Gallien-Kruger 400RB 2x10 combo problem, no signal to speakers.

[jbow]

Hi,
I bought a G-K 400RB from a yard sale. It has a problem that I can't figure out. I'm not much of a tech but I do know a little.
I think I am getting a signal from the preamp because when I dime the volume, master, and other tone controls I get a lot of hiss from the speakers, when I set everything at 12:00 it is quiet. G-K support said that this is normal. However when I plug into the input jack I get nothing from the instrument, or even from touching the cord.
I removed the PCB where the input and all the pots attach and nothing looks loose or burnt. I looked close for loose solder joints or anything looking wrong. The input jack is the style that is black plastic with prongs soldered straight to the board. I am familiar with the old style input jacks but not really with these.
I'm not sure how to check to see if the jack is bad or really what else to check.
Does anyone have any idea what I might check and how to check it? I really don't think the board is bad because of the EQ pots sending hiss through the speakers. It makes me think that it is an input problem but I don't know what, beside the jack itself, I should check or how to check it. I found an internal fuse, it looks fine. I didn't have the exact value but I did have another that fit. I swapped it, it didn't blow but didn't make it work either.

edit> I read some other posts. I used a little deoxit in the input jack, did nothing. I plugged into the return jack and got instrument sound but it was low volume and was only affected by the Master and Boost. The power light is blue. I get a slight pop when I press the Pad button. I get an enormous pop if I press the Post EQ or Tuning Mute buttons, a really loud pop.
edit> I forgot to mention that when I had the amp open I noticed that the fan was not working. It moved slightly when I hit the power but stopped and did not turn. I don't know it some automatic protection could make that stop the amp from working or if possibly something overheated due to the fan not working. Is there anything that this points to that I can check? I can check readings with a multimeter.

Thanks for any ideas. I am not far from Atlanta and will take it down to Wizard if I can't figure it out. I know to use a chopstick as a probe and to keep one hand away from the amp, also it has been unplugged for over a week. I guess I'll have to plug it in to check anything but I am careful.

Thanks again if you have any ideas for me to try.

Julien

[J M Fahey]

Usually first suspect is a "mechanical" , meaning bad/worn/dirty jacks/pots/switches/connectors but in this case, it looks *to me* like a bad Op Amp , because the loud pop you hear when using certain switches hint at them switching some DC (15V?) which should not be present there.
I would go as fas as suspect the first Op Amp in the signal chain for a couple reasons:
1) your description of a slight pop from Pad switch which comes before it
2) the LOUD pop from the Mute one which comes after it
3) the fact that the first Op Amp lives in a dangerous place, connected to the Outside World through the input jack.
Sometimes, say, a speaker out is connected to it, or 2 amps are on opposite power lines and a big spark jumps when connecting 2 amps together, etc.
Or a protective diode shorted, and now it has + or - 15V at its input.

But I donīt know how to explain you how to measure and check that.

Do you have a multimeter?
Are you experienced in its use?
Can you find your way in a preamp board and measure voltage at specific points?
When necessary , can you safely replace an Op Amp?
Notice I mean safely to the part and PCB.

Glad to help, but you need to have at least a certain level of experience.

As an example, I can NOT land a plane safely (as shown in many Hollywood Films) even with constant verbal explanations from the best Pilot Trainer in the World.
To each his own.

[jbow]

Hi Juan,
Thanks for the reply.

As to your question overall, maybe.

Yes I have a multimeter.
I can use it if I know what to measure and how.
I am not very good with schematics but can build an amp a wiring diagram. I have built tweed amps from parts and have done micro soldering with ICs. I suppose I could replace an OP amp. I have all the equipment I would need to solder and clean away old solder. It looks to me like an OP amp is configured like an IC, or is an IC, so I think I could do it.
Yes, to the PCB and OP amp. I know to not burn the PCB or the OP amp or get them too hot. I could repair a printed circuit board if it isn't too badly damaged, I've done that.
If all else fails I can take the PCB out and take a picture of it and post it to get help on which part is the suspect, or guidance on where to measure voltage. From what you've said I suppose it resides on the board between the pad and the tuner mute buttons. I can follow the etching on the board from button to button to OP amp. I think I can find it.
I'm not sure where I would get a replacement though, Mouser I assume? I have an account with them and Mojo and a few other places.
Again, thanks for the answer. I hope it is something I can do. I picked this amp up for 20 bucks, I figured if I couldn't fix it, the speakers are worth more than that.
Julien

[jbow]

I cannot find the schematic for the 400RB but I think this one is very similar. From looking at it I 'think' the OP amp I want is one of the TL072, the one where 6 on the OP amp goes to R5 and C4. Would that be a good guess or am I off? http://bmamps.com/Schematics/Gallien...20206-0181.pdf

Thanks,
Julien

[jbow]

I found a schematic. My best guess is I am looking at U2. It is a TL072. It's a Motorola TL072CP. Am I right that this is the suspected OP amp in the schematic, or is it a different one. GallienKrueger 400RB-IV Service Manual free download,schematics,datasheets,eeprom bins,pcb,repair info for test equipment and electronics There is a DL link near the bottom. It has a small "?" under it.
I found the part at ACK in Atlanta. I'll go down and pick up a couple of them. If there is a way I can test the one on the board, I'll try it. I'll be happy if I can fix it with a part that costs a couple of bucks! I am pleased that I was able to figure out which part is the OP amp from the schematic too!

