Can Anyone Figure Out What Might Be Wrong?

We have an old Yamaha BK7I electronic organ and it just broke. It emits a constant loud noise all the time, won't play.

Well in fact it will play - for maybe one second when you first switch it on. If you quickly press a key you'll hear the tone for that key alright. And then the sound comes.

So that's a clue. Shows the power is there, etc. And sounds to me like a capacitor charging up and going bad when it gets enough charge.

But I'm very, very ignorant.

There's no Yamaha outlets hereabouts, no service centres and actually no electronics technicians or shops hereabouts either.

So I'll have to go it alone. I have the service manual is all.

I wonder if anyone is interested in perhaps looking at the circuit diagrams and taking a guess at what part might have gone bad?

It is a 62 page manual so I won't put it up here. I could of course pass on to any interested parties any part of it they'd like to see.

Right now I'll attach the circuit diagram for the main amplifying circuit - which is where I'm thinking the trouble probably lies. I wonder if anyone agrees with me on that, for starters?

You haven't given much to go on, but your assumption that it might be a bad capacitor could be the accurate, although not for the reason you described.. Electrolytics are a very common point of failure. Also check for cracked solder joints in areas that might see power/heat, and also check your grounding.
Also, very carefully -- to avoid getting a shock, or shorting out your circuit -- with a non-conducting object, you might want to try putting gentle pressure on various parts of the board while it is powered up to see if any changes occur. This procedure can also be mimicked by using a heat gun or cold spray if you have it. Again, use extreme caution!
Also, check for rodent damage!!!
Maybe you can send some close up pictures of the board?

Is 'PJ' (Schematic upper left) a plug/jack? If so, unplug it and see if the noise goes away or stays the same. If you can't unplug it, disconnect the wire at 'PJ'.

If it goes away, we will need to look at an earlier section. If it stays the same, then...

Using a voltmeter (and caution as hyedenny said) measure the voltage at the collector of TR4 (or equivalent point). Should be +32 volts. Negative lead of the voltmeter on chassis ground - (or US1-S pin 8 or US2-S pin 2). Then check the voltage at US2-S pin 8, should be -32 volts.

Thanks a lot, guys, that's great.
I will follow your suggestions.
Meanwhile I've put some pics up on imgur, I'll link to them in this post.
I'll get a cotton bud or something and clean up the dust and dirt on the thing a bit.
I'm a bit wary of those big metal capacitors there. They could be dangerous? I should short the cans to earth?
The thing is running on + or - 32 volts once we get away from the transformer, is that right?
So this should be the link if things work right:

main amplifier circuit yamaha bk7i

abrogard said:

Thanks a lot, guys, that's great.
I will follow your suggestions.
Meanwhile I've put some pics up on imgur, I'll link to them in this post.
I'll get a cotton bud or something and clean up the dust and dirt on the thing a bit.
I'm a bit wary of those big metal capacitors there. They could be dangerous? I should short the cans to earth?
The thing is running on + or - 32 volts once we get away from the transformer, is that right?
So this should be the link if things work right:

main amplifier circuit yamaha bk7i

Click to expand...

shouldn't be dangerous. old CRT TVs are dangerous because the stored charge is thousands of volts.

Check to make sure none of the keys are damaged and in the on position, there might be a rubber mat type thing under the keys with one of them collapsed and constantly making contact, also might be coffee or something spilled onto a contact.

dr pepper said:

there might be a rubber mat type thing under the keys with one of them collapsed and constantly making contact

Click to expand...

that's the type of contacts in newer Yamaha keyboards, but that one has TO-5 style transistors in it, most likely keyswitches in it are either microswitches or contact busbars. i think the most likely cause of noise or oscillation is going to be dried out electrolytic caps. TO-5 style transistors would make that keyboard at least 35- 40 years old. electrolytics are the first thing to check in something that old.

also, what might help is a description of the noise it's making. there are a lot of things that could be abnormal noises:
1) RANDOM NOISE
a) white noise... a hissing sound like air escaping from a tire
b) pink noise... a full range rushing sound like an FM radio not tuned to a radio station, or like a waterfall
c) red noise.... a rumbling sound like an earthquake

2) REPETITIVE SOUNDS
a) repetitive pinging, clicking or ticking sounds
b) motorboating... sounds like a boat motor, or motorcycle engine
c) "blocking oscillation" where an interrupted tone is heard with a repetition rate from once every few seconds to 2 or 3 times a second (any faster than that begins to sound like "motorboating")

3) CONTINUOUS SOUNDS
a) high pitched whine or screeching noise
b) midrange whine or buzzing
c) low pitched hum or buzzing (two types, first is hum or buzzing at the AC line frequency or it's second harmonic. i.e. 60 or 120hz in usa/canada/japan, and 50 or 100hz most other countries.... second type, hum or buzzing at a frequency NOT related to the AC line frequency)

i tried to keep the list simple, i know there are audio engineering textbooks that have a very long list of abnormal sounds an audio system can make. i'm trying to think off the top of my head what the most common failures that make noise through the speakers, and the 3 categories and 9 (and 1/2) specific types of noises seem to be a fairly broad set of descriptions. there could also be RF getting picked up by the amplifier, but the OP didn't say he was hearing a local AM radio station through the amp.

EDIT: 4th category:
4) POWER CYCLING SOUNDS
a) turn-on thump
b) turn-off thump
c) a chirp when unit is turned on (or off)

when there are turn on thumps, if the speaker cone returns to where it was before the power was turned on, that's ok. if the cone remains displaced after the thump, the amplifier has a DC offset problem, and should not be used until the cause has been found. sometimes there will be a thump followed by a hum twice the AC line frequency, and the speaker cone remains displaced (and may smoke too). that's an indication of a shorted output transistor. i call that the "expensive sound" because it not only indicates shorted output transistors (as well as the driver transistors, and everything before them) but it usually fries speakers as well.

Yes your right unc, I just looked it up.
I'd say its busbars with wire contacts to each key.
The only ones I've repaired were ones you can carry.

That unit has a built in amp, if you can try the keyboard via its line out, if that works its the power amp causing it, and thats where the fattest electrolytics are.

Edited, Off topic

Here's the block diagram of the Yamaha Model B4B.
Is there a similar block diagram for your BK7i model?

I used to have the BK4C service manual in hardcopy, but couldn't fine it anymore. The organ from this age used many small cards connected together with bundles of wires and was very hard to trace the problem when there's a fault. I think all the boards use transistors except the main tone generator which uses ICs.

abrogard said:

Well in fact it will play - for maybe one second when you first switch it on. If you quickly press a key you'll hear the tone for that key alright. And then the sound comes.

Click to expand...

Yes the problem could be from the Main Amp board but it can be also from further up-stream.

When the expression pedal was at the minimum, is the loud tone still there?

Allen

ok, well now i get the idea that the OP has moved on.... no reply to questions about what the nature of the "noise" or "sound" was. without a description of what he was hearing, any other speculation about it is really pointless.

May be you're right; or it could be that something more important came out and he was distracted. Lets wait at least a week or two and see if he does come back.

Hopefully the problem was just a capacitor in the PA stage.