Help troubleshooting old AM Radio

Discussion in 'The Projects Forum' started by spinnaker, Nov 11, 2009.

  1. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I dragged out my old AM radio that I built (many years ago) in High School and I am trying to get in working again.

    My problem is that the only test equipment I currently have is a digital multimeter. If I had a scope or even and RF generator, I think I might stand a fighting chance after all these years of not touching electronics but I am at a loss.
    Hopefully someone can help me.

    I am including a schematic with actual voltages I measured.

    Here is some background.

    TR3 was just replaced. I had to replace it with a 2n410 of different packaging. This transistor has a red mark. I assumed that was the emitter. Is this assumption correct?

    TR5 and TR6 were just replaced.

    I am getting hum out of the speakers. Hum changes and speaker crackles when I measure voltages on TR4.

    If you look at the voltages, they seem to be off a bit but the most significant one is the emitter of TR4, Is it possible that TR4 is bad too?


    Any tips on what else I should check?
     
    Last edited: Nov 11, 2009
  2. KL7AJ

    AAC Fanatic!

    Nov 4, 2008
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    Is it totally dead or just lame? A local A.M. broadcast station makes a fine signal generator~

    Eric
     
  3. spinnaker

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    Sorry I don't understand? "Totally dead or lame" ???

    A radio station doesn't do me much good if I can't measure the RF through the radio. Or can I any tricks other than to use a scope?


    Some other info

    When I adjust the volume, I can hear scratching (dirty pot).

    Also I probably should have posted this is the RF forum. If a mod wants to move it feel free. Sorry.
     
  4. beenthere

    Retired Moderator

    Apr 20, 2004
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    If you have a small screwdriver, hold it by the shank and touch it to the positive end of C16. A buzz indicates lame, vice dead.

    Does the collector voltage on TR1 stay a volt low even with an antenna connected and while you tune up and down? That would seem to be an indication that the RF amp is not working. That is, the transistor is biased into conduction, but has no AM signal to work with.

    The RF and IF cans should have visible tuning slugs present. Look and see that a dark object with a hex indentation for an allen wrench (actually a nylon diddle stick) is visible in the core of each shielded can.
     
  5. spinnaker

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    Thanks Beenthere. This is what I am looking for.

    Yes I do know about the RF and IF cans but I but not have the right type of tuning wand on hand. They should be adjusted properly any way since they were tuned years ago.


    If I touch the + side of C16 with a screwdriver while touching the shank, I do get a buzzing. It is not loud at all just barely there but it is noticeable.

    The voltage at the collector of TR1 remains at -9.33 (for some reason it is measuring -9.33 now) regardless of where I tune.

    But there is a SIGNIFICANT amount if buzzing from the speaker when I measure the voltage.

    Please don't tell me TR1 is bad because it is the only one I do not have. :)
     
  6. Wendy

    Moderator

    Mar 24, 2008
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    You can also make a 1Khz square wave generator with a 555 or whatever else you have laying around. Square waves are loaded with harmonics, and you can use this to inject signals around to see if they flow through. Don't use a direct connection (we are talking radio), just touch the square wave to various points inside the circuit.
     
  7. spinnaker

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    Hey I am just getting started here again after a 35 year hiatus. I don't have anything "laying" around. :)
     
  8. Wendy

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    Mar 24, 2008
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    I still have one I built in college kicking in the bottom of an old tool box. Don't you have any pack rat genes at all? :D

    Seriously, if you're going to work on radios these are must have gadgets, and are simple to build.
     
  9. k7elp60

    Senior Member

    Nov 4, 2008
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    My old data book shows the 2N410 leads looking at the bottom where the leads come out of the case in a triangle pattern. The peak is the base, the lead to the left is the emitter, and the other lead is the collector.
    TR1 voltages on base and emitter may be okey, as it is a mixer oscillator.
    I think that TR3 base voltage is a little low. If I read your post correctly the base voltage is 0.122 volts, this may be a little low. TR4 the other IF amp is 0.2.
    You may want to put another AM radio near by and tune the other radio to about 600KC. It's local oscillator will probably be 600+455 or 1055KC. It will act as a signal generator with no modulation. Tuning the defective receiver you may hear a hetrodyne.
     
