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
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
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............
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............
rmsmax said:
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.
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.
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.
Ive heard bad things about the pre amp, most probably uses old type opamps, nothing to do with the amp.
I would suggest rmsmax to replace the output transistors and driver which is most probably the cause of the fault and take the amp to be auditioned against others in its price range. Remember this amp cost a measly 170 euros or so when new, what can you get for that price nowadays. Those slitfoils might still be ok for use, I have some rotels 15 years old now and the caps are still good. The parts could always be used to construct another amp if the results arent good. Couple it to a better preamp for testing.
I would suggest rmsmax to replace the output transistors and driver which is most probably the cause of the fault and take the amp to be auditioned against others in its price range. Remember this amp cost a measly 170 euros or so when new, what can you get for that price nowadays. Those slitfoils might still be ok for use, I have some rotels 15 years old now and the caps are still good. The parts could always be used to construct another amp if the results arent good. Couple it to a better preamp for testing.
Evaas ...no I probably wasn't prepared for such enthusiastic input as ..."taking a saw to it"..... but good to see an optimistic outlook to what I saw as gloomy!
At the time when I bought the combo they were well rated and they were not cheap. As you say Jacco given replacement parts to repair and balance the output stages may be expensive it may be more economical to get a second hand quality amp.
For interest the schematic is attached (I hope).
Anyway great to have your input guys and thankyou.
Gavin
At the time when I bought the combo they were well rated and they were not cheap. As you say Jacco given replacement parts to repair and balance the output stages may be expensive it may be more economical to get a second hand quality amp.
For interest the schematic is attached (I hope).
Anyway great to have your input guys and thankyou.
Gavin
Attachments
Not what I thought but still worth a fix, and easy to upgrade.
For less then 10 pounds you can get get it going again with original parts and if you know how to handle a soldering iron. If you take it for repairs via rotel expect it to be expensive, thats normal, they are dealers and want to make big profits. The ouput trannys cost less than 2 pounds each and the drivers around 80 cents each. Easy to upgrade too, just have to do some work on the compensation. What preamp do you have, here is the weakest link. Have a schematic of that ??
Its up to you what you want to spend. A second hand unit can be had for around 70 dollars off ebay. There are guys selling amps with this topology and some refinements for 300 dollars a channel, complete working amp would cost you over a 1000 dollars. So much for low fi topology.
Going jacco way s would most probably set you back around 70 pounds a channel and the sound quality improvement is debatable. Replacing those power supply caps is what is expensive but this you would have to do even with jacco s suggestion. Maybe they are still ok, because they are high quality slitfoils but its something you will have to do eventually.
For less then 10 pounds you can get get it going again with original parts and if you know how to handle a soldering iron. If you take it for repairs via rotel expect it to be expensive, thats normal, they are dealers and want to make big profits. The ouput trannys cost less than 2 pounds each and the drivers around 80 cents each. Easy to upgrade too, just have to do some work on the compensation. What preamp do you have, here is the weakest link. Have a schematic of that ??
Its up to you what you want to spend. A second hand unit can be had for around 70 dollars off ebay. There are guys selling amps with this topology and some refinements for 300 dollars a channel, complete working amp would cost you over a 1000 dollars. So much for low fi topology.
Going jacco way s would most probably set you back around 70 pounds a channel and the sound quality improvement is debatable. Replacing those power supply caps is what is expensive but this you would have to do even with jacco s suggestion. Maybe they are still ok, because they are high quality slitfoils but its something you will have to do eventually.
Hi Homemodder. I think I'll have a go at what you suggest. I know you aren't in the UK but where should I look for the bits? (used to be local component shops now long gone.... so out of touch!!)
Probably the pre-amp (RC850) is the weakest link and would need replacing to upgrade. Thought about a Lindsey-Hood but I'm sure there are more modern designs. However I can always use the modded power-amp it for bi-amping.
Thanks for your advice and I look forward to having a play!
Gavin
Probably the pre-amp (RC850) is the weakest link and would need replacing to upgrade. Thought about a Lindsey-Hood but I'm sure there are more modern designs. However I can always use the modded power-amp it for bi-amping.
Thanks for your advice and I look forward to having a play!
Gavin
Best to test whats wrong first, although if it was a short on the output, you can bet the output transistors are dead.
The schematic looks typical of most of Rotel's amps of that era. Rotel seem to love the 2SD1047/2SB817 from Sanyo.
Farnell has them, and I'd buy them from there rather than anywhere else, as Japanese transistors are notorious for counterfeiting.
The driver transistors might also be damaged, but I can't make out the part numbers on them to advise. I would guess they are 2SD600/2SB631. Fairchild's KSA1220/KSC2690 would be a good replacement, but Farnell don't stock them (although you could try buying them as samples from Fairchild Direct)
The schematic looks typical of most of Rotel's amps of that era. Rotel seem to love the 2SD1047/2SB817 from Sanyo.
Farnell has them, and I'd buy them from there rather than anywhere else, as Japanese transistors are notorious for counterfeiting.
The driver transistors might also be damaged, but I can't make out the part numbers on them to advise. I would guess they are 2SD600/2SB631. Fairchild's KSA1220/KSC2690 would be a good replacement, but Farnell don't stock them (although you could try buying them as samples from Fairchild Direct)
The suppliers Jacco suggests are good ones, I have purchaced from criclewood and dalbani. Tell them you want original Sanyo parts and not second source, if they have from same batch and group gain its the best bet. I find criclewood to be cheaper in most instances. Test the transistors first, the drivers and outputs, all might not have blown, thats the 2sd1047, 2sb817, 2sd600 and 2sb631. Maybe only one of the output tranies have blown together with its driver, has happened to me many times with rotels.
You could always try get them from rotel, they will sell them to you as well, maybe its cheaper, shop around. Farnell is a safe bet if they have the output transistors. You could also get those equivalent drivers like jaycee has suggested, that might work you out cheaper, depending on shipping charges.
Nothing wrong with linsey hood, I have a preamp of his which is still my reference from 1988. Purchased it as a kit when still teenager, I have upgraded the parts with modern japanese transistors and high quality caps and resistors, it sounds awesome, so far I have only encountered two preamps that to me sound as good or better, both are selling for over 5000 dollars. Linsey hood was a master, I use some of his technics in my own designs, some of them are 30 years old and till today cant still be bettered. A 1969 designed amp of his is still loved, highly regarded and used by thousands around the world.
You could build a simple preamp and put it in the case of your current 850, easy upgrade and wont cost much. Try get the service manual on this preamp, maybe a simple change of opamps will do the trick, as it was an entry level seperate, they might have used cheap low cost opamps on this model.
You could always try get them from rotel, they will sell them to you as well, maybe its cheaper, shop around. Farnell is a safe bet if they have the output transistors. You could also get those equivalent drivers like jaycee has suggested, that might work you out cheaper, depending on shipping charges.
Nothing wrong with linsey hood, I have a preamp of his which is still my reference from 1988. Purchased it as a kit when still teenager, I have upgraded the parts with modern japanese transistors and high quality caps and resistors, it sounds awesome, so far I have only encountered two preamps that to me sound as good or better, both are selling for over 5000 dollars. Linsey hood was a master, I use some of his technics in my own designs, some of them are 30 years old and till today cant still be bettered. A 1969 designed amp of his is still loved, highly regarded and used by thousands around the world.
You could build a simple preamp and put it in the case of your current 850, easy upgrade and wont cost much. Try get the service manual on this preamp, maybe a simple change of opamps will do the trick, as it was an entry level seperate, they might have used cheap low cost opamps on this model.
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.