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Vintage Schematic Questions

Discussion in 'Datasheets, Manuals and Component Identification' started by KilgoreCemetery, Dec 22, 2017.

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  1. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    I'm rebuilding a mid-fifties Airline GAA-953B portable turntable and have a couple of questions about the schematic I found.

    1. I've replaced the original Selenium Rectifier with a diode and I'm just now noticing that the plus and minus on either side of the diode symbol are backwards. Is there something to this, or is it inconsequential?

    2. Is the 25VAC listed after the motor supposed to be actual voltage at that part of the circuit, or the amount of volts used up by the motor (AKA voltage drop)?

    3. Would I be able to remove the vacuum tube and power on the unit without damaging anything?
     

    Attached Files:

  2. HellasTechn

    HellasTechn

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    Apr 14, 2013
    I am not 100% sure about what i am going to write but...
    I think that since it appears to be a half wave rectifier (AC voltage) then it does not really matter which way the diode will be orientated.

    Again i am far from professional. just a thought.
     
  3. dorke

    dorke

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    Jun 20, 2015
    1. Ignore the ± sine on the diode,the symbol itself is correct (cathode to the right).
    2. 25VAC is the actual voltage,it is the heater voltage of the V1 tube.
    3. Yes, but take care,C1A and C1B voltages will have no bleeding path!
     
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  4. dorke

    dorke

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    Jun 20, 2015
    No,
    You can't reverse the diode !
    The Anode voltage of V1 needs to be positive ,not negative.
    There will be smoke(or a blast) out the electrolytic caps C1A,C1B !
     
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  5. Bluejets

    Bluejets

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    It does insofar as the electrolytic caps are concerned.
     
  6. kellys_eye

    kellys_eye

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    Another important point to note is the potential for a LIVE CHASSIS situation as the wiring from the mains supply can be crossed without affecting the operation of the equipment - other than making touching it potentially lethal!

    Do not use a non-polarised AC mains plug. Use one with an earth pin (unconnected) and observe correct wiring of hot and neutral.
     
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  7. HellasTechn

    HellasTechn

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    Apr 14, 2013
    Shouldnt have spoken at all besides i know nothing about tubes...
    You are right about the capacitors. I just noticed they are polarized.

    Im sorry !
     
  8. dorke

    dorke

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    Jun 20, 2015
    No need to be Sorry !
    That is the way to learn,sometimes we are wrong.
    My experience clearly tells that we learn more from our mistakes...smoke as well;)

    More than the capacitor polarization,
    the basic issue is to supply +Vdc at the output of the rectifier,reversing the diode direction will produce -Vdc.

    Look here for a simplified educational video.
     
    HellasTechn likes this.
  9. debe

    debe

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    Oct 15, 2011
    If you remove the Tube the motor wont run as the tube filament is part of the motor circuit. The motor is being used as a voltage droper for the 25V filament. This unit is an exercise in how to build a record player as cheep as possible.
     
  10. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    The company that built and sold this record player was probably either inspired by Muntz TV or it perhaps was Muntz immediate predecessor. Certainly the "design philosophy" is similar: make it as simple as possible with no redundant parts. Read an opinion about this from (deceased) analog guru Robert Pease.

    Ignore the plus and minus symbols. What is important is the symbol, usually a line of contrasting color around one end of the diode, that represents the cathode of the diode. The cathode provides a rectified positive DC output with respect to common when an AC voltage with respect to common is applied to the anode.

    Be careful replacing selenium rectifiers with modern silicon diode rectifiers because the forward voltage drops are quite different... about 0.7 V for silicon diodes and several volts to a dozen volts or more for selenium stacks. This will cause the rectified DC voltage to be greater with silicon rectifiers than with selenium. Some folks put a resistor in series with the silicon diode to simulate the forward drop across the selenium rectifier and preserve the original power supply voltage, but the value depends on how much current is normally drawn from the DC supply. Your circuit already has a 150Ω resistor, R5, in series with diode, M1, so perhaps the Sams schematic already incorporates a silicon diode as a replacement for a selenium rectifier.

