Archive for analog

RCA BA-72A Preamps

Posted in Racked Modules, Restored with tags , , , , , , , , , , , , on January 22, 2014 by AnalogAudioRepair

A client in Los Angeles wanted a pair of RCA BA-71 preamps racked with the option of using them to run some stereo tracks through.  These use the same circuit as the BA-31 (minus power supply) and the BA-72.  I had a few BA-72A modules in stock and came up with this single space rack mount solution.  First the modules were tested, re-capped and tested again.  Then I built a power supply with 30 VDC for the preamps and 48 VDC phantom power.  The modules were wired with a switchable 20dB pad as well as a switch to change the input transformer from 150 ohms to 600 ohms.  With both the pad and 600 ohm setting, the preamps can take up to 0dBm input without clipping.  Of course the output is +20dBm at that point, so I added 600 ohm attenuators to the output.  Everything fit nicely in the single rack unit and I put collars around the +48 switches, original RCA knobs on the outputs, and a black and silver RCA “meatball” logo on the front panel for good measure!

recapped RCA BA-71 72 31 transistor preamplifier

Recapped Preamp

connecting RCA preamps BA series discrete transistor

Wiring in Progress

Wiring RCA preamp rack BA-72A

Wired Up

RCA BA-71 vintage preamplifier front

Front View

RCA BA-72A Preamp back panel

Back Panel

RCA BA-72 discrete preamp controls


RCA meatball logo on BA-72A preamp

Meatball Logo








Suzuki Omnichord OM-84

Posted in Repaired with tags , , , on April 28, 2013 by AnalogAudioRepair

I recently serviced two vintage Omnichords, both model OM-84. One had been attached to the wrong power supply, the other cut out intermittently and had to be turned off and on to reset.

The first one had an obvious problem – the power amp chip had literally blown! The power supply circuit has some regulation for the digital logic and other circuits, but the power amp gets the full supply. The part is obsolete but I sourced a genuine replacement to get it playing again.

The second unit had some intermittent issues. I found that pushing on the front panel or tapping the side would cause a playing rhythm/chord to freeze and require re-booting. Opening the unit it became obvious that the connectors between the two PCBs were at fault. Cleaning and reseating did not solve the issue so I touched up all of the connector solder joints on both boards – problem solved!