Amplifier speaker outs shorted

Philip Ferro

Estimable
Apr 21, 2014
5
0
4,510
Sherwood 4 channel home amplifier. Connected b channel left speaker neg to neg and right speaker pos to pos. horrible sound ensued. Figured there was no protection and I fried transistors.
Also figured that as there are 8 separate output transistors, I would still have 6 channels functional. Not so.
Prior to this mishap, once the amp was powered up there was a 5 second delay before a relay would click and sound would come. That doesn't happen, seems that whenever power comes on the speakers are powered immediately, but no audio. Ideas?
 
Solution
Right,so that is in fact a stereo or two channel amp that is switchable to four sets of speakers .
If I understand correctly you have connected a speaker to left pos and right pos instead of right pos and right neg.This in effect would connect the two channels together and could cause output transistor failure .
Since both channels have been affected none of the four output zones will work.
As you say there are 4 delay relays and only one should have been affected,so there must be a problem with the time delay circuitry which would appear to be powering the relays instantly,these delay circuits are normally controlled by a capacitor which may have been damaged by the fault condition.

makkem

Distinguished
Hi
Could you clarify some points.
You say it is a 4 channel amplifier at the top but lower down you seem to think that you should have 6 functional channels suggesting an 8 channel.
Could you supply a model number of the amplifier.
Not sure what you mean about connecting the speakers neg on speaker to neg on amp and right speaker pos on speaker to pos on amp?
Now if you did cause a major short enough to take out the output transistors then there is a decent chance that the time delay relay (which allows the capacitors to charge before connecting the amp to the speakers and so preventing the thump on startup)contacts have been welded so that it is in an always on condition.
Also output transistors are normally used in pairs so there would be two for each of the four channels.
Sometimes the output transistors are protected by fuses so it may be worth checking inside the amp as it may be just a fuse that has blown.
 

Philip Ferro

Estimable
Apr 21, 2014
5
0
4,510


 

Philip Ferro

Estimable
Apr 21, 2014
5
0
4,510
I. Believe it to be Sherwood RX 5502. I said 8 channels, meaning left and right x 4 channels. I opened it up and could not locate any fuses blown fuses or any clearly burnt resistors. There are what look like relay at each speaker out terminal set. Would on set of fused contacts here keep all the other sets from operating? There's is the thump when it's powered on when there wants before.
I was hesitant to get too far into disassembly to check for individual fuses at the power transistors, but will check that when I get home in Friday. Any other suggestions would be appreciated. Thanks for taking the time.
 

makkem

Distinguished
Right,so that is in fact a stereo or two channel amp that is switchable to four sets of speakers .
If I understand correctly you have connected a speaker to left pos and right pos instead of right pos and right neg.This in effect would connect the two channels together and could cause output transistor failure .
Since both channels have been affected none of the four output zones will work.
As you say there are 4 delay relays and only one should have been affected,so there must be a problem with the time delay circuitry which would appear to be powering the relays instantly,these delay circuits are normally controlled by a capacitor which may have been damaged by the fault condition.
 
Solution

Philip Ferro

Estimable
Apr 21, 2014
5
0
4,510
Alrighty then. There are actually 8 relays, one for each set of spkr outs with 8 accompanying power transistors. I should have also mention this is also dual zone.
I really like this unit as it is perfect for its present use. Would hate to have to replace the unit instead of a cap. Thanks for the guidance. I guess I should attempt to locate a schematic. Any suggestions there?