70% vol.loss in clean ch. crate gx212 amp

[mjojo51]

Need help figuring out how to find the problem in this circuit. The only thing i have is a multi meter for testing. Its the model on the bottom half of the attached schematic. Replaced ic1 for the heck of it to no avail. thanks

[Enzo]

Poke a plug in and out of the INSERT jack a couple times.  Did the amp wake up?


In fact, plug the guitar into the INSERT jack, does that sound clear and strong?


If you have an extension speaker jack, poke a plug and and out of that a few times and then try the amp.

[mjojo51]

Did all that. Amp just a little louder than the maxed clean channel when plugging into the pwr. amp insert.

[mjojo51]

Lead channel normal/ no volume loss

[Enzo]

The entire clean channel is nothing more than the tone stack, the volume control, half of IC3, and Q7.


Ignoring your loss of volume, do the tone controls and volume control seem to function?

Explore Q7.  When you switch channels, does the gate voltage come and go?  Does the resistance between source and drain go to a low value when the Q7 is not suppressed at its gate?

Stick a more or less stable signal in the amp input, and trace it through the channel, see where it goes away.

[mjojo51]

yes tone stack and vol.work fine. With vol. on ten it's only as loud as maybe at 2. and sounds fine but not loud enough to play with band. Though a novice at electronics wouldn't that mean that Q7 is actually working properly and the problem is either ic3 or something in that section before Q7 thanks

[mjojo51]

Just thinking out loud i'll check what you mentioned tomorrow. thanks

[Enzo]

Q7 acts like a valve - it is turned on or off to pass signal or not.  If instead of turning to a low resistance, it is stuck at a fairly high resistance, the signal through it could be weak.   COuld it be something other than Q7?  yes, of course, but we need to check it to be sure.

[mjojo51]

Will check that first ,thank you

[J M Fahey]

Fully agree with Enzo (of course  :tu: ) , just throw some numbers into the test to make it more certain.
Specially because the stage does work, simply not enough.
Follow the *excellent* instructions written by Crate, which ususlly no other manufacturer includes.
First and foremost, you may not have a scope, but your meter *must* have a 200mV AC scale, those cheapest of the cheap with only 200 and 750VAC or even 20VAC scales are useless for audio, are mean for electricians .
If not, go get one.
1) inject 40 mV 1KHz signal.
No audio oscillator? .... thought so  .
Don't worry, download a 1KHz MP3 tone, and play it with any cheap "stick" MP3 player.
They put out around 200mV RMS max.  (which you should be able to measure with your multimeter  ) and then you lower volume until you are roughly around 40/50 mV
Of course you'll need to make a stereo mini plug to mono guitar plug adapter.
2) then measure audio at TP1, you should have around 150mV RMS (1/3 the indicated Vpp) or 500mV RMS, depending on "condition".
3) then measure audio at TP6,you should have either around 1V RMS or 3V RMS. (remember the 1/3 rule)
4) measure TP7, you should have either distorted sounding 2V to 3V  RMS or clean sounding 2V RMS.

No need to test "somewhere else", as Enzo suggested, the problem, if real, should appear in that area.

There's a couple extra tests but we need the answers to these questions first.

[mjojo51]

Thanks for all the info but this is way beyond my understanding so i'll just make it easy on myself and everyone else and replace Q7 and if that doesn't work i'll change ic3 And eliminate those from the equation. Thanks guys

[mjojo51]

Replaced  IC3 problem solve. thanks guys.

[J M Fahey]

 :dbtu: