low hum on Realistic receiver

The problem here is almost certainly the main smoothing capacitor. This will be the biggest electrolytic capacitor in the unit. Value will be 1000 or

2200 or 4700uF or something similar. Replace with similar value and voltage rating, and there's a 95% chance that the hum will go away.

Arfa

As Arfa said, it is almost certainly an electrolytic cap. This is a situation where an ESR meter would be helpful. In fact, I had a receiver that developed the same type of problem and found it in minutes by using an ESR meter. Just one problem found could pay for the meter in time saved.

"buddy" bravely wrote to "All" (14 Jan 06 23:10:07) --- on the heady topic of "low hum on Realistic receiver"

bu> From: "buddy" bu> Xref: core-easynews sci.electronics.repair:355180

bu> I just acquired an old Realistic STA-45B stereo receiver. It works bu> fine except that it produces a low hum to the speakers as soon as it bu> is turned on. It does not get louder as the volume is turned up. It is bu> also in the headphones.

bu> The hum occurs even if no inputs are connected, just the speakers or bu> headphones. I took the cover off and sprayed everything with bu> electrical contact cleaner. All screws from boards to chassis were bu> tight.

bu> It is annoying but if I can't fix it I will use it like this if I bu> must. I only want to use it to go from my computer's soundcard to the bu> Aux inputs, to use stereo speakers instead of PC speakers. I do home bu> recording and need better sound to mix/master.

bu> I do not have any experience to diagnose/repair audio equipment, but a bu> little long-ago experience testing components on computer boards. I bu> have a multimeter.

bu> Can someone describe what to do to track down the problem?

There might be an open rectifier but if the rectifier isn't discrete then test the bridge rectifier. However, given the age of the equipment, it is more likely to be a problem with dry electrolytic supply filters. Test all electros with an ESR meter. (beg, borrow, but don't steal!)

A*s*i*m*o*v

... A stereo system is the altar to the god of music.

Ac hum in an audio device is very easy to find with an o'scope. It is obvious which filter capacitor is not doing its job when looking at it with the o'scope. It is also possible that a power supply bypass filter near the audio circuit is also weak and will also be easy to find in this way.

In addition to the main large filter capacitors, also look for any other power supplies that might have bad caps. I found a small capacitor that was high-ESR in a Reaslitic STA-20.

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Change the main filter capacitors, and most likely the hum will be fixed.

--

JANA _____

The hum occurs even if no inputs are connected, just the speakers or headphones. I took the cover off and sprayed everything with electrical contact cleaner. All screws from boards to chassis were tight.

It is annoying but if I can't fix it I will use it like this if I must. I only want to use it to go from my computer's soundcard to the Aux inputs, to use stereo speakers instead of PC speakers. I do home recording and need better sound to mix/master.

I do not have any experience to diagnose/repair audio equipment, but a little long-ago experience testing components on computer boards. I have a multimeter.

Can someone describe what to do to track down the problem?

Thanks to all who replied, and "southmetro" who emailed. Sounds like the first thing I should try is:

Replace main "smoothing" "large filter" capacitors.

So I googled to get an idea what they look like. I saw photos of the small "M&M" ones, and the cans. I assume I am looking for the cans.

Could someone help me identify the ones to replace?

Please download these 2 photos:

In the "top" photo, there are 2 large cans next to each other, in the middle of the board at the top of the photo (which is rear of the receiver). They are lavendar color, and say

1000 uF 25 V CE 1 9.

In the "bottom" photo, there is a small board in the top of the photo, just left of center. The board says "AC Input". The cans are:

(lavendar one)

CE (W)

1000 uF 35 V

looks like it has some kind of glue on the bottom in addition to being soldered

(silver one)

• Nippon Chemi-Com

• CE W

• 35 V 470 uF

which are the one(s) to replace?

Questions:

-To replace, should I :

1. clip leads, leaving about 1/8 " nub to solder new one to (seems to me this is easiest way) or
2. unscrew the board and unsolder where the lead comes through the board on the other side

- do I need to do anything to "discharge" them first? leave unplugged for some amount of time, or something else?

- unrelated question - there is no light for the FM scale and indicator. Is this the way the receiver was made? In the "top" photo you can see the bulbs for the two meters, but I don't see anything to light the FM scale and indicator. Hard to believe it was made this way. But no biggie, as I said in my OP I only want to use the Aux input.

Thanks, Rod

I would go for the big grey one in the middle of the underside photo first. This would appear to be the main smoothing block. The board at the back left on the topside photo, looks like it is part of the output stages. The pair of axial caps in the middle of that board may be supply rail decouplers or output caps. Likewise the pair of radials at either end. These caps are much less likely to be the cause of the problem than the grey one.

If you can get access to a 'scope, or have a buddy with one, I would feel inclined to have a look at the ripple levels on the grey cap. It looks as though it may be a ' dual ' type having 3 tags. One is usually common negative, whilst the other two are the positives of two different caps in the one can. They may or may not be both the same value, but are usually the same working voltage - ie could be 2200uF + 2200uF @ 35v, but could also be

2200uF + 1000uF @ 35v.

If you do finish up replacing the wire ended caps, they should be replaced ' properly ' that is by removing the old one from the board, clearing the holes, and resoldering the new caps' wires directly back to the print. Leaving a cut end from the old one, and trying to tack solder the new one is going to give you problems with the original solder crystallising as it melts when you try to solder the new wire. As far as I can see, both sides of all the boards are get-at-able, without having to unscrew them. Or am I missing something in what you're asking ?

Arfa

No, that is what I was asking. But I realized that it's fairly easy to do the "right" way. I replaced the two caps in the Input board in the "bottom" photo before I saw your post, but no change.

After I saw your post I removed the big gray one - had to drill out the screw $%& for the clamp - it was just a regular one, 2000 uF and 50 V. I replaced it, but still no change.

I think I'm at the limit of what I can do with no test equipment so I'll have to use it as-is for now.

I really like the look of the old Realistic with machined Alum knobs and brushed Alum faceplate and solid wood case. The sound is very "clean".

Reminds me of a separate tuner/amp combo from Technics I had in the

70's.

Thanks for trying, Rod

"buddy" bravely wrote to "All" (17 Jan 06 13:55:16) --- on the heady topic of "Re: low hum on Realistic receiver"

bu> From: "buddy" bu> Xref: core-easynews sci.electronics.repair:355552

bu> I really like the look of the old Realistic with machined Alum knobs bu> and brushed Alum faceplate and solid wood case. The sound is very bu> "clean".

bu> Reminds me of a separate tuner/amp combo from Technics I had in the bu> 70's. bu> Thanks for trying, bu> Rod

IIRC Realistic were likely re-branded Sharp. You will find that styling and look in Sharp components of the same period.

A*s*i*m*o*v

... When I was your age, we carved transistors out of wood.

Transistors !! Wood !! ??

We had to make tubes out of old jamjars and paper clips, all whilst we lived in a cardboard box in the middle of the highway, watching the snow come in through the open top. Sounds to me like you had it easy ...

Arfa

Tape decks, yes. Receivers were made to RS specs by unknown factories.

Some of the mid-70's ones were identical to Hitachi models....

jak