Solid State Guitar Amp Forum | DIY Guitar Amplifiers

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!

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.

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?

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.

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.

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

Does the distortion work?
Phil.

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.

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.

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?

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.

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

Quote from: ryanckelso on July 14, 2019, 01:26:52 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?

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.

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

If you have full rail voltage at +C0 (the output) then it's a fair bet at least one power transistor is dead.
The problem with these circuits is that they are DC coupled and if one Transistor blows it usually takes out (or severely stresses) the other components.

In this case *DC coupled* stands for Direct Coupling of active devices.
There are no interstage capacitors to block the DC voltages so if a resistor or transistor goes out of spec then high currents flow and burn up transistors.
As these Amps are getting old then it's common for things go wrong.

As you can't take voltage readings with the board in place then it might be easier to replace TR20 to TR23 and build yourself a light bulb limiter Shown here; https://www.ssguitar.com/index.php?topic=2093.0

Then if things are still not right you won't blow up brand new components when testing.

Depending on circuit design and what has actually blown, some Power amp circuits will still pass signal even with blown output Transistors.
In this case maybe Tr20 &Tr21 are still working giving the low output volume you noted.

You won't be able to balance the output (with VR2) at C0 to half voltage until the offending devices have been replaced.

And same goes for VR1 which adjusts the idle current for the power transistors.

Better minds here might have easier/better ways to fix this but I'd just replace the right hand side TR20 to TR23 and fire up with the limiter and see if you can centre the C0 point and set idle current.
Then run a signal through it.

Don't worry about VR3 until you get the DC working,,VR3 is just for balancing the AC signal entering the amp.

Phil.

Hi Phatt,

I managed to purchase some crocodile clips and detach the power amp section to a more workable state.

Unfortunately there was a small pop and no there is no guitar signal at all. So something definitely blew.  :'(

I checked the fuses and they are all okay.

I managed to take voltage readings as best I could across the transistors:

Rail 6 voltage = 123V

TR24 = IN 123 / OUT both terminals 119V
TR16 = IN 119V / OUT both terminals 118V (Working)
TR17 = 117V all terminals
TR18 = 117V / 117V / 12V
TR15 = IN 119V / 104V / 97V
TR14 =  IN 117V / 115.5 / 103V
TR13 = All 117V

TR19 = collector 124V
TR20 = collector 124V
TR 21 = collector = 0.59V

I have to deconstruct the MA board even more to get the emitter and base reading for these, so I could only take the metal plate readings, which to my best known are the collectors?

TR22 = 124 in / 115 / 116
TR23 = 116V CV / 0.044V TP / 0.58V

I hope someone can help decipher what the issue is now, and explain it if possible.

Thanks.

Hi All,

I thought I would give this another shot with some time on my hands.

AN UPDATE:

G100 410 series 1 UK / Europe version.
Powers up - but no sound through the speakers.
+CO voltage reads 115v when should be 45v.

disconnected PA board from MA and DC. Direct input to MA board with orange wire from schematic - still producing no sound.

Transistor Test undertaken for all - good readings throughout - except:

TR 19 - Collector (Neg) to Emitter (Pos) read 0.59v - unsoldered the joints and retested.
TR19 is okay when out of the circuit.

TR13 JFET 2SK30
with negative on the gate readings - 1.0v & 0.45v (I believe this should be OL?)
with positive on the gate readings - 0.73v & 0.64v

Other Diodes ISI55 & D1 working.

I have built a lightbulb limiter and an audio probe.

I still need help with figuring out why the power supply at +CO is off and where the audio signal is failing.

Many Thanks!

+CO   positive output cap.  Not a power supply.

This is a single supply power amp.  The supply appears to be about +137v.  That means normally the output bus, which here is +CO, should sit at about half that, or +70v.  Yours is slammed up towards the supply.  SO divide and conquer.  Find TR19, and see the 220 ohm trim pot bias adjustment and diode D1?  Pick one leg of the trimmer or one leg of the diode, whichever is easiest to get a meter on, and what voltage appears?  If it is also +100 or so, then the outputs are probably just doing their job.

There are voltages all over this drawing.  SO look at TR24, the regulator for the front half of the power amp.  Is the voltage on its emitter close to its collector voltage?   You have two diffy pairs of transistors, how do your voltages compare to the drawing on those?

