stereo

Phonic 1500 Rack Mount Stereo Amp Repair and Update

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Billy was running this amp as part of his PA for years until one channel quit.  Could the Unbrokenstring Crew bring this inexpensive amplifier chassis back from the e-waste pile?
To some folks, Phonic and Professional do not belong in the same sentence.  But this unit has been working very well behind the scenes, for more years than many of its Brand Name Brethren have been working.

 

Simple controls on the front.  Cranked to eleven, of course.

 

It doesn’t get much simpler than this.

 

Billy asked to have an IEC power cord installed on this when (not if) we fixed it.

 

The Neutrik power connectors are a nice touch, almost required at higher power levels.

 

Inputs are either balanced quarter-inch TRS or XLR connectors.  The usual set-and-forget controls are found here.

 

The power transformer seems adequately-sized for this power level.  AC-to-DC duties are performed to the lower right, and one audio channel is seen between the transformer and the aluminum heat sink.

 

With the unit turned around, the other audio channel is found here.

 

The front panel controls are here.  These potentiometers will get a million-mile cleaning and lubrication.

 

The dark charcoal-colored ribbon cable connects signals to the two audio channels.  The flat ribbon is Just The Thing here, because it does not block the path of cooling air into the unit.

 

The electrolytic capacitors are bulged, which is not unusual for a unit that has seen this many years and this much use.

 

These electrolytics will be replaced.

 

These rectifier blocks are wired in parallel.  Yeah, Baby!

 

I was kinda wondering where the line fuse was located.  It’s under this bundle of cables.  Yes, that says 30A at 115VAC.

 

Before the unit comes apart, I need to document where all these cables go.

 

These cables need to go back where they started.

 

Behind the colorful bundle in the foreground is the circuit board that connects the rear-panel output connectors.

 

I need to remove the larger circuit board to get to the solder-side of the PCB.  The dirty little ribbon cable comes off first.

 

These output cables come off next.

 

And now, we begin.  There are about thirty screws that hold the circuit boards in place.

 

These little screws are everywhere.  Like that’s a bad thing…

 

More screws.

 

The heat sink is split into two sections, one for each channel.  They need to come loose from the chassis as well.

 

This aluminum block bridges the tops of the heat sinks to add strength and rigidity to this unit.  Nice!

 

OK, the main board is out of the chassis.

 

Interestingly, some other version of this amplifier uses more electrolytic capacitors.  In this version, the pads are jumpered.

 

Turning the board over, the solder joints to be cleared are ‘marked’ with some rosin solder flux so I can find them if I look away to grab the soldering iron and braid.  Yes, I’ve unsoldered the wrong solder joints in the past.

 

The old caps at the top of the picture are out and the holes in the PCB are cleared.

 

Meanwhile, back at the ranch, we will remove the captive line cord and add the IEC connector here.

 

This cord has 14AWG conductors in it.  A matching large molded IEC power cord will be supplied with this unit when it is returned to the customer.

 

Here is our new IEC male socket.  Some of these come with flange ears, but spacing on the rear panel is too tight to allow the use of one of those.  This one snaps into place.

 

This looks a little rough, but this is the approximate outline of the rectangular cutout for the new IEC connector.

 

While we are hatchet-ing on the rear panel, these magnets will catch any chips or bits of steel removed from the hole.

 

Here is the outside view of the first trial fit.

 

And this is what it looks like on the inside.  This IEC socket snaps into the hole, so the hole size needs to be right.

 

Here is the new IEC socket wired into place.

 

And this is the closeup of the finished installation.  That silver thing next to the CE mark is a ground point.  I temporarily removed the thumb screw while grinding on the chassis.  It goes back on the unit next.

 

Now it’s time to reassemble.  Screws, anyone?

 

Glad I took all those pictures of where these wires went!

 

The unit is now reassembled.

 

Here, the terminal block outputs are tested at 250 watts.  So far, so good!

 

The Neutrik connectors are tested next at 750 watts per channel.  All is well!

