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MR-67 Detector

The first time I rebuilt the 19K can I did it that way, gutted the micas from the bottom of the can and put it back together. Then came the fun part, with a pile of various values I started tacking them across the pins and attempted to align, almost wore out the slugs adjusting them after each cap change. Started with 470pf and ended up at 1000pf with the slug in the middle of it's range. This is one of the jobs that requires a good stock of various value capacitors!!
 
I got this meter today that will return values for pf size caps. In the pic I've got a 6.8 pf cap clamped in the 'gator clips. Pretty close, eh?

The meter is extremely sensitive as you have to keep your hands off the test leads in order to zero the meter and you have to have the distance between the leads about the same distance as when they're clamped or the reading will be off when you move the leads.

Meter comes in a kit or assembled. Will test inductors as well. Cool.

Murray
 

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I've had my eye on that meter myself. For the money, I don't think you can do better. It will measure smaller inductances than many other meters. I normally use my Tektronix 130 LC meter, but that would be nice to have. I was unable to measure the "built-in" capacitors in the detector, I guess due to the parallel inductance. I tried my Meterman and my Tek but no soap. I wonder how that unit would do?

Good investment.

Bill
 
Whenever I take the tf out of the '67 again, I plan on taking some measurements first with the coil wires disconnected and the internal caps in place. Then again with the cap out. Perhaps I'll get some "close enough" reliable readings to work with.

I bot the kit and got it put together in a little less than 2 hours so no big deal. First class pcb and pretty clear instructions/pictures as to where stuff goes as the pcb is not labled. Worked first time so it can't be that complicated. :D

Thanks for the kind comments.

Murray
 
What is this meter called exactly? I have a good capacitance meter already, but I could really use a good inductance meter.
 
I think I will stay with my Sencore LC53 "Z Meter" it should be 1% accureate from 1pF to 200,000uF, part I like the best it tests caps at operating voltage up to 600V for both leakage and capacitence.
 
dewickt said:
I think I will stay with my Sencore LC53 "Z Meter" it should be 1% accureate from 1pF to 200,000uF, part I like the best it tests caps at operating voltage up to 600V for both leakage and capacitence.
Click to expand...
I've lusted after one of those units for many years. Can these ever be found at reasonable prices used?
 
Ok, here it is. I cut the brass rivet with a carbide burr on a Dremel tool. The green plastic piece, which is somewhat translucent, is one of the capacitors although I would never known just looking at it. It is slightly dented by the metal tabs on the top of the clear plastic connector.

I tried measuring the capacitance by just clipping the meter leads to the green cap and got nothing. The only way to get a reading between pins 3 and 4 is to hold the green cap against the clear bottom and measure from the solder connectors. Then, it seems that the capacitor between pins 3 and 6 is integrated in the clear plastic bottom. :scratch2:

I'm consistently getting around 28pf between pins 3 and 6 and around 33pf between pins 3 and 4.

I just now did this so that's as much as I know. I really haven't thought much about putting it back together although it looks like the brass rivet is going to have to stay as it will not come out unless the slugs are removed.

Murray
 

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I plucked this piece, in the foreground, out of the clear plastic base. It seems to make a connection between pins 3, 4 and 6 and may be the actual cap and the green piece isn't a cap like I thought. It may be that just squeezing the green piece on the bottom causes this clear/white piece to make a better connection between pins 3 and 4 and I can then get a measurement.

Sorry for any confusion as I'm just trying to figure this out. Sheesh.

Murray
 

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You got it !! Mica sheets with silver plating, combined with the little fingers that come in from the side comprise each capacitor. Some last forever and some in the right humidity conditions fail, hard to predict, can get silver migration, contact corrosion, or punch thru.
 
Thanks for stopping by Terry. I was sorta wandering around in the forest and was having trouble finding my way home. :tears:

I didn't realize the sheet of mica was even there as it's held down by one of the metal clips and only noticed it when I was probing around with an Xacto knife trying to figure out what the white "paint" was. Bend up the long metal clip up and it slides out.

I may try to put it back together tonite. BTW, the sliver mica sheet and the rest of the components surrounding it were wet with oil from the DeOxit that I sprayed on it several times.

Murray
 
While putting the transformer back together, one pair of magnet wires on the top of the coil broke, right at the point where the winding stops and they head down to the solder point. I must have over manipulated them.

It appears that there are 2 wires wound on the top, red and green, and I'm getting .005" which would make it 36 gauge. I need to double check this with a mike instead of dial calipers but I'm pretty sure it's .005" I did find 36 gauge magnet wire at a couple of places but only in one color so that'll make it more complicated.

It should be fun trying to re-wind it. :tears: I think I'm going to stop for tonight.

Murray
 
Just got this wound up and am waiting on the glue on the windings to dry (the new wire is on the top). Epoxied the 3 pieces together, leaving the cut off brass rivet in place. 11 turns of #36 magnet wire and I only have about 3,000' left. Your prayers that this will work will be appreciated.

I performed a test wind on the ball point pen, in the background. :D

Murray
 

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Ok, back together. The pic of the bottom shows 2 33pf mica caps added to take place of the internal mica caps.

Tuner works but when a stereo signal is received (with the mpx lamp lit) the center tuning meter is way off to the right. If I try nudge the meter back to the center by tweaking the top slug of the transformer, I lose the signal. Quite a bit of distortion too. Hmm.

It didn't blow up or catch of fire so that's a good sign. :D Some more, umm, adjustment seems to be indicated, however.

Any suggestions?

Murray
 

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Best thing to do is pull the oscillator stage and pump 10.700mZ into the IF, use a scope to peak the can, keep the level low to stay out of limiting, this will let you see if the can peaks at 10.7 or the slug is in the middle of the coil and needs more C, sounds like this is where you are. For best tuning the slug should be about 50% into the coil, you should be able to tune thru the sweet spot with ease.
 
So today, I decided to "work" on this tuner a little more. I was getting quite a bit of tingle from the chassis and I'd been wanting to tend to this so I changed the 2 .005µf caps (Sprague 250v AC ceramic) that are connected between the chassis and the 120v line. Matched up 2 within about 2%, soldered them in and it works! No more little zap when touching the chassis. I also changed R32 (part of the power supply to 6CS6)which was showing 2.5 M when it should be 2.2 M.

Back to that pesky transformer. The center tuning meter was pegged on the right side and turning the top slug clockwise would move the meter back to the center but I was losing the stereo signal (some repetition here) and retuning was zero help. So, I started turning the slug counter clockwise and boom, the meter zipped back to the center and stereo lamp came back on. Woo ho! Dumb luck wins again! :D

Anyway, it sounds pretty good! At first, some of the high frequency extension was gone but after having it on for 15 minutes it's come back to life.

Whew!

Murray
 
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