Sunday, 27 August 2017

Boss DD-2/DSD-2 repairs

Once again, I have a backlog of photos of things with very poor notes - hopefully I haven't forgotten everything.

Early Boss Delays

I've played a few of the early Boss delays, but oddly enough I've had none of the more common modern versions (DD-5/6/7 etc). The DD-2 is regarded as the first digital delay in a stompbox, using some of the tech from Roland's larger rackmount delays (see Boss's article on the history of delays). The architecture is similar to early Japanese delays, with one large custom controller IC doing all the digital work - a sigma delta ADC (with an external comparator), DRAM interfacing & parallel outputs for an resistor network DAC. This is the same RDD63H101 custom used in the famous Roland SDE-3000, often referred to as the "long chip" as it barely fits in the Boss stompbox case. Everything is synchronised off a single master clock which is directly controlled by the delay time knob - changing the delay time smoothly pitch-shifts whatever audio is memory without any glitching.

Digital section, from Boss Service Notes

Unlike many other 80s delay pedals, the audio is quantised to 12-bit samples (8 is more typical) for lower noise and there is also more memory than I would expected, 3 64k DRAM chips (a single 64k IC is common in some 1 second digital delays from the era).

Parts of the schematic look more like what would be expected from an analog delay, there is a NE570 compressing on the way into the digital delay line and expanding on the way out to further help with noise, and pre-emphasis and de-emphasis filters at 7 kHz to hide any sampling bandwidth limitations.

The custom chip is also a weak point in repairing these. If it's dead then the only way to find a replacement is to pull one from another pedal. I am hoping that most of these are still working and that faulty pedals just have power or memory issues.

DD-2 #1

This was another eBay purchase. It powered up, but only produced a whining noise. The noise would change in volume with the Level knob and change in pitch with Delay Time knob, so the digital end was the first suspect. Power from the 5V regulator looked good.

DD-2 #1

I had seen something like this before on an 80s Digitech PDS delay (write-up to come at some point in the future). In that case I was getting massively distorted repeats and I narrowed things down to bad memory, data was beings written in but garbage was being read out. I also knew that DRAM failure was very common in the late 70s/early 80s so I decided to try swapping out new memory chips on this DD-2.

All memory ICs desoldered
The 3 DRAM ICs are directly below the main controller. These pedals are absolutely crammed with through-hole parts, it's impressive that so much could be fit into the standard BOSS housing when you consider how much more simpler the earlier Boss pedals were in comparison.

Socketed memory ICs - a dead end.

Initially I tried to install sockets to make troubleshooting easier. After a struggle to get socket into the board I realised this wouldn't work, the flying leads over ICs would need to replaced with longer wires and it was possible the PCB wouldn't fit back inside the case.

Replacement memory installed.

I removed the sockets and installed 3 MK4164 64K DRAMs with a compatible pinouts. Switched the pedal on - absolutely nothing had changed. Shit.

Going over the each pin of the controller IC with an oscilloscope, I found that even though 5V was present, when I had a test signal connected to the input (a triangle wave in this case) I could see the triangle wave riding on top of the 5V supply! I traced this back to a cracked solder joint on the 5V regulator, which was then reflowed. The power filtering electrolytics were replaced at the same time. Another delay working again.

DD-2 #2

This much dirtier DD-2 was not mine, but was bought broken at a market for €5(!) and eventually found it's way to me to take a look.

DD-2 #2

This was a lot easier to figure out, the pedal wasn't lighting up and was shorting out my power supply. The reverse polarity protection zener diode (D6) was burnt out and failed as a short. It looked like someone had been in here before and had tried to jumper over a PCB track with a piece of wire and some cold-looking solder joints.

Previous attempt at a fix
 After removing the jumper and the diode (which tested as a short out-of-circuit) the board looked a little rough. I added a 1n4001 as a replacement and tried it out.

Some PCB foil damage
This time the LED would only come on when the pedal button was held down, it wouldn't latch and stay on. Usually Boss use a discrete flip-flop for switching (an excellent Geofex article on this) but in the DD-2 they use a BA634 flip-flop in a SIP package, probably just to save some real estate on the PCB. I thought this IC was bad but it just had a broken ground trace - another small jumper wire fixed it.

Both of these pedals sounded identical, and when I checked the calibration routines in the service manuals they were both still perfectly dialled, with max delay coming in at 800ms on both.

Bonus Repair: Boss DSD-2

This pedal is a couple of years newer. The problem was pretty obvious, all of the 1/4" jacks and the DC power jack were broken and had to be replaced. I can only guess someone was putting the pedals into a bag without unplugging the cables and they managed to break them all.


Guts - note broken jacks everywhere

What is interesting is how close the DSD-2 is to the DD-2. There are a couple of part changes for cost and space savings (the RAM chips have been replaced with SIP packages that are a better use of space) but it's basically the DD-2 without the hold mode and with an external trigger input instead as a "sampler" mode. The service notes even shows that the only differences in the digital side is that a couple of pins are wired differently on the controller to trigger a delay from the Trig In jack instead of the footswitch.

The DSD-2 should perform identically to the DD-2 in delay modes, and to my ears they sound identical. If you like this sound it may be worth finding a DSD-2/3 as they don't command the same prices as the DD-2.

Monday, 14 August 2017

Electro-Harmonix Pog (Big Box)

Following the Nano POG & POG2, here is the original big box Electro-Harmonix POG. The big POG has mostly the same set of features as the POG2 - it doesn't have presets and the filter has fewer settings, but there are separate detune sliders for the 1 & 2 octave up harmonies.

No slide pots were placed this time
In keeping with a trend I've noticed with EHX products, it's completely different on the inside than the other POG pedals. The POG2 and Nano POG use Analog Devices BlackFin DSPs, the OG POG is much more late 90s and has a DSP56364 and a 8051-style P87C52 MCU, just like some Line 6 products, or the Digitech Whammy. I have no idea why this cheap microcontroller family is so commonly found alongside the DSP56K series but I'm starting to suspect there is an application note somewhere that shows how to use these parts together. The AK4552 codec and 2272 & 5532 opamps are also very familiar by now. A TLV1543 ADC reads out the slider positions.


This pedal is also an outlier in that the complete factory schematic is online at Freestompboxes. The P87* series MCUs are OTP (but could be replaced with a compatible programmable part), so I have dumped this one. There is no other programmable part on the board so this should hold firmware for both the MCU and the DSP. The binary is here.
There are a lot of unpopulated parts in the PCB layout, some may be used for development and some look abandoned. There are footprints with silkscreen labels that indicate that they are for MIDI control and the schematic also indicates MIDI was planned at some point. I didn't experiment.

Trashed slide switch

No complicated fix for this one, once I had rigged up an 18V supply I found that the LPF mode switch was broken and a couple of wires looked like they were about to come off the board. After some solder touch ups everything worked fine. It sounds very much like the POG2, but as I don't own one anymore I couldn't do a direct comparison.