Moog Source Troubleshooting Challenge

[MC]

The EPROM is the oddball 2532, which is getting really hard to find and a lot of EPROM burners won't handle that one.

Even the 27xxx EPROMs are getting harder to find.

[Anthropic]

Thanks for all of the tips. I did a little investigation with my scope; perhaps one of you could enlighten me a bit on what I should be seeing. When probing the pins where I was told to expect a pulse stream, I saw what looked like square waves. But, when I pressed a key, those "square waves" (I put in quotes because I couldn't see the vertical portions of the waves- only the flat tops) turned into a solid horizontal line on the scope. Does this mean something is working in the opposite way it should be?

I've gone through the schematics and looked for points on the schematic that specify certain voltages to be present at that particular location; the results conformed to the schematic. Also, at the points on the OSC (1 and 2) portions of the synth board which show triangle, sawtooth, and square waves, I am seeing those waveforms on my scope.

What I probably need is some guidance as to what steps I should take to really diagnose this problem. Given that the power rails, etc. are all checking out as correct, are there certain points on the boards that can be scoped (or the voltage measured) to narrow down the problem to one particular area? For instance, is there a measurement that I can take to be assured that the digital portions are working correctly, or the components are receiving a proper signal from the keyboard, etc.? I feel like I need to create a checklist for myself that will help me divert my attention away from circuits that are already working perfectly, to ones that need investigation...

Keep those beautiful minds churning!

Dan

[Kevin Lightner]

What you're seeing is most likely what's called multiplexed data.
It may not even be necessary to look at this data to repair the instrument, but I'll give you a really quick explanation...

In synths like these, they have a thing called a data buss.
Obviously what goes on the data buss is data. +5 or zero (1 or 0s.)
Where it all gets crazy is that another buss called the address buss can choose where the data above comes from or goes to.
So in a literal fraction of a second, the CPU (which controls both the address and data buss) could tell it to check if any buttons are pressed as an input, send out to the display, read the jog wheel and send out data to control the pitch.
In reality, they usually do many more operations as well.

But the big issue is knowing where to look for what data and when.
That requires advanced knowledge of a scope and understanding gates, clocked logic and other deep subjects.
The important thing is that you're seeing some data. This can suggest the CPU is working and you should look elsewhere.
I personally would check to see what you have and what you don't and find some commonality to it all.
For example, do you have CV hitting the VCOs? Envelope voltages going to the filter and/or VCA?
Do certain buttons on the top panel work, but others don't?
Does the wheel work? (LED count up or down?)
Are the VCOs putting out waveforms?
Document everything you can to see if you can narrow it down to something that might cause all of the faults you're experiencing.
After that, look at the schematics to see if there are parts common to whatever failures you've experiencing.
One bad 4051 chip, for example, could take out 8 different ouput signals.. or refuse to read 8 different input signals.
Look for things like that.

[Anthropic]

UPDATE:

For those of you just joining us, here are my non-functioning Source's symptoms:
- No output when keys are pressed.
- Panel functions appear to be operating correctly.
- All power rail voltages are correct.
- The synth emits a quick "woop" sound upon turn-on (see below), but then only a VERY low constant tone thereafter. Keyboard has no effect.
- Jog wheel fully functional.
- No burned/missing components or traces, no loose/corroded connectors, no leaking caps, functioning battery with no leaks.


Some troubleshooting I've done since my last post:

1. I did a few temporary IC replacements to see if any particular chip was malfunctioning. U32 on the Digital Board was swapped with no effect on the problem. On the Synth Board, U3, U4, and U5 and were swapped with no effect.

2. Another hint that may (or may not) be related to my problem: the Mod Rate LED will respond differently every time I power up the Source. Sometimes it remains steadily lit, sometimes it flashes rapidly (regardless of the positioning of the Mod wheel), and sometimes it flashes slowly (regardless of the position of the Mod wheel). When I first power up the Source, I hear a rapidly descending "shooowwwooop" sound coming from the speaker and the Mod Rate LED "sputters" as the aforementioned sound is present (2-3 seconds), then returns to a steadily-lit state when the sound stops.

I used my scope to trace backwards from my output jack. I noticed that with the scope on a very sensitive setting, I can see some miniscule output from the synth which changes slightly when a key is pressed. However, that tiny signal is not heard through my amplifier at all (yes, the amp is functioning properly and is turned up loud).

Tracing back, I can follow that incredibly small signal back through to the oscillators. At a certain point though, I reach the perfect sawtooth/triangle waves produced by the oscillators before they were robbed of all recognizable features, culminating in the synth output I am seeing.

Here are some pictures of my oscilloscope screen while testing the points at which the signal changes:

Picture 1= The junction of U32 Pin 2 and R117 (both on the Synth Board)
http://www.flickr.com/photos/100410681@N03/9546903183/

The OTHER end of R117 shows NOTHING. Is this proper? I, at first, thought R117 might have failed, but perhaps there is another reason for not seeing an attenuated triangle wave on its other side?



Picture 2= What I'm seeing at Pin 6 of both U33 and U24 on the Synth Board
http://www.flickr.com/photos/100410681@N03/9546904507/




As a result of these pictures, I'm curious to know what is causing the nice big triangle and sawtooth waves I'm seeing coming out of the OSC circuits to be reduced to (seemingly) random noise.




Some other questions:

- What kind of "reset pulse" should I look for on power up? What should that look like on a scope? Is it long enough (in duration) to see?

- Does the inconsistent "Mod Rate" LED flashing indicate a problem with the CPU (even though all of the front panel controls seem to be working)?


Thanks for your help. I'm sure I could take this synth down to a local repair shop, but I'm enjoying the learning process and appreciate all the insight I get from the experts here.

[Kevin Lightner]

- What kind of "reset pulse" should I look for on power up? What should that look like on a scope? Is it long enough (in duration) to see?

Short spike that ends usually in a ground. (low)
Doesn't matter though.
You don't need to check it.
If your controls work or the keyboard plays, the cpu is working.

- Does the inconsistent "Mod Rate" LED flashing indicate a problem with the CPU (even though all of the front panel controls seem to be working)?

I can't say. Could be something loose or something taking a lot of the CPU cycles as a broken state.
Normally it should just flash.

[Anthropic]

UPDATE:

Couldn't find the cause of the problems. Decided to take it to an experienced tech (Corey Davidson at Davidson Electronics). After tons of frustrating troubleshooting, he narrowed the problem down to the D/A converter. As original AM6012PC chips are quite expensive nowadays (around $60 for NOS or new), I did some research and found out that the DAC312 is supposedly a direct modern replacement. So, the tech is going to install one of those and calibrate it. Hopefully, that solves the problem. We'll see!

Thanks for all of the help!