circuit modification, help Please

[Pauld35]

hello,

I bought what was advertised as a stereo power old amp with power supply (all one unit) from an auction site for ten bucks. i found out its from an old yamaha organ.
I got the schematic for it and it has two 30 watt power amps and a 4588 pre.

I have modifed the schem. to show what i wish to do. the insert points to the power amps will be switched jacks to disconect the pre amps if a different pre is used.

Can anybody see any possible problems with mods before i start?

here is a link to the original schem if the photos arent clear enough thanks.

YAMAHA ELECTONE C-55N Service Manual free download, schematics, eeprom, repair info for electronics

 

[Pauld35]

also I noticed a couple of caps near the power supply looked a bit cooked. are these a special type or could i replace them with any 600v film. i think the values were 22nf and 47nf? thanks

 

[Enzo]

The amps are pretty much self contained. Maybe you never got them, but I see no point in eliminating the volume control that was just before it. I do this sort of thing just for the heck of it now and then. And while this one already has it, sometimes clients ask me to add line in jacks for aux inputs on keyboards. It turns out the same as what you present.

 

[JonSnell Electronic]

also I noticed a couple of caps near the power supply looked a bit cooked. are these a special type or could i replace them with any 600v film. i think the values were 22nf and 47nf? thanks

They are class X capacitors. The rating must be the same as the original. Class X is 'self healing'. Looks like the 22n is class Y, the service manual will explain it.

Where your external pre in is you will need to place a resistor in series at pin 1 and 7 of the 4558 otherwise your signal will be shunted. I would use 22k or better still, remove the 10/50 caps.

 

[Pauld35]

The amps are pretty much self contained. Maybe you never got them, but I see no point in eliminating the volume control that was just before it. I do this sort of thing just for the heck of it now and then. And while this one already has it, sometimes clients ask me to add line in jacks for aux inputs on keyboards. It turns out the same as what you present.

hello, the action was only for what was in the photo, there was no volume controls or speaker conections or crossovers with it so i was just going to use another pre amp with controls. cheers

 

[Pauld35]

They are class X capacitors. The rating must be the same as the original. Class X is 'self healing'. Looks like the 22n is class Y, the service manual will explain it.

Where your external pre in is you will need to place a resistor in series at pin 1 and 7 of the 4558 otherwise your signal will be shunted. I would use 22k or better still, remove the 10/50 caps.

thankyou i will just remove the cap straight after the 4588 outputs then and not install the resistors if this is better. cheers for the advice

 

[Pauld35]

thanks for having a look over the circuit

 

[Pauld35]

hello,
i noticed one channel preamp has a 33pf and a 390pf. bridging pins
the other channel has a 33k series a the input. RI input is not used.
could you please take a look and advise me if this will unbalance the channels and what the best solution would be if you have a moment to take a look, thankyou

 

[Ketje]

I would use 22k or better still, remove the 10/50 caps.

No no, don't remove the caps, there is -7volts on the output of the pre
I think you can do it this way,
Mona

 

[Pauld35]

thankyou for drawing that up