Roland CUBE-40 Chorus Issue

[smilan]

Hi and happy new year!
I have a Roland CUBE-40 Chorus Amp with an annoying problem, when I turn the chorus switch on, there's a strong electric pulse at the speaker.
Please watch the attached clip:
https://www.youtube.com/watch?v=sAXPQ5jPs3I
I've already replaced all the electrolytic caps and measured the values of all the resistors at the chorus circuit and tested all the transistors and all of them seems to be good.
When I plug a guitar to the amp the chorus work as long as the signal is strong enough, but from a certain point when the signal become weak the noise starts.
Attached photo of the schematic
If you have any idea how to solve this issue please let me know.
https://www.youtube.com/watch?v=sAX...Mw18tSK6K5-sH6rlXuZQDWP8QZ7-1lIaanPax2_my2HMw

 

[Whoops]

Do you have the schematics?

What IC or ICs does the chorus circuit uses?

 

[smilan]

Do you have the schematics?

What IC or ICs does the chorus circuit uses?

The schematic attached to the first post.
It seems like IC2 and IC3 responsible for the chorus.

 

[CJ]

those are a pain to take apart, and once you get in there, it is all surface mount, technicians nightmare, i feel your pain, i would start with making sure you have 15 volts feeding the circuits,
Fets are notorious for popping, so next bet would be on Q27. spray out all the jacks with contact cleaner, input, effects loop, ft switch, pots if you can reach them, you do see replacement boards for sale once in awhile, but they are expensive, good luck!

 

[smilan]

those are a pain to take apart, and once you get in there, it is all surface mount, technicians nightmare, i feel your pain, i would start with making sure you have 15 volts feeding the circuits,
Fets are notorious for popping, so next bet would be on Q27. spray out all the jacks with contact cleaner, input, effects loop, ft switch, pots if you can reach them, you do see replacement boards for sale once in awhile, but they are expensive, good luck!

This is an 80's amp, all components are "thru hole", no vicious surface mount components.
I've checked all voltages and the voltage dividers resistors and all seems to be good.
I tried to replace Q27 and it acts the same.

 

[Whoops]

A lot of strange problems and noises come sometimes from a broken or colder solder joint,
I advice you first of all to reflow all the solder joints in the Chorus circuit area

I checked the schematic, IC4 and IC5 are simple 4558 opamps, replace them, they're cheap

I suspect something wrong with the BBD delay chip (IC2) or it's clock (IC3),
it's not rare that BBD chips fail after so many years.
Don't know where you're located but Banzai stocks both chips, I would replace them also

 

[smilan]

A lot of strange problems and noises come sometimes from a broken or colder solder joint,
I advice you first of all to reflow all the solder joints in the Chorus circuit area

I checked the schematic, IC4 and IC5 are simple 4558 opamps, replace them, they're cheap

I suspect something wrong with the BBD delay chip (IC2) or it's clock (IC3),
it's not rare that BBD chips fail after so many years.
Don't know where you're located but Banzai stocks both chips, I would replace them also

I replaced both 4558 with no change.
I'll order MN3007 and MN3101 and hope it will solve this issue.
I'm trying to understand how the chorus works when there's enough signal but when the signal is low it start to produce this noise?

 

[Whoops]

I'm trying to understand how the chorus works when there's enough signal but when the signal is low it start to produce this noise?

Did you reflow all the solder joints in the Chorus circuit?

 

[smilan]

I went on all the solder joints, all the electrolytics and all the transistors and two of the op amps are new, so many on the solder joints are actually new.

 

[Whoops]

It's because the problem you describe could be a cold solder joint or broken joint, that starts to make contact when more voltage is applied (more volume) and looses contact with lower volume.

BBD delay chips also fail, it might actually be a Clock problem, try changing those 2 chips

 

[CJ]

if you do not have a scope, then a regular voltmeter could see those spikes,
download the data sheets for the mn chips and start poking around,