Author Topic: What are the steps involved in repairing digital equipment? (Read 8792 times)

obiwanjacobi · « **on:** April 08, 2015, 03:25:24 pm »

I have a digital mixer on my desk and was wondering if there are some common things to check or do with repairing digital devices?
Besides checking the power supply voltages

Thoughts?

plazma · « **Reply #1 on:** April 08, 2015, 03:35:32 pm »

Check for leaked capacitors. Measure voltages also with an oscilloscope. DMM may show correct voltage even though there may be huge ripple.

nctnico · « **Reply #2 on:** April 08, 2015, 06:08:02 pm »

What do you mean by digital? If it has a lot of discrete logic chips (74xxxx) then you can see if you can find logic levels which are around half the power supply rail. If there are just a few big chips you are limited to just checking the power supplies. A lot depends on what the problem is though.

obiwanjacobi · « **Reply #3 on:** April 08, 2015, 06:30:53 pm »

With digital I mean it has a CPU. The mixer I have has a 386-ish CPU and 4 DSPs. There is some discrete logic and a FPGA as well as a couple of MCUs. Problem is that it doesn't boot up...

nctnico · « **Reply #4 on:** April 08, 2015, 07:39:49 pm »

Doesn't boot... does that mean no lights, some lights, message stuck on a screen? Nothing obvious like a serial port which could output startup messages?

obiwanjacobi · « **Reply #5 on:** April 09, 2015, 04:13:27 pm »

Technically it probably boots: it shows a splash screen (LCD is driven directly by main CPU) but then gets stuck. Some lights are on but not all.
There is activity on the address bus but I suspect the program loops - because I sort of see patterns on the scope while probing each address line...?

There is a serial port (RS232) but haven't checked for diagnostic messages. Good idea, will do that.

BTW: it's a Behringer DDX3216.

Things I have tried (back on topic)
- Check supply voltages - also with scope to check for ripple
- Check clock for main CPU
- check RESET for main CPU
- check Address bus activity (just probe some address lines with the scope)
- disconnected other sub-systems from the CPU board in order to isolate the problem.
- firmware upload - PC software (for the mixer) does not recognize the connected (over RS232 and MIDI) mixer.
- Removed flash RAM chips I think stores settings, kept the program chip in-place - got an error message that the program was invalid (so that works)
- Checked the current draw on the +5V rail for just the CPU board - was within specs (157mA).

nctnico · « **Reply #6 on:** April 09, 2015, 04:48:14 pm »

Just an idea: Try and find if there is some I2C device connected to the processor. Check whether that works (no I2C timeouts).

jlmoon · « **Reply #7 on:** April 09, 2015, 04:50:45 pm »

Quote from: obiwanjacobi on April 08, 2015, 06:30:53 pm

With digital I mean it has a CPU. The mixer I have has a 386-ish CPU and 4 DSPs. There is some discrete logic and a FPGA as well as a couple of MCUs. Problem is that it doesn't boot up...

Yes the term "Digital" can end up being a loaded question or statement.

jlmoon · « **Reply #8 on:** April 09, 2015, 04:53:25 pm »

Quote from: obiwanjacobi on April 09, 2015, 04:13:27 pm

Technically it probably boots: it shows a splash screen (LCD is driven directly by main CPU) but then gets stuck. Some lights are on but not all.
There is activity on the address bus but I suspect the program loops - because I sort of see patterns on the scope while probing each address line...?

There is a serial port (RS232) but haven't checked for diagnostic messages. Good idea, will do that.

BTW: it's a Behringer DDX3216.

Things I have tried (back on topic)
- Check supply voltages - also with scope to check for ripple
- Check clock for main CPU
- check RESET for main CPU
- check Address bus activity (just probe some address lines with the scope)
- disconnected other sub-systems from the CPU board in order to isolate the problem.
- firmware upload - PC software (for the mixer) does not recognize the connected (over RS232 and MIDI) mixer.
- Removed flash RAM chips I think stores settings, kept the program chip in-place - got an error message that the program was invalid (so that works)
- Checked the current draw on the +5V rail for just the CPU board - was within specs (157mA).

How about the CPU getting stuck due to a stuck keyentry button / pots.. etc. . I would use the 90% rule of mechanical (stuck buttons, faulty input conditions) failure to start my diagnosis after first checking all supply rails. Regarding those buttons, take a look around the most used buttons on the system and work backwards.

Hope this helps.

obiwanjacobi · « **Reply #9 on:** April 09, 2015, 05:29:11 pm »

With the CPU board disconnected from all other sub-systems (just power and LCD connected) the problem still occurs.

My guess is that one of the flash chips is corrupt. That would mean that just by reflashing them it should start to work.
Now hunting for the binary files to use in the programmer...

EDIT: just got hold of the binary dumps for the chips. Too bad my programmer isn't in yet O0

Anks · « **Reply #10 on:** April 09, 2015, 06:26:12 pm »

Normal fault if the psu is clean with these is the clocks. Change Q3 and associated caps. No real way to tell as your scope will load the circuit but the clocks go our of spec. Cheap just to change it and see. Also the other clocks can cause the fault but that is the main culprit.

The program could also be the issue. I'm assuming you have checked for a halt state on the processor.

Once you have changed the crystal and confirmed the program I would check the ram is switching cleanly.

When you said the address buses where switching what voltages were you getting. Got a scope screen shot.

obiwanjacobi · « **Reply #11 on:** April 10, 2015, 05:21:46 am »

What do you mean with "RAM is switching cleanly"?

