Yamaha G100-112 Repair

We were on the lookout for another Guitar amp that has a clean tone. Well truth be told, Im ALWAYS looking for a guitar amp in my budget that has a better tone.

We found this Yamaha G100-112 first series. This was probably made around 1980. Yamaha came out with a Series II and a III version of this amp. The II series looks a little different, the III looks a lot different on the outside.

This particular amp needed some work.
I had just purchased Fender that also needs work, so I talked my Father into buying this Yamaha in the hopes I could repair it for him.

It was/is a very clean looking amp. The guy that had it before us really took care of it. Talking to him, he really did not want to sell it. It was not working right and the estimate to repair the amp was more than he wanted to pay. He decided to sell it.

What's wrong with you you ask?

1. It was not amplifying the sound like it should, you had to turn it all the way up
2. loud Popping and crackling noises
3. The pots are real stiff
4. One of the knobs will not stay on
5. Minor - some very small nicks in the outside.

I have very little experience in repairing guitar amps. Only very simple stuff like cleaning pots (I know how to do the dishes, but not that kind of pots). I actually hope to repair some old radios and CB radios in the future.

Old Flux everywhere

I tested the amp and sure enough it was having those issues.
So, go with what you know. The pots are stiff, so clean the pots and see what happens. I've been in contact with the Seller Via Text Messaging. The seller is very upfront and honest.

Pulled the amp out of the cabinet, and noticed it is very dirty. There is old flux everywhere.
I went ahead and sprayed some contact cleaner in the holes on the pots and worked them a LOT. I spent over an hour cleaning the pots  until they move very freely.
Put it back together and tried it out. The popping was less, but still there was some crackling.

wire damage, maybe done with pliers.

Knowing the inside was in a dire need to be cleaned, I grabbed some isopropyl alcohol and an old tooth brush and went to town. Once I got some of that old flux off, I noticed some of the soldering did not look like it was making good contact anymore. I also noticed a few wires that the insulation is nicked off. Im not sure what causes damage like that. See the picture to know what Im talking about. I unsoldered that wire, cut the end off, and soldered it back on.

The back of the PA Board cleaned up.

I do not recommend the following! but on the same aspect, Im on a journey to learn more. 

Who knows maybe someday someone will want me to fix their amp. The more you learn the better you will be.

I started testing components with the new tester I bought LCR-T4 tester. 

I also wanted to clean the glue off that was all over the place. I had read that some glues becomes conductive when it gets old, and could cause a shorts.

Cracked Input jack housing

In the mean time, I was still googling around and found this Forum Solid State Guitar Amplifiers

First I want to say the people on that forum are AWESOME! They love fixing Solid State Amplifiers, HEY just what I want to get into and learn as a hobby. A Win Win for me!

Glue and Flux everywhere.

Scraped the glue off.

See all the Gunk on these Diode Leads?
That stuff was on all the components.

Yup, they told me the errors in ways of striping these boards apart. I agree. I do not plan on stripping the next ones, only if then really have a problem and need it. But again, Im in learning mode and want to see everything.

But while striping the boards and testing the components I found a bad transistor, and a cracked input jack. Plus all the bunked up parts.

After isopropyl alcohol cleaning

I could not find the right transistor to buy, they do not make them anymore officially. I saw some off brands, but also read some bad reviews from people buying those. I did not want to take a chance. I was on a hunt to find a replacement.

Parts back on the DC Board

 One of the guys on the forum looked at the wiring schematic and noticed that 2 of those JFET transistors really were not needed. That I could just take the pair out.
I just could not understand why you could take out 2 transistors that were put there by the factory. SO I found a so called "replacement" transistors. I put them in and put the amp back together.

I made sure to use my Dim Bulb Tester when I turned the amp back on after working on it. This will protect the components by detecting any shorts that I may have inadvertently added. Happy to report no shorts were detected.

