Education please...

Posted: 4/16/2016 4:07:31 PM EDT

I'm dumber than a bag 'o hammers, so apologies in advance.

Given the way this capacitor is marked, what is its value? Is this 5 milifarads or 5 microfarads? I've seen references that say it can mean either on a 1960's component. It's a wee tiny thing, about .325" in diameter by .600" long.

The back story: I'm tinkering with an old Swan 260. The volume control is non-responsive, sounds as if it's on high all the time. I took the pot apart and cleaned it. It's supposed to be 10K ohms, but measures 39K. I jumpered in a new 10K pot, but there is still no volume control. So I'm looking at this coupling capacitor which connects to the volume control pot.

Thanks in advance for the edjumaction! Jim...

Posted: 4/16/2016 5:10:36 PM EDT

[#1]

5 microfarads is how I would read it.

I don't think a bad coupling cap would cause that symptom.

Posted: 4/16/2016 5:18:12 PM EDT

[#2]

Microfarads.

Millifarads would be 1000 times the capacitance, and even at 50V would be a hell of a lot bigger than that.

Posted: 4/16/2016 5:29:19 PM EDT

[#3]

I agree with micro..

if it was mili it would mmfd.. ie micro-microfarads

Posted: 4/16/2016 5:36:17 PM EDT

[#4]

If you have an oscilloscope you can isolate which section the problem is in. That control is early in the receiver and not at the final audio amp. The manual seems to be pretty well written. It even talks about the capacitor values.

ETA: I would be suspect of one of those carbon resistors going really high in value with age.

Posted: 4/16/2016 6:13:22 PM EDT

[#5]

Quote History

Quoted:
I agree with micro..
if it was mili it would mmfd.. ie micro-microfarads

View Quote

Negative.

Mmfd would be 1/1000 of the capacitance.

Decimal points are important.

Posted: 4/16/2016 8:49:33 PM EDT

[#6]

Really appreciate it gents!

I figured that capacitor was not the likeliest of suspects, but I was ordering some other electrolytic caps for replacements, and thought I would plug in a substitute to check.

No, no oscilloscope here, nor would I know how to employ it. My current electronic understanding is rather Neanderthal-like. I poke at something with a stick and see if it comes to life. But I'm reading some old texts and trying to learn.

Thanks for the tip, I'll check all those resistors next. The few I looked at were good, but I wasn't very thorough.

Again, thanks... Jim

Posted: 4/16/2016 9:11:21 PM EDT

[#7]

Be careful in there, Jim, there are some voltages that can kill you. Like I said, that manual is pretty good. This would be a perfect time to get some very inexpensive older test equipment and learn how to use it and some basic troubleshooting skills if you want.

Posted: 4/17/2016 1:11:56 PM EDT

[#8]

Thanks for the warning, I know there is some pretty high voltage in there.

it's funny, pretty much anything mechanical I can fix it or make it from scratch, but when I try to get my head around these simple electrical circuits, my brain turns to mush. Oh well, I'm still having fun trying!

Cheers... Jim

Posted: 4/17/2016 1:44:17 PM EDT

[#9]

Hi Jim,
It's just a matter of learning, do mind the cautions about high voltage. Just about any resistor in a circuit will measure it's shown value or lower due to the interactions of other components. Resistors in that vintage equipment will go bad as extremely high values in ohms. A cheap DMM will help a lot, do your measurements with the power disconnected and wait for the power supply caps to discharge. To be safe, measure for any high voltage at the caps with the radio unplugged. The high voltage should be bled off after 5 min or so. If it does not one of the "Bleeder resistors" is open or extremely high in value.
73,
Rob

Posted: 4/17/2016 7:23:29 PM EDT

[#10]

Order this book. It contains a wealth of information about how various tube-based stages operate. Well worth the $10.

This is another good one to have on the workbench.

There are only a few of these books left as the person who reprinted them decided to go Galt, closed his business, and sold his remaining stock to this store.

"The Impoverished Radio Experimenter" series is very also good to have around.

Posted: 4/18/2016 12:20:09 AM EDT

[#11]

I hope you gentlemen understand that I DO appreciate your help and experience.

Going to get that first book on order tomorrow if I can get all my chores caught up. Have been slowly reading through "Elements of Radio Servicing," by Marcus.

Am cleaning up my reloading / workbench so I can get the Swan off the floor and am going to check every resistor in the little devil. I had no idea they would most likely fail by going high value. Also have a capacitor checker now, but I think they have to be taken out of circuit to test.

What fun!

Thanks again... Jim

Posted: 4/18/2016 4:34:15 PM EDT

[#12]

I bet your an Extra.

Posted: 4/18/2016 6:19:53 PM EDT

[#13]

11b4v-

I think you're poking at me!

I'm a General who has been halfheartedly banging his head on the Extra question pool with out much success. I can't get above about 70% on practice exams. But, I know it's my fault, I just haven't been industrious enough. I've got too many other things on my plate, and a lovely XYL who needs a lot of my time and attention. I need to follow the good example of the guys who have been shooting for the "30 Day Upgrade Challenge."

Frank_B-

I did order a couple of those books. Thanks for the recommendation!

Cheers... Jim

Posted: 4/18/2016 6:31:18 PM EDT

[#14]

Volume always high ---> possibility is that the grounded lead of the volume pot became disconnected .

Posted: 4/18/2016 7:49:20 PM EDT

[#15]

KnowFear- Thanks for the tip. I'll double check that.

I made time to play with this project a bit today, and spent an enjoyable hour or so checking the values of every resistor I could get to. I used both my ChiCom digi-meter and my Simpson 260. It was fun to note that the lower ranges of the Simpson were very useful in a few places.

I didn't find any resistors that were really high value. There were a half dozen that seemed a little low, and then two that seem to read dead short, including the one pictured below. This one has two capacitors soldered up tight against it. Do these caps prevent me from properly reading the value of this one?

Thanks for the patience with my ignorance. Jim...

Posted: 4/18/2016 8:00:26 PM EDT

[#16]

Checking voltages at the tube pins is a quick way to isolate problems.
Schematics and voltage charts/diagrams can sometimes be found online. The control grid voltages are the ones where an high input impedance meter is required. Most VTVMs are 11 Megohms.
If you can't find the voltage charts, the correct voltages are usually pretty close to those recommended in the tube manual.

BINGO! Check this out: http://www.lost-manuals.com/equipment.php?id=7755 Manual in .pdf format, complete with schematic and voltage/resistance charts.

Posted: 4/18/2016 8:06:57 PM EDT

[#17]

That typically indicates that there's a coil across it. Glancing at the schematic it looks like that photo is part of the bandswitch circuitry and there is probably a coil in parallel with them depending on the bandswitch position.

ETA: RE Frank's comment - There are voltage charts in the manual.

[ARCHIVED THREAD] - Education please...

Outdoors » Ham Radios

Warning

Confirm Action

About AR15.COM

Stay Connected

Newsletter

Contact Us