yamaha g100 1x12 version 3, amp issue - no sound (sometimes)

[mentalmidget]

hi, I bought used yamaha g100 1x12 version 3. sounds great when it works. im having a few issues :

- sometimes, when turning on the amp (it lights up) it either takes a couple mins to start producing sound, or not at all. it'll stay dead silent, then a clicking noise will occur and then its able to start making noise..
- sometimes (usually) it starts producing sound instantly.
- just now, i played for about 5 mins, no issues at first, and then it went silent again. now when i try and turn it on right after, it wont produce sound. this happened before, its like i had to wait a day or at least a few for it to start working again.

not an amp tech at all. no idea what this could be. its like its a loss of power to the preamp section.

https://imgur.com/gallery/yamaha-g100-112-iii-krQQ02U

any tips?

it's an incredible amp. want to fix it.

[Kaz Kylheku]

The flow of signal through an amp is pretty linear, like a factory assembly line. All of the blocks in the line have to pass signal, or else there is no sound. A no-sound-at-all problem could be anywhere.

If the amp starts passing signal without you banging on it, it looks strongly like a thermal issue. Somewhere there is a bad contact: some kind of mechanical hookup, or a cracked solder joint, or a bad circuit board trace, where a slight thermal expansion improves the contact.

One form of contact that is common in amps is the contact between a potentiometer wiper and the resistive track. Amps have multiple level controls at various gain stages, and if any of those lose contact due to oxidation or contamination, then you get no signal. Next time you fire it up, give every single knob a good spin.

Input jacks can be intermittent, and same with speaker hookups.

Are you able to switch channels? Are both dead at the same time? If so, the problem is in whatever is common between the two: any common preamp path and power amp.

Tapping on things with the eraser end of a pencil can help find a problem like this. Like when the amp is working, and you tap on a certain component and there is a loud crackle or outright relapse into nonworking, then you almost certainly found the culprit. Or vice versa, when it's not working, if tapping on something gets it to work or at least produces a noise.

[g1]

Another thing that can lead to turn on delays, or random issues, can be defective capacitors.
You said you hear a click when it starts working.  This may be the protection relay engaging.  It should engage right at start up, but sometimes does not, and will sometimes turn on and off at random (rather than when it receives command to protect).
The relay is controlled by IC111, and it is not uncommon that the capacitors in it's circuit go bad.  Those would be C183 thru C186.
Service manual attached.

[mentalmidget]

thank you. i played the amp yesterday, no issues at all. i dont get it.

[Kaz Kylheku]

thank you. i played the amp yesterday, no issues at all. i dont get it.

Sometimes electrical contacts improve through use. The person who sold the amp might have had it lying around unused.

If it appears to be improving through daily use (you're seeing less and less of the periods of signal dropout) then just keep doing that. If it stops improving before becoming satisfactory, then back to investigation and repair.

I recently got a 2016 Kia Rio, which had mostly nonresponsive cruise control buttons. I thought of opening it to clean out the microswitches, but just by using the buttons everyday, they vastly improved. It is almost 100% reliable now.
I discovered a trick that when the "Set/-" button is not working to initiate cruise, I can hit "Cancel", after which "Set/-" works. This was true even while "Cancel" had not been reliable for its actual job of canceling cruise, though it's working much better now.  The cruise buttons are keyed to a resistor network, creating different voltage levels on a single control line. I think "Cancel" has the lowest resistance, and so most easily discharges a stray gate charge on some CMOS input, helping the next keypress to be read more correctly, or something like that.

Maybe some relay contacts in the amp similarly just need regular exercise?

The previous owner likely disused cruise control. Which kinda makes sense since it's a 6 speed manual. I love cruise, manual or not, but maybe some manual people want to be all manual all the time, gas pedal included.

Another example is the amp that I use for playing guitar. It's a stereo amp but I only use the left channel. The master volume on the opposite channel has totally rotted from disuse. It is horribly intermittent; you have to spin the knob wildly this way and that just to get it to "bite" and pass signal. One day I will wire the cab for stereo and start using that, so I will have to give it some TLC.  However, I think it's going to be coming up to two years pretty soon that I have not used any time-based effects, including reverb! Just distortion. No effects means stereo is pointless.