Hi everyone, I'm new to this site and I'm posting for the first time here, to ask about DX7 repair.
Issue:
I got a DX7 (Mark I) which seems to be working but no audio, even in test 1 of Test Mode. All other self test pass and MIDI is working, but I got nothing on the EGS output to OPS (EC bus, F bus). The OPS is outputting SYNC pulses, no output to DAC though.
Action done:
The supply voltages, reset and clocks (9.4265 tree, 500k tree and SUB-CPU 4M) are fine. The board have some logic ICs not presenting correct output, which I replaced. I assume there has been an overvoltage event caused by the +5V voltage adjustment pot on PSU board, which I deleted. I then went ahead to replace all logic ICs, socketed the 8255, CPU, RAM, DAC, ADC. I replaced safety cap on mains input and used 3-wire mains cable with ground added since the unit has electrocuted me more than once. I'm now running on replacement CPU, 8255 and ADC. No luck.
Questions and thought:
At this point I think the EGS is bad. Unlike OPS who have a sync output, the EGS have no output other than those buses, which means I'll need a logic analyzer with many channels to confirm and I don't have one. Maybe some of you will have a better idea? I'll also consider buying an EGS chip if you have one to offer. If I can't buy one and I'm convinced it's a bad EGS I'll just give up.
Another thing I'm not sure about is the voltage levels on buses. The data bus have voltages all over the place with high level ranging from 3V to 5V. I know it's probably okay for TTL but is this also the case with your DX7 boards?
The timing for the data input to EGS is also a little bit off, the falling edge of EGS's CS (#Y6) is right at the transition of data bus, maybe it's because the logic chips are too modern.
Next plan:
Desolder and dump the SUB-CPU HD6805S1 internal ROM and post is somewhere. AFAIK there is no such dump available online. Then it's possible to replace the 6805 with MC68705 if someone ever runs into problem with their SUB-CPU.
Thank you!![:)]( "Smile")
Ryan
Issue:
I got a DX7 (Mark I) which seems to be working but no audio, even in test 1 of Test Mode. All other self test pass and MIDI is working, but I got nothing on the EGS output to OPS (EC bus, F bus). The OPS is outputting SYNC pulses, no output to DAC though.
Action done:
The supply voltages, reset and clocks (9.4265 tree, 500k tree and SUB-CPU 4M) are fine. The board have some logic ICs not presenting correct output, which I replaced. I assume there has been an overvoltage event caused by the +5V voltage adjustment pot on PSU board, which I deleted. I then went ahead to replace all logic ICs, socketed the 8255, CPU, RAM, DAC, ADC. I replaced safety cap on mains input and used 3-wire mains cable with ground added since the unit has electrocuted me more than once. I'm now running on replacement CPU, 8255 and ADC. No luck.
Questions and thought:
At this point I think the EGS is bad. Unlike OPS who have a sync output, the EGS have no output other than those buses, which means I'll need a logic analyzer with many channels to confirm and I don't have one. Maybe some of you will have a better idea? I'll also consider buying an EGS chip if you have one to offer. If I can't buy one and I'm convinced it's a bad EGS I'll just give up.
Another thing I'm not sure about is the voltage levels on buses. The data bus have voltages all over the place with high level ranging from 3V to 5V. I know it's probably okay for TTL but is this also the case with your DX7 boards?
The timing for the data input to EGS is also a little bit off, the falling edge of EGS's CS (#Y6) is right at the transition of data bus, maybe it's because the logic chips are too modern.
Next plan:
Desolder and dump the SUB-CPU HD6805S1 internal ROM and post is somewhere. AFAIK there is no such dump available online. Then it's possible to replace the 6805 with MC68705 if someone ever runs into problem with their SUB-CPU.
Thank you!
Ryan
Statistics: Posted by ryan_zheng — Fri Dec 06, 2024 11:39 am