Troubleshooting 35V on speaker output

[Wikeysz]

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

 

[east electronics]

.....

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

 

[unclejed613]

i agree. find a schematic and put the shotgun away.

 

[Wikeysz]

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.

 

[east electronics]

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

 

[Wikeysz]

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

 

[Wikeysz]

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.

 

[Jimmy154]

I have one good channel, what should I start replacing if anything?
Which resistors were bad? Small ones? Big ones?

I have really loud noise from 4 channels (center is fine) when I plug in an input.
Really loud buzzing popping and high pitched squeal.

My time, patience and troubleshooting skills are pretty limited though

 

[jaycee]

Schematic at:
https://www.hifiengine.com/manual_library/rotel/rb-985.shtml

If you blew a 2SA1942 that means you blew one of the transistors in the output stage. There are 2 of them per voltage rail - chances are the other one is damaged as well. Check the opposite transistors 2SC3856 as well, and their drivers 2SC4381/2SA1667.

A pertinent question is "what happened when the amplifier blew up?"

 

[jaycee]

Meh. Shotgun repair in the end. Chances are if all you did was replace resistors and capacitors, it isnt "fixed"... shrug

 

[jaycee]

Jimmy154 you already have a thread on this. DC on the output is not your problem. As it only happens with connected inputs i'd say it's more likely you have a ground loop being formed. The squealing noises could well be oscillation though.

Honestly if you dont have any experience with electronics, and the appropriate test gear, you're not going to be able to repair it. It is not as simple as just replacing parts.

 

[Jimmy154]

Jimmy154 you already have a thread on this. DC on the output is not your problem. As it only happens with connected inputs i'd say it's more likely you have a ground loop being formed. The squealing noises could well be oscillation though.

Honestly if you dont have any experience with electronics, and the appropriate test gear, you're not going to be able to repair it. It is not as simple as just replacing parts.

I built a LM3875 amp before from schematic and with PCB's
Had a ground loop, but it was very faint.
This is very loud.

My friend told me I couldn't change timing belt, water pump, etc in a parking lot and I rebuilt the whole engine almost in the parking lot
But I needed to do that, since I need the car more than an amp.
Although the timing belt looked really good.

But I hear you, it is hard to do without proper equipment/knowledge.
I guess what any one making these threads is hoping is to get an idea what commonly causes a problem in a specific product from some one that has had the same problem.
Same products have the same problems.
At least with cars it's like that.

Yeah, you could figure it out yourself, but that takes skill, time and expertise, so you're looking for a short cut, right?

Sounds like this will not be worth my time then.

I relearned that from my Velodyne SPL800 amp repair.
Quite time consuming.
Actually the whole DIY audio hobby is quite time consuming.

 

[jaycee]

Ground loops can be small or large, depending on whats happening. The nastier the ground loop, the larger the hum is.

One thing you can try is disconnecting all signal grounds between the amp and the source apart from ONE of them. See if that quietens it down.

Mechanical stuff is often easier because you can just see the problems with your bare eyes. With electronics, its invisible until you break out a meter, a 'scope and know how to use them and where to look with them.

Also in the same way that a car engine will have your fingers off or throw chunks of metal about if not repaired right, an amplifier will go bang with sparks and flames and lots of ruined parts if something isnt quite right. That even happens to the "experts". You'll never quite forget the first time you've had a large electrolytic capacitor explode near you for example

 

[Jimmy154]

Ground loops can be small or large, depending on whats happening. The nastier the ground loop, the larger the hum is.

One thing you can try is disconnecting all signal grounds between the amp and the source apart from ONE of them. See if that quietens it down.

Mechanical stuff is often easier because you can just see the problems with your bare eyes. With electronics, its invisible until you break out a meter, a 'scope and know how to use them and where to look with them.

Also in the same way that a car engine will have your fingers off or throw chunks of metal about if not repaired right, an amplifier will go bang with sparks and flames and lots of ruined parts if something isnt quite right. That even happens to the "experts". You'll never quite forget the first time you've had a large electrolytic capacitor explode near you for example

Funny you mention that.
First thing I thought when I opened it was: I wonder if this thing can kill me?

The caps are pretty big and the transformer is very big, easily biggest I've seen.

I mostly just try to get lucky with these repairs.
Rarely works.
Well I'd like to think I mostly succeed in figuring it out, just not fixing it.

 

[jaycee]

Yes the power supply voltages inside a power amplifier can kill you. There's 57v to ground which would burn you, and 114V rail-to-rail which is practically US wall voltage. And it's DC, so it burns as well.

You can even get a pretty nasty jolt from the speaker outputs if you touch them while the amplifier is operating.

 

[Jimmy154]

Jimmy154 you already have a thread on this. DC on the output is not your problem. As it only happens with connected inputs i'd say it's more likely you have a ground loop being formed. The squealing noises could well be oscillation though.

Honestly if you dont have any experience with electronics, and the appropriate test gear, you're not going to be able to repair it. It is not as simple as just replacing parts.

Well you were half right.
In fact, it was simpler than just replacing parts.
Maybe so simple a crave-man could do it, specifically me

 

[jaycee]

I take it those wires were broken or something ? Where do they connect, to the chassis?

 

[Jimmy154]

I take it those wires were broken or something ? Where do they connect, to the chassis?

Same problem as this guy:
Rotel RMB-1075 noise issue ? Polk Audio

Broken RCA block grounds.
Side PCB's (everything but center channel) and heat-sinks shifted at some point.
I went around the PCB with the ground wires.
Not sure if length of wire matters; but I can make them a lot shorter.

You were right it was a ground issue

Guess I was right too

But I hear you, it is hard to do without proper equipment/knowledge.
I guess what any one making these threads is hoping is to get an idea what commonly causes a problem in a specific product from some one that has had the same problem.
Same products have the same problems.