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
