Archive for discrete transistor

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

Controls

RCA meatball logo on BA-72A preamp

Meatball Logo

 

 

 

 

 

 

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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.

Image

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

Neve 5316 Diagrams

Posted in Uncategorized with tags , , , , , , , , , , , on July 21, 2013 by AnalogAudioRepair

I have been working on some modifications to the Studio G Neve 5316.  The board came from BBC Scotland and did not have any schematics.  While it is certainly possible to figure out a lot of the signal flow without block diagrams, it is sort of like driving across country without a map – you know where you start and end, but you have to find signs in between.

I posted on Geoff Tanner’s vintage Neve forum at http://auroraaudio.net/forums/ and frequent contributor Mr. Blake Devitt was able to provide some helpful documents.  The forum did not handle large images well, so I volunteered to host them here for anyone who needs them in the future.

Block Diagram

Block Diagram

Circuit page 1

Circuit page 1

System Diagram

System Diagram

Telefunken ELA V624 Discrete Preamp & EQ

Posted in Repaired, Restored with tags , , , , , , , , , , , on June 29, 2013 by AnalogAudioRepair

Bad packing, or should I say NO packing, is to blame for the damage this preamp suffered.  It was shipped to me in a small flat rate Priority Mail box with nothing to protect it.  As you can see, the frame was bent and one of the circuit boards was broken into six pieces!  The frame was easy enough to bend back, but the PCB was like a little jigsaw puzzle that required epoxy and jumper wires to make it structurally and electrically sound again.  While I had it open, I replaced the electrolytic caps and added XLR ins and outs as well as a power jack with regulation.

The ELA V624 is a nice little module from later Telefunken mixers.  It has a mic pre with 60dB of gain, high and low EQ (I believe at 60Hz and 10kHz), and a line input as well.  Both the mic and line have transformer balanced inputs and I wired the unit with an impedance balanced output.  The circuit is fully discrete transistor and runs on +24v DC.  They are not as huge sounding as the earlier discrete modules (V672 Pre, W695 EQ, etc.) but very useable on a variety of sources.  Certainly worth the time spent repairing such unnecessary damage!

Bent Frame

Bent Frame

Broken PCB

Broken PCB

Broken PCB 2

Broken PCB 2

Before

Before

Fixed Frame

Fixed Frame

Fixed PCB

Fixed PCB

Recapped

Recapped

XLR Box

XLR Box

Completed, Front

Completed, Front

Completed, Back

Completed, Back

Zim-Gar Vintage Transistor Amp

Posted in Modded, Repaired, Restored with tags , , , , , , , on May 4, 2013 by AnalogAudioRepair

This is a funny little solid state amp I bought from a fellow on Craig’s List.  I bought it “as-is” and it hummed and made noise but passed no signal.  At first I thought it was Japanese, but when I took off the back panel there was a tiny schematic marked “Paris Corp. Salina, Kansas”!  I replaced the first transistor which took care of the noise and lack of signal, but the tremolo wasn’t working.  I rebuilt that section with some new caps and a resistor and dug up a vintage footswitch to turn it on and off.  The power supply filter cap had already been replaced so I can only think that the amp has always had some hum (it was certainly not high-end when new).  Adding a second stage of filtering killed the hum and made this into something that could actually be used.  Honestly it was more work than the amp is probably worth, but a fun project nonetheless!

Zim Gar Front

Zim Gar Front

Zim Gar Panel

Zim Gar Panel

Zim Gar Panel

Zim Gar Panel

Zim Gar Schematic

Zim Gar Schematic

Zim-Gar Guts

Zim-Gar Guts

Sphere 1014 and 910 Equalizers

Posted in Racked Modules with tags , , , , , , , on May 3, 2013 by AnalogAudioRepair

This is a rack full of vintage Sphere equalizers.  My client made the wooden case from zebra wood and walnut.  I wired and mounted the EQs and transformers and built the power supply.  The 1014s are IC based parametric EQs with high and low cut filters.  The 910s are fully discrete graphic EQs with inductors for each band.  We used transformers from Orphan Audio – Quad Eight inputs and Reichenbach outputs.  Unfortunately the Reichenbachs were PC mount so I had to attach them to some perfboard.  The Quad Eights mounted right below the input jacks on the back panel.  Power requirements were +/- 24VDC for audio and +5VDC for logic.  It was a lot of wiring to fit in the box, leaving length for servicing.  They are pretty fantastic EQs, especially the graphics!

Sphere On

Sphere Back

Sphere Back

Sphere Wiring

Sphere Wiring

Sphere Xfmr Board

Sphere Xfmr Board

Sphere PSU

Sphere PSU

GATES Discrete Microphone Preamps

Posted in Racked Modules, Restored with tags , , , , on December 29, 2012 by AnalogAudioRepair

This is a pair of Gates broadcast discrete transistor mic pres.  These microphone preamplifiers were used in Gates radio boards and have transformer balanced inputs and outputs.  They have 37 dB of gain and I racked them with a level control ranging from 20-37db.  The position of the level control is immediately after the input transformer so there is no need for a pad on the input unless you are overloading the transformer!  These units were re-capped and wired for 150 ohms in and 600 ohms out.  This was a super simple and clean rack job so they only have XLR in and out, level control, and a power switch for the outboard +30v power supply.