RCA Console Stereo has loud "Hummmming" Noise

IowaFightFan

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So, I had an RCA Console stereo given to me a few months back. It worked great for a while, just using it at work to listen to AM/FM radio. We noticed fluctuating volume levels when on FM, but it didn't seem to happen on AM.

Last weekend, we turned it on and there was a liyd humming noise .It doesn't matter if the volume pot is completely down, or turned up loud, it's there.

I believe, in my research, it's likely the Filtering capacitor. It's a 4 prong capacitor, with 4 different values. It has a part # 972187-25 on it, but I've been unable to find any.

I know you can use 4 individual capacitors, and I have a rough idea how to do this, but need a little help. I'm not sure how to figure out which capacitor goes where, since they are all 4 different.
 

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If the filter cap has failed suddenly whilst in regular use, it's a good idea to check the rectifier diodes for shorts or open.

Then replace the cap. The cap is four electrolytics in one can. Three caps are indicated with a symbol -- a semicircle, a square, and a triangle. One cap has no symbol. The caps with the symbol are rated for 350 volts maximum working voltage. The fourth is 16 volts if I'm reading the picture correctly.

What is not shown is their capacities in microfarads. That will be written on the can before the working voltage (W.V.) rating. Usually, they'll be something like (but will almost certainly be different for your unit) 80 mfd, 40 mfd, 20 mfd, and 200 mfd corresponding to the semicircle, square, triangle, and blank, respectively.

You'll then need to buy modern equivalents. Slightly higher capacity is fine; higher working voltage is ideal.

Solder each new cap to the corresponding old cap's terminal wires. E.g., if the semicircle is 80 mfd and 350 W.V., solder the +ve terminal of the new 80mfd 350 W.V. cap to the wires formerly connected to the semicircle terminal, and the -ve terminal of the new cap to the ground terminal of the old cap.
 
If the filter cap has failed suddenly whilst in regular use, it's a good idea to check the rectifier diodes for shorts or open.

Then replace the cap. The cap is four electrolytics in one can. Three caps are indicated with a symbol -- a semicircle, a square, and a triangle. One cap has no symbol. The caps with the symbol are rated for 350 volts maximum working voltage. The fourth is 16 volts if I'm reading the picture correctly.

What is not shown is their capacities in microfarads. That will be written on the can before the working voltage (W.V.) rating. Usually, they'll be something like (but will almost certainly be different for your unit) 80 mfd, 40 mfd, 20 mfd, and 200 mfd corresponding to the semicircle, square, triangle, and blank, respectively.

You'll then need to buy modern equivalents. Slightly higher capacity is fine; higher working voltage is ideal.

Solder each new cap to the corresponding old cap's terminal wires. E.g., if the semicircle is 80 mfd and 350 W.V., solder the +ve terminal of the new 80mfd 350 W.V. cap to the wires formerly connected to the semicircle terminal, and the -ve terminal of the new cap to the ground terminal of the old cap.

Thank you so much for the information!

The values are:
80 MFD 350 WV (semi circle)
40 MFD 350 WV (Square)
100 MFF 350 WV (triangle)
150 MFD 15 WV

Are they marked on the bottom somewhere with the symbols? I flipped the power board over, and I sure don't see any markings.

I assume I will be removing the whole can, and then placing new individual capacitors underneath, correct? There's no way to reuse the old can with new capacitors, right?

Is there any video or literature on checking the rectifier diodes? I'll admit, this is new territory for me. I have a very basic understanding of electronics, and can solder fairly well. I have a digital multimeter, but nothing else in the way of diagnostic equipment. Even with the meter, I'm not sure what/how to test most of these components. I attached a pic of the underside of the power board.

I really appreciate your help!
 

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The bakelite base insulator of the filter capacitor will normally have cut-outs or etching adjacent to the terminals in the same shape as the symbols. Occasionally, the base of each terminal is in the shape of the symbol, or has the symbol etched into it.

You may be able to fit the new caps into the old can. This is called capacitor restuffing, and it's popular on vintage radios, amplifiers and test equipment where preserving the original appearance of the gear is important. Preserving original appearance is rarely much of a concern with vintage console amplifiers since they're hidden away and generally aren't an aesthetic focal point.

Note that you have two 100mfd 200 W.V. capacitors under the chassis -- the large brown cans held in place with metal straps. They should be replaced too.

Google "test diode with multimeter" for information on testing rectifier diodes. I have circled the two rectifier diodes in your image in red:

IMG_20181021_163514982.jpg
 
The bakelite base insulator of the filter capacitor will normally have cut-outs or etching adjacent to the terminals in the same shape as the symbols. Occasionally, the base of each terminal is in the shape of the symbol, or has the symbol etched into it.

You may be able to fit the new caps into the old can. This is called capacitor restuffing, and it's popular on vintage radios, amplifiers and test equipment where preserving the original appearance of the gear is important. Preserving original appearance is rarely much of a concern with vintage console amplifiers since they're hidden away and generally aren't an aesthetic focal point.