Hi Enzo,

I set up the amp to read with the light limiter on.

42W bulb. (it's all I have at the moment). The bulb flashes bright when the amp turns on and then fades to a dim glow.

I took a reading on the leg of the B220k trim pot that goes to D1.
Voltage = 100V

TR24 collector = 106V
TR24 emitter = 101V

+B Voltage = 106V

Perhaps the voltage is down from the limiter?

Hi All,

I took a few more readings with the limiter on. Most readings seem about 30V down from the schematic voltage. I've put in brackets what the schematic says at these points.

+B = 108V (137V)

TR24 collector = 107V (137V)
TR24 base = 103V (130V)
TR24 emitter = 103V (136V)

TR16 / 100 resistor = 103V (130V)

TR13 TO TR17 Base = 101V (128V)

TR13 / TR14 / Capacitor = 101V (100V)

TR15 Base = 83V (95V)
TR15 Emitter = 90V (99V)

TR17 / TR18 shared point = 101V

TR17 Collector = 100V (88V)

TR18 Collector = 6V (48V)

All help is appreciated!

Thanks.

Yes, the bulb drops your voltages.  If the amp is not trying to blow fuses, ditch the bulb.

You have 100v on the trimmer, so you expect 100v on the output.

TR17,18 are not cooperating here.  All your readings are low, but the 88v on TR17 is 100v in yours.  That 100v is controlling TR19 and so causing the 100v output.  Are the bases of TR17,18 more or less balanced?

Hi Enzo,

I retested those points without the limiter:

TR17 base = 116.6V
TR18 base = 116.6V
+B = 123V
+CO = 115V

TR17 Collector = 117
TR18 Collector = 11.7V

Shared point between TR17 & TR18 emitters = 117V.

I think the bases should be about the same - if I am reading the schematic right.

Is there any significance / coincidence that TR17 collector is a 10th of the voltage?

I also found 41V on the circuit after D1 following the 10K 1W resistor connected to the 10/80 cap but then jumps back up to 115V on the 5 rail.

Thanks!

Quote from: ryanckelso on May 13, 2020, 08:25:54 AM
I also found 41V on the circuit after D1 following the 10K 1W resistor connected to the 10/80 cap but then jumps back up to 115V on the 5 rail.

What is the voltage at the D1/10K junction (TR21 base) ?

Hi g1.

TR21 Base = 115V
TR21 Base / D1 / 10K junction = 115V

Thanks

Hi All,

I replaced TR13 as it failed the diode test.

Original Readings:

TR13 JFET 2SK30
with negative on the gate readings - 1.0v & 0.45v (I believe this should be OL?)
with positive on the gate readings - 0.73v & 0.64v

This has now resulted in the following readings:

TR13 Source = 90V
TR13 Drain = 116V
TR14 Source = 90V
TR14 Drain = 116V
* these could be mis-named, but the voltages are correct in relation to the schematic.

TR17 / TR18 emitter = 117V
TR17 base = 116.5V
TR18 base = 116.5V
TR17 collector = 117V
TR18 collector = 30V

TR19 base = 117V

+B = 124V
+CO = 116V

All in all, the voltage readings are much closer to spec, with drain and source voltages on TR13 / 14 now reading close the the schematic. And the collector of TR18 now reading 30V opposed to 11.7V.

The collector reading on TR17 is still 117V where it should be around 75V when everything is approx 15V down from the schematic.

Do we think TR17 is now the faulty transistor?

Thanks!

TR17 shorted emitter to collector could give you a voltage like that.
But TR19 shorted collector to base could also.
Check both for low readings with your diode check function.

Hi G1,

I removed TR17 & TR19 from the board and tested with diode function. They both give good readings. However as mentioned before TR19:

TR 19 - Collector (Neg) to Emitter (Pos) read 0.59v when in circuit.
OL when out of circuit.

I think that these transistors are fine.

After getting better results changing TR13 I thought I would try the same with TR14.

Again this has helped:

TR13 Source = 90V
TR13 Drain = 116V
TR14 Source = 90V
TR14 Drain = 116V

TR17 / TR18 emitter = 117.7V
TR17 / TR18 base = 117V
TR17 collector = 61V (should be around 80V)
TR18 collector = 75V (should be around 40V)

TR19 base = 74.7V

+B = 124V
+CO = 74V

TR24 emitter = 119V (should be 123V?)
TR24 base = 119V
TR24 collector = 124V

TR16 / 100k = 119V

33 / 160V Cap = 119V


Finally the +CO voltage is closer to centre. But I am still not getting any signal / sound.

