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Author Topic: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.  (Read 620 times)

ryanckelso

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Hi All,

I'm new here, but need the internets help to fix my amp.

I've got a Yamaha G100 410 - Series 1. General Model with a DC board.

Everything works, tremolo, reverb etc however the volume is lower than it should be for a 100W amp.

I've done some initial scoping. But the problem alludes me.

When I turn off the amp, the volume first goes to nothing then surges and gets really loud! How loud it probably should be when its at full volume. Then it kind of fizzes out towards the end. Which sounds like a diodes clipping from a distortion pedal. So could be the electrolytic capacitors, discharging at the end? But why does it only get loud when its powering down?

Also another thing that confuses me is the 'Pre-Set Volume' knob does nothing to change the volume when turned. But I think that could be because its meant to be set by a footswitch, so you can boost when you want. A little clarity on that would be great.

I've got some basic knowledge and have built / fixed pedals before. But fixing this is the next step in the journey.

All help is appreciated.

Schematic Attached (hopefully)

Thanks!


g1

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #1 on: July 08, 2019, 02:08:44 PM »
You are correct, the pre-set only works in conjunction with the footswitch.  You can plug an extra cord in that preset footswitch jack and short out tip to sleeve to see how it works.

A good place to start with troubleshooting is with power supply DC  voltages.
Looks like there are 12V, 36V, and 136V supply levels.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #2 on: July 08, 2019, 03:07:30 PM »
Hi G1,

Thanks for the clarification.

Ive measured the DC voltages, from the DC Board points, here goes:

+36V (Brown Wire) = 39V
+12V (Red Wire) = 13.9V
+136V (+B & C+) = 123.5V

These don't seem too far away from the original specs.

What are your thoughts / next moves?

phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #3 on: July 08, 2019, 10:36:27 PM »
OK, maybe check those voltages shown on schematic,, see if any are way off.
TR5 looks like it's switching the Dist on/off ,,maybe it's not working right.
Others here will know more,, Phil.

Jazz P Bass

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #4 on: July 08, 2019, 11:30:19 PM »
Quote:"lower than it should be for a 100W amp. "

Can you quantify this statement?

@ Max volume, what is the AC voltage at the (speaker) when running a 100 mv test signal through the amp.

(preferably you would want to run the amp output into an 8 ohm dummy load for this test, instead of the speaker. To preserve your hearing.)

100 watts will equate to roughly 28 Vac output.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #5 on: July 09, 2019, 02:48:18 PM »
Jazz P Bass -

I dont have a signal generator or a dummy load to test.

But I plugged my guitar into the high input, and turned vol, treble, mid, bass to max as stated in the service manual.

I took readings of 1.58Vac at the speaker terminals.

The speakers are Speakers JA-2556
8 Ohms
R.M.S for Speakers = 30W

I took resistance readings across the speakers = 5.8Ohms.

Thanks


phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #6 on: July 09, 2019, 07:12:21 PM »
Does the distortion work?
Phil.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #7 on: July 10, 2019, 03:40:31 AM »
Phatt,

The distorion worked when I plug the guitar in and turn the pot for it.

I didn't have a chance to check the transistor.

phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #8 on: July 11, 2019, 06:34:28 AM »
Divide and Conquer.
To test the power amp separately from the preamp just lift that orange wire (comes off TR10 to point 0 on pcb)
That is the preamp out/power amp input. So rig up a temporary input socket, make sure it's got a ground connection to amp chassis and plug your guitar into that,,,
you should get a clean undistorted signal. it won't be super loud but if it can hardly be heard then the power amp has a problem.

Or better still plug in your mp3 player or other audio player with line out/headphone out  and it should blow you head off at full volume on your player.
If only weak signal is present then you know it's a power amp problem.

If Power amp test is good then you know the preamp is at fault.
Phil.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #9 on: July 12, 2019, 03:56:42 AM »
Phatt,

I am going to try that test later today.

I thought I would just update everyone.

I took some extra readings. The +CO Voltage is supposed to read +45Vdc according to the service manual.

The reading was +115V, which is way off.

There is a pot in the MA board (VR2) which is a B470k trimmer. I turned this anti-clock wise to the max and now the +CO Voltage = +90Vdc

This has resulted in a louder sound than before, and stopped the surging when powering off. The volume is still too low, and when maxed is a reasonable room level sound.

So I feel that the MA circuit is the culprit.

Any thoughts on what might have failed on the MA board?

phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #10 on: July 12, 2019, 08:13:39 AM »
Sounds like you are mixing up test points;
CO is the Poweramp output which sits at Half the supply voltage.
The highest voltage in the amp is at +B (next to the 3 Amp fuse,, which supply the collectors of TR20 & TR22,, as well as TR24 & TR19.
Or pin 6 on the board.

PIN5 should read half voltage which is the *CO* voltage,
So if you have 100VDC on +B supply then *CO* will read 50VDC.
IF point *CO* also reads 100VDC then likely the power transistors are dead.

The fact that you have altered that trim pot means the amp is now overheating and pulling a LOT MORE Current dragging the supply lower likely to kill even more parts.

As I said before check those voltages which are all over the schematic,, the ones in little boxes,, they are the DC voltages you should be seeing if they are way off it gives a clue as to what might be stuffed.

If the Amp is working close to spec then you should see some where close to 137VDC at the Collectors of TR24,19,20,22. with all controls at zero.
if you only get 90~100 then something is likely blown and dragging the supply down.
Phil.
« Last Edit: July 12, 2019, 08:14:59 AM by phatt »

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #11 on: July 14, 2019, 01:26:52 PM »
Hi Phatt,

The CO voltage was definitely 115V before I changed anything on the pot.

I couldnt take voltage readings inside the MA due to the construction when connected to the DC board.

But I took it apart and checked all the transistors in diode mode.

I was getting some interesting results hope you can follow the abbreviations:

R24 = NPN = C-B=0.560 / B-E=0.575 / C(N)-E(P) = OL / C(P)-E(N) = 2.11 = dead

Based on similar reading here are the list of shorted / faulty transistors.

TR19  = Dead - 2SC783
TR20 = NPN = - 2SC783
TR21 = PNP - DEAD - 2SA818
TR16 = dead - 2SA561
TR13 = dead - 2SK30A
TR14 = dead - 2SK30A
TR15 = dead - 2SC1000
TR24 = dead - 2SC484

Here are the remaining working transistors.

TR23 = Good
TR17 = good
TR18 = good
TR22 = Good

Is there anything else I should check on the board? Should replace all the faulty transistors found?

Thanks

g1

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #12 on: July 15, 2019, 01:35:10 PM »


R24 = NPN = C-B=0.560 / B-E=0.575 / C(N)-E(P) = OL / C(P)-E(N) = 2.11 = dead


Not sure why you are calling it dead?
2.11 is higher than many meters will register on diode function, lots of meters would call that 2.11 reading 'OL'.

Are any transistors you listed as bad giving readings significantly lower than .500 for any junction?

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #13 on: July 15, 2019, 02:05:43 PM »
g1.

Thanks for helping. I thought that 2.1 was indicative of a bad transistor.

After running through again. only TR13 and TR14 read 0.4 across one junction each.

Im going to go back and divide the power and preamp section now.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #14 on: July 15, 2019, 03:05:35 PM »
Hi all,

I just split the pre and power output stage.

I couldn't hear anything when I set up an input jack to the orange wire to the MA board.

I then re-soldered the orange wire back to point O on the pre-amp and it produces sound at low levels like before.

So I guess it is the power section that has the problem.

Thanks for your help phatt

 

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