 

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Behringer T1953 Preamp Refurb

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This preamplifier and microphone processor is in the audio chain of every hit produced by Majic$tyle Studios over in Houston’s 3rd Ward.  But it quit one day.  So the music quit.  Could the Unbrokenstring Crew help the Majic$tyle Crew bring back the music?
Like many recording facilities around Houston, the modest studio in the back room has a lot of history.  A lot of talent in the Houston Urban Contemporary and Hip Hop scene have stood in front of this box, layin’ it down.

 

This unit needs an overhaul.  A quick tour of the front panel reveals input level controls for two channels.

 

Phase and tone controls are here.  The tone control can be completely bypassed, which can be useful.

 

These inexpensive meters actually do a pretty good job of displaying dry level and processed (wet) levels for each channel.

 

The ‘warmth’ is another term for the wet signal, routed through a starved-cathode tube circuit.

 

Power input and device info is found on the back.

 

All of the inputs and outputs are on the back.

 

The cabinet screws are a different length.  This is noted.

 

The DC power management and toroidal line transformer are located in the center.

 

All of the rear panel wiring is done with a circuit board, with the wiring harnesses glued to the chassis.

 

We are missing some DC voltages, so our investigation will focus on this circuit board.  What a heat sink!

 

Toroidal transformers are the way to go in high quality audio equipment.

 

Behind the front panel, we see a pair of 12AX7 dual triodes living on this circuit board.  The incandescent light bulbs are burned out, but are for aesthetic purposes only.  We will have some fun with these!

 

The rest of the front panel circuit board is almost entirely encased in sheet metal, as shielding.

 

I documented these cables, in case I needed to disturb them.

 

Can you see the bulged electrolytic capacitor?

 

Hot glue is used to keep all the connectors connected and all the big parts from moving around.

 

With the wiring harnesses removed, this circuit board is held in place by screws that fasten the power semiconductors to the heat sink.  When those screws are removed, the circuit board assembly can be repaired.

 

Remember the bulged capacitor?  It leaked out the bottom.

 

This power supply will get a full cap job.

 

The electrolyte from the leaking capacitor has chewed away some metal from the circuit board and a couple of the solder joints.  This will need to be taken into consideration when the new parts are installed.

 

The blue jumper wire restores continuity on the corroded trace seen in the previous picture.

 

New caps are installed!

 

Here is another view of the new skyline of our power supply board.

 

Heat sink grease is used to insure thermal transfer from the power semiconductors to the aluminum heat sink.  This tube of compound will probably live longer than I will.

 

The tabs of the power semiconductors SHALL BE isolated from the heat sink.  Here, an ohm meter checks for isolation.

 

Those incandescent light bulbs are not part of the signal chain, but just serve as a back light behind the vacuum tubes.

 

These are 24v bulbs.  I could just replace them, but why would I pass up a little fun with just a stock replacement part?

 

The tip of the base of the bulb is actually a specially shaped bead of tin/lead solder, which holds one of the terminals from the filament of the light bulb.  So, I desoldered it.

 

One of the leads from the filament goes to the side of the metal base of the bulb.  So I unsoldered as well.

 

This is a better view of the base, showing the central terminal.

 

The glass bulb comes out.  Besides the broken glass, you can see bits of glue on the rag.

 

The plan is to add LED lights in place of the bulb.  This is the series limiting resistor for 24 volt service.

 

It hides inside the base, as shown.

 

These red LEDs are built on top of the resistor and are supported by the bulb base.  This is gonna be cool.

 

The meters are also internally illuminated.  These meters will get yellow LEDs.

 

The front panel will get a million-mile cleanup and polish.  Off comes the knobs.

 

These bushings keep all the knobs turning smoothly.

 

Now that the front panel is free, it can be cleaned and polished with Gibson Guitar Polish.

 

Removing the knobs is the only way to get the grime out from down around the knob shafts.

 

The clear windows are polished on both sides.

 

Behind the aluminum panel, we have rotary encoders for the wet level control duties.

 

All the low level audio is contained on this double-sided circuit board.  Any copper that isn’t signal is audio ground.

 

The switches are cleaned and lubricated.

 

With the front panel removed, this chassis is just floating.

 

Let’s take a quick peek underneath the audio shield.

 

Audio gain duties are handled by TL074 opamps.

 

Here are the microphone preamp chips.  These are surface-mount JRC4580s.

