Thursday, 29 June 2017

T Rex Mudhoney II, Again

I'm in two minds about posting about repairs on stuff I've already covered, but maybe highlighting common faults will help someone else troubleshoot. I got another Mudhoney II to look at, and this had the exact same issue as the last one.

Bypass didn't work. No power at the flip-flop IC that drives the switching JFETs. The trace to this IC was broken in the exact same way as the last one. It runs right along the edge of the PCB and underneath the jacks so this is probably a common fault.

A jumper to the jack restores power and everything works.


This is kind of a shame, as it sounds great (dual RAT). If you have one with this problem then check this connection. If I had one that was working I would probably add a wire as insurance.

MXR EVH Flanger

Another eBay pickup. This is a slightly modified re-issue of the MXR M117 with an "EVH" button that adds a fixed preset for a Van Halen sound. Dead, as usual.

This pedal uses an 18V supply or two 9V batteries, like some of the original M117s. The insides however are the standard modern MXR stye - almost entirely surface mount, a red PCB and board-mounted jacks and pots. It's a very tight fit to the enclosure, but it looks fairly robust as everything lines up very well. Turning it on lights up the LED but there is no signal.

Before soldering the new DC jack

The fix for this one was fairly simple. The DC jack was broken, so that was replaced. Still no power at any ICs. The traces from the DC jack & battery harness run to a 15V linear regulator, which feeds a 9V linear regulator downstream. Replacing the 15V regulator (78L15) brought it back to life. The LED is powered from the unregulated 18V supply, everything else needs working regulators. Drop-out voltage on a 78L15 is usually only ~1.5 - 2.0V so if you use batteries you probably can't drain them below ~8.3V each before the pedal will stop working.

It's possibly that this was killed by an incorrect power supply, but there is a reverse polarity diode which is still intact and the 15V regulator should handle up to 30V

The insides are pretty packed but this is more-or-less a standard Flanger design. Here is an overview of the ICs used:

V3204. A Bucket-Brigade Delay chip, almost definitely made by Coolaudio, but they don't list it on their site. As the original MXR Flangers used a Reticon SAD1024 (dual 512 stages) this is probably a 1024 stage BBD. Coolaudio make a V3207 chip with 1024 stages though, so I can't be certain.
SA572D compandor. Compressing and expanding before and after the BBD for better SNR, Electricdruid has an article on how this works.
MC33178, MC33179, TL072: opamps.
MC14504b level shifter. I was surprised to see this, I think it's translating low voltage clock signals to the higher voltage ranges that the BBD wants. This could be done with discrete FETs but maybe this solution was cheaper.
HEF4013 dual D flip-flop. I would have guessed that this used for bypass, but looking at older 117 schematic this is used with an opamp LFO for generating the complementary BBD clock signals.
HEF4053 triple SPDT analog switch. This switches out some of the pots for fixed-value resistors when the EVH button is pressed. This is a neat solution, I think more pedals could use this for "channel switching" by having two sets of pots.

It sounds great, it's a classic design and the controls are broad enough to go from subtle to over the top which I always enjoy. I can't hear any clock noise or bleed-through. The EVH button may be a bit of a gimmick as it's not easy to activate by foot, if you bend down to push the button you could just turn the knobs.

Tuesday, 6 June 2017

Electro-Harmonix Freeze

The Freeze launched in 2010(?) and has been very popular. It's essentially a very short looper with some windowing to hide the "jumps", giving a smooth drone of whatever was captured. There is a decent claim for this being a genuinely new effect, even though there have been reverbs, delays and granular synth effects that can get similar results there is nothing that competes with it directly. I have never played with one so I've been looking out for cheap broken units.

This one passes no signal, crackles a little bit and I probably overpaid for it. The insides are more simple than I expected, a 56k-series DSP (DSP56374), a PCM3052A ADC/DAC, a serial EEPROM for the DSP code, an LM317, a TLC2272 opamp and a tiny SOT23-5 package that I am fairly sure is a NC7SZ66M5X SPST switch. It's surprising to see the DSP56374 as these are fairly long obsolete now, and other EHX products are using Analog Devices DSPs that are still in production. This might have been a 2010 purchasing decision, or they may have large stock as the 56k series is also used in other older EHX products. The PCM3052A is also fairly old but I have seen pictures of other Freeze revisions that used different codecs.

This Freeze passed no signal in bypass or effected mode, just some crackling noises. The LEDs did light up according to the Slow/Fast/Latch switch, and as there is no microcontroller I have to assume that the DSP handles this and that it is probably working correctly.

I fed in a test signal and traced it with an oscilloscope. I could see it at the input jack, through the ferrite bead and then at one of the input pins on the opamp (U2). It appeared to be wired as a buffer (makes sense) and there was nothing on the output pin. So the digital end was probably fine, it was just not being fed an input signal. Replacing this opamp fixed it.

In process of replacing U2

 To be honest, sometimes the process of fixing stuff is more fun than playing with it afterwards. The appeal for me is a mixture of curiosity about the technical stuff and the musical/creative aspect, and it can be easy to switch the things off after testing if it's "just" another delay, overdrive or whatever. The Freeze has consistently been a lot of fun every time I plugged into it. On the day I got it I spent a lot more time playing with it than I did repairing it (that has to count for something). I have used loopers before and found that they need a bit of planning and practice to use well, the Freeze is very immediate and it lends itself to noodly bullshit in the first couple of minutes of use, especially in latching mode. Thumbs up.

I expected to see some kind of microcontroller as the DSP 56k series have usually been paired with one in the previous pedals I've looked at, usually some kind of 8051 core (Line 6, Digitech examples). As the Freeze is a single processor design it might make this an interesting one to reverse engineer, there should only be code for a single architecture in the EPROM. I would guess that the effect is fairly simple, but the magic is in the delay length and the filtering to avoid clicking or popping. There is an assembly language manual available from NXP, I've looked for a disassembler and have found that it is supported by Ida Pro (great, but the full version is out of my price range) and some pretty old 90s tools that I haven't tried yet. If there is a decent free tool please let me know.

First step is dumping the EPROM and figuring out if the code is on there or if the DSP is pre-programmed. OpenOCD does support the DSP56374, so the programming header may be another option.