This was a email rejected by workbench for the time being so please see if you can help me I have read all the places on internet including how things work very good article on shadow meters but they all talk about circular or horseshoe magnets under vane but mine doesn't have that please read on:
Hi I have a Philco 630F with a shadow meter different from the one described in the very helpful article it is the old-style box type and it was working until I messed with it figuring I could make the internal permanent magnet stronger by place a super magnet close to it .Well, dumb and dumber idea it stopped working, and became un-centered. So I took it apart. The coil test good at a little over 1k. I very carefully cleaned and set the tension so the needle is very free moving this is correct right? I took the coil apart from what I surmise to be the permanent magnet post which has no magnetism. I have tried several times to magnetize this using a very strong Niobium magnet with no luck. I have tried wrapping wire around it hooking it up to first dead short across 12v car battery briefly several times, no luck hooked up a test light in series with this setup (16 gauge wire) ran that for several minutes ,nothing. I then tried running a headlamp in series with it hooked up a battery charger ran that for at least three minutes. I can't figure out why it won't magnetize is it the copper plating? As I inspect this at the pivot point of the vane there is a copper plated little bar is this the heart of the unit? Does this need to be in the center of the two little tabs that are at the end of one end of the copper plated iron u shape core and the metal screw holding the coil and vane bracket in a magnet flux to keep it centered without power. I understand that when radio is off station you get more voltage from AVC kicking in, this deflects the vane from center position and gives more shadow Is this correct? So, my question is, does this type use the permanent magnet. I know this is lengthy should've just left it alone . Regards. Paul
(This post was last modified: 02-16-2021, 06:06 PM by frogpond.
Edit Reason: Clarification
)
Did you ever get your shadow meter figured out? I see that this was a post from a couple of years ago and didn't see any replies. I have the same type of meter out of a 29X model that seems to have gotten demagnetized (moves a bit with the 9V test but seems pretty weak).
The pm are there to bring the vane back to the center position. The coil moves the vane to the left or right not both depending on which way the coil wires are connected to the hv.
The pm doesn't have to be a super strong one as it would be fighting against the power of the electromagnet in my estimation. There a level of balance between the two. I haven't had to fool with the pm's so far. If it's too weak the vane will wobble and won't settle down for a few seconds after tuning in a station.
GL
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Hey, great info…thanks! This is my first time fiddling with this meter. Haven’t tried it in the radio yet as I’m still in the process of working on the chassis. Thought I’d take a look at this contraption to see if it worked. Was glad to see the coil was intact (a bit over 1k Ohms) and the vane moved when some voltage was applied. Didn’t really know how much movement to expect. Thanks again!
Thanks
Well, I hooked up the shadow meter in the 29x I’ve been working on and it doesn’t seem to work. Out of the circuit, I get a reasonable deflection when a 9v battery is applied, but nothing with it in the radio. I measure around 6.8v from the leads going to the meter when on a station (in the radio) and 8.5v off. The radio appears to receive well otherwise and I’ve done a full alignment. These voltages don’t seem right, but I don’t know what I did wrong…not sure what I’m missing. Thoughts?
Ok Here's some things to check. Pin 3 of the 42 tube should have about +320vdc from chassis. If it's lower than +300vdc find out why. If your 6A7 or either of the 39/44's have low emission that could cause it. Weak 6A7's are common. Check #'s 66,26,33,34,and 37 for proper resistance, they may have drifted.
Now I'm assuming that you have replaced all of the foil/paper caps that are inside the bakelite blocks. If you haven't it's a must!
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Terry
(This post was last modified: 04-27-2023, 09:19 AM by Radioroslyn.)
Thanks for the suggestions. That voltage is a tad low at 306v but should be ok, I think. As a matter of course I replace all caps and out of tolerance resistors, although I didn’t do the ones in the tone control…didn’t seem to be crucial and the control seems to be working fine. I have a couple of other 6A7’s and 39/44’s that I can try switching out. I don’t know the status of the latter that I have as my tester (Precision 640 emission tester) doesn’t have setting data for that tube. Hopefully, swapping out the tubes will help. Otherwise, as previously mentioned, the radio sounds good but it’s just annoying that the meter doesn’t function.
Your output transformer and the 80 tube is going to hate you when any of them short.
306v seems reasonable.
The meter movement is dependent on the plate current level on the mixer and two IF amps. The more current the wider the shadow. This is because the plate current is controlled by the AVC voltage. It's the dc voltage found on the left side of #26. If you follow it you find it goes to the control grid of the the previously mention tubes. The more negative voltage on the grid is less current/the more positive the more plate current. Also the level of signal at the ant determines it. It takes a strong enough signal to make the meter get a narrow line at it's center.
Something else that determines AVC voltage is the diode section of the 75 tube.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
Yeah, I was being kinda lazy and rather anxious to get her up and running, so didn’t dig into to tone control. Will do those caps next. After rooting around in my tube stash, I found 8 6A7’s and 3 39/44’s, so will start sequentially subbing them and see what I get. I seem to be getting strong signals from a local AM station that I’d think would be enough to generate sufficient plate current, but who knows? The fun continues…
So, I swapped out the 6A7 with 8 other ones I had on hand…no change. I then subbed both 39/44’s (and what the heck, the 75 as well) and now get deflection of the vane when on station. Not as much as I expected, but it definitely moves. So that’s good. Now my question is about the lamp. I put a #47 in but with the little plastic (?) window in place (I was watching the vane move with it removed) nothing shows through. My understanding is that the lamp and its placement is critical for this to work. Any suggestions?
Yes you'll want to manipulate the lamp bracket to get the best amount of deflection. Also turning the bulb's socket to rotate the lamp's filament position can give you a better looking and brighter shadow meter. The object is to align the filament to the slot at the rear of the sm. You could use a #44 lamp they are a bit brighter than the 47's.
When my pals were reading comic books
I was down in the basement in my dad's
workshop. Perusing his Sam's Photofoacts
Vol 1-50 admiring the old set and trying to
figure out what all those squiggly meant.
Circa 1966
Now I think I've got!
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