Beginner trying to do a simple (I think) repair.

[Jebus0000]

k, I have an old Boss OD-1 overdrive pedal.  It works fine when it's on but when it's in bypass it only passes a very faint and distorted signal.  I have the schematic for it but don't really know what I'm looking for.  How do I start to troubleshoot this?

[freebird1127]

Do you have tools?  A multimeter?

IMO, one of the best checks you can start with for a problem like this is continuity of the input signal to the output jack when bypassed.  That is to say, find out how far the signal is getting from the input...  use a multimeter set to resistance to find this out.

Connect one lead to the input signal line and use the other lead to probe down the signal line and switch connections.  If the resistance is 0, that means the signal will get through to there.  If it is very high or if the meter shows OL (overload), that means the signal will not go there...

other than that I would say check the physical connections of the signal line to the input/output jacks and switch, maybe just reflow the joints to make sure the connection is good.

[petemoore]

Sounds like a Jfet bypass problem, or like other ones I've seen typed about.
 I don't know how to fix that, but knock on wood and say I'm glad none of my pedals [with Jfet  switching]  have had problems.
 Good idea to check wire connections...could be that.

[MartyMart]

Hi Jebus,
I had a similar problem with an old Boss SD1, though i got clean signal with a touch of drive in the background when bypassed.
To get rid of it I had to change both "switching Jfet's" which look like small black plastic "three quarter moons" on three legs ! ( in case you dont know what they look like )
They are a pair and will probably have "2SK30A" or very similar written on them, i dont have a schem, but if you follow the trace on the circuit board from where the foot switch leads enter the board, you should be able to track them down.
Note: there will be other similar looking transistors on the board so make sure these are the correct ones !.

When replacing them, be VERY careful to not overheat them ( poss use small sockets)  as you can burn them up very easily.
Good luck !
Marty.

[AL]

I can't say it enough. Build RG's audio probe at GEO FX. It will take 5 minutes to build and save you hours of debugging time. I don't know how I got along without it. YOU NEED ONE!!!  

AL

[jplaudio]

k, I have an old Boss OD-1 overdrive pedal.  It works fine when it's on but when it's in bypass it only passes a very faint and distorted signal.  I have the schematic for it but don't really know what I'm looking for.  How do I start to troubleshoot this?

In bypass mode the signal passes thru Q6 (buffer) Q2 (FET) and Q7 (buffer)
You can check if the FET is not turning on by shorting the two outside legs of Q2 together. If the signal sounds good with the FET shorted you may have a bad FET or the drive signal from Q3 is not turning Q2 on. If the bypass signal is still distorted you may have a bad Q6.
Check R11 and R26 for the proper resistance.
Check the board for any solder bridges and touch up any dull looking solder joints.
Some times it is quicker to "shotgun" a problem by replacing every part that *could* be bad if you have the parts on hand. Replace Q2,Q6 and Q7 and see what  happens

[petemoore]

use sockets...
 or heat sinks on the transistor legs while soldering.
 I mount the iron myself, in a brick, Id hold one of the 3-4 items without my hand...pulling the transistor, if you can clamp the board...that's way you can pull the part with the 'diving board' effect' = solder splash thing.
 I like clamped and braced at the other end [even taped down] for nice PCB re-worx.

[Darren N]

One thing to check for: on old boss pedals, the pots can spin around internally and touch the metal sides of the box. Makes for a quick repair.

[Jebus0000]

Thanks for the help.  i did find one bad resistor and changed it but didn't solve the problem.  I'll try changing the jfets.  i do have some transistor sockets to use but I don't have the jfets.  They have K30A written on them.  Are those common/still made?

[MartyMart]

MartyMart wrote:
"They are a pair and will probably have "2SK30A" or very similar written on them,"

They are the ones !
You can probably get hem from a good parts supplier, smallbear dont have them, i got mine from Olaf Nobis at  www.banzaieffects.com  he's in Germany and is a very helpful guy.
Cheers,
Marty.

[Mark Hammer]

K30A FETs are the most widely available of the K30 types (there are AY and others).  I get mine for 30 to 45 cents each at Supremetronic in Toronto.

How on earth was there a "bad resistor"?  Perhaps more importantly, WHERE was said component in the circuit?  Identifying the resistor would help in isolating which aspect of the circuit is misbehaving and maybe even why.

Bear in mind that the FET performs its switching function by changing from a low resistance ("on") to high resistance ("off") state.  In the case of the OD-1,  when Q1 is "on" and Q2 is "off" the distortion signal passes to the output stage (Q7) but the clean signal does not.  Step on the pedal and Q1 goes off at the same time that Q2 goes on, blocking the distorted signal from reaching Q7 but allowing the clean signal to do so.

From the sounds of it, IF there is an issue with a FET, then the culprit is likely to be Q2.  You can confirm this by *carefully* desoldering Q1 and Q2, swapping and reinstalling them.

The service notes and board layout are posted around (perhaps at GGG).  If you can't find them anywhere, I have them.

[Jebus0000]

The bad resistor was somewhere near the beginning of the input signal path.  I measured it and it was 2M ohms and is supposed to be 470k so I changed it but it didn't do anything to solve the problem.  I measured the bad resistor while it was still in the circuit is that ok or would that @#$% up the measurment?  I'll go measure it again hold on.....nevermind I threw it out.

I have a print out of the schematic and layout.  It was posted by jplaudio on a different forum. here it is:

[Jebus0000]

I'll try switching around Q1 and Q2.  And also track down some new ones.  damn! I'm learning lots here.  Thanks guys!

[Wolfman922]

just a quick thank you to jebus. he just mailed me a link to this thread and made my day! the info and rare schematic here should solve my
OD-1 problem. thx to Rhys W. for his help as well.
awesome! thx guys.  

now, if i can find a schematic for my super-rare LOCO BOX Tubemanix OD unit...i'd be set..; )

http://www.locobox.com/70s.html

[modmod]

which forum is that ? can you post the link here ? thanks

[Jebus0000]

which forum is that ? can you post the link here ? thanks

Do you mean me? like where did I get the schematic?  It was just the harmony central effects forum.

http://acapella.harmony-central.com/forums/forumdisplay.php?s=&forumid=29

Mike Hammer - I changed Q1 and Q2 around and nothing changed.  umm...what does that mean?  it's not those?  also when I short out Q2 it does nothing.  

I'm going to Toronto soon so I'm gonna stop by supremetronic.  The store here are not so good.