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
Grab this;
http://www.schematicsunlimited.com/download.php?manu=gallienkrueger&file=gallien-krueger-ml-series-service-manual.pdf (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).
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?
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?
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?
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.
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. :-X
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)
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.
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.
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.
Quote2012/7/30 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://rsk.imageg.net/graphics/product_images/pRS1C-2265643w345.jpg)
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
(http://rsk.imageg.net/graphics/product_images/pRS1-7922775w345.jpg)
Quotep_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
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.
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?
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
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.
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.
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$
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 .
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?
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 :o
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.
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?
Quoteu18 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.
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?
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.
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?
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.
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.
No worries.
Just woke up for a few minutes (it's 4 AM here).
Checking Pg3 of the schematic: Power amps + PSU I see R112 100 ohms is in the base of transistor Q17 (TIP34C) and emitter of Q16, also a TIP 34C.
It's a bad sign and means that probably at least one of these is bad.
Please re-confirm that this channel still works driving a speaker, even if at reduced power, and that at least at low level (say a couple watts) it can sound clean.
R112 scan0004.gif
That is exactly what I was afraid of. I'd guess that Q16 is boofed {that's the technical term for it ;) }, and things don't look too great for Q17 either. Both will need to be removed and tested out of circuit (being very careful to note where each came from, and which way they go back).
Q12 and Q16 are the drivers for that output channel and Q13 and Q17 are the main output transistors.
I'd also resistance test probe Q13 and Q12 with the amp dead to try and confirm that neither are shorted, and if in any doubt test them out of circuit also.
Use this dead time to make up a limiting lamp - you're going to need it.
http://www.ssguitar.com/index.php?topic=2093.0 (http://www.ssguitar.com/index.php?topic=2093.0)
general question:
OK the schematic shows both q17 and 18 are TIP34c but Q16 is in reality a TIP30c according to the assy drawing and looking at the actual pcb. does this matter?
JM:
yes there is still clean sound until you turn the volume way up it just isn't anywhere near the volume of the other side.
Roly:
pulled q16 the TIP30c How do I tell what pins are B,C, & E? What kind of values am i looking for? will pull q17 next.
I will build the limiting lamp in the next few days.
I always suspected a drawing error.
Q16 *should* be a TIP30C and Q12 a TIP29C (or 31C)
Please confirm.
Now to the half good half bad news : your amp is broken, yet still working somewhat, but in a very unreliable way.
Any moment it may crack down in full, and complicate repair a lot, so for now I'd stop using it, build the lamp limiter, and order
1 TIP34C
1 TIP30C
1 0.33 ohms 10W ceramic resistor
1 100 ohm resistor, 1/4W
EDIT: and search "how to measure a transistor", start here and if not, google it.
I'm tired ow writing *always* the same 17538 times.
Not your fault, of course.
1 0.33 ohms 10W ceramic resistor
is this for r123 isnt it a 0.33ohm 5w 10% resistor?
1 100 ohm resistor, 1/4W
is this for r112? I replaced this already and have a bunch left over in case it burns up again, they come in packs of 5.
dont wann buy extra stuff for nothing.
as for the drawing the values you state are correct. so the schematic has typos. the small schematic with everything crammed on one page lists them correcty just really small.
Drawing is somewhat fuzzy so I specified a conservative (safe) value.
If the actual part says 5W on it, so be it.
As of the 100 ohms resistor, you'll use up hundreds of them unless you repair the main problem.
I forgot to add to the list insulating micas for the transistors, plus nylon bushings to insulate the transistor mounting screws, plus some thermal grease.
The amp already uses that, after the repair it must be left in the same way as you found it.
As of "not buying unnecessary parts" you really don't know until the repair is finished ;)
It ends up being much more expensive if you are "almost there", find you need a 2 cents resistor, and have to spend 10U$ in postage or drive 15 miles to get it, besides the extra delay.
If I need 32 screws for a cabinet I'm building, I buy at least 40, because the one that falls from the table usually manages to hide very well.
Any leftovers go to a spares box which sometimes proves useful.
Same with any other parts.
Good luck.
I get your point. I will order what I cant get locally and make up my limiting lamp. I'll let you know when I have all the parts and i'll post all the values on the transistors at the same time.
ok I made my lamp limiter and pulled the transistors. both check out ok. they are only conducting in one direction and the voltage drop is .5v so that is ok. I have ordered all the parts. they should be here tomorrow. should I just replace them when they get here?
Aso check the 0.33 ohm 5W resistor, I suspect it may be open.
Lift one end "in the air" to be sure and re-measure it.
Ok the .33ohm resistor is bad testing it next to a new one. I replaced that. should I replace the Transistors also? They are not in the board yet. Then what? I dont want to move forward without instruction.
