Unidentified Magical Tesla Coil

[GasMask52]

Picture this:

Just finished re-soldering and working on this Marshall Preamp , no value mistakes with resistors , no polarity fails with caps , went to mount everything up and when I turned the guitar amp on [which powers up normally, fuse doesn't blow] a continuous sparkle starts somewhere on the PCB Preamp..

What would go through your head? What causes sparkling? Can it happen when component leads are not cutted short enough and are too close to each other but not touching? Or when solder doesn't properly apply to the holes?

Was thinking about powering up the guitar amp again with the chassis out, to see where the sparkle starts

Please enlighten me

[Enzo]

ANy of those. YOu have an arc forming between two points. If you cannot see it, turn out the room lights. Could be on either side of a board if you used one.

[Leo_Gnardo]

I saw a similar effect one evening some 40+ years ago, when I was working on an Ampeg SVT. There was a crackling noise from the preamp and I couldn't track it down. For some reason I switched off the lights in the workshop, then I observed a 47K resistor that had a spark show in a spiral pattern along the length of the resistor. Replaced it and no more crackle noise.

On rare occasions since then, when I'm trying to track down a similar problem I'll do the same - observe the circuit in the dark. Every now and then the problem reveals itself that way.

On your Marshall it may be a failing component, a bad solder connection, an instance where high voltage is in close proximity to ground and an arc has set up, something like that. Dust or some other contaminant on the board may also be the cause. Further close observation under dark conditions will reveal the source. BE CAREFUL! and continue your investigation.

[GasMask52]

Thanks a lot Enzo & Leo ,
I've just tested it with the chassis out in a dark room , as soon as I powered it up the sparkle appeared for 2 seconds, clearly visible even on the surface of the PCB , being a Marshall VS100 the sparkles showed up on the preamp in a ~4cm diameter that involved 2 100uF 35V Caps , 2 tension regulators and the 2 wires [OR1&2] that go to the X-former..

Sadly as soon as I bent my head down to see under the PCB (to literally see the arc) the spark was already dead... this time for some reason the spark just stopped , I re-powered up the amp and no spark shows up anymore and the LEDs light up normally... should I just forget about it and plug in my guitar and test it or does that mean that something shorted?

[Chuck H]

Well "somewhere on the board" implies that you can't see it.?. So it's a sparkling sound? Like a static sound? A frying sound? Is it a snap, crackle and pop kind of thing or a steady modulation? When you equate it to a Tesla coil I get the impression it's a buzzy/crackly kind of thing.?.

Sometimes new tubes will little tinkling noises as they're warming up for a while.

[Chuck H]

Thanks a lot Enzo & Leo ,
I've just tested it with the chassis out in a dark room , as soon as I powered it up the sparkle appeared for 2 seconds, clearly visible even on the surface of the PCB , being a Marshall VS100 the sparkles showed up on the preamp in a ~4cm diameter that involved 2 100uF 35V Caps , 2 tension regulators and the 2 wires [OR1&2] that go to the X-former..

Sadly as soon as I bent my head down to see under the PCB (to literally see the arc) the spark was already dead... this time for some reason the spark just stopped , I re-powered up the amp and no spark shows up anymore and the LEDs light up normally... should I just forget about it and plug in my guitar and test it or does that mean that something shorted?

What are the two 100uf/35V caps doing? Near "2 tension regulators" (I don't know this term). Are these power supply caps? A 35V rating would seem very low for any tube amp power supply.

[GasMask52]

What are the two 100uf/35V caps doing? Near "2 tension regulators" (I don't know this term). Are these power supply caps? A 35V rating would seem very low for any tube amp power supply.

You've never heard of a tension/voltage regulators??

I already clarified that it's a VS100...

[Chuck H]

I already clarified that it's a VS100...

Oh! Of course. I should have known then. All of us here are intimately familiar with every piece of gear ever created. I shall go and boil my feet now as penance for my ignorance.



