Boss SD-1: D3 and R31 Question- Why not in new SD1's??

[dlilly69]

Hi folks,

I own 2 stock Boss SD-1's. One is much older than the other. Both are made in Taiwan. The older one has MUCH more volume output than the newer one (the new one is not loud enough).

I noticed that the older SD1 has a diode in D3 and a resistor in R31. The newer one does not- it just has jumpers in those areas. Why is this? Will putting the diode and resistor in the new one instead of the jumpers increase the volume?

Finally, what is the value of R31 and what kind of Diode is D3 in the older SD1?

Thanks for your help!

[cd]

Your old one w/R31 and D3 is designed to run off an ACA power supply.  The newer ones are PSA so they don't require those parts.  Should not account for the extreme volume difference though.

IIRC D3 is a vanilla diode, 30 or 40V rating, R31 is 470ohms.  If you use a battery in both pedals, these parts are not active.

If you turn the Drive knob all the way down and the Tone and Volume knobs all the way up, is one pedal still a lot louder than the other?

[dlilly69]

Cd,

I just tested them. With the drive all the way down and the volume and treble all the way up, the volume is the same between the 2. When I turn the drive up on the older one, there is a HUGE volume boost as the gain goes up.

Why is this? Are they different in some way? Any idea how I can get a little more volume from my newer one? I like the tone of the new one, it's just that I have to crank the volume just to BARELY get line level. The older one sounds a little fuller as well. They are both stock- no mods.

Thanks for your help!!!

Dan

[cd]

I don't have an SD-1 right now, but when I did, running all the knobs at 12 o'clock provided a good volume boost.  With the level knob up full and the drive all the way down, there was not much of a volume boost until the drive was turned past 9 o'clock.  If this it not occuring on your new SD-1, it leads me to believe there may be an issue with the drive pot - open it up and check the pot with a multimeter, it should sweep from 0 to 1Meg ohms.

Also the behavior of your old SD-1 sound strange, I would open that one up too and check if someone hasn't modded it before, like removing the clipping diodes.  Make sure both pedals work normally before you go hack modding them!

[dlilly69]

Thanks CD!

I opened both pedals and they are the same- no mods that I can see. The volume does not increase very much even after I dial in past 9 oclock on the new one. Even with the gain and volume cranked- it just about makes it to guitar level.

Sorry for the noobie questions.

So do I just take a multimeter to the new ones pot and put it on the outer lugs? Then just roate the pot to see if it goes to 1 on the mm? If it doesn't, I assume thats a problem?

Should I maybe remove a diode or replace one with an LED? Which diode- D4 D5 D6?

You are right about the old one. The old one has TONS of volume compared to the new when you dial in gain- even before 9 oclock. Weird.

Thanks so much!

[cd]

So do I just take a multimeter to the new ones pot and put it on the outer lugs? Then just roate the pot to see if it goes to 1 on the mm? If it doesn't, I assume thats a problem?

Should I maybe remove a diode or replace one with an LED? Which diode- D4 D5 D6?

Yup take a multimeter and put it on the middle lug and one outisde lug.  You'll probably notice one outside lug is either not connected to anything, or it's already connected to the inside lug.  That pot is wired as a variable resistor which controls the gain of the clipping stage.  The resistance should go up as your turn the Drive pot up.

Alternately, measure the resistance between the pins of the 4558 chip.  With the drive control all the way down, the resistance between pins 6 and 7 should be around 39k.  With the drive control at half, it should be around 500k.  With the drive control up all the way, it should be around 1M.

If the clipping stage looks OK then there's probably a short somewhere else or one of the FET switches is not turning on all the way.  I would get it working properly first before mucking around with anything else. 

[dlilly69]

Thanks again CD, but I'm not sure how I can even get it working properly? How would I check to see if the FET you mentioned is working (not even sure what that is)? ALso, If I take the measurements with the MM and they are not 1 meg, then what would be the next step?

Thanks for your patience and helping me on this!

[cd]

If the pot is bad, the pot would have to be replaced.  That's easier than worrying about the FET, let's take care of that for now

[cd]

FYI stock SD1 board:

[dlilly69]

Great! I will test the pot, etc and post back my findings, cool?

One quickie- At what point on the volume dial on a normal SD-1 is it the same as guitar volume? 12 oclock? 3 oclock? I would assume afteer 12 oclock it should be MUCH louder than  guiatr volume.

Thanks for your help CD!

[analogmike]

It depends on the guitar you use, if you have HOT HBs the sd1 may not be able to equal the unity gain volume of the pickups.

one more thing - best to test the pedals one at a time, not both in a row, when checking volumes. The buffer in the 2nd pedal or the pedal order may change things.

[dlilly69]

CD,

I measured the "drive" pots on the 2 outer lugs as you suggested on both SD-1's to see if they read 1 meg. Here is what I found:

OLD SD-1: .856 m-ohms

NEW SD-1 with low output issue: .750 m-ohms

Is this that much of a difference? Could this be the cause of the lower output on the new SD-1 when gain is turned up as you suggest? The old SD-1 has tons of volume when gain is dialed in past 8oclock.

If the pot is the problem, is there an easy fix to get it up to 1 meg? Maybe a resistor across the 2 lugs?

Analog Mike,

I am using Gibson LP's and an Explorer with Gibson 57 Classic p'ups in all. Those are not that hot, right?

Thanks for your help!

Dan

[cd]

It depends on the guitar you use, if you have HOT HBs the sd1 may not be able to equal the unity gain volume of the pickups.

one more thing - best to test the pedals one at a time, not both in a row, when checking volumes. The buffer in the 2nd pedal or the pedal order may change things.

Hot HBs would not account for one being louder than the other.  Remember both pedals are equally loud with the DRIVE turned all the way down.  One is obviously normal, the other is not.  Also both in a row should not make any difference, there's buffers feeding the clipping section in both pedals.

The drive pot difference between 856k and 750k is negligible.  Have you checked the diode voltage drop between pins 6 and 7?  They should be roughly equal in both pedals, around 1V in one direction and 0.5V in the other.  At this point I would build an audio probe:

http://geofex.com/FX_images/audioprb.gif

And see where the signal is actually being dropped in level.  Both pedals should have similar outputs at pin 7 of the 4558.  If not check C4 (.018uf) and see if it's shorted, or C3 (.047uf).

Q1 would be the "effect on" switching FET - when the pedal is ON, from the wiper (middle lug) of the volume pot to R14 (22k) should measure very low resistance, <1000 ohms (drain to source of the FET).