Multiple identical-looking VTVM's.

Discussion in '"Boat Anchor" & Classic Equipment' started by W1GUH, Sep 16, 2014.

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  1. KB1WSY

    KB1WSY Ham Member QRZ Page

    Bingo! This one's inside my Eico Oscilloscope Voltage Calibrator (I haven't restored this unit yet):

    Selenium.JPG
     
  2. N2EY

    N2EY XML Subscriber QRZ Page

    Seleniums have some advantages for particular applications. Plus the silicon replacement requires some thought and testing, to get the right value resistor.

    There's also the desire some folks have to keep something as original as possible.

    As for tube manuals, go here:

    http://www.tubebooks.org/tube_data.htm

    for a couple of gigabytes worth of info. In particular, note the many RCA Receiving and Transmitting Tube Manuals, plus HB-3 sheets, available free for the download.

    (The site has been around a long time - it cautions that 25 Mb is a big file!)
     
  3. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    I haven't seen one explode, but they sure do flare up big for a very short while. I've been told they smell like garlic. I like garlic, and I think I know what it smells like. I've never smelt garlic when a selenium rectifier burnt. It is a nose-stinging smell, but not like garlic, in my opinion.
     
  4. K8ERV

    K8ERV QRZ Member QRZ Page

    Seleniums will take a beating in the reverse direction that silicons won't. I had to use them in a microwave oven power supply where if (when) a maggy backfired it put a BIG current spike back into the supply. Seleniums worked.

    TOM K8ERV Montrose Colo
     
  5. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    Also, the internal resistance is ideal for battery chargers. I always wondered why you couldn't use a silicon rectifier in series with a resistor, but I don't design battery chargers.
     
  6. KB1WSY

    KB1WSY Ham Member QRZ Page

    So, this thread has prompted me -- finally -- to replace that selenium rectifier in the Eico 232 VTVM.

    I understand that the ohm value of the resistor has to be found by trial and error, until you get the same voltage drop that you had with the old rectifier. In terms of the power rating of the resistor: the original selenium has a max current rating of 35mA, according to Eico's parts list. Can I safely use that as a clue to the absolute maximum current that might flow through the B+?

    I have an excellent article that I printed out a couple of years ago: "Replacing Selenium Rectifiers" in the AWA Journal, January 2007 (I cannot find a version online, as of today). It gives step by step directions for calculating the wattage of the resistor (fairly obvious Ohm's law stuff) but it assumes that you already know how much current flows through the B+.

    The other alternative would be to measure the current by inserting a milliammeter into the circuit, I guess, and then multiply by three or four for a safety factor in calculating resistor wattage.

    I have no high-wattage, low-ohm resistors in the junkbox so I'll have to go out and buy one, since it's likely to be somewhere between 20 ohms and 200 ohms. I'm thinking of using a potentiometer in the B+ circuit to determine the correct ohm value to buy: that should be OK, at these low currents, shouldn't it?
     
  7. KC9UDX

    KC9UDX Platinum Subscriber Platinum Subscriber Life Member QRZ Page

    What convinced you to replace it? I sure wouldn't. Someday it might burn up; replace it then.
     
  8. AF6LJ

    AF6LJ Ham Member QRZ Page

    The voltage drop will be about a volt per cell, so calculation will be easy.
    For that VTVM you should only need a 2 Watt resistor at most I would think.
    If your supply current draw is more than about four to six ma I would be surprised.
     
  9. K8ERV

    K8ERV QRZ Member QRZ Page

    I have a bunch of pots in small boxes with terminal screws, and I label the max current based of the resistance and pot wattage. But if it is not marked, just a plain carbon control, it is likely to handle about 1/2 watt. Someone may have a better estimate.

    If 1/2 watt and you have a 500 ohm pot then the max current on ANY terminal is 32 mills. Probably take a bit more briefly.

    Don't forget if you have the room and a good assortment of resistors you can series/parallel them to get your final value.

    TOM K8ERV Montrose Colo
     
  10. K7MEM

    K7MEM Ham Member QRZ Page

    I don't know about the fumes being poisonous or not. I have never heard of anyone dying from the fumes. But it certainly smells terrible.

    As with my previous post, when I was in high school (1964-1967) we had shop (Electronics Lab) for half of the day. The other half was dedicated to the normal high school subjects. Usually, the seniors got morning shop and the sophomores/juniors had afternoon shop. The shop had a main power switch that shut down the entire shop, except for overhead lighting. When the morning students left for lunch, the power was always shut down and turned on again when the afternoon students arrived. It was common for the morning shop to leave a little something for the afternoon students. You just wait until the power is shut down and plug in a selenium rectifier, wired across the AC, and go to lunch. When the afternoon turned on the power, it didn't take long for the rectifier to go up in smoke, but I never saw one actually explode. High voltage filter caps, yes. The big aluminum cans can rocket right across the room and leave a shower of foil and cardboard.
     
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