SA212 - 50W - Crackling noise on certain notes

[jujub]

Hello to all.

I'm calling for help. It been a month since I try to find the case of a big Crackling noise on certain notes, mostly low ones.

To this day we already done the following job with the help of sometimes a friend sometimes a lopper playing the 'deadly' notes or chords:

• Cleaning every mecanical connection (jack, switches)

• we try to change every tube without any success.

• We connected on another speaker cabinet with finding the same problems.

• I've double check every solder joint with a sharpie but we didn't find any faulty one

• We've already had retighten every knob or bold and measure every resistor a found some faulty ones (broken or Out of spec) we replaced (highlighted in yellow in the attached photo)

• All filter caps have been changed to F+T or TAD I can't remember.

• we also have add an electronic board inside the chassis (with any drilling or invasive or destructive méthode/device be assured) for implementing the 'Improved bias circuits' found on Mr Huss almighty website

To give some clues and other conclusions we found :

• The issue happened suddenly one day without any strange noise before.

• The noise disappear when the chassis is out from the combo: so the noise should be related to a bad connection...We supposed

• The noise only happen Wen I play notes an can be reproduce by touching any component or wire.

• we connect directly after the preamp with crocodile clamps : the noise was still there. so It most happened in another place than the preamp

• no matter If the amp is cold or as been heating for hours : the noise occur.

The amp is an SA212 50 from 1974 that was stored inside my house at suitable temperature and humidity

Even after replacing those the nose is still there..can you help me?

Did anybody had the same issue?

Could it be the output transformer

Thank you for reading me

[mhuss]

hi jj,

That's a bit of a puzzle. Perhaps check carefully in and around the power tube sockets for any sign of arcing (carbon spots). The fact that it only happens in the box is odd. Did you try out of the box, but 'right side up' (tubes down)?

[jujub]

Hello yes we tried with the chassis on top of the combo...the noise is there . Mostly because of vibration I think

I'm puzzled too!

[jujub]

hello unfortunately there's no sign of arcing on the tubes...

Could it be a fault in the output transformer?

[mhuss]

Try loosening and re-tightening all the ground connections (near the inputs, and near both ends of the 'skinny' board.)

[jujub]

Hello...Just found that a cap had a broken leg.... Just had to résilier it properly and the amp is back un bussines!


Note: tonight my DR103 Blown. main fuse went out suddenly