K4700... Wait LED

I have built this kit and on testing I have a problem…
At switch on, the yellow led lights. Then after six seconds the relays click in and the yellow led goes to half brightness.
It then stays at half brightness. It comes up to full brightness again when the red led is triggered.
The red led operates correctly when triggered…
In other words, the yellow led stays at half brightness when it should be fully off.

Any ideas please?


This seems to be normal behaviour of the kit.
Most likely, base of T3 does not go low enough to fully turn it off, due to the fact that T4 is a darlington.
At the time the kit was designed, leds required 10-20mA to achieve reasonable brightness, so it was not a problem.
Today, they require very little current, so this could explain why it stays on at half brightness.
You could experiment with different values of the series resistor R20 (i.e. increase the value), or with the value of R17.

Thank you for the prompt reply…
I will try a little experimenting with some old leds I have…
The important thing is, that my amplifier is now well protected.
Kind Regards,

Hello Team,

is there allready any experience from the values?
I want to change the LED from yellow into blue and the LED is after the wait time allmost fully on.

So tried to change the value from R20 and was allmost at 10kOhm with the same result.
After that I tried R17, after I increased to allsmost 680kOhm I stoped.
Strange behaivor?!

Now I have a 680kOhm with a 100kOhm to ground.
Maybe I go back to 47kOhm with 10kOhm to ground.

It was like try and error.

Maybe you have better values for me?


Hi, I too was irritated at the yellow LED not fully switching off.
A quick check of the circuit reveals T3 will not be able to turn itself off, if the relay driver T4 has too many volts across it. These volts are caused by 2x relay ON current flowing through it.

Leave the circuit values as-is and solder a 10K ohm … 33K ohm resistor (value is not critical) across T3 base and emitter. The easiest way to do this is to solder it onto the resistors R17 and R7, the legs closest to T3.
This ensures the “off” voltage at T3 base is significantly reduced and yellow LED should now be fully extinguished when wait period is over.