Loud Crackling On Power Up... Sometimes

[Hicks86]

So I just fixed my Crate GTD 120/212 that had an issue with a bad cap in the overdrive channel EQ. Now that that issue is resolved, and its been that way for some time, I've been reminded that sometimes when I power up the amp, it make a super loud popping/crackling noise. Maybe once out of every 15 times I power it up, but its not predictably regular. I've read a little bit online on similar issues that suggest it could be a bad cap in the main power circuit, but I don't know. If the cap was bad, my guess is that it would happen every time I start it up.

[Jon Snell]

The only soft ish start is ICL1, a mains inrush NTC. I agree with you that is a capacitor was faulty, it would happen all of the time.
If it bangs after it has been running for a short while and you turn it off then back on again, that could be due to the NTC being warm.
There is no soft start on the amplifier apart from that one component and it only works when stone cold.

[nickb]

I take it the noise is coming from the speaker. First test, short out R48 on the preamp and see if that stops it. If it does suspect the power on mute circuit around Q16 and Q31 especially check / change C61.

[Hicks86]

Okay, cool. I'll try that. Thank you.

[Hicks86]

I haven't messed around with anything yet, but it was doing the crackling thing yesterday so I took a video. The guitar is still audible when this is happening. The crackling has what kinda sounds like reverb on it to me.

[Jazz P Bass]

Sounds to me as if the issue is with FX circuit.
Turn down the FX Leve control when the noise occurs to see if that reduces the noise.

[nickb]

Oh! That is completely different from what I understood from the OP which was a singe loud crack at power on. This is a continuous sound rather like a low frequency oscillation. Of particular note is the reverb it which tells us that it's not after the effects. Forget shorting out R48 (since we know it's before that). Short out the input to the DSP i.e R123 instead.

[DrGonz78]

Here is the other parts of the schematic that I did not post on the other thread. The effects schematic might prove helpful but we'll see.

GTD120-212-Effects.pdf
GTD120-212-LEDdisplay.pdf

[Hicks86]

Awesome! I'll open it back up and give it a shot as soon as I can find some time. Thanks!

[Hicks86]

I still haven't had a chance to take it apart yet, but messing with the knobs while playing I've figured out that its something to do with the delay. What I found is that the popping is from the delay, and the reverb is picking up on that. The chorus doesn't make any change to the sound. Should I still short / replace R123?

[nickb]

Can't be sure from that info. The chorus, reverb and delay are all done by the same digital signal processing chip. When these fail it usually breaks all the effects in a major way. So, it is still worth doing the short test. If it really is constrained to the delay effect the problem may turn out to be a bad delay pot or a connection involving the delay pot.