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 Another Cisco Problem :(

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postie



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Join date : 2010-10-19

PostSubject: Another Cisco Problem :(   Thu Oct 21, 2010 2:47 pm

Hello Everyone,
Now that you guys have helped me identify the Cisco...actually a Detrola 571...I decided to start working on it a little. Please remember I am really new to this stuff so thought this might be a good practice radio.

Plugged in it does nothing. I started out with a variac, then just plain plugged it in. Nothing. So, first thing I replaced the power cord, which was totally dry rotted. Next thing, I replaced the lamp. Powered up the radio and the lamp lit briefly and blew out quickly. I have not tested any tubes yet.

Thought someone here might know right off something that would commonly cause the dial lamp to blow immediately. I'll proceed on...hopefully with some help here Smile If it's a short, I already know I'm sunk...hoping for some other answer...lol
Thanks ahead!
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jack shirley



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Location : SE USA

PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 2:56 pm

The lamp acts somewhat like a fuse for the B+ voltage in this circuit. First order of business would be to replace the filter capacitors, c10 in this schematic.
http://www.nostalgiaair.org/PagesByModel/075/M0004075.pdf

Here's a good page you should read up on. Note particularly the section about replacing capacitors. They get old and dried out and produce a partial or full short on the B+ voltage. Hence the 'fuse' blowing. The C10 filters are first out of the gate and they are electrolytic types which DO commonly dry out or leak internally.

http://www.antiqueradio.org/begin.htm
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postie



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 3:01 pm

Thank you very much and that does make sense. I have just finished changing out the capacitors EXCEPT the electrolytic. I will have to wait until tomorrow and go buy it/them. Just glad someone didn't jump right in with a "short" as I can't trace shorts for now. We'll see what happens with the Electrolytics changed. Thanks Smile
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ggregg



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 4:22 pm

On an AA5 (and most other sets too), I always replace the electrolytic first. They are usually the first capacitors to go bad. If they open, the radio will hum really bad but if they short, you will have a condition like yours.

FYI, AA5 means all American five. It is a transformerless chassis that was made by many manufacturers. It was very popular because of it's simple design and common tube line up. They were made in versions with octal tubes and miniture tubes. Most smaller table radios are this design. They came out in the thirties and lasted until into the sixties. There was also an AA6.

If you have an VOM, you can trace for shorts.
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postie



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 5:15 pm

Well...here's my REAL stupid question of the day: The electrolytics are in an upright canister on the top of the chassis. I've brought just a few simple little radios back to life, but they all had the electrolytics underneath along with the other old wax capacitors. Now my dilemma is how to get to these upright electrolytics. Tear the cardboard canister off? The base is riveted to the chassis. I know this is a simple thing...I just haven't done it yet. Thanks again Smile
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ggregg



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 5:34 pm

The simplest way is to get single capacitors. Your can will have two or three capacitors in it. It will either have wires going into the center or most likely terminals in the middle. The terminals are usually marked by a symbol that matches a symbol on the side. They are usually triangles, half circles, and squares. Sometimes the wires are color coded with the code on the can.

Say the can says 50-40-40 or 50mfd@160, 40mfd@160, 40mfd@160. This means that it has three capacitors in it with those values. The first number is the value, the second is the voltage so the first capacitor would be 50 microfarads at 160 volts, etc. You can go a little higher in value if you have to but don't go lower than the original. For example a 50mfd could replace a 40mfd if you had to but I always try to go exactly the same if I can. Voltage always needs to be the same or higher. 160 volts is common in AA5's but sets with a transformer (your doesn't) will be much higher. By looking at the schematic, it looks like yours is 20mfd @160 and 40mfd@160. Should not be too hard to find. I've been looking at too many TV's and jukeboxes lately which have 3 or 4 stage cans and forgot that most AA5's have a two stage.

You can order single capacitors and usually make them fit or you can find a multistage capacitor that you can mount underneath. This is what I usually do as it is easier. You may even be able to find a can that will replace the old one. I leave the old capacitor in place for looks if I go underneath. Take the wires from the corresponding terminal of the old capacitor can and solder them to the appropriate new capacitor on the + side. Make sure this connection is isulated or well taped or you may hear a big bang when you fire her up. You will probably hear the same thing if you hook one up backwards, ask me how I know. Electolytics have the polarity marked, usually the - or ground. The grounds from all the capacitors or the ground wire from the multistage capacitor would be grounded to wherever the original was grounded. Usually, but not always, to the chassis itself.

