Boss DS-2 Always On, Switch is Good, Flip Flop Won't Flip

[JVO]

Going off of a problem very similar to this post that was never resolved: https://www.diystompboxes.com/smfforum/index.php?topic=117008.0

I've read this excellent article about how the circuit works: http://www.geofex.com/Article_Folders/bosstech.pdf

And by going off of the schematic here: http://guitar-fx-layouts.42897.x6.nabble.com/Boss-ds-2-for-vero-td35382.html

The pedal starts up on and despite the switch being good, it will not turn off. I've checked the two transistors (Q1, Q2) and get the following values:

Q1

E = 0.01mV
C = 47.1mV
B = 631mV

Q2

E = 0.01mV
C = 5.23V
B = 16.9mV

I am by no means an expert but it seems to me that the Q1 base being higher means that it is being used as a switch and reducing the flow between the collector and emitter. According to R.G. Keen's article, when the base of both of them are grounded by the switch it will flip to Q2 having a higher voltage, Q2 being switched off, and a high value going through the collector of Q1.

I can get the LED to go off momentarily by touching the base of Q1 with a probe connected to ground.

He mentioned at the end of the article that he could get it so that he could consistently have the flip flop start in a position by having "the transistor with the bigger capacitor connected to its collector tends to come on (that is, collector low) all the time when the power supply comes on."

My going theory is that something like that is going on here but I'm not entirely sure.

Does anyone have any ideas here? Any suggestions would be greatly appreciated and thank you in advance.

[Rob Strand]

The pedal starts up on and despite the switch being good, it will not turn off. I

The power-up state for Boss pedals is not guaranteed to start-up in the off state.    The circuits
with the 1M resistor and extra transistor power-up in the off-state more often than the slight
variants like the DS2 circuit.

The power-up state isn't the problem.

https://elektrotanya.com/PREVIEWS/45612216/23432455/hangszer/boss/boss_turbo-distortion-ds2.pdf_1.png

There's been a few cases of this type of behaviour and I rememeber one where the transistors needed re-soldering.

If you short the base and emitter of Q1 the pedal should go into bypass mode.   If you remove
the short and the state immediately changes back to effects mode then the circuit has fault.

The fault is Q2 is not staying on.    With the Q1 short present it is worthwhile measuring the
voltage on the transistor Q2.   It could be the case Q2 never turns on.
The fault is likely to be Q2 open, R6 (100k) open, R8 (56k) short, maybe C5 short.

If the voltages check out then C3 (220pF) open might do it (in this case the fault is Q1 not turning off, which lets Q2 turn on.)

[JVO]

When connecting Q1 base to ground, I get the following values:

Q1

E = 0.0mV
C = 5.13V (up from 47.1mV)
B = 0.0mV (grounded, down from 631.0mV)

Q2

E = 0.01mV
C = 5.00V (down from 5.23V but not what expected)
B = 412.0mV (up from 16.9mV)


The fact that Q2's collector didn't drop is not what I expected which would be a number similar to what Q1's collector was with voltage applied to the base.

So, maybe bad transistor? Finding the 2SC-2458 isn't easy (small bear, tayda, mouser, digikey don't have) but I can go to eBay or something. This post (https://www.diystompboxes.com/smfforum/index.php?topic=72916.0) says that something like a BC549 would work but I see the collector and base have switched positions. Anything else with the BCE configuration that I'd be able to swap out?

[JVO]

I switched out the 2SC-2458 with a 2SC945 that I had from a different project and it flips on and off again!

Thank you very much for the assistance and I hope this helps someone else out who is googling in the future.