U13 is driven by latch U18 (schematic p2) called PD (Port D). Here are some notes for the schematic (I always do this to help me understand the schematic): Analog circuit: LF353N Dual Opamps LF311N Voltage comparitor LF357N Dual Opamps Digital circuit: 74HC157 Quad 2-input multiplexers LH2464-12 (DRAM) (4464 in schematic) VGT7764-6024 (DSP) LH5116-12 (RAM) HD63A03RP (CPU) HY6264LP-12 (Static RAM) PCM54HP (DAC) - warms up - replaced it.Ī8, A9 and A10 are not wired to the DSP!, but come from U13 (Schematic p1). So anyone willing to take a look at the schematics and help figuring out what could be wrong. Control of the unit, recalling presets and responding to buttons, driving display etc. I thought this might be oscillation in the analog circuit but to be honest, I have no idea how that works. After one second or so, the scope image becomes a jumble of lines. When switching the unit on, after the CPU/DSP has booted up, there is a clear tone 'visible' on the output. When I insert even one, the problem comes back. I have tried to switch around the DRAM the DSP uses but only when I remove them all, is the tone gone. The problem is currently that it generates a tone without any input. I have replaced the DAC (U39) with a new one and that fixed a distorted sound (when none was desired -) that was the initial problem.
The GSP-5 is quite a complex beast and I cannot say that I have all parts of the schematic (attached) figured out. Although I am not willing to spend a whole lot of money, I wouldn't mind taking it further than you would usually. Note that this repair is more for my experience/learning than it is to make it profitable. I have been sitting on this for some time now and I decided it was time to either fix it, or get rid of it.
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