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 Post subject: First Restoration
PostPosted: Jun Thu 17, 2021 3:29 pm 
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Joined: Feb Tue 16, 2021 12:52 pm
Posts: 32
Hi Folks,
I bought a little Motorola A2G and a GE Model 200 at an estate sale last weekend. I put the GE 200 in storage and began the restoration of the Motorola. This the first time I've tried a radio restoration and I was hoping for the encouragement an easy one would bring. The circuit in the Motorola was what looked to be a classic America 5 and I figured just simple enough to get me started.
I lifted one side of all the resistors off the board and found that pretty near all of them were pretty wide of spec. so I began a list with DigiKey. It went well and I moved on to the Capacitors.
There are only 4 fixed capacitors in the model. When I tried to lift one lead off the board to measure them two literally broke in half no matter how gently I handled them, so I began "trying" to add them to the DigiKey list...and that brings me to the root of the reason that I am writing.
Here is the way they are listed in the Photofact Folder:

Fixed Capacitors
Capacity values given in the rating column are in mfd, for Paper Capacitors and in mmfd foe Mica and Ceramic Capacitors.
1. C2/ .05 400v
2. C3/ 8.2 N750 +- 5mmf
3. C4/ 100 N750
4. C5/ .02 400v ...C5 is a Can (which isn't there) and the schematic says that it has two values of 30mf and 50mf. There is a big Blu-Seal dry electrolyte capacitor that the wrapper has been torn off so I can't read it's value but I assume it belongs in the missing can.
The tubes are:
12BE6
12BA6
12AV6
50C5
They all look good. Can you test them with a Multimeter or an Oscilloscope or must you have a tube tester.

I guess what I most need is a decent line on the caps. Digikey and Mouser are just not all that helpful.
Thanks for your help.
Dave Lowa


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 Post subject: Re: First Restoration
PostPosted: Jun Thu 17, 2021 3:36 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30055
Location: Annapolis, MD
Capacitors and other parts:
---Just Radios or Sal's---see the ads at the right.

Test tubes with multimeter or oscilloscope? No
The best tube tester is a radio. If the radio does not play with all the tubes in, then there are specific diagnostic procedures to see if a tube is part of the problem

"They all look good."----So what?---that maybe just tells us the radio was not used a lot.

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-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: First Restoration
PostPosted: Jun Thu 17, 2021 5:11 pm 
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Joined: May Thu 14, 2015 4:15 pm
Posts: 3599
Location: Dallas, TX
AirScooter wrote:


Fixed Capacitors
Capacity values given in the rating column are in mfd, for Paper Capacitors and in mmfd foe Mica and Ceramic Capacitors.
1. C2/ .05 400v
2. C3/ 8.2 N750 +- 5mmf
3. C4/ 100 N750
4. C5/ .02 400v ...C5 is a Can (which isn't there) and the schematic says that it has two values of 30mf and 50mf. There is a big Blu-Seal dry electrolyte capacitor that the wrapper has been torn off so I can't read it's value but I assume it belongs in the missing can.

I guess what I most need is a decent line on the caps. Digikey and Mouser are just not all that helpful.
Thanks for your help.
Dave Lowa

Mica and ceramic capacitors seldom fail. I'm not saying they can't, but odds are that they are fine.
Do you realize that N750 is a temperature coefficient? That is another property that you would have to match if you replaced them. C5/ .02 400V wouldn't be in a can, probably a tubular (paper).
The dual value 30mf and 50mf would be a dual section can electrolytic. Maybe you made some typos there.
NOT TO MENTION THE STICKY THREAD AT THE TOP OF THIS SUB-FORUM.

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Tim
It's not the Destination, It's the Journey.


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 Post subject: Re: First Restoration
PostPosted: Jun Thu 17, 2021 11:38 pm 
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Joined: Apr Wed 10, 2019 3:48 am
Posts: 609
Surprisingly, tubes are generally good. The many radios that I have worked on have had bad capacitors, out of value resistors, and dirty pots, but very few bad tubes.

Make sure the tubes are in the proper sockets. Pull them, clean the pins, make sure they have not ‘gone to air’ white color at tube top, get the datasheet for each tube and identify the heater pins, and use your ohms meter to make sure the heater is not open. Clean the tube but do not wipe off the markings and replace and test in the radio as Pix suggested.

If the radio is not functioning as expected, you can compare the tube pin voltages to the schematic as a troubleshooting reference.


Last edited by Codepug on Jun Thu 17, 2021 11:42 pm, edited 1 time in total.

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 Post subject: Re: First Restoration
PostPosted: Jun Thu 17, 2021 11:41 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30055
Location: Annapolis, MD
AirScooter wrote:
H

I guess what I most need is a decent line on the caps. Digikey and Mouser are just not all that helpful.
Thanks for your help.
Dave Lowa


Idon't use Digikey, but I can recommend Mouser without reservation...they have everything you might need, and they have a really good search engine

_________________
-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: First Restoration
PostPosted: Jun Thu 17, 2021 11:55 pm 
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Joined: Jul Mon 26, 2010 8:30 pm
Posts: 30055
Location: Annapolis, MD
Quote:
acity values given in the rating column are in mfd, for Paper Capacitors and in mmfd foe Mica and Ceramic Capacitors.


Watch out for units and exponents....popular methods include the old-time mmF, which is now pF, meaning picofarad. You'll also see a pseudo-scientific notation in which 1 mF becomes 1E04 nF, where n is nano. The common shorthand is simply 104, which translates to 1.0 E4, which reads as 1.0 * 10^4. Spend some time with the different formats, and you'll appreciate things like:
"1 megaphone = 10^12 microphones"

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-Mark
"Voltage is fun to watch, but it's the CURRENT that does the work."


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 Post subject: Re: First Restoration
PostPosted: Jun Fri 18, 2021 4:42 am 
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Joined: Dec Sat 28, 2019 4:18 pm
Posts: 1116
Location: Corinth, TX
AS, my suggestion (it's worth what you paid for it):
C2: replace with .047 mfd/630 volt unit. .047 is the new "standard" value.
C3 & C4: They are temperature compensation caps. If they are not broken, they are probably OK. If you need to replace them, try to get N750 parts. If you cannot, replace them with silver micas. Heresy, I know, but except for a little warmup drift, once the radio reaches operating temperature, it will work fine.
C5: replace with .022 mfd/630 volt unit.

The new 630 volt film caps are physically smaller than the old lower voltage paper caps and even if you could find 400 volt caps in small quantities, it's really not worth the trouble.

Replace the two electrolytics with 33 mfd (today's "standard" value) and 47 mfd (same) 200 volt parts.

I am a little surprised that there isn't a capacitor bypassing the cathode resistor in the 50C5 stage. If there is, a 25 or 50 volt cap will work there.

To expand a little on pix's comment:
Because a farad is a REALLY large amount of capacitance, back when I was young (when snakes walked the Earth), the standard units of capacitance were the microfarad (mfd. or mf.), 1X10^-6 farad or one millionth of a farad, and the micro-micro-farad (mmf), or 1x10^-12 farad or one millionth of a millionth of a farad, today called the picofarad (pf). There is now an intermediate unit called the nanofarad, or 1X10^-9 farad. A .022 mf cap (multiply by 1000 to convert from md to nf) is also a 22 nanofarad (nf) cap.

John


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