Echo unit now working properly - MN3005

[guidoilieff]

A friend gave me this thing to repair.

The delay is stuck at a high repeat rate. The delay circuit seems to work from no delay to a lot of feedback.


Are the 4049 and 4013 the ones that control the repetitions? I read that 4013 is a flip flop device and it needs a pwm signal (provided by the 4049 maybe?) to control it.

I replaced the 4049 but I can't find a 4013 in my country.... Also I cannot import or buy from another country.


All voltages seems fine except transistors that idk much about correct voltages on those...


There is no info on the internet about this device.

[j_flanders]

4013 provides the clock signal for the mn3005 bbd delay chips. If you have delay then it's most probably working correctly. It has nothing to do with the number of repeats, only with the delay time of/between the repeat(s).

A lot of analog bbd delays have a very similar setup.
In most circuits the repeats are controlled by a 'mixer' pot that acts like a volume control for the delayed signal output, being fed back into the start of the circuit.
Turn that pot to 0 and no delay signal is being fed back into the start of the circuit and you'll have 1 repeat going to the final output (mixed with the dry signal through another mixer pot).
The more you turn the feedback mixer/volume pot up, the louder the delayed signal being fed back will be and the longer it takes for each regenerated repeat (coming back through this attenuating mixer/volume control) to come down in volume until you no longer hear it.
If you have the pot at 50%, the first repeat will re-enter the circuit half as loud. When that repeat comes out of the delay section again, it will be re-enter at 25%, then at 12,5% etc. Which is what causes both the multiple repeats and also the lower volume of subsequent repeats.
Apart from that there are also losses throughout the delay circuit, and a moderately loud delay signal will eventually 'fade away'/die out but you'll have a few repeats.
A very loud delayed signal being feedback can lead to infinite repeats.

Check some 'random' bbd schematics (EHX memory man, Boss DM2, Ibanez ad etc) for how it's done. I don't think I'm allowed to post them here.

For your circuit check or trace the part involving that repeat pot.
The input for that pot comes from the output of the delay section (bbd > filters).
The output of that pot goes to the input of the delay section (gain section > filters >  bbd)

Too much gain in a section before the bbd chips can also cause this.