SMR-4 BP Mode problems

Hi there.

I seem to have an issue with the BP Mode mods on my SMR-4 board.

I wired a switch to select between 2/4 pole on the LPF, which worked fine.
I then added a centre-off toggle to switch between the BPF modes which also works great.
The problem is, now when the BP switch is in the off position, I get no 4 pole LP… only 2 pole.
When 4 pole is selected there is no sound.

I can’t really figure out what’s going on :-/

If anyone has a any suggestions, I’d be well chuffed!


Are you sure you have a centre off switch and not a centre on (on-on-on)? Did you measure continuity for the 2/4 pole switch in 4 pole postition? Test it at the solder points on the PCB.

It’s definitely on-off-on.
I’ll give the continuity another check, but not sure that’s the problem as switching between 2 and 4 pole low pass works fine when either band pass is engaged.

Right, no bad joints between the switches and PCB… and all poles/throws are wired to the correct pads.
Very confusing.

Just checking to be sure. Do you have the 10k resistor at the mode point?
Can you measure for continuity between the yellow arrows with the switch set to 4 pole? Like this you measure the whole circuit from pcb to solder joint to the switch to solder joint to the pcb.

Hey Shiftr.
The 10k mode resistor is in place… and there is a continuous circuit between that via and the other 10k. What I have just noticed is that it is in fact the 2 pole that has the problem, not the 4 pole! Doh!
Same issue, just with the 2 pole lowpass… it won’t work unless the bandpass is switched in.

and between these points in 2 pole?

Hi shiftr.
Nope, there is no connection between those points, in 2 pole or at all.

there should be with the switch to 2 pole. Maybe there is a problem with your switch or there is a bad solder joint.

I have double and triple checked the soldering… definitely no problem there.
And I’ve tested the switch, it works fine.

Can you confirm that when the bandpass switch is off, there is no signal on the “Mode” pin? And do you always get a signal on IC7 pin 7?

When the switch is in the centre (off) position, there is no signal at the mode pad.
And yes, there’s always signal at pin7 IC7, no matter what configuration the switches are in.

Now that’s interesting, because IC7 pin7 is hooked to the LP2 point!

In the pole selector = LP2 ; BP mode selector = OFF configuration, do you get the same signal at the VCA input (upper pin of C4, or upper pin of the 10k resistor near the C7 label)?

There is no signal there when the switch is in 2-pole position.
There is when it’s in 4-pole.

Which means I’ve probably got the problem I originally though I had with the 4-pole not working?!

With the switch in 2-pole mode, do you get signal at the middle pin of the pole selector?

It seems to me that something broke in your 2/4 poles switch wiring.

With the switch to 4-pole, I get the following - normal signal at 4p and centre, low level signal at 2p.
With the switch to 2-pole, I get - normal signal at 4p, nothing at centre and 2p.

Post a photo of your board and wiring!

I would happily do that, if I had a camera!