    The 25 VAC after the motor is the nominal voltage across the cathode heater filaments of the 25CA5 pentode vacuum tube power amplifier. This tube has an indirectly heated cathode (pin 1 or K) internally connected to the pentode suppressor grid. The motor windings act as an inductive reactance in series with the tube heater filaments to lower the voltage supplied to those heater elements. This is a lot cheaper than a dedicated filament transformer.

    You must leave the 25CA5 vacuum tube inserted in its socket, and its filament heater must be intact and conducting current when power is applied. The voltage drop across the filament is part of the circuit for the turntable motor. You may be able to "short out" the tube filament with a jumper wire and run the turntable motor directly from the mains 115 VAC power, but a better idea is to replace a defective tube filament with an 82Ω, 10 watt, power resistor and remove the tube from it's socket. The motor windings may not be capable of operation directly from the AC line power without overheating.

    Note: In the interest of providing a low-cost solution, the manufacturer used a high-output-level crystal monophonic transducer in the tone arm of this turntable. Attempts to "play" a vinyl stereo record with this turntable will likely "plow up" the stereo track and render the record useless because of the needle style and tone arm pressure used. Should be okay for ancient "big hole" 45 rpm records though.
     
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  11. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    That is a lot of interesting information. I could tell it was a very simple schematic, but I guess I didn't realize quite how simple.

    The good news is that I just finished building a better Current Limiter to test with so I'm far less apprehensive about powering it up with the tube in place.

    How would I know what resistor to put in? Should I hook the Selenium Rectifier back up?

    I know very little about why they designed it the way they did, but I can say that the 150Ω resistor was being used with the selenium rectifier. Does that mean that I would have to add another resistor to compensate for the silicon diode?
     
  12. duke37

    duke37

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    Just chose a resistor to give 110V across th reservoir capacitor when the valve it taking current, try 470Ω 5W.
    With the chassis connected to the mains, it looks like a good alternative to an electric chair.
     
    hevans1944 likes this.
  13. hevans1944

    hevans1944 Hop - AC8NS

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    Jun 21, 2012
    If you leave the silicon rectifier diode in place, you are on your own in selecting a resistor. Try @duke37's suggestion to replace the 150Ω with a 470Ω, 5 watt resistor. Measure the power supply voltage, and if it is close to 110 V DC you are good to go. If you still have the selenium rectifier stack, putting it back would be the easiest thing to do. You can always replace it later after you get the turntable working.

    Are you going to use the original 25CA5 vacuum tube power amplifier or go with something more modern? What is the purpose of "restoring" this vintage piece of junk (VPOJ)? Are you going to use it or sell it? Do you want a "realistic" restoration, preserving as much of the original circuitry as possible? It is likely that the three electrolytic capacitors represented by C1A, C1B, and C1C have failed by now.

    Finding an exact replacement may be difficult to impossible. Avoid buying New Old-Stock (NOS) electrolytic capacitors. If you aren't interested in authenticity then a set of new (NOT New Old Stock or NOS) individual capacitors could be wired in. That might require the installation of a small terminal lug, connected to the steel chassis, to provide an electrical connection to the negative terminal of each new replacement capacitor and the installation of a "three-eye" terminal strip to anchor the positive terminals. Should be plenty of room under the steel chassis for that.

    Does your portable turntable look anything like this?

    [​IMG]

    There were zillions of these (or similar) manufactured and sold in the late 1940s to 1950s, mainly for use by children or (ahem!) young adults purchasing single hit-song 45 rpm records. They predated the "hi-fi era" but were much less expensive than the "big box" combination radio and record player consoles, sold as much as for their use as furniture as they were for the electronics inside after the end of World War II. The hi-fi crowd ruled during this era for a very short period of time, but contributed much to the recorded music art. This record player was not even remotely related to hi-fi, but is WAS cheap and functional. Thanks for sharing this little bit of nostalgia and history. I owned one of these, probably purchased by my parents at "Monkey Wards" in the early 1950s.
     
