Boss ODB-3 bass overdrive

Hosebass57 · Post by **Hosebass57** » 19 Oct 2020, 22:48

Greetings

Re: Boss ODB-3 intermittent sound.

I have recently found in my collection of pedals, one of them.

The problem is that when I step on the pedal to turn it on, the sound is faded. It sounds like the effect is only kicking in very little. If I keep my foot down on the pedal, the sound Is perfect. I believe something is wrong in the switching circuit. I've tried looking for a schematic but haven't had any luck. Any insight would be helpful.
I'm using a Voodoo Labs ISO 5 power supply to run it

Thank you!

Post by **The G** » 20 Oct 2020, 13:17

Google Search "boss odb-3 schematic" > Images:

DIYStompboxes.org: Boss Bass OD; any mods??

: From diystompboxes.org

Imgur: Boss ODB-3 low gain smooth mods

: From imgur.com

guitarristas.info: Boss Bass Overdrive ODB-3 (needs registration)

: From guitaristas.info

Others:

musicforums.ru wrote:[...] you change location C10 to 1uf or 2.2uf to add mids,warmth and fatness, C8 to .001uf to remove highs that make your ears bleed,and D2,D3 to 1n4148 this will yield less distortion and allow more clean tone coming through, and a little smoother!! let me know if this makes any sense!! it help get me closer to tim C. sounds from audioslave!! that boost with a little bit of overdrive but still full and articulate [...] (link)

Hosebass57 · Post by **Hosebass57** » 20 Oct 2020, 16:48

Hi,
Thank you for the schematic and mods. I do appreciate it.

Although this is great, the schematic doesn't show the foot switch section. I want to repair the problem of the intermittent sound.

Hosebass57

Post by **The G** » 20 Oct 2020, 22:01

Very true.

But looking at the gutshot, it seems that ODB-3 is using the same JFETs switched by a BJT flip-flop circuit as most of the classic Boss pedals (R.G. Keen's article here).

: The purple wire seems to go to the switch.

So, the only part not shown would be the flip-flop containing the two BJTs and the associated passive components. That's where I would poke. I'd disconnect the flip-flop first and check that by manually biasing the JFETs, the switching works as it should. If it does, the flip-flop needs more poking, if not, the JFETs need more poking.

george giblet · Post by **george giblet** » 21 Oct 2020, 00:57

There's no doubt it has the common two transistor flip-flop. Here's a good schematic (no diode in the emitters on the ODB-3).
The ODB-3 is likely to have another the 1M, 47n cap, and Diode off the other collector

[Warning: CE-2 uses the older flip-flip circuit. New ckt has no emitter diode and has 2x220pF going to the switch, 100R on the switch not 1k;
checkout a modern schematic.]

The key point is you say the pedal works if you hold the foot-switch in. That's weird behaviour as the flip-flip toggles and the switch is coupled to the transistors via two 470pF caps. A solder bridge or junk on the board around the 470pF caps would do it. A bad solder joint might do something weird as well.

For the ODB-3 the flip-flop is circuit in the bottom left-hand corner, around Q14 and Q15. It shouldn't be too hard to find the two 470pF caps. You want the caps that are going to the 1k+ switch.

Hosebass57 · Post by **Hosebass57** » 21 Oct 2020, 01:57

Hi
Thanks to all for the input. Much appreciated! I'll keep you posted.

george giblet · Post by **george giblet** » 21 Oct 2020, 09:14

The 2x470pF caps to the switch are actually 2x220pF caps on most of the Boss pedals.

Hosebass57 · Post by **Hosebass57** » 22 Oct 2020, 05:26

Just wanted to update everyone about my little project.
I finally got the opportunity to open the pedals up. Was I in for a suprise.

Someone was trying to do a Wampler bass mod and was not very good in doing so

Cold solder joints in several areas. I have even found parts not soldered in.

Well, at least I have something to do this weekend!

Thanks again for everyone's input

george giblet · Post by **george giblet** » 22 Oct 2020, 11:09

Frightening. And all this time you were thinking it's a stock unit.

The best thing about the problem is it has given you an opportunity to clean-up the mess.

Hosebass57 · Post by **Hosebass57** » 23 Oct 2020, 18:00

Greetings.
Back again!

I’m doing the wampler mod for the ODB-3.
I want to a mini toggle switch with different parts for D2 And D3.
I want to put the pair of Red LEDs,( like stock version) and a pair of 1n4148’s. ( Wampler mod).

I’m not sure how to exactly to wiring this switch. Any suggestion on how to wire the parts to the switch would be appreciated

I also need to know where to put the switch.
Viewing the diagram in the previous post, do I add the leads to the D2 spot or the D3 spot?

Thanks in advance!

