Pages: 1 [2] 3   Go Down

Author Topic: Kokusai mechanical filters  (Read 1379 times)

KM1H

  • Member
  • Posts: 11155
Re: Kokusai mechanical filters
« Reply #15 on: December 04, 2019, 07:48:33 AM »

Zenith Radio invented a mechanical filter just prior to WW2. They tried to sell it to Collins, who refused, and it didnt get any other traction.

I dont remember when it was patented but it has been discussed on forums before....probably not here.

Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #16 on: December 04, 2019, 04:41:59 PM »

One problem with mechanical filters is that over the last century, they have taken many forms.

The oldest patent I have found for a mechanical filter was filed in 1924 by Western Electric.

https://patents.google.com/patent/US1666681A/en?q=rca&q=mechanical&q=filter&oq=rca+mechanical+filter+

It does not resemble the modern Collins or Kokusai filters in that it is made of plates with round disks that connect them. What it has a strong resemblance to is the modern ceramic filter as shown here.

https://sv8ym.blogspot.com/2010/07/mysterious-case-of-withering-filters.html

This is the only patent I found by Zenith ( filed July 19, 1946 ) but it is an important one.

https://patents.google.com/patent/US2501488A/en?q=zenith&q=mechanical&q=filter&oq=zenith+mechanical+filter+

In it the author describes many implementations. He mentions using wires to connect the elements but his elements are rectangles in the drawings. Towards the end, he shows an implementation with disks and a central rod connecting them. This RCA patent for a mechanical filter filed just after Collins on May 14, 1949 uses the disk with central shaft concept. The Collins patent uses the wires and disk concept.

https://patents.google.com/patent/US2578452A/en?q=rca&q=mechanical&q=filter&oq=rca+mechanical+filter+

I suspect what Toko called "mechanical filters" back in the 1960's are really ceramic filters.

Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #17 on: December 04, 2019, 05:09:07 PM »

This is a capture of the 600 Hz filter replaced in its housing with new foam. The insertion loss increased a bit. This data was taken with a nanoVNASaver 0.2.1 software release which occurred just about 14 hours ago. This release solved the problem with setting a marker crashing the program.



Discovered the problem with a 2 kHz sweep is a problem that has been fixed in firmware.

« Last Edit: December 04, 2019, 05:29:21 PM by HAMHOCK75 »
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #18 on: December 06, 2019, 06:01:21 AM »

Updating the firmware on the nanoVNA was an adventure in itself but here is the result of scanning the 600 Hz filter with the latest firmware. Notice the red circled term “Segments”. The number of segments refers to how many 101 point segments make up the sweep so five segments means the 2 kHz span is broken into five, 400 Hz segments of 101 points giving rise to the 3.96 Hz step size.

The horizontal frequency labels are still strange but the marker frequencies are accurate.

Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #19 on: January 12, 2020, 03:42:04 PM »

Some further investigation was done with regard to the 600 Hz filter. Varying amounts of foam was used to pack the filter. Less packing resulted in more passband ripple. These photos also show the updated firmware for the nanoVNA allows plotting the input resistance instead of relying on the Smith chart.



More foam packing was used below with the result that the frequency shifts up slightly, passband ripple is diminished, and passband loss also decreases. The phase response also become more linear.

Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #20 on: February 17, 2020, 02:00:29 AM »

This is a further examination of how these filters work regarding the mode of vibration and how the bandwidth is determined. The photo below is from the Collins patent of 1949 listed earlier. Figure 7 indicates that the bandwidth is determined by the distance from the center of the disk at which the connecting rods are placed but perhaps the most important detail is the reference to a “nodal  plane”.



The nodal plane refers to a circle about the center of the disk at which there is no motion. The animation at the link below illustrates this,

https://www.acs.psu.edu/drussell/demos/membranecircle/mode02-2.gif

Placing the coupling rods near the nodal plane where there is little motion, mechanically, lightly couples to the vibrating disk. The animation also explains the insensitivity of the filter to being touched at the edge of the resonators since the edge is also not in motion.

Add a comment
« Last Edit: February 17, 2020, 02:04:56 AM by HAMHOCK75 »
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #21 on: January 15, 2021, 03:23:16 PM »

I always wondered what a Collins mechanical filter looked like inside, the failure mode, and the solution. W1BR posted this recently. His first sentence seems to apply more to the Kokusai filters.

https://forums.qrz.com/index.php?threads/r390a-mechanical-filter-failures.744273/
Logged

W1BR

  • Member
  • Posts: 4422
Re: Kokusai mechanical filters
« Reply #22 on: January 16, 2021, 06:18:13 AM »

The foam issue also occured to the mechanical filters used in my JRC-515.  I did an article for Popular Communications on to repair them a long time back.

Folks are telling me the R390A filters usually have the hard rubber supports, so the decreased filter performance
I encountered seems to have been caused by out gassing contaminating on the discs.

