Need advice fixing Boss Dr.55 drum machine

[methodofcontrol]

i have a late 70's analog drum machine made by BOSS, it's a small unit about
8"x4"x1.5". i would really love to have it in working order again!
when i picked this up a year or two ago and it had no problems. i built a battery-clip-to-power-jack coupler for it and i believe i may have reversed my polarity. i don't have the coupler anymore, unfortunately. so i can't confirm 100% the cause of failure, but i was using a reliable 6v power supply so i can't imagine what else would've went wrong.
basically, i hooked up my coupler and plugged in the power source. and when i went to test the pedal, it didn't work. i immediately unplugged the power source and retested the unit with batteries. in it's present state, it makes sound when you press the "play" button, but there's no sequence of sounds. it just makes one sound when you press the play button. i already checked to see if there was a protection diode that blew from reverse polarity, i didn't find anything. and obviously, the unit still powers up, so getting juice is not the issue.
i have a Boss write-up for the drum machine but wasn't able to make it of any use. wondering if anyone can tell me where to go from here?

[Maschinengeist]

Sure thing, the service note will come in handy: http://technopolitan.se/manuals/Boss...e%20Manual.pdf

I can't write much right now about the issue but iy seems to me that you should check the diode D5. I wonder if the memory (IC1, TC5501P-1) has not been affected by the reversed voltage(i.e partially erased). Just a quick theory... don't take my idea for cash.

Find the RAM datasheet here: http://www.datasheetarchive.com/pdf/3913144.pdf

Hope that helps

[methodofcontrol]

thanks for the links!
i already have the service notes, and i wasn't able to make use of them. i'm not sure there's enough information in there for me to trouble shoot the unit w/ a multimeter and probe. perhaps if i had an ocilloscope...

anyway, the datasheet might help. thanks for that!

i'll post back my findings soon, thanks again for the help.

[Sock Puppet]

By the looks of it IC2 and IC3 are the most likely culprits to have died from reversing the polarity. I would change these out as a matter of course.

S.

[nathanscribe]

Interesting. I just picked up a second one of these as a spare/project, and it's doing the same thing.

After opening it up and probing around a bit, I see the counter (4520) is still working - the bits count up fine. In Write mode, the LED lights when it should (at the start of the pattern) and it triggeres the counter - but all I get is a BD sound, along with the hats if they're switched on. This doesn't change no matter what sound the switch is set to.

In Play mode, the flip-flop (half the 4011) does not stay latched up when I press the Start button. The buttons themselves are fine though. I can't get the rhythms to either program properly, or trigger or run.

I connected the trigger inputs of each sound to +V, and the sounds themselves fire up OK, so the analogue side is working.

I suspect a slightly troubled 5501 (memory IC), which over here costs about £10 to replace if you can find one. I need to do some more testing though. Can't see any shorts or broken tracks.

One option I am considering is to turn this unit into a module with trigger inputs for each sound (if the 5501 is dead) or to add Sync24 input if it isn't.

Any thoughts?

EDIT: Cleaned the tracks associated with the clock and flip-flop, and now that part works properly! There was no visible short-circuit, but obvioiusly something was awry. What it does now is sound all the sounds on every step, whether in play or write mode - but at least I have the two modes running correctly. I'll try to clean the rest of the board later, but I still suspect the 5501.

EDIT 2: BTW, there are some errors in part numbering in the service manual. I'll list those later.

[methodofcontrol]

man, i have been SO lazy and/or busy regarding this project. thanks for reviving the thread it made me remember that i've got to fix this thing!

nathanscribe....it sounds like you're getting very familiar w/ the unit! i hope you don't mind if i pick your brain once i can sit down and open it up again.

what should the voltages be on IC2 and IC3? i'd like to check them before replacing anything that might not be broken. thanks!

[nathanscribe]

what should the voltages be on IC2 and IC3?

If you turn on the unit, set it to PLAY and check IC3 (4011) pin 3, it should be low (0V). Hitting START should sent it high (5V or so). STOP should send it low again. You'll want to check that pins 1 and 4 follow your button-pushes, each going briefly low when you press START and STOP respectively. The output of the flipflop (pins 1 to 6, the o/p being 3) trigger the clock (pins 8 to 13). Put the machine in PLAY mode. Check pin 11 - should go high when you push START, low at STOP.

The counter is IC2 (the 4520). Pins 11-14 should go high and low in a binary pattern to represent 0-15. These pins provide the step number to the memory. If you set WRITE mode, and push the buttons (either will do), trace the high/low pattern of these pins. Bit 1 is pin 11, bit 4 pin 14. The bits should count up one each time you press a button.

Supply voltage should be 0V on pin 7 of the 4011, 8 of the 4520, with positive (5 to 6V) on pin 14 of the 4011, 16 of the 4520.

Supply voltage on the memory IC should be pin 22, ground pin 8. The write inputs are pins 9, 11, 13 and 15; the trigger outputs pins 10, 12, 14 and 16.

