Boss delay DM-2 question

[tubenit]

I've built at least 8 pedals over the years with success, but I have almost NO idea how to trouble shoot them?

My friend has one of the original Boss DM-2 pedals. The pedal works. It is the sweetest warmest delay I've heard from the 6 different delay pedals I've played.

The ON/OFF switch works fine and will turn the delay on and off.

All the pots work fine.

It sounds lovely.  Almost like a warm delay with a splash of reverb in it. Sorta hard to describe but a beautiful tone.

However, the LED bulb does NOT light up when the pedal is on.  I measured some voltage there on the solder terminals of the LED light.  You can tell it's on by the tone (similar to a wah pedal) but no ON indicator light comes on.

What is odd to me is if I touch the ground of the 9v adapter terminal to ground that the bulb will actually light up. Which informs me the bulb is working.


I am not adapt at reading solid state PCB board effect pedals schematics.
Boss DM-2 Delay Guitar Pedal Schematic Diagram (hobby-hour.com)


I am wondering two things:

IF the input jack is wonky could that prevent the LED light to come on?  It's the original (maybe 1983?) jack and looks to be cheap plastic.

IF the CLR  current limiting resistor and/or diode is bad could that prevent the LED light from coming on?

I know I could move all the guts into a new pedal box and use a 3 DPDT to install another LED light, but the owner doesn't want to do anything like that.


Thoughts? 


Thanks, Jeff  aka Tubenit

[pdf64]

I think the collectors if q6 and q7 should work in opposition, when one pulls low, the other goes high.
Can you note the V DC on each with the effect on and off?
V DC with respect to the battery negative / case metalwork.

[tubenit]

pdf64, 


I appreciate your willingness to help me!  Thank you!  I can see that I am quickly (maybe instantly) getting in over my head on this. 

I have no idea what/why the term collectors?  Well beyond my understanding. I realize from the schematic info those are silicon transistors but I don't know where the measure voltage & on which leg?  Or whether it would cause a problem if I measured on the wrong leg.

I'm taking it that they shut on and off and perhaps play a role in the shutting on/off of the LED bulb? 

You've given me enough of a response to realize that I need to simply return this "vintage" pedal back to the owner which is actually a useful response from you so thank you.

IF it were my own pedal, I'd pursue trying to troubleshoot and risk breaking it permanently but since the pedal works ....... I'm not gonna take on a risk that the owner would lose the ability to use the pedal at all. I don't think he'd be very happy with me.


Thanks, Jeff

[pdf64]

Sorry, I’ll dial it back a bit :)
See the 56k resistors R42 and 43?
The transistor leg they connect to is the collector.
We’re looking for the voltages at those circuit nodes in bypass snd effect engaged modes.

[sluckey]

Look at this before you give up...

[tubenit]

pdf64,


I'm thinking you're meaning R62 and R63 which have 56k resistors?  I'm running out of time today to troubleshoot but will measure voltages tomorrow and post them.  Thanks for the help!


Jeff

[tubenit]

Steve,


OK, yep I will do that 1st chance tomorrow. I've run out of time today.  I was interpreting those as a current limiting resistor circuit?  Would that be correct?

I'm embarrassed to own this ............... 

Checking the resistor involves measuring it and making sure it's value is correct. Right?
What am I looking for in checking the diode? Never mind, I found a YouTube explaining that.


That zener diode is 5.6v.  I have one on hand that is 12v and 500ma.  Can I substitute that if need be? 

Thank you,  Jeff 

[sluckey]

Yes, R40 is the current limiter. D3 drops the battery voltage a certain amount regardless of the amount of current flowing through it. So just think of the zener and R40 as series current limiters. You must unsolder one end of the zener to check it. Then just select diode check function on your DMM. Connect probes across the diode. You should get about a .5 reading or OL. Now swap the probes around and you should get an OL or about .5 reading. IOW, a good diode will read about ,5 in one direction and OL in the other direction. If you get the same reading in both directions the diode is bad.

While you have one end of the zener disconnected check R40. I suspect either the zener or the resistor is open.

[tubenit]

Thanks!  I am understanding the direction you're giving me and can follow that well.

Will trouble shoot it tomorrow and post the results.  THANK YOU!!  This is a very very nice delay pedal, IMO. 

IF needed, can I substitute a 12V 500ma zener diode in place of the original which is 5.6v?   

IF not, how about a 6v zener 500ma which I can get off Amazon.

