Yamaha G100II Head
Just like most of my projects these days, I found this Yamaha G100II 100W, solid state, 2-channel, with parametric EQ, workhorse of an 1980's guitar amplifier. I picked it up along with a virtually destroyed 1970's Ampeg 4x12 cabinet. I'm not mentioning the price, because it's just plain not fair! It was so ridiculously low that I'd really not want to insult or offend anyone. Let's just use the term "insane deal" for this article!
Thanks again Dave, for spotting me the cash, helping me load and unload, and for spending the day with me trying out Grammatico Amps and doing all this guitar related stuff!
Yamaha G100II Guitar Amp Head DIY Repairs
It was busted and not working because the signal wasn't coming out. I say, "the signal" because I could hear some hiss and some very nasty speaker bumps when I turned some of the controls. Most of the really bad issues seemed to come from the parametric EQ section. I thought, "Bummer." There wasn't much more thought, because I had already been prepared to disassemble it anyway. It's how I roll.
So, I opened it up and made several observations:
- There had been a mouse living in there (or at least pooping in there).
- It had been quite some time since this thing had seen the light of day.
- It had sustained some form of high impact. Doesn't show on the outside.
- There was hope for its restoration.
Pictures of Yamaha G100II Amp head.
First off, there was a clean break in the large board, which equates to the right-hand section of controls, including the parametric EQ section. Luckily, though, the break did NOT include any damage to the traces on the board. I planned to fix that with some super glue and set the busted circuit board piece in my now filling bag of random screws, knobs, and other parts.
The next thing I did was to very closely inspect every component, where I found something even more alarming. The parametric EQ "Level" pot had snapped apart, and seemed unrepairable. I made note of that, and the fact that the pot has 4 pins. An odd one indeed. Upon further inspection of all components, I determined that there were no other obviously broken or damaged components.
Part of every DIY electronic project that I undertake involves doing something I learned very many years ago in the US Air Force Basic Electronics training I received ... I learned to carefully inspect EVERY solder joint, and every trace for any signs of stress or bad solder joints. This may seem tedious up front, but it has paid great dividends in the long run! I may not find anything obviously wrong, but in way more circumstances that I'd like to admit, I have found tiny cracks where the solder meets the pin or wire, and that is potential for failure. I touch-up every solder joint that is even slightly suspect. That's a trick I always use to get the job done correctly the first time. It also lets me move forward in the project with confidence that I have covered everything that "might" be a problem.
This time it paid dividends, because not only had the amp taken a hit at some point, it was old. There were quite a few weak, pitted, loose and discolored solder joints on the pots and some of the inter-connecting wires. I touched them all up and moved on.
I removed the reverb tank and found more mouse poop nad an even more disturbing issue. The tank had a broken spring, broken in such a way that the spring is unrepairable. New reverb tanks are about $40 shipped, so that's cool. I did a bit of research and found that the same model is still available at a moments notice, so I removed the broken spring and associated pieces, cleaned it up real good and set it aside. I certainly had bigger fish to fry here.
Yamaha G100II Head Repair Parts
Well... Let's take inventory of parts needed to begin repairs. Parametric EQ Level Pot - 1. Reverb Tank (still a maybe) - 1. So, two parts. That should be easy. Yeah, right.
As I mentioned earlier, the parametric EQ level pot was 4 pins. It has the standard 3 pins that mount to the circuit board, and one pin on the opposite side. The purpose of the single pin is to divide the value of the pot in half, which allows the parametric EQ level to add level in db, or to subtract level in db. It also has a center detente which represents "0" level change from signal input. The Pot and related circuitry was a great solution for that need, but after a short bit of research, not only did I find out that Yamaha wasn't supporting the repairs of these amps, they didn't even have any spare pots to sell me one.
Normal, standard pots can be located, purchased and be at the house in just a few days, but not this one. I researched and found very little on these amps. I hadn't given up yet, though. So, I called my local Austin music shop repair departments. No one had the part, but I got one tip that I followed up on. Some dude in California sells these kinds of Yamaha parts, and actually has a searchable online parts listing. That did me some good.