Anks · « **Reply #12 on:** April 10, 2015, 06:51:11 am »

Sorry I wasn't very clear on that point. What I mean is make sure the data lines are switching cleanly. Ive not looked at one of these desks for awhile so I'm trying to remember. I've had a service manual for this desk in the past but I seem to have lost it. I'll check my other computers if you really need it.

obiwanjacobi · « **Reply #13 on:** April 10, 2015, 07:26:50 am »

I have the schematic pdf document (including PSU and test-load), but if you have more information (test procedures, test point values etc) that would really be most welcome! So I would really appreciate if you could try to find that. Thanx.

Anks · « **Reply #14 on:** April 10, 2015, 08:09:16 am »

I've got very little notes from back then but as I said it was common knowledge that the crystal q3 failed and as is a 30p part i would just change that. If you have a old battery powered clock knocking around it probably as the correct part inside it.

Have you checked and reseated all the ribbon cables? Also try a factory reset ch1-16 + setup while turning on.

From what I remember the desk will throw a error message if it as other problems other than the clock dropping out and bad power

obiwanjacobi · « **Reply #15 on:** April 10, 2015, 09:45:57 am »

- I did already change the 32.768kHz crystal (not the caps though).
- I have currently disconnected all ribbon cables from the CPU board. Only the LCD and the power is connected. All other boards are still powered on, though - I wanted to keep the PSU under load.
- I am looking into recapping the PSU, there are at least two bad caps (waiting on my ESR meter) - but I cannot see any ripple on the 5V and 3.3V rails...

Thanx.

Anks · « **Reply #16 on:** April 10, 2015, 06:31:30 pm »

Have you got the data sheet for the am386sc300?

This processor also controls the lcd. Have you confirmed the lcd is working correctly? Is there any sign of life from the LCD

Also check that the cpu reset ic is working. you should be able to force a reset by studying the datasheet.

Have you got scope shots of the psu lines for me to look at. These desks are finicky bugars about the clean power.

obiwanjacobi · « **Reply #17 on:** April 10, 2015, 06:54:37 pm »

The LCD shows the splash screen. I was not able to find the datasheet for the CPU, so if you have that I would be interested.

The scope images are just straight lines in all time divisions.

Anks · « **Reply #18 on:** April 10, 2015, 07:21:05 pm »

Quote from: obiwanjacobi on April 10, 2015, 06:54:37 pm

The LCD shows the splash screen. I was not able to find the datasheet for the CPU, so if you have that I would be interested.

The scope images are just straight lines in all time divisions.

Here is the data sheet https://drive.google.com/file/d/0B-eqpfg5gsWbang2bk04VV9YMm8/view?usp=sharing.

With it showing a splash screen and halting it points to somthing locking up after intial startup. This could be due to not reseting cleanly or for the correct duration or currupt data. I would myself force a reset by pulling the reset line low after it as got stuck. This should ensure that all subsystems are up and running before the processor comes online.

It maybe that the processor is starting from a unknown state. This can be checked by scoping the 3.3v on one channel and the reset on the other with the scope in single shot waiting for a rising edge on the reset line and turning on the power.

What your looking for here is that the reset happends after the rail as stablized.

This may not be the issue as you say you have a splash screen but its easy to test and confirms another part of the circuit is working.

Another thing I would try is adding back all the other components as the system maybe trying to comunicate with other parts of the system as a startup self test but I would expect a error.

obiwanjacobi · « **Reply #19 on:** April 10, 2015, 07:41:28 pm »

Thanx for the datasheet. I often wonder how manufacturers find their parts when datasheets are that hard to find

The guy I bought it from said he'd tried swapping boards with a working unit until he found that swapping IC15 fixed the problem. That could very well be what we're looking at - a corrupt memory content that screws with the execution flow of the program. I have obtained the binary dumps for the flash chips and when my programmer arrives, I will try to reflash each chip (starting with IC15) and test in between.

In the meantime I will look into recapping some (or all) of the PSU outputs - I saw at least 2 bad caps there.

Anks · « **Reply #20 on:** April 10, 2015, 08:02:29 pm »

One of the program IC's being bad would certainly explain your issue.

What format have you got the files to reprogram the flash IC's. If it is somthing other than the BEX format off behringer I would be intrested in obatining these off you for future repairs even though im very unlikely to see another one of these desks.

Also I would order another flash chip at the same time you obtain a programmer as it could also be the chip that as died.

obiwanjacobi · « **Reply #21 on:** April 10, 2015, 08:19:34 pm »

The guy I got it from had it password protected. Please go through him for obtaining the password.
http://music-electronics-forum.com/t39175-post381508/#post381508

I already have a spare flash chip - I ordered it as soon as I bought the unit. Only later I realized that the chip might still be good and perhaps only the data corrupted...

Anks · « **Reply #22 on:** April 10, 2015, 08:30:25 pm »

Have you tryed the chip blank just incase its for storing user data. it may cause the system to reset also try a factory reset like i explained before with it in place.

obiwanjacobi · « **Reply #23 on:** April 10, 2015, 08:35:32 pm »

I think so. I keep forgetting to write down the things I try and the results I get - should really work on that

If I remember correctly the factory reset did not reinitialize the empty chip...


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Author Topic: What are the steps involved in repairing digital equipment? (Read 8792 times)

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