Thanks,
julien

[J M Fahey]

OK, found the service manual for GK 400RB III .
Hope itīs the one.
It has a discrete FET/bipolar input stage.
Iīm posting DC voltages you *should* have there.

Mind you, GK shows some AC voltages but does not explicitly say so ; I put them inside ovals (what Fender does, by the way )
We are not yet worried about them, so first check DC voltages posted by GK which I enclosed in rectangles, plus what Iīd expect , in simple red.
No, not relation to one of my favorite singers

[jbow]

Thanks Juan. Actually is is a Mark IV, I found a schematic and posted a link in the previous post, maybe it is the same.
It will be this weekend before I can get in there and check. I think I can find everything from this, I'm assuming that the PCB will have the resistors numbered, if not I still think I can do it. I have a few questions that are not critical to fixing this amp but will help me with general schematic knowledge of a SS amp circuit. There are a few components that I think I know what they are and one I don't recognize. R50 is tied to pin 3 of the tuning mute button. What is the component that the other side of R60 is tied to? It says +15v, it is right up at the top of the schematic.
Is the J406 Q2-A and Q3-B the rectifier? edit> After thinking about it, I know this isn't a rectifier. That would not be in the preamp and it isn't the symbol for a rectifier, but what is it?
Is MPSA56 a preamp transistor that would be the SS equivalent of a 12a_7 tube ?

Also, part of the schematic is not on the screen. I'm not sure if there is anything there I need to check. I cannot see the direct out, just the top of he words so if there is anything down there I need to check, I cannot see it.

Thank you for the help!
Julien

[audiopete]

I own that same amp (mark III actually) and I had an issue with it where one of the coupling caps in the early stage of the preamp was intermittent internally - can't remember which, but it was something like C40 there. It was a yellowish axial AVX cap (see below). Sometimes it was fine, but I found out one rehearsal that it would cut out and I would lose signal. In the moment I used a Yorkville amps preamp and used it to drive my GK power section. It was a good workaround for the moment.

Wiggling the input jack would bring it back because it was flexing the PC board and causing that cap to make connection briefly. Inject a test signal and poke around the components at the beginning of the preamp section and see if you can find a bad part.

You need a audio test probe at least to trace the circuit at least, but ideally you should use an oscilloscope. Instead of guessing its an opamp, you can test the in and out of the opamp and see if its doing what it's supposed to. You can also see where you are losing your signal.

Use an mp3 player or whatever and inject it into the power amp in jack and make sure your power section is good. Keep the master volume turned down at first.

[J M Fahey]

Thanks Juan. Actually is is a Mark IV, I found a schematic and posted a link in the previous post, maybe it is the same.

Itīs practically the same, only the first FGets are fed from +43V instead of +35V , and a couple resistors and maybe a capacitor different value.
But they are so close that for a Tech, having the "wrong" schematic is 90% of having the right one.
One problem for you may be that parts numbers differ, although actual position relative to each other is the same.
As in: the resistor which goes from the input jack towards the first capacitor is R70 in the III and R9 in the IV ... but still 12K ... si sane value and same connection ... and so on all over the place.

It will be this weekend before I can get in there and check. I think I can find everything from this, I'm assuming that the PCB will have the resistors numbered, if not I still think I can do it.

Ypuīll have to "translate" meaning you print the schematic, preferrably large enough to fill 2 pages (which you tape together), find the points where I asked you to measure, and "relabel" parts in red ink with IV ID ,,, then search for them in the IV PCB and measure voltages.
That is enough to point to the gross voltage errors I expect to find.

I have a few questions that are not critical to fixing this amp but will help me with general schematic knowledge of a SS amp circuit. There are a few components that I think I know what they are and one I don't recognize. R50 is tied to pin 3 of the tuning mute button. What is the component that the other side of R60 is tied to? It says +15v, it is right up at the top of the schematic.

It means that itīs a wire/track/connector which comes from the power supply, bringing +15V in ... so we expect to find +15V (or very close) there.
Of course, we might not ... after all we are repairing a bad amp and we expect to find fishy things somewhere.
As you see, itīs not simply "parts replacing" at all but *functional* testing.

Of course, 90% of the time there will be some bad part(s) "somewhere" ... but how do we find them?
It might be "miles away" (in amp scale) ....