A schematic is always helpful. Though I know they're not always available. Short of being able to provide one you shouldn't expect anyone to be aware of the circuits inside.

[GasMask52]

I saw a similar effect one evening some 40+ years ago, when I was working on an Ampeg SVT. There was a crackling noise from the preamp and I couldn't track it down. For some reason I switched off the lights in the workshop, then I observed a 47K resistor that had a spark show in a spiral pattern along the length of the resistor. Replaced it and no more crackle noise.

On rare occasions since then, when I'm trying to track down a similar problem I'll do the same - observe the circuit in the dark. Every now and then the problem reveals itself that way.

On your Marshall it may be a failing component, a bad solder connection, an instance where high voltage is in close proximity to ground and an arc has set up, something like that. Dust or some other contaminant on the board may also be the cause. Further close observation under dark conditions will reveal the source. BE CAREFUL! and continue your investigation.

Just tested the Amplifier on a dummy speaker that I don't care about , it made a small swirling sound and then a very loud pop sound, it made me think that some DC is getting through

[Leo_Gnardo]

Near "2 tension regulators"

Tension = French for voltage. "Haute Tension" high voltage signs on fences surrounding power distro facilities, switching stations & hi voltage lines in France, Canada & other Francophile countries. Get some of that "haute tension" on ya, see how your tension fares, guarantee it will increase. Just before it lets go for good... OK language lesson over for today.

I'd clean off that portion of the circuit board with some solvent & a cotton swab, also inspect carefully for any bad soldering. Maybe re flow the connections to those caps, regulators & incoming transformer leads.

Back to haute tension, here's one of my favorite descriptive warning signs:

[Chuck H]

DC is usually heard as a steady state. What you describe sounds more like what I'm familiar with when an electrolytic capacitor is failing.

[GasMask52]

Oh! Of course. I should have known then. All of us here are intimately familiar with every piece of gear ever created. I shall go and boil my feet now as penance for my ignorance.



A schematic is always helpful. Though I know they're not always available. Short of being able to provide one you shouldn't expect anyone to be aware of the circuits inside.

Don't get upset sweety, the reason why I didn't provide any schematic is because this is no technical component issue, it's more of a crappy soldering/mounting issue, knowledge of the schematic is futile in this case - as I already stated, I wasn't able to precisely detect in time where the arc started 'cause it died off quickly, hence the reason why I listed the components "involved" in the spark

[GasMask52]

DC is usually heard as a steady state. What you describe sounds more like what I'm familiar with when an electrolytic capacitor is failing.

Thanks dude , I'll definitely consider that

[GasMask52]

Tension = French for voltage. "Haute Tension" high voltage signs on fences surrounding power distro facilities, switching stations & hi voltage lines in France, Canada & other Francophile countries. Get some of that "haute tension" on ya, see how your tension fares, guarantee it will increase. Just before it lets go for good... OK language lesson over for today.

I'd clean off that portion of the circuit board with some solvent & a cotton swab, also inspect carefully for any bad soldering. Maybe re flow the connections to those caps, regulators & incoming transformer leads.

Back to haute tension, here's one of my favorite descriptive warning signs:

[ATTACH=CONFIG]58078[/ATTACH]

Leo I've detected the culprit, flipped the Pre-Amp over and saw 2 burn marks on two terminals, that belong to two 1n4007 diodes that were also near the spark spot , funny, honestly I thought it was either the caps or the regulators

[Chuck H]

Leo I've detected the culprit, flipped the Pre-Amp over and saw 2 burn marks on two terminals, that belong to two 1n4007 diodes that were also near the spark spot , funny, honestly I thought it was either the caps or the regulators

Diodes don't usually fizzle. They just go open or short. It could be like Leo said and there's some contaminant on the board. Sometimes boards can go conductive with burn marks. Gotta clean all that or even remove the burns with a Dremel like a melon baller.