Vox AC15 CC1-Crackle on certain notes

[Skeeter_pdx]

Someone brought me an AC15 for repair but I'm stumped at this point. Theres a hiss/crackle that shows up when certain notes are played at higher volume. Here's a quick rundown of how it showed up and what I've done so far:

-Came in with one of the power tubes cracked, put new power tubes in and there was no sign of redplating with the new tubes.
-Tried swapping preamp tubes for new ones but the issue still showed up.
-Cleaned the tube sockets, but I'm not sure how to retension the style of sockets on this amp. No noises when I wiggle the tubes around though.
-Found several microphonic capacitors in the circuit so I replaced those--as well as the filter capacitors while I was at it.
-Tried a different speaker cabinet and got the same issue.

I also reflowed a few solder joints that looked suspect, but I very thoroughly prodded every component with a chopstick and couldn't get any noises to show up. It looks like someone has been in here already and replaced the plastic input jack with a switchcraft one, and replaced a couple 5W ceramic resistors.

What are the next moves from here? Should I be checking voltages to make sure they're good? Diodes? Ground connections? Or am I finally going to have to learn how to use an oscilloscope? Would I have to run a signal generator with the speaker on to recreate the scratching sound? 

Any advice or direction is appreciated!

[Latole]

Did you put brand new EL84 or second hand ones ? Brand new are a must.

Clean all the connectors to PCB board. I have issu with those.
Clean tubes socket contacts.
Clean input jack.
Check EL84 bias resistor R38 on schematic. Read voltage ; 11.62 volts.

See a good amp tech

[Merlin]

Crackle immediately makes me think bad contact. Bad connector, cracked solder joint, wire broken internally, something like that... Sounds liek one of those situations where you have to go round tapping anything and everything with a pencil for ages!

[BrianS]

You mentioned cleaning tube sockets, but not jacks.  Did you clean ALL the jacks?  Did you clean/lube ALL the pots?  Virtually all amplifier repairs start with PROPER cleaning of sockets, jacks, pots, and some switches (slide switches typically).  You can't really skip around; you follow the servicing protocols before going deeper.  If after proper servicing the amp still has the problem, then you start dialing down from there.  Good luck.

[Latole]

You mentioned cleaning tube sockets, but not jacks.  Did you clean ALL the jacks?  Did you clean/lube ALL the pots?  Virtually all amplifier repairs start with PROPER cleaning of sockets, jacks, pots, and some switches (slide switches typically).  You can't really skip around; you follow the servicing protocols before going deeper.  If after proper servicing the amp still has the problem, then you start dialing down from there.  Good luck.

You are right.
And doing good cleaning need to know how thing work , how to do the job and need some experience too

[Skeeter_pdx]

You mentioned cleaning tube sockets, but not jacks.  Did you clean ALL the jacks?  Did you clean/lube ALL the pots?  Virtually all amplifier repairs start with PROPER cleaning of sockets, jacks, pots, and some switches (slide switches typically).  You can't really skip around; you follow the servicing protocols before going deeper.  If after proper servicing the amp still has the problem, then you start dialing down from there.  Good luck.

I just cleaned all the pots, input and output jacks, tube sockets, and the impedance switch. Also found out the hissing/scratching sound will show up when the amp volume is high even if the guitar volume is low. This is all with the chassis on my bench and the cab on the floor. The treble knob affects the hissing sound so I'm going to check upstream from there.

Bias voltage and all tube pin voltages are good.

I have thoroughly poked around with a chopstick and reflowed any suspicious looking or sounding solder joints.

My next suspicion is a noisy resistor or coupling cap in the signal path. Are there any other obvious causes I am overlooking?

Appreciate the input

[AlNewman]

Also found out the hissing/scratching sound will show up when the amp volume is high even if the guitar volume is low.

Does it make noise when nothing's plugged in?

The schematic Latole shared has a lot of IC connections and ribbon cables.  Those connections need to be cleaned and chopsticked, (jiggled) as well.  They are probably the most likely place for a fault to happen, along with dirty pots and jacks, (or bad guitar cables.)

[Skeeter_pdx]

Does it make noise when nothing's plugged in?

The schematic Latole shared has a lot of IC connections and ribbon cables.  Those connections need to be cleaned and chopsticked, (jiggled) as well.  They are probably the most likely place for a fault to happen, along with dirty pots and jacks, (or bad guitar cables.)

I tried different speaker cables, different guitar cables, etc., but it didn't cross my mind to try a different guitar. Plugged in a different guitar, had a slight crackle, but haven't had the issue come back in the past 15 minutes or so of playing. I'm going to leave it  on for a while and play it and see what happens. If there was absolutely no issue after plugging in the new guitar I would feel more confident, but I guess fingers crossed for now.

I prodded the daylights out of the ribbon connectors and didn't have any noise. It appears to me from the schematic that the ICs all drive/recover the reverb--I originally had the issue with the reverb completely off so I figured I could rule those out.

Might also plug in a signal generator and see if I can make the issue appear again.

[BrianS]

After trying the new guitar, did you re-try the first guitar you were using?
What was the original problem that caused the amp to come in for repair?

[mresistor]

If the first guitar was a stratocaster sometimes the pickguard gets static buildup and if you are one that rests a finger on the pg it will make crackling sounds through the amp.

[Skeeter_pdx]

After trying the new guitar, did you re-try the first guitar you were using?
What was the original problem that caused the amp to come in for repair?

I did--it seemed to crackle again but a little more subtly. This was after putting the chassis back in the cabinet too, but I thoroughly checked to make sure my speaker cable extension wasn't the issue either. The amp came in with a complaint of low output volume and intermittent crackle. One of the EL84s was completely cracked--I put in a new pair of JJs and the output was back up. Not sure how long it was being played with only one power tube. Voltages on the tubes and the bias voltage were good with the new tubes in though.

Poking around with the multimeter it appears the diodes in the heater circuit (D3 and D4) were testing open. I'm not entirely sure what the function of those diodes is. Others were testing strangely and showed ~0.6V in the forward bias and similar in the reverse bias with the voltage continuing to climb up to 2+ volts or so. Should I try to test the diodes again or maybe look for AC voltage where there shouldn't be any?

If the first guitar was a stratocaster sometimes the pickguard gets static buildup and if you are one that rests a finger on the pg it will make crackling sounds through the amp.

I was using an SG

[BrianS]

I think at this point you may be chasing ghosts.  I have no idea why you'd test the diodes in the heater supply.  They seem to be part of a rectifier circuit for the heaters, i.e., they are running the heaters with DC voltage rather than AC.  You could have checked the heater voltage first to see if it was what it was supposed to be.

This amp will most likely hiss when turned up loud; most amps do.  The "crackle" you hear may be something in your shop mains.  It may be some kind of EMI in the air...you said this "crackle" changed when you changed guitars...hmm...

When a tube amp comes in for repair, you should have basic servicing protocols to follow before you start chasing down "noise".  You start with the obvious things:  proper cleaning and checking/replacing tubes.  Once that is done, if the customer complaint persists, then you have to do more serious, pinpoint troubleshooting.   


Can you be sure that the "crackle" you are currently hearing is the same "crackle" that the customer complained about? These things are very hard to troubleshoot through a message board.  I have seen on this board where problems are solved by uploading a video or sound file that shows the problem you are having directly.  "Crackle" can mean different things to different people.