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Allied SX-190

Discussion in '"Boat Anchor" & Classic Equipment' started by N4KDF, Apr 30, 2021.

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  1. N4KDF

    N4KDF Premium Subscriber QRZ Page

    I just received an Allied/Radio Shack SX-190 and despite needing a little TLC it is in really nice condition. I've removed the covers and nothing has been touched. Nice to see that on an old rig for a change. I suspect filter caps (although there is no hum in the audio) and an alignment may be required as it is pretty deaf. Calibrators run but no signals from the outside world.

    The one thing it IS doing is giving me a little bit of a buzz when I touch the chassis. Just enough to know its there. I've not dug into t he schematic yet but I suspect it is related to having the 2-wire AC plug and the cap and resistor to ground on one side of the power transformer.

    What are things to look for? Any options to convert to a 3-wire plug?

  2. WA1GXC

    WA1GXC Ham Member QRZ Page

    Getting a 'bite' from the chassis is very, very common in a receiver from that era but totally unacceptable, just the same, from a safety standpoint.

    The parts list tabulation specifically shows the line-bypass cap as an 'oil' type, probably a Sprague "Vitamin-Q" brand or its equivalent.
    Paper-in-oil type; kind of an odd choice of component, but no matter.

    Remove and replace that capacitor C-89. It is electrically leaky- it's 45 years old. The ideal and proper choice using today's available components would
    be a "X / Y "safety-rated ceramic disc cap specifically designed for that application. If you can't obtain that or not sure what that is, you can use
    as a substitute a 1.6 kiloVolt disc ceramic, value not critical, .001-.005 would be fine.

    Check the value of R-91. If you can get a replacement, change it out. If not, make sure its value is up there at 3M ohms +.

    Change out the power supply electrolytics. They will fail, probably sooner than later and likely take-out other components along with them.

    Install a 3-wire line cord for safety. First thing on your agenda. There is no other option. Ground lead to chassis/common and you're done.

    Last edited: Apr 30, 2021
    W7UUU likes this.
  3. G3YRO

    G3YRO Ham Member QRZ Page

    Surely ANY Mains by-pass capacitor connected to chassis will give you a shock if the chassis isn't earthed? (ie it doesn't mean that the capacitor is leaky)

    It's been a legal reqirement here in Britain for over 70 years that ANY equipment sold that has any metal parts accessible MUST have a 3-core lead, so that the chassis is earthed. (this includes Fires, Cookers, Washing Machines, Light Fittings, Irons, Toasters, Kettles, Hair Straighteners, etc, as well as all Electronic equipment)

    Roger G3YRO
  4. WA1GXC

    WA1GXC Ham Member QRZ Page

    "If there is no other use discovered for electricity, this however is something considerable, that it may help make a vain man humble."
    --Benjamin Franklin 1747

    I don't go around doing primary research on minimum discernible current-flow perception in pre-geriatric males by acting as artificial
    Earthing for my 1966 Drake T-4X.

    There's lots of scientific literature around on AC current perception; eschewing Internet 'expert' opinions, I settled on "Minimum Thresholds for Physiological Responses to Flow of Alternating Electric Current..." published by the US Naval Medical Research Institute in 1969. The data and literature
    widely available publicly suggest that perception is widely variable between individuals and genders. The US standard ANSI C101.1 effective
    in 1969 limited maximum acceptable current between device and safety ground as 0.5mA for US standard mains service (120v/60Hz).

    I'm not going to test my hypothesis because to do so would be arcane and tedious, but I don't think a modern .001uF line bypass capacitor should
    allow an uncomfortable AC 'bite', chassis to Earth.

    Whether or not the cap as installed is leaky is really moot. It's untrustworthy and a device of that type construction has no business
    being installed as-wired 48 years after it rolled off the assembly line. Even in as-new condition, it is unsatisfactory by modern design and
    engineering safety standards. Get rid of it.

    Last edited: Apr 30, 2021
  5. WA5VGO

    WA5VGO Ham Member QRZ Page

    Connect the chassis to an earth ground.
  6. G3YRO

    G3YRO Ham Member QRZ Page

    Well most rigs use .01uF decoupling capacitors on the mains to Chassis . . . and I can tell you from bitter experience - having worked on lots of Imported rigs with only 2 core mains leads - that they give you quite a big shock if you don't earth the chassis first !

    Roger G3YRO
  7. N7EKU

    N7EKU Ham Member QRZ Page

    I seem to remember that it depends also on which way you have their non-polarized plugs plugged in?

  8. N4KDF

    N4KDF Premium Subscriber QRZ Page

    Good comments all.

    The first thing I did was to 'turn over' the AC plug in the socket as I too was aware that might be contributing to the buzzy feeling I was getting. And indeed that took care of the issue. I think I had measured about 117VAC to ground, which in this case was the top of my clothes dryer/ workbench. After flipping the plug it was down to around 2 or 3 volts. Still I was curious about the two parts involved or if maybe I was getting some leakage from the primary winding.

    The capacitor is a .001 mfd rated at 600V. Not very big physically and has a ceramic body. Physically it looked fine. I don't have an ESR meter so I just used my DVM and it measured at 1.6 megohms. Not oil-filled and not an old wax and paper guy either. The resistor which is supposed to be around 3.3 megohms is at over 4 megohms so it is fine.

    The cap/resistor is wired from the neutral to ground. For noise I figured it would be from the line side to ground. I don't know.

    My next decision is whether or not to remove the cap and resistor and go to a three-wire power cord. I may just go with a two-wire polarized plug setup to keep things looking a bit more original and to lessen any extra chassis work.
  9. G3YRO

    G3YRO Ham Member QRZ Page

    Well most rigs have a .01 uF capacitor from BOTH sides of the incoming Mains to chassis . . .

    But if the chassis isn't earthed you WILL get a nasty shock !

    Well I wouldn't ! And if that radio was sold in Britain, it would come with a 3-core mains lead.

    Roger G3YRO
    WA1GXC likes this.
  10. NN5AA

    NN5AA Ham Member QRZ Page

    ALWAYS repair/modify a piece of gear so that it is safe, even when not turned on. Someone else may get 'bitten' or worse, and all for the mere price of a 3-conductor power cord ?!?!:eek:


    73 , Vince - NN5AA
    WA1GXC likes this.

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