In my opinion, the only stupid question is the one you DIDN'T ask. Everyone wants to help and all of us were at the stage you are now at one time. Good luck.
Greg
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postie



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:07 pm

This particular canister is unmarked, it's just totally black (cardboard). It sits on top of the chassis right along with the tubes and there are several wires running underneath, but I would have no way to know what wire is what without knowing what is in that cardboard canister. Wonder if I could carefully cut it off (the cardboard canister), then seamlessly glue it back into place with new electrolytics inside? I'm real good with glue...lol Also, I don't see any way to remove anything from underneath the chassis. Thanks again Smile
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ggregg



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:13 pm

If it's riveted it can be a little difficult to get off. What you are talking about it called restuffing and many people do it, especially on sets that they want to look totally original. You should be able to tell by the schematic which cap goes where. I would get a 20mfd 160 and a 40mfd 160 or a two stage with those values. Even if you hook it up wrong, as long as the polarity is correct, it should still work. Just not as well as it would if you hooked it up correctly and it may have a little hum. You could probably even get by with two 40mfd@160volt capacitors. Then it wouldn't matter.

I can understand your hesitation to move parts, etc. but this is the next step in your repair training. You can do it. What you proposed may seem easier to you but I can assure you that it will be more work than you think. You can buy two stage capacitors in the values you need that have insulated wires so you should be able to hook that up easily.


Last edited by ggregg on Thu Oct 21, 2010 6:21 pm; edited 1 time in total
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jack shirley



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:15 pm

ggregg wrote:
I leave the old capacitor in place for looks if I go underneath. Take the wires from the corresponding terminal of the old capacitor can and solder them to the appropriate new capacitor on the + side. Make sure this connection is isulated or well taped or you may hear a big bang when you fire her up. You will probably hear the same thing if you hook one up backwards, ask me how I know. Electolytics have the polarity marked, usually the - or ground. The grounds from all the capacitors or the ground wire from the multistage capacitor would be grounded to wherever the original was grounded. Usually, but not always, to the chassis itself.

The easy way if you're not into restuffing the original can is to add a new terminal strip nearby to mount the new caps. Well, I understand that a first-timer might not have a bag of terminal strips laying around the bench for convenience but you get the idea.

The important thing is to have the old cap positive connections disconnected from the circuit as you re-arrange things. You can often put your new caps somewhere else by following a lead from the old can. The negative connections are often isolated from the chassis, and seem to be in this case according to the schematic, so you have to respect the terminology that negative doesn't always mean chassis or ground although we tend to refer to them loosely in discussion. You can keep the same negative tie-point in this particular radio...and make sure its not grounded to chassis via a solder blob Smile You may find the most convenient spot for a new filter cap is halfway across the chassis if the + and - connections from the old cap are available there. Feel free to use some physical placement ingenuity Smile but get the pos/neg connections right!
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postie



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:16 pm

And how do you remove the can entirely? It's riveted on. I want to see what's inside.
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jack shirley



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:24 pm

Trust me/us/we. Leave the old can intact unless you're restuffing. Old radio folk hate the sight of a hole on the chassis where something "used to be" Smile

Inside the can is a tightly wound glob of layers of foil and wax paper. Sure, its worth a visit for experience sake but find a loose one to hack open rather than hacking up an original set.

Sometimes the diamond and square indications are not printed on the outside cardboard cover but if you slip it off you might see them printed on the can. They put the cover over the cap in radios like this because the cap can IS NOT grounded to the chassis and could present a shock hazard.
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ggregg



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:26 pm

Cut up a different one. You will probably never find one to fit exactly and then you will have an ugly hole in your chassis. Also Jack's points are very valid and we seem to be answering at exactly the same time. Don't try to overthink it. Terminal strips are a great idea and I usually use them. Understand the schematic and hook up the new parts exactly the same.
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postie



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 6:47 pm

Ok...so I won't cut off the cardboard cylinder...and I'll go by the schematic for wiring the replacements and also relocate them in a convenient place. Not sure I understand the diamond/square thing yet...but I'll learn about it quickly! Smile Thanks so much for all the help...the cabinet is coming along nicely for those of you who saw the awful picture...lol Now, if I can just stretch the grill. Laughing
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ggregg



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PostSubject: Re: Another Cisco Problem :(   Thu Oct 21, 2010 7:37 pm

The diamond, square thing was a way to tell which post was which on the bottom of a capacitor can. If you look at the terminal, look at the way the bottom of the can is cut out where the terminal comes out. The cut out will be diamond shaped, square shaped, etc. Not all cans had this marking scheme but many of them do. Yours may not, it may go by a color code.
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