crate glx 65 half the volume, then slow fade to silence

[DiverDown]

Hi guys! Brand new here, and so glad i found this forum. you can't find jack for info on solid state amps. lol. Well i'm still a noob with electronics, but i'm learning. the hard way it seems. lol. i have a kinda recent crate gfx 65. it sounds fine, nice clean signal and all effects work and sound like they should. except there's only half the volume until it get warmed up after about 20 minutes then the sound slowly fades away to nothing. everything visually looks fine. i was thinking maybe it's the big caps in the power stage. which i can't seem to get loose since they glued the bajesus out of it. lol. any suggestions on how to get them out, or maybe i should be checking other areas as well?

thanks,
mike

[Enzo]

Leave them alone.   Why do you think they are the problem?


If main filter caps fail, you get loud hum from the unfiltered power that results.   You don't get fading output.

First suspects are always the jacks with cutout contacts.   You have a jack on the panel for extension speaker?   It has a cutout contact, and if that gets dirty, you can lose level or sound altogether.   Easy test is to just plug so0me extension cab into that jack.  If all the speakers now sound, the jack is the issue.

You have an insert jack that can cause the same issues.  Poke a plug in and out of that jack a couple times and see if it wakes up the amp.

Right next to the extension speaker jack is a large 5 watt power resistor, R77.  Check it for opens.   It is a 0.1 ohm or 0.2 ohm, so it will read like a short on your meter if it is good.   Also check its solder or wiggle it to see if it is loose.  Does wiggling it restore sound?


Those are things to check first.

[Roly]

after about 20 minutes then the sound slowly fades away to nothing.

Not the power supply caps.

Don't have a circuit for this one, but it sounds like it could be some chip with internal thermal protection shutting itself down - wild guess.

Need to a) know if it's still alive from the Fx Return (if it has one) and b) confirm that you still have main and preamp supply voltages under fault conditions.

[DiverDown]

Thanks guys.

Leave them alone.   Why do you think they are the problem?

I had thought that maybe once the amp warmed up the caps would become leaky and drain out. But thinking about that now that would drain the power from the whole amp, not just the output.

You have a jack on the panel for extension speaker?

yes it does. as well as a effects loop.

b) confirm that you still have main and preamp supply voltages under fault conditions.

gonna check up on all these asap. I had thought about a chip overheating. Since it has been to a tech before it was given to me to learn on. He never found the problem. I thought that might have been easy to overlook? As I said though i know jack by comparison. Will return with results. thanks again.

[Roly]

it has been to a tech before ... He never found the problem.

While we are waiting, pardon a tiny rant about people who call themselves "tech's" who seem unable to find a blown fuse.  It's not like this is a subtle or intermittent fault in a seriously strange and complex item of gear.  Signal tracing should be able to quickly locate the offending stage, and that should confine it to perhaps half a dozen possible components (most likely an active device of some sort).  Even then, using the dumb "blunderbuss" method and replacing everything in that stage should have got him somewhere.   {Sheeeee...} :grr

[DiverDown]

 Well so far I haven't been able to get the amp to fail, besides the quarter volume problem. i had the chassis out of the cabinet at first, so i put everything back together, and still no failure. so i never got the voltages under failure. the 5w resistor tested ok, but was super fricken hot. wow. 5w of dissipation must be alot. lol. i rigged up a cable to hook up an extension speaker, and yes the speaker in the combo did not cut out. also found that there is no signal from the fx return. so went to the jacks. they look good. i sprayed them all with deoxit. no change. hit it with some fresh solder. nothing. i guess i'm gonna have to get under the board to check where the ribbon cable connects. i did have to re solder the power caps because i was being hastey and not methodical. bad moves on my part. won't do that again, i promise. lol. and as far as the tech guy goes. i don't think he took any real time with this amp. there is noone within 50 miles of where i'm at that works on amps. so he gets flooded. why i want to be a amp tech.

*note* i found that with a vibro pedal in the loop, and the amp fx set to delay. even though the return isn't working when i pulled the plug out to where only the sleeve is making contact in the jack. it makes this awesomely wild noise. 

*note2* the amp is not a gfx65, it is a glx 65. sorry about that. i just realized i wrote the wrong amp model.  :duh

[phatt]

Sorry can't help with the problem but note that Your efx loop explanation is rather confusing.
EFX loop has an Output (Send) and a Return (Input).
So highly unlikely you will get a signal *Out* of Return socket.
If that is what you are trying to achieve?
Phil.

[DiverDown]

sorry i've never used the effects loop on any amp. so i don't know alot about it. well with a pedal hooked up in the loop. the sound does not come through. the mention of the crazy noise was created by the delay and hum from the plug, not the actual vibro sound. actually when the fx return jack has a plug in it, it kills all sound. i guess signal was the wrong term to use, lol. though i'm not real sure how to test a jack with a multimeter. what i did was while the amp was on i plugged into that jack, and set my meter to 200v dc since that is the next setting beyond 20v (which i tried as well). and put my leads on the 2 tabs on the jack and got nothing, 0v. i figured there should be some voltage going across the jack when plugged in.

[phatt]

OK then get a guitar cord (That is known to be good) and plug one end into the *Send* and the other end into the *Return*.

