uPD6376 DAC- Roland E-70 Synth -

[michel74]

Hello here,

I have a Roland E-70 which doesn't work anymore ....
So first, (thanks to frisc0), the service manual can be found here  (https://www.synthxl.com/roland-e-70/) )
When switched on, everything lit on. LCD is fine all buttons ok etc. etc. (I did the test from the Service manual and everything is ok except the MIDI test because i have no cable to loop back In / Out)
So, PSU is ok.
The amplifier is Ok: when I remove the output from the DAC and connect a microphone with an pre-amp, it works fine.

So, obviously, I suspected the DAC area (GSE board) and its uPD6376. As far as I could check, the uPD is operating with L-ch and R-ch input alternatively. At least, I could check with a scope on pin 13 that I get two set of data bits.
Pin 13 gets a clock, ok
But from here, I have now idea on what to check.
I was hoping to see "something changing" while hitting some keys, on pin 13, but ... nope. Nothing change.
So, either it's because it's not the uPD which is faulty (and then something before), either I did not understand (this is much more "probable" :-) something in this DAC
Any advice would be more than welcome.

Thanks a lot to the community

So

[Audiorepair]

Test Mode in the service manual includes tests to output tones through the speakers via the DAC.

Do these tests work? 
You said everything works except you didn't test MIDI.

[michel74]

Ok, my bad. I wasn't clear enough :"all the tests in 'Test mode' section were ok, except the MIDI as I do not have any MIDI loop-back cable"
For me, these tests only show show that most of the I/O are ok, and PSU has no issue.

In the "Adjustment" section, the audio test didn't work (neither for High-midle range nor for the mono-woofer).

Today I did connect the scope to the MIDI Tx (and even Rx in case I missed something), but nothing was shown on the screen.

I also tested the MUTE Pin (we never know ...), it is high, so not mutes. In any case, 1-2 sec after switch on, I can hear the relay, so the amp is definitely not muted.

Again, the amplifier itself is ok. When I put a source to CN2 (R/L input) the amp works fine

[Audiorepair]

Ok, so if there is no audio in "adjustment mode" then likely there is no internal audio, and the DAC is a good place to start looking.

There are too many things that might prevent audio by just trying to play the keys, so you'd be better off at first just troubleshooting using the adjustment mode tone generation which is guaranteed to work.  (e.g. keyboard can be disabled by the setting "local off", MIDI volume can be set at zero by faulty fader, or otherwise set to zero etc.).
You need to be using known knowns when troubleshooting or you could end up chasing your tail.

The MIDI out circuit is a constant current loop which drives a LED in the receiving units opto isolator, so rather than trying to scope activity you're better off just shoving an LED in the MIDI socket pins either side of centre, the LED will flash when MIDI data is present.  (can flash quite dimly, unless you shove your whole hand on a whole bunch of notes when it becomes much more obvious). 
Note that the LED is polarity sensitive, it will only flash one way round.

[michel74]

So, in this board, the MIDI interface is no working with an LED Rx/Tx mode but with a direct serial to parallel driver (an ACIA) output to a DIN connector.

After a proper check, I could observe activity on the output of the device. So, this one is not having issue.

I finally managed to probe the input of the DAC, with a DSO, and data are properly “entering” the DAC but nothing on the output which clearly makes me think that the DAC is having issues.

I ordered one; and I’ll try to exchange it. I’ll let you know when it will be done (2 weeks ?)