M70 Hummmmmmmm ?

picked one up local today --

norm ramon anybody --need an opinion -help

on AC ( only ) --it has a hum - not terribly loud --but very bothersome

here are the symptoms

1. the hum is equal thru both woofs
2. it does not increase with volume - at all
3. it happens at zero volume --as soon as the power turns on
4 its on all modes and all settings --ie - its always !!

5. its totally gone with batteries
6 and its gone on both AC and DC thru the headphone jack

all i tried so far is to spray the phone jack --and that fixed nothing -

whats next ?
thanks

loose transformer?

then why would the hum be gone -- thru the phone jack ??

you mean the phono jack?

headphone jack --the is no hum at any volume thru headphones on AC

I think it is either a bad smoothing capacitor on the secondary of the power supply transformer, or the bad core of transformer causing it.
Some headphones cannot detect very low frequency sound.

It looks like the power supply needs to be rebult.

C681, C682, C683 and C684 are the electrolytic caps in the power supply. Norm will have better suggestions.

ok - thanks

just tried some GOOD low headphones -- NO hum

also --tried the line-out - NO hum on ac or dc

Pre out shouldn't show that hum. If you hook an external speaker it will show that hum.

It could be a problem with the power supply. That is my guess!


If you connect an AC adapter and if the hum goes away, we can confirm it is the problem with the filter caps.

throw it on ebay, you will still get $300 for it

[quote="BoomboxLover48"

If you connect an AC adapter and if the hum goes away, we can confirm it is the problem with the filter caps.