Trouble diagnosing bad power transistor GFX212, please advise

[jimbrady57]

Hi guys! My amp just started doing this thing where it would play fine for a couple minutes then lose all it's volume. Originally it would play fine if I turned it off for like 30 seconds, then a couple minutes, and now not at all.
I read a bunch of other posts and decided that I might have a failing power transistor, here's a picture of the 4 in my amp;

https://drive.google.com/open?id=0B9...Ud2MjVKd1RndXc
(I tried inserting the image from my hard drive and it didn't work)

Either way, from left to right there are 2 TIP142's (PNN), then two TIP147'2 (NPP). I started testing them with my multimeter set to 2000k ohms, with the following results;
Both TIP142's tested the same.

P-(005)-N-(007)-N
"------(002)------"

I also got those exact same reading either way the polarity was switched.

The TIP147's also tested exactly the same as each other, but with one number different after the polarity switch.

N-(-519)-P-(see below)-P
"---------(001)---------"

When testing between the emitter and collector on the 147's I got;

Collector(P)-(-511)-Emitter(N) and,

Collector(N)-(No reading)-Emitter(P).


Overall, it seems to me that if I've tested them right that all 4 of my transistors are toast. It just seems strange to me and they would all 4 go bad so suddenly, any thoughts?

P.S. Below is a link to the schematics, then 2 links to pictures of my notes (not really important)

https://supportloudtech.netx.net/lou...category/19786

https://drive.google.com/open?id=0B9...G9YMGQ1UFhLVGs

https://drive.google.com/open?id=0B9...3Y5WmhCLXVBR28

Thanks!

[Enzo]

Transistors don't heal themselves, so if they are all shot, it wouldn't work fine for a while then quit, over and over.


If I have vanishing output, I do not think of output transistors.

[jimbrady57]

Thanks Enzo, I should clarify that for the past couple days my amp hasn't worked at all. It just has a nasty humming and a very quiet output.

If not the (output) Transistors, is there a more likely component that I should test?

P.S. is it the proper nomenclature to call just call those (power) transistors?

[olddawg]

Thanks Enzo, I should clarify that for the past couple days my amp hasn't worked at all. It just has a nasty humming and a very quiet output.

If not the (output) Transistors, is there a more likely component that I should test?

P.S. is it the proper nomenclature to call just call those (power) transistors?

Have you tried plugging a signal into the effects return yet? Run a chord from the send to the return? Used the effects send to drive another amp? Divide and conquer. Troubleshoot the problem. Shotgunning parts most likely cause you more grief than good.

[jimbrady57]

Thanks olddawg, I'll go try that right now and I'll get back to you.
I did so far try plugging the output into a separate speaker cabinet and the output remained the same.

I should also mention a perhaps more obscure symptom in this situation, that being that when i turn the effects knob up, it makes the humming slightly louder and I have occasionally heard a phaser effect while playing with the knobs, this is all while the effects are supposedly turned off. Either from a third party pedal or having the knob turned all the way down on the amp.

[jimbrady57]

Have you tried plugging a signal into the effects return yet? Run a chord from the send to the return? Used the effects send to drive another amp? Divide and conquer. Troubleshoot the problem. Shotgunning parts most likely cause you more grief than good.

Hi olddawg, sorry about this but I put everything back together to try your idea, but then I got confused as to where to actually plug things into. Here are pictures of the front and back of the amp.

Front;
https://drive.google.com/open?id=0B9...UV6UkwwVDBtRW8

Back;
https://drive.google.com/open?id=0B9...0lwLUF0WjloX1E

[olddawg]

Hi olddawg, sorry about this but I put everything back together to try your idea, but then I got confused as to where to actually plug things into. Here are pictures of the front and back of the amp.

Front;
https://drive.google.com/open?id=0B9...UV6UkwwVDBtRW8

Back;
https://drive.google.com/open?id=0B9...0lwLUF0WjloX1E

This amp is a little odd. I can't remember seeing an extension speaker jack on a front panel. Anyway... you need to find the owner's manual. That "insert" jack is probably it but I bet it is a TRS jack with a common ground and your send and return signals on the tip and ring. Also... is this a DSP amp? If it is there is probably a reset procedure that may fix your problem.

[g1]

The insert has send on the ring and return at the tip.
https://www.manualslib.com/manual/33...?page=3#manual

Try plugging your guitar all the way in to the insert jack.

[jimbrady57]

Olddawg, g1 is working off what you were recommending (you are right about the insert jack),
So anyway, g1, I just tried that and it didn't seem to make any noticeable difference.

I also read through to figure out how to reset the DSP settings. As far as I can tell you just have to go through all the channels and put the knobs where you want them, which I did both with and without the foot switch to no avail.
The only difference I noticed was that turning the level up while have the built in distortion on changed the pitch of the humming, but didn't fix anything.

