Wednesday, July 6, 2016

Part 4: MCI JH110B Rebuild - Smoke in the studio!!!

By Tim Dolbear

So... first the good. I built the switching system to allow me to route to/thru/from the JH110B tape machine, adding it to the signal path of the Mix Buss.

I decided to do a switching system and not a patchbay as I am not a fan of patchbays or connectors.

See my blog about connectors and patchbays:

I also decided to do a terminal connection instead of XLR ins/outs. Connectors just have such a small point of contact verses terminals.

I did a test measurement prior to adding the switching system,  I figured the first thing many would think was the same thing I thought about; will it add noise or a loss of sound quality?

 Adding the Switcher into the signal path, only added 1 balanced cable to my mix buss signal path when its going straight through, meaning no tape machine in line:  Mytek D/A feeding the GR EQ-2NV (in Bypass for the test) into the Manley Mastering Edition Variable Mu into the switcher, out to the Apogee A/D. Of course it added to it all the wiring for the Switcher box.

Crazy as it seems, My noise floor went down from -82.6 db to -83.7 db and my volume back into the DAW from the mix buss is .2db louder.  I can not explain it as all the cables are the same except for the 1 new 3ft balanced cable added between the Manley and the Switcher.  It may be the difference between the Variable Mu being warmed up for 1 hour verses 4 hours. (first test was at noon, 2nd was at 4pm).  Regardless, sound perfect and did not add any noise issues.

While then testing the route to the tape machine, setting levels, and working with a TC Electronics Volume Pilot that is inline between the tape machine and the monitor send, audio was working nicely on the JH110B, still not bias/aligned, but passing sound both in INPUT and REPRO (while Recording) modes. Suddenly a bad sound came through the right side speakers, and smoke poured out of the Audio Penthouse of the JH110B. I quickly killed power to the JH110B and opened the door for some fresh air...

Another Resister blew, this time one on the 18v input to the I/O card for the right side or Ch2.  I checked the 2 tantalum caps that are inline with it and booth are dead shorted now... good grief!

At this point I will of course replace the resister but also plan to replace all the tantalum caps on the audio boards (repro, rec, bias, I/O).  Fun!  But apparently needed since I have so many bad and going bad daily.

Till next time!

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