After staring at the schematic for a couple of minutes it dawned on me that perhaps it was just sampling garbage on the inputs.? I started to look for the ADC but couldn't find any. So finally the DRAM memory I ordered came in this week and I have tried it today. So perhaps something is drawing more than it should? Thanx for reading. I also noticed after having the unit on for a while (like an hour or so), that the little transformer gets warm and the voltage regulators (+5/+12/-12) also get warm. They get the warmest -not hot exactly, you can still touch them, but warmer than I would like. If I had to guess I would put my money on the DRAMs and replace them all. It is connected to the DSP ENEX pin 81 (schematic p1) though. However, I can find no way how the CPU manipulates this signal. One thing I don't understand is the XENEX signal that seems to give the CPU access to the SRAM. Reinserting the latches brought back the problem (of course). However it may simply be that the SRAM data is zero'ed out and that this will simply not generate a tone. When I lift these latches, the tone is gone. Some presets seems to continuously activate these latches, probably to write expression pedal data (Midi) into the DSP memory (not investigated). When switching presets these latches are clocked.
The other SRAM address lines are accessed through latch U17 (Schematic p2) and the 16 bit data lines with latches U16 and U30 (Schematic p2), all controlled by the CPU. These lines are used as sort of a memory page selector by the CPU. 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.
0 kommentar(er)
