# Amp repair questions



## Rick31797 (Apr 20, 2007)

On Saturday I drove 3 hrs to Toronto to buy this old Yamaha amp. It’s a G-100-115 from the 70s .. I had one of these amps years ago and really like it and then sold it for some reason . So I wanted to get one back and come across one in Toronto. 
so today I take it too rehearsal and it was sounding great for about 1 hr . Then , the volume on it cuts down to lower then a 1 watt amp. Also when I turn it off it makes a lingering sound , a bit like TV static for 10 seconds .......

my question is , Can a solid state amp from the 70s be repair , or are there parts that are not available. I know the amp is not worth much but I would like to fix it if it could be.


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## player99 (Sep 5, 2019)

Did you check for fuses?


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## RBlakeney (Mar 12, 2017)

well we can rule out bad tubes at least!


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## nonreverb (Sep 19, 2006)

If it has an FX loop, put a jumper between the send and return to eliminate that as a possible cause.


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## greco (Jul 15, 2007)

Rick31797 said:


> Can a solid state amp from the 70s be repair , or are there parts that are not available.


I'm betting that it could be repaired. You are in Kingston...Correct?

Maybe a forum member knows of a good amp tech in your area.
At least get an assessment and cost estimate for the repairs.

Please keep us updated.


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## Rick31797 (Apr 20, 2007)

I just checked the fuses , there are two in the back and they tested good . There is no effects loop in this amp..
I am in Kingston and will need to send it in for repair. I will update when I find out what it wrong with it .


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## Alan Small (Dec 30, 2019)

did a speaker wire wiggle loose?
Change guitar cable
Clean input jack
Hook up a known good speaker?

All things you can do before the shop fee looms


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## Alan Small (Dec 30, 2019)

Some speaker models have an extra ground wire to the frame that can come loose and need to be resoldered..pic

5 jacks on the back can also be cleaned

Manual attached


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## Thunderboy1975 (Sep 12, 2013)

Plug your git into the FX return and see if no sound persists.


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## Paul Running (Apr 12, 2020)

Let it cool down, a couple hours and then power-up to see if it recovers.
If it recovers, the fault is intermittent which can be elusive in troubleshooting IMO, in that case make sure that it goes to a tech that is experienced with intermittent faults.
These are nice amps for discrete, solid-state technology; well-designed:


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## Latole (Aug 11, 2020)

player99 said:


> Did you check for fuses?


It can't be the fuse.

On a second hand amp, it is good to check if fuse is the rignt one to protect the amp


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## Latole (Aug 11, 2020)

Thunderboy1975 said:


> Plug your git into the FX return and see if no sound persists.


OP wrote ; There is no effects loop in this amp..


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## Latole (Aug 11, 2020)

Rick31797 said:


> On Saturday I drove 3 hrs to Toronto to buy this old Yamaha amp. It’s a G-100-115 from the 70s .. I had one of these amps years ago and really like it and then sold it for some reason . So I wanted to get one back and come across one in Toronto.
> so today I take it too rehearsal and it was sounding great for about 1 hr . Then , the volume on it cuts down to lower then a 1 watt amp. Also when I turn it off it makes a lingering sound , a bit like TV static for 10 seconds .......
> 
> my question is , Can a solid state amp from the 70s be repair , or are there parts that are not available. I know the amp is not worth much but I would like to fix it if it could be.


Answer to your question ; yes
For how much ? 

In my opinion it can come from several things; power transistors overheating; thermal paste replaced? Speaker impedance too low? 
A resistor in the power supply to replace? 
Solder in the power supply or copper trace to repair?


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## ampaholic (Sep 19, 2006)

I suspect I'm over simplifying but I've seen (heard) dirty/bad pots do some weird and wonderful things, especially on a 50 year old amp. On/off switches (and Standby on tube amps) too.


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## Hammerhands (Dec 19, 2016)

Alan Small said:


> did a speaker wire wiggle loose?
> Change guitar cable
> Clean input jack
> Hook up a known good speaker?
> ...


It could also be the speaker output jack or the speaker input jack if it has one.


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## Hammerhands (Dec 19, 2016)

How did the amp sound before it fizzled? A little like a JC-120?


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## Robhotdad (Oct 27, 2016)

I've been repairing amps since the 80's. I rarely take on a solid state amp unless its my own. A tech might find the problem within 15 minutes or 8 hours. There's a good amount of DC coupling in there. In some sections the failure of one transistor could bring down several others. If you can find a tech who doesn't mind earning $8/hour, it's fixable. I have an idea how much work could be involved here (could, that's the key word here). It has to be taken apart, troubleshot, parts need ordering and replacement, and cross your fingers. I wouldn't touch it for less than $400 if I was desperate for work.


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## Thunderboy1975 (Sep 12, 2013)

ampaholic said:


> I suspect I'm over simplifying but I've seen (heard) dirty/bad pots do some weird and wonderful things, especially on a 50 year old amp. On/off switches (and Standby on tube amps) too.


I got a no sound issue with my 2525c, i rolled all the knobs back and forth and the sound came back, hasnt done it since. 😮‍💨


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## 2N1305 (Nov 2, 2009)

in repsonse to Mr Running, thank you for the schematic, I love schematics, they excite me.

It's a quasi-complementary output?! YESSS!! With a JFET diff input for the power amp? sexy....


