Help with Yamaha RA50

[piney]

Hi, this may be off here but I need some help. I´m currently troubleshooting a Yamaha RA50, which is a keyboard amp (with a built-in leslie). It´s from around 1975, I think.

At first, the problem only showed up after the amp had been switched on for about an hour or so. Now, however, it´s there invariably...

Firstly, there is a rather loud noise, with crackles and pops and the AC hum becomes pretty evident. This noise is not affected by volume or tone controls in the preamp.
One can also disconnect the preamp altogether and the problem is still there, which, of course, suggests the error is located in the power amp.

Secondly, there is an alarming DC offset at the speaker output of approx. -5V.
Interestingly enough, however, the amp does work and sounds pretty good, if it wasn´t for this "overlayered" noise...

Since it´s pretty old, maybe one should exchange all the capacitors..?

Any help appreciated.
I´ve attached a schematic for the power amp.
Cheers

[Netlist]

I you have access to a scope you can measure the ripple from the supply rails.
I would however change the small electrolytic caps first.
Keep in mind that the 10µF at the base of TR2 is a bipolar.

/Hugo

[dmitririzzo]

I just got a Yamaha RA50 that has exactly the same problem. Did you fix yours? What was the problem?

Thanks very much!

[pinofluvius]

Hey, that´s interesting!

Yes, I found the problem, it was one of the transistors in the
input differential stage of the power amp (TR1 or TR2, can´t remember which...).

Couldn´t find the right kind so I found a new equivalent pair, and they worked fine (can´t remember what they were, either, sorry).

Hope this helped!
Cheers
Martin

[jasonm]

hi

i have a yamaha ra-70r combo guitar /keyboard amp , has a fault in the amplifier power section huge hum and 3amp fuse blows , someone changed the output transistors and the collector load resistor from .47 to a 2.2ohm. this is wrong as it must be the same as the other side to produce a 0 volt dc on the speaker.

so i am looking for a circuit diagram ... maybe i can fix the amp..
i checked the ra50 and is not similar. as mine has 11 transistors in all ,

thanks