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Old 25th February 2013, 06:00 AM   #1
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Default Synthesizer power distribution

I was able to repair the power supply on my synth. The 5VDC and the +/-15vdc are both good. Power seems to be where it should be on the Oscillator, filters etc. The 5VDC for the logic board is present but the machine is still dead. Many of the ICs have no available datasheets. Poking around blindly, there seems to be 5VDC in alot of places. Sanyo LC3517AL-15 Cmos is one example of and IC with 5 VDC on most of the pins I am not certain if this is normal. There is one diode on the board that is illuminated. I have never seen an illuminated diode before now (not an led)
I have tested for the obvious, simple defects and have found none. There is a transistor A564 that has 5 volts on the emitter, 5 volts on the collector and 4.2 volts on the base. This does not seem nornal to me. Any thoughts on this.
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Old 25th February 2013, 06:47 AM   #2
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You seem to be continuing from another thread I'm not familiar with, but I can make a couple of (hopefully) helpful suggestions.
Provide numbers for some of those "no available datasheets" ICs. It will help determine what you're working with. They may be obscure obsolete parts, or they may be "in-house number" chips.
5V in alot of places is a good sign actually; the system seems to be powered normally. The 80C31 MCU and 74xx chips are 5V-compatible.
The illuminated diode is, technically, a LED. Is it surface-mount, or just an uncommon package?
Those transistor is a PNP and likely operating correctly.
Is the clock oscillating?
What datasheets can't you find? The MCU and logic should be easy to find. The programmable memory should be easy, too, but if they've gone bad the chance of replacement success is likely slim to none.
If you hadn't noticed, I'm poking around blindly also.
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Old 25th February 2013, 07:15 AM   #3
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I just downloaded a 10 page pdf. Everything is there but I am overwhelmed by it. The troubleshooting page says "No lights, replace CPU-09 which is the entire logic board. I feel like diagnosing the board id beyond me and I am tempted to pull the SSM2044 filters and build a vcf. I know it is a copout but this unit is discouraging.
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Old 25th February 2013, 07:51 AM   #4
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All 16 pins on the lefthave voltage. Does this make sense. When the switches close, 5 volts would meet 5 volts. How would that work or is this related to my problem?
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Old 25th February 2013, 07:56 AM   #5
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That could well be a multiplexed "keyscan". You need to scope the data lines to see. A DVM may just show 5 volts when it is in fact a pulse.
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Old 25th February 2013, 08:09 AM   #6
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"On the left" means pins 2-40? That doesn't seem right at all.
Did you check the clock? That's the first thing to check for a dead system (well, first after the power supply).
I'm afraid I can't be of much more help.
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Old 25th February 2013, 08:52 AM   #7
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It may not be as dead as I thought. From that era with not much going on. I have been fighting for a few hours and found no positive voltage on any l.e.d. s. The transistor is 2SA874R. I can find no information on the transistor. There is 12Volts on either side of the transistor but nothing coming out. I do not quite understand PNP and BCE but hopefully I will soon! Does this diagram specify PNP or NPN?
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Old 25th February 2013, 09:17 AM   #8
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I think I understand that 5 Volts goes to the emitter, the base is supplied by HC139 and I should have 5 Volts at the collectors supplying the Leds. How do I go about selecting a transistor? Pin position, voltage, current?
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Old 25th February 2013, 12:59 PM   #9
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2SA874 will be PNP. That means the emitter (99% will be one of the end pins) will be at some supply voltage (you say 5 volts). The base needs to be pulled "low" to turn it on. Voltage wise E and B may appear similar but the base should be around 0.7 volts lower than the emitter when its turned on. That should put 5 volts on the collector.
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Old 25th February 2013, 04:19 PM   #10
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I became excited when I found no power coming from the LED driver. I prematurely pulled all of the PNP transistors. Without the transistor installed, I have 5 volts at the Emitter and at the and at the base. Base voltage is supplied by HC139. How can I check the clock?
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