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







Gates Sta-Level

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

A client gave me a beautiful original Gates Sta-Level to bring back to life.  It had all original components but had not been turned on in over ten years.  After testing all the tubes, I brought it up on my Variac with just a solid state rectifier installed.  The power supply filter caps were good, but not great so they got replaced first.  With the new PSU caps and the tubes installed the unit was working ok, but had a very strange frequency response.  Above 1kHz it was fairly even, though not quite up to spec.  Below 1kHz it increased steadily to about +12dBm at 30Hz!  It turned out the feedback caps from the 6V6 output stage to the 12AU7 were leaky.  I replaced those and the coupling caps and things are now back to factory spec.

Vintage Gates Sta-Level

Original Gates Sta-Level

Gates Sta-Level original guts

The guts – dead stock!

Gates Sta-Level power supply re-cap

New Power Supply Caps

Gates Sta-Level capacitors

Replaced Film Caps


4 Channel Mic Pre & Limiter

Posted in Modded, Racked Modules with tags , , , , , , , , on December 31, 2013 by AnalogAudioRepair

Another fun project for engineer and lover of odd gear, Dan Physics.  He dubbed it the Mic RipPre 4.  These are funky little portable mic preamps with opto limiters that were designed to run on two 9v batteries.  They have Beyerdynamic input transformers and 600 ohm output transformers.  I gutted four of them, recycling the XLR connectors and circuit boards, and fit them in a 1U rack case with a +/-15v power supply and +48v phantom power.  The only other mod was a control for the amount of limiting.  It’s not exactly threshold, nor is it really ratio – just “amount”.

They are pretty clean sounding preamps, no surprise with such a simple circuit.  All new capacitors didn’t hurt either.  One board needed a new diode in the limiter, but otherwise they were in good shape.  Fitting all of the controls on the front panel was the hard part!  The original preamps had nice little collars around the toggle switches so I used those to prevent accidents with phantom power.

mic preamp/opto limiter

Stripped down boards

laying out 4 channel mic preamp/opto limiter

Making sure it will fit.

mic preamp/opto limiter control panel layout

Control layout

hand drilling panel for mic preamp/opto limiter

Why I need a Drill Press

wiring 4 channel mic preamp/opto limiter

All wired up

4 channel mic preamp/opto limiter

Completed unit

knobs and switches


Neumann CMV-563 Recap

Posted in Repaired with tags , , , , , , , , , , , on December 3, 2013 by AnalogAudioRepair

The Neumann CMV-563 microphone body can be used with different capsules including the famous M7 (cardioid) and M8 (figure eight).  This unit was getting noisy and swapping the tube did not entirely solve the problem. The two critical capacitors are a polystyrene cap which couples the capsule to the tube, and a film cap connecting the tube to the output transformer.  We decided to replace the third cap as well, although it was not original (green chicklet cap in the Before photos). The polystyrene cap requires extra care. A heat sink during soldering prevents damage to the component, then everything in this high impedance area must be carefully cleaned.  Any flux or oils from fingers can cause noise.  The other two caps were replaced with Wima film caps, increasing the size of the output cap to 1mfd. This was a really tight fit, but worth it. In the end, with a fresh NOS tube (not shown) the mic sounds great and the noise floor is much lower.

Before - cheap replacement cap

Before – cheap replacement cap

original Neumann CMV 563 polystyrene capsule coupling capacitor

Original polystyrene cap


Heatsink for Neumann CMV 563 tube microphone capacitor replacement

Heatsink on the new polystyrene capacitor

Neumann CMV 563 vintage tube condenser microphone output coupling capacitor

Original output coupling capacitor

Neumann CMV 563 vintage tube condenser microphone Wima output coupling capacitor

Wima 1.0uf output coupling capacitor

Calrec PQ 1161 Mic Pre & EQs

Posted in Repaired with tags , , , , , , , , , on September 29, 2013 by AnalogAudioRepair

I have always been interested in Calrec modules.  They are touted as being “as good as Neve”, “built to BBC specs”, and on and on.  Recently I found myself with a few modules on my bench from two different clients.  The first two were racked and powered but the second unit was noisy.  The other module came from a larger rack with 1181 and 1161 modules.  It had a different problem which I will get to later.  Besides the beautiful construction of these units, the first big thing I noticed was that the pair had all discrete amplifier cards, while the single unit had ICs (chips) on the cards!  It seems Calrec changed over at some point between the serial numbers in the 450 range and those in the 600 range.

