Yamaha DX27 won't boot up

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Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Sat Feb 16, 2013 2:15 am

Hi,

I have a DX27 I am trying to repair at the moment. Basically when you power up The screen just fills with mostly black squares. This is as far as it gets.

I have verified that voltages are present around the board so the PSU seems fine. I am about to start looking at the CPU etc.

I was just wondering if anyone could tell me is there an easy way to verify that the CPU is functioning correctly. I will check for the clock signal but am wondering what else I should be looking for. I have a scope.

Cheers,

Luke
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Re: Yamaha DX27 won't boot up

Unread postby Clyde » Sat Feb 16, 2013 4:05 am

Hi Luke,
Welcome to the Forums! Have you checked the voltage of the internal battery? What you are describing sounds very much like the symptoms for a dead or low voltage internal battery. Bad battery can cause all sorts of symptoms.
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Re: Yamaha DX27 won't boot up

Unread postby Miks » Sat Feb 16, 2013 5:35 pm

Yes mate, I'm absolutely with Clyde!
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Re: Yamaha DX27 won't boot up

Unread postby koa » Sat Feb 16, 2013 6:53 pm

I had a DX21 with black squares but the battery was fine. The display would come back to normal when would reload the sounds by pressing Character Function (Compare on the dx27), then 12, then Yes to confirm.
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Sat Feb 16, 2013 10:53 pm

Hey guys,

Many thanks for the speedy replies!

Have checked battery voltage- sitting at 3.3V so I think thats OK. However on closer inspection around the battery area there is some suspect soldering. Will suss this out when I get back home tomorrow and will report back.

Thanks again.

Luke
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Fri Feb 22, 2013 11:17 pm

Hey,

Finally got time to get check out the DX27 again. I put a fresh battery in, cleaned up the suspect soldering around the battery, and checked the tantalum cap which is adjacent to it.

Still black squares though.

I am wondering could this be a CPU fault or something similar? Anyone got any ideas for where to go from here??

Thanks again.

Luke
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Re: Yamaha DX27 won't boot up

Unread postby Miks » Sat Feb 23, 2013 2:06 pm

Did you follow the instructions given by koa:

I had a DX21 with black squares but the battery was fine. The display would come back to normal when would reload the sounds by pressing Character Function (Compare on the dx27), then 12, then Yes to confirm.


This should normally bring your DX back to life...

A weak battery affects the sounds stored to the RAM. So you have to reload them (by using a RAM/ROM card or via MIDI (sysex file). Then store it to the internal memory, enable the internal memory protection... and all should be fine...
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Mon Feb 25, 2013 3:13 pm

Hi,

Yes the method for reloading the factory sounds on the DX27 is simply to put the unit into test mode apparently. So you just have to hold 2 buttons and power the unit up.

However this unit does not get passed the black squares screen, so it is impossible to even get it into test mode. I got this from the service manual, is there something I'm missing as you mention RAM?ROM cards etc?

I really get the feeling this is a bit deeper than that. I'm wondering if there's some simple way to check the ROM/RAM and CPU with a scope?

Thanks again,

Luke
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Re: Yamaha DX27 won't boot up

Unread postby Clyde » Mon Feb 25, 2013 11:39 pm

The DX27 I'm pretty sure was the same for patch storage as the DX21. Internal ram for storing patches (this is what the battery powers to save patches after power off), there were no ram cards, etc. I know the DX21 was able to save patches to cassette tape with a special adapter that Yamaha included when they were sold new (I did save and load some patches via cassette many years ago on the DX21 I had back then).
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Wed Feb 27, 2013 2:14 pm

Yes Clyde this is true... The DX27 does have an internal RAM for storing patches.

So the method of restoring the patches is just to enter the test procedure.

Which leaves me kinda stuck... Anyone else got any ideas as to why a DX27 goes to these black squares and doesn't boot up???

Or any ideas where I should start looking around the CPU and RAM with my scope to see whats not working??

Cheers,

Luke
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Re: Yamaha DX27 won't boot up

Unread postby db7 » Wed Feb 27, 2013 7:56 pm

So, just to confirm, does the method recommended by koa not do anything on the DX27?
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Re: Yamaha DX27 won't boot up

Unread postby koa » Thu Feb 28, 2013 1:31 am

I just noticed the DX27 has button 11 for loading a sound vs 12 on the DX21.
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Fri Mar 01, 2013 11:36 pm

Hi,

Yes to be clear this unit cannot get passed the black squares stage. Once power is applied, black squares appear and no controls change anything.

I wonder if maybe there is an issue with one of the ram/rom chips...

Anyone got any other ideas for how to chase this issue??

Thanks again,

Luke
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Re: Yamaha DX27 won't boot up

Unread postby synthjoe » Sat Mar 02, 2013 1:02 pm

I'd first check pin 8 of IC7, when powering up the synth this pin has to go from 0V to 5V in a very short time (few 100 ms) and then stay high. This same signal should appear on pin 6 of IC1 (the CPU). If that works you'll need to check pins 51-58 of IC1 for activity on the scope (right after swithing on, again). The same goes for pins 47-50. If there's no activity, you might want to check pin 3 of the CPU (3.58 MHz clock) and pin 11 (500 kHz clock). You'll need some experience to check and evaluate these signals. Let us know of your findings.

If the reset signal works and clock signals are there, then the processor should start reading the memory for executable code. This must result in some activity on low order address bits (A0-A3), and depending on memory content the same should happen on data bits 0-7. If memory chips are dead or there's some other problem (eg. break on the data bus or bad control signals), then the processor will very quickly come to a halt. Therefore it is pretty difficult to diagnose, because these initial events happen in only a few microseconds. However, if your scope is triggered, then some activity must be present on the bus (actually, triggering the reset signal might be better than switching on/off the whole synth, as the supply voltage going on/off might render reading the signals difficult).

Hope this gets yo going with the first steps.
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Re: Yamaha DX27 won't boot up

Unread postby chen_hobgob_inc » Tue Mar 05, 2013 12:35 am

Awesome thanks synthjoe. This is just the kind of info I was looking for. My weakness is definitely the digital side of things... but I have a decent knowledge of analog synth electronics.

Will try this stuff out and get back to you in the next couple of days.

Cheers
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