Archive for the Repaired Category

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

 

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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.

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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.
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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.

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

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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

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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