TC Electronics Nova Drive NDR-1

Programmable overdrive & distortion from TC Electronics. All overdrive/distortion parameters are controlled digitally, so presets can be saved and recalled, the order of the two circuits can be changed, they can be run in series or in parallel. All of this can be triggered by MIDI.

 TC did a Nova series of pedals and multi-effects units, but I don't think they were all built around a common digital design (like the Line 6 4x4 series). TC stuff is interesting as they have a history of high quality and targeting high-end markets and they tend to use fairly modern designs and parts. Unfortunately they don't seem to release much information or contract out any repair services, so I have not come across any modern TC Electronics schematics, either official or leaked.
TC were sold to Behringer in 2015 and their latest range of pedals look to be more standard stuff, a bit watered-down compared to the Nova series. I'll reserve judgement until I see one.
This series used a 12V power supply for some reason (why not 9V, like everything else?)
This Nova Drive started up but only passed a signal in bypass mode. The LED display and indicators did react to the knobs and to button presses, which at least meant that the digital section is working and the problem was likely in the analog end.

Top PCB (digital)

The build quality is really nice, they use a folded and tapped sheet metal chassis instead of cast box. There are two PCBs inside, the first is a "mezzanine" board with the encoders, buttons and displays. There is an Atmel ATmega168 and some '595 and '165 shift registers for digital IO. I'm pretty sure the ATmega is reading out pots and buttons and controlling something on another PCB to vary the effect parameters, as well as driving the LED displays and responding to MIDI commands. I would guess that this top PCB is probably very similar across all the Nova pedals. I am assuming the ATmega has some code-protection so I haven't tried desoldering and dumping it.

Lower PCB (analog)

The bottom PCB fills the whole enclosure - it has a bunch of opamps (presumably the analog overdrive and distortion circuits), DG441D quad analog switch ICs, switching power supplies, the MIDI optoisolator, a relay and a Cirrus CS3308. The CS3308 is a cool part, it's an 8-channel digitally programmable volume control. Presumably each channel is mapped to one encoder (4 knobs for overdrive and 4 for distortion) and the ATmega sets the levels over SPI or I2C. TC have used the +-5V version with 123 dB of dynamic range, which is kind of ludicrous for a distortion pedal.

Lower PCB, backside

U3 generates -12V from 12V input

All parts on this side are glued in placed before soldering

The bottom PCB is very parts-dense, but helpfully there are test points for all supply voltages. I found that -12V was reading very low and so all of the negative supplies that are derived from -12V were also missing. -12V appeared to come from a switching supply controlled by a CS51411 on the underside of the PCB, the circuit looks liked a close match to the inverting converter in the datasheet (Fig 25). The inductor was getting burning hot, so the switching IC was a pretty likely culprit. TC used a double sided load for this PCB, and parts on the underside are wave-soldered instead of reflowed, which means they are all held in place with a dot of red epoxy. Desoldering the IC took a lot more heat and force than I'm used to but it did eventually lift off. It still didn't work with a new chip, solid 12V DC across the inductor with no switching happening, which explains the heating. The SYNC pin did have a ~340 kHz square wave - I don't know if this was also present on the original chip. The huge number of test points on the back of this larger PCB suggests TC use some kind of bed-of-nails jig for testing during manufacturing.

Around this time I held my hand over the board when powered up and found another hot spot. One of the two DG441D switch ICs was also running hot, so I desoldered it. This part does run off +12V and -12V rails, so it may have killed the -12V supply when it failed.

There is another switching power supply using a L5970D controller - this one is generating 3.3V for the digital parts and was working correctly.

As I knew there may have been a short from -12V to ground or some other supply, I wanted to see if I could completely isolate the CS51411 from the -12V supply and use an external power supply to provide -12V. I removed the output capacitor and the inductor. Using an bench supply wired to the board and providing -12V, the pedal worked. Surprisingly, it pulled over 100 mA which seemed very high for some opamps and switch ICs so there may have been some other damaged part on that -12V rail.

The series/parallel switching did not work, only series mode passed a wet signal. This confirms the DG441Ds are routing the signals into series or parallel combinations, one of them was still missing. A new IC here restored all modes.

The CS51411's inductor measured 33uH out of circuit which seemed correct. I decided to try swapping it with this part, which fixed all the power supply issues. I am guessing that the old inductor got so hot that it reached it's curie temperature and it's magnetic properties changed (??) The new part runs cool. Looking again at the CS3308 datasheet shows it pulls 36 - 50 mA on both positive and negative analog supplies (!), which explains the high power consumption seen earlier, and why the pedal originally shipped with a 12V supply rated for 400 mA. This chip does run noticeably warm, but the power consumption matches the datasheet figures. I can only guess that power consumption is targeted  high to help lower noise and increase dynamic range. For a distortion pedal a low-power version with poorer specs would probably be a much better fit. The pedal will actually work with 9V input, and will generate a -9V supply instead, but will need a beefier supply than is usually seen on pedalboards.

All good again.

I broke the ribbon cable connecting the two boards when I was working on this. I just replaced it with individual wires - I would really like a tool that strips ribbon cable so I could just buy a small reel, if anyone knows of one please let me know.

EBS ValveDrive

This is another 2017 leftover, I want to write it down before I forget it.

