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Analog meters deceased. Substitute strategy, sorta.

Discussion in 'Amplifiers' started by nomadradio, Feb 16, 2018.

  1. nomadradio

    nomadradio Analog Retentive

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    This post started as another "Wacky Project" thing. Has to do with using parts you can get to replace parts you can't.

    Any more.

    Good example is the two analog meters on the front of the Heathkit SB-220, 221, HL2200, etc.
    The plate meter on the left has no protection from overload if a tube breaks down, a relay gets dirty, or the coax forgets to get plugged in to the "Ant" socket.

    If it goes, it's gone forever unless you can turn over the right rock. They exist as NOS if you have the hundreds of bucks to hold up that rock when you find what you want under it. Fixing a blown meter is like repairing a wristwatch. No point to it, can't get the parts.

    The meter on the right has three functions selected by the knob under it. Plate voltage, Relative output and Grid Current.

    Again, no protection at all. There's a bonus, though if you keep that selector on "Grid". A tube with a grid-to-cathode short tends to blow out the coil in the meter. There are other, more expensive failures that can also do this, like an arc inside the big (expensive) high-voltage transformer. When (not 'if') the high-voltage secondary winding jumps an arc to the grounded frame of the transformer, it will ALL go through the coil in that meter. And through the 0.82-ohm resistor right behind it. If that one is exploded, the grid meter is almost-certainly toast.

    Forever.

    Always wasting time looking for a deal on Ebay. I found a batch of partial analog meters. No scale plate inside, no black cover plate snapped over the bottom part of the face.
    Most replacement meters packaged in a nice box for individual sale also include nuts and washers to hook it up and mount it to a panel.

    These are naked. As you see.

    [​IMG]

    Yes, they're cheap. But the labor to make them presentable as replacement parts is part of the bigger picture.
    [​IMG]

    Ebay still shows nine lots of 20 like this one, as of Feb 15.

    Let's see if the forum clobbers this URL to that sale https://www.ebay.com/itm/Lot-of-20-New-Yokogawa-255209FAZZ8-UL-MN-2-5HL-1-1mA-DC-1B-Panel-Meters-F022/201553529302?ssPageName=STRK:MEBIDX:IT&_trksid=p2060353.m2749.l2649&autorefresh=true



    Feel free to buy as many as you wish. I have tied up all the money I can in this project as it is for now.

    I decided to buy a compatible-type meter that HAD a metal scale plate inside. Seemed the most-reasonable way to obtain a template to make my own.

    Shopped around, found one for 43 bucks.

    And some others that were higher.

    Sounds expensive, but lumped in with the rest of this project's expenses, doesn't look like a big penalty.

    Haven't cut a plate from 1/32-inch aluminum yet.

    [​IMG]

    Here is a plate/grid current pair set up for four 3-500Z. The plate is white plastic sheet, with a monochrome laser-printed scale held with double-sided tape.

    The "0-15 DC Volts" is what the scale said. Turns out that it's a 1 mA meter coil like the ones I bought a hundred just now. But with a 15k resistor inside it.

    The white plastic sheet turns out to be a bit brittle, so it's time to find another material for this.

    The 15-Volt meter also had the black plastic "skirt" that wraps around the lower half of the face and snaps in place. Too bad I don't have a 3D plastic printer handy.



    Might just use wide black tape for that.

    But this post is getting long enough. And I haven't even mentioned that a 1 mA meter with a 17-ohm coil won't just drop in and match the old meters, with 1300 ohms at a 200 uA sensitivity.

    Takes only seventeen MILLIVOLTS to push this new meter to the peg.

    Takes the original meter around 0.26 Volts DC to push 1 mA through the coil and push the pointer near the peg. That's two hundred and sixty millivolts.

    Plenty of leeway here to adapt the new meter. General rule is that if the old meter took a higher voltage on the coil than the new one, all you need to adapt it is resistors.

    The right ones.

    But what aobut adapting the other way, where a new meter requires more drive voltage than the old one?
    Special case. A hostage to how the arithmetic turns out, and how much of the meter-drive circuit you can hack.

    But that's what should make these easy to adapt to the Heath SB220. It fits the original opening and mount-stud holes.

    The math behind this will just have to wait for now. Tends to put people to sleep, anyhow.
    If I can get these to look reasonably legit I'll resell some of them. For now, they will only get used for repairs here.

    Maybe not so wacky, this project?
    73
     
    #1 nomadradio, Feb 16, 2018
    Last edited: Feb 16, 2018

  2. nomadradio

    nomadradio Analog Retentive

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    P.S.

    The meter scales were printed from Tonne Software's "Meter" program.

    His web site: http://www.tonnesoftware.com

    73
     
  3. BlowinSmoke

    BlowinSmoke Member

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    Honorable work but a few corrections. The SB-220/1 meters are 1400 ohms and 200ua and the FS voltage is .280v FS. A true "replacement" meter is one that looks pretty much exactly like the original and can barely be discernible unless one is purposely looking. Any other meter is just a " add on". The way to go about fabricating a true "replacement" meter is to use the original meter housing so that the meter is actually "original" and finding a movement that fits inside using the original scale so it all looks correct. The Heathkit SB-220/1 meters are particularly hard to "clone" since they are highly specific in design. The shape,color, and opaque scale are truly "dead special" items. I have been "cloning" SB-220/1 meters for a long time and had someone custom make an opaque meter scale to fit the slightly different movement I was using to replace the blown ones. Only a super trained eye with these amps would ever pick up the difference. If a non standard meter is OK for you then what you have created is a fine add on meter for a blown one. Unfortunately as you say these meters are scarce and if you can find an original, it's going to cost you. My "clones" aren't cheap either. I have cloned the Kenwwood 922 plate current meter also. Amp Supply amps like the LK-500 use standard issued still available meters. As for the HL-2200, those meters are an easy substitute if you can find the old SWR meters with the dual meters. A simple scale change and you are in business.
     
  4. nomadradio

    nomadradio Analog Retentive

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    That's very cool! Do you have the meters for the Kenwood amp for sale on line anywhere?

    Don't need any right now, but you never know.

    I ordered prototypes of a small pc board to go on the back of these meters, with the scaling resistors and protection diodes on it. I'll have to "jump around" the 3600-ohm resistor that scales the original Plate meter. Too high a resistance for these meter coils. The pc boards will be labeled with the terminal letter where it should connect to the SB-220's meter board.

    Gonna just "fly" the parts on the back of each meter for the first guinea pig. Have a customer's SB-221 with both meters blown out. He'll get a real deal on them for being the guinea pig.

    Film at 11,
    73
     
  5. BlowinSmoke

    BlowinSmoke Member

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    At the moment I do not have a 922 meters available but I usually make them up when one is requested. I only make the Ip meter since it only serves one purpose unlike the multimeter which does 3 things. It's basically a mechanical drop in replacement but does require changing R7 on the meter board which is a simple task. In the event that one needs a multimeter for a 922 the protocol is to remove both meters from the amp and swap the scale from the good Ip meter for the multimeter scale. Both meters are the same except for the meter scale. Then my replacement meter takes the place of the Ip meter. I usually have SB-220/1 and HL-2200 OEM meters in stock as well.
     
  6. nomadradio

    nomadradio Analog Retentive

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    That's very cool. What kind of price do you get for those?

    73
     
  7. BlowinSmoke

    BlowinSmoke Member

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    The OEM Heathkit meters are $85.00 shipped and the 922 meters are $65 shipped
     
  8. nomadradio

    nomadradio Analog Retentive

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    Finally got the first test of the substitute SB220 grid meter.

    Works just like it's supposed to.

    Sure is cool to have something work as calculated the first time, even for something this dirt-simple.

    The digital meter is showing the (nearly) 0.287 Volts that will drop across the resistor in the Heathkit that drives the stock grid meter.

    [​IMG]

    This tiny pc board makes for a neater conversion.

    [​IMG]

    Using 1-percent tolerance resistors probably helped.
    Now on to the Plate-Current meter.....

    73
     
  9. nomadradio

    nomadradio Analog Retentive

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    Wow, two for two.

    Ideally, this meter should show full scale with one Volt across the adapter-board input.

    Close enough for government work. Didn't have the right 1-percent resistor for this one. Had to use a 5-percent part.

    [​IMG]


    Next step is to see if they work in the amplifier.

    Here's hoping.
    73
     
  10. Onelasttime

    Onelasttime Well-Known Member

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    To be honest, I am not someone that see's most devices from a romantic perspective it either works or it does not. For me beauty is in the utility of the thing not the cosmetics of it. For a tube amp it is rather handy to have built-in meter's for tuning and so you know what is going on. If it's meter's are not working then some of it's utility is missing.

    Anything Heathkit is about as special as anything with Chevrolet on it. In fact outside of owners mechanics and engineers will often tell you off the record that the Corvette is just another Chevy.

    There is no magic in the name Heathkit in fact that only reason they are even around today is that they were cheap and readily available. Just like most of the CB radio's that fetch insanely high prices today that no one cared about when they were in production. It is only after things are gone that suddenly people want to pay top dollar for something old and well used that is for instance why it is almost impossible to find old SSB rad's that have not been hacked and butchered to death base or mobile. When they where in production they where just another POS CB fast forward 20, 30, 40 years and now everyone will pay top dollar for beat up units and pay to rebuild them.

    If people had been this supportive of the hobby all along maybe we would still have the good factory-built radios and plenty of amplifier companies around. I understand romance is good for business but let's be clear a pretty meter that looks exactly like OEM is not as important as the overall performance of the device.

    Some of you guys are as bad as Corvette Owner's tring to find data plates to put on a replacement starter to make it look exactly like it did in 1968. Restamping parts and blocks ect...It does not matter how convincing the counterfeit item is it is still a deception a lie a fake just one person's fake part looks more original than another fake part!! Basically what we are saying is we like to support the person who has the best fake "insert item here" often times people do not even care how functional the part is as long as it looks good! LOL

    Heathkit will never grace the concourse at Peeble Beach! LOL....Not Autorama....
     
  11. Low_Boy

    Low_Boy Well-Known Member

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    Nomadradio,, You are the man. You come up with some great ideas and all look nice.
     
  12. BlowinSmoke

    BlowinSmoke Member

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    Speaking of Corvettes, if you want to score a high score in a Bloomington Gold event you will need an original battery. It is probably hard to find a good original battery for a 1950's Corvette. One trick was to take the old battery which will no longer hold a charge and cut a large section of the bottom of the battery and carve out all the guts like you would with a pumpkin when you carve it. Take out everything and then buy the most powerful tractor battery you can find and within the battery connect the plus and minus terminals of the original battery to the tractor battery. The tractor battery will fit right inside the case of the old original battery. Tighten down battery hold down and presto the car will start and the battery will pass with full points. They won't remove the battery to look underneath. If you want to get real nasty you can make a surgical cut out of the bottom and replace the cut out piece and glue the old piece back in and do a little body work on it so it doesn't look like it had been cut away. Re-stamping blocks is tricky the judges may catch a poorly re stamped block. I don't think you can even obtain a Gold if you get bumped on your engine. I used to own the 1995 SIlver Salute Corvette obviously a 1970. 1970 LT-1 Roadster Mulsane Blue.
     

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