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## Paul Running (Apr 12, 2020)

Yes, the only part that I would change because I prefer the complementary, symmetry output configurations similarily, I prefer operating opamps with dual supplies.


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## Latole (Aug 11, 2020)

Thunderboy1975 said:


> I got a no sound issue with my 2525c, i rolled all the knobs back and forth and the sound came back, hasnt done it since. 😮‍💨


No sound is totally different than OP amp issue ;"....it was sounding great for about 1 hr . Then , the volume on it cuts down to lower then a 1 watt amp. Also when I turn it off it makes a lingering sound , a bit like TV static for 10 seconds ....... "

And no sound is most of the time easier to fix.

Robhotdat is right.


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## Rick31797 (Apr 20, 2007)

Update ... So I took to a retired neighbour technician, and he had it solved in about 40 mins ... he first checked all fuses , there are 6 fuses , Two the user can check and another 4 fuses inside . He started to use his ohm meter and then put one of these Magnifying head set on , and noticed a crack solder joint .. in the power supply . He solder it and it’s works great .. but the true test is at rehearsal because I will be pushing this amp ..
Thanks so much for all the suggestions...and the schematic is great to have. I think I got lucky getting this repaired so quickly.. 
I have had issues with both my Tube amps and the second one is at long and Mcquade now getting diagnosed. My other amp , Hughes and Kettner had two cracked tube sockets before the Blackstar when in last week With a cracking noise. If this Yamaha solid state amp falls apart on me I will likely buy a new amp and pay there performance warranty.


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## Rick31797 (Apr 20, 2007)

Hammerhands said:


> How did the amp sound before it fizzled? A little like a JC-120?


It does sound like a JC-120 ... it has a lot of treble , even a bright setting which it don’t need .. I keep the treble and mids down to 1 and Bass at 7 .. The built in distortion is terrible ! I use a digitech 1101 rack unit ... sounds great , a lot of power and the 15 inch speaker gives me the low end I like . The Yamaha 115 is harder to find , most seem to be 2x12s.. They are very heavy , this one is 68 lbs ....and They only sell for around 300.00 .


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## Paul Running (Apr 12, 2020)

Rick31797 said:


> The built in distortion is terrible


If the distortion is too harsh, you could consider replacing the 2, 1S1555 diodes (blue-highlight) with LEDs...that would soften the clipping characteristics.


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## Latole (Aug 11, 2020)

Rick31797 said:


> Update ... So I took to a retired neighbour technician, and he had it solved in about 40 mins ... he first checked all fuses , there are 6 fuses , Two the user can check and another 4 fuses inside . He started to use his ohm meter and then put one of these Magnifying head set on , and noticed a crack solder joint .. in the power supply . He solder it and it’s works great .. but the true test is at rehearsal because I will be pushing this amp ..
> Thanks so much for all the suggestions...and the schematic is great to have. I think I got lucky getting this repaired so quickly..
> I have had issues with both my Tube amps and the second one is at long and Mcquade now getting diagnosed. My other amp , Hughes and Kettner had two cracked tube sockets before the Blackstar when in last week With a cracking noise. If this Yamaha solid state amp falls apart on me I will likely buy a new amp and pay there performance warranty.


 You have found an excellent technician and the problem was a possibility mentioned earlier.


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## Rick31797 (Apr 20, 2007)

Thanks for all your help , so far it’s working great ...I have a question about if I could hook an external passive speaker to this amp.. 

the only output I see is the Rec out .. it says in the manual , Nominal +4dbm output for connection to 600 ohm line input to tape recorders or amplifiers. For non-Professional a 20db attenuation pad will prevent input overdrive. To feed a microphone level input with the record output either a direct box or a 50 db attenuation pad should be utilized.


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## Paul Running (Apr 12, 2020)

If you feel confident, you could install a 1/4" shorting-jack (3-terminals) for an external speaker option.
Remove the hot-wire (+), at the speaker and connect it to the tip connection of the shorting jack.
Then connect the sleeve or ring connection from the shorting jack to the wire that was removed from the + or hot side, of the speaker...all grounds are common.
During normal operation, there is nothing plugged into the shorting jack so, the amp output loads the internal speaker.
When you plug an external speaker into the shorting jack, it opens the connection to the internal speaker and diverts the amp output to the external speaker.
This would be easier if you could read a schematic.
OR
If you are not using the Record out jack, you may convert it to an ext, speaker jack, by shorting out the 560Ω, 3W resistor.
However, be aware that the internal speaker will always be in circuit and therefore always active, in which case, amp loading should be taken into consideration.
The stock speaker for your model is 8Ω and it appears that you could parallel another 8Ω, for 4Ω, total load, which the 212s are loaded.


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## Rick31797 (Apr 20, 2007)

Paul Running said:


> If you feel confident, you could install a 1/4" shorting-jack (3-terminals) for an external speaker option.
> Remove the hot-wire (+), at the speaker and connect it to the tip connection of the shorting jack.
> Then connect the sleeve or ring connection from the shorting jack to the wire that was removed from the + or hot side, of the speaker...all grounds are common.
> During normal operation, there is nothing plugged into the shorting jack so, the amp output loads the internal speaker.
> ...


I could install an external speaker jack , that is about the level of my skill .. Thanks for all your help


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