  14. KilgoreCemetery

    KilgoreCemetery

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    Apr 12, 2017
    The turntable looks almost exactly like the one in your picture. It's even about the same shade of green.

    I replaced the electrolytics with new ones when I added the diode and the C1 replacement caps all share a common ground just like before. I'm going to continue to use the tube, but was planning on adding a polarized plug with a fuse on the hot side for safety.

    I haven't decided whether I want to keep it or sell it at this point. It's pretty neat and the speaker still works, but I've had a lot of people ask about it. Evidently, it still has that "like new" smell that reminds people of when they were kids.
     
  15. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    In my opinion, it would be best to add a zener diode in series after the silicon diode with a value similar to the forward voltage drop of a selenium rectifier stack.

    Loading the power supply by reducing the value of the resistor across the capacitors is just going to increase ripple and possibly overheat the 1250 ohm resistor in series with the mains.

    edit: See here. The drop is likely to be 5 to 15 volts, and the article gives you a whole lot of specs on selenium rectifiers which may allow you to spot the one that was fitted. They recommend a series resistor (but the value -- often between 20 and 200 ohms -- will depend on the current and that's why I recommend a zener). This circuit already has significant series resistance to limit the surge current.
     
    Last edited: Dec 29, 2017
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  16. duke37

    duke37

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    Jan 9, 2011
    I may have expressed myself poorly. I did not intend to add an extra load to drop the voltage. The 470Ω resistor which I suggested would be in series with the diode. This will reduce the HT voltage, reduce the ripple and reduce the heating in the transformer.
    The resistor value should be chosen when the load is active.
    Using a Zener in series with the rectifier could increase the ripple and power loss in the transformer.
     
  17. Petkan

    Petkan

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    Feb 9, 2011

    Petkan:
    The original rectifier might have been full wave (bridge). What your schematic shows is a one way diode rectifier.
    Modern silicone diodes have lower "resistance" and may see higher inrush current. You have ballast resistor, so it may be fine, or you may have to oversize the diode (on peak current) or increase the ballast resistance.
    You can expect at the capacitor at the output of the rectifier 1.41 times (peak value) higher voltage than the applied input voltage. For 220Vac it is around 310V. If your capacitor has at least 400V rating, power with valve (tube) removed is fine. Note that these caps degrade over time (increase equivalent series resistance). You may check it for excessive ripple or measure it with ESR meter. Valves have filament heated cathode, typically 6.3Vac, around 0.3A consumption. Normally this comes from mains transformer. No harm using 6.3Vdc as filament heating. Typically the same transformer, providing the heating, provides also 250V for main anode voltage. It is conceivable to directly rectify (as you show)...but the common of your amplifier would be connected to the neutral (N). God forbid if the plug is plugged wrong way around - it will become the live (L). Normally we avoid connecting to the neutral in favor of the Earth terminal. In short if you repair an existing design-most likely it was transformer isolated. Moreover it might have a tap from the primary for the motor. The motor most likely is 'split pole" AC induction motor. Its speed is defined by the mains frequency and if originally it was designed for 60Hz and you are using it at 50Hz you may have to change the motor driving hub. It is conceivable to use a motor as transformer as well. Some like Grunding have done it in the past. You have to clarify if you want a complete restoration of the original design (retaining everything), or you are salvaging the box with mechanism and looking for replacement electronics. Note that the amplifier has very specific frequency response (gain vs frequency), matched with the pick up head. On cheaper designs these used to be piezo-ceramic, on more advanced - moving coil types.
    Contact me at <removed> for more details on valve (vacuum tube) based designs
     
  18. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

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    I've never seen a diode made of silicone.
     
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