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Old 13th January 2009, 02:14 AM   #1
rmsmax is offline rmsmax  United Kingdom
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Default Problem with Rotel RB850

Evening All. Found you whilst doing a search on the above piece of kit. Hope you may be able to help me.

Have the above amp as part of 850 system and the left channel seems to have gone dead. Not only that but it also took out the speaker !
The LH channel fuses blew as did replacements. The RH channel work ok.
One cause could have been a short on the speaker leads? Family have been using it!javascript:smilie('')

I used to do a lot of home electronics and have built a number of HI Fi systems in my time, but now a little rusty. Any help would be much appreciated to point me in the right direction.

I have contacted Rotel who have kindly sent me a copy of the Tech Manual which has all the schematics, parts list etc.

On another aspect, is this worth repairing. How does this system match up to the modern amps. I've been out of touch so don't really know.


Many thanks
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Old 13th January 2009, 09:31 PM   #2
mjf is offline mjf  Austria
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hello.
yes,a short on the speakerlead kills many amps,that means the outputtransistors (on the heatsink)are most probably dead.
i have no servicemanual with me,but i would do so: solder off the powertrans. and test them with an ohmmeter or a diodetester (on the dmm).probably there are any other trans. that have failed too (around the powertrans).same test or with the hfe-tester.sometimes there are some resistors damaged.........
new parts built in you can test the left channel without a (speaker)load:measure offset and so,compare the working channel with the repaird........
if you like your amp try to repair him,if not.............
greetings............
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Old 14th January 2009, 04:04 AM   #3
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Hi,

If the manual is in electronic form please email it to me and I will have a look.

Best,

Mitch
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Old 14th January 2009, 08:17 AM   #4
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Quote:
Originally posted by rmsmax
On another aspect, is this worth repairing. How does this system match up to the modern amps.
Poorly, the RB-850 was one of the best deals for a true dual mono power amp 20y ago, but the amp circuitry itself is pretty low-fi nowadays.

The cheapest solution would be to fix the left channel by exchanging the output devices and drivers, would still cost you in the whereabouts of 20 pounds, the original Sanyo output devices will be costly.
Given the age of the power amp you'd likely have to refresh the big electrolytics, also if you merely repair the faulty channel, four 10.000uF/50V caps will not come cheap unless you buy them NOS.

The RB-850 would make a rather smashing conversion project ; twin transformers, quality heatsinks, a great amp layout, and a fine amp case.
You could re-use the electrolytic cap bank section of the circuit board by sawing the amp circuit parts off on both sides.
An example of a possible exchange contender is the low build cost SymaSym amp.
I'm into big boyz power amps, but inbetween a project i assemble small time amps for fun, a circuit as Mikey-B's SymaSym is close to unbeatable in value for money.
With a bit of luck you might find a set of boards at this bum club.
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Old 14th January 2009, 09:25 AM   #5
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Hi jacco what is the topology of this amp, because its old doesnt mean its bad, the caps will be tired though, the rotel uses 2sd1047 and 2sb718 output transistors cheap and not hard to find, easily substituted with even 2sa1943 and 5200. A short at outputs is catastrofic for these amps as is for many of the latest ones, some have now included output protection.

Audio hasnt come up with new better toplogies for many years now and I frequently see better topologies used in older amps. They have mostly been refined and been upgraded with better parts, in the case of rotel they already used some of the finest parts available. You also forgot to mention that for the new amps one still has to buy the same expensive caps, output devices etc.
For the price of new boards and parts for it one can buy the original excellent slitfoil caps for the power supplies or use cheaper high quality ones.
What is special about the SymaSym, its just a typical ltp vas amp as maybe used 35 years ago also low fi if you judge it by the topology. If this rb850 has the topology I think it may have I wouldnt say its inferior over a SymaSym, old tired parts must just be replaced and a tweak or two made for some things which werent clear about audio design of 20 years back.

Rmsmax could you email me the manual too please.
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Old 14th January 2009, 09:51 AM   #6
rmsmax is offline rmsmax  United Kingdom
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Thnks for the reply guys. Attached is the manual pdf.

Gavin
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Old 14th January 2009, 10:04 AM   #7
Evaas is offline Evaas  Canada
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Quote:
Originally posted by jacco vermeulen

You could re-use the electrolytic cap bank section of the circuit board by sawing the amp circuit parts off on both sides.
the poor guy just wanted help with a repair, and you're already taking a saw to his amp! on the fourth post of the thread! why did you wait so long?


welcome to the dark side, rmsmax
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Old 14th January 2009, 10:09 AM   #8
rmsmax is offline rmsmax  United Kingdom
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Sorry bounced - file too big...new to this forum business. I'll try to extract schematic ard re-post.
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Old 14th January 2009, 10:10 AM   #9
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Excuse me, for a moment there i thought i was answering one of the questions he asked.
If you are the kind of guys who exchange the entire EF output stage of one channel and leave the other oldy one intact, be my guest.
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Old 14th January 2009, 10:11 AM   #10
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true Evaas but Jacco is right - this is an ordinary sounding amp. I remember auditioning it with the matching pre and it was obviously inferior to the higher model 870 pre power and also the cheaper integrated (RA840???).

sp
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