Maxon Ibanez CS 505 Chorus Stereo - Distortion on wet channel - HELP

[Astuck]

Hi folks,
I'm trying to repair a Maxon CS505 : the dry channel is OK but the wet channel has a slight distortion mixed with the chorus signal.
I'm using both outputs so 1st jack is dry only while the 2nd jack is wet only.

I've changed all IC but still distortion, same with FET switching transistors : light distortion.
Polarised capacitors have been changed too. BIAS and CLOCK trims are checked and OK.

The CS505 has surely been repaired once, the 78L12 was replaced with 78L15 with a 2.7v zener diode in serie on 78L15 out to get a Vcc=12vdc I presume.
I switched it back to 78L12 and I replaced the 2.7v zener with 1N4148 diode, the ICs receive 11vdc and 5.5vdc after the voltage divider.
Still distortion...

I'll try tomorrow without the 1N4148 and check again.
If anyone have an idea, any help will be appreciated.
Thanks, cheers.

[lowell]

Can you please provide a schematic so we can better assist you? Sounds like you're using the "shotgun" approach. Schematic will help target the cause.

[Astuck]

Hi lowell, of course sorry, here's the files :
I'd say "Machine gun" approach

CS505 Schematic :
CS505schem_highlight.pdf

CS505 Service Manual :
Ibanez_CS505sbfi_servicemanual.pdf

Thanks

[Astuck]

Hi,
So the problem was the regulator and the diode !
Everything's back to the original drawing and the distortion is gone 78L12 is all the CS505 wants !
BDD MN3007 had 11v and that was just wrong.

The last problem is the LED, it blows after 5min ON... 3rd change for it and still the same thing.
LED is powered by the 2x9v batteries then 5,1k and switching FET.

Cheers

[lowell]

Measure voltage across the LED... or better yet make sure that 5k1 resistor is still correct...measure it. It's there to limit the LED current.

[Astuck]

Hi lowell,
Voltage across the LED is 16vdc and the 5k1 is OK so 3mA on the LED.
So my guess is around the LED itself, I use a violet ultrabright, modern one but I don't have any data for them,
maybe 3mA is too much, I'll try to increase the 5k1 to 47k.

[lowell]

How did you get 3ma? Did you calculate it via the v-drop across the 5k1?

[Astuck]

I get 3mA by the math, I may be wrong, electronic theory was not a step during my studies
I=(Ubat-Uled)/R Ubat is 2xbatteries 9V Ubat=18v Uled=1,5v R=5k1
I=3,25mA

So I choose to go with a violet LED ultrabright 1100 mcD : switch on at 0,1mA and bright at 1mA,
voltage drop 3,6v.
R=(18-3,6)/0,001
R=14,4k

I put a 10k in serie with original 5k1 and everything works fine !
Thanks for your help.
Cheers

[lowell]

I don't follow your math...doesnt mean it's wrong. I would measure Voltage across 5k6 and divide by 5k6. That will give you the current thru the LED. Nonetheless you got it working!