Yamaha Electronic Organ Main Amp Circuit. Fault Somewhere.

[abrogard]

 Can anyone help me fix my Yamaha organ?  It emits a loud 'white' noise when switched on.  And I find fuse 3 on the main amp board was burned out and burns out again when I replace.

 I don't know enough electronics (virtually none) to be able to figure out what to look at next.

[commongrounder]

Do you own a multi-meter?  It’s pretty likely that at least one or both of the output transistors (tr6 and tr7) in the amplifier circuit have failed.  You are going to need a meter to look for shorted devices. Don’t replace the fuse again until you’ve corrected the problem, to avoid stressing the power supply.

[abrogard]

 Yes, I have a multimeter.  Not sure how to use it to test the transistors though.

 And I'll have to pull the chassis out to get at the board and would it work then if plugged into the mains but the 'input' and 'output' plugs not plugged in?

 I've got pics of the machine and you can see the location of the board down there at bottom left of the cabinet.

https://imgur.com/a/DZGxouz

p.s. I see these very cheap DSO's on ebay and such.  Would I need one of those?

[bsfeechannel]

What is the model of the organ?

[DaJMasta]

With a multimeter you can check using resistance or diode modes to see if transistors are shorted - which is a symptom of most total failures.  Just check each pin with both of the others, looking for shorts.

[abrogard]

What is the model of the organ?

 BK7i

What is the model of the organ?

thanks for that. will do as soon as I've got time free of all these xmassy things...   

[bsfeechannel]

BK7i

Thanks. I saw your posts on other forums. It would be a good idea if you could publish the whole service manual with the complete schematics, not only the circuit you posted above. Although in good probability the problem, I think, is related to the output transistors, as mentioned by the other members, there must be stuff connected to connectors US₁-S and US₂-S that we need to rule out as a source of problems.

[abrogard]

Happy to. 

 I've put it here. It is a pdf file:  http://abrogard.com/

  If you just click on the link on that page it should start downloading.  I tested and it worked for me.
 

[bsfeechannel]

Cool. With a bit of luck, it won't be necessary to pull the chassis out, at least for the time being.

Before we proceed, let me ask you a question. The loud white noise you heard was continuous or just a burst?

Meanwhile, show us, please, a close up picture of your multimeter so that we can see its range switch, its binding posts and the display in detail, together with its leads (but not connected to the binding posts).

[abrogard]

 That's pretty easy.  There's pics on the web.   It is a DT-9205A

  And the noise?  It is continuous.  We left the thing on all night by accident and the next morning it suddenly burst into life with this loud continuous noise.   And then it would do it again whenever I switched it on,  with only a few seconds before it began during which I found I could press a key and get a note just as though it was okay.



https://www.petervis.com/meters/Excel_DT9205A/Excel_DT9205A.html

Mine doesn't have the 'auto power-off', unfortunately.  And that push button on/off means tossed in jumbled work bench drawer it can all too easily get switched and drain the battery away.....  I found out....   the hard way....   

[bsfeechannel]

That's pretty easy.  There's pics on the web.   It is a DT-9205A

Brilliant. I have exactly the same meter.

And the noise?  It is continuous.  We left the thing on all night by accident and the next morning it suddenly burst into life with this loud continuous noise.   And then it would do it again whenever I switched it on,  with only a few seconds before it began during which I found I could press a key and get a note just as though it was okay.

That piece of information is important and will help us understand what is going on later.

Mine doesn't have the 'auto power-off', unfortunately.  And that push button on/off means tossed in jumbled work bench drawer it can all too easily get switched and drain the battery away.....  I found out....   the hard way....   

Alright. Let's get your meter to work.

First off, for your safety, please, turn off the organ, if it is not already, and disconnect the power plug from the wall socket. There are hazardous voltages around that area of the circuit, even with the organ turned off.

Now turn the range switch of your meter to the 20K position (next to the Ω sign). Connect the black lead to the COM binding post, and the red one to the VΩ binding post. Turn on your meter on and short the probes out to make sure it is indicating zero or close to zero ohms.

If that is ok, disconnect both the "IN PUT" and "OUT PUT" plugs from their sockets on the board. That way we will prepare it for probing.

Now, let's concentrate on the "OUT PUT" socket. What you probably have is an octal socket. Locate a notch in the center and count the number of pins like in the picture below.



Connect the meter probes to the pins and report what you read:

Black probe 4, red probe 3
Black probe 4, red probe 2
Black probe 4, red probe 8
Red probe 4, black probe 3
Red probe 4, black probe 2
Red probe 4, black probe 8

We say pin, but of course the socket has holes. If you find it difficult to insert the probes due to the small diameter of the holes, and/or the constraints of space, we will have no choice but to pull out the chassis.

In that case it will be better to probe the socket from the underside, in which case the numbering of the pins will follow what you have in the schematics (just flip the picture above and count 1, 2, etc. clockwise from the notch).

If you do that you have to take care not to bend any components or change the trimpots (those with a red blob of paint).

We're probing for short-circuits. So good electric contact may be established between the probes and the pins.

[abrogard]

 Hi..........  I'm back again.    11 months later.  Sorry.  Don't know how I lost touch in the first place but I do know I've had a hell of year this last year.

 So I'm about ready to have a go at this organ again.

 Just wondering if this 'connection'  is still good or I should start another thread?