crackling Laney

[azatplayer]

Got a buddys sons VC30 on my desk.
His lad said it was crackling badly in and out, originally.
By the time i got to it tho, he said the tubes were not lighting up.
I got it open and only the first EL84 had heaters working the rest of the string was not. Pretty simple fix, the daisy chain had a bad joint at the first tube.

But now im at the original issue, this crackling.
Amp is fine till its turned right up.
Ive swapped all the tubes for known good ones, and have the chassis out of the cabinet, so its not vibration related.

Its mainly on the thinner strings/higher frequencies, and is friggin ugly.
Its initially over the top of the note, but soon replaces the note alltogether.
Voltages all seem ok, ive repaired one of these before so have replaced the load R's in V1 and V2 as they have a tendency to burn out.

WHen it happens, its a loud crackle, and has a kinda blocking vibe about it. As the note dies down, it clears up again. Its super fine right up to maybe 3/4 volume, after that its horrendous.

I placed a meter on each filter cap, they drop maybe 10-15 volts when playing hard, nothing huge.

Im wondering if maybe a coupling cap could be failing at a given point? Clutching at straws.
It does it on the clean channel as well as the dirty channel.
They use V1a on both channels, and split them before V1b, one side to V1b then the PI, the other side of the switch to V2a-V2b then the PI.
So its either in V1a or its from the PI on.
Anyone had one on their bench?

http://www.bymcomunicacion.com/pvecino/ ... eyvc30.pdf

[Kregg]

I had that problem once, it turned out to be a bad speaker. Have you tried it with a different speaker cabinet?

[Phil_S]

I'm assuming you don't have a scope, which would be a helpful tool for this.

Just a wild guess here. Pull V1 and V2 or disconnect it at R28 so you have only the PI and PA active. Scab in a preamp section from another amp into R28. A solid state preamp unit would be fine. If the problem is still there, think about doing a cap job. If the problem goes away, you know it's in the pre-amp and likely at V1 as you indicated.

[M Fowler]

I had that on a Marshall Vintage Modern and it was the preamp tubes.

[azatplayer]

Thanks Phil, i was thinking of that, will check it tomorrow. I do own a scope, bought it a couple years ago, but have yet to work out how to use it...
Yep Kregg, ive had it thru different cabs.
Been thinking about my description, its probably more like a shorting noise really. Has a massive crackle that when really pushed does that blocking fart out and drops power, sounds like youve stuck your finger where you shouldnt, dont ask me how i know that..

Got me thinking maybe the OT is worth a look. Cos it does it when the amp is pushing hard, at or near its limits. I cant see how a preamp tube would feel like it cant go on. Schem has 100R screens, it has 82R fitted like the last one i was in.

Think i will touch up the solder joins i can get to, rewire the screen R's standing up, after i try another preamp into it.

I have changed out ALL the tubes for known good ones.

[azatplayer]

Google shows they have a issues with crackling.
Also a few guys have had to replace the OT's in them.
Seems like there are weaknesses. But a pretty decent sounding amp tho.

[selloutrr]

After ruling out the usual causes, tubes, cables, instruments, power, speaker....

if it's a PCB double check any discolored or burn marks on the board it could easily be a solder point that over heated and makes intermitant contact with vibration.

chopstick the board?

[azatplayer]

Ive chopsticked like a Chinaman!
I have the chassis out for troubleshooting, to make it easier also to eliminate mechanical noises from vibrations. Damn combos are so hard on glass.

The board is pretty easy to get in behind, the control plate drops off with it, easy to desolder from the back. Looks really clean in there. But i will touch the joints up.

[Cliff Schecht]

I think it's the OT transformer as well. Sounds like they didn't Hi-Pot test the damn thing well enough and it eventually breaks down internally and starts arcing. Flybacks from a speaker through your OT can be a bitch!

[azatplayer]

Yep Cliff, im leaning that way too. After the other checks, im gonna jerry another OT in there, see what happens.

[Cliff Schecht]

Be weary of false-positives when measuring out that OT transformer too. It could read perfect with a low voltage DC voltmeter but when you put any substantial voltage through it then it will start to act up again.

[Phil_S]

Thanks Phil, i was thinking of that, will check it tomorrow. I do own a scope, bought it a couple years ago, but have yet to work out how to use it...

Youtube is your friend on how to use the scope. There are a number of instructional videos. I wish I had bookmarked the one I liked. A number of them are done by blowhards who talk too much and don't get down to business fast enough. Pass those by. I recall finding one that was about 15 minutes or so and covered the basics very well. He wasn't too goofey looking either.
I think this was it:
http://www.youtube.com/watch?v=rC8HBcsN ... re=related

I think the scope can save you a lot of time, as you should be able to see where the problem is. You may need someone to run the guitar while you probe

[azatplayer]

So, thats what a buggered OT sounds like! What a shame. I didnt want to find a wrecked OT BUT im glad i can stop searching, spent more time than i can be recompensed for. BUT i now know the sound of a stuffed OT, and thats a lesson worth learning
I patched in one i had for a build, sounded killer too. No crackles. And louder.. struth, loud!
Gotta get something that will fit, will have Alden do one i think, the thing is positioned between V2 and V3, very tight fit.
Thanks comrades!