This bypasses the internal switching setup and of course should guarantee signal passes from preamp to power amp unaltered.

It is possible the internal switch may not be connecting properly and display the symptoms you describe. (unlikely but certainly possible)

The idea being lets get the easy stuff out of the way first before resorting to open heart surgery of the gizzards. 8|

I personally would love a dollar for every efx loop that eF's up,, it is a common cause of heart ache for the novice.

If I could shoot the idiot that ever invented such a daffed idea I would

Am I being too harsh?
Phil.

[Enzo]

Ah geez, um... it is kinda important to use the right model number, if nothing else, any reference to resistor number so and so will be meaningless.

Nonetheless, any intermittant level problems or loss of sound is still most likely a jack problem.  The FX return may work fine as a return jack, but the problem is when the jack is not in use, there are contacts inside it that carry the signal on past.  If those get dirty, they don;t always work.  Same with extension speaker jqack.  It may work with a speaker plugged into it, but when that jack is not in use, it has contacts to complete the circuit to the main speaker.  Those contacts get dirty, you lose level or get silence.

[DiverDown]

i tried the loop bypass trick. no change, but a faint hiss. i also tried plugging everything in, and nothing. i took some pics of the amp and schematic i have, but they all take too much memory to post. xP

[DiverDown]

hey i figured it out. lol. the 1st pic is of original fx loop jacks. 2nd is of pcb with replaced jacks, the jack without the nut is the speaker extension. 3rd is the power supply side of amp and 4th and 5th pics are a continuing of the 3rd. the 5th has the effects stage with the blue and white braids. the schematics are of amplifier stages. if you guys want the fx stage pages i'll get those up. on the fx board in the 3rd pic there is a grey ribbon cable with a white connector. can someone please tell me how to disconnect those without damaging them? lol 

[Enzo]

Push down on the top of the white connector, it feels springy doesn't it?, hold it down and pull the wires out of the holes.  PLEASE mark the wires and connector somehow so you do not install the wires backwards later.


SOo what exactly did you figure out?  What solution is underway?

[DiverDown]

oh, lol, i was talking about figuring out how to get the pics posted. i had to resize them to reduce file size. the amp has me thrown for a loop. lol ah geez i can't believe i didn't think of that.  :loco

thanks

[Roly]

why i want to be a amp tech.

In the land of the blind the one-eyed man is king.

"Why do you rob banks?"
"Coz that's where the money is."

I personally would love a dollar for every efx loop that eF's up,, it is a common cause of heart ache for the novice.

If I could shoot the idiot that ever invented such a daffed idea I would

Am I being too harsh?

Me too.

Just a tad; it's a basically reasonable idea but the typical problem is lack of use so the switch contacts don't get scrubbed, oxidise, and there we have yet another Fx loop socket fault.

To be specific, Fx Send is the signal from the preamp to the outboard Fx, Fx Return is the signal coming back in to the power amp, so the Fx Return is often effectively the input to the main power amp and therefore a handy point to inject a signal (from guitar, radio, cassette, CD, iPlod, &c) to determine if the power amp section is healthy.  This is normally after the amp volume control, so when injecting a signal you need to have a source (like an MP3 player) that has its own output level control, and start with it set zero/very low if you don't want to scare ten years growth out of the cat.

The huge blurt you got, for whatever reason, tends to suggest that the power amp in this one is okay and that the signal is getting strangled somewhere upstream, and as @phatt has observed, the bridging contacts that are supposed to carry the signal across from the Fx Send to the Fx Return when nothing is plugged in, are a very common cause.  By plugging a (known good) guitar lead into both you provide an alternate signal path that bypasses the jack switch contacts.

Another thing to try is to see if you can get a signal out of Fx Send to drive another amp (e.g. its Fx Return).  If this is healthy it tends to point to the Fx jacks themselves, but if it's still dodgy then it suggests the problem is in the preamp before the Fx jacks. (you can also do this in reverse, other amp Fx Send to faulty amp Fx Return)

The only voltage you should see on the Fx Send should be about 1 volt of audio (fully driven), no DC.  Signal tests across to/from another amp with Fx jacks are generally more reliable than using a multimeter, but then the ideal is to have an oscilloscope (CRO), and IMO any CRO is better than no CRO; however something like an old cassette deck with VU meters (in record mode) can be pressed into service as a poor mans audio millivoltmeter, and if it has a headphone socket you can also monitor what it is indicating for noise and distortion.

Circuits.

"Doctor, my eye-ess..."

Okay, well those are (somewhat) better than nothing.

Locate Test Point 9 (TP9) near the Fx Return.  This is the main power amp input and a volt or so of signal there should produce pretty much full output (and blow down a wall).

An early check to make are the two main DC voltages +/-40-(something) volts at TP14 and TP15, and the preamp supplies which are ... er ... um ... (blink blink) ... on the two zeners just to the right, I dunno, whatever it says, but normally around +/-15 volts.  Making a habit of checking that these supplies are about right early can save a lot of wasted time.

{Nothing ever goes according to plan, and if it does, you've missed something - Murphy's Law Rules, okay?  :lmao: }
HTH