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YO-100 with positively charged case!

Discussion in '"Boat Anchor" & Classic Equipment' started by KI7WQH, May 19, 2018.

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  1. KI7WQH

    KI7WQH Ham Member QRZ Page

    oh the joys of buying things on ebay. So I was pretty excited when I first powered up my yo-100 scope and saw that the CRT was working and everything seemed to be in order... ...but I was less excited when I felt what seemed like a mild shock when i touched the back of the case. I broke out the voltmeter and sure enough there were about 6 volts or so running through the case. I opened her up again and I don't see any obvious issues so I've downloaded the manual and it's time to start troubleshooting.

    As you can probably tell by all my recent posts asking questions I'm rather new to this. I was a computer tech when I was younger and my dad was an electrical engineer that had my brother and I building simple stuff when we were kids and I've restored electro-mechanical pinball machines but I am mostly a software guy in my professional experience. This dive into ham radio that is driven by my compulsion to preserve these old radios and the story they tell of what I see as the first "internet" is going to be a bumpy road so I really appreciate the help I've received on this forum.

    I hardly expect that anyone will know what the problem is off the top of their head and I'm probably just writing this to put off having to go through the intimidating schematic, but if there is a methodology or some words of wisdom you would like to impart, now is the time! "fixing" the ft-101 has been simple thanks to the removable boards but the guts of this thing is quite a messy mass of wires and components.

    Sorry for the "blog post" I'm off to test circuits!
  2. KA9JLM

    KA9JLM Ham Member QRZ Page

    You should check C28 and C29, And install a grounded AC cord.

    Have Fun.
    KI7WQH and K7TRF like this.
  3. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Follow Don's advice and replace those caps with Y rated capacitors in addition to a 3 prong grounded line cord.

    A hot chassis is almost certainly a result of a leaky common mode bypass capacitor. You can read about X and Y rated capacitors and why they're custom designed for AC supply filtering applications here:
    KI7WQH and KA9JLM like this.
  4. KI7WQH

    KI7WQH Ham Member QRZ Page

    Thanks for the info guys, looks like I have several bad caps. I'm new to identifying capacitors for replacement, several of them say "WV" for example 1500 WV 0.1 μF. I know that is a 0.1 microfarad cap but im not sure if "WV" is volts or if the W is something else? Also, do you recommend a vender?
  5. KA9JLM

    KA9JLM Ham Member QRZ Page

    Working Volts. Anything with a higher WV voltage is good.

    I normally go with a higher voltage. Many times the caps used are under rated, That is why they are bad.

    Those caps on the AC line should be updated to new types used today.

    Mouser is a good supplier.
    KI7WQH likes this.
  6. KI7WQH

    KI7WQH Ham Member QRZ Page

    Awesome, thanks! Last question (for now) There are some capacitors that are obviously axial and marked as such but there are others that are the same shape) (tubes with connections at either end) that don't have arrows or any noticeable indentation at one end. Are all capacitors that are structured like this axial or do they need to be marked as such?
  7. KA9JLM

    KA9JLM Ham Member QRZ Page

    For electrolytic types, normally the end that is negative is the end that is all aluminum.

    The end that comes out of the middle of the rubber is positive.

    If you connect them backwards you should find out fairly quick. :eek:

    A radial is normally marked, On a new one the long leg is positive.
    Last edited: May 19, 2018
    KD2ACO and KI7WQH like this.
  8. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I think you're asking if the capacitors in question are 'polarized', axial refers to the geometry of construction. If the leads stick out the ends of a cylinder then the capacitors are axial by definition. In terms of polarized it depends on whether the caps in question are electrolytic, tantalum (rarely axial, mostly radial in construction) or something else. For instance there are an awful lot of mylar and metal film caps used in older rigs that are axial in construction but not polarized at all and installation direction doesn't matter.

    If you can post some photos of the caps in question folks can tell you whether they're polarized or not. Either that or look up the part number in the rig's manual to find out what kind of capacitor it is.
    KI7WQH likes this.
  9. KI7WQH

    KI7WQH Ham Member QRZ Page

    These are the caps I'm looking to replace, thanks for the help guys

    Attached Files:

  10. K7TRF

    K7TRF Premium Subscriber QRZ Page

    All of those with the obvious polarity markings are electrolytics and polarized (there are non-polarized electrolytics for some applications but they're not common). Similarly in the top shot the radial capacitors with the metal top surfaces and shrink wrapped outer shells are also electrolytic caps and no doubt polarized.

    The lower value caps are not electrolytics and not polarized. The black 600 WV 0.1uF caps are likely some kind of metal film caps. The 0.1uF 600WV black caps and the 0.1uF 1500wV gray Fujicon are likely some kind of metal film caps and not polarized. Same for the 0.012uF 50WV radial caps which appear to polystyrene or similar metal film capacitors.

    Best bet is to find the service manual online which should list each component type and value but many of those caps you're looking at are not polarized and thus not marked with a polarity indicator.

    It's nice to match the original cap types for looks as much as anything but the key thing is to match the capacitor value in uF (or pF, nF, whatever units) and meet or exceed the voltage rating of the original caps. In some unique situations like in free running oscillators that need good frequency stability across temperature the specific capacitor type such as NPO or silver mica or similar might be important but for most signal coupling and supply filtering applications the specific type (e.g. polystyrene vs polyester) isn't critical.
    KI7WQH likes this.

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