Gallien Krueger ML250 output issue

[pmarchione]

it has a 4 volt ac setting, I bought the one you posted from radio shack. ill return it and get a different one from lowes. I get no reading from playing a guitar chord. ill retest the u18 u19 ic in the am after work and getting a better multimeter.

ill try this multimeter

http://www.lowes.com/pd_136122-12704-61-340_0__?productId=3127727&Ntt=mulitmeter&Ns=p_product_price|1

[J M Fahey]

Incredible.
The user manual states it does have a 400mV scale, I attach part of it.
Please post the link to the Lowe one.
EDIT: I am re-reading the user manual, now I guess your multimeter *does* have a 400mV AC scale, but it has to be manually selected.
Otherwise it gets autoset in the "up to 600V" scale, which obviously is deaf to a 200 mV guitar signal.
Look at my corrected picture and try the following:
1) set the rotary selector to ~mV
2) push the Select button once so a nº "2" appears (I guess) indicating it will measure AC (or the letters "AC" or a "~"symbol) Hate this stupid manual.
3) press the "Range" button once so the "Auto" label dissappears, then push it again 1 or 2 times until the screen looks like the one I show.
You should see the decimal point as "---.-"  and the "mV" little sign should appear.
Hate this stupid manual and meters that think they are helping me. They don't.
So now you should have succeeded in setting:
400 - mV - AC
*Now* it should indicate the output of your guitar (which obviously has a pickup selected, and volume/tone on 10).
The bar graph display on top should show the output of your guitar, like a Vu Meter.
With the meter so set, touch the output pins I suggested earlier, which had around 7.5V DC on them.
The meter (specially the bar graph) should show a pulse, while you touch the 7.5V, and then drop to almost zero, if it really can differentiate between DC and AC.
All this without injecting any AC in J9? (the aux in).

As you see, I hate automatic stuff (meters and software) which auto set, "to help me", to what I don't need or want.
Post results before sending it back, It *might* do, after all.
Also post the link to the other multimeter.
Oh well.

[pmarchione]

I tried that it didnt work. so i returned it. I sent the link to the lowes one in the post above. I think one of these should be ok. It states that it does 400.0m ac voltage. its only 15 bucks or so more than the radio shack one.
heres a link to the site from the company.:

http://www.idealindustries.com/products/test_measurement/multimeters/test-pro_340_series.jsp.


or this one.
http://www.extech.com/instruments/product.asp?catid=48&prodid=277

ill pick one up in the am and retest all of those voltages and repost.

[pmarchione]

ok here are the results with a good meter.
U18 pin 5 = 7.56dc
u19pin 5 = 7.09dc
u18 pin 6 = 15.09dc
u19 pin 6 = 15.1dc
u18 pin 5 = .1m ac
u19 pin 5 = .1m ac
I went with the Ideal meter from Lowes for 58$ works great and sturdy, well worth the extra 20$

[J M Fahey]

Good.
This Lowe one is the real thing.
As you see, even when touching Pin 5 and its 7 V DC present, it shows 0.1 mV AC (please always shows it's "V"olts or whatever you are measuring) which probably is noise.
Now to useful measurements:
Inject your 1KHz tone into J9 , measure the AC voltage there, I guess you will find around 200mV (most MP3 players put out about that) , and then measure on Pin 5 .
*Maybe* you have over 400mV there, in which case you should rise the measuring scale to 4.0V AC (maybe the multimeter does so in Auto).
If value on each channel is about the same, fine, these LM386 are working as intended, then the next suspect is the Power Amp itself.
So far I suspect these LM386 because they are used as headphone outputs, have direct connection to the "outside world" and a bad connection might have killed one of them.
Good luck.
And congratulations on buying a good multimeter.
Which, by the way, should be able to measure your MP3 and guitar output .

[pmarchione]

the above numbers are what I got with the tone injected. unless i change the scale again then i get .05v ac and .047v ac respectively. I also tested the guitar cable and it works as you stated when playing a chord. now what?

