I've been working on trying to get this Organ going for some time. Its an Eminent 7500 and I managed to get most schematics.
I've been rebuilding the oscillator board but I'm having issues diagnosing why one octave is dead. The organ uses a circuit on a insertable card - like a PC ISA slot. Each card does one octave or other function.
They all use TMS3617's and all the other octaves are working fine.
I'm not sure where the problem is on the board - I've repalced two chips that allow you to play the pedal section from the keyboard but still nothing. I can see that signals are reaching the chip from the keyboard with my DMM but no sound.
My knowledge of this chip is minute so I'm struggling to see where the issue is.
I've been rebuilding the oscillator board but I'm having issues diagnosing why one octave is dead. The organ uses a circuit on a insertable card - like a PC ISA slot. Each card does one octave or other function.
They all use TMS3617's and all the other octaves are working fine.
I'm not sure where the problem is on the board - I've repalced two chips that allow you to play the pedal section from the keyboard but still nothing. I can see that signals are reaching the chip from the keyboard with my DMM but no sound.
My knowledge of this chip is minute so I'm struggling to see where the issue is.
That circuit is similar to the Roland keyboard. There is a master oscillator and the TMS chip gives the required notes. If the TMS has an input and doesn't produce an output, it could be that the bias voltage is wrong on the control pin, pin 4. See the Roland VK-09 for a better idea on how it works.
Thanks for the information - I'm looking for some information on the VK-09 now.
Meanwhile I measured some voltages. The Bias on pin 4 measures at 15VDC
I measured the voltage coming in to a few keyer pins and the voltage coming in ranges from around 13.5 to 14VDC seems to vary a little from keystroke to keystroke.
I had my scope connected to the 8' output pin just to see if anything would display. I got no reading from the output in AC but there was 5VDC on the output.
Does this hint to anything?
Without more circuitry and information...
It looks like a digital circuit to me, so voltages are not so important. As long as the voltage on a line is above a certain threshold, the line is "high" and below a lower threshold, the line is "low". So if you have a 15v circuit, then a line can be 12 or 14v and either will still be a high. Digital is on and off, no shades of gray.
Think of a toggle switch, you can push it up to a certain point where it snaps on. You can push it just that far or push it farther, but either way it snaps on at the one point.
When you close keys, do you see any outputs on the right side of th IC? The lines that feed into the transistors? Do you have a scope? It can be difficult if nt impossible to see little digital spikes on a meter.
It looks like a digital circuit to me, so voltages are not so important. As long as the voltage on a line is above a certain threshold, the line is "high" and below a lower threshold, the line is "low". So if you have a 15v circuit, then a line can be 12 or 14v and either will still be a high. Digital is on and off, no shades of gray.
Think of a toggle switch, you can push it up to a certain point where it snaps on. You can push it just that far or push it farther, but either way it snaps on at the one point.
When you close keys, do you see any outputs on the right side of th IC? The lines that feed into the transistors? Do you have a scope? It can be difficult if nt impossible to see little digital spikes on a meter.
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