Greenious wrote:Actually the ACIA is MC6850, the particular model used is 68B50, not 68850, which is why you can't find the datasheet.
Fujiyama wrote:I've opened up the PSU and am in the midst of measuring the cap sizes and will check those voltages when done.
May I ask if the MOSFET transistor listed there was also included, or just the caps?
Did you notice any difference before/after?
siriushardware wrote:I've looked at the wider area around the sound section and I see that it's not only the LMC1992 which runs on the 8.2V supply.
All four elements of IC 605, LF357 (Two immediately to the right of the LMC1992, and one to the right of each of the two DACs on the circuit diagram) run from a +8.2V supply, so we can guess where that must come from. (The little 78L82).
You should check to see if there is 8.2V not only on LMC1992 pin 28 where you already know there is, but also on on pin 4 of IC605.
While you are looking in that area, check that the positive supply to IC607 (also LF357) pin 4 is +5V.
Finally check that the minus supply for IC605 and IC607, on pin 11 of both of those ICs, is minus 5 volts. The diagram set I found online does not include the power supply but the pin markings on the power connector J201 (sheet 2 of 11, centre) show that the minus 5 volt supply comes from the main power supply itself, unlike the +8.2V supply which is derived locally using that 8.2V regulator.
With respect to the 7406, you seem pretty unlucky to have three plain 7406 devices which all appear to be faulty in different ways. According to my databook the main difference between the 'original' and LS versions of the 7406 is the current consumption, about 32mA for the original device and 6mA for the LS version. The propagation delay for both types is given as 12.5uS, so there's no apparent speed difference. Both are inverting buffers with open-collector outputs rated at 30V maximum. It would do no harm to try a 74LS06 if you have one.
As to exactly what the 7406 (IC301) is involved in, as well as the four elements used in the MIDI out / Midi thru buffers I see one element of the same 7406 (IC301) in the output from the 'power-on-reset' circuit (upper left area, sheet 2 of 11) which then goes to U801, the TTSCU? IC. (Sheet 3 of 11).
If one or more of your suspect 7406 ICs have the 'A' element (pins 1 and 2) damaged that might explain why the MSTe will not even boot. I can't find the missing sixth ('B') element of IC301 - you might have better luck if you try. It may not be used.
I notice there is another 7406 in the machine (IC 406) which is mainly concerned with buffering signals between the FDD controller IC and the FDD, where five of its elements are used for that purpose. (Sheet 4 of 11). If you put that IC in a socket as well it will give you the option to swap IC301 and IC406. However, I haven't yet found the sixth element of IC406 either, so I'm not sure if that IC is involved in anything else.
With reference to IC303 (Midi UART) and IC304 (Keyboard UART) these are apparently (EDIT) identical 68B50 ICs, so if they are socketed or if you are prepared to put them in sockets you could try swapping those over to see what happens.
Fujiyama wrote: Checking the PSU could also be useful when troubleshooting. Using the Mega STe PSU (Phihong PSM-5341) schematics I've been able to determine what the wire colours of its connector correspond to. With the negative probe of my digital multimeter attached to one of the four black wires (GND) I've used the positive probe to measure each of those other pins in turn as follows:
+5V line (three red wires): +5.025V
-5V line (white wire): -5.036V
-12V line (blue wire): -12.572V
+12V line (yellow wire): +12.120V
There's also an additional (orange) wire which the PSU schematic has labelled "Power good". I have no idea what this is for but it shows +4.892V
IMHO the PSU output appears to be within specs but I suppose there's more to it than that, like spiky/noisy power, right? Is this something I can check simply by attaching my oscilloscope to GND and one of the PSU power lines? With limited knowledge of using a scope and the knowledge that you should keep away from switched PSU units unless you know exactly what you're doing I've refrained from a "trial and error" type approach here, not wanting to risk blowing up the scope.
siriushardware wrote:Been thinking further about this: Assuming you don't have an oscilloscope or a logic probe (stop me if you do have either of those things), there is a way you can test the PC900 in-circuit and the MSTe does not need to be fully working, as long as the +5V supply is OK, and assuming that at least some of the 7406 (U301) is OK. You can make sure of that by replacing U301.
This test takes advantage of the fact that the MSTe has hardware MIDI THRU. Any MIDI data received on the MSTe's MIDI input through the PC900 is retransmitted through U301F and U301E to pins 3 and 1 on the MSTe's MIDI OUT port, from where (with a suitably rewired cable) it can be sent back to the synth / keyboard.
siriushardware wrote:Sorry about all the mistakes. My copy of the diagram is seriously difficult to read so '8', 'B', and '3' are all easily confused.
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