The popping noise was worse than before.
Figured I would just take those transistors out and see what would happen. Like Enzo from the Forum said in the first place.
low in-behold it works, and it is Amplified like it should be! Of course Enzo knew it would, but the inexperienced is just that.

BA Board

Cleaned BA Board

BA Board back together

HEY! Im learning and excited to tear into the next one.





What's the deal with this knob!

On further inspection - this pot is different than the other pots. Someone replaced this pot with one that probably had a long shaft and then just cut it off. The shaft is also little thicker.

I was texting the previous owner, because he seemed interested in if I could get it repaired and what I found. Very nice guy.

He told me when he took it in to get the estimate, the amp originally would not work at all when you messed with that pot. So I guess the tech, just replaced the pot with one he had to rule it out for further diagnoses. The tech probably discovered it needed a lot more time to diagnose and clean. I did spend a lot of hours on this amp cleaning and checking.

I did not search for a new pot to buy, figured I would just fix this one. I taped around the pot to protect the cabinet. Took a hacksaw blade and cut a slot. After getting the slot cut in, I took a small file around the shaft to decrease the diameter. Worked it until the knob fit on snug.

Showing how the heat sink fits between the transistors
and the circuitboard. That is really cool idea to get the
transistors further away from the circuitboard.

As for the nicks - I just used some wood glue and tape to hold stuff where I wanted it until the glue dried.


Took a sharpie and colored it black.







Why even bother with such small nicks and holes?
Sure it will make it look a little better, but the purpose of the covering is to protect the wood from moisture.

Hopefully the glue will keep out moisture for a little longer until it needs to be re-applied.


I put some pictures of my progress. Got any questions - just ask.

Over all Yamaha made a fantastic amp. I put some pictures below of how thick the plywood they used for the front. It sounds amazing!

If your interested in repairing SS Amps, check out that forum at ssguitar.com

Heavy duty, looks like the front is an Inch thick

Just cleaning behind the grill and looking

Can't wait to get to this one!

Dim Bulb Tester

Some scrap parts

The DBT - Sounds like a pro-wrestling move, but its not. Well if it is I'm not aware of it.

The DBT (Dim-Bulb Tester) I'm referring to is a type of electrical short tester. If the electronic device you are plugging in has an internal short, the DBT light will be bright, and draw the power. It will help protect the device from further internal damage. 

If the light is dim or off that means the device you have plugged in does not have a direct short.

One of my projects is to repair a couple of old radios and record players. I know they need a lot of work and I want to be safe. As I work on them, I can test them to make sure I do not have a short somewhere. Also if I come across an old radio or amp I want, I can them using a DBT.

DBTs are cheap to make yourself, and very handy.

1. Detects Shorts.
2. Protects the device if it has a short.
3. Could help prevent you from getting shocked.

The version I made requires;

1. small electrical box
2. outlet and cover
3. old light fixture
4. some wire
5. some scrap wood
6. few screw
7. Light Bulbs - Must be incandescent light bulbs.

Low wattage bulbs (florescent, LED) defeats the purpose of the DBT. We did not have any incandescent bulbs in the house. You need a bulb that will draw the current.

I bought some 60W incandescent light bulbs. I got the Edison style. I thought it l would look cool.

I did some research and saw that some people also had a switch to switch off the bulb circuit. I was afraid I'd forget which way was on or off. So I made mine so that if you're using it, its on. I also wired the bulbs in series instead of parallel. Series will draw more current.

Electrical Schematic

I was thinking if I had a bad bulb, I would have to unscrew it and check it with an ohm meter or plug it into a lamp or something. So I made a shorting plug so I can test my DBT. With the shorting plug it simulates a short.

Added a shorting wire

Using a Variable Autotransformer would be also idea (Its on my list) to be able to slowly increase the voltage. Variac is often used to describe a Variable Autotransformer. Variac is a trademark name. You can use a variac to slowly turn up the voltage on new capacitors as well.