Gallien Krueger ML250 output issue

[J M Fahey]

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.

[Roly]

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

[pmarchione]

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.

[J M Fahey]

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.

[pmarchione]

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.

[J M Fahey]

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.

[pmarchione]

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.

[pmarchione]

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?

[J M Fahey]

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.

[pmarchione]

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.

[pmarchione]

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?

[Roly]

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.

[pmarchione]

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.

[Roly]

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?

[pmarchione]

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.