Tech advice needed for Rotel RB-850

[pjeaton]

Hello All,

My Rotel RB-850 amp has an annoying problem.

Either of the two sound channels keep switching themselves off during operation, after, say 30 monutes of operation. Sometimes the channel will come back on again after a couple of minutes. They've never gone off at the same time.

Giving the case a whack with my hand doesn't seem to help, but if you power it off and a fraction of a second later power it back on again, both channels come back on again.

Can anyone give me an idea as to what might be wrong?

Does anyone have any circuit diagrams for this amp?

I have a degree in electronics, but I got that 15 years ago and have never really done anything since then with analogue stuff. (I'm pretty good at fixing old computer circuit boards though :-) )

I really like this amp and have owned it from new since the late 80s so I'm pretty happy to get stuck with the soldering iron.

Any thoughts very much appreciated!

Thanks,
Phillip Eaton
London, UK

[Netlist]

Hi Phillip and a warm welcome.

Does the amp have an output relay? Does it turn on when the amp switches off?
Probably not as most amps have one relay for both channels but let me ask just in case this one has two of them.
What DC do you measure at the output? Did you check for bad solder joints? Are your interconnects OK?

/Hugo

[tubenut]

If there is an output relay the contact could be dirty/worn....

[anatech]

Hi Phillip,
Take the cover off and locate the relay. Give it a "twack" with your finger. Try from different angles with the volume low. Also, slowly operation the various switches. The tape loops are good for this type of thing.
-Chris

[mpopovics]

There is no output relay in RB850
Check the electrolytics, this is an older power amp, and could bethe root cause, that they are just simply dry out.
My brother's RA-8xx-series integrated had same problem.

[anatech]

Right you are, no relay on this. I have seen some with bad solder connections to the speaker terminals. Of course you may have bad connections anywhere that don't always react to a sideways slap to the case.

-Chris

[mastertech]

could be the overload protection shutting down the channel
sensing some kind of fault

[anatech]

I think what we need now is a schematic.
-Chris

[gni]

I have been researching the Rotel RB850 amplifier for several years.

No relays in this model.

Maybe a transistor on the heat sink is overheating since it
might have come loose. .check the mounting screws on
the heat sink (while the power is off). Also, take the cover off
while operating and put your fingers near similar transistors on
both sides. . .there should be equal heat coming off of them.
This could account for the left channel cutting out at different
times than the right channel.

A bad capacitor is the next suspect. . . the transistors
could be shutting down since there isn't enough voltage to keep
them open. Take the time and replace the electrolytic caps.
There are better caps out there. . .re-solder all contact points.
Tthis could take an hour or two but on older amps. . .worth the time.

The Rotel RA-02 has a similar schematic. . .a few extra
parts and some missing in other areas. . .This site offers
for free schematics of some similar amps to the RB850.
www.eserviceinfo.com/browse.php?id=32

I will answer any questions you may have (as far as I know).

Sincerely,

Chris Browne
Ground Noise Industries

[AKN]

Hi There!


Check bridge mode switch also.

Only have schematic on one amp section.