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 Post subject: Electrolytic source for transistor radios
PostPosted: Jun Thu 07, 2018 7:04 pm 
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Location: Minnetonka, MN 55305
I, unwisely perhaps, offered to repair a friend's 1958 General Electric - P-745-A radio. The electrolytics are 6 volt, 3, 6 and 35 mfd. I can't seem to find similar values on Mouser or Digi-Key but will admit I have never tried either source and find their website to be confusing.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Thu 07, 2018 7:31 pm 
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Location: Stone Mountain, GA
Look at 3.3, 6.8 and 39uf and through hole.

You will probably find 16 and 25 volt caps near the same size.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Thu 07, 2018 9:42 pm 
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Transistor radios are very forgiving regarding capacitance values. I've used 10 uF in place of 40 uF and vice versa. Except for the low value needed to the VC wiper, most values will work. I don't think working voltage ratings below 15 or 16 even exist anymore.

Try All Electronics for the caps if you don't have any available. Their site is easy to navigate, and the radial caps start at a dime.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Thu 07, 2018 10:07 pm 
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For the GE P805A through P808A series, I've found doubling the value if audio coupling from 5mfd to 10 improves low freq response. After a wholesale recap had one that was noticeably lacking low end. I checked the removed caps and found both fives were nearer nine, added a second 4.7 across ones I installed and Bingo back to it's old self. Now I just use 10 mfd. For a couple that's playing fine with orig, I ain't touching them.

I'll guess a P-745 has the white IEI/Nashville caps, 99% those are open. My P-745B would not make a sound on old caps.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Thu 07, 2018 10:55 pm 
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I use Newark.com. Their site is easy to navigate, with no minimum order and they have several choices for both radial and axial. Very reasonable prices.


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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Fri 08, 2018 12:11 am 
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I went on Ebay and recently bought a "variety" package of small lytics in a plastic box with little compartments and a list of the caps inside the lid to show you what is what. They seem pretty small and would fit fine in transistor sets. Being that they are Chinese caps, like pretty much most you find nowadays, I think I would test them but the random samples checked on the LC 102 seemed good. Otherwise in the past, with RS gone, I had just gone thru the process of ordering what I thought I needed each time I had a project, which is inefficient and costly for shipping. For critical stuff I might even continue on that route. As my stash of low voltage caps was pretty thin. I have hoarded plenty of caps for tube radio resto, but not much for solid state. Like this kit, but from a US seller- https://www.ebay.com/itm/24value-550pcs ... SwjVVViptH

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Fri 08, 2018 1:32 am 
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As Richard says... all electronics is an easy site to navigate... :)
https://www.allelectronics.com/category ... ial/1.html

Modern day electrolytic capacitors have become so much smaller since the 1960s - 1970s
Some of the old μF (microfard) values are no longer common... so choose the next nearest capacitance value, up or down.
... I buy the 35V and 50V ones today to replace the old 3V, 6V + 10V ones and 85°C is good enough for transistor radios.
It's foolish to hunt down 3V 6V or 7V caps :shock:


Aluminum-capacitor Parametric search at Digi-Key with radial + 50V selected→ here. Dimensions listed.
Aluminum-capacitor Parametric search at Digi-Key with axial + 50V selected→ here. Dimensions listed.

If you look at the lead spacing, diameter and height data - (for the 50Volt capacitors referenced above) - you will get an idea how small today's electrolytics have become.
Image
Also, if you have trouble locating axial capacitors, you can bend the legs of a radial cap to suit, just be sure to use some spaghetti or heat-shrink tubing on the bent over leg.


"Good sizes to keep on hand are:
2.2 μF
4.7 μF *
10 μF *
22 μF
33 μF *
47 μF *
100 μF *

Those with an asterisk are the ones I use most often. YMMV.
Anything rated 16V or higher is fine. I tend to use higher values because
the lead spacing matches better and there is virtually no difference in price".

Mitch.

Greg.

Edit: If you need the 1958 four-page service manual, shoot me an e-mail address via PM private message.


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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Fri 08, 2018 6:31 pm 
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Many thanks to all of you who offered your insights. I ordered from All Electronics because, as stated, their website is easy to navigate. They have a large catalog so I ended up ordering several additional items. The caps are indeed the "Nashville" variety. As a result the radio is completely silent at this point. I am a 30's era tube guy so only occasionally take on a transistor. I view electrolytics as part of the AC to DC smoothing technology so am a bit surprised to see them in a DC circuit. The high capacitance in a small package must be the reason?

I'll report back when the caps are installed. Thanks again everyone.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Fri 08, 2018 7:24 pm 
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Quote:
The high capacitance in a small package must be the reason?

