• The move to the new server is done. There are some software and database maintenance updates in process. This has us passing the hat around to help out. We appreciate any donations. Seriously, even a dollar helps. The payment page may be found here - https://www.audiokarma.org/support.html

MR-67 Detector

pathologymd

pathologymd

Active Member
Hi all,

Some time back a thread discussed the silver mica capacitors inside IF and multiplex transformers occasionally going bad. My MR-67 has exhibited (I believe) symptoms consistent with this, including sudden changes in volume heralded by a soft "thump" and apparent restricted bandwidth when tuning and observing the center tune meter.

I went into the tuner slightly twisting transformer cans and then probing the connections and components with a stiff plastic alignment tool. I had the tuner hooked up to a ST1000A with the tuner output monitored on a scope. All the poking and prodding showed no effect until I reached the detector (T6), when the level jumped back and forth on the scope with poking. I touched up the solder joints and inspected carefully, but to avail. I subbed the preceding tube/cleaned the socket etc to make sure movements weren't being coupled to the tube pins. No change. Took the transformer out (see pix). There is one 33 (assume pF) ceramic cap across the secondary (effectively across the anodes of the two diodes), but I cannot find any other caps inside. The schematic indicates one across the primary and secondary. Terry mentioned they were sandwiched in the base but I don't understand!

I attached a couple of pictures. Outside of replacing the one cap and carefully touching up the internal solder joints I'm not sure what to do next.

I am open to suggestions. After the transformer was out, I tested each component that had been attached and all seemed stable and on value.

Thanks for reading this,

Bill
 

Attachments

  • MR67-1.jpg
    MR67-1.jpg
    27.4 KB · Views: 93
  • MR67-2.jpg
    MR67-2.jpg
    25.4 KB · Views: 114
Register to hide this ad
Same deal with mine. Wiggling the can back and forth would raise/lower the volume substantially and change the character of the output - ok one way and not so good the other way.

I took the Al top off (slipped an Excto knife blade between the base and can) and looked exactly like yours. The sheet infers that there's 2 caps inside but only one. I left the oem cap in place.

After some back and forth ems with Terry, I sprayed the bottom clip with DeOxit, cleaned the can with steel wool and put it back together. Then, touched up the solder joints underneath.

Problem eliminated. The center tuning meter was then way off, however, but a little tweeking of the top part of the coil whilst observing the multipath trace on my MI-3 and I got it re-centered.

I know this doesn't make a lot of sense, but there it is.

Murray
 
In the bottom view the silver looking flat sheet is the capacitor, it makes contact with a finger that is part of the terminal. The brass rivet in the center and the metal plate provide the sandwich pressure on the silvered mica sheets. Soak the bottom section in DeOxit - wiggle the pins, and hopefully the fingers get better contact and all will be well. The silver can also migrate and cause the capacitor to arc and eventually short, this problem will show up as a steady hard static and even a glow inside the bottom area of the can, this is when you gut the caps out of the bottom of the can and use external ones on the pins.
 
Be careful tweaking these old cans, the material in the core can dry out and crack without much warning. This happened to me years ago with an MR-71. Lucky for me at that time Mac still had the replacement can in stock!!
 
Thanks everyone for your input, I do appreciate it.

Terry, should worse come to worse, given the internal nature of these caps, how do you "gut" them to eliminate them from the circuit?

Bill
 
Brass rivet is soft and easy to cut, once free of the rivet it will come apart, the lugs will be loose without the support of the plastic plate so I use some epoxy to hold them in place.
 
I could have looked at the bottom of that transformer and never known there was a cap there.

If anyone knows or figures out what the value is, please post it.

Murray
 
OK,

I soaked the base of the transformer in deoxit and wiggled the pins (didn't have much "play" and I sure didn't get too vigorous). Cleaned the clip and transformer case. I replaced the internal 33 pF capacitor for "cause" and touched up the internal solder joints. I tried to measure the internal capacitance without any success. It seemed to "swamp" my digital capacitance meter and I couldn't get any reading on my Tek 130 meter. Not sure about that. I guess the inductance messed things up.

I reinstalled the transformer and cranked up the tuner and it seems very stable with twisting and poking. I adjusted the detector for minimum distortion and double checked separation. Sounds very good right now! I hope I bought some more time.

Thanks guys for your help.

Bill
 
Now only time will tell if this was a brain storm or one of my brain farts, lets hope the miricle cleaner D5 holds up to its reputation.
 
Ok, I've got mine out. I was thinking that the cap was on the bottom of the clear plastic base under the eyelet, but it isn't.

Is the cap the dark green part immediately on the top (to the right in the pic)? Sheesh! It seems that to get the thing apart, the coil windings have to be undsoldered from the pins. Otherwise, it doesn't look like it will come apart as the brass "tube" won't come out the other end if the bottom is cut off.

Any help here?

Murray
 

Attachments

  • t-6.jpg
    t-6.jpg
    61.3 KB · Views: 64
On the left under the green plastic, they are sheets of silvered mica, the solder pins have contacts built into them that make contact with the silvered mica sheets.
 
Thanks Terry. Any idea of the mica cap(s) value? Seems like I saw in another thread that it's 100 pf?

Murray
 
Wish I remembered the value, I wrote the value for the caps in the 19K can on the schematic but never did add the value for the IF cans, start with a 100pf and see how it tunes. Remember top of can is secondary and is tuned for 0V at the audio out test point, the bottom of the can is the primary which I tune for maximum audio out and minimum distortion.
 
Well, I chickened out on taking the transformer apart. I stared at the bottom of it with a magnifying glass several times and just never had the balls to cut it apart. Once the brass eyelet is cut off, there's no going back it would have to be epoxied back together.

The schematic shows 3 caps as part of the transformer, one is the 33pf that is soldered to pins 1 and 2. The other 2 are apparently the internal caps of unknown value.

I cleaned, as good as is possible, the bottom part contaning the problem connections first with "electronics cleaner" then gave it a healthy dose of 100% DeOxit. Replace the internal 33pf cap and the 2 6.8pf caps that are soldered on the bottom. The diodes and resistors that connect on the bottom all checked out ok so I left them alone.

Put it back together, checked voltages (115v coming in and voltages at 6CS6) and hooked it up.

So far so good - no volume popping up and down like it was doing before. Center tuning meter was way off but I've got that dialed in pretty close now. Sounds great. :music:

Much thanks to Terry.

Murray
 
This is one of the most educational threads I've seen on AK (or AA, for that matter)! I am always put off by the 'cans' in tuned circuits, but they're a bit less mysterious now. Thanks for sharing this information!
 
Lol. I would never have attempted this without Bill doing the first exploration and Terry holding my hand along the way.

Murray
 
Great work, Murray--

Your intestinal fortitude is right on par with mine! I just couldn't bring myself to mess with that brass rivet either! Mine is still doing great. I now need to tackle one of my MR-71's with the up and down volume problem. I just don't want to go there right now!

Again, fine work indeed.

Bill
 
Ha! Now that I have it put back together and working, I'm a lot more brave. :D If it acts up again, I'm now thinking that I'll take it apart. We'll see.

My Fluke dvm won't return a value for pico - the lowest it will go is 1n, so that's part of the problem, for me anyway. I did get several silver mica caps with values from 47p to 150p or something like that. It actually wouldn't be too hard to temp solder them in place (after removing the internal caps) and road test the tuner. I think.

Murray
 
You must log in or register to reply here.
Back
Top Bottom