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.

Dunlop Cry Baby 105Q Bass Wah

Unlike the traditional Crybaby, the Dunlop bass wah doesn't use a stompswitch to switch the effect. Instead the pedal is spring-loaded to return to the heel position, where it engages a small momentary switch which bypasses the pedal. You only have to lift a foot off to disengage the pedal, which makes using it for just a few notes much easier (maybe this was thought to be more useful for bass players). The downside is that it can't do cocked-wah sounds without a foot holding it in place.

This one doesn't switch at all, the output signal is always in bypass mode. It does seem to work in that it buffers the input signal, and when power is removed no signal is passed. This is likely to be a problem with the switching system.

The insides are very different to the Crybaby. This is an inductor-less wah, with 2 quad opamps doing the filtering, presumably with a gyrator design. The normal Dunlop Hot Potz is used.

Momentary button desoldered

The switching system uses a small momentary button soldered on to the hidden side of the PCB and poking through the top of the enclosure. One side of the switch looked like it was always closed, so this was replaced with a new one - without fixing the problem. The transistor is just buffering the switching signal, with the new switch installed this could be seen changing from 0 to 9V as the pedal was lifted and dropped, so this part of the PCB is working correctly.

Main board.

All of the rest is done with 4 ICs. The MC33174 opamps are almost certainly doing the filtering, the CD4066 analog switch is routing signals when changing from bypassed to effected modes. The signal from the momentary footswitch is running into a CD4049 buffer. There are some Rs and Cs around the buffer, I would guess it is de-bouncing the switching signal to cut out any popping. There was no activity on any of the CD4049 pins.

4098 buffer desoldered.

I took a picture after I removed U3 but apparently forgot to take one after I soldered a new part. The new chip correctly sent on the switching signals and the pedal wahs once again.

This is a pretty nice design, I only tried it with a guitar as I didn't have a bass around at the time. I would have guessed that the pushbutton is the weak point, but in this case it seems an IC died first.

Empress ParaEQ Repair

This is another case of getting to look at something  nicer than I would normally get to play with. When it comes to guitar gear I am pretty cheap, and it's hard to justify spending a lot on "simple" pedals (this is a dumb distinction to make). Empress' ParaEq seems like it's probably worth it, it's a good idea executed really well.

This is basically a 3 band EQ (low, mid & high) with adjustable centre frequencies and independent Q settings. There is an input pad and a clean boost as well. The build quality is very high - the toggle switches are positioned behind a row of knobs to protect them from stomping feet, and the pots all have nice metal knobs. Empress seem to do pedal graphics with enough personality to not be completely boring to the eye.

Revision 7, Jan 6 2014. Copyright date is 2013...

Inside we have:
- OPA4134 & 2274A quad opamps, buffering, amplifying & filtering.
- an ADG442 analogue switch for the buffered bypass
- TC7760HE charge pump controller for a negative voltage rail.
- 16F630 Pic microcontroller, monitoring footswitches to change between relay and buffered bypass.
- a SOT223 package, marked "4576". I'm certain this is an LDO and I think I had the part number and looked over a datasheet but I have lost it again.

Plugged in, the pedal was completely dead. My little test amp was humming, which usually means a power supply is shorted out somewhere and some filter caps can't do their job. Measuring inside the pedal, the DC jack read at ~2.5V (using a 200mA Boss power supply).

Flush mounted DC jack

No through-hole anchors for the jack.

Something is pulling power to ground, usually this is a bad power filtering capacitor, bad IC or a shorted protection diode. Empress are using a surface mount DC jack which must be desoldered to remove the PCB from the enclosure. I guess they really wanted the power jack to sit flush with the edge of the case. I took it off to see if there are more parts or any damage on the backside.

There is nothing on the backside. I took pictures of pots while I was here.

Gain pots are B5K.

Frequency pots are marked "B1 P1416C"

As there are not too many ICs on the board I tried to order them in likelihood of failure and then removing them to see if the short cleared. Pulling these chips off is pretty fast with hot air.
The 7660 was first, as I have seen these fail before. No change. Then I removed U1, the sot-223 LDO and the short was gone - but as this chip is powering all the opamps I couldn't say of this was the problem or not. U101, a 2274A quad opamp had a mark on the casing which may have indicated some kind of failure, so I removed it and replaced U1. Short was gone, I now measured +8V at the opamp positive rail and -6V at the negative. It passed a signal, the LEDs now lit up when the footswitches were pressed, and the boost worked. The EQ controls did nothing, so obviously this opamp was handling some of the filtering.

U1 & U25(?) removed.

U101 removed, short cleared.

After soldering in a new opamp everything worked as expected. Re-soldering the power jack is easy with some kapton tape holding it flush to outside face of the enclosure.

This is a more useful design than I had expected, parametric EQ solves a different problem than a graphic EQ. It's much easier to sweep the frequency control with a high Q and identify exactly what will be boosted or cut, it always allows adjustment of frequencies "between" the bands available on a graphic EQ.

The only downside for longevity is the PIC - again, this is a programmed part. If it was bad I would not have been able to replace it and would have had to re-wire for true bypass.