Trucker PA
An affordable, no frills PA/Disco amp from 1978.
The shop sticker on the back panel and the Custom Sound sticker (which for some reason was on the inside) are interesting. 'Salop' became the official county name in 1974 but was changed back to it's original 'Shropshire' in 1980. Just another example of politicians with bugger all else to do but waste taxpayers' money (quick rant there).
Salop Music Centre is still going strong in new premises:
Things to watch out for:
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Corrosion. The set bias control, RV301 on the power board, had disintegrated and read 1K2 end to end instead of 470R.
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Noisy / broken pots
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Missing knobs. The knobs are a loose fit on the shaft and were sometimes glued on. If you take them off for cleaning, use a small spot of glue on the end of the pot shaft or they'll just spin round.
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The edges of the vinyl covering come unstuck and make it difficult to get the chassis out of the sleeve.
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Be careful around the mains transformer. The live connections are easy to touch when you're working on the boards.
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The original screen printing shows the speaker protection fuse as 3A. On this chassis it's been stamped over as 5A.
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1970s electrolytics. One had burst and leaked into the case and the other one was starting to bulge.
The main electrolytics are ITT 4700uF 50volts in a 35mm diameter can. The wiring loom in that area is tight and it can take a while to replace them. The other potential issue is the length of the pop rivets that hold the clamps onto the chassis. A couple of them were digging into the side of the cans.
Features
Front Panel:
CHANNEL 1
Two inputs
Volume, Bass, Treble
CHANNEL 2
Two inputs
Volume, Bass, Treble
CHANNEL 3
Two inputs
Volume, Bass, Treble
CHANNEL 4
Two inputs
Volume, Bass, Treble
MASTER:
Presence, Volume
Illuminated mains power on/off rocker switch
Rear Panel:
IEC mains input
Mains fuse 2 amps
Output Fuse 5 amps
2 x Loudspeaker Outputs. 8-16 ohms total
Send, Return
Booster (slave) O/P
Measured output power:
66watts rms into 8ohms at clipping with a 1kHz sinewave.
IMPORTANT: These drawings and notes are provided for interest only and are not guaranteed to be accurate. Use at your own risk and discretion. The equipment shown contains lethal voltages and should only be worked on by people with suitable training and experience. Schematics and layouts are typical but will vary.
PRINTING
Wix (bless 'em 😖) have changed their image format - again - to avif just to make them awkward to print. Unless you want to buy a converter, the easiest way to make a hard copy is:
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Left click when the magnifying glass icon appears and wait for the picture to clear.
(On a touch screen, tap on the image). This will display a higher res file. -
Right click and 'save image as' to somewhere as an avif file.
(On a touch screen, touch and hold down to view the menu) -
Upload the saved file to your Google Drive.
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Open the file and use the print menu to make a hard copy or save as a pdf.
Custom Sound Trucker PA General Layout
Custom Sound Trucker PA Input Board Schematic
Note: The Motorola MPS3904 transistors on this chassis can be replaced with 2N3904s with no issues.
Custom Sound Trucker PA Input Board Component Layout
Custom Sound Trucker PA Master Board Schematic
Custom Sound Trucker PA Master Board Component Layout
Custom Sound Trucker PA Power Board Schematic
Custom Sound Trucker PA Power Board Component Layout
