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Author Topic: Otari MX-5050BII-2 Modification for Heads direct to XLR  (Read 8661 times)

Offline AZ_Gary

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Otari MX-5050BII-2 Modification for Heads direct to XLR
« on: July 28, 2008, 11:48:33 AM »
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

« Last Edit: July 28, 2008, 02:56:02 PM by AZ_Gary »
Gary Marklund
Sun City, AZ - USA

Offline joeljoel1947

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #1 on: July 28, 2008, 12:27:20 PM »
Hi Gary,
I am having a hum issue with my Otari head out wiring.  I have both a B3 and a Mk.3 Otari console and on both units a friend of mine (who knows a lot more about this stuff then I do) and myself are getting severe hum unless either:

a) a cheater plug is used with the head out wiring connected
b) the head out wiring is disconnected all together.  Then, no hum.

We're using shielded wire internally and grounding the shield inside.

Were you getting hum before you used the lamp cord and that helped solved it?

And when you say "no hum from the listening position" does that mean that you are getting a 60Hz hum when up close to the speakers and say their is a quiet passage or in between track silence?

I will have my friend read over your post and perhaps we are doing something else wrong.

Thanks for the excellent post!!!!

Joel Kozlowski
Regards,
Joel Kozlowski

Offline AZ_Gary

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #2 on: July 28, 2008, 02:05:48 PM »
When I first fired it up, the gain was not high. I think it was just because I had just turned on the Seduction. I had the line stage volume cranked up a bit and could hear some hum at the speakers. I tested by just touching the ground wire to the terminal and it made a noticable difference, so I connected the ground wire. The hum level was not terribly high at the beginning. If you are getting a lot of hum, I would look at other possible causes. After an hour of play, remember this is also a new Seduction, I had very acceptable gain and no discernible hum.

Factors to consider 1) my speakers are only 94dB at 1 watt/ 1 meter 2) I'm just running a low power 2A3 Paramour I and 3) I'm 67 years old and may miss low level hum or hiss.

I'm not sure what you mean by cheater plug at the head out wiring. That is the place (B9 pins 2 and 3) that I grounded to the internal ground connection. Those leads run through the shielded wire and out to the XLR where they are strapped to the low side of the balanced output connector.

Gary
« Last Edit: July 28, 2008, 02:09:18 PM by AZ_Gary »
Gary Marklund
Sun City, AZ - USA

Offline docb

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #3 on: July 28, 2008, 03:47:20 PM »
For what it's worth, our experience with Technics internal head cables proved that grounding the shield of the internal head cable at the RCA output jacks rather than at the head connection end of the cable was critical to keeping the hum out. Also, even in cases where you don't hear hum, we have found that you can see a reduction in the noise floor on a scope if you connect the ground post of the Technics to the ground post of the Seduction.

Unfortunately this may or may not apply exactly the same way to the Otari. Ground loops and hum pickup are what I would deem the most gnarly aspect of audio design. You have come up with a methodical approach to connecting and disconnecting the shield, chassis and safety grounds in various combinations to solve the problem.
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline joeljoel1947

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #4 on: July 28, 2008, 03:55:47 PM »
Hi Gary,
Thanks for the info.

What I meant was, if I float the ground to the deck itself---using a 3 to 2 prong cheater plug--then I was able to almost completely eliminate the hum.  But, it was still there when the volume was cranked and I was within 3-4 feet of the loudspeakers.  Like you state, the gain on the Seduction is somewhat low.  I was told to perhaps add this buffer circuit, which will not only improve gain but also help impedance matching as well:
http://db.audioasylum.com/cgi/m.mpl?forum=bottlehead&n=92206&highlight=FET+buffer&r=&session=


In my particular situation, adding a ground wire between the Seduction and the Otari increased the hum tremendously, so I left it disconnected.  The other issue that I am actually addressing is that even WITHOUT using a seduction---just the Otari electronics themselves---there is a 60Hz hum that get's "added" when I add the head wires out.  This is in about 4 different systems, including my friends house.  However, if a cheater plug is added, the hum disappears.  So, my friend and I are in search of how to eliminate this mysterious hum that does not exist UNTIL WE add the direct head out wiring on the Otari.  Then, the hum issues set in.  This is even using shielded wiring.

My Seduction had a different issue and needed to be sent in for repairs.  I'm hoping when I get it back the slight hum will also have vanished.  BTW, another thing I learned was to remove those springs from the tube shields.  There were a few posts a while back on the Bottlehead forum by people who professed that removing those springs eliminated all their hum/microphonic issues.  The springs are coupling the physical tube to the large metal plate that everything rests on.  YMMV...

Thanks again for the help and post.  I was told to seek out a "Mike Paschetto" (Mikey) who posts here and is an expert at head out wiring but I haven't got that far yet.

Joel Kozlowski
Regards,
Joel Kozlowski

Offline docb

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #5 on: July 28, 2008, 07:30:17 PM »
Hi Joel,

Where exactly is the shield wire of the direct out head cable attached?
Dan "Doc B." Schmalle
President for Life, Bottlehead Corp.
Managing Director - retired, The Tape Project

Offline steveidosound

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #6 on: July 29, 2008, 12:06:46 AM »
Grounding is always an issue with low level signals. Sometimes just having 2 items in a given system tied together with the grounds of their power cords will create a ground loop. When I was connecting equipment for example with grounded power cords that was also grounded chassis to chassis in the same rack and grounded through pin one of XLR cables or other balanced connection, I almost always had to eliminate the balanced line ground at one end for lowest hum. Sometimes eliminating grounds helps more than adding more ground paths.
I will be going through the same issues with my Seduction kit soon. I wonder if instead of a single conductor unbalanced shielded cable wired to the RCA  jack inputs from the heads, if a 2 conductor balanced cable wired from the heads with the outputs to the RCA jacks and the shield terminated at the central chassis ground point of the preamp and floated at the tape head end would work better? That is to say, no connection to tape deck chassis ground at all except through power ground. Anybody tried this scheme?
Steve Williams

you don't want to know what equipment I listen to...

Offline joeljoel1947

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Re: Otari MX-5050BII-2 Modification for Heads direct to XLR
« Reply #7 on: July 29, 2008, 04:25:07 PM »
Hi Doc,
Thanks for the advice.  My buddy solved the issue!  It turns out that the RCA connectors he installed were interfering with the chassis ground of the unit---metal to metal contact.  To fix it, he used some isolation washers around the RCA connections he installed on the deck to eliminate the metal to metal contact.  This was the source of most of the hum.  He then tied the shield connection to the chassis ground on the RCA end (like you mentioned) which eliminated all the hum.

If anyone wants to see a neat little video on all this (and my lovely Otari Mk.3 console) then PM me offline.  It's a 11.2 MB file. 

Joel Kozlowski
Regards,
Joel Kozlowski