Repairing equipment is a lot like solving a crime. Except it is a lot messy! My dishwasher was giving problems recently and after a lengthy faultfinding, I realised that the hose connecting the pressure sensor was leaking because of a bad hose-clamp. No sensors... not the relay on the control board which I replaced... nothing sinister in the software... a plain old cheap hose-clamp! Great repair video, thanks Gary! Let's hope no.3 will be the last one for this instrument! Good luck!
You might want to look into those MOSTEK ROMs on the main board. These are known to fail over time getting corrupt bits. If any ROM has incorrect contents the processor will most likely crash and go into an endless loop. This unit appears to have a 6800 family microprocessor and these typically go into an endless loop cycling through the whole address space reading each memory location sequentially. Probing the address lines with a scope can check this with each address line having a square wave on it halving in frequency for each successive address line from A0 through to A15.
@@stephenbell9257 It looks like it is doing exactly as you describe. The address lines are all toggling like they should, but the all 8 data lines have some invalid logic levels (+2 V) at times. I doubt that the 6800 is bad; and I’m hoping it’s not the ROMs. Could be anything on that data bus causing this problem. I don’t have a logic analyzer, or a signature analyzer either (they never did work the way they were intended anyway.) But, right now, I have bigger issues: recovering from Hurricane Beryl, which came right through my area. It will take me a little while before I can get back to work on this unit.
@@garyramsey4275 Some invalid logic levels are normal during times when the data bus isn't being driven by a device. You can better check the logic levels by triggering your scope off the chip select line of a device and verifying that the logic levels are OK towards the end of the chip select pulse. Repeat this for each memory and peripheral device on the microprocessor bus to verify that all devices are driving correct logic levels onto the data lines. It is highly likely that you have a bad ROM as these MOSTEK ROMs are notorious for failing like this - it is so common that the term "ROM rot" has been coined to describe it. There is a procedure for checking the ROMs on power up. K6JCA has a detailed repair log for this exact problem so it would be worth checking this log. It refers to replacement ROM images being available on the KO4BB web site but this appears to be non-functional at the moment. However, there may be other sources out there for getting these ROM images since this seems to be a relatively common problem. I hope you have success in bringing this instrument back to life!
Repairing equipment is a lot like solving a crime. Except it is a lot messy!
My dishwasher was giving problems recently and after a lengthy faultfinding, I realised that the hose connecting the pressure sensor was leaking because of a bad hose-clamp. No sensors... not the relay on the control board which I replaced... nothing sinister in the software... a plain old cheap hose-clamp!
Great repair video, thanks Gary! Let's hope no.3 will be the last one for this instrument! Good luck!
At 5:20, you should be saying to yourself, “That’s when he realized, he F’ed up.”
You might want to look into those MOSTEK ROMs on the main board. These are known to fail over time getting corrupt bits. If any ROM has incorrect contents the processor will most likely crash and go into an endless loop.
This unit appears to have a 6800 family microprocessor and these typically go into an endless loop cycling through the whole address space reading each memory location sequentially. Probing the address lines with a scope can check this with each address line having a square wave on it halving in frequency for each successive address line from A0 through to A15.
@@stephenbell9257 It looks like it is doing exactly as you describe. The address lines are all toggling like they should, but the all 8 data lines have some invalid logic levels (+2 V) at times. I doubt that the 6800 is bad; and I’m hoping it’s not the ROMs. Could be anything on that data bus causing this problem. I don’t have a logic analyzer, or a signature analyzer either (they never did work the way they were intended anyway.) But, right now, I have bigger issues: recovering from Hurricane Beryl, which came right through my area. It will take me a little while before I can get back to work on this unit.
@@garyramsey4275 Some invalid logic levels are normal during times when the data bus isn't being driven by a device. You can better check the logic levels by triggering your scope off the chip select line of a device and verifying that the logic levels are OK towards the end of the chip select pulse. Repeat this for each memory and peripheral device on the microprocessor bus to verify that all devices are driving correct logic levels onto the data lines.
It is highly likely that you have a bad ROM as these MOSTEK ROMs are notorious for failing like this - it is so common that the term "ROM rot" has been coined to describe it. There is a procedure for checking the ROMs on power up. K6JCA has a detailed repair log for this exact problem so it would be worth checking this log. It refers to replacement ROM images being available on the KO4BB web site but this appears to be non-functional at the moment. However, there may be other sources out there for getting these ROM images since this seems to be a relatively common problem.
I hope you have success in bringing this instrument back to life!