Vintage Synth Repair

Hi Royce,

i´s been quite a long time, but now I could get back to my project bringing the sy77 back to life. So what did I do? I bought a second one an now, after having swapped boards, I´m able to tell for sure that the malfunction is caused by failure on on the DM1-board. I hope this will bring myself closer to the solution of the problem.

> I'm not sure what your level of electronics is so forgive me if I suggest the obvious.

Well I´m tying hard to gain knoledge. ;)

> If you do have an oscilloscope look at the central CPU (IC101) pins that cause freezing, pin 7 WAIT, 8 & 9 INT (not sure if pin9 is connected but it should be pulled high) and see if any of them are held low.

I do have an oscillocope but never used it for checking ic-pins so i´d appreciate further instructions on how to do this.

best regards

moe


--- In vintagesynthrepair@yahoogroups.com, "Royce" <rpcfender@...> wrote:
>
> Hi moe
> 
> > another detail i may not have mentioned yet is that when powering up the sequencer leds flicker for a shorter or longer period of time. And only during this time slot it is possible to execute an "init ram$" which is part of the hidden functions. Could this be a further hint leading to the cause of the malfunction?
> 
> The SY77 has 3 CPUs. A central one for the display, the wheels and rotary encoder and co-coordinating the two others as well as the M3 chips on the DM2 board.
> The second is the switch, led and keyboard one, mentioned before.
> The third is the sequencer CPU which controls the flashing Run LEDs as well as the Record LED and well as doing the record to memory and the playback. It is like a separate  sequencer, but needs to chat to the main CPU.
> 
> I suspect the led flicker is just part of the boot routine for the sequencer CPU (IC145). 
> 
> Because neither of the other two CPUs is connected to an INT line of the central CPU they must be polled.
> So if the switch CPU freezes you couldn't play the keyboard and the switches wouldn't work, but the Midi in should still function.
> Is this is what is happening?
> 
> I did miss an input that might cause freezing on the CPU. There is a WAIT signal line which I am assuming is for the timing for access to the memory and perhaps the other CPUs. So only the M3 chips are time critical and are therefore connected to the INT lines of the CPU.
> 
> I'm not sure what your level of electronics is so forgive me if I suggest the obvious.
> I would start with the following.
> 
> Measure the power supply and the each board's voltages with a digital multimeter (DMM). Note that there is and analog and a separate digital ground. Measure using the correct ground. Check the power pins on EACH of the boards.
> 
> If you don't have an oscilloscope you should check continuity of the connecting cables between the boards with your DMM. I would start at the switch board connectors. Remember to have the power off (perhaps unplugged, as I zapped myself fixing mine). The mode switches you are having trouble with are connected to the main switch board and then to the DM1 board so you have to check two sets of cables.
> 
> If you do have an oscilloscope look at the central CPU (IC101) pins that cause freezing, pin 7 WAIT, 8 & 9 INT (not sure if pin9 is connected but it should be pulled high) and see if any of them are held low.
> 
> Like the keyboard, the switches are read in blocks. In a group one side of the switches are connected together and the CPU polls them as a block and the other sides are read as data lines. Check the switch board to make sure the switch bank lines are being scanned. If they are the Switch CPU is doing something and it is likely you need to look elsewhere. 
> 
> Royce
>