I must mention here that I did a couple of forum posts asking for anyone that knew anything about parts for these amps. [Can you hear the crickets chirping?] Finally, I had a response from a very cool guy who encouraged me and sent me the repair manual complete with schematics! Hooray! That alone was an obstacle removed from the restoration of this amp. What I wanted the schematic for was to determine if I could re-design or retrofit the circuitry to achieve the same result with another solution. A bit ambitious since I'm NOT an engineer.
So, after determining that the re-design was a bit more difficult than I had hoped, I moved back to attempting to find one of those pots. The schematics have a detailed parts list including all the model numbers for all components. This was where I had resumed hoping that I might be able to restore this amp head back to its original glory. I secretly wanted to jam through it, too! duh.
Remember that dude in California who stocks Yamaha Amplifier and other music gear parts. I researched his website and found exactly the part I was looking for in his database! I thnk I shouted something like, "Hell yeah!" Yes, out loud. My family already knows I'm a bit nutty, so I'm in a safe place to do something like that! I emailed him with the part number, he emailed me requesting a picture, which I had a detailed close-up already.
I emailed him with the pictures, and received a response the next day. I think - Yeah, there's life on this planet, and hope that I can still restore this Yamaha G100II Amp head to its full glory... I opened the email and read "ain't got one" or something to that effect, I'm loosely translating on 1-1/2 cups of coffee, months later, so give me a break here... So, he knew the exact part, recognized it for the specialty part that it is, and he is pretty sure he doesn't have one. I'm feeling a bit more hopeful (desperate, pushy, freaked-out), so I pick up the phone during his business hours. He answers, remembers the email like it was only yesterday (because it was), and restates that he does NOT have one of those pots in his parts bin. [sigh]
Still, being desperately hopeful, I started a series of questions like, "Know anyone else who might have one?" and "Do you know anyone overseas who might have one?" and finally "Is there any chance you have one in some random, miscellaneous parts bin on the back of a work bench somewhere?" Yeah, how desperate does that last question sound, huh? I really must want to fix that Yamaha G100II amp pretty bad. He said he wasn't sure, but he'd keep an eye out for one. Yeah, right.
Yamaha G100II DIY Project Stalls
I stopped even thinking about the G100II for a while. It sat in the corner of my combination studio/office/repair shop - still torn apart and un finished for quite a while. I did several other projects during that time. Picked up a couple of Epiphone Valve Jr. amps and experimented with true 2-channel guitar playing, which I now love by the way. I also found and pulled apart a JR beck tele for a mesquite burl lamination DIY project.
Yamaha G100II DIY Project Continues
I don't remember how many weeks had passed when I received a phone call out of the blue...
"Hello? Is this Scott?"
"yes, it is."
"You were looking for the Yamaha G100II parametric EQ level control pot?"
"That's me."
"I found one in a miscellaneous parts bin on the back of my bench. It wasn't in the right place. Would you still like it?"
(HELL YEAH!) "Oh, yes, I'd still like it. How much and what do I need to do?"
Please call NTS Tech Services, Inc. (714)990-0004 for your older Yamaha amplifier parts.
After working out the details, the pot was expensive, it was here in three days. I slapped it in and hooked everything back up. Viola! Sound in the speakers! I played through it and the sound was sweet and clean. I tweaked the knobs and everyone of them made scratchy, poppy noises except one. The parametric EQ "Q" knob when moved, made the speakers bump so loud, that I thought it might blow them if I moved it again.
Yamaha G100II Head Cleaning the Pots
First, I removed the Q pot and removed the bell housing so I could inspect the inside of the pot. Everything looked good, no cracks in the board nor the carbon element. The wiper looked to be undamaged, as well. So I ran down to my local Fry's and picked up some NU-TROL Control Cleaner with special lubricating oils...
I treated all the pots including the Q pot to the cleaning, and re-assembled the amp to test. WOW! What a difference! The amp sounds awesome and loud. I swept every control through 100% of each pot's range and there was virtually no dirty pot noise.
I have read in a couple of reviews of this amp that dirty pots is a huge problem with these amps. This repair verifies those reviews. If you are lucky enough to find a Yamaha G100II amp head or 212 combo. Be prepared to clean and lubricate the pots. It will virtually come alive again. Mine is alive now, and is ready to be immortalized in this article and in an upcoming review article...
I am truly impressed with my Yamaha G100II amp head!
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