Sharp SG-FR35 loud hum
 

Hello

Can anybody give me some pointers help eliminate a loud hum when switching on this device? I have replaced the 2200uf capacitor in the power supply side as the existing one was bulging and leaking grunge. The hum is very loud across both channels and sounds like a failing smoothing capacitor. Disconnected the transformer is giving out about 28volts.

Your assistance is appreciated.
Dave

Re: Sharp SG-FR35 loud hum
 

I rather suspect it's the LA IC since it is across both channels and you have eliminated the 50hz hum by replacing the capacitor.

Re: Sharp SG-FR35 loud hum
 

Hello and thank you for your response

I have an LED light plugged into the same socket as the Sharp and when I power up the Sharp the LED light dims. Would the L.A. chip draw a large current like this?

Re: Sharp SG-FR35 loud hum
 

Quite possibly since it's the amplifier chip!

Re: Sharp SG-FR35 loud hum
 

Unplug the LED lamp and you may find that the hum disappears!

Re: Sharp SG-FR35 loud hum
 

I have a LED equipped wandering lead lamp at my workbench, Ken, and have never known it to inject hum or any other kind of interference to any radio or stereo I have on the bench. The lead lamp is usually left on when the bench is powered. It's plugged into a socket strip on the back of the bench into which other items, including a variable voltage linear PSU, a RF Sig. Gen. an A.F. Sig. Gen., etc. are permanently plugged, each being switched on as and when needed.

Re: Sharp SG-FR35 loud hum
 

It was just a thought, l got rid of an Ikea led lamp as it made a noise like the Russian woodpecker on a medium wave radio on the same ring.

Re: Sharp SG-FR35 loud hum
 

Tried that, it made no difference.

Re: Sharp SG-FR35 loud hum
 

What is transformer voltage when connected?

Suggest checking the four rectifier diodes. If one (or more) is short-circuit it will either put AC on your new cap or overload the transformer.

Re: Sharp SG-FR35 loud hum
 

I haven't checked that. The hum is so loud and occurs immediately the power button is pushed. The unit was working as I had just remade the solder joints to the aux input as they were cracked. The unit worked until i powered it down to refit the case then the hum started across both channels regardless of input selected.I noticed that the 2200uf cap was leaking grunge over the circuit board so I replaced it thinking this would cure the hum. It also causes my LED lights to dip as they are fed of the same socket. It comes down to either diodes or the L.A. chip. I could do with a circuit diagram.

Re: Sharp SG-FR35 loud hum
 

If one of those diodes had failed short circuit it could have caused the capacitor to fail. Depends if the muck was new or old.

Re: Sharp SG-FR35 loud hum
 

Hi George

The muck was very hard and dried on the circuit board. I managed to get virtually all of it off. I cannot read the value of the diodes can you suggest any that would do? The 2200uf was rated at 50v.

Thanks for your help
Dave

Re: Sharp SG-FR35 loud hum
 

I would not start replacing components without further evidence - especially on a pcb. If the unit is making a lamp dim I am surprised the fuse has not blown. You obviously have some sort of meter so a couple of voltage checks on the transformer output and across the 2200uF would be a good start and diode checks on the bridge.
Of course someone who knows something about the particular Sharp would have experience of their faults. I notice mention of the amplifier chip has been made and I know nothing about that side of it.
Maybe wait for a few more opinions.

Re: Sharp SG-FR35 loud hum
 

Hi!

Get the S.M. from my Dropbox here. . .

https://www.dropbox.com/s/5qoo9aljvi...DE-FR.pdf?dl=0

. . .have a read thro' the circuit diagrams in it, than if you're still none the wiser afterwards, I'll go thro' the fault with you!

Chris Williams

Re: Sharp SG-FR35 loud hum
 

Looking at the photo of the capacitor you removed, it looks more like the type of hard glue that was in common use rather than leakage from the capacitor. That's not to say the capacitor hasn't leaked, just that it's not obvious from the picture (I can't see the top to see if it's bulging or vented for instance).

Is there any audio at all behind the buzz, or it is so bad that you can't leave it on for more than a second or two?

Can you measure any DC voltage at the speaker terminals?

Re: Sharp SG-FR35 loud hum
 

So you say that the unit worked until you re-fitted the case. This rules out that you may have fitted the new capacitor the wrong way round by mistake, but re-check anyway. I think you may have done some damage to the circuit board print when you were cleaning that glue off. It's worth noting that you must be able to determine the difference between glue used to hold large components in place, (which is put there by the manufacturer to protect the circuit board connections from stress breakage) and leakage of electrolyte from a capacitor. Note that now there's no glue holding the new capacitor, there will be more stress on it's solder and print trace connections, although the new capacitor may well be smaller and lighter.

Taking note of the above, I would take a careful and close look at the electrical connections and print traces going to and from that capacitor you've just fitted. With the unit disconnected from the mains and the electrolytic discharged by the brief application of a shorting link, check the entire length of each print trace from the two connections of the capacitor to each point they terminate on the circuit board with your DVM on the lowest ohms range - obviously all readings should be close to zero ohms.

The thing with it dimming a lamp plugged into the same socket when powered on seems absolutely ridiculous to me, but stranger things and all that!

Re: Sharp SG-FR35 loud hum
 

The old capacitor when removed was bulging from the bottom so was probably due for replacement.
The buzz is so loud that I couldn't leave it on fearing something would go bang!

Re: Sharp SG-FR35 loud hum
 

Hi!

There are two basic versions of this system - the SG-FR35E which gives 2 x 7W nominal RMS per channel O/P and uses a Sanyo LA4282 output IC with a 31.5V supply, and the higher power SG-FR35H which uses a Sanken STK4362 hybrid output device operating from a 42.5V power supply - circuit, layout and parts-list details for both versions are given in the Service Manual I posted last night, please ensure you're working from the correct diagrams!

LA4282:-

Datasheet:-

http://www.onsemi.com/pub/Collateral/LA4282-D.PDF

Device:-

https://www.ebay.co.uk/itm/LA4282-SA...UAAOSwiCRUddyv

STK4362:-

Datasheet:-

http://pdf.datasheetcatalog.com/data.../1638798_1.pdf

Device:-

https://www.ebay.co.uk/itm/STK4362-I...4AAOSwCE5bmoEC

(Links posted to replacement devices posted in accordance with Section C Forum Rules which permit eBay links to spares directly related to the fault under discussion)

I'll go thro' it in more detail if you still need help!

Chris Williams

Re: Sharp SG-FR35 loud hum
 

Thanks Chris. I have sent you a PM. Any help would be appreciated as I feel it's something silly that's causing the hum

Re: Sharp SG-FR35 loud hum
 

I think fro memory it's a LA4282 giving about 10w per channel.