Quote from: phatt on August 29, 2016, 08:23:46 AM
A quick google,, tells me it has a fancy effects loop.

Does the FX control knob have any impact on sound level?
Or turning any other (not so obvious) knobs have an effect on sound produced?

Sometimes the FX loop *Sockets* can cause loss of Volume and just by bridging those loop sockets with a guitar cord can bring it back to normal.

Just trying to cover the easy stuff first.
Phil.

Hi Phil,

The effects knob does not seem to be doing much of anything at all. The bass, treble, and mid knobs seem to be working, but just barely making a difference in the sound.

Jim

Hello,

I have a Marshall MG100DFX combo amp that isn't working properly. I'd appreciate any help troubleshooting and repairing this amp.

Here is the issue I'm having: When using the clean channel, I have to turn the volume and gain almost all the way up in order to hear anything. When using the overdrive channel, there is a lot of hum, as if the guitar cord was unplugged and someone was touching the tip.

Any ideas as to what the issue may be?

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

Quote from: Enzo on July 27, 2015, 05:28:15 PM
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.

Quote from: Enzo on July 26, 2015, 07:31:00 PM
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

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!

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

Quote from: Enzo on July 24, 2015, 06:36:48 PM
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.

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.