[J M Fahey]

Now too busy, will recheck your schematic later.
Anyway, you *must* have audio (AC) on those points, after all at least one output hannel does have sound 
What do you hear on the speakers?
Please check that you are actually injecting a tone there.
What do you read on J9 pins?
Unplug the cable from there, you should read around 200mV from plug ground to tip or ring.
Remember setting yourself in a basic troubleshooting "state of mind", meaning different tests should match and tell you about the same, reinforcing each other.
In this case, if you inject audio in J9 you must read audio signal downstream and hear audio even more downstream, i.e. the speakers.

[pmarchione]

I finally got the voltages off the pin correctly. not sure why, but i had to make contact before changing the scale or it would not read correctly.

the prior dc voltages are no different but the ac voltage is as follows,

u18 pin 5 .92V ac
u19 pin 5 .84v ac

a few questions:

If im starting to understand how to read the schematic this Is a little low since is says 1.1v on the schematic. Correct?
Is this is acceptable?
If not where is "the power amp itself" you said may be the next suspect and where do i start checking?

[J M Fahey]

u18 pin 5 .92V ac
u19 pin 5 .84v ac

Good, that's what I was expecting.
These values are fine.
The schematic shows 1.1V AC with a given signal at the input at certain volume and tone settings.
But now you are injecting your own signal (we still don't know the value, measure it as I said before) but I estimate it around 200mV *at J9*.
What's important is that we find it amplified by a factor of 4 or 5 , meaning U18/19 are working properly.
I suspected them bad, now I discard that theory.
Troubleshooting usually means not only measuring a few values, *but* checking that stuff *does* what it's supposed to do (in this case, amplify), and if not , analyze it and guess why.
Not random guessing but something related to the problem we see.
And then we measureor replace, testing our hypothesis.
Not magic at all, but brainwork.

Back to your amp, it can be roughly divided in a few blocks, simply for easier analysis:
Preamps > switching > effects > headphone amp > "big" power amps > speakers.
I *hoped* to find one of the headphone amps dead or defective; easy/cheap to replace ... but they work, so the next suspect is the power amp belonging to the bad channel.
Pity, because it's a complex unconventional one which not even me understand fully.
Didn't put time analyzing it, because I don't like it much anyway, and it uses a "pet idea" from GK which they obviously love (they have used it many times) and I find iffy at best.
Oh well.
Let me have a look at it and suggest something.
Beware that besides complex, GK's are very tight , working on them is uncomfortable.

[pmarchione]

I got .8mv ac on j9 the source is from my laptop at max volume. would it help to know the location of the resistor I found burned on the main board?

[J M Fahey]

I'm sure you have much more than .8 mV there.
Unplug the cable from J9 and measure AC millivolts from ground to tip and from ground to ring.
I expect at least 100mV and as high as 400 or 500mV.

[pmarchione]

Ok how about .450v ac on the solder joints which is the same as tip and ring on the cable prior to plugging it in. guess where i was probing was not the input huh?

Shame it wasnt the cheap parts. whats next?

[pmarchione]

The resistor that was burned was R112.

Located near J4 on the drawing of the circuit board, top left of the drawing. J4 would be the right external speaker output jack (this is the bad one by the way).

It is the 100 ohm resistor (I believe) on page 3 of 3 in the schematics. Located on the left side of the page near the external speaker.

I wont to proceed without advice, but since we were testing voltages on u18 and u19 and those (headphone amps) were ok, should I be testing pin 5 voltages at u20 and u21? If I am understanding the diagram correctly u20 leads to the side where the resistor was burned and there should be .16v here. just an educated guess on my part, but if u18 and u19 are "amps" then I would guess that u20and u21 are also.

Please advise and correct me if any of my observations / assumptions are incorrect.

[J M Fahey]

Weekends are complicated for me, after all my niche customers are Musicians who "go out and play live", but on Monday or Tuesday I'll have a good look at the GK schematic.

[pmarchione]

No worries.