Mystery crackle

JohnPerry

Can anyone advise? My old '71 silverface Deluxe Reverb has a low-level crackle I can't shake off. It's not related to volume, nor any of the controls nor cables. The tubes have all been swapped out one by one. Just keeps crackling away, even with no cable plugged in. 

It's not noisy enough to make it a great problem - it's a magnificent amp - but it's just there, this noise, like an annoying fly. Though that would be a buzz. A crackling fly, then.

Babones

Power tubes swapped out too? Loose input jack?

viz

Rattling transformer? Or is it coming out of the speakers?

ICBM

Most likely a plate resistor, or more than one. Notorious problem with old Fenders.

That's if you're lucky… if you're unlucky the eyelet board has become conductive and is 'leaking' slightly between a couple of the eyelets. Harder to find usually, and a bit of a faff to fix thoroughly.

DJH83004

I am totally with ICBM on this one, regarding the eyelet board, I have a '69 Deluxe Reverb and Vibrolux Reverb, that have done the same. Just as a test slacken off the fixing screws and squirt WD40 between the two boards to see if it reduces or clears the crackle, other than that carbon comp resistors are the next likely suspects.

JohnPerry

Thanks gents. Very kind. I will check all these one by one.

JohnPerry

Finally got round today to acting on this excellent advice and achieved a result i did not expect.

Took the amp out, squirted switch cleaner or WD40 in every orifice and socket and copiously between the eyelet boards. I found a small piece (maybe an inch square) of screening foil on the underside of the top of the amp hanging down and taped all over it with gaffa. Then reassembled.

In the last month this crackle had become noisy enough to be seriously tedious, virtually ruling out low-level home practice. And it was always there, including in my final test before taking the amp out.

Anyway, after reassembly it was quiet as a mouse on idle for the first time in months. Marvellous. Couldn't believe my luck. I expected to either effect zero change or to cure it entirely. I seemed to have done the latter.

Turned it on tonight. Still silent. Then after a few mins the crackling/frying bacon sound started back up, but much much quieter than before (perhaps 20 per cent of the original volume). There's no doubt this investigation has vastly improved it. What i didn't expect was for it to still be there, albeit now at a low level.

Can anyone speculate on how the problem might have been improved but not cleared? It might help me to focus on a specific area if i have another look inside it.

ICBM

That does sound like you've proved it as board leakage. It's actually often caused by the underlayer rather than the main board - if so you can fix it by adding a new layer between the two. Cut a piece of cornflake packet to the same size as the boards, and slide it in from the input jack end - you can lift the end enough to get it under, then punch the screw holes back through.

If it turns out to be the main board, it's also sometimes effective to heat and re-melt the wax on the board using a hairdryer, although ideally you should remove the electrolytic caps first since they don't like high temperatures.

JohnPerry

Thanks, that's great. Will do the cornflake trick asap. The latter will need an expert 

DJH83004

To be honest the spray between the boards was never meant to be the complete fix, it was just to see  if it improved matters, which clearly it did. The next stage as ICBM says is to introduce some physical separation, but being a SF Fender I would personally go for a more up market cereal like a Museli packet    

JohnPerry

I was actually thinking of some slightly thicker, bespoke cardboard unrelated to cereal of any sort. I wonder if there is a vintage-correct kind you can buy very expensively 

DJH83004

You could source some thin phenolic sheet <1mm, which should be flexible enough to slide in between from the jack end.  

martinw

Google 'plasticard'.

JohnPerry

I found the plastic cover of a diary, cut it to shape and inserted. Reluctant to count chickens but, by God, the crackle has not been there for the last hour. In which case, thanks very much gents. Huge help.

Gassage

JohnPerry said:

Thanks, that's great. Will do the cornflake trick asap. The latter will need an expert 

Won't you get a snap, crackle and pop if you do that?

SimonC

No, but it might improve the crunch channel.

Gassage

SimonC said:

No, but it might improve the crunch channel.

Flakey notion.

ICBM

Gassage said:

JohnPerry said:

Thanks, that's great. Will do the cornflake trick asap. The latter will need an expert 

Won't you get a snap, crackle and pop if you do that?

No, that's Rice Krispies. Hence why it's important to use Cornflakes...