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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

    So according to this, that AC power should be going to the transformer before it goes to the fuse? I'm going to have to have another look at the wiring because prior to all this fuse nonsense the unit would power on and give me a trace and I didn't change the wiring. I wish I could take a look at the inside of a working unit to be sure.
  2. K7TRF

    K7TRF Premium Subscriber QRZ Page

    No, look at the schematic again. AC comes in from the wall outlet plug and is bypassed to chassis on both the hot and neutral leads by the Y capacitors. One leg goes straight to the transformer primary (presumably neutral) and the other lead (presumably hot or the black wire in US AC wiring standards) runs to the front panel switch (not shown in this segment of the schematic). Another wire returns from the front panel switch and runs to the fuse and then the other side of the transformer primary.

    IOW, picture a SPST switch that shorts out those two wires that run off the schematic thereby connecting the hot lead on the plug to the fuse which then runs to the transformer primary.


    Attached Files:

    KI7WQH likes this.
  3. KI7WQH

    KI7WQH Ham Member QRZ Page

    ohhhh, ok. Got it, thanks! That makes more sense, time to hunt down this short. I hope it's not the transformer. I don't see any burn marks on it but that doesnt mean too much. Would you advise taking the transformer out and then running power to it and seeing what it's putting out?
  4. K7TRF

    K7TRF Premium Subscriber QRZ Page

    I'd probably do a couple of things before pulling the entire transformer out.

    - First I'd either bring up the rig on a variac or run the old school light bulb current limiter where you run a light bulb (a real incandescent light bulb rated somewhere around 60-75 watts or so) in series with the AC hot lead. If there's a dead short the light bulb will light at full rated intensity but fuses and circuit breakers won't blow. If there's a partial short or the rig is simply drawing well beyond expected idle mode current the bulb will burn at moderate intensity but it allows you to at least make some measurements without popping fuses and circuit breakers every time you try to power the rig.

    - Without pulling the actual transformer out I'd disconnect at least one leg of each of the secondary circuits. IOW, desolder one or both output wires that run to the tube filaments, disconnect at least the ungrounded wire that runs to the B+ circuitry, etc.

    - See if the rig draws excessive current (light bulb burns brightly if using the series light bulb current limiter) with the secondary circuits disconnected. If current basically drops to zero with all the secondary circuits disconnected you could either ohm out each secondary circuit, especially those that are rectified for DC supply usage or you can reconnect them, one circuit at a time till you find the culprit. It's very likely one of the DC circuits with bad rectifiers and or shorted filter caps as that's fairly common in rigs of this vintage but it could also be something dragging down the filament supply.

    - If the rig still draws excessive current with all the secondary circuits disconnected then yeah, a bad transformer is likely, either that or a gross wiring error right where the AC enters the rig or connects to the transformer as shown in that schematic segment. At that point pulling the transformer (after a very careful inspection to make sure nothing got wired incorrectly when replacing those caps) would probably be the next step. But hopefully you'll either find a big wiring error in the primary circuitry or disconnecting the secondary circuits will help point to the problem.
  5. G3YRO

    G3YRO Ham Member QRZ Page

    If both the new Filter capacitors you fitted have gone short-circuit, THAT would blow the fuse, even if the case wasn't grounded.

    Did you alter any wiring OTHER than just replace the Filter caps?

    Other most likely fault that would blow the mains fuse is that one or more of the PSU rectifier diodes have blown - simple to test these with an ohm-meter.

    Roger G3YRO
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  6. KI7WQH

    KI7WQH Ham Member QRZ Page

    The filter caps seem fine, but I can only test them with an ohm meter at the moment (until my tester arrives). I think the diode idea makes sense because what happened initially was after I had replaced all the caps and grounded out the case I powered it up and heard some crackling/buzzing for maybe a couple of seconds and then the internal fuse blew. No smoke that I could see though. The second time I powered up (after double checking all my wiring and making sure I didn't accidentally solder any points together) the fuse went instantly. Could it also be the CRT? When I first got the unit and it was giving me a trace (with the case charged) it was taking a long time to warm up. I've seen people power these things up on youtube and they seem to give a trace a lot quicker than mine did. I was wondering if perhaps it was a weak CRT and replacing all the caps somehow gave it more juice and killed it but I'm totally just pulling that out of my rear because I've never worked on CRTs beyond just recapping a few arcade machines.

    Also to answer your question, I didn't fiddle with any wiring. Just swapped out caps.
  7. K7TRF

    K7TRF Premium Subscriber QRZ Page

    It could be the CRT but I'd strongly suspect one of the supplies or the transformer itself before suspecting the CRT but it's an easy test to disconnect the CRT via it's socket and power up the unit (with some form of current limiter unless you want to keep popping fuses). If the CRT is internally shorted and drawing excessive current then disconnecting it will remove that short but that seems less likely than a shorted rectifier, shorted supply cap or shorted transformer.

    One way or another check one circuit at a time. That could be careful ohm meter measurements looking for a near DC short in one of the supplies or it could be disconnecting all outputs from the transformer, testing to make sure the transformer itself isn't creating the large current draw and then reconnecting the secondary circuits one at a time until you find the culprit.

    FWIW, I had a similar problem with a Kenwood SM-220 that turned out to be one of the metal film caps in the HV supply section. I was able to find it with an ohm meter as it was a near DC short but my symptoms were similar, slow warmup for a while, sometimes with a jumpy display trace and then the monitor started blowing fuses when I tried to power it on. Your situation could be quite different but there's only a handful of rectifier diodes and capacitors to check.

    Here's info on how the homebrew a light bulb current limiter which is useful when troubleshooting these kind of supply shorts:
    Last edited: Jun 1, 2018
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  8. G3YRO

    G3YRO Ham Member QRZ Page

    Even if you have a short in the CRT, this may ALSO have caused one or more of the rectifiers to have blown (usually short-circuit . . . so either way, I would check those first (very simple with an ohm-meter)

    In my experience, it is VERY rare for mains transformers to fail . . . the mains fuse will blow quick enough to protect it.

    Yaesu always used very high-quality transformers back then . . . for example, the only time I have EVER known an FT101 transformer to fail is where a customer kept blowing the 3 amp Mains Fuse (due to a rectifier blown inside the rig) and so stupidly fitted a 10 amp fuse ! (and therefore ended up frying the transformer)

    Roger G3YRO
    K7TRF likes this.
  9. K7TRF

    K7TRF Premium Subscriber QRZ Page

    Agreed, pretty much every blown power supply transformer I've come across also had an oversized fuse in the fuse holder. That's true of gear from many vendors including: Drake, Heath, Kenwood and others. Oversizing fuses that keep blowing is a really bad idea.
  10. KA9JLM

    KA9JLM Ham Member QRZ Page

    Did you check the polarity of your ac outlet and cord ? Did you replace the cord ?

    The caps that you replaced are not the problem, They are before the fuse. They would trip the breaker that feeds the outlet.

    Good Luck. Be careful when playing with electricity.
    Last edited: Jun 1, 2018

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