Humming from Mordaunt Short MS502 Speaker Amp

[AudioGuy88]

I have a pair of Mordaunt Short MS502 speakers.
These have built in amps for the side firing subs.
One of the amps has developed a loud hum.

The hum is quite strange and I can't figure it out.
This is happening to 1 speaker only, the other one has no hum.

The speaker has a Int and Ext switch for choosing the signal. Int uses the speaker inputs and Ext uses RCA.

If you connect just the power, the amp will power on and no hum at all, on either Int or Ext.
On Int input, as soon as you connect the negative (-) of the speaker, then loud hum. Like a ground loop.

If you connect an RCA (from the sub out of the pre/avr), then the hum goes away, even though the input in on Int and is not using the RCA.

I thought this was a filtering problem so replaced all the caps in the path of the Int connection, but this has not changed anything.

Is anyone able to work out what is going on using the schematics?

TIA

[R_J]

Re: Humming from Mordaunt Short MS502 Speaker Amp

Is your amp output connected to the speaker properly or are the wires reversed? Or maybe speaker wire from main amp is not making connection or is open?

[AudioGuy88]

Re: Humming from Mordaunt Short MS502 Speaker Amp

Is your amp output connected to the speaker properly or are the wires reversed? Or maybe speaker wire from main amp is not making connection or is open?

Yes I believe so. There are 2 speaker connections, both have different sized terminals.
The incoming from main speaker terminals/XO only fit 1 way. Have checked that that is correct.
The output to the sub is also correct.

[CapLeaker]

Re: Humming from Mordaunt Short MS502 Speaker Amp

Could be a broken cord or jack too.

[AudioGuy88]

Re: Humming from Mordaunt Short MS502 Speaker Amp

I have taken the humming amp to an old school tech who repairs at a component level. He said he couldn't find anything wrong with it and gave it back to me.
Of course it still humms loudly whereas the other amp is fine.
Unfortunately the tech is 'on holiday' till next year! He's semi retired and only opens when he feels like it. I don't know of another tech who does component level repairs. So I'll have to find this myself.

When the amp is connected to power, it is fine, no humming. As soon as the (-) negative terminal is connected, it hums. As soon as the negative is disconnected, it's fine again.
The hum is related to the volume. If the volume is down, no sound and no hum. If the volume is up, lots of sound, lots of hum.

I've checked polarity of the woofer and it's fine. I've checked the connections and they look fine. I've swapped the amps over and the other amp is fine, and the humming one still hums.

So I am certain it's somewhere in the amp. I'm just not experienced enough to work it out.

Any additional help would be appreciated.

[CapLeaker]

Re: Humming from Mordaunt Short MS502 Speaker Amp

The speaker has a few inputs... test and see if the problem is the same using the INT setting. Take the amp out and check the grounds on the RCA jacks. While you are at it, upload some straight shot, high resolution pictures using the attachment function.

[redwire]

Re: Humming from Mordaunt Short MS502 Speaker Amp

The schematic shows the MS502 is not earth-grounded, it has a two-prong power cord.
But your MS502L pic shows a three prong IEC connector.

I would use an ohmmeter and verify the power cord ground prong connects to the amp chassis, and readings to the RCA jacks.
The hum problem you are describing is likely an open-circuit on the ground.

Also, the schematic shows the RCA jacks and pre-amp are grounded (to something...) via RX01 47Ω so if that resistor was burnt or open-circuit that could do it.

[petehall347]

Re: Humming from Mordaunt Short MS502 Speaker Amp

"On Int input, as soon as you connect the negative (-) of the speaker, then loud hum. Like a ground loop."

is your amp common ground on speaker outs ?
make sure wires are correct way around . try different wires too
is the source machine earthed at the mains ?
is the sub earthed ?
keep just the source earthed and lift the ground on sub ..

[redwire]

Re: Humming from Mordaunt Short MS502 Speaker Amp

INT is the high-level loudspeaker input that is differential. So it should normally hum when only one of INT(+) or (INT-) is connected. Lifting the ground on the sub would not work and may even be the problem here in the first place.

If it's humming with both speaker inputs connected then the diff amp is having some problem, or the (common-mode) noise on ground is too great. In other words, either end (main amp or MS502) is not earth grounded.
Because the other MS502 does not have this problem, it's this particular sub with a ground or diff amp problem.

[AudioGuy88]

Re: Humming from Mordaunt Short MS502 Speaker Amp

I never did find out what was going on. I'm sure it was a very small grounding issue.
However due to the lack of time and the speakers taking up so much space, I sold them as is with the humming for cheap. I'm sure the buyer will fix them and get them running. They are great speakers.
Thanks everyone for your help