Owners Manual;
http://pdfstream.manualsonline.com/4...23832dfff2.pdf

[olddawg]

Many times in these DSP amps the signal is always running through the DSP Chip. If the programming gets corrupted they will not pass signal. Usually there is a reset procedure that restores default programming. You generally have to hold down 3 buttons while powering the unit up. But you OM doesn't list a procedure. I would call the company and ask if there is one. Also... if I had to make a wild guess. I would check if your single input jack hasn't become unsoldered.

[Enzo]

The DSP seems to be a branch, with the dry signal going past it.

OK, so plugging into the insert still leaves just hum? Is it just hum or is it very loud HUMMM? I am wondering if there is DC voltage on the speaker.

At least if the speakers are making noise, then they are working and their wiring is working. Even if they are worn out speakers, they still should make music if you feed them any.

Look just right of the insert jack on the drawing. See Q9? That is a power up mute. It mutes the amp for a moment as it powers up to prevent big WHUMP noises. But we want to make sure it is working. +40v comes through R71 to charge cap C28. Q9 needs a positive voltage on its gate to turn it OFF. In the on state, it mutes the circuit by grounding the signal path. I expect maybe +10v on that gate. Only takes a second to charge up the cap.

So, first are your power supplies up to voltage and clean? The power amp - wher you have a problem - runs on +40 and -40v. If either or both of those supplies is not right, then the amp won;t be either. I don't care if they are 39v or 42v, but I do want then to be about the same as each other. I want them free of ripple. If one measures about half what it should, you probably have lost a filter cap. Even if the supplies are crappy, the amp ought to make some sound, however, back to that mute and Q9. What if the +40 haas an issue? the so does the mute that runs off it.

You want to make sure the 15v supplies are right too, but the powr amp doesn;t use them. I see four test points in the drawing, how are they? I am mainly looking for the DC ones at the moment.

If all that seems right, then apply a signal to the insert jack, and see if you can traces it through the power map.

[jimbrady57]

Olddawg - I just tried calling crates support number (866-858-5832) twice, and both times I was on hold for 45 minutes. I did however check all my jacks and they are still soldered on good.

Enzo - You gave me some hardcore stuff, before I go much farther I checked the for voltage going to the speaker wires in both a/c and d/c mode; With my volume knob halfway up I read 0.3VAC and .021VDC to the speakers;
A/C - https://drive.google.com/open?id=0B9...zdBdXYxWTV5V3c
D/C - https://drive.google.com/open?id=0B9...HhVRzd3c2dBS0k

I also checked the voltage output of my transformer and both leads read 28.8vac, should I put in a new transformer before anything else? Here's a picture of how I tested it;
Tranfsformer - https://drive.google.com/open?id=0B9...Eg2MHlOenJ5Nm8

I also decided that I should post a little video about how it sounds, I kinda feel stupid for not being more specific from the beginning. It will almost play quietly fine, but when I try to play anything loud the signal is immediately cut down. The way I was describing it before "just that it was playing quietly" was taking for granted that I was trying to play loudly. I almost feel like it's the "power up mute" kicking in when it shouldn't.

Video Example - https://drive.google.com/open?id=0B9...2FFb3hBUk1qa28

p.s. I'd call it "just a hum"

[Enzo]

also checked the voltage output of my transformer and both leads read 28.8vac, should I put in a new transformer

Why would you want to replace the transformer? 28.8vAC rectifies and filters to just about exzactly 40 volts. 40.7 really. But take away a volt or so for diode drops and 40. (Note the test points are called out at 35v while the schematic calls it 40v.) Just get close is all that matters, Ther is nothing wrong with your transformer.

Unless the DSP is programmable, it won;t have a reset. As far as I know it is just a stock suite of FX.

AC readings are taken with test signal applied as specified on the drawing. DC readings are taken with no signal.

0.021vDC on the speaker is just fine.

[jimbrady57]

Hi Enzo, sorry for being so slow. I just started working at a job that's 2.5 hours away.

Anyway. I wanted to actually read the schematic for once and look things over in advance (because i haven't been, lol), and I noticed that one of the test conditions is the 1khz 40mv sine wave input. Do I need some special device to do that? Also, do I need a multi-meter that analyzes waveform to test for ripple or is it just as easy as checking for the dc on the ac circut?

[Enzo]

That test signal input is what needs to be there for the AC signal readings at the test points to apply. Doesn't affect DC readings.

I just apply my music signal. I look at relative voltages. If 40mv in makes 80mv somewhere, then chances are 100mv in might make 200mv at that point. In other words I am looking for evidence of gain where it belongs. If in doubt, stick to the specs.