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 Post subject: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 2:02 am 
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I'm re-capping an Eico Signal generator and it has one hard to find .5 Mfd capacitor. it appears to be electrrolytic because the positive side is marked but it looks like a regular wax paper cap... I'm trying to find the closest I can get to it because I can't find .5 Mfd anywhere. I'm not too good at math.. is .68 the best choice?

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 2:36 am 
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The modern replacement for an 0.5 would be 0.47 or 0.56. The stripe on the end of the old paper caps indicates the outside foil. They are not polarized. Some circuits wanted the outside foil grounded to reduce hum.

RRM


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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 2:38 am 
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You can use a .47 MFD. Telling us the model number of the signal generator would also be helpful.

Pete

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 2:53 am 
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Even if it was an electrolytic cap, a modern polyester or poly .47uF cap would be a far better
replacement. They will last much longer. Less ESR, less leakage, and they don't age.

Peter


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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 3:36 am 
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oookay, I was always told if you have to use a different capacitance, always go higher not lower. is it different for test equipment? I have an Eico model 320.

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 3:53 am 
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Daniel Schwank wrote:
oookay, I was always told if you have to use a different capacitance, always go higher not lower. is it different for test equipment? I have an Eico model 320.


It is the application of the device that matters. Depending on the tolerance of the circuit for a change in engineered value.

I looked at the 320 schema and that capacitor is part of a Colpitts oscillator for the audio modulation, C5. A .47 is a good choice and well within a 20% tolerance for unscreened paper caps. Be sure to replace C4 also with a .22 and C3 with a .047. Use the inexpensive metalized 630 volt, can be dipped, or wrapped, as stated they are very low leakage which is important to a Colpitts. That should pep it up. Be sure the tube is in good condition with no heater/cathode leakage...

GL

Chas

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 3:14 pm 
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The second part of your question - This capacitor is part of a frequency-detremining circuit, so you would go with the best quality you can find and use two smaller caps in parallel if you were really a stickler for decimal-point accuracy of the dial.
Since everything is so old anyway it would be sheer luck to hit the dial calibration exactly.
Your earlier information was right for coupling and bypass caps. One size larger would insure enough bypassing of lower-frequency signals and usually not add too much bass boost to coupling caps.
Pat

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 4:01 pm 
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If this cap is in the audio oscillator section, I think the exact frequency of the audio modulation of this RF generator would be fairly non-critical in this case.

The "always go higher" rule mainly applies to electrolytic filter caps in power supplies. A bit of circuit analysis determines if it's OK to use this rule for other types of caps.

Dave


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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Sat 10, 2012 6:16 pm 
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I have the same Sig Gen and recently re- capped it. I used a 0.47uF 400 Volt from Mouser "Polyester Film Capacitors ยป 150474J400LF". It works and the capacitor value is not at all critical as it is only for the audio oscillator

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Wed 14, 2012 1:14 pm 
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Ok thank you all for the info!

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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Wed 14, 2012 8:27 pm 
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As noted earlier, the dial calibration on such an old generator may not be perfect, regardless how well you restore and calibrate it.

I have used an EICO 324 generator for years, and it's stable enough for everyday use. I always double-check its dial setting, however.

This morning, I happened to be aligning a Stewart Warner radio whose IF frequency is 465 KHz. In this photo, my EICO 324 is sitting on top of a BK Precision 1801 frequency counter which shows me that the output is set to the correct frequency.

Image

When the measured output was 465, I looked at the EICO's dial setting out of curiosity, and it was around 461. Not a huge difference, but given the choice, I'd rather be precise than not.

You can also check the output with a modern multi-band radio that has digital tuning. If it doesn't receive at the desired frequency, try tuning in a fraction or multiple of that number, to receive the harmonic. For example, twice 465 is 930, so if I set my digital radio to 930 KHz and put the generator lead next to it, I can hear the generator's tone when it hits 465.

Phil Nelson
Phil's Old Radios
http://antiqueradio.org/index.html


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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Mon 19, 2012 4:52 am 
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The "always go higher" doctrine applies most importantly to the voltage rating. Unless we're dealing with ultra high-precision circuits (and sometimes even then), ten percent either way will be just fine.

Shawn


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 Post subject: Re: Closest value to this cap for an Eico signal generator?
PostPosted: Mar Wed 21, 2012 8:16 pm 
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The 0.5-uF capacitor you found in the generator was an old value which was once commonly available. About 40 years ago, component manufacturers standardized on what are known as the "E3, E6, and E12 series" of numbers. They are geometric progressions which were intended to cut down on the variety of parts having only slightly different values which had to be manufactured and stocked.

In the E3 series, there are only three values per decade, going 10, 22, 47 then 100, 220, 470, 1,000, and so forth.

In the E6 series, there are six values per decade: 10, 15, 22, 33, 47, 68 then 100, 150, 220, 330, and on and on.

The E12 series has 12 values per decade: 10, 12, 15, 18, 22, 27, 33, 39, 47, 56, 68, 82, then it rolls over to 100, 120, etc.

Most of the common lines of capacitors have E3 or E6 values. E12 values are used for resistors and for certain precision capacitors. You'll note that 50 is not a standard value in any series.

When replacing non-standard values, the rule of thumb is to consider the tolerance of the original part. This is 20% for most paper capacitors. Then use the closest standard value that is otherwise acceptable (i.e. has the correct voltage rating, is of a compatible type of construction, physically fits, etc.) In this case, 0.47 is 6% off from 0.5, but it is a lot closer than 20%, so it should be fine. Had the original been a mica cap with a 5% tolerance, you could not make that substitution and still expect the circuit to work as it should. If necessary, two smaller capacitors would need to be connected in parallel to arrive at a closer value.

Electrolytics are a special case because they nearly always have higher than marked values when new. This compensates for the loss in value that they exhibit as they age. So using a slightly higher value replacement electrolytic cap only restores the conditions which could have existed when the equipment was built. This is why it is the recommended procedure. On the other hand, putting a slightly smaller electrolytic cap in may work for a while, but could prove to be inadequate over the long term.

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