have any saying if the Otari is too close to anything heavily magnetic like big speakers? Just a wild guess.

Have you demagged the machine since buying?
 
It's about 2 or 3 feet from my monitors but thats it... could that cause it? I use my 4 track in the same area and it doesn't seem to be affected.

Thanks toddyjoe, I'm going to run those tests tomorrow and see if I can narrow it down some more. As far as doing more in depth inspection, I'm going to get a voltmeter tomorrow and I downloaded a Mac-based program called AudioXplorer which is supposedly an oscilloscope.
 
Additional to the suggestions from above:

0. Find out if the noise is only in playback or in monitor mode as well. One way to do this is to put the recorder with no tape, but holding with your hand the tension roller, so the deck thinks there's tape, listen to the output in both repro and monitor mode, to see if the noise is there in both cases or only repro. If both, then the issue is in the preamp or power supply likely. If only on repro, then you have to look at the repro amplifier. I doubt it will be zero, as the noise is present with prerecorded tapes (like the MRL). Check if the noise presence is related to motors running, both capstan and the reel motors. Maybe they are inducing noise (power supply/capacitor problem)

1. With the scope, look for the noise, to see how it looks, meaning with no tape as mentioned above, you dont want the audio signal of a tape on top of it to debug this. The idea here is for you to see how the noise looks like in the scope. If you have a spectrum analyzer app, that will be nice too, as you can understand the offending frequencies, and then look for the same pattern when looking at different signals in the circuit.

2. About using the computer as a scope, keep in mind that it may be ok to look at the audio signals coming from the plugs, audio outs. But not to troubleshoot the circuit in other places, levels maybe too high and can kill your soundcard.

3. Check the power supply for the noise, I think this machine has a +15VCC, look at the schematics and put the scope there, look for the noise as you saw it in step 1. From what i can hear it is a random noise, so you may not be able to reproduce it 100% of the time i am guessing.

You may need to follow the audio path to find the problem in the actual cards. Schematics will be a most for this if none of the steps above work. Not a big deal, but some basic electronics experience is needed, if you have zero experience on that, i suggest to not try to debug the internal electronics and find some help. Doublecheck the switches and use deoxit, you will be surprised and it is something that you have to do anyway for any old machine, why are you sure that is not related to contacts ?

good luck
 
ok, Just tried a few more tests...... The MRL tape sounds the same coming off the repro and record head. I put on a fresh reel of RMGI 911 and listened to it... I could hear some crackling but I couldn't tell if it was the same sound I heard on playback. Then I recorded my synth holding a tone and as it was recording it sounded fine when switched to SEL-REPRO ( the record head I think... i could be wrong) but I could hear the popping when I switched to REPRO. I'm going to try spraying some contact cleaner on the input pots now .....
 
999imw said:
ok, Just tried a few more tests...... The MRL tape sounds the same coming off the repro and record head. I put on a fresh reel of RMGI 911 and listened to it... I could hear some crackling but I couldn't tell if it was the same sound I heard on playback. Then I recorded my synth holding a tone and as it was recording it sounded fine when switched to SEL-REPRO ( the record head I think... i could be wrong) but I could hear the popping when I switched to REPRO. I'm going to try spraying some contact cleaner on the input pots now .....
Click to expand...

And what does the synth sound like when you switch the Otari to monitor from the input source directly instead of from the tape? Incidentally, what tone were you holding on the synth? Your initial posts suggest the noise was frequency dependent on the MRL, so you may wish to be consistent with the MRL in the notes you are using on the synth to minimize the number of variables for trouble-shooting purposes.

If you are spraying the inputs pots you should probably spray all of the pots and all of the switches as well. Also make sure you are working the pots and switches with the cleaner and not just spraying them.
 
999imw said:
It's about 2 or 3 feet from my monitors but thats it... could that cause it? I use my 4 track in the same area and it doesn't seem to be affected.

Thanks toddyjoe, I'm going to run those tests tomorrow and see if I can narrow it down some more. As far as doing more in depth inspection, I'm going to get a voltmeter tomorrow and I downloaded a Mac-based program called AudioXplorer which is supposedly an oscilloscope.
Click to expand...

Sounds unlikely to be causing the problem then. I was thinking about big, heavy PA speaker standing close to the deck.

Did you demag? You should do that frequently no matter if you're encountering problems or not.
 
toddyjoe said:
And what does the synth sound like when you switch the Otari to monitor from the input source directly instead of from the tape? Incidentally, what tone were you holding on the synth? Your initial posts suggest the noise was frequency dependent on the MRL, so you may wish to be consistent with the MRL in the notes you are using on the synth to minimize the number of variables for trouble-shooting purposes.

If you are spraying the inputs pots you should probably spray all of the pots and all of the switches as well. Also make sure you are working the pots and switches with the cleaner and not just spraying them.
Click to expand...

I was using a 1k tone, the same as on the MRL. It sounds fine from the input and but when monitoring from the Repro head in record mode I can hear the popping. When I playback I can hear it off the Repro head and Record head.... Actually... I can hear a low frequency rumble on the blank tape if I turn the gain up enough but it doesn't sound quite the same as when there is a tone on the tape.

should I spray and turn all the bias/calibration pots (Low F level, VU level, peak level) on the back of the machine? I'm afraid to start turning those and mess anything up...
 
WarmJetGuitar said:
Sounds unlikely to be causing the problem then. I was thinking about big, heavy PA speaker standing close to the deck.

Did you demag? You should do that frequently no matter if you're encountering problems or not.
Click to expand...

I demaged yesterday.... unless the demag device didn't work properly. It seems kind of old but I could hear it humming. Would magnetization cause this sort of problem? I thought that mainly affected the high frequencies....
 
if you mess up the bias and eq pots, you need to recalibrate the machine, even if you mark the locations of the pots. Be prepared for that. Dont touch those yet as you may not be able to reproduce the problme in the same way, one change at a time. If the noise is only on playback on either head but not in monitor mode, the problem is in the play/repro amp section likely. where are you located?
 
Cleaning the trim pots didn't see do help anything.... A friend of mine is going to come over with his oscilloscope and although he's not a tape machine tech he knows his way around electronics. Hopefully we can figure out whats going on. I'd really like to try to get to the bottom of this myself in order to gain a better understanding of my equipment. The funny thing is that it's happening equally on each track so the problem must be before the signal splits off into each individual track..... closer to the head. does that make sense?
 
+100. Trouble shooting should always start with the power and power supply.
 
ok. As soon as I can get my friend over here with his scope, thats the first thing we'll check. I know he's pretty busy but hopefully by next week sometime... thanks everyone

oh... and I'm in Montreal fgonza.
 
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