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.

Before

A jumper to the jack restores power and everything works.

After

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.

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.

Electro-Harmonix Pog2

Much like the Nano Pog repair, this one is also written up after the fact and I have probably forgotten a lot of important details. I sold this one on, so this is mostly based on blurry pictures I took.

I got this with one of the slide pots broken off, and dead on powering up. The picture below is out of order and was taken after it was repaired.

All good.

Inside, the problem was immediately obvious. FB3 near the DC jack is missing but the pads have visible solder on them. It's very common for high-speed digital pedals to have ferrite beads installed on the 9V input and ground lines, to keep high-frequency noise out of the power wiring and comply with EMI/EMC regulations.

Guts

FB3 is gone.

My guess is that someone broke off one of the slider pots and tried to remove the board for a repair. It looks like an attempt was made to desolder the 9V jack and FB3 was lost in the process. I added an 0805 ferrite bead and the Pog powered up again, everything working.

 The architecture seems to be similar to the Nano Pog, with a larger Analog Devices ADSP-BF531 DSP, and additional controls for filters, volume swells and presets.

After desoldering the broken pot.

Replacement slider pots are available from Small Bear, replacements should be 10 kOhm. They also sell the rubber slider tips.

Update: I came across a similar repair at this blog that states the pots are 5k. My order confirmation email says 10k, I'm certain I matched the new part to the old one. It's likely that these are just used as voltage dividers and both values may be used in different batches or versions.

Compared to the Nano, I much prefer this version. The additional octaves are nice but attack/filter controls really take this from being a novelty to something pretty inspiring. Fun!

Rozz R8 Repair

Another analog delay with no delayed signal. I had never heard of Rozz (not Ross), apparently it was a brand applied by a Japanese OEM that released pedals under many different names. Effects database found quite a few variants, it seems that the Ampeg rebrand is sought after nowadays.

http://www.effectsdatabase.com/model/guyatone/ps00x/ps006
http://www.effectsdatabase.com/model/westbury/06
http://www.effectsdatabase.com/model/ampeg/a/8
http://www.effectsdatabase.com/model/tora/ts006
http://www.effectsdatabase.com/model/rozz/r/8

Freestompboxes has a schematic for the Guyatone variant. It looks like it was drawn using the official Boss DM-2 schematic as a starting point.

http://freestompboxes.org/viewtopic.php?f=1&t=21461

The narrow English font looks very Japanese.

Inside is a Mitsubishi (Panasonic!) MN3005 BBD and MN3101 clock generator, which is nice. The only other IC is an NE570 compander, all buffering and filtering is done with discrete amplifiers. Otherwise it's similar to the DM-2/AD80 style delays of the time, except that there is no "cancel" trim pot to dial out any clock noise, and the wet and dry signals are passively mixed with no output buffers. This might have been a lower-cost design.

PCB is the usual single-sided type used by Boss in the 80s.

Looking around the board I could see clock signals at the 3101 and BBD, but I was confused as they seemed to disappear when I flipped the board over to see the traces on the solder side. Pushing down on the MN3005 would give a momentary burst of delayed noise, so the delay circuit appeared to work, only with a broken connection somewhere. I moved all the wiring from lap-soldered pads to run through the PCB in case a bad joint was causing the problem. No dice.

Eventually I realised that the trimpot for setting clock rates was broken, it only made contact when the pedal was upside down! After desoldering it literally fell part, the wiper only contacts the conductive track when held in place by gravity.

Original timing trimpot.

With a 100k replacement pot the delayed signal was back. The available delay times were not quite right, it would happily go to ~1/2 second at one end of the pot but the other was not much shorter. Fast short delays were not possible. Clock bleed-through was also really bad, it would whistle at the clock frequency even in bypass mode.

100k temporary replacement.

After finding a 50k replacement things were a lot better. Clock rates were manageable from ~8 kHz up to 15 kz or so, allowing long and short delays. Clock noise dropped a lot but is still audible at long delay times - I think this will need a trimpot added to the BBD outputs to dial this out.

