Sansuiman, you’d think that with the unit shut off that the logic circuits or COU would reset such that with a known state (tone arm on its rest and turntable motionless) it would behave appropriately when turned on, assuming that the sensors and motor were not faulty. What gets me is that this unit works, sometimes. But not usually.
Qsiris, thank you very much for your offer. PM incoming.
Cheers,
John
Yes, it should default to a known logic state at reset (power up) and it appears to do just that. The trouble only manifests when you hit the start/stop button to initiate playback of a record. The platter starts rotating at the selected speed, and then the computer LED on the control panel blinks as if it is "thinking", but nothing more happens - the tonearm rest fails to elevate, the tone arm doesn't rotate and it waits several seconds until the unit times out in an error state. At that point, the computer LED stops blinking and the platter stops rotating.
I should note here that the tonearm rest can be operated by applying voltage directly to the solenoid, so mechanically I've verified it's capable of working. It just never gets the drive voltage from the CPU at the correct point in the operating sequence. Likewise, the tonearm can be operated by hand from underneath (with unit housing open) so nothing is evidently seized or broken. Again, it just never gets going under CPU control.
What is unknown to me is whether this is a result of bad sensor(s), bad timing from otherwise working sensors (mechanical mis-adjustment of portions of the mechanism will result in sensors not reporting the expected state when required), belt and clutch slippage, a failure of the CPU itself, or some combination thereof.
As there is no integral diagnostic or test mode for the CPU, one can only verify the state of I/O pins at the CPU are showing the state expected at a given point in the cycle of operation. If one or more inputs is found to be absent, then either a faulty sensor or mis-timing is likely the cause. If all inputs are being supplied per design, and the needed outputs are not found in turn (through the use of scope) then one could surmise the CPU is faulty, as it is failing to follow through on the input data.
In the case of a faulty CPU chip, there is little to be done as the CPU is a proprietary part made for Sansui by NEC. Based on the part number, it is of a series of microcontrollers made by NEC, but programmed - likely with a mask ROM or OTP ROM - to implement the functionality required by the customer; in the case, Sansui.
I've fought with these models for years (Q9 and Q11, I own 6 of them in total) periodically putting them back on the bench hoping to have a break through or moment of clarity that would finally give me the clue I needed to get them working. But, there are always other projects of higher priority, so they get a few hours of attention once every couple of years now maybe, then go back into storage when I get frustrated again, as I can't stand to just junk them.
I really just need to commit to sitting down and plowing through an extensive troubleshooting session, going over each part of the circuit and mechanism until I find the weak link(s). I simply lack the time and motivation to deal with these complex beasts right now. Life gets complicated - more so than it was 15 years ago when I first tried to get these working - and so stuff like this has a way of getting pushed to the bottom of the priority list, never to re-emerge.
I've not found a tech willing to take them on either, despite trying a number of times. Lots of guys who'll do parts swaps, but no one with the troubleshooting chops who will babysit the thing until they find the root cause of the failure. Time is money and it looks like no one wants to get sucked into a hard case like this, when they can turn around normal repair work all week long.