And 10% of the time, no part will be bad but some connector will not be fully in, rust/grime might be killing some contact or even worse, some pot or switch is set the wrong way

Is the J406 Q2-A and Q3-B the rectifier?
Is MPSA56 a preamp transistor that would be the SS equivalent of a 12a_7 tube ?

Ouch !!!
No, but what worries me is that you can not recognize parts

Thatīs why we can usually lead an untrained repair guy to clean/replace a pot, switch or jack, maybe replace a bulged or leaky large electrolytic, replace a socket or pwer IC (think TDA2030), check that connectors are fully seated ... all large visible easily recognizable things ... but not much further.

As in my earlier "Airport movie" passenger-as-pilot example: I might, on cue from control tower, say, find the autopilot button, turn it on, and type "fully automated landing.... enter" , but as soon as they start telling me "ok, harmonize the 4 engines ... set rpm to xxx ... raise hydraulic pressure to yyy ... lower the landing gear ... etc. etc. etc. ... Iīll start looking for a parachute pack and what door I can jump from

Also, part of the schematic is not on the screen. I'm not sure if there is anything there I need to check. I cannot see the direct out, just the top of he words so if there is anything down there I need to check, I cannot see it.

The schematic was chopped to show the area where I *suspect* the problem is.

Ok, correct your schematic, identify the test points and post results.

And good luck

PD: and re re reconfirm itīs a IV.
Sometimes itīs printed in some PCB corner, top or bottom.
May say it clearly (GK MB400 IV) or give a PCB number and revision which matches the published schematic or at least a date.

[audiopete]

Chances are that you will get lucky and just swap out the bad part is pretty slim, btw, as Juan mentioned. I can't remember the last time I found a bad opamp. It doesn't happen often.

[g1]

The full version of the 400RB-IV service manual is available here:
http://elektrotanya.com/gallien-krue.../download.html

[jbow]

Hey, thanks Juan and everyone else. Like I said, I m experienced with building 5f1 amps and 5e3 amps, they are simple and there is lots of info that is easy to understand. Several years ago I took an electronics course at a local technical college. It helped me a little with learning to work on PCBs and with small parts but beyond that it was not geared in any way toward guitar or bass amps. As far as SS, I know less than about tubes. I surely am weak on schematics but I try to learn, that's why I ask questions. I realized after I asked that the schematic was not showing a rectifier, I do know that symbol, just had a moment... I looked up the OP amp numbers and transistors... I'm just not sure what the transistors do, of course I also remembered that there wouldn't be a rectifier in the preamp (after I posted the question).
Be assured that I WILL be very careful. I may buy an oscilloscope, I have wanted one for some time.
@Pete, I can pick up an audio signal tracer locally. Perhaps if I have the equipment I can have a better chance of learning to use it. I can get some help from Richard Goodsell if I push it a little, he is busy but has helped me a little before and is not too far away.
I've found it really hard to learn electronics that apply to instrument amplifiers. I have tons of books from Inside the Vacuum Tube to more recent works... they still only go so far in theory and troubleshooting. I think I need to find someone local who can teach me hands on, that way I will learn. I learn more from videos than anything else but they always leave me with questions.
One other thing. I know how to drain the caps in a tube amp but I have no idea where to connect a clip to drain the caps in a SS amp like this one. If I do find the part that is loose or not working I will want to connect a lead to drain the caps and keep them drained while I am in there. Do you know where that would be? Do I use the same value resistor between the clips as I would with a tube amp or is it even necessary.
In any event. I will use a chopstick and keep one hand behind my back. I've been shocked once (by accident) and it was enough for me. I learned about electricity when I was about 6yo I put one finger on a prong of our Philco console TV and plugged it into the wall. This was before three way plugs. It was on the right side. It knocked me across the room into the footboard of my parents bed... I didn't do that again.
I have some radio and stereo projects I want to work on in the future so the equipment and whatever I can learn here will help me. I have my old Hallicrafters SX-43 and I have a spare. I intend to try and build one good one. I know I'll be replacing all the resistors and caps, I busted the meter cover on my old one so I'll be replacing that... and that reminds me one other time I was accidentally shocked. The signal strength meter needle on an SX-43 is HOT. LOL.
I will be careful, I promise and I will ask questions. I also realize I am fully and solely responsible for my actions.

Thanks again,
Julien

[52 Bill]

Before you get too far along replacing things, I'd try a few simpler things first. Try plugging your guitar into the tuner jack, any sound? Try cleaning the mute switch with a little DeoxIt. Any help?

[J M Fahey]

Sorry but the Tuner jack is an output only.
If you try to use it as an input, signal will try to get in through a 100K resistor and will be swallowed by the discrete Op Amp's low output impedance which is always connected to it.
Agree that a bad/dirty switch may open the signal chain towards the rest of the preamp.

[audiopete]

You will be amazed what you will learn with a tracer (or scope) and a schem. It will open your eyes to how to follow the road map through an amp.