I performed these mods to my machine and am very happy with the results. Doc, if you want to edit this for proper terminology, especially things like "terminal blocks", be my guest, I'm not an engineer.
This Otari is connected to the Seduction I recently built with the NAB/IEC equalization at the following link:
http://www.marklund.com/images/Audio/Bottlehead%20Gear/Tape%20Seduction%202/Otari MX-5050-BII-2 tape head output modification:These instructions assume the reader has the service manual for the Otari. My copy is Edition 8 dated October 1987.
Referring to the view in Figure 5-1 on page 5-2 lay the unit on its back, remove eight screws to remove the bottom plate and pull down the bottom panel providing access to the amplifier board, A10620B foldout, following page 10-1.
Using the circuit board diagram PB-16EB on page 8-3, unplug terminal blocks 9, 13 and 14. They are numbered on the blocks. The pin numbers are located on the board next to the PCB-mounted male connectors. Note these numbers. Loosen the screw that attaches a ground wire at at the right end of the chassis. Block 9 is the channel 1 and channel 2 connections from the heads. Pin 4 is channel 1, pin 1 is channel 2, pins 2 and 3 are grounds. Block 13 is the harness connecting the line output to the channel 1 XLR and block 14 is for channel 2.
Prepare two lengths of Doc's STP. I just cut the excess from my Seduction kit in half yielding two 6 inch lengths. Remove the outside jacket and foil for a length of ? inch on one end and trim the red and black leads 3/8 inch on one end. On the other end, remove about 1 ? inches from the outer jacket and foil. Trim the red and black leads, leaving all but 3/8 inch bare.
Wrap the drain wire of the long end tightly around the black lead two times. You can solder it if you like, I didn't.
Just push the leads of the short end into blocks 13 and 14, the wire diameter is a good match for the block connector and should hold it well. The red wire of one goes into pin hole 2, the black to 1 and the drain to 3. The other end of the red wire going block 13 is attached to pin 4 block 9 and the black to pin 3. The red wire from block 14 is attached to pin 1 of block 9 and the black to 2. The drain wire of one is wrapped twice around the drain wire of the other and soldered. Trim the excess from the end you wrapped. Put a ?j? hook in the remaining drain wire and attach it to the previously loosened ground wire connection. It will work best of looped under the existing ground wire connector. Note that the Block 13 and 14 numbers DO NOT correspond to the XLR pin numbers. Aside from that Otari has done an excellent job of part identification.
The above assumes you have wired your XLR connectors for unbalanced single conductor interconnects as shown in Figure 2-4 on page 2-8.
I also connected a length of lamp cord between the ground posts of the Otari and the Seduction.
This procedure should produce a hum-free connection. My seduction has ample gain and is hum-free at the listening position. I may add the Hammond box as additional shielding on the Seduction.
I hope this will be helpful for anyone using the Otari. I like it because I can revert to the original configuration in about 15 minutes with no visible scars.
Gary
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