The collector voltages on TR17 & 18 are still off and the emitter of TR24 should only be 1V below the +B / collector voltage.

I took TR24 out of the circuit and tested with diode function - all good readings.

Looking at the schematic - I don't have the right voltage hitting the 2 x 33uF /160 Caps - currently 119V whereas should be 123V.

I took all the caps out that are connected to TR24, discharged and took capacitance readings with the DMM.

Value | DMM Reading

4.7uF = 7.9uF
33uF = 39uF
33uF = 46uF

I dont know much about capacitors, so could anyone tell me if these are too far away from their values and could therefore be affecting the voltage readings from TR24?

Don't worry about TR24, that is an error on the drawing.
Notice above TR16 it also says 130V on the same line that says 136V by TR24.
Yet TR24 base is shown as 130V.
There should be roughly .5V between base and emitter.  So the 136V is wrong.  Coming out of TR24 should be around 130V, with .5V more at the base.
Your TR24 numbers sound ok with lower B+.
So now you have higher collector voltage on TR18 than TR17?  Before TR17 was the higher one (normal)?  Please double check.

Hi G1,

I put the 3 electrolytic caps back into the circuit that link to TR24 back into the circuit and the voltages are now:

TR13 Source = 82.4V
TR13 Drain = 106V
TR14 Source = 82.4V
TR14 Drain = 106V

TR17 / TR18 emitter = 106.4V
TR17 / TR18 base = 105.7V
TR17 collector = 63V
TR18 collector = 53V

TR19 base = 62V

+B = 123V
+CO = 62V

TR24 emitter = 107.9V
TR24 base = 107.6V
TR24 collector = 122V

From what I can tell now the circuitry seems to be working better. The voltage from the emitter and collector of TR24 has now dropped, which could be to do with the capacitors being discharged / could be past their life.

TR18 & TR17 collectors are now closer to spec but I feel there should be a larger difference between them.

I tried running through the MA section only and still nothing.

Thanks!

To add to the previous post:

I went through the board with the audio probe connected to a separate amp that I know works well.

There is audio at points 5, +CO and even on the white cables connected to the speakers.

So I think the MA board is now working fine - it is amplifying between TR13/14 - TR17/18 and TR19.

I guess the issue is that the speakers are not being driven for some reason to create the sound.

I looked back at an old post to test the AC voltage on the speakers.

Unfortunately there is a 0V reading which I presume is the problem now to create the movement in the cones / generate audio.

All help appreciated!

What are you using to create signal and what level are you applying when you inject straight into the MA board?
On the MA board there are 2 power resistors, .47ohm 2watt.
Measure the DC mV across each of them (one probe to each end).

Remember, it is all about the CIRCUIT, not the parts.   You report you have signal all the way to the white wire to the speaker.  OK, keep going.  Is there audio right on the speaker terminal with the white wire?  Is there audio on the other speaker terminal?  If so, the ground is missing.  Current ONLY flows through the speaker if there us a complete circuit out to the speaker and back.  Is there continuity, and I mean zero ohms, between the negative speaker terminal and the E (earth) on the MA board?   It is not enough to get signal TO the speaker, we also need a return path.   Just like an effect pedal won't work if we only connect ONE snap on the battery.

And did we test the speaker?  Unplug the speaker from the board, and touch a 9v battery to the speaker terminals for a brief moment.  A working speaker will pop or thump when you do this.  A silent speaker is a dead speaker.

Hi All,

Just to let you know, it was the earth connection back to the board.

Some of the cables became disconnected. I sorted this and now it plays!

Heck it gets LOUD!

Thanks for all your help.

Quote from: ryanckelso on May 20, 2020, 09:21:11 AM
Hi All,

Just to let you know, it was the earth connection back to the board.

Some of the cables became disconnected. I sorted this and now it plays!

Heck it gets LOUD!

Thanks for all your help.

Glad to hear you got it sorted!  Seems like it almost always ends up being a simple thing in the end.   :dbtu: