NZGuitars.com

So slightly weird issue with the Calvin v3m I picked up.

Bad hum unless I pull the input lead to a side (hum then goes away until I release it). Hard to play while holding lead in that pose

I expect there’s an issue with the input jack but that seems likely to be a PITA to get to?

I’d add a video but don’t know how to do that.

Take out the PCBs and have a look. I don't think you'll be able to repair anything from the outside and I suspect the ground connection isn't as reliable as it should be.

Ok that’s been helpful. I’ve opened up the amp. I can see that when I pull the lead to the side the sleeve is touching the metal in the hole which results in ground being connected. But it seems that ground is not connected the proper way ie through the socket.

Here’s the socket. From my limited electronics knowledge it appears that the signal is meant to ground via this yellow capacitor 224J63 (based on tracing the signal route from sleeve connector in the socket through to the ground connection to the chassis). But the capacitor is giving me a nil reading on my multimeter’s capacitance and resistance settings.

I’m thinking the capacitor is toast. My questions:
1. Where am I going to get one of those to replace it?
2. What is its actual purpose in the signal route to ground? (Google tells me it is a 0.22 uF film cap)?

Capacitor sourcing wise: https://www.jaycar.co.nz/220nf-630vdc-p ... r/p/RG5222

I'd be tempted to replace that jack, jaycar have those sockets too: https://www.jaycar.co.nz/high-quality-p ... k/p/PS0195

It's a 220n cap rated for 63v. I have about 100 of them if you get stuck and are in Auckland. Although mine are 100v and a little fatter to reflect this.

Then reflow those solder joints around the jack and you should be sweet. Solder does harden and crack over time.

It won't be grounding through the cap. I would just re-flow the solder joints on the board and clean the contacts in the jack first.

Presumably that's C106? That goes between lugs 2 and 3 of the jack.

According to Aiken amps it's there for added protection against noise/interference:

"...Fortunately, there is a simple addition to the star ground scheme that will make for a very quiet amplifier with no RFI. Simply add a 0.01uF capacitor from the chassis to the ground lug of the isolated input jack..."

Although 10n to 220n is quite a big difference.

And the cap will show as shorted when there is no input jack as the connections will be closed.

This is the now removed cap. If I test it using my cheap jaycar multimeter it gives no reading (neither short nor anything).

It is C102 on the board and the schematic I’ve looked at for the V3M. I think it goes from lug 1 (sleeve?) to c102 to QC2 ground which is consistent with both the schematic and what I see physically.

Can you post a photo of the underside of the input jack pcb? It would help to see the solder pads and the circuit traces.

It would be unusual for the virtual ground cap to be a problem (first time for everything though) and it sounds more like a problem with the jack itself - either it needs resoldering or the tension isn’t right so the ground tab isn’t touching the sleeve of the cable plug, which means it needs replacing.

Remember that the ‘ring’ and ‘sleeve’ of the jack are connected when you insert a mono plug. Sometimes the ring is wired to a muting circuit and the sleeve is a standard ground, to keep the amp silent until a jack is inserted.

Here’s the input part of the schematic. I was wrong about the ring - it does nothing. But I still think the jack is your most likely suspect at this stage.

Thanks for the help. I replaced the cap and touched nothing else on the basis that the less I touch, the less I might break! But seems to be fixed now

That is odd, I wouldn't expect the cap to be an issue, maybe just unplugging the board has cleaned the connectors and now its ok.

I suspect a bit of opening up and reconnecting fixed something that was dodgy. I don’t care - so long as it works

Great outcome regardless!