Help Diagnosing a Yamaha B100-115III Bass Amp

[CarverZ]

Hello,

Can anyone help diagnose a Yamaha B100-115III bass amp? Here are a few pictures of the amp. The third shows a resistor that appears to be burnt (doesn't show up in the pic well). I also have a service manual with a schematic. I'm not experienced with electronic repair, so any help getting this amp working again would be greatly appreciated.

[Enzo]

Can you post the schematic for us?

This looks familiar, did you post this question on another forum?

[CarverZ]

Can you post the schematic for us?

This looks familiar, did you post this question on another forum?

Here is the schematic.

I did have this posted on another site but didn't receive any help.

[phatt]

Can I suggest you explain what is actually wrong with the amplifier otherwise it's a guessing game.
Like ; any sign of life,, does it power up or is it completely dead?
History of the rig is also useful. :tu:
Phil.

[Enzo]

Does the speaker relay click on after a couple seconds?  If so, that resistor is OK.

I agree with phatt, it is difficult to assist until we know what is wrong that you are trying to fix.

[CarverZ]

The amp powers on but there is no sound. I plugged the speaker into another head, and it works fine.

[J M Fahey]

Just tom begin with something:
1) put your ear close to the speaker and turn it on.
Hear *anything*  at all? : even sight hum/hiss/pop/click/buzz .
There is not such a thing as a 100% silent amp ... if it's alive that is.
2) as Enzo said, there is a muting relay connecting the power amp board to the speaker jacks ... if it does not get activated (check for a mechanical click sound) , it stays muted
3) that relay is activated (or not)  by a speaker protection circuit, which will keep speaker OFF if there is any DC voltage at the speaker out (which otherwise would destroy the speaker)  so measure DC voltage at the PCB itself, at the speaker out rail.
One convenient point may be where the "2p 0.22" resistors meet, they are right and left of the small heatsink, in the middle of picture 1.

Ok, start with that.

[CarverZ]

OK, so there isn't any sound from the speaker at all. Also, I don't hear a relay click.

The resistor that looks burnt, got extremely hot, it actually burned my finger!

[Enzo]

Do me a favor, I am an old guy, humor me.  Look at your schematic, that resistor is in series with the relay coil.  parallel the coil is D110, a snubbing diode.  Measure that diode to see if it is shorted.  No need to remove it, if it is shorted it will measure so, and if it is not shorted you will just be measuring the resistance of the relay coil, probably a couple hundred ohms.

[CarverZ]

Do me a favor, I am an old guy, humor me.  Look at your schematic, that resistor is in series with the relay coil.  parallel the coil is D110, a snubbing diode.  Measure that diode to see if it is shorted.  No need to remove it, if it is shorted it will measure so, and if it is not shorted you will just be measuring the resistance of the relay coil, probably a couple hundred ohms.

Enzo,

That diode is measuring .002

[Enzo]

Sounds shorted to me.  But did you use the ohms scale or the diode test? I am looking for low ohms.  If it is shorted, I would replace the diode.  Since they cost 2 cents, I'd probably replace it either way at this point.

If that diode shorts, it will cause the 390 ohm resistor to run hotter, plus it will prevent the relay from energizing.

[CarverZ]

Sounds shorted to me.  But did you use the ohms scale or the diode test? I am looking for low ohms.  If it is shorted, I would replace the diode.  Since they cost 2 cents, I'd probably replace it either way at this point.

If that diode shorts, it will cause the 390 ohm resistor to run hotter, plus it will prevent the relay from energizing.

I used the diode test.

[DrGonz78]

Yup the diode is shorted. Oh and add that it will measure .002 either way you place your meter leads on either side.

[CarverZ]

You guys are the BEST! I replaced that diode and the amp is sounding great again! Thank you ALL for your help!!

[Enzo]

it's almost like we've been doing this a while...

I'm glad it worked out so well.