OK. I couldn't wait any longer for a response. Sorry im not the most patient person.
I soldered in the chips and fired up the amp with the limiter lamp. If I use only an external 8ohm speaker and turn off the internal with the switch there is now sound, but it is distorted. If I unplug the external cabinet and use the internal. the sound is distorted and the limiter lamp lights up when I play a chord. the more signal in, the brighter the light.
Where do I start looking next?
The three voltage of interest are the +ve rail, the -ve rail, and the mid-rail or output. These can be located on diodes D20 and D25 (top-left scan3 circuit). The two supply rails should be roughly equal above and below ground, and the mid-rail, speaker output, should be very close to ground, say within about +/-0.1 volt (speaker disconnected). The distortion now could be due simply to reduced supply rails, but we need actual voltage measurements to know.
The telling point is the mid-rail voltage. If that is close to ground then you can try fitting a higher power limiting lamp and checking it again. If it isn't, say it's much closer to one of the supply rails than the other, this is a sign you still have a problem on that side, e.g. if it's almost +ve then you have a problem on the "pull up side" (Q7, 12, 13), and that must be located and repaired before moving to a higher power limiting lamp.
If the mid-rail is still in the middle (ground) with a higher power lamp you can either move to a really high power lamp, or take your heart in your hands and try it on full power - but I stress, if the mid-rail isn't very close to ground do not try direct power, you will only fry something.
Roly,
A few questions:
1. I am checking voltages on both sides of d20 and d25 also at the speaker output. Do I need to have the 44100Hz tone injected?
2. Should the voltages be 45v or am I misunderstanding the diagram? If so Is this the voltage comming from the big 50v caps
3. Should I just order q7,12 and 13 ahead of time since they are so cheap. How about the 1N4002 (these are the rectifiers right) should I just order those and the 50v caps to save time, or am I getting ahead of myself?
I figured Id check the resistors at r106 and r98 since they are kinda a mirror to the bad ones on the q16/q17 side and r106 was bad so I replaced that.
Sorry for all the questions, but im learning a TON and enjoying the repair. I may have to build something after this. lol. Thank you guys for all your help and patience.
1. No, these are DC no-signal checks at this stage. Signal test come after we get the DC conditions right first {and I can't imagine why you would want to inject a supersonic 44.1kilohertz CD sampling frequency anyway; a reasonable test frequency would be 1kHz or 440Hz, the A above Middle-C}.
2. We know what the voltages are on the diagramme; that's for a good amplifier in working condition - what we need is for you to measure the voltages on your particular chassis and report them to us so we have some small idea of what is going on at your end - WE AREN'T BLOODY PSYCHIC. With the limiting lamp in circuit we have no way of knowing what the supply voltages will be, other than somewhat less than normal. Yes, these are the voltages, +ve and -ve, coming from the main power supply filter caps.
Now, again, specifically, what voltage are you measuring for +ve supply, -ve supply, and speaker connection (without speaker), to ground?
3. Yes, you are getting way ahead of yourself. First we identify which components are faulty, then we take Berlin - okay?
I tried testing the voltages. I get around 35.8v ac on both d20 and d25. when I attempt to get dc voltages the meter starts out reading like 15v then really slowly drops i get as low as 9.47v or so and the meter wants to do its 5 min shut down. I get -.27v at any of the speaker connection wires.
What am i doing wrong? black is clipped to chassis and I have changed the scale the ac voltages are consistent. I'm not sure what im missing.
as for the parts. I only asked because it cost like 4-5$ for shipping each time and the parts cost less than ten $ last time. I can order every part in that section of the diagram with duplicates of like 5 or ten pieces for like 20$ and only pay shipping once. then Id have spare if I accidentally fried something and wouldn't have to wait 5 days for parts and pay shipping again just thought it would be cheaper and quicker.
Dear pmarchione, please be patient. ::)
We all have other pressing things to do elsewhere and SSGuitar is (should be) a relaxing hobby in the (unfortunately spare) free time. Stay tuned and if possible, you'll get the answers.
And it's good you saw that sometimes it's better (and cheaper) to buy a couple extra spares than pay needless repeated postage , not to mention the extra time loss. :tu:
Quote from: J M Fahey on August 16, 2012, 10:37:42 AM
...And it's good you saw that sometimes it's better (and cheaper) to buy a couple extra spares than pay needless repeated postage , not to mention the extra time loss. :tu:
True. Any time I order parts for something I'm repairing now, I almost always order 2 or 3 of each thing (unless its cost prohibitive). Then if I mess one up I have a spare. And even better, next time I need that part, I already have it on hand!