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Author Topic: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.  (Read 2767 times)

phatt

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #15 on: July 17, 2019, 05:17:25 PM »
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.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #16 on: July 20, 2019, 07:00:20 AM »
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.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #17 on: May 11, 2020, 12:56:46 PM »
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!




Enzo

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #18 on: May 11, 2020, 03:04:39 PM »
+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?

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #19 on: May 12, 2020, 07:03:26 AM »
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?


ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #20 on: May 12, 2020, 10:21:11 AM »
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.





Enzo

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #21 on: May 12, 2020, 06:22:28 PM »
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?

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #22 on: May 13, 2020, 08:25:54 AM »
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!

g1

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #23 on: May 13, 2020, 03:24:35 PM »
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) ?

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #24 on: May 13, 2020, 05:14:49 PM »
Hi g1.

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

Thanks

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #25 on: May 14, 2020, 09:10:12 AM »
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!



g1

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #26 on: May 15, 2020, 11:04:26 PM »
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.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #27 on: May 17, 2020, 08:19:43 AM »
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?






g1

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #28 on: May 17, 2020, 10:49:50 PM »
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.

ryanckelso

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Re: Yamaha G100 410 - Series 1 - Low Volume / Surges when turning off.
« Reply #29 on: May 18, 2020, 06:51:47 AM »
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!