The discrete amplifier cards are easy to pull out and swap for quick troubleshooting.  That was good because it allowed me to pinpoint the output amplifier as the noisy board.  Then probing with the oscilloscope tracked the noise down to the first transistor on the card.  Replaced the part, and it spec’ed just like the good channel again.

Thankfully the module with ICs had a much simpler problem, though one to watch out for!  These modules were sold with no light bulbs in the EQ and phase reverse switches.  The owner bought bulbs, put one in the EQ switch and watched smoke come pouring out!  Fearing the worst, he brought me the module.  Thankfully, there is a current limiting resistor which gave it’s life (and gave up the magic smoke) to save the day.  The problem was the way that the bulb socket mounts on the switch.  It is a plastic holder that straddles the switch and is soldered to the first pin on either side.  However, on this one module the other pins were not bent down for some reason (as they were on all the other modules) so pushing in the bulb caused it to short against the second pin!  Replacing the 100 ohm resistor and bending the pins was all it took to get things back to normal.


Calrec PQ 1161 Preamp & EQ

Calrec PQ 1161 Preamp & EQ

Calrec chip amplifier cards B202/205/210

Calrec chip amplifier cards B202/205/210

Calrec chip amplifier cards B202/205/210

Calrec chip amplifier cards B202/205/210

Calrec discrete amplifier cards B202/205/210

Calrec discrete amplifier cards B202/205/210

Calrec discrete amplifier cards B202/205/210

Calrec discrete amplifier cards B202/205/210

Calrec discrete amplifier card B202/205/210 PCB Front

Calrec discrete amplifier card B202/205/210 PCB Front

Calrec discrete amplifier card B202/205/210 PCB Back

Calrec discrete amplifier card B202/205/210 PCB Back

Lots Of Gear

Posted in Modded, Repaired with tags , on September 17, 2013 by AnalogAudioRepair

Last month was super busy with work and a short vacation, so here are a few quick pics. This is the first post done from my phone so I apologize for the brevity.

First is a 1950s Gibson Les Paul tube guitar amp full of NOS RCA tubes.  It had been improperly repaired previously (component soldered to the wrong side of a cap) and it had a 6V6 with an intermittent short. Found the fault and replaced the tube with a matching vintage RCA.
Next was a Drip 175 in for servicing. These guys are very particular about the 6BC8 that you choose! Calibrated nicely with a vintage GE tube.

Next is another Allison Labs filter mod with UTC transformers. Followed by another CBS Volumax mod, just because these pieces are so fun.



Vintage Tube Mixer Direct Ouputs (RCA, Ampex, Altec, Etc.)

Posted in Modded with tags , , , , , , , on July 30, 2013 by AnalogAudioRepair

Not a lot of great pictures this time, but something I do get requests for.  There are quite a few nice tube mixers around that sum two or more inputs into a single output.  Often folks would like to use these as individual mic preamps with their own outputs.  Your only choice is to use them as a single channel pre, or add some sort of direct out.

The issue is that these preamps use a single tube (or half of a dual triode) to amplify each input, then one or more tubes for EQ and output stages.  Of course it would be costly to add identical output stages to each channel, even if the power supply could handle it.  So the quick and dirty method is to take an unbalanced signal right off the coupling capacitor from the first stage.  The downside is that these outputs can’t drive much of a load, and have a reduced level compared to the main outs.  The upside is that they generally work well into the high impedance inputs of most converters and other modern gear.  Also, by using a switching jack, the signal can be connected to/disconnected from the mix buss automatically.

I have done this recently with a vintage RCA mixer (MI-C38690) and an Ampex MX-35 and both clients were quite pleased with the results.  The RCA also got input pads and a nice panel for all the connectors as well as some power supply cleanup.  The Ampex was additionally modded with line inputs on the first two channels.

RCA tube mixer with input pads and direct outs

RCA tube mixer with input pads and direct outs

RCA Tube Mixer Wiring

RCA Tube Mixer Wiring

RCA Power Supply Mess - Before Servicing

RCA Power Supply Mess – Before Servicing

Ampex MX-35 Line Inputs

Ampex MX-35 Line Inputs

Ampex MX-35 Direct Outs

Ampex MX-35 Direct Outs