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

AKG BX20 Spring Reverb

Posted in Restored with tags , , , , , on May 3, 2013 by AnalogAudioRepair

So a client brought me a BX-20 – the largest of the AKG BX series spring reverbs.  I have always loved the BX-10 and was interested to work on the big daddy.  The amplifier cards were partially recapped, so I completed the job.  I even replaced the tantalum caps on the preamp daughterboards because they were getting leaky.  With a little adjustment the boards now sounded good but one side of the reverb was spitting out random noises.  After chasing ghosts for a while, I determined that the problem was in the spring assembly and proceeded to take it out and open it up.  It was measuring like there was a broken wire or burned out coil.  I was hoping for the wire, but unfortunately one of the tiny red coil windings that hangs in the magnet gap had gone open.  There are no replacements for these so for now it will be a mono reverb!

AKG BX20 Panel

AKG BX20 Panel

AKG Daughterboard

AKG Daughterboard

AKG Inside

AKG Inside

AKG Spring Tower

AKG Spring Tower

AKG Spring Base

AKG Spring Base

AKG Coils

AKG Coils

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!

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Ampex 601 Tube Preamp & DI Mod

Posted in Modded, Restored with tags , , , , , , on March 22, 2013 by AnalogAudioRepair

This was a fun project that involved removing some tape deck electronics and modding the remaining circuit into a tube mic preamp with DI.  As you can see, several of the tubes are not needed when using this as a preamp.  A whole bunch of other parts came out too which cleaned up the crowded underside of the chassis a good bit.  The headphone jack is now the DI input and the Input/Tape switch selects either the mic or DI.  The Line Rec Level pot is now the DI level control and an XLR output jack has been added on the side.

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Telefunken V72S Transformer Replacement

Posted in Repaired with tags , , , , , on February 26, 2013 by AnalogAudioRepair

This is a rare Telefunken V72S – the “S” is a higher gain version.  Unfortunately this one had a bad input transformer.  The frequency response of the transformer looked like this: /   A rebuilt original was too costly for my client, but Sowter offers a replacement that works just fine.  The hard part is removing the original and mounting the new one!

RCA BA-2C Tube Preamp

Posted in Restored with tags , , , , on February 26, 2013 by AnalogAudioRepair

This vintage RCA vacuum tube preamplifier was missing it’s power transformer.  Fortunately I had the schematics and a transformer that would fit the bill.  A couple power resistors were required to get the voltages spot on, but in the end it worked out well.  The unit got a full re-cap and a fresh set of New Old Stock vacuum tubes.  The new owner even received an original manual!

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.

CBS Volumax Limiter Mods

Posted in Modded with tags , , , , , , on December 29, 2012 by AnalogAudioRepair

This was a project for a client from Brazil – modding a CBS Stereo Volumax discrete transistor limiter.  The Volumax is a two band limiter with transformers in and out.  You can do fun stuff like turn off the high or low frequency limiting, vary the HF level, change the release times, and much more.  This was a pretty standard mod that also included XLR in and out.  Amazing units when you do a bit of work to them!

Fender Re-Issue Amp Repairs

Posted in Repaired with tags , , , , , , on December 29, 2012 by AnalogAudioRepair

I recently serviced a pair of Fender Re-Issue tube amps from the 1990’s.  One was a Deluxe Reverb with a shorted output tube, the other a Vibrolux with a blown cap (literally).  These ’90s re-issues are much like their predecessors, but with printed circuit boards which make it a bit more of a pain to work on them.  At least the Deluxe has connectors for the front panel controls rather than soldered wires like the Vibroverb!  It had some 2 watt resistors that were so charred they almost burned through the PCB.  I had to scrape away as much carbon as possible and seal the board with epoxy before replacing the resistors and power tube.  The Vibrolux had a capacitor which actually burst and both amps had very weak Chinese made 12AT7s.  While I was in the Deluxe I added the de-clicking cap to the vibrato circuit.