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I'm working on a Model123 - 1x12" combo amp from Acoustic Control Corp (circa 1983?). The amp showed up with a non-working pre-amp section but the PA was okay.
The preamp board is a real mystery. It appears that Acoustic re-designed the board but kept the same # of transistors and Op-amps. The JFETs on my board are 2N5457 and 2N5462 and the BJTs are 2N2484 and PN4248 the OPAMPS are LF351. Note that the JFETs and OPAMPs are different part numbers than the schematic calls out. Of course this means the pre-amp schematic is almost useless (Thanks Musicparts).
I have attached a pdf of a crude reference designator for the preamp board. The OPAMPs are clearly being used as amplifiers and NOT comparators as shown in the schematic. I've chased the signal across the board and believe that their might be an issue with the socketed OPAMPs.
Okay got the thing working but I have a very loud hum coming from the pre-amp section only if I DO_NOT isolate the input connector from ground. The original design has the input connector isolated from chassis ground. Also when in the cH1 MODE the CH2 led will glow dimly as well??. What am I missing?
img198.pdf
img200.pdf
img201.pdf
img184.pdf
The preamp board is a real mystery. It appears that Acoustic re-designed the board but kept the same # of transistors and Op-amps. The JFETs on my board are 2N5457 and 2N5462 and the BJTs are 2N2484 and PN4248 the OPAMPS are LF351. Note that the JFETs and OPAMPs are different part numbers than the schematic calls out. Of course this means the pre-amp schematic is almost useless (Thanks Musicparts).
I have attached a pdf of a crude reference designator for the preamp board. The OPAMPs are clearly being used as amplifiers and NOT comparators as shown in the schematic. I've chased the signal across the board and believe that their might be an issue with the socketed OPAMPs.
Okay got the thing working but I have a very loud hum coming from the pre-amp section only if I DO_NOT isolate the input connector from ground. The original design has the input connector isolated from chassis ground. Also when in the cH1 MODE the CH2 led will glow dimly as well??. What am I missing?
img198.pdf
img200.pdf
img201.pdf
img184.pdf