Grateful,  Jeff

[sluckey]

A 12v zener would never conduct in a 9V battery circuit. Therefore the LED would never light up. 6V zener would be fine. Personally, I think using a zener in the current limiting circuit that only lights a LED is a bit obsessive. I would throw the zener away and just replace R40 with a 5.6K resistor. Replace the zener with a jumper wire.

[tubenit]

I would throw the zener away and just replace R40 with a 5.6K resistor. Replace the zener with a jumper wire.

That definitely works for me!  I've never used a zener in the CLR circuitry of any of the pedals I've built. Only used a resistor.

I'll repost later today what I find out.   THANKS!   I'd love to get this fully working right for my friend.

Jeff

[tubenit]

I am testing everything with a jack plugged into the input jack on the pedal.

Q6 & Q7 alternate between 590 millivolts and 0  as I push the on/off footswitch.

3.9k is correct for the CLR resistor

The zener alternates between 847 on one end and reading OL on the other.  I did not lift one end of the zener up to test this. The reason was the node 14 was not changing voltages so I wasn't certain if the zener diode test was called for?

node 14 reads 21-22mv regardless of whether I have the on/off switch in either position. NO change with the switch.

On the back of the AC adapter there are three posts.  Two red ones. And a blue post. My understanding is the blue post is a ground connection?

IF I jumper that blue post to the pedal casing which is ground. Then the light goes on/off with the footswitch going on and off.

Grateful for the help!   Not sure where to go from here?

IF it were my pedal, I'd put it in another effects enclosure and use a 3 DPDT and have the light and CLR bypass the PCB board.  However, my friend wants to keep everything vintage.

Thanks, Jeff

[tubenit]

See my "testing" above post.


I discovered that if I jumper the "ground" on the AC adapter to the effects pedal casing, that everything works. Light works, pedal delay works, potentiometers work.

Any problem simply doing that and putting the pedal back together? 

I'm wondering if this could be problematic for the LED light and would be bypassing the CLR circuit?  The light doesn't seem bright jumpering to ground. It looks pretty normal to me.

Thoughts???

Jeff

[pdf64]

That indicates either the input socket sleeve contact isn’t making a good connection to the case, or the input socket ring contact isn’t making a good connection to the sleeve.

[tubenit]

 I checked with my voltmeter and there is a good connection between the input socket sleeve and the case. And with a short plug in, there is a good connection between the sleeve on the other end of the jack not plugged in with the case.

What about this idea of simply grounding the blue terminal on the AC adaptor?  Right now I'm using a battery checking all of this out?

Would there be an issue in using a AC power supply  OR a concern about bypassing the CLR circuit? 


OK, I tried an AC power supply instead of a 9 volt battery and this seems to work well.

Kinda grasping for straws here ...............

Thanks, Jeff

[pdf64]

Are D1 and R2 good?
You should be able to check them in circuit.

[tubenit]

D1 is good. The only resistor connected to D1 is a 330 ohm marked both by the color band and measured by the voltmeter and it is good.


What about the idea of grounding that blue wire off the AC supply adaptor?  Everything seems to work fine with that approach.


See any downside or concern with using that to resolve the issue?


Jeff

[sluckey]

I would forget about the ac adapter and just use a battery. Put a jumper between pin 14 and pin 12 on the board. A paper clip would work fine because you only need to touch the pins and then release. The light should come on with the jumper regardless of whether the effect is on or off. Does it?

[tubenit]

Jumpering pin 12 & 14 does not allow the light to come on. I did the foot switch numerous times and have a jack plugged into input.  To make sure the battery hasn't worn down, I jumpered that blue terminal on the AC adapter to ground again and the light instantly came on.


 



This makes no sense to me?!  Since pin 12 is ground .................. why wouldn't this work? But it honestly doesn't. Now I am wondering if the output jack ground is not good. I'll check that out and report back IF I learn anything new about it.
 

Any problems you can think using this approach?  I have grounded that on the effects pedals I've built in the past and it worked just fine. See attached diagram. This pedal has two red and one blue instead of two red and one black.


The reality is that I almost never use anything except a delay or reverb. Once in a while, I use a wah or overdrive but honestly never more than two pedals.


In contrast, my friend has a big pedal board with maybe 9 pedals on it all run on AC adaptors. So, he's not gonna want to use a battery.   


With respect and gratitude, Jeff

[pdf64]

… I jumpered that blue terminal on the AC adapter to ground again and the light instantly came on.


Any problems you can think using this approach? …

Sorry for overlooking this, I can’t think of a problem with it, it’s just it’d be nice if possible to find the actual fault.

