Troubleshooting a Rotel RA-931 amp/finding replacement parts

[lars667]

Hi,

my first post here. I joined just because I've got a problem with my Rotel RA-931, and want to try fixing it myself. I have some DIY skills (know how to solder and reading basic schematics, I have built amp kits and some guitar effects pedals from schematics/layouts).

The symptoms of this problem, is that the right channel seems to drop out intermittently. So far I haven't had this problem on the left channel, and the problem appears both on remote, direct and on the headphones, so I'm pretty sure it is in the preamp section.

I have opened the amp up and tried the "chopstick" method, which I thought isolated the problem to the caps c505 and c506 (I noticed a difference when I poked at them with a wooden stick while playing music). I found a couple of caps with the same values, and besides a perceived drop in sound quality thought that fixed it, but now the problem has reappeared again, on the same channel, so I'm not sure.

I'm still thinking this could be due to bad capacitors, so I'm prepared to change all the electrolytic ones.

My questions are:

* Do you have any other suggestions as to what be wrong, or any other troubleshooting procedures?
* Where do I get good replacement caps for this amp? If not the same caps (Rubycon and Nichicon), are there any that might soften/warm the sound a bit? I've llways found this amp a bit sterile, but I can't afford another in quite some time.

I'm grateful for any help so that I might prolong the life of this amp. And yeah, it's just about 10 years now, I think. I have tried searching, but not found the same symptoms for this amp.

Lars

 

[jaycee]

My personal guess would be that the Tone Defeat switch could be faulty. You could try bypassing this if you don't use the tone control.

I have the service manual for this amp if you want it.

If it is some years old then replacing all of the electrolytics may well prove beneficial. Any good quality capacitors will do - Rubycon, Nichicon, Nippon Chemi-con, Panasonic.. I will leave suggestions for sourcing these up to others who will know what's available in norway better than me

(edit - cough, got the flag wrong!)

 

[lars667]

Thanks, jaycee. I will try bypassing the tone on/off switch (that's the one you mean, right?) to see if that makes a difference.

I think I have the service manual, it's basically schematics and 4 pages? I downloaded some rar files with a pdf after searching through this site. If there are other manuals, I'd take those too

Now let's see if we can find some cap suppliers that don't only do orders for 100 or more.

 

[jaycee]

Yeah thats what i mean

It sounds like you have the same manual as I do. I did get it from some schematics website at some point, but couldnt remember where. It has 4 pages, first is a general overview, second is a parts list, third is PCB layout and fourth is schematic.

The only supplier I know about offhand in your neck of the woods is ELFA. They have Elna capacitors there.

 

[mihaip]

Hi jaycee,

What makes you believe that tone on/off switch may be the cause of one channel dropping out every now an then ?

I am having similar symptoms in my ROTEL RA-971 :
- Left channel makes some noises and drops every now an then, nothing periodic but more often lately.
- The problem appears on direct, remote and phones outputs.
- I think I noticed it for the first time 2 weeks ago late night when the amp was on but nothing was playing.
- This week I played some louder music (not maxed out, just louder than usual) in the morning and after that I started to notice the problem more often.
- If music is playing, the symptoms are noise as if a corroded potentiometer is used, and sometimes I noticed decreased volume.
- If there no input signal only the sound similar to 'corroded potentiometer' noise appears and goes.