Behringer mixer effects DSP crash

[Th3_uN1Qu3]

Got a Behringer Eurorack UB1222-FX Pro with a bad effects processor. Owner said he found two bad caps in the PSU and replaced, but still no go. All caps are Xunda, the replacements are Jamicon which aren't exactly adequate, but that's not the problem here.

The effects will sometimes work, sometimes output really loud white noise, but most of the time, when trying to select an effect, the DSP just crashes and the display goes out. There was significant ripple (about 100mV) on the 3.3v regulator when the DSP was operating, most of it making its way on the 5v rail as well. The ripple disappears when the DSP crashes, so it appears it shuts down entirely. However, that isn't the issue as i've added caps to the 3.3v rail only to have it behave the same. Haven't tried adding caps to the 5v rail yet, will do...

Power cycling or toggling the reset line of the DSP board brings it back up. Most of the time it just crashes again. I've attached a schematic of the DSP board. I see a DSP, an audio codec, an EEPROM, a couple chips of RAM and some miscellaneous logic for driving the display, reading the encoder and such. Fortunately all chips are available to buy, but for instance replacing the main DSP (100 pin TQFP) would be a major pain in the ass. I think i'm gonna try swapping the RAM first, but if anyone has a better idea i want to hear it.

[tom66]

Re: Behringer mixer effects DSP crash

If the DSP has a PLL, that ripple can cause the PLL to be unstable, which may trigger an oscillator trap (many DSPs have this feature - for safety critical systems.) With the new caps, is the ripple lower? You're looking for very low ripple - perhaps only a few mVp-p, at high frequencies (>~1kHz) and while low frequency ripple isn't as critical, it can still cause issues...

I wouldn't immediately suspect the RAM to be honest; likely that only holds the audio data, the failure of which would only cause corruption to the audio (not the DSP crashing.) Unless the DRAM locks up and fails to satisfy some condition (but I don't know if that's even possible.)

Another possibility is the RESET IC. MCP809-460 suggests to me a 4.6V reset; check the IC isn't toggling the RESET line, and that it is getting at least 4.6V (the IC appears to be connected to 3.3V though, so no idea if that's a schematic error or if 460 means something else entirely.)

An intermittent MLCC on the PCAP or a core regulator (if the DSP has it) might also cause the issue.

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Since the DSP works on 3.3v, the reset line is at 3.3v too. I've checked that already, it isn't toggling. A reset will cause the DSP to reset (as i mentioned above), it doesn't do that, it just crashes.

[tom66]

Re: Behringer mixer effects DSP crash

If the RESET line gets stuck low, the DSP can do odd things - also, some have "compliance times" on the reset line, it must go low for x ms for it to count as a valid reset (otherwise weird stuff could happen.)

Check its crystal is stable too.

I doubt the DSP itself is failed.

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Crystal is good, that's the second thing i checked after the caps. Reset line is high all the time so we can rule that out. Thing is, it'll sometimes start up fine, work for a while then crash when trying to change effect. Most of the time tho, it doesn't even start properly. This is signaled by a burst of white noise at startup. When it does start up correctly, it only makes a short pop.

I think the audio codec is alright. Playing with my finger along its pins makes noise even when the DSP has crashed. Playing with my finger along the DSP pins when it's crashed makes no sound at all. When it locks up making weird noises, touching the DSP pins changes the noise. Touching the RAM does not change anything at all.

I think i'm gonna go ahead and order the DSP.

[tom66]

Re: Behringer mixer effects DSP crash

There is a line LRCLK going to the audio processor. This should be clocking continuously (at twice sample rate if stereo), does it? And SCLK should be at 48 x sample rate. Does the particular effect chosen matter, or does any effect cause it to crash?

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Most effects cause it to crash. The ones that i have better luck with are the flanger and delay effects, those sometimes work. If it hasn't started up properly, any effect will cause it to crash. And it takes like 20-30 resets to get it to start up properly.

Edit: Just checked pin 4 of the CS4220 audio codec: I get a nice clean 48kHz clock signal all the time, regardless of the state of the DSP.

[tom66]

Re: Behringer mixer effects DSP crash

Very odd, makes me think that the software on the DSP might be crashing - not the actual DSP. Otherwise, the I2S peripheral would shut down.

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Hence my RAM comment... I have some SIMMs laying in a drawer, maybe the ICs match.

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Good news. Swapped out the DRAM and it works now. First it worked for a few minutes and then started whacking out (flanger sounded like a loud police siren, reverb like bursts of white noise), and one of the chips i'd put in was marked a bit differently than the other. I swapped that for one 100% identical with the first, and no more glitches. I've also added more caps and bypassed a tiny SMD inductor on the DSP board which was dropping too much voltage for my taste.

It still clicks once or twice on some effects, but at no point does it develop in full-blown bursts of loud noise, and starts up reliably every time. This likely also has to do with the fact that the original RAM was 60ns rated and the ones i put in are 70ns. I'll have to see if i can get better chips, but so far it looks promising.

[tom66]

Re: Behringer mixer effects DSP crash

Very interesting, though I would doubt it's near the limit of the RAM speed, audio isn't really high speed. Might be normal behaviour...

Honestly I wouldn't have immediately suspected the RAM, I would have considered the flash memory maybe, possibly dodgy firmware? Glad it's working now.

A failure of RAM ICs on a Samsung DLP:
http://www.youtube.com/watch?v=fLKtoEYnE_8

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

I don't think it's normal behavior. As far as i understand the FX section, audio goes in the A/D, gets processed by the DSP, and comes out via the D/A to be mixed in with the original signal. So the RAM holds digital data, not audio per se.

The modules i've pulled the chips from have been sitting for years, and IIRC i've never tested them, as i had 16 and 32MB modules in my last oldschool SIMM-based machine. In fact, i think these modules came from a box of random computer junk i got a couple years ago, along with some ISA cards. So for all i know they could be bad as well - and the incident with the first chip supports this idea.

However, for the application the mixer will be used for (live sound), it isn't likely to require more than simple delay or reverb, and those are fine, so i think i'm going to leave it like this. I might try to bump the 5v rail a bit higher (it uses a 7805), as other than the logic and DRAM, there's nothing else on it.

[Th3_uN1Qu3]

Re: Behringer mixer effects DSP crash

Started acting up badly again today, went thru another 2 pairs of DRAM. This time i found some that work solid on each and every effect without fail.

The first pair i put in, was of the kind with the dodgy printing. It didn't like them at all. All reverb effects were borked, but all flangers worked, so this was a consistently bad area in those chips. Other than that it was solid, and you could switch effects without it crashing, even when you had a bad one.

Pulled 2 more chips from the 4MB SIMM module, with nice printing. These fired up first time, and all FX work without any crashes, clicks, pops or other noises. We can call this a job done now.

[ivcha]

Re: Behringer mixer effects DSP crash

I have a wired problem with my DSP on Behringer UB2442FX-PRO DSP works, effects are good, it does not crash. The problem is that on return from DSP i get very big noise, I think to start by replacing the caps on the Electrolytic DSP Board and in the PSU, is there any other reason why can this noise occur should i replace voltage regulators in the PSU.