Gallien Krueger ML250 output issue

[pmarchione]

kinda a newbie, but i have built a few effects from kits and such.

I just aquired an old Gallien Krueger ML250 from my dad. the input jack was broken and I replaced it with no problems. The issue Im having is the output jack on one side is maybe half the volume of the other side. the circuit board look ok with no burns at any of the componet sites. it will drive a 2x12 on one side well. I have a multimeter, a good soldering iron, and some knowledge. where do I start? Pix will follow if anyone can help.

Thanx all

[Roly]

Grab this;

http://www.schematicsunlimited.com/download.php?manu=gallienkrueger&file=gallien-krueger-ml-series-service-manual.pdf

...and try to identify the circuit(s) that match yours, extract them from the PDF (copy, paste into something like IrfanView, and save as a GIF's) then post here so we can all work on a common basis.

A first test is to try a jumper lead from Fx Send to Fx Return, except with this amp it might require a stereo lead, TRS (I'm assuming for the moment it's one of their "stereo" systems?).

Is the weak output channel still clean, or distorted?

If you feed in equal signals to both L&R Returns (at the same time) is one still weaker than the other? (if so it means the problem is in the main power amps, if not it's in the preamps).

[pmarchione]

Roly,
Yes the is a stereo model, but according to the user manual the effects loop uses standard guitar cables so trs stereo cables are not required if I am using the effects loop. So if i understand you correctly i should connect a cable from send to return and see if the weak side is distorted or not? there are no left and right returns. there is however an aux in/stereo out 1/4 inch jack on the back also. i guess i should check to see if the signal is the same comming out of that to my pa or something huh?

[pmarchione]

one more thing. there are two internal 6.5inch speakers that I just assume have equal output but it is hard to tell just by listening. I guess i could hook a multimeter up to the wires going to each internal speaker and check that the output is the same also. correct?

[pmarchione]

I think I found the culprit. After testing the amps internal speakers I found that only one was working so I opened it up and  followed the path from the speaker contacts and found this burnt resistor/inductor. not sure which it is.

the green one in the center of photo one with the red/gold/black/brown bands.

Can anyone help me ID this so I can go to radio shack and order one?

[pmarchione]

After googleing around I found the service manual... long story short it has a pic of the PCB and a part list. hopefully this is the bad part and i'll be set after replacing it. I'll update after I get it soldered in.

[Roly]

This burned 100 ohm resistor is almost certainly only a symptom of deeper trouble, but if you are not bothering to actually read my posts I can't help you.   

[pmarchione]

Sorry Roly, got excited when i saw the burned resistor. I did read the posts.

Here are the schematics.

As for the effects loop.... I jumpered it and the weak channel sounds the same, no distortion. it is not a stereo effects loop. the send /recieve jacks are the same part as the 1/4inch input jack. they are not trs.

(Side note I replaced the resistor to no avail)

[J M Fahey]

1) Plug stereo headphones (which should mute the speakers) and play: do you hear the same on both earbuds?
2) make a cable with a stereo plug on one end, join the tip and ring contacts and apply signal (an MP3/CD/guitar through a distortion pedal used as a preamp) into J9 (external audio input?).
You should hear the same on both speakers.
Post results.

[pmarchione]

OK JM,
the out to the headphones actually needed to be powered. i sent out to my pa good both sides. in from a cd player the same side is distored and weak. j9 is both the stereo input and the stereo output on this model.

[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]

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.??

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]

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.

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?