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Otari MTR-10 voltages

I recently acquired an Otari MTR-10 tape machine on Ebay. While it was promised in pristine condition, I am finding a lot of issues. One thing I ran into is that the power supply voltages labeled + and - 26 volts on the schematics (and also on the front panel indicator lamp) are actually coming out at + and - 22.5 volts. I removed the power supply and checked it fairly thoroughly, and it seems to be working fine. In fact, when I look at the regulator circuit, based on a 15 volt zener diode and a resistor divider network, I estimate that it is designed for 22.5 volts. (These are the voltages that power all the audio circuits and most of the op amps in the sensor circuits. However, they are regulated down to 20 volts for the audio circuits and 15 volts at the individual board level as far as I can tell.) The other voltages are very close to the labels on the schematics. Does anyone have experience with these machines to know if there is something wrong I am missing? Thanks.


Boswell Wed, 03/30/2011 - 02:31

I don't know the inside of that particular machine, but I would not be too worried by the difference between the measured rail voltages and the documentation you have. It's quite common in electronic equipment for the designers to carry out partial updates (i.e. updating one part of the system leaving the rest untouched), so it would not surprise me to learn that, for example, there had been a heat dissipation problem when using 26V rails and they reduced them to 22.5V without re-working the front panel. You may have the schematics of an earlier version that had the higher rails.

Until you have evidence of an operational fault, I would enjoy using the machine. Unless, of course, your "lot of issues" get in the way.

marklang Wed, 03/30/2011 - 06:44

Thanks for your comments. I am not too worried because the 26 volts seems to be regulated down to at least 20 volts at the board level every place I can see it used, and 22.5 would be enough for that to work. However, the 22.5 is much lower than marked on the schematics of 26 volts. The schematics I have are probably not for the exact same series of MTR-10. However, I did verify that they represent the power supply circuits I actually have except for one small change in a current limiting part of the circuit. Also, as I noted, when I try to estimate the voltage that would be produced by the circuit in my schematic based on the 15 volt zener reference and the resistor divider network, I come up with 22.5 volts. (There could be a nuance in the circuit I am missing.) I suppose Otari could have reduced the voltages as Boswell suggested and updated the details of the schematics without changing the voltage labels on the rails. Anyway, if anyone recalls measuring these voltages on an MTR-10 or 12 I would still be interested in knowing. In the meantime, this seems to be an awesome machine now that I have gotten the digital servo system to work properly.