Boss BF2 MIJ. No sound :(

[FingerBlisters]

Hello guys and happy new year.

I acquired a 1982 MIJ Boss BF2 Flanger. It does not pass signal when engaged or when bypassed.

The LED does light up when the switch is pressed, and turns off when pressed again as normal.

When it arrived and I took off the back Q6 fell into my hand. I have replaced this with a 2N2222 and twisted the legs to match the pinout. While in there I replaced a couple of resistors that had some corrosion on them and also switched out the electrolytics in the power supply (C40 and C49) for higher voltage electros (having a 6.3v on a 5v line or a 18v on a 12v line doesn't sound amazing).

No dice.

I switched out the 11v zener for a fresh one thinking it could have been wrong power that fried something.

No dice.

Following some reading on here about Q4 being necessary for the switching in/out I replaced that with a 2N5485 (only jfets i have on hand), twisting the legs to match the pinout of the 2SK30 which is stock to these units.

No dice.

BUT, an interesting thing that may shed some light.

If the pedal is unplugged from the power and I strum very hard I do get a gated signal like an underpowered fuzz coming through the amp, but the amp must be quite loud for this to be audible.

Then if I strum the strings and plug the power into the pedal I get sound for about half a second along with a whooshing sound that increases in pitch until it fades again to silence, then no matter what I do there is no signal.

I see there are 3 trimpots. I haven't touched these.

What I know: seller didn't know much. Said they acquired it as a defunct unit, which it is. Beyond that there is no usage history.

I've powered this both 12v dc off my CS7, and also daisychained it in a regular 9v setup on a pedal board.

All ideas gratefully received! I have a DMM but do not have a scope or anything fancy. If there's any hints where to look, what to test, that'd be really helpful. Thanks, guys.

Here's some pics I tried to be as clear as I could get them. The blue caps and resistors are the new ones.





2n5485 at Q4:



2n2222 at Q6:



Here's the Schematic:

[Rob Strand]

For the LED to work the PSU must be OK.

You need to build an audio tracer and trace where the signal dies.

I'm suspecting a dead JFET or even a dead opamp.  With some old pedals it can even be the 6.5mm sockets.

[FingerBlisters]

For the LED to work the PSU must be OK.

You need to build an audio tracer and trace where the signal dies.

I'm suspecting a dead JFET or even a dead opamp.  With some old pedals it can even be the 6.5mm sockets

Yeah I did wonder that with the PSU. Power is going somewhere, I was just dealing with the easy wins first.

How would one test for a dead opamp? Hook it up to power, and see what the voltages are at the pins?

Good shout on the jacks. That had occurred to me, I just don't seem to have any like for like replacements but do have some mono open frame jacks which I think will do the job (?) if they fit in there.

[Rob Strand]

How would one test for a dead opamp? Hook it up to power, and see what the voltages are at the pins?

Good shout on the jacks. That had occurred to me, I just don't seem to have any like for like replacements but do have some mono open frame jacks which I think will do the job (?) if they fit in there.

Well the easiest way is to measure the DC voltages at the output pins of the opamps.  The opamps in the audio signal path should sit at mid supply rail.   That's a quick test.    Don't worry about the LFO opamps, they will be doing something different.

I wouldn't go changing out a whole lot of "possibly" suspect stuff.  If you build an audio probe you can pin point the issue in no time without doing any soldering.

Image:

Direct link:
http://www.geofex.com/FX_images/audioprb.gif

[FingerBlisters]

How would one test for a dead opamp? Hook it up to power, and see what the voltages are at the pins?

Good shout on the jacks. That had occurred to me, I just don't seem to have any like for like replacements but do have some mono open frame jacks which I think will do the job (?) if they fit in there.

Well the easiest way is to measure the DC voltages at the output pins of the opamps.  The opamps in the audio signal path should sit at mid supply rail.   That's a quick test.    Don't worry about the LFO opamps, they will be doing something different.

I wouldn't go changing out a whole lot of "possibly" suspect stuff.  If you build an audio probe you can pin point the issue in no time without doing any soldering.

Image:

Direct link:
http://www.geofex.com/FX_images/audioprb.gif

Well that's interesting.

I checked the volts at pins 1 and 7 of each of the 4558s in the signal path. They're all at 1.33v/1.32v. My supply voltage (from a 12v DC) is 10.4v. If the outputs should be at mid-supply and my calcified brain still works that 1.33v should be ~5.2v, which it isn't.

As a precaution I cleaned the jacks inside and out too, removing some white crusty corrosion between the jack and bare metal of the enclosure.

[Rob Strand]

Well that's interesting.

I checked the volts at pins 1 and 7 of each of the 4558s in the signal path. They're all at 1.33v/1.32v. My supply voltage (from a 12v DC) is 10.4v. If the outputs should be at mid-supply and my calcified brain still works that 1.33v should be ~5.2v, which it isn't.

If they are all like that then there's a chance there's a common cause.   If the voltage on C25/R35 is 1.3V it will force all the opamps to be at 1.3V.

The next step is to work out what is causing the C25/R26 to get pulled down.

[FingerBlisters]

Well that's interesting.

I checked the volts at pins 1 and 7 of each of the 4558s in the signal path. They're all at 1.33v/1.32v. My supply voltage (from a 12v DC) is 10.4v. If the outputs should be at mid-supply and my calcified brain still works that 1.33v should be ~5.2v, which it isn't.

If they are all like that then there's a chance there's a common cause.   If the voltage on C25/R35 is 1.3V it will force all the opamps to be at 1.3V.

The next step is to work out what is causing the C25/R26 to get pulled down.

Yep I was doing that in the interval between replies

I pulled R34, R35, and C25 which are all responsible for the voltage divider. The supply voltage was coming in to the top of R34 then I couldn't get any readings downstream. Also, after prodding around some more the voltages on the opamps would swing wildly from 1.3v to 7.5v on the outputs.

When I pulled C25 it left behind some crusty residue and the underside of the cap is covered in the same crust.

I replaced the 2 x 33k with new metal films, and a 47uf/50v for C25 (I don't have any 33uf). And.... it works perfectly

The 'preventative maintenance' I carried out with the replacement of filter caps etc probably could have waited, but I'd prefer to know there's nothing underspec'd in there after 41 years of flanging.

Something I mocked up in PS that might help other people is the overlay of the board layout over the back of the PCB. I mirrored it so the text is backwards but it works well:

[Rob Strand]

replaced the 2 x 33k with new metal films, and a 47uf/50v for C25 (I don't have any 33uf). And.... it works perfectly

The 'preventative maintenance' I carried out with the replacement of filter caps etc probably could have waited, but I'd prefer to know there's nothing underspec'd in there after 41 years of flanging

Way to go!

[Chillums]

I would assume that all the older caps are on there way out or are already dried up as well seeing as they were all the same type.  IMO it's good you replaced them too.   Nice job getting her up and flanging again!! And thanks for the great pics!!

[FingerBlisters]

I would assume that all the older caps are on there way out or are already dried up as well seeing as they were all the same type.  IMO it's good you replaced them too.   Nice job getting her up and flanging again!! And thanks for the great pics!!

Funny you say that... I replaced all the electrolytics after posting my last post. I figured they're all 40+ years old and if one has failed, the others could too. Plus, they weren't quality parts when they were new let alone now.

It's also really nice having a big open board like this to work on.