EBS's ValveDrive is a valve overdrive/preamp that uses a single 12AX7 dual triode. This is the original version, there is a newer model that adds a balanced XLR output and has an internal switching power supply.

 The "vintage" circuit is similar to various Fender Baseman preamps, the "modern" setting adds some diode clipping - the original schematic is available at Freestompboxes.org. It uses a 12 VAC power supply and has an internal 12 VAC transformer wired "backwards" to supply the high voltage for 12AX7 plate. This used to be a very popular pedal with bass players, especially as the high voltage supply means it can get really loud and will happily drive a standalone power amp. Maybe it is not as fashionable right now, I don't see them as often as ~5 years ago.

Like nearly everything that comes across my bench, it doesn't work.

After verifying that my 12 VAC supply was working, I removed the top shield and looked at the valve. No wonder it does nothing, the vacuum has escaped from this one.

I put in a Jet City RetroValve that I keep around for testing, and still nothing. After pulling the valve and probing the socket I measured 0V at the plates - something wrong with the high voltage supply. The heaters are getting 12VDC, so low voltage power should be OK.

A leak has developed somewhere...

The build quality is very good, a nice folded steel enclosure and multiple PCBs connected together with ribbon cable (switching signals) and runs of coax for the audio signals. This was probably expensive to manufacture but it's hard to kill and convenient to work on.

The high voltage transformer is on the lower left of the topside of the PCB. The surface mount fuse on the backside is not blown, but the transformer looks weird, as if it had been re-soldered or replaced. The output winding only measured around 0.5 VAC with the power supply connected.

Transformer pins are not coming through the plated holes...

Additional holes drilled in PCB...

Once it was desoldered, it was obvious why things looked strange. The transformer pins were not going through the plated through-holes, instead someone had drilled holes in the PCB (!) and bridged over to the pads with some bus-bar to make the transformer fit. The low voltage measured at the transformer made sense now, scaling 12 VAC to 230 VAC is a roughly 19.16 ratio. If the transformer was scaling 12VAC down by the same ratio I should get 0.6 V AC, which is pretty much what I had measured.

I reinstalled the transformer the other way round and now measured 230V AC, and 400+V DC at the valve plate with no valve installed. With a new valve installed, this dropped to around 350V, and everything worked and sounded great. The original filter capacitors were only rated to 400V, so I replaced this with 450V rated Nichicons in case this was ever left running with a dead or missing valve.

The part number on this transformer is slightly different from the schematic's part list, and obviously the footprint is different as well. My guess is that someone attempted to replace the transformer, installed it backwards, and then gave up and sold the pedal on. The slightly higher plate voltage(~350V vs 300V) is probably due to small differences between transformers.

Klon KTR

Another quick one. Another pedal that's a lot fancier than I normally get to play with. This is the reissue/sequel Klon that "inspired" the EHX Soul Food and dozens of other klones.

Not of Bill's making, but it probably helps.

The build quality is pretty high, everything is board mounted and the footswitch is on it's own small board with a ribbon cable and snap-in connectors providing strain relief. The only wires are for the battery snap. Despite the "essential" diodes, there are no exotic or outrageously expensive parts. It looks like surface mount film counts are used throughout.

 

The circuit seems to be similar to Centaur traces found online, and pretty close to the Soul Food except for the magic diodes. The 7660 doing voltage doubling and inversion seems to add a lot more ultrasonic switching noise (~26-27 kHz) to the power rails than I remember seeing in the Soul Food.

This one had a bad TL072 opamp (U2). I would guess that someone used a power supply above 9V and the charge pump supplied a voltage above +18V (the upper limit for the TL072) and popped it. The pedal works after replacing it, so an easy enough fix once the problem had been found (I just guessed when everything else around the opamp measured OK). I forgot to take many pictures of this one, the noisy one above is an "after" shot, you can tell the alignment of U2 is a little off.

Side-by-side it's very close to the Soul Food in sound. Connecting to a signal generator and scope, both can do ridiculous 25+ Vpp output with the right input settings and signals, which should overdrive the shit out of pretty much anything.

Electro-Harmonix Soul Food

I don't think I need to talk too much about this one, there's a lot of info online already (not enough pictures though. Photograph your stuff). This is EHX's budget version of the Klon Centaur, apparently with very few changes from the original.

This one arrived dead, nothing happens when power is applied. Probing around with a 9V supply connected doesn't find voltage anywhere. I started at the DC jack, and found that there was no continuity between the tip and the PCB. After desoldering, the DC jack still had an open circuit internally. This was probably mechanically damaged.

The 3PDT footswitch board has the True Bypass/Buffered Bypass switch of the original Klon and KTR.

D2 & D3 do not look like the "magic" Klon diodes. Internet consensus says they are Schottkys.

With a new jack installed it still had no power to the ICs. Diode D4 is connected across 9V to ground as reverse polarity protection, but as there is no series resistor D4 had become shorted, presumably by an incorrect power supply. I pulled it and replaced with 1n4007 as I have a lot of those. Works as expected now.

I would imagine there were two incidents that caused it to end up like this. Someone used a power supply of the wrong polarity and burned out D4, and someone broke the jack by forcing in the wrong connector or stepping on the power cable while it was plugged in.

For anyone else working on these modern EHX pedals, I have found that plastic spudger tools that are sold for opening electronics (particularly phones) work quite well for pushing on the DC jacks to remove the PCB from the enclosure, and don't leave any scratches.