JC120 Loud Buzz

[J M Fahey]

While the amp is on the speakers do push out

If cones push forward or backward and stay there, you *do* have DC.
You will also have loud hum.

Not sure what are you measuring.
Set meter to 200VDC scale, press firmly one leg against one speaker terminal the other one to the other terminal, turn amp on for 10 seconds (you needn't more than that and we do not want to kill speakers) and get reading.
Then repeat on the other speaker.

There's also no justification for the 30V DC measurement, appearing *after* amp turns off, and *slowly* falling to 0 .

Can't trust your measurements because they don't match reality or each other.

[Tate]

Ok so fist off, the measurements i took earlier were in fact DC on the speakers, i just double checked. Sorry for the confusion. I just took a look at Q27, and it tested fine, at those voltages. R151 was at 40V. Now, Q11 appears to be shorted. I took an ohm reading between the emitter and collector (pin 1&3) and got a reading. If Q11 is in the preamp, then bypassing the preamp by plugging a cable into the main inputs should eliminate the buzz correct? Im not completely sure that I'm testing these small transistors correctly. By testing between pin 1 & 3 nearly every transistor on the board has tested as a short. Could this be due to excessive supply voltage going to them?

[Tate]

*Update* I just hooked the speakers back up to see if bypassing the preamp would get rid of the hum. As soon as i bypassed the preamp, the hum stopped. About 5 seconds later, I got that familiar burning electronic smell. I checked the speakers, and they are fine.

[oc disorder]

Well Q 11 provides power to the pre-amp and doesn't to the power amp.The schematic says its a 2SD667.
The two outer terminals in this case are the emitter and base and if you look on the diagram you can see a resistor of 4.7k half watt across them.
Your meter may measure this.
There are plenty of sites and the odd video detailing how to check transistors with a multimeter.
It helps if you have a diode checker built in and know the "pin outs" of the transistor.
To properly test it you may have to remove it from the circuit.
I think the transistors might not be damaged... for instance that q11 is rated up to 80 volts it possibly your testing methods.
The higher ohms scale on your meter will read surrounding components connected across or to the transistor you are testing.
If you have a diode test function check the zener diode going to the base of Q 11.
In one direction you should get a voltage drop of .5/.6 of a volt. (of course with the amp off.)
So there is no 30v where the emitter of Q11 joins the positive terminal of C 13?
You could try to see if the Main in plug test eliminates it but being a power supply fault I find it hard to tell.
Of course you are using your Light Bulb Limiter you recently made ar'n't you?????

[oc disorder]

Can we have a second attempt at a DC reading from the speakers please as Juan suggested.

Sorry, I missed where you said the readings were DC.

So this is now correct?

On channel 1 (brown and red) there was 0.01VDC at 0.05A. On channel 2 (black and orange) there was 0.067VDC at 0.44A.

Don't comprehend the current reading but I'll ignore it .

See if you can sniff out what burnt!

[Tate]

The current reading was from red/ orange to ground. With those readings i did not have the amp connected to the speakers. I realize that is a bad idea but at that point I was more concerned about saving the speakers then running the amp in a healthy way. Ill try again tomorrow with the speakers connected as i assume that had an impact on the readings. Thanks for the pinout diagram for that transistor, I was in fact testing it wrong. Plugging into the main in did eliminate almost all of the hum. Shortly after plugging into the main in is when i smelled the burning, not sure if its related or relevant. And yes! I absolutely am using my new lightbulb limiter! Pardon my lack of knowledge here, the whole reason I'm doing this myself is to learn more about all of this. I really want to have an in depth understanding of solid state and tube amps. I really appreciate the help!

[g1]

Current readings can only be done when you break into the circuit and place the meter in series. When in current mode, your meter is like a straight piece of wire. So if you put your meter in current mode and connect some point to ground, you are shorting that point to ground and things get fried.
If you are not familiar with doing current measurement, it is very hazardous and you should not do it.

[oc disorder]

Well if something bursts into flames we'll probably be able to find the fault !
Switching to the current range is not a good idea when across a transistor amplifier output.
It's shorting out the hot output to ground.
It does have to be in series but with a load connected .
That way we can see how much current is passing through the load in this case speakers.
A couple of large wire wound load resistors in place of the speakers (a dummy load) would be
the better way to do it or a network of smaller wire wound resistors.

I was looking for a way to separate the two pre-amps while we are trying to narrow down the fault.
There's two connections from terminals 5 and 7 which run to the second channel board to terminals 11 and 9.

If we temporarily disconnect them at preferably 11 and 9 (whatever is easiest) hopefully we can then determine
which board carries the fault.

Another quick hum test.
While I was looking at that I noticed that Channel 1 has it's own 30volt supply for the IC's
coming from the main 37volt supply to the first power amp.
Q4 is the transistor there and circuit is identical to the one with Q 11.It's a 2sc667.

So do we have 30 volts there as well as the one with Q 11?

Attaching that section from Revision 5 schematic .. which I think yours is.

[oc disorder]

It maybe worth disconnecting the whole supply to that board which will stop the power amp
there working too.
Terminals 12 and 19.
Make sure the chassis isn't grounded otherwise a grounded soldering iron will ark at the terminals
as it shorts the charged power supply capacitors to ground.

If that makes no difference and it still hums the fault must be on the channel 2 board.

You could leave channel one disconnected in that case while you concentrate on the other board.

[AmpFix]

You need to stop turning the thing on and remove the output stage.

1) If the main transistors are gone, you should also consider replacing the driver transistors.
2) Check all resistors and caps around those transistors (out of circuit).
3) Check the the speakers (dc ohms) are still good and move freely.

[mikeeboy]

Greetings of Peace! im trying to fix the same amp model with the same schematic, can somebody explain to me what happens if instead of +30V supply for the CH2 preamp is only +15V? i am currently working on the same amp, and found a problem on zener D16

[Enzo]

Hi Mike, please start a new thread for your amp.

A bad 30v zener would certainly compromise the 30v supply.