Correct---and also because higher capacitance is required for equivalent functions. (S/S circuits are lower impedance)

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Sun 10, 2018 12:54 am 
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Location: Palos Verdes, CA
There was a Radio Shack located about a mile from my home and I would go there to look at their selection of axial electrolytic capacitors, resistors, battery holders, etc. When it was announced that all of the Radio Shacks in my area were going to close, I stocked up on as much stuff as I could find. It was sad to go to the stores at the end when they were selling the furniture and fixtures. I recently went to Fry's Electronics in El Segundo and was disappointed to find that they had little of the electronic components that I previously purchased at Radio Shack and their offerings of NTE items was next to nothing. I needed to replace a diode in a Sony TR-72 transistor radio and decided to look if I had a suitable replacement in my collection of parts radios and did find the correct NTE-109 equivalent diode.


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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Sun 10, 2018 6:20 pm 
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3Transistors wrote:
I recently went to Fry's Electronics in El Segundo and was disappointed to find that they had little of the electronic components that I previously purchased at Radio Shack and their offerings of NTE items was next to nothing. I needed to replace a diode in a Sony TR-72 transistor radio and decided to look if I had a suitable replacement in my collection of parts radios and did find the correct NTE-109 equivalent diode.

Well that's surprising; coincidentally I was at Fry's in Burbank last night to take advantage of a Canon printer they had on sale for 29 bucks (about the cost of two cartridges, which were included), and in wheeling the cart down one of the component isles, was amazed at how extensive the selection was.
Of course if I was needing something in particular, I might be singing a different tune, lol. At least though, you got what you needed. Short of the internet now, where else locally ya gonna find it?

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Mon 11, 2018 5:33 am 
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Fry's is weird---there never seemed to be a rational or consistent pattern to what was or was not stocked. But i still miss it.......

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Mon 11, 2018 5:03 pm 
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fifties - I tagged along with a friend who wanted to buy a HDTV antenna and converter box for his Sony television, and I went by Fry's selection of NTE components, which looked like it hadn't been stocked in months.


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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Mon 18, 2018 5:59 am 
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My caps arrived and are now installed. Unfortunately the GE 745 is still dead. Those who are familiar with this set know that three AA batteries slide into a tube with no markings on either end. Naturally I managed to reverse polarity. Have I damaged anything?

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Mon 18, 2018 6:26 pm 
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I doubt that you harmed anything with 4-1/2 volts.

The second biggest cause of a dead Transistor set is the contacts in the earphone jack not touching each other. Use a screwdriver and push the inner blade against the outer one just below it, to see if you can get audio.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Mon 18, 2018 11:40 pm 
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fifties wrote:
I doubt that you harmed anything with 4-1/2 volts.

It isn't voltage that causes issue, it's current. A good set of AA batteries can easily produce a ½ amp, exceeding limit of early transistors by 5-10x their rating.

If there isn't a small pop/thump in speaker when rocking the on/off switch back and forth, explore speaker, wiring, jack etc.. If that seems ok check to see if osc is operating.. With another radio tuned between 1200-1600, your radio should produce a signal tuned around 750 to 1150Kc. The osc was dead on my P-746A, after recapping which got audio working, I wound up replacing osc transistor.

GE gives a warning regarding polarity reversal.

Yes I had to look to remember exactly which my model is.

Image

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Tue 19, 2018 9:58 pm 
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You can juggle your capacitor values by either parallel or series connection. The rule is if you add your capacitors in series, follow the same maths for resistors in parallel. So 10 microfarads in series with another 10 should give 5. Yes, mixing up polarity on an old radio can happen. Worst mistake I ever made was applying 1.5 volts to the 2 volt filaments of a 1938 battery tube set I rebuilt. The filaments just stopped drawing current and died. Later I found under voltage was as bad as too much.


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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Tue 19, 2018 11:27 pm 
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I installed batteries when I had the battery holder out of the case so I didn't have the mounted reference to guide me. Further more the + end of the case has the spring that in my experience always denotes the negative. What's worse is that I installed them correctly initially and did experience a faint click in the speaker when toggling the on-off switch. After the recap is when I reversed polarity. No click now I'm afraid. Started checking voltages at the output transistor. The emitter is OK at 4.5 but the base is supposed to have 4.4 and it is a fraction of 1 volt.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Wed 20, 2018 12:24 am 
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Trace what components are feeding that base. The failure could be upstream from the Transistor.

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 Post subject: Re: Electrolytic source for transistor radios
PostPosted: Jun Wed 20, 2018 3:11 am 
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If any of the caps in audio output are reversed that will pull down voltage.

Easiest test is check the B-C forward & reverse resistance of output transistor(use meter diode scale). If you measure near zero ohms in both directions, likely the xistor is shorted. To be sure, either B or C should be disconnected. I gently use wick for desolder to let appropriate lead "float".

BTW for std AA batt the spring is negative... For whatever infinite wisdom, mercury batteries had connections reversed..

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