With 50k trimmer. Much better.

This sounds very nice, it does DM-2 style delays on the verge of oscillation very well. With some minor mods to remove any clock sounds I think these would be really nice analog delays.

T Rex NeoComp

Another T Rex pedal with problems. This is a VCA-based compressor, and it was really noisy, like white noise with some low-frequency popping and crackling. Fortunately the noise was there in both effected and bypassed modes - this meant that everything was more or less working and that the source of the problem could be isolated to parts of the circuit common to both.

On the inside there are some similarities with the previous T Rex pedal, there is a CD4013B and momentary footswitch for latching bypass, so there are probably some JFETs nearby. MC33174 quad opamp. The compression is done with a THAT 4316 which seems like a nice chip for pedal applications. It's sold as an "analog engine" and contains a VCA and the RMS detector. The QSOP package will probably turn people off. An LM393 comparator is used for something, but I have no idea what for.

Main PCB.

Noise problems like this are usually caused by a bad semiconductor or a missing or failed capacitor. I like to find the source of the noise with an oscilloscope and then remove parts to see if the source disappears. Opening the circuits isolates things into two sub-circuits and narrows things down. In this case I ended up removing nearly all the ICs to figure it out. I started tracing out the circuit as well but solved the problem before I got very far with that. The SOT23 transistors in the corner of the board are driven by the 4013 and are connected together in the same way as the switching JFETs in other T Rex pedals ADD LINKS One of them showed a lot of noise when probed with the scope, so I replaced it with a J201 and that killed the noise. The originals are marked "G5   7" which I can't identify, but for switching it seems a J201 will work just fine.

Switching FETs are in the top right corner, faulty FET has been removed.

Apparently this noise problem is a common failure with T Rex pedals. Swap the JFETs.

T Rex Mudhoney II

This is something like two slightly different Proco RAT circuits in one box. As received it only seemed to work with very heavily attenuated output volume. Maybe it was something to do with an electronic switching system.

Shown here without footswitches

The PCB photo came out very noisy, but the inside is just a CD4013 dual flip flop IC and 2 OP7 opamps driving some diodes to ground - I believe this is standard for modern reissue RATs. The red wire and replacement capacitor was a previous repair. The switches are held by rectangular cut-outs in the PCB, I like this method.

 

As well as being very quiet, the LEDs underneath the tone pots were not lighting up. I guessed that the 4013 was driving some switching FETs and it was dead.

First attempt at a fix was replacing the CD4013, as these are cheap and I can replace SOIC parts pretty quickly nowadays. No change. Probing the new chip I saw it was getting no power - I should have checked this first. I followed the traces from the 9V input outwards and they were hidden underneath the input jack. That had to be removed.

Input jack removed

Underneath was a trace carrying 9V that ran right next to the edge of the board and had been cut right through. I added a wire from to restore 9V to a power bypass cap at the 4013 and reinstalled the jack. Now the pedal lit up when engaged, and the left side (channel 1) was loud and sounded pretty great. The right side still had a very low output volume, much less than unity.

I hooked up a signal gen and tried to follow with an oscilloscope to see where signal was lost. Thi schematic was helpful. The opamp has outputting a 6Vpp signal but this was lost at the tone control, so the problem was probably somewhere in between. Testing the clipping diodes showed that one was shorted. I guessed and lifted one off.

One clipping diode removed.

This was lucky, that diode tested as a short out of circuit. This side of the pedal was now much louder, but less distorted than the other side as only half of the signal is getting clipped. These diodes are MELF packages and I can't find any identifying markings on them. The remaining diode measured around 0.5-0.6V on a DMM, so probably something silicon. Internet consensus seems to indicate that these pedals used 1n4148, and I had MELF 4148s at hand, so I used that.

My hand soldered replacement looks different to the reflowed diode. The good news is that it sounds the same as the other channel, and now both will get fairly loud. I forgot how nice a RAT can sound.