Crate CR2R Combo Refurbish

The oak cabinet houses a simple solid state amplifier and a Celestion G12 loudspeaker.  The rustic look is intriguing; the wife agreed that this matched the decor and would therefore be allowed into the house.  This particular amp was languishing in a pawn shop until Dave called me to bring the truck and refurb this guy at the Unbrokenstring Shop!

The grille, metal handle, and metal corners had plenty of surface rust.  The wood hadn’t been conditioned in, like, forever.  And, it didn’t play.  When shaken, loose parts were heard rattling around.  So we have a starting point for electrical test…

A scan of the front panel shows typical controls found on 1970s era units.  The LO input has a 10dB pad in series with the input jack.  The BRIGHT switch is a throw-back to earlier times.

This unit has a three-band EQ and a reverb tank.  As we will see later, the electronics were built by St. Louis Music.  Crate had licensed Ampeg’s intellectual property and were rebranding it into their own units.

Here’s a quick peek at the rear panel.

A pair of TO3 transistors handles the push-pull duties.  Nice to see insulators over the cases of the transistors, which may be at 40 volts or more with respect to ground when operating.

A foot switch can be plugged in here to enable or disable the reverb.  Don’t know if the LINE IN and LINE OUT signals are really ‘line level’ e.g. 2v or not, but back in the 1970s, who cared?

A utilitarian Celestion loudspeaker handles the electricity to sound transducer duties. This particular loudspeaker is in good shape considering that it is probably original for that time period.

I took this pic to preserve the location of the reverb tank wiring and the cable to the loudspeaker.  They are all RCA males.

The output from the reverb tank was manually marked with a Magic Marker. Interestingly, the Magic Marker ink has dispersed into the plastic jacket of the cable but is still visible.

I unscrewed the chassis and removed it out the back of the amp.  Yeah, we’re a little dirty and dusty.

Now we have a little more room to remove stuff to clean up the cabinet.  Two screws hold the reverb tank in place vertically along the side of one wall.

The grille is held in with some of these screws.  Everything is coming off in order to clean this cabinet and oil it.

The grille frame is pine, painted black.

Here we find Vincent Price’s “funk of forty thousand years.” TIP: If you don’t get the association, Google Michael Jackson “Thriller” lyrics.

This portion of today’s program is brought to you by Pledge furniture polish.  It contains enough petroleum solvents to clean the wood and condition the exposed wood in the scratches.

The bottom of the cabinet hasn’t been this clean since 1978.

Now, to start on the electronics. I took this pic to document the orientation of the BRIGHT slide switch.

Rather than unsolder these wires, I’ll remove the front panel jacks to liberate this side of the circuit board.

I’m going to disconnect this internal wiring to free the circuit boards.  This pic is for documentation purposes.

All the knobs pull off, no set screws.  These knobs have an interesting tri-lobed scheme to hold to the knurled shafts.

Removing the nuts will free this side of the circuit board.  This socket is covered in felt to keep from scratching the paint.

Remember this switch? The BRIGHT switch needs to be detached from the front panel.

Aha! Here’s one of the loose parts rattling around inside the cabinet. Only the component leads secured this part to the circuit board. Over the years, the leads were repeatedly bent by vibration, work-hardened, and eventually broke. No audio passes through a broken lead!  There are two of these guys running loose inside the amp.

The preamp board is loose from the chassis.  These controls will be cleaned and lubricated before reassembly.

Some service has been performed in the past.  Why can’t these people remove the old rosin solder flux?

More solder rework is apparent here.

The white stuff is RTV adhesive, commonly used to secure components in place so they don’t vibrate loose.  We have a problem here.  Can you see it?

More tech porn.  The topology of this preamp is pretty standard, with op amps performing the heavy sonic lifting.

Everything here looks pretty good.  Let’s fix what we’ve found so far.

This cap came unsoldered.  It checks OK otherwise so this will be reinstalled right where it is.

Remember those green parts that were found rattling around inside the chassis?  My fingers are pointing where they go.

Radio Shack had these parts in stock.  They are installed here.  This board is the power amp assembly.

The first amplifier IC right at the input is bad. It goes here, to the left of center of this picture.

My reassembly helper has decided to perform a Quality Audit.

The controls were cleaned with Blue Shower, then lubricated with Rid-Ox, sprayed directly into the control. The control shafts were twisted from one extreme to the other several times until the Rid-Ox evaporated.

The grille is firmly reattached to the cabinet. Loose grilles are often the source of mysterious buzzes and rattles.

This yellow gripper is handy as a screw starter.  The power amp board is being installed here.

And our old friend, the BRIGHT switch, is reattached here.

The reverb tank is refastened to the side wall here. The Pledge furniture polish does a good job of removing crud from the tank sleeve.