Regarding errors on the service book schematics, if you look at the lower left corner of the diagram you'll see a diode and resistor connected to IC3 pin 6. The diode is labelled D18 - read it as D19 instead. The resistor is labelled R78. There are 2 of these, so I called this one R78A. The other R78, R78-B in my designation, is connected to IC3 pin 2.

Hope that helps for now.

[methodofcontrol]

ok i checked voltages on the 4011 and the 4520. you lost me a little on the memory IC, i'll get to that in a moment.

RE: 4011 (IC3): machine set to "play" mode:
pin 3 was 0v at idle and jumped to 5v when i hold down "start". if i just press and release "Start", the voltage stops increasing the moment i release the button.
pin 1 was 5.8v at idle, dropped to 3.2v if i hold "start".
dropped to 5.3v if i hold "stop".
pin 4 was 6v at idle, dropped to 5v if i hold "start"
stays at 6v if i hold "stop"
p11 was 0v at idle, up to 6v when i hold "start"
no change when i hold "stop".

the 4520 seemed to react as you predicted. in "write" mode, i start at pin 11 with a 0v reading. when i push "Start" it goes up to 6v. i push start again and the 6v reading moves to pin 12, with pin 11 returning to 0v. i was able to find the 6v reading on a pin between 11-14 (sequentially) after every time i hit a button.

supply voltages on IC2 and IC3 were as you had indicated they should be.

on the memory IC, pin 22 was 5.5v and pin 8 was ground. but i wasn't sure what to do with the "write inputs" and "trigger outputs" info.

thanks for the edit on the service diagram, makes sense.

where should i go from here? unless i'm missing something, IC2 and IC3 seem ok.

[nathanscribe]

Hmm. Sounds like there's something going on with your start/stop section.

Regarding IC3 (4011): pin 4 should be high (5 or 6 volts depending on battery levels) normally, and low (0V) when start is pressed - it should continue to be low when start is released. When stop is pressed, it will revert to high.

Pin 3 is the opposite - normally low, but goes high when start is pressed and released. Pressing stop will send it low again.

Pins 3 and 4 are the "truth" outputs for the run status, if you like: 1 & 0 respectively for running, 0 & 1 respectively for stopped.

This is in PLAY mode.

Check the tracks/components for shorts or bad joints etc., clean the PCB around the associated components (times I've 'fixed' something "broken' just by cleaning it), and maybe replace the parts if that still doesn't help. To be honest, I haven't paid much attention to mine in the last couple of weeks. I'm considering just forgetting about fixing it for programming, and adding some trigger inputs instead, which I could trigger from various interfaces/other drum machines. It's such a cheap machine, I don't know if it's worth spending weeks troubleshooting! Shame to see 'em keel over though.

[methodofcontrol]

do you think the 4011 itself is bad?
the traces and pads all look great, i can't imagine what i'd be cleaning off that could be causing any problems.
and don't forget....the unit worked perfectly until i carelessly tried to hook up a wall adaptor. (i may have reversed the polarity when doing so)
i'm wondering if the 4011 took a hit of wrong polarity even though the protection diode is still ok.
i'll check and clean the traces. but i'd like to hear any opinions about the possibility of a bum 4011 (IC3).
thanks again!

[nathanscribe]

The datasheets for the 4011 and 4520 are readily available online for free at various places, just stick'em in Google and see what comes up. From those, you should be able to see how they work. The DR-55 schematic shows how the start/stop setup goes together, and the service notes detail readings at various points in the circuit as they correlate (the diagram with the square wave type readings lined up). If you've only got a multimeter and logic probe, you might have problems accurately reading things. An oscilloscope is your best bet. Multimeters don't respond fast enough to get small pulses, and the readings you get might be misleading. Your logic probe should help you debug/test the logic ICs.

Beyond that, I've said as much as I usefully can up to now. I need to poke around in mine some more. Keep us updated on what you try and how it goes.

[methodofcontrol]

when it comes to oscilloscopes, i definitely don't have "experience with" or "access to" .
i'll see what i can do w/ the 4011 datasheet, but i'm having trouble finding one. google gives me several hits, but none have taken me to a datasheet yet.
if you happen to know of a good site in particular to check, please let me know!
thanks again!

[methodofcontrol]

i'm happy to report that my dr55 is back up and running.
a few weeks ago i ordered a 4011 IC. i put it in this afternoon and my unit came to life. thanks for everyone's help!
i ordered 2x 4011's to be safe, so i still have 1x new one i probably won't need. nathanscribe....i'm wondering if you need one of these for unit? let me know. i know you said your dr55 was working after you cleaned some traces, but if it's not reliable...i can send you my extra chip.
thanks again guys!

[Enzo]

For future reference, if you found a 4011 - or any other IC you ever need - from a vendor, there is a good chance there is a link on the vendor's page for the part to the manufacturer's data sheet. Mouser for example will have a little box that says "data sheet" next to the part number for most parts.

When I need a data sheet, I often start by looking the part up in Mouser and using their links.

[methodofcontrol]

thanks enzo.
i forgot about that info. consider it noted for next time!