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Old 5th November 2008, 01:20 PM   #1
Wikeysz is offline Wikeysz  United States
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Default Troubleshooting 35V on speaker output

I have a Rotel RB-985 amp that has lost one channel.The channel that was lost shows about 35V DC at the speaker outputs.I also have voltage of 15 and 35 at places on the board that show 1.7 on the unaffected areas.I had one bad 180v transistor which has been removed,but there is still a problem somewhere.I have tested remaining transistors with a Ratshack transistor tester,and checked resistors.Could only do simple tests of the caps with a DMM.I also removed the OP Amp with no change.Somewhere voltage is being introduced ,but I have not found the source.The good news is this amp seems to have independant areas for each channel,so comparing voltages has been easy, however I have failed to find the bad part.Could someone point me in the right direction? I tried searching with a couple of key words,but did not come up with a good match for my problem.I have found the replacement transistor(2sa1492)but would rather wait to order till I find out what else I need to replace.Thanks in advance.Jeff
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Old 5th November 2008, 04:23 PM   #2
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Default .....

dont expect any fastfood solution to your problems by any forum member .....

it could be a miilion of things from very simple till combination of things ....

to my understanting you obviously have a leaky ( or more ) devices to your amp like output transistors like driver transistors small other transitors used in vbe ccs and so on ....

one simple resitor that is open or leacky .....one capacitor anything that puts your amp out of balance will have the result of DC in the out .....

google answers to problems like that dont exist and if they do they will refer to a specific problem ....

you should first start with a schematic and then if you are able to read it you will have to isolate part by part every stage of your amp and start checking one thing at the time

regards sakis
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Old 5th November 2008, 11:04 PM   #3
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i agree. find a schematic and put the shotgun away.
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Old 6th November 2008, 10:22 AM   #4
Wikeysz is offline Wikeysz  United States
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Thanks for the replies,No shotguns or fast food here! I would like to find a schematic for this thing,but no luck with google, and Rotel has been rather unresponsive(not bashing,just reality).Just thought maybe there was a common failure point when one (power?) transistor goes bad,but all others seem to test fine.I admit I do not totally understand amplifier theory,but do know the basics of how it works,and understand the different components on the board. I had hoped the one bad transistor was it, but there is another problem somewhere.I have 4 working copies in this 5 channel amp,as it seems that only one channel is effected, the voltages on other channels appear fine.Almost as good as a schematic,but not quite.
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Old 6th November 2008, 10:32 AM   #5
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Default then .....

if you have another 4 working chanels there is a simple and very clever way to show you exactly where the trouble is it will only require a multi meter ....

usually when you use a multimeter you place one lead to ground and the other one for example to psu caps to mesure voltage ..... correct ????? yes !!!!

if you have next to you a working channel you will have to use the multimeter in another way you put one lead to one resistor example : R128 and the other lead to the exact same spot one the exact same side of the same resistor on a working amp .....

if you read a diference of a few millivolts all i fine
if you see a lot of voltage then trouble is arround there

regrds sakis
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Old 9th November 2008, 09:34 AM   #6
Wikeysz is offline Wikeysz  United States
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sakis,your help is greatly appreciated.I have not had time to test since my last post.I will try your testing method. I did not know of that way of testing components,Thank You.Another positive note,the support person at Rotel was able to E-mail me a copy of the service manual after all,which includes a full schematic and parts list.This will be of great help to me, and I want to thank Rotel support for that.I will post my progress. Thanks again,Jeff
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Old 29th November 2008, 09:00 AM   #7
Wikeysz is offline Wikeysz  United States
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Thanks sakis again for your help.I have fixed the amp, although I do not know the exact cause of the problem.I found a few resistors out of spec, and had no way of testing the caps,so all the caps and the resistors out of spec got replaced.It was interesting to find that a few components on the schematic did not match what was on the board.So much for following a schematic.Thanks again.
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