More reverb tank action.  Exciting, huh…

Those cables are finally put back where they belong.

This guy doesn’t look too bad now. This is just a straight-ahead practice amp, but the cool factor is bumped up because it’s a Real Crate. And it plays well!

Thanks for reading all the way to the bottom!

Contact – David Latchaw EE
cell – 281-636-8626

DigiTech TSR-24 Repair

This stereo rack mount effects processor had a couple of problems. When turned on, the display would flash. And the rotary encoder is busted.  And we really need to get this fixed this week.  And many parts are obsolete.  Can it be fixed in time?  The Unbrokenstring Crew to the rescue!

These units are, in my opinion, highly-under-rated. When they work, they sport an innumerable list of effects. When they don’t work, they are still prized for spare parts.

This unit needed a new rotary encoder.  If there is one mechanical design problem, you are looking at it.  This big knob sticks out farther than anything else on the front panel, and therefore takes a beating if the unit is abused.  And, of course, this exact part is not available anymore.  Let’s open up the unit and go to work!

A firmware upgrade kit is still available from an eBay vendor, that adds delay time and new effect algorithms.  The new parts go here.  This unit will not be upgraded at this time, but if you are considering the upgrade, it’s not difficult at all.

More tech porn.  This is a custom processor flanked with other members of the DSP chipset.

We need to remove the circuit board entirely from the chassis in order to replace the failing voltage regulator.  These nuts and washers all come off.  Fortunately, they are finger-tight.

Interestingly, the screws holding the front panel in place are Allen head cap screws.  They look very cool.

More Allen head cap screws secure the external heat sink.  The voltage regulators are behind this heat sink.

Just documenting where all the cables go.

The configuration of the wiring harness determines if the particular unit is 115vac or 230vac.  No switches or jumpers!

This pin was loose in the circuit board.  We’ll repair this solder joint when we get the board out.

OK, I think I got it out.

The voltage regulators are inside this sandwich of aluminum channel.  The big heat sink seen in a previous picture bolts behind the aluminum angle bracket, seen at the top of this photo.

With the top piece of aluminum channel gone, the voltage regulators are easily accessible.  DigiTech used a grey silicone pad underneath the regulators instead of messy grease.

The new regulator is soldered in from this side.  Can you tell that the three joints to the right are ‘shinier’ than the others?

The aluminum sandwich goes back together.

Now that the mechanical sandwich is secure, I am reflowing the solder joints to remove all mechanical stresses.

The loose pin seen earlier was not the only issue.  Two pins fell out of their holes!

The solder joints beneath pins at Q11 and Q12 were re-flowed with rosin solder.  The flux seen here will clean up with isopropyl alcohol.

This sub-assembly, called the “Pot Board,” carries the rotary encoder, the control with the white shaft seen in this pic.

That’s the problem!  Of course, this part is no longer available.

Here is a better look at the damage.  When the knob is banged inward, the phenolic circuit board used to implement the rotary encoder takes most of the stress.

Let’s clean up this mess.  No sense in leaving the broken pieces in the unit!

I prefer braid and rosin to clear out plated thru holes in circuit boards.

The plan will be to transplant the white nylon shaft and bushing into the new encoder body.

The new rotary encoder was dis-assembled.  The old shaft assembly is on the left, and will mate directly with the circuit board.  Note that this rotary encoder has a different land pattern than the original.  This circuit board will make fewer pulses per revolution.  In my opinion, slowing down the action of the knob is a Good Thing on this unit.

Here’s the completed “V’Ger” rotary switch.  The tabs on the back hold everything together, assimilating the two encoders; a tip of the hat to the Star Trek movie when the Voyager space probe ‘merged’ with an alien probe, then returned to Earth to ask the question, “Who’s my daddy?”

Here is a good view showing how everything lines up.

The new encoder board has a slightly wider pin spacing, but nothing that will keep us from finishing this project.

Here is the pot board assembly with the new encoder assembly installed.

That pot board assembly shoe-horns behind the front panel.  All the electrical connections are made thru the ribbon cable.  The cable needs to be mated first before the pot board is installed, because it won’t mate after the board is installed.  Don’t ask me how I know that…

Now everything fits.  This is a good view of the little bit of gun-smithing necessary to get the new encoder board onto the site on the pot board where the old one was located.  We’re on the home stretch!

Re-using the original shaft and bushing allows us to stay with the original knob and hardware.

The instructions to perform a factory reset are freely available online and in the manual.

All done!  No flashing display, and the rotary encoder knob steps through all 255 different banks of effects at a sane rate.  This unit is ready for another decade of duties around the studio.

Thanks for reading all the way to the end!

CONTACT – David Latchaw   EE

cell – 281-636-8626