Otari MX-5050 help needed: replace HA12017 with LME49710

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Brad McGowan

Well-known member
Joined
Apr 14, 2006
Messages
113
Location
Palo Alto, CA
Hi all,

I'm seeking some help from some opamp gurus. I'm in the middle of upgrading the signal path of my Otari MX-5050 1/2" 4-track and am trying to swap out some opamps.

I just kludged a LME49710 on a Brown Dog SIP adapter so that the pins of the DIP are rerouted to match the old Hitachi HA12017 SIP (see pic). I ended up installing the adapter backwards in the socket and then jumpering some pins to unused pins (tie DIP pins 4&5, 6&8, and 7&1, and then clip pin 2). It was quite the mental challenge but it worked out! So functionally everything lines up: +V, -V, inverting input, non-inverting input, and output. The bias and compensation signals (SIP pin 3 and 5)are no longer being used since I removed I removed C104=22p and R104=24k. It seems to pass audio and nothing has exploded...yet. However, I am noticing that when monitoring that channel there is a faint sweeping sucking noise that seems cyclical. It sounds like someone is trying to tune in a radio. Not good! What's causing that? Bad cap? Bad opamp replacement job. :?

I'm attaching a picture from that portion of the schematic. Besides the changes I made above, everything else has been left the same. IC101 is now the LME49710, but with the pinouts remapped as I described. Anything else I should change or remove. How about C102 or C137?

49601d1201583077-replacement-njm4556-4559-otari_repro_amp_schem.jpg


Any help to get this working properly would be greatly appreciated.

thanks,
Brad
 

plexi

Active member
Joined
Feb 7, 2006
Messages
31
Location
St. Louis
Sounds like it is oscillating. Maybe try a .1uf film cap across the power pins. Can you look at it on a scope to verify the oscillation? Maybe try 22 or 47 pf across C105.
 

Brad McGowan

Well-known member
Joined
Apr 14, 2006
Messages
113
Location
Palo Alto, CA
I'm going to try that tonight hopefully. I haven't yet put it on the scope, but I suspect that it is indeed oscillating.

I'll report back with what I find.

Brad
 

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