SMR-4 board with only "plops"

Sorry if my english isn’t perfect. I’m french.

I’ve just finished my Shruthi but I can only have some “plops”.

The control boards seems to work perfectly. This is not the same with the SMR-4 board.

I had the right voltage at power supply test points.I’ve checked solders and swap resistance. IC, diodes and transistors are in right orientation.

I do not have access to an oscilloscope but I’ve checked the sound path with a Jack and here is what happens:

At point 1 : Seems ok
At point 2 : Seems ok
At point 3 : No sound at all
At point 4 : No sound a all
At point 5 : No sound a all
At point 6 : No sound a all
At point 7 : Only plops
At point 8 : Only plops

I’m far from beeing an electronic guru but some points looked strange to me while I was mesuring resistors:

-When mesuring R28, I should have 56k; I have only 14k while the color code seems to be correct : Green , Blue , Black , Red.

-When mesuring 10k resistors, many of them give me only 5k. As I find this results in severall 10k resistors , I assume this is normal?

Please tell me what to do , I want this little box to play nice sounds ;).

Thank you for your Help

Regarding the “wrong” resistor values: an ohm-meter will measure the total resistance between two points of the circuit - and it’s likely that there are many more paths in the circuit between those two points than the resistor you’re trying to measure. For example, if two 10k resistors R1 and R2 are in parallel, how would you measure R1 with an ohmmeter? If you put the probes on both ends of R1, you’ll also be sending current to R2, and you’ll end up measuring R1 // R2 = 5k. Use the ohmmeter only on individual parts outside of an actual circuit (unless you’re pretty sure that there is only path between the two points you’re probing).

Things to check:

  • 2N3904 swapped with 2N3906?
  • Badly oriented transistors?
  • IC7 orientation

Thank you for the ohmmeter precisions and for your prompt response.

IC7 orientation is ok
Transistors orientation are ok
No Transistors have been swapped.

What is the voltage at pin 1 of IC3 when you sweep cutoff?

I am at work for a few hours but i’ll check this as soon as I get back.

This is what I could measure at pin 1 of IC3:

Cutoff = 0 --> 478mv
Cutoff = 127 --> 53mv

It works now :wink:

I simply re soldered R19,R44,R45 C11 and both 2N3904



I’ve found resoldering to be the solution on many occasions, even with SMT chips. Having said that, I’ve also managed to wreck quite a few ICs, possibly by over-heating them, which nobody else seems to have managed, so maybe it’s not always the solution…