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.

DSD-2

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.


9 comments:

  1. I also have a pair of these that need to be repaired. One initially came to life after changing the shorted diode, but died again shortly later maybe from a power supply problem. The second one I don't think I ever got it going, it didn't spring to life with a diode change.

    ReplyDelete
    Replies
    1. I would double check that the large controller chip is getting powered correctly. The traces around the reverse polarity diode are pretty fragile.

      Delete
  2. I have a DSD-2 that I had modified by a guy to run the Delay time 800ms/200ms and the sampler/Rec/play out to an external footswitch so that I could change the delay time from 200ms and 800ms with my foot. This was revolutionary back in the early 80's when it was done...lol;) Any how, I had use for this pedal recently and removed the external switch and wire from the unit. After doing so, the unit does not work correctly. Something is wrong with the trim pot that controls the delay times and sample/record. When the delay time is selected (i.e.800ms) when you press down the pedal the LED light comes on but only stays on while you are pressing it down. Once you release the LED light goes off. So, not sure where to look for a DYI repair. Is this similar to the unit you repaired in this article? Any help would be much appreciated. Thanks - Bob

    ReplyDelete
  3. Other clues: When plugged in and the Delay/Sampler selected to "Record/Play or Play only" the LED light comes on after each depression of the switch but goes off when released. When the Delay/Sampler selected to "Delay" the LED light does not come ON after the initial depression or release but rather it comes ON after the 2nd depression and goes OFF after release. I hope the problem is a simple one that you may recognized. Thanks in advance for reading my post. -BOB

    ReplyDelete
  4. update: After spraying electronic spray into all (4) trim pots the LED light works correctly across ALL (4) positions for the Delay/Play/Rec selector. But, when the Delay side of the switch is selected for either 200ms or 800ms, no delay happens through the pedal. This is the MAIN and only issue with this pedal. Hopefully, I've narrowed down my question to you. THX.

    ReplyDelete
  5. I guess you don't monitor this site any longer?

    ReplyDelete
    Replies
    1. Hi Bob,

      have you tried the calibration routine in the service manual linked above? I would make sure the clocks signals are present and dialed in correctly before looking at anything else.

      Delete
    2. Any step-by-step tips on how to do a calibration routine to check the calibration routine? I'm very handy but not an electronics guy.

      Delete
    3. You will need an oscilloscope and the manual (https://drive.google.com/open?id=0ByVCt2OFhXnyeS1RQWFlUlpWRTA). You will need to probe one pin of a chip with the oscilloscope and check the frequency of a clock signal as you adjust the delay time. If the frequency is off at the low or high end then you will need to adjust some trimpots that are on the PCB. Since you have no delay it is possible that this clock signal is dialed completely incorrectly or is missing...

      Delete