Yamaha G50 112 parametric EQ squeal

[jjm_335]

Hi all,

New to the forum. I just picked up an old Yamaha G50 112 for $50. Though I haven't tried it out yet, the previous owner tells me that it works fine except that when you turn the Q or frequency knobs of the parametric EQ section, the amp starts to squeal loudly, and the only solution is to turn the amp on and off. I'm OK with using the amp without the parametric EQ (especially for $50), but wouldn't mind troubleshooting it. I found a copy of the service manual online, and I see that there are couple of ICs in this part of the circuit. I know some of these parts can be hard to find now.

Any quick ideas as to what might be the cause of such an issue? My instinct tells me it's some kind of feedback loop, but beyond that I'm just guessing.

I know this isn't much info, but if any of you has any thoughts I would appreciate hearing them. Thanks!

Justin

[nevetslab]

Welcome to the Forum!

Well, we can be sure the single-band parametric EQ circuit did work correctly when new, so it's possible one or more components have drifted since then. Usually, the tendency to squeal or oscillate occurs at the high Q settings, but odd that it would squeal by just adjusting the Frequency control, unless it's in max boost & high Q. I'd start by exercising the controls for that parametric EQ section (rack them ack and forth a bunch).. Then, I'd look for solder fractures on the pot terminals...a common place to find such faults. If all is ok there, check to see if the resistors in that network are what they claim to be. Then, it might be you would want to substitute the film caps...though I doubt they have changed in value...very rare to find that.

Does the circuit behave correctly at low Q?

[Enzo]

You haven't tried it yet? So please DO try it, then we will know exactly what it is doing rather than someone else's report.

When it acts up, ball up your fist and whack the top of the amp. Does that affect the problem in ANY way?

[Leo_Gnardo]

These are great amps - of course when they're working right. A customer, who's a well known guitar builder, has one and he loves his to pieces. That parametric EQ makes it super versatile for tone choices. His G50 was doing the same annoying tricks, maybe 20 years or so back. If I recall correctly the tracks of the Q and frequency pots were worn out, and there were places where the wiper wasn't contacting the conductive part of the track anymore. Again, IIRC, I had to transplant tracks from other pots in order to get the parametric to work. After that, the frequency markings on the dashboard no longer matched the actual frequency the filter was operating at, but at least we got the EQ working again.

[jjm_335]

Thanks all for the help. I did finally get to plug it in and try it. What a great sounding amp! I don't hear the squealing the guy described to me. Rather, I hear a lot of noise from the pots when turning them. The noise sounds like dirty or worn out pots, so I'm going to pull it apart and just clean it. If I'm lucky, that will take care of it!

Thanks again,
Justin