Solid State Amplifiers > The Newcomer's Forum
Gallien Krueger ML250 output issue
J M Fahey:
OK, google and download a sinewave MP3
A free 1KHz one is
http://www.mediacollege.com/audio/tone/files/1kHz_44100Hz_16bit_30sec.mp3
Or download the WAV and play it on your PC
http://www.mediacollege.com/audio/tone/files/1kHz_44100Hz_16bit_30sec.wav
Set any of them as Play and autorepeat.
You should get around 200mV audio from a standard MP3 player, inject that into both tip and ring of J9.
1) do you still get 1 channel good and the other weak and distorted?
2) if so, measure the AC voltage on pin 5 of U18 and U19.
I'd expect around 1V AC there.
You should use a reasonably good multimeter with AC voltage scales which can be set to 200mV AC.
The cheaper ones with only a 200V AC and (usually) 750V AC scale are not suitable for this job.
EDIT : also check that U18/U19 get around 15V DC on pin 6 and have around 7.5V DC on pin 5
Post results, good or bad.
J M Fahey:
--- Quote ---2012/7/30 p_marchione@yahoo.com <noreply@ssguitar.com>
also my meter has settings for ac-0,10,50,250,500. will that be acceptable or should i run out to radio shack and get a new one.??
--- End quote ---
This one has a 400mV AC scale and will do.
http://www.radioshack.com/product/index.jsp?productId=2103176&retainProdsInSession=1
In general, "Home" use cheap multimeters do not really have an AC scale, but the much cheaper DC one.
To measure AC they still use the DC one, but slam a diode in series with the input and multiply readings by roughly 2.5 .
Fine, if you are checking a wall outlet or a transformer winding, which are *pure* AC, but measuring inside an amp, where there is DC (as in a transistor collector or the pins I told you to measure) it gets cheated.
As an example, on pin5 which I told you to measure, I *expect* around 7.5 V DC and around 1V AC but a cheap multimeter will give proper DC readings but crazy 20V AC even without signal which is impossible ..... !!!!
Raw diodes used in cheap multimeters "lose" 0.7V (meaning they introduce an important error) so they are only suitable for the 200V and 500 or 750V AC scales.
Yours goes down to 10VAC which makes me suspect it.
Suitable-for-audio ones have an extra AC detector circuit which amplifies a weak AC signal so it can be measured .... and usually *block* DC so it's not a problem anymore.
As an example, the next cheaper one does not:
http://www.radioshack.com/product/index.jsp?productId=4214667&retainProdsInSession=1
J M Fahey:
--- Quote ---p_marchione@yahoo.com noreply@ssguitar.com
07:09 (14 horas atrás)
para mim
one side goes to the pin on the chip ie pin 6 on u18 but where do i place the other side when checking voltages? as I said im kinda new to this. I have only worked on dc effects pedals building them from kits.
--- End quote ---
Usually negative/black probe to chassis or to a ground track on the PCB, red one measures.
Be careful not to slip a pin while measuring and causing a short or explosion by touching something you shouldn't.
Real easy to happen, even more so on the ultra compact GK 250.
EDIT: check our friend Petey Twofinger's videos
http://www.ssguitar.com/index.php?topic=2555.0
pmarchione:
Ok I got a new multimeter. here are my results....
yes i still hear the same with the 44100hz signal into j9.
u18 pin 5 yeilds 7.5v dc
u19 pin 5 yeilds 7.09v dc
u18 pin 6 14.9v dc
u19 pin 6 19.95v dc
here is the ac part im not sure about. when i set the meter to measure less than 4v ac i get 0.05 on pin 6 on both u18 and u19,
but i read in the manual for my multimeter that to measure ac voltage on a dc source bias i need to connect a 0.1 microfarad mylar cap at the end of the positive test lead. isnt that what im measuring on this circuit? Do i need to go buy a cap to test the ac voltage correctly?
P.S. thanx for all the help.
J M Fahey:
Probably.
Cheap multimeter, *they* should have included that .1uF cap inside at the AC measurement position.
I guess your multimeter is still from the "home" type , which gets cheated by DC and is essentially "deaf" to low audio signals.
I told you these "lose" around 700mV (0.7V) from what you measure.
Not important when measuring a transformer or repairing a toaster, but here we are working with around 200mV (0.2V) signals :(
Just a quick test: connect it to a guitar cable and plug it into a guitar , then play a full chord: what does it show?
Black probe to plug body; red probe to plug tip.
The DC measurements you posted are fine, just a little worried because U19 has 20V on pin 6, it should have the same as U18 (15V), they are (or should be) fed from the same supply, please recheck.
As of your multimeter, what's the lowest AC scale it has?
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