Sonab R3000 repair

Hi,

I have just got a second hand Sonab R3000. Apparently this was a very good amp for its time, and it sounds very nice except that one of the channels (back right) only plays very quietly. Adjusting the various controls and trying a different audio source (both the internal FM tuner and an MP3 player) doesn't cure the problem, which leads me to think something is broken internally.

I would very much like to fix this amp, but I can find very little information on it. I have some knowledge of electronics but have never repaired anything like this. The amp is all very well constructed using discrete components (only one IC in the whole thing), so hopefully repair shouldn't be too hard once I can track down the source of the problem. If anyone can supply/link to schematics, manuals etc., or just point me in the right direction to get started, then I would be very grateful.

Btw, there is one small capacitor that looks blown out/damaged (see pics), but I don't really want to touch anything until I at least know a bit more about the circuit layout.

Thanks


Pics:
amp open
output transistors (sorry about the poor quality image)
blown capacitor (?)
 
having left it playing for a while the quiet channel came back to full volume again. later the entire left channel went dead, but after some plugging around with the speaker connectors i've isolated the problem to the speaker connectors (they are DIN sockets, and i've heard they tend to go faulty).

what concerns me more though is at one point the entire sound went dead, and was then replaced by a loud static-type noise, which wasn't affected by the volume control (so the problem must be after the preamps?). however, it started working again and right now is playing fine. i think i'll keep listening and if it does it again then i'll post back here. any info on the amp (schematics, manuals etc.) would still be helpful though.

thanks
 

Tekko

Banned
2005-01-01 3:33 pm
One thing you can try is to replace all the lytics with fresh new ones and see what that does, also there can be loose solder joints that make and break contact as the insides of the chassi warm up.

Another thing is spraying the pots with contact cleaner.
 
i think you might be onto something with the solder joints and thermal issues. i just ran the amp for about three hours without the top of case off without a problem, but within ten minutes of putting the cover back on the static-type noise started up again. the only thing i can think of that would have changed putting the cover on is the temperature inside.

i've also excluded the preamp as the source of the problem. handily, the amp has wire leads connecting the preamp to the power amp, so disconnecting them was easy. even when all of the leads were disconnected the crackling sound continued, so i'd presume that means the preamp isn't at fault.
 
i recon that you should stop looking at a damage to repair .... to make sure that this amp will be also working next week you need to totaly restore it ....

all electrolytics ( all means all ) will need to be replaced .... soldering needs to be very well checked resistors need to be sampled for drifted values and pots but mostly switches will need the best available cleaning ...

after the above you may also start looking for any failures

here is some information that might be handy http://www.diyaudio.com/forums/solid-state/136261-vintage-amplifier-repair-upgrade-manual.html