[sluckey]

...it’d be nice if possible to find the actual fault.

I totally agree.

This ain't complicated. Make the checks in the attached pic.

[tubenit]

Well that was interesting!   Using an 8.85v battery.

D1 was ZERO ............... NO volts at all! 

Until, I grounded that blue terminal on the AC adapter.  Once that was grounded then:

D1 was 8.63v
juncture   5.06
pin 9     5.05


Also without the blue terminal grounded, the voltage on pin14 did not change. With the blue terminal grounded, the voltage did change from .03v to 5.6v using the footswitch to go from off to on etc...

Whatever the reason is ............. that blue terminal on the AC adapter needs to be grounded and when it is everything works properly including the delay, the on/off bulb and potentiometers. Seems to work fine with battery and AC adapter.

My friend and I were jamming when the delay pedal "went out".  It was on a board with I think 8 other pedals?

My guess is that whatever method Boss used to ground everything, that the original ground broke but doing this restores it.

I've spent a bunch of hrs on this. Unless there is a really solid reason to continue troubleshooting, it seems reasonable to me to ground that blue terminal and call it a wrap. What I mean by that is IF you don't see a direct logical concern using this approach, it makes sense to me to go with this solution and use the pedal and watch what happens.


Does that seem reasonable?    Jeff

[sluckey]

D1 was ZERO ............... NO volts at all! 

Until, I grounded that blue terminal on the AC adapter.  Once that was grounded then:

D1 was 8.63v
juncture   5.06
pin 9     5.05

Nobody talking about D1. The diode I was interested in is D3.

What's this blue terminal on the AC adapter? The schematic you posted only shows two wires coming from an AC adaptor. And when you plug in the AC adapter, the battery is disconnected by the switch in the AC input jack and the AC adapter feeds the rectifier/filter (D4, C37, R27, and C38) to produce the voltage to run the unit.

If this unit was working properly you would not need an AC adapter. Just unplug it and put it out of sight. Troubleshoot using the battery only.

[tubenit]

I would like to convey that I am truly very grateful for the help and I've tried to follow the advice given to me.

This is a lovely sounding pedal and my guitar buddy is a good friend so I've wanted to find the original problem and put everything back in order.  I have had NO formal training ever in electronics. Zero. None. So while I can build an effects pedal following a layout diagram, I actually have very very little idea of what I'm doing and why it works.

My apology.  I typed the wrong diode.  It was D3 that is part of the CLR circuit & connected to the 3.9k resistor that I checked ........ not D1. Sorry.

I am using the battery NOT the AC adapter for every test that has been suggested.

And I also then after testing hooked up the AC adapter to see if that worked also when playing thru my amp. But doing the tests that were suggested, it was the battery alone.  The AC adapter was not involved in the testing just the battery.


In other words, as long as I had the "blue terminal" of the AC adapter grounded to the pedal casing .......... the battery alone worked AND the AC adapter without a battery also worked.  Everything worked properly that way.

I have built around 8 effects pedals over the years and I always grounded the AC adapter as shown in the below diagram. This Boss pedal was not grounded there, so I gave grounding it a try and everything works perfectly from what I can tell?

So my presumption (which may be dead wrong) is that whatever ground connection this Boss pedal had wasn't working properly?

Respectfully and with gratitude,  Jeff

[sluckey]

Finally I understand. The blue wire is on the power input jack, not on the AC adapter. Thanks for helping me understand. I have been thinking the AC adapter is the external transformer that plugs into the power input jack.

[pdf64]

Whatever works, works  :dontknow 

[tubenit]

Gentlemen,


I'm very grateful for the help!  I know more about troubleshooting an effects pedal than I did before. I was eventually able to confirm certain junctures of components were working properly.  ALL good! Thank you! It was a useful process to be in.

In the end I have a working delay pedal to return to my friend. Perhaps he will get some more yrs out of it?

The communication was stifled on my end by not knowing what to call components and junctures, by not knowing what certain things did like diodes, by accidently typing D1 instead of D3 ....................... and so on....   It wasn't intentional but I made your roles much more difficult in helping me. I apologize for that.

You guys were phenomenally patient!      I would have given up on me early in the thread and you guys hung in there. Super appreciative of that and the generous information and help that is offered.

Steve, you especially have gone the extra mile with me and bailed me out of more problems and trouble shooting than everyone else combined. That is a generous gracious gift and not one I take for granted. Your kindness and help goes back 15 yrs now.  Wow!


Best regards and my respect, Jeff