Yamaha G100 - fried voltage regulator?

[sound=waves]

Hi all,

this is my second amp repair project – I'm zeroing in on a diagnosis here & wanted some feedback.

Amp is a Yamaha G100-212 III. My bandmate loves to run the thing at full blast, which he was doing when it died during band practice. The sound sort of fuzzed away then disappeared. The power light still comes on and I can hear the relay click. Preamp section works fine - tested via the amplifier's headphone jack - but there is nothing getting to the speakers, not even the usual pop when the relay comes on. Speakers check out, so I'm assuming trouble in the power amp section.

Looking at the board traces by the speaker jacks, I can see a spot that looks burnt - components there are a 7815 voltage regulator and a 24vdc relay. Input voltage on the regulator is ~15VDC, but the output is like 24V which is out of spec. I can't seem to find a pinout for the relay and I have no schematic so it's hard to say what should be going thru it.

My questions to you: is the problem likely just the voltage regulator, or is the relay potentially fried too? Should I just go ahead and replace the 7815 and see what happens? Should I tell my bandmate to turn the **** down?

Cheers,
Jesse

[Jazz P Bass]

I think you are reading the 7815 incorrectly.
Looking at the front of it:
Left pin: input Vdc (24V)
Mid pin: Ground
Rht pin: output Vdc. (15V)

[olddawg]

It is not uncommon for regulator circuitry and the pcb in that area to become discolored over time. They carry a lot of current and disipate a lot of heat. If your speaker protection relay is not clicking on, my guess would be that circuit is detecting a DC offset in the output stage usually caused by component failure. Check the speaker first and verify that it is not open. I say this because you say that the headphones are working. Some amps use a separate active circuit to drive the headphones. Most use a voltage divider off of the output before the protection relay. You may simply have blown a speaker. If the speaker is good, find the protection relay and see if you measure any DC on the input side. If there is, the problem is with the output stage. At this point you need a schematic, experience, or both. If you can find a schematic we can probably walk you through the repair if you have basic electronic skills. Be aware that they write "Lethal Voltages Inside" on the case for a reason.

[sound=waves]

P Bass, you're correct, I was reading it wrong. Input voltage is 24.3V and output is 14.8V.

olddawg, I thought I had verified speaker continuity before, but I must have been doing it wrong – this time I removed all connections from the terminals of both speakers and tested for continuity with the multimeter. Guess what? Open circuit.

Thanks for the responses guys, I am definitely still a noob at this stuff! Do you think it's likely that the amp is damaged as well as the speakers? I've been powering it up and have played a bit through it while listening thru the headphone jack - would this be enough to damage the amplifier since it wasn't actually connected to a load?

I have a 4x12 cab at the practice space that I could run the amp thru, but don't want to destroy those speakers as well...

[olddawg]

P Bass, you're correct, I was reading it wrong. Input voltage is 24.3V and output is 14.8V.

olddawg, I thought I had verified speaker continuity before, but I must have been doing it wrong – this time I removed all connections from the terminals of both speakers and tested for continuity with the multimeter. Guess what? Open circuit.

Thanks for the responses guys, I am definitely still a noob at this stuff! Do you think it's likely that the amp is damaged as well as the speakers? I've been powering it up and have played a bit through it while listening thru the headphone jack - would this be enough to damage the amplifier since it wasn't actually connected to a load?

I have a 4x12 cab at the practice space that I could run the amp thru, but don't want to destroy those speakers as well...

It's a solid state amp so it doesn't care that you have been running it with an open (infinite) load. As long as the speaker load that you plug it into is not less than the minimum load the amplifier is rated for the amp will be fine. If the headphone jack is working the amp is probably working. But before you hook up your nice cabinet, use your multimeter to see if there is any DC voltage across the output connections. That may have been what killed the speakers. But if the guy was always diming it like you said earlier, the speakers might have just failed. If there is no DC present with the amp on and the speaker cab is of a reasonable load for the amp (and it probably is), hook the cab up. It will probably sound better than the original speakers.

[sound=waves]

Oh yeah duh, open circuit = infinite load, I was thinking the opposite for some reason. Anyway, there doesn't appear to be any DC across the speaker out, so I'll try hooking it up to the external cab. I bet it'll sound great.

Thanks for the prompt and helpful responses! I appreciate it!