 

Here is the other mic preamp 4580.  OK, we’ve done our gut shots for the day.

 

Now let’s get these meters done.  Hot glue is everywhere.

 

Once these meters are removed from the front panel, they can be easily serviced.

 

The lens and front bezel come right off.

 

So far, we haven’t destroyed anything.

 

The meter scale just lifts out of the body of the meter.

 

Here is where we are so far, keepin’ it real.

 

If we peer inside the body of the meter, we can see a ‘grain of wheat’ lamp.  These are all burned out.

 

I will attempt to reuse the leads to each of the bulbs.  The burned out bulb is removed.

 

To get a little bit more room to move around, I’ve removed the meter movement.  Be very careful!

 

The meter bulbs are 12 volt units, and two of them are in series because only 24 volts is available.  This is the LED current-limiting resistor for one of the meter illuminating LEDs.

 

I purchased a bunch of these 1% resistors years ago from Texas Instruments (TI) and so these are probably collectors items.  Why not use them on this project?

 

Here is the yellow LED and resistor installed inside the meter body.

 

Now the meter face is reinstalled.

 

The clear lens is popped out of the bezel for cleaning and polishing.

 

Everything is reassembled and the meter movement is zeroed.  Ahhh, that’s done!

 

This shield was loose under the circuit board assembly that held the tube sockets.  What’s up with that?

 

Turns out, these standoffs were unscrewing from the bottom.  More LokTite, please.

 

This is going to take three hands.

 

Unfortunately, I only have two hands.  But The Unbrokenstring Crew can make it happen!

 

Let’s reinstall the front panel and do a final assembly.

 

The meters are gently torqued onto the panel.

 

The knobs and bushings are restored.  This is starting to look nice!

 

The red and yellow LEDs are doing their job.  Unfortunately, the .jpg engine in my phone camera is making the red LEDs behind the vacuum tubes look a little gaudy.  Oh, well.  More importantly, this mic preamp works well and is DEAD SILENT with no input signals.  I think the rebuild went pretty well!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626

Peavey Stereo Chorus 212 Head Stuck In Mono

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The owner of this amp was so anxious to get it fixed, he removed the chassis from this Peavey Stereo Chorus 212 combo and even helped load it!  The amp plays, but only one stereo channel is operational.  Can the Unbrokenstring Crew help?

A quick look of the front panel shows the inputs and the normal (clean) channel controls.

Did you catch the missing knob?  The Unbrokenstring Crew will find a replacement, which is no small thing because these are no longer available new and are only available from spares or salvaged from a non-working unit.

This amp uses a digital signal processing (DSP) module to make all this audio magic.

Interestingly, the DSP module creates two unique audio signals (stereo) from a single mono input.

The ground lift switch is on the front panel, which is a nice touch from many other designs.

This dual OPAMP is non-functional.  However, this is not the root cause of the missing stereo signal.

This is the DSP module in this amplifier.  We have only one audio stream as an output from this device.

A piece of cardboard serves an an insulator so that the bottom of the DSP assembly can be probed.

Here is where the DSP processor outputs both audio as a stream of ONEs and ZEROs.  We are OK here!

This chip processes the two DSP streams in a manner similar to a successive approximation analog-to-digital converter.

The bit streams are split using an analog switch.  Here is the output of the switch assigned to the working channel.

And this is the output of the switch on the non-working channel.  Time to change the switch!

Desoldering today is performed with a traditional iron, rosin flux, and copper braid.  This keeps solder balls to a minimum.

The circuit board is cleared of excess solder and flux.

The new MUX chip is available through the usual sources.

With the tweezers, the new MUX chip is oriented properly on the circuit board pads.  See the tweezers?

After soldering, the workmanship is inspected with this hand-held microscope.  The high-powered white illuminator is on the left, and the black cylinder next to the big IC chips is the optical microscope itself.

Any remaining solder flux is removed with alcohol.  The rag gives the dissolved mess somewhere to go.

Both channels are operating at full power.  The knob is coming from an eBay auction.  When it gets here this head is ready to return to the eager customer!

Thanks for reading all the way to the end!

CONTACT – David Latchaw EE
281-636-8626