Trouble shooting relay with ne555

[idy]

Student brought in a fuzz ("Black Mass Missionary Fuzz) with bad relay switching bypass and I don't know what to look for. Bypass signal is ok but footswitch produces no LED, no change.

The relay is Nexem EC2-5NU.
The 555 reads
1: 0
2: 4.9
3: 0, and shorting 3 and 4 makes LED light, and engages pedal
4: 9.36
5: 5.58
6: 4.91
7: 0
8: 9.36

[Kevin Mitchell]

That appears to be the non-latching model of the relay.
I'll guess there's a diode near the 555. Does the cathode show flipflop voltages when you press the bypass switch? Does it do anything at all?

[idy]

There are two diodes close to the relay. Neither shows voltage on K either when clicking footswitch or when shorting pins 3 and 4. But shorting those pins produces a nice audible click as relay closes.

[Kevin Mitchell]

Okay so the relay is fine.
It is possible that C10(I think that's what it is, can't be sure without further details) isn't pulling enough juice to hold  the pulse from the switch long enough to actuate the latching circuit.
Would have to scope to investigate further.
Or just replace C10 and see what happens.

[JerS]

Do we know if the footswitch is any good? Try shorting the two pins that the footswitch connector is tied to - perhaps it is just a bad switch.

[idy]

Footswitch is good, provides solid non-latching contact, and the contacts are connected to the board.

Yes, C10 is the only large cap near the relay/555. Just touching a large (10uf) cap to its leads doesn't help... Is the next step to remove the SMD cap and try to kludge an electro there?

[idy]

Can anyone point to a likely starting place in understanding this kind of circuit? 555 with non-latching switch driving a relay, without a transistor in between the 555 and the relay.

I'm pretty sure there isn't one: the board has 3BC109 as a TB type fuzz and an MPSA18 as a buffer at the volume control, no Q close to relay or hidden on the other side of the board.

[Rob Strand]

Student brought in a fuzz ("Black Mass Missionary Fuzz) with bad relay switching bypass and I don't know what to look for. Bypass signal is ok but footswitch produces no LED, no change.

The relay is Nexem EC2-5NU.
The 555 reads
1: 0
2: 4.9
3: 0, and shorting 3 and 4 makes LED light, and engages pedal
4: 9.36
5: 5.58
6: 4.91
7: 0
8: 9.36

Can anyone point to a likely starting place in understanding this kind of circuit? 555 with non-latching switch driving a relay, without a transistor in between the 555 and the relay.

There's many possible minor variants but this one at least has the NE555 driving the relay.

https://www.zorgeffects.com/index.php/en/blog/143-true-relay-bypass-with-ne555-flip-flop

There's two ways to approach it:
- The circuit has fault.
- the circuit has a design issue making it flaky

If the circuit worked before then you might assume it should work now and there is a fault.

If the LED isn't working then the problem isn't the relay, unless the relay is shorted.  You could check the DC resistance of the relay against the model number.

It could be the switch, but that checks out OK.

So that leaves perhaps shorted diodes or a faulty NE555.
BTW manually shorting pin 3 and 4 can fry the NE555!

To prove that you would have to look more closely at what is going on in that part of the circuit.  You could just swap it out but you might not want to do that so readily on an SMD board.

I know for a fact the NE555 bypass circuits which drive the relay directly may operate a little marginally.   The problem shows up more at low supply voltages.

Another version of the circuit was discussed not long ago.  The basic idea is the same but the output section is different. 

https://www.diystompboxes.com/smfforum/index.php?topic=129003.0

During that thread I remember analyzing the circuit, although I didn't post all the calculations.   The part values on your board might not agree with the values in this schematic. 

https://www.zorgeffects.com/index.php/en/blog/143-true-relay-bypass-with-ne555-flip-flop

However, I seem to recall the values of R3 and R4 are important and if the circuit is operating marginally you might be able to add a high value resistor, say 10 times R3,  in parallel with the R3 (100k on this schem 10k on other schem). 

To make a better suggestion I would have to go through all the details of the circuit and have the actual part values on your board.

[Rob Strand]

To prove that you would have to look more closely at what is going on in that part of the circuit.  You could just swap it out but you might not want to do that so readily on an SMD board.

If you hold pin 2/6 to +9V (whatever pin 4/8 is) and ground you should be able to force the output at pin 3 to be in the off or on states.  Don't press the switch.  That would at least show some parts of the NE555 are working.