Pete
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #23 on: January 16, 2021, 02:19:44 PM »

W1BR what did you use to clean the resonators? Did you dip the entire resonator in it or did you apply the cleaner only to the resonators as with a brush?
Logged

N8YX

  • Member
  • Posts: 2449
Re: Kokusai mechanical filters
« Reply #24 on: January 17, 2021, 07:18:39 AM »

W1BR what did you use to clean the resonators? Did you dip the entire resonator in it or did you apply the cleaner only to the resonators as with a brush?
I'm not Pete ('BR) but I am a fellow NRD-515 owner who has encountered the Kokusai filter problem. Seems they began to follow me home last March and between now and then I came into 3 receivers plus several accessory filter boards, loose Kokusai filters and whatnot. Altogether I've refurbished at least 8 - possibly more - of the 455KHz units.

My cleaning arsenal consists of isopropyl alcohol, toothpicks, a couple different types of cuticle sticks and small cotton swabs. When the filter enclosures are removed I use the toothpicks and cuticle sticks to carefully remove as much gooped-up foam as possible from the resonators and the filter base. I'll then apply a healthy dose of alcohol to a section of disks with a swab and use the pointed ends of the wooden tools to get the remaining gunk out from between and off the disks. Likewise the transducer coils and the base. Keep at it until completely clean. An alcohol dunk may work for any film still present on the disks, though none of mine appear worse for me not performing this process.

I've repacked a few filters with pieces of closed-cell, non-conductive foam and others with cotton. An early lesson learned was to sweep the filter BEFORE soldering the thing to the board.  :-\ Sometimes the packing needs a little adjustment.
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #25 on: August 25, 2022, 08:54:35 PM »

Although the Kokusai filters shown on this thread did suffer from the disintegration of the foam packing, other failures are possible like the transducers. I had never seen a description of how the transducers work in the Kokusai filter but fortunately, Kokusai patented the transducer. The patent linked below describes in detail how it works,

https://www.freepatentsonline.com/4140984.pdf
Logged

W9AC

  • Member
  • Posts: 373
Re: Kokusai mechanical filters
« Reply #26 on: August 26, 2022, 06:04:19 AM »

The same Kokusai filters are used in the Yaesu FR-100B receiver.  After removing the decomposed foam, I replaced it with cotton which won't decompose any time soon. 

To deal with the filter impedance mismatch issue for common 50-ohm measurements, look at the Q3 2015 issue of the Collins Signal:

https://www.immigration-lawyer-us.com/images/q32015.pdf

Paul, W9AC
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #27 on: August 27, 2022, 10:55:54 PM »

This photo shows what the patent refers to as the older Langevin transducer. It appears to be ceramic with two gold endcaps. The reddish wire is the drive wire. The photo tries to show an unusual characteristic of these filters. What appears to be a copper and green wire is actually just one wire which is green on one end and copper color on the other. This is done only on one end of the filter. The opposite transducer only has a copper colored ground wire. The patent never mentions why this might be done.

Also of interest is that the coils are glued around the resonator endcaps only to serve a strain relief function.



W9AC. Thank you for the paper. I was unaware that Collins filters looked like a 940 uH coil at the input. I suspect that is because Collins uses a magnetostrictive transducer. A variable capacitor was added externally to resonate with the coil forming a high pass filter which transforms 50 ohms to 145 kohms with the filter actually presenting an impedance of 12-16 kohms with resultant mismatch loss of 4-6 dB.

The Kokusai filter looks more like shunt capacitor across the piezolectric element which the patent mentions is “several kil-ohms or less” at resonance. I decided to create a lowpass matching network consisting of a shunt capacitor across the filter input which absorbs the transducers capacitance with a series inductor. With the ability to control both component values, I found that a 100 uH inductor with a 1000 pF capacitor could both resonate and match at the same time to about 1.7 kil-ohms. The result is below is almost a perfect match in the filter passband to 50 ohms. Unfortunately, this filter still has problems. It is more than a kHz too low in frequency and the passband ripple is severe. I suspect that the two problems are related but will need more investigation.



If you noticed the white strands attached to the weld for the ground wire connection to the adjacent resonator in the first photo; I tried your suggestion of a cotton ball instead of foam.
« Last Edit: August 27, 2022, 10:58:35 PM by HAMHOCK75 »
Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #28 on: August 31, 2022, 01:00:29 PM »

Generous use of a spray cleaner resulted in a large reduction in the insertion loss even though there is no visible contamination. The input resistance of the filter is also lower at the points of lowest insertion loss as suggested by the peaks in the resistance curve.

Logged

HAMHOCK75

  • Member
  • Posts: 1297
Re: Kokusai mechanical filters
« Reply #29 on: September 01, 2022, 03:03:37 PM »

The matching network was changed to transform 50 ohms to about 800 ohms using 70 uH in series with a shunt capacitance of about 1,660 pF.  Below is the measured that 50 ohms is transformed to about 762 ohms. The change had little effect on ripple, center frequency, or loss.



I used some 600 grit wet/dry sandpaper to clean up the resonators but that also had little effect. When I polished the endcaps of the transducers produced a dramatic change in both loss and flatness as shown below. Frequency is still low by about 1 kHz however.

Logged
Pages: 1 [2] 3   Go Up