Boss DM-2 Agony, MN3205, V3205

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Crispybacon501
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Post by Crispybacon501 »

Ok need some help here. I have built the DM-2 circuit on breadboard as per the schematic here: https://www.hobby-hour.com/electronics/s/dm2-delay.php

I have 2 x MN3205 BBD ICs and 8 x Cool audio V3205 Ics so hopefully I have ruled out a dodgy IC. Just to clarify the MN chips were bougfht from AliExpress so could possibly be dodgy, the coolAudio ICs were bought from a reputable source.

I have tested the entire circuit and for some reason I am not getting any wet signal from the BBD IC. With the oscilloscope I have traced a 10Khz sine wave (from function generator at the input) all the way to pin 7 of the MN3205 but there his no signal at pins 3 or 4. I have bypassed the IC to make sure that I have signal all the way back to the Opamp so I know that there is no problem anywhere else in the circuit. Swapped out the MN3205 with another and several V3205s with no success. Also just to clarify I have checked for signal with the ongoing wires to VR2 (cancell) pot connected and disconnected.

There is a healthy clock signal on both pins 2 and 4 of the MN3102 clock driver that I am using which I can also clearly see on pins 2 and 6 of the BBD chip. I have set up the frequency between 10Khz and 100Khz as per the setup guide. I have also tried Vdd voltages between 4 - 8v and still no success. I have bypassed the diode on the MN3102 pin1. I have also slowly changed the bias voltage on RT1 (bias) full sweep with no success.

Am I missing something here? The datasheet for the MN3205 is shows than pin 1 is GND and pin 5 is Vdd but the Boss schematic has 8v supply on pin 1 and GND on pin 5?? Any reason for this? Then, when you look on the same data sheet at the “application circuit” the notch is now on the opposite end of the IC, WTF??

I also had a look at other schematics for analog delay such as this one from BYOC: http://byocelectronics.com/analogdelayschematic.pdf

In this case he has the supply reversed? In either case I have tried both with no success.

If anyone can help here I would be very grateful this it quite frankly doing my head in! :)

Thank you

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stolen
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Post by stolen »

Hello!

Oh no... we think we know what happened here... We're very sorry to tell you that you probably have fried your BBDs. MN30xx and MN32xx are not directly interchangeable: It is rather cruel that at first glance at the datasheets they look identical in pinout, but where VDD for MN30xx is negative, it is positive for MN32xx! This is a terribly misleading notation as one would usually expect VDD to be negative. If you have used a BBD with reversed supply (as you have if you have used a MN32xx in a MN30xx circuit) it is very likely to be dead and should not be used for further testing.

If you have any 3205s left that you haven't plugged into the board yet, rewire your power supply to have positive VDD, check socket voltages before inserting the IC and then test again. Also take note that BBDs are somewhat sensitive to static electricity, so we'd recommend wearing a ground bracelet or take similar precautions.

Hope this helps!

PS: We don't know how you're powering this, but bear in mind the limited supply range of the MN32xx series!

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Fender3D
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Post by Fender3D »

:applause: What stolen said.
Another tip, since filters cut-off is way lower than 10KHz, you'd better feed ~200Hz signal to check BBDs functioning, it will dodge any pre/post enphasys filters and anti-aliasing filters cut-off.

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Crispybacon501
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Post by Crispybacon501 »

Guys thanks for the help. Yes you are right I have the supply back to front and I can also see from the BYOC schematic that the 100K pullup resistors on the outputs shoulld now be pulled down to ground.

Ok so the V3205s were still not working so I switched back to the spare MN3205 I had and it worked. I then decided to try the original MN3205 (which I could have sworn I had fried because it was hot to touch) and bizarrely that also worked!! :-)

Ok so more troubleshooting required to get the V3205s working but we are on the right track at least.

Thanks again for your helpful suggestions

B

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Post by stolen »

Crispybacon501 wrote: 26 Jul 2021, 21:23 Guys thanks for the help.
*and girls! You're welcome! Glad it worked out, and it's super interesting one survived, thanks for the feedback.

All the best,
stolen

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Crispybacon501
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Post by Crispybacon501 »

Ok another quick one....hopefully. I had set this aside for a while whilst busy with other things but coming back to it there seems to be an issue with the clock signal from the MN3102. Whilst adjusting the frequency with either of the pots I noticed that the clock frequency seems to jump around and will not increase/decrease proportionally. I disconnected it from the BBD and got the scope on it and its the same. Tried different pots, checked all values etc and can't see what could be causing this. Also tried a different 3102.
Has anyone else had the same issue?
My next step would be to use an alternative to the MN3102 but curious to know why this is happening.

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Post by Fender3D »

There's just ONE pot to adjust frequency or repeat rate. Trimmer is used to set the minimum clock freq. You set trimmer to achieve minimum clock then use pot to select your desired delay

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Post by Crispybacon501 »

Thanks Fender3D I understand that. My circuit is still on the breadboard so I have both the trim pot and external pot next to each other on the board. I have adjusted the trimpot as per the BOSS instructions and set the frequency/time to 146uS but when I then operate the external pot its jumping all over the place so I then tried just about all combinations from both pots to no avail.

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Post by Fender3D »

It shouldn't act like that... either your pot is faulty or dirty, or blame usual breadboard gremlins...

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Crispybacon501
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Post by Crispybacon501 »

Yes it could be gremlins indeed. I already had those problems with the compander ic so mounted it on a socket, the only one i had left though so the other ICs had to go straight on the board. Some creativity required, i'll figure it out. Thanks again

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Post by Crispybacon501 »

Update: If anyone is still interested :-)

This problem went away when I bypassed the 22K resistor on pin 7 (as per the datasheet example circuit). With the resistor in place it seems to pick up a lot of interference which was noticable when I touched it. I'm curious to know why Boss used the 22K resistor in the first place? Perhaps the problem only exists on breadboard, we'll see when I build the pedal.

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