Line 6 MM4 Repair and DL4 Conversion

 

I think these are fairly well known. The big 4-switch pedals have been very successful, especially the Big Green Delay (DL-4) which was something of a game-changer in popularizing live looping. The basic idea of the series was large, robust floor units that grouped Line 6's modelling technology into different categories bases on effects type. This is the "Modulation Modeler" and does chorus, phaser, flanger, tremolo etc. The FM-4 did filter effects, and apparently there were Amp Modeler (AM-4) and Distortion Modeler (DM-4) units that didn't sell well. I guess the people buying these pedals didn't want Line 6 POD-style sounds, and everyone else saved some money and just bought PODs.

This is a beat-up unit I got from eBay, sold as not working and missing the footswitches and knobs. It doesn't seem like a lot of people are buying these as projects, despite used units holding their value very well for a late 90s digital effects (the DL-4 especially). You can see why: digital, surface mount, fairly complex.

Backside of the board just has a socketed EPROM

Main board

There is quite a lot of info on this series online. This thread at freestompboxes.org has a complete service manual that includes full schematics and BOM for most revisions http://www.freestompboxes.org/viewtopic.php?t=23282&p=256501 for anyone who wants to play along.

All the pedals in this series are built on the same PCBs and have the same housings. The differences come down to which parts are populated (the DM-4 and AM-4 are mono only) and which firmware is in the EPROM chip. There's an NXP DSPB56364 DSP doing the audio processing, an 80C32 microcontroller for reading inputs/housekeeping and a Cirrus CS4223 24-bit 48kHz audio ADC/DAC. The rest of the board is some CMOS logic gates for sequencing LEDs and relay switching and some reasonably complicated voltage regulators for power. Nearly all parts are available as replacements, except for the DSP which is discontinued but still for sale from surplus and grey-market sellers.

This board seemed to have power issues, as I couldn't find any voltage rails that seemed correct. The Line 6 power supply is 9VAC and the internal DC supplies are pretty convoluted. 9VAC is fed through a 3.6 V zener and 1N4002 diode and connected to a MAX887 buck regulator (U14) producing 3.3V. The 3.3V supply powers some of the digital side, and also supplies a MAX660 voltage doubler (U20, giving 6.6V output), which is further regulated by an LM3480 5V regulator (U21). Finally, a LM2664 charge pump inverter (U15) produces -5V. The + and -5V rails are used for opamps throughout and 3.3V and 5V power the digital ICs. There's a very good post at the diystompboxes forum covering troubleshooting of these supplies http://www.diystompboxes.com/smfforum/index.php?topic=41142.msg297051#msg297051

" ..so the most likely suspects are
1: 3,6v Zener ( D16 )
2: MAX887 ( U14 )
3: MAX660 ( U20 )
4: LM3480 ( U21 )"

It's worth noting that the MAX887 has a shutdown pin that is switched by the input jack. The voltage regulators won't start up unless you have a cable in the input jack.

I had no 3.3V supply so I suspected the MAX887 and the diodes at the input. I replaced D11, D12 & D16 and applied power. The MAX887 immediately blew a hole in it's case and started smoking.

I removed the MAX887 and R49. R49 measured good out of circuit so I put it back. I tried powering up without the MAX887 and C40 (1000 uF) exploded. After removing it I had a close look at the board. There were some jumpers from battery contacts to the DC jack that look like a previous repair attempt. There was another jumper on the topside from the DC jack that I removed.

WTF

After buzzing out the connections from DC jack to the voltage regulators I found a burnt track from the input choke (which is not populated) and D16. Maybe this was an attempt to fix that? I have no idea. I am lucky to have a spare DL-4 parts board courtesy of Moose Electronics Dublin (check him out https://www.facebook.com/mooseelectronicsdublin/?ref=br_rs). I swapped over the MAX887 from the spares board and jumpered over the damaged track. Now the 887 regulator circuit buzzed audibly. OK, it probably can't start up as C40 is missing. I swapped over C40 from the scrap board and now I had a correct set of voltages at each regulator. I pushed one of the switches and the relays clicked and LEDs lit up, working again. I have to use a screwdriver to push the switches until I can get some replacements but everything functions correctly.

There are 4 testpoints on the main PCB that can be used to validate if things are working. When viewed on a scope, MCLK should show a 12 MHz clock signal, SCLK a 3 MHz clock, LRCLK a 47.8 kHz clock and ADC and DAC should show data changing at around 1 MHz. DAC will only show data if you have a cable in the output jack.

DL-4 Conversion

Ok, so the difference between pedals in this series is just what software is loaded, so can we convert one to another by swapping ROM chips? I tried the EPROM from the scrap DL-4 board (marked DL-4 V1.3) in the MM4 and it doesn't work, the pedal is non-responsive. Comparing the two boards it looks like the only difference is that U5 is not populated in the MM4. This makes sense, U5 is a 16mbit DRAM. The DSP has a small amount of RAM that can hold enough audio for effects like flangers, but the DL-4 has long delays and a 14 second looper so some extra memory is needed. I attempted to transplant U5 from my scrap DL-4 to the working MM-4.

U5 on scrap board. Pots and some caps had already been scavenged.

U5 desoldered.

U5 came off with hot air after a few minutes. Unfortunately the two boards are of slightly different revisions and the footprints for U5 are different, so this was a tight fit on the MM4.

MM4 prepped for populating U5.

U5 soldered.

After soldering I tried the pedal with the original MM4 ROM chip (the socketed PLCC32 on the backside of the board) and it still worked as an MM4 with the extra memory added. I swapped in the DL-4 chip from the scrap board, and now it works as a DL-4 as well. I thought I might have gotten the pedal to lock-up in DL-4 use but this was probably because I didn't know how to use a DL-4 looper, it seems to work correctly after playing around for a half hour.

So it seems you can convert these pedals as long as the memory needed for the delay is present. I dumped the contents of both chips to PC. I also have a dump of an FM-4 I got online, but all of the PLCC chips in these pedals are one-time-programmable and can't be overwritten. I'll have to order some EEPROMS to try the FM-4 firmware. If anyone has dumps of the AM-4 or DM-4 please get in touch, it would be fun to try these, even if just for completeness.

I think it should be possibly to modify any of these pedals to switch out firmware and reset the DSP to go between DL-4/FM-4/MM-4/whatever at will, possibly on a footswitch.