Ferrite Ring in Marshall VS 265

[guitarkitbuilder]

A reader of ours wrote me to ask about a ferrite ring that broke while he was opening up his Marshall VS 265 amp.  I have no idea how it broke, but he was replacing potentiometers when it happened.  Does anyone know which wires/cable (AC mains?) go through the ferrite ring , and if it's just straight through or wrapped or what? 

[guitarkitbuilder]

Now that I have a photo of the whole chassis I can see that the cable that was using the ferrite ring is the audio cable for the reverb tank. 

[J M Fahey]

It's there to keep RF interference away, but usually he should have no problem at all if he does not replace it.
There's also another ferrite ring at the power(mains) cable and now they also add ferrite beads at the input jacks.
Not bad at all, but amps have worked without them forever so ....

[guitarkitbuilder]

Thanks JM - he's already noticed a higher level of hum when he turns up the reverb so he'll want to fix it in this particular case. 

[phatt]

The Rev design maybe the problem,, the ferrite might have been a patch up quick fix.

If I remember this issue came up some time back and that one was actually squealing like a pig.

I recall Mr Fahey picked up on the bad design. :tu:
Phil.

[guitarkitbuilder]

You can see the photos of the reverb cable and missing ferrite here:

http://www.guitarkitbuilder.com/content/mailbag-marshall-valvestate-vs265-repair

[phatt]

Hi Guitkit,,
just Be aware that hum from the reverb is not uncommon.
Yes the ring may help but I did have a qucik look at the schematic.

For those interested I found it Here;
http://www.schematicsunlimited.com/?z=marshall

I'm fairly sure it is the problem reverb circuit I mentioned.

Cheers, Phil.

[teemuk]

Wow, I have never seen such a small repair covered in such detailed extend.
Great work!  :tu:

[teemuk]

Not bad at all, but amps have worked without them forever so ....

To be honest, in this issue I don't know how comparable stuff of today is to stuff of yesteryear. Back in the day there weren't as much SMPS in gear, class-D amps, cellphones, RF interference from everywhere, etc. Heck, even my toothbrush today has a radio transmitter inside it. I think regulations of how much interference the equipment was allowed to pass into mains were less strict too.

I don't know if Marshall, Blackstar amps and handful of other brand gear using an abundance of ferrite rings will squeal without them or not but I'm pretty sure that this time and age the chance for it is a lot greater than it was, say, ten years ago, and it will get worse all the time. Plus they have to meet with compliance specs and since more and more stuff is starting to implement SMPS or class-D I think we'll see more and more ferrite rings in the future.