  10. CDRIVE

    Senior Member

    Jul 1, 2008
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    Wow, are those old leaky Germaniums? I do a lot of yearning for the good old days but I miss them like I miss the Typewriter!:D
    Anyway, those old carbon resistors can change value quite a bit and old paper caps will leak. Ceramic caps should not deteriorate with age though.
     
  11. CDRIVE

    Senior Member

    Jul 1, 2008
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    I took another look and I would not trust any electrolytics that old. You would be wise to check or simply replace them.
     
  12. SgtWookie

    Expert

    Jul 17, 2007
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    They most certainly do, even if they aren't in use!

    I had a variety of mil-spec carbon resistors that were made in the late 1960s and early 1970s that were still sealed in mil-spec packaging for protection against moisture.

    I opened up some of the packages a few years ago, and measured a number of them. What were supposed to be 1% tolerance resistors were anywhere from 10% high to well over twice their marked value.

    And yes, caps will become "leaky", too - particularly aluminum electrolytic caps. Sometimes you can re-form the dielectric of electrolytic caps by slowly charging them to their rated voltage using a current limited source, but if the electrolyte has leaked out or dried up, there is no hope of restoring them.

    It's best to buy replacement caps from high-volume authorized distributors, such as Digikey.com, Mouser.com, Newark.com, etc - as they rotate their stock on a regular basis, and you will be much less likely to wind up with caps that have been sitting on the shelf for a long time.
     
  13. spinnaker

    Thread Starter AAC Fanatic!

    Oct 29, 2009
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    I got it working!

    Well sort of. There appears to be am intermittent connection some place. The whole works is built on a metal chassis. If I press on the chassis near the power transformer, I can get it to work.

    Also in this area is the audio section.

    I guess perhaps it is a bad solder joint?

    Oh and it turns out I had the radio near a really good RF source. My LCD monitor. The Tivo remote makes a pretty good on too, though since it is IFR, I suspect it ti the IFR to RF remote device I have setup that is generating the RF.
     
  14. spinnaker

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    Turns out no matter where I press on the chassis, I can get it to work.

    Just a little more info on the construction.

    The thing is built on a metal chassis as I wrote above. The components are mounted to terminal lugs.


    The ground lugs are screwed down to the chassis.
    Is it possible that one or more of those ground lugs are corroded between the lug and the chassis?

    I barely need to touch the chassis to get it to work.
     
  15. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Are you perhaps becoming an antenna when touching the chassis?

    Try touching the same spot with a non-conductive item, like a pen, and see if that also makes it work or not.
     
  16. Duane P Wetick

    Active Member

    Apr 23, 2009
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    I used to repair old car radios and here's my method: Start with some type of signal generator (something simple) and inject the signal at the 1st rf stage and work thru the stages until the final amp. Where you lose the signal, thats where a component has failed. I made a lot of money doing this simple detective work.

    Cheers, DPW
     
  17. spinnaker

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    Don't think so, I have to press on the chassis just a bit to get it to work. I can do the same by moving it around on my desk.

    There is a loose connection in there somewhere.
     
  18. SgtWookie

    Expert

    Jul 17, 2007
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    Try moistening your finger slightly before touching the chassis lightly.

    If your fingers are dry, you have to press down some to lower the resistance of the connection.
     
  19. spinnaker

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    I really think it is a loose connection somewhere in the area of TR1. I was poking around a bit tapping connections with a pen and I could get it to work then not work.

    It very well could be the the transistor socket. I know I had troubles with some of them when I first built it. I had to double over the leads to get a good connection inside the socket.
     
  20. CDRIVE

    Senior Member

    Jul 1, 2008
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    Transistor Sockets! My, you do know how to push the buttons on old men. Actually, since you built it in the early seventies, you're an old fart too! :)

    Really though, I have boxes of very old components (some dating to the fourties) and I have found that many of the leads have oxidized to the point of not being able to take solder without giving them good cleaning first. Our S. Florida climate didn't help either. By the way, a common pencil eraser is a very effective cleaner while not damaging what left of the lead plating. A residue less electronics cleaner prayed on your sockets while working the transistor in and out (while the cleaner is still wet) is also is beneficial. In a pinch I've been known to use Mineral Spirits.
     
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