Manfred · Post by **Manfred** » 23 Oct 2020, 19:08

You can add the 1N4148 diode pair to the LED pair connect in parallel.
Since the LED's have a higher threshold voltage than the 1N4148 diodes, they have no effect when the switch is closed.

Hosebass57 · Post by **Hosebass57** » 25 Oct 2020, 19:53

Good morning

So, I sat down this weekend and finally got the pedal to work.

Mostly cold solder and made sure all componets were soldered in correctl..

It now works like a charm. (I did the Wampler mod)

i would like to see if there are any mods for the balance control circuit.

The sweep on the control is strange ( distortion kicks in when the blend control is at 8-9 o'clock Is there a way to have it at 12 o'clock with equal levels between clean and distortion?

Thanks in advance

Hosebass57 · Post by **Hosebass57** » 25 Oct 2020, 19:59

Greetings

I wonder if I can just replace the Blend knob with 250k M/N post?

george giblet · Post by **george giblet** » 26 Oct 2020, 11:25

i would like to see if there are any mods for the balance control circuit.

The sweep on the control is strange ( distortion kicks in when the blend control is at 8-9 o'clock Is there a way to have it at 12 o'clock with equal levels between clean and distortion?

I wonder if I can just replace the Blend knob with 250k M/N post?

An M/N pot probably isn't the way to go since it basically shorts.

The 47k resistor R28 sets the amount of blend when the Balance pot is in the centre. If you want to knock-back the drive level you could increase that resistor to say 100k, 220k, or perhaps even pull it out. If your saying 9 O clock to balance then the range is probably more 220k to removing it. Easy to do by ear.

The pot actually loads the filter C10, R22 etc depending on the position of the pot so it's a bit tricky to suggest a mods doesn't change anything since the circuit is changing depending on the pot position. All I can say is the effect should be fairly minimal.

One caveat, the part references I'm using are for the top schematic. I do not know if those part designators are the same as the actual lboard. Some drawn-up schematics have part designators which do not match the actual board. So when I say the 47k resistor, it's the one across the balance pot *like* R28 in the top schematic.

george giblet · Post by **george giblet** » 26 Oct 2020, 12:48

Perhaps a slightly better or equal way is to add another 47k (tweak to taste) resistor just like R28 but on the other two terminal of the balance port. So add a 47k (or whatever) across the counter-clockwise and wiper terminals of the balance pot. The reason it might be slightly better is this set-up is loaded less by the following stage when the Balance pot is centered.

DavidRavenMoon · Post by **DavidRavenMoon** » 15 Jun 2022, 21:18

Here's a question... I did a partial Wampler mod. One of the things I did was swap the LEDs for 1N4148 diodes. That sounds much smoother to me. I also changed C10 = .1µF and
C8 = .033µF, the second one after trying a few values. I like the tone, but I'd like MORE distortion, so I'd like to increase the gain on the clipping stage. I'm not all that knowledgeable on transistor circuits though, Which parts would I change for a little more gain? I'm assuming it's one of the resistors.

george giblet · Post by **george giblet** » 15 Jun 2022, 23:18

I'd like to increase the gain on the clipping stage. I'm not all that knowledgeable on transistor circuits though, Which parts would I change for a little more gain?

Many later Boss pedals like this one use discrete opamps: 2xJFET input stage (with some gain) and 1xPNP gain stage. Look at these circuits as if they are non-inverting opamp circuits. In this case, referring to the schematics earlier in the thread, the gain setting resistors are:

Gain Stage 1: Feedback Resistor 2k2 + half of 250k gain pot, Ground resistor 1k5 (and 10uF cap).
Gain Stage 2: Feedback Resistor 100k resistor, Ground resistor 1k8 (and 10uF cap).

gain = 1 + Rf / Rg; Rf = feedback resistor, Rg = ground resistor

To increase the gain, increase the feedback resistor or decrease the ground resistor just like you do on a non-inverting opamp circuit. One big caveat through these discrete opamps have limited "open-loop" gain in the sense the actual gain levels off before an opamp circuit. So if you set Rf to a very high value or Rg to a very low value there will be little effect on the actual gain. If that occurs, in order to maximize the gain of a two-stage amp it is best to set the maximum gain of each of the stages to the same value. That minimizes the chance of the gain open-loop pooping out.

FYI, if you are making DIY versions of these discrete opamps the choice of two JFETs in the discrete opamp is important. It has a strong impact on the open-loop gain. Boss have chosen those JFET carefully to help maximize the gain. The wrong JFETs will cause a noticeable loss in gain compared to the original.

Having said that, I suspect you might have a level issue more than a gain issue. Replacing the LEDs with Silicon diodes causes the clip level to be reduced and that reduces the output level of the gain section. The non-inverting second stage can make the overdrive sound weaker as well if the clip level is low. Better would be to replace each LED with two silicon diodes in series, you could even try three.