Boss DD-20 repair

[beatnik]

Found a non working Boss DD-20 pedal for very cheap

As I was expecting, the power supply has been damaged.

I have managed to fix the 8.6, 11.5 and 23 V points. The problem now is with the 3.3V point

I took out IC12 an Q17, they are responsible of regulating 8.6V to 3.3V for the DSP and CPU chips

The problem is, these components are now obsolete and impossible to find in quantities less than thousands and I can't find any substitutions either. Strange pin configuration.

What I'm thinking to do now, is just building a simple LM317 based power supply section that will take care of the 3.3V rail

My question is if the output from the LM317 is good enough in terms of current and other parameters to not damage the DSP and CPU chips.

Here you can find the DD-20 schematic, if anyone has time to give it a look really appreciate that

https://www.dropbox.com/s/mkbfe0nptbqcmoy/boss_dd-20-t.pdf?dl=0

[duck_arse]

I don't think the 317 will get down to 3V3, will it? there should be something suitable cross ref wise, lotsa processors run at 3V3. I found this:

http://www.piclist.com/techref/postbot.asp?by=thread&id=Re%3A+%5BEE%5D+5V+switching+regulator%3A+low+quescent+curr&w=body&tgt=post

..... may prove useful. Q17 looks like a power fet. that should be fairly easy ....

I hope this isn't the dd20 I rebuilt recently.

[slacker]

An LM317 will go down to 3V3 the minimum is about 1.25 Volts. I can't see any reason what you suggested won't work, it's probably worth doing it as a test at least, if you're unlucky you might find what ever killed the other stuff also killed some of the DSP chips. If it works then you could look at a better replacement.

Have you got any idea how the power supply had been damaged? The schematic shows a series protection diode (D9) so reverse DC or AC wouldn't kill it, too high a Voltage perhaps?

[beatnik]

i really don't know but the guy who sold it said he left the wrong power supply plugged in for a while....

i think i will try the lm317 route and see if i can make the 3.3 rail work

will let you know how it goes. thanks guys

[PRR]

I do not see an easy accurate way to estimate the current load on the 3.3V rail. I don't even, offhand, see the part-numbers for the CPU, DSP, RAM or ROM, or display controller. If I did, they'd probably be hard to find data on, and maybe in Japanese.

However we know the total load is 9V 200mA.

And the analog stuff on the 22V rail can't be all that much.

So they were chomping, say, 9V 180mA down to 3.3V, with an efficient "switting" supply. With one-thumb accuracy, that could be 600mA at the 3.3V rail.

The LM317 sure can supply 3.3V and at 600mA (to 1A).

One issue is heat. A 6V drop at 600mA is 3.6 Watts of heat. The LM317 tab will not throw-off that much heat, it will shut-down. You want a heat-sink, a 2"x2" square of heavy metal sheet (tear-up some old PCs and look for finned slabs). Space and trapped heat are further issues.

Another is battery life. The original plan was to drain a handful of AA cells in a few hours. Without the efficient switching conversion, this will be less than a 40 minute set.

I would assume that these days you have a ready supply of good 9V power on your pedalboard, so battery life is not an issue. If you commit to that plan, then the huge battery tray is waste space. Cobble-up your heatsink to replace that battery door so it can throw most heat out under the pedal. That's not as ideal as a heatsink on top (heat rising away from the works instead of seeping out under the works), but perhaps the only way it will work. Unless the main case is truly thick metal, and you can find a place to bolt your '317 to it.

Look for "fully insulated" LM317. It will need to be insulated from anything else (especially ground/case). If you must use bare-metal-tab '317, then get the insulating things (sheet and washer), but that's annoying. At just 3.6 Watts, heatsink grease is not essential, and messy.

For pedalboard planning purposes, note that this is now a *600mA* pedal, not 200mA as it says. And 600mA is a heavy load, about half the total capacity of a 1-Spot. If your other pedals are thrifty, one good supply is fine; if you have other power-pigs you may be looking at a second 9V supply.

[duck_arse]

the 29 page service manual has all the dull stuff like part #'s listed on page 10. and 11, and 12.

UPD703105AGJ-019-UEN DD-20 1., TC220CCA0AF-B01(MR3), M12L6416-7T, CAT24WC02J, BU2090F (led) + BU9728AKV-E2 (lcd)

[PRR]

> manual has all the dull stuff

Zzzzzzzzzzzzz...... 

Thanks!

[beatnik]

i've tried plugging 3.3v for the digital power rail through a LM317 circuit but the voltage drop down again

at this point i should consider that dsp or cpu chips are burned so i abandon the project

i will put the pedal for sale on ebay if anyone is interested in the case for a diy project

thanks to everyone helped