Note that you have two 100mfd 200 W.V. capacitors under the chassis -- the large brown cans held in place with metal straps. They should be replaced too.

Google "test diode with multimeter" for information on testing rectifier diodes. I have circled the two rectifier diodes in your image in red:

View attachment 1311692

Once again, thank you very much for your time and information!

I assumed the brown capacitors needed replaced too. One of them has cracks on the end, where the positive lead comes out .

I noticed when I replaced capacitors on a turntable that I have, the new ones were mostly all smaller than the originals. I assume the new capacitors that I replace the 2 brown cans with will be considerably smaller than the originals, right? As long as voltage and micro farad values are the same, physical size isn't an issue, right?
 
Todays capacitors are smaller. Chances are that you have a bad can filter. Its usually the first section (40MFD) that opens and causes power supply hum but since it is the original capacitor, it will need to be replaced for reliability and safety anyway.

And of course, all others will need to be replaced also.
 
Right. And you can increase the microfarad values a bit, and the voltage rating a lot, without any problem.

Where do you recommend purchasing the capacitors from? I've used Digi-Key (their website is a bit confusing at times) in the past, and most recently I used Allied Electronics. I wasn't sure if you had another source that these are easier to find.
 
Where do you recommend purchasing the capacitors from? I've used Digi-Key (their website is a bit confusing at times) in the past, and most recently I used Allied Electronics. I wasn't sure if you had another source that these are easier to find.
I'm in the UK so my choices might be different from yours. For mainstream components, I use Mouser and Farnell. When they don't have what I need, I Google for the part I need and that usually turns up specialist providers.

For example, I just googled 80 mfd 350 v and it came up with https://www.justradios.com/capacitors.html, which looks promising.
 
So, I started working on this tonight. I'm having a heck of a time getting the solder on these old joints to melt. I know better than to leave the iron on too long and heat up the parts, but the majority of this old solder just won't melt.

Ant suggestions? I'm using a Hakko FX-888D adjusted to 750, and it's still not melting it at all .
 
I like to get a small blob of solder on the tip of the iron, melted and hot, put it right on the old joint. This gives you a good sized contact point and very quick heat transfer.

Great advice! I did this and was unable to get all but 1 solder joint desoldered. The last one is giving me a hell of a time, as it was a huge glob. I've got it about 80% done, but had to take a break...lol
 
I want to replace these resistors. The brown with different colored rings. Anyone know how to identify these so I can order replacements? They have no numbers on them.

One of them got pretty hot yesterday when I was desoldering a large joint. It may be fine, but I hate to chance it .
 

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I want to replace these resistors. The brown with different colored rings. Anyone know how to identify these so I can order replacements? They have no numbers on them.

One of them got pretty hot yesterday when I was desoldering a large joint. It may be fine, but I hate to chance it .
3.9k ohm 10% tolerance, likely about 5 watt.

They're made to get hot, and solder heat is unlikely to have damaged them. Resistors do sometimes drift in value, though. Measure them with an ohmmeter and see if they're still within 10% of 3.9k ohm.

If it were me I wouldn't have bothered unsoldering; just snipped close to the joint and soldered new connections.

Use this to convert resistor colour codes: https://www.digikey.com/en/resource...version-calculator-resistor-color-code-4-band
 
3.9k ohm 10% tolerance, likely about 5 watt.

They're made to get hot, and solder heat is unlikely to have damaged them. Resistors do sometimes drift in value, though. Measure them with an ohmmeter and see if they're still within 10% of 3.9k ohm.

If it were me I wouldn't have bothered unsoldering; just snipped close to the joint and soldered new connections.

Use this to convert resistor colour codes: https://www.digikey.com/en/resource...version-calculator-resistor-color-code-4-band

Thank you for the great information!

I desoldered the joints because I'm restuffing the can that those resistors were connected to. If it weren't for that, yeah, I'd definitely have cut them.
 
I'm in the UK so my choices might be different from yours. For mainstream components, I use Mouser and Farnell. When they don't have what I need, I Google for the part I need and that usually turns up specialist providers.

For example, I just googled 80 mfd 350 v and it came up with https://www.justradios.com/capacitors.html, which looks promising.


So, I am restuffing the can but have one other question. My original showed an 80uf 350v and a 100uf 350v , I know it's okay to replace with similar values, and increasing on the voltage side is fine. Could I use 100uf 350v to replace both of those, or is the 20uf imcrease on the one capacitor too much?
 
So, I am restuffing the can but have one other question. My original showed an 80uf 350v and a 100uf 350v , I know it's okay to replace with similar values, and increasing on the voltage side is fine. Could I use 100uf 350v to replace both of those, or is the 20uf imcrease on the one capacitor too much?
A 100mfd 350v would be fine for both.
 
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