Otari MTR-10 Voltages - diyAudio
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Old 29th March 2011, 09:56 PM   #1
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Default Otari MTR-10 Voltages

I am working with an Otari MTR-10 tape machine. I noticed that the power supply that is labeled + and - 26 volts both on the schematic and the front panel indicator lamp only puts out + and - 22.5 volts. However, I have checked the circuit, and it seems to be working. It is based on a 15 volt zener diode, and calculating the resistor divider in the circuit suggests that it is designed for 22.5 volts, not 26. Does anyone have experience with this unit (either MTR-10 or MTR-12) who can confirm what voltage should be there? Thanks.
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Old 30th March 2011, 08:37 PM   #2
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I have an mtr-12 and manual.
It does show 26v.
What voltage do you measure coming off of the power transformer?
And what voltage is your line source input voltage as there are three taps for 100v,107v and 117v.
Is it possible that that the wrong tap is selected?
You should have 21 vac feeding each half of the regulator circuit.
acording to the manual.
The 26v power supply feeds a bunch of 78xx type regulators and the voltage drop could be due to a large current draw and is probably no cause for alarm.
What happens to the voltage when the power supply is seperated from the circuits it is feeding(if they can be seperated)?
I will pull my machine (mtr-12) out and verify these voltages for you.
The difference (ratio) of 26v/22.5v is 1.15 which is why I question the input taps.
what is your line voltage?

Last edited by geraldfryjr; 30th March 2011 at 08:42 PM.
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Old 30th March 2011, 09:40 PM   #3
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Thanks for your help, geraldfryjr. My voltage selector plug seems to be set for 118 volts, and I probably have 120 volts coming in. The transformer taps for this circuit are labeled 21 volts, and I have about 24 volts AC on both sides. After the rectifier and main capacitors, I have + and - 32.5 volts DC driving the regulation circuit. The regulation circuit then puts out the + and - 22.5 volts DC as I noted. I get this 22.5 volts under full load or with the power supply removed and no load connected, so it clearly is regulating to the 22.5 volts. The regulator circuit has a 15 volts zener diode that is actually measuring 14.5 volts and two transistors. The output voltage is divided across a 3.9 k and and 5.6 k resistor to feed back to the regulator. With the 15 volts zener (14.5 v actual) and the two transistor junctions, the circuit cuts off when the feedback voltage reaches about 13.2 volts. Interestingly enough, if you backup the drop across those resistors, you get that 22.5 volts out will give the 13.2 volts to the regulator, so it appears to be designed to regulate to 22.5 volts.
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Old 30th March 2011, 10:58 PM   #4
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Mine is a MTR-10 II.

I get 22v using my analog meter measured on pin 1 on the takeup tach board.

The thing is built like a tank I'm suprised they didn't just weld the thing together and save on the cost of the screws.

I knew you were waiting and I didn't have the proper tools handy so I traced it from the topside. jer
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Old 30th March 2011, 11:03 PM   #5
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Let me know if there is anything else I can help you with as I have the following decks and manuals. jer
TASCAM 38
TASCAM MSR-16
OTARI MTR-15
OTARI MTR-10 II
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Old 31st March 2011, 01:30 AM   #6
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Thanks so much for checking for me. Measuring under the deck is just as good as any. I am not sure what the difference is between the MTR-10 and the MTR-10 II. However, the fact that your schematics show 26 volts but measure 22 volts gives me confidence. I was pretty much convinced that must be the case based on my review of the circuit, but this relieves any doubts I had. I had a suggestion that Otari might have had some issues with heat dissipation and so lowered the voltage on later model MTR-10s without changing the labels on the rails either on the schematics or the front panel indicator light. I have run into several other things that suggest Otari might have made more changes than one would expect between the different series of these machines. Anyway, thanks again. At this point the only other analog tape recorders I have are Otari MX5050 series (II and III) and two Magnecord tube decks (1028 and 728). And, yes, I agree that the MTR-10s seem to be very well built.
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Old 31st March 2011, 06:46 AM   #7
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Your very welcome!
I also have some teac 4 tracks and 2 tracks as well as a TASCAM 22 1/4" half track that recently found schematics for too.
Good luck. jer
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Old 3rd April 2011, 10:12 PM   #8
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Geraldfryjr, One more question if you don't mind. I started calibrating the audio electronics, and ran into an issue with azimuth. The manual says to put the playback of the outer two tracks on a dual trace oscilloscope, play back first 8 kHz and then 16 kHz tracks on alignment tape, and adjust the head screw until the two sine waves align. When I did this, the lower track would not stay still on the scope. It danced around a probably about a quarter wave of the sine wave. I tried to set it so that it danced equally around the center of the top channel, but this did not feel good. Have you set the azimuth on your MTR, and did you see something like this? Thanks.
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Old 3rd April 2011, 10:47 PM   #9
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I haven't done an alignment on the otari yet but I have done them on my teac's years ago.
I what you are seeing is the actual flutter that the machine creates.

Here is why,
Your scopes trigger is set to one channel and what ever that channel is, the wave form on that channel will remain to appear stable and locked.

If you switch the trigger source on the scope to the other channel, then the results would be switched and the bottom track would remain stable and the top track would appear jittery.

If you set the trigger to both channels the both would remain rock soild stable and any azimuth adjustment would go undetected.

Only one channel can be set to trigger as this becomes the refference.

Are you working with the two track version or four track version? jer
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Old 3rd April 2011, 11:29 PM   #10
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You are correct that whatever channel is the trigger appears stable because it is the trigger. The other channel bounces around a bit. I had not thought that it is reflecting the actual flutter. I guess that means it is inherent. It is also worse with higher frequencies, based on the space on the tape. I had always just set azimuth by maximizing the output on both channels, which is not as precise. Have you learned any tricks to make it easier? I have a four track MTR-10 but I have converted it to a 1/4 two track by replacing the guides and head assembly.
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