Old philco radio power upgrade

Discussion in 'General Electronics Chat' started by matthew798, Sep 14, 2013.

  1. matthew798

    matthew798 Thread Starter Member

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    Hello all.

    I have scored a really nice old Philco 1008 Radio. It worked in the antique store but when i brought it home I noticed that the power cord was crumbling. I decided to upgrade it with a new 3 conductor ac power cord, like the ones used for computers.

    Here is the shematic:
    SCHEMATIC

    I just wanted to know if it would be safe to connect the ground (green) wire to the chassis. I know some of these old radios were chassis hot, but I can`t seem to find anything indicating whether or not it is in the schematic. I did notice that a few components seem to be connected to one of the coils in the transformer via ground symbols... I`m not quite sure if i`m right though. Thanks guys!

    P.S. the power circuitry is in the lower right hand corner of the schematic.
  2. ronv

    ronv Senior Member

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    Wow, an oldie and goodie. ;)
    It has an isolated supply so you can use your green wire ground.
  3. matthew798

    matthew798 Thread Starter Member

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    Thanks for the reply ron!

    I did however notice a few lamps connected directly to mains. Could this indicate that only most of the circuitry is isolated?
    Also, about 1/3 random wires, joints and metal bits seem to be connected to the chassis... Everything from caps to resistors... I'm having trouble understanding this.
  4. ErnieM

    ErnieM AAC Fanatic!

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    Before you cut the cord... see if it connects to the chassis. Use an ohmmeter!

    The old standard was to connect the AC line neutral direct to the chassis. Needless to say...

    If it is not connected then you should be good, connect the hot and neutral to the transformer. I would leave the green ground unconnected; there is a ground symbol between the caps on the power transformer but I have no idea where that goes, it may be the chassis (and that is a little scarey).
  5. matthew798

    matthew798 Thread Starter Member

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    That's what I felt as well (scared)

    The primary of the transformer is not shorted to the chassis on either side, according to my MM. But as you said, quite a few components are...

    So, I guess my next question is: Is it dangerous to operate the unit without a proper chassis ground? I can't see it being terribly life-threatening considering it was manufactured that way for many years...

    Am I being too picky by wanting to install the earth ground?
  6. ronv

    ronv Senior Member

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    They may have used the chassis to distribute DC ground as well as wires. Since it has a power transformer for isolation the only way the chassis could become hot is if something on the primary side shorted to it.
  7. matthew798

    matthew798 Thread Starter Member

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    alright, well i've wired the LINE to the on/off switch and left the ground disconnected. Neutral is obviously wired to the appropriate spot. Originally, the power plug was non-polarized... Neither prong was shorted to the chassis. Is my set-up in any way dangerous?
  8. #12

    #12 AAC Fanatic!

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    Technically, it's wrong. The 2 capacitors across the primary of the transformer will pass some house current to the bond wire. A GFCI would detect that. The thing to do is remove one of the capacitors and the center connection to the chassis. Use the remaining capacitor directly across the power line at the primary of the transformer to absorb high frequency noise spikes. Do not connect anything on the primary side to chassis.

    All the other case connections are isolated from the electrons in the wall outlet by the power transformer. Leave them alone. Now. Connect the bond wire to the chassis and expect zero point zero current in it.
  9. vk6zgo

    vk6zgo Active Member

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    I wrote an extensive run-down on this circuit,& tried to post it,but Dear old
    AAC decided it would be fun to lose it in cyberspace-AAAARRRRGGGGHHHH!!!
  10. vk6zgo

    vk6zgo Active Member

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    Technically,you're right,as the incoming Mains are not a balanced system.
    In practice,the Reactance of the capacitors at supply frequencies is too high to have any great effect.

    How do I know?

    Years ago,I bought a Drake SSR1 Radio which was made for the US 240v balanced system.
    It had a mains filter,part of which consisted of capacitors from each side of the Mains to chassis.

    In the US 240V system,the chassis would have been at "virtual earth",so I wouldn't have received a little zap when I touched it.
    Yes! it had a 2 core cable.

    As the Oz 240v system is unbalanced,I did get zapped.so I fitted a 3 core cable,& have used it many times on RCD/GFCI protected outlets without any issues.
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  11. #12

    #12 AAC Fanatic!

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    Did the math. At about 380 ua, a GFCI would not detect the fault.
    My bad.
  12. MaxHeadRoom

    MaxHeadRoom Well-Known Member

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    First off the wrong symbol is used, Earth symbol instead of chassis.
    It appears that the chassis is the common reference but on the DC supply the C.T. the common is raised above chassis by 3 reference/bias resistors (77)?.
    Max.
  13. #12

    #12 AAC Fanatic!

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    True, the center tap of the high voltage winding is raised above ground by 3 resistors which apparently make some DC bias voltages, but that means nothing about how to apply the power cord.
  14. MaxHeadRoom

    MaxHeadRoom Well-Known Member

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    True, Just an aside comment.
    I never meant to muddy the water!
    To OP, you may want to check or replace the two electrolytics either side of the DC PI filter, ones of that age tend to be a cause of failure, and take out the rectifier tube.
    Max.
  15. #12

    #12 AAC Fanatic!

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    and THEY (the capacitors) take out the rectifier tube.
    Just clearing up your phrasing.:)
    (It sounded like you were telling HIM to take out the rectifier tube.)
  16. vk6zgo

    vk6zgo Active Member

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    It wasn't the wrong symbol back then.:D

    The symbol used in this schematic was commonly used for both types of "Earth",as well as for a real "earth" stake used with an external antenna.

    It was usually obvious which was meant by the context.

    The resistors between the transformer centre tap & chassis were for "back bias" which was used to simplify bias circuitry.

    The chassis is several volts +ve w.r.t. the centre tap,so connecting the cathodes to chassis & returning the grids to various points along the voltage divider is the same as providing a separate -ve grid supply,but cheaper.

    This is fairly easy to see in the audio stages,but is a bit messy in the IF stages with the additional circuitry for AGC.
  17. MaxHeadRoom

    MaxHeadRoom Well-Known Member

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    I wonder what the excuse is now though for current literature that makes the error? ;)
    Max.
  18. vk6zgo

    vk6zgo Active Member

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    It is still usually obvious from the context .
  19. MaxHeadRoom

    MaxHeadRoom Well-Known Member

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    ??????????
    You mean I have to make assumptions?
    Max.

    Attached Files:

    Last edited: Sep 16, 2013
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  20. vk6zgo

    vk6zgo Active Member

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    Yes,Max!
    I stand my ground on this!:D
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