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post #1 of 124 Old 09-28-2012, 06:21 PM - Thread Starter
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I am thinking of getting an FM filter from RadioShack to see if it may help with reception of distant channels. But do you install the filter before the preamp at the antenna, or before the power supply closer to the TV? I am in zip code 29607 in Greenville, SC, and apparently there are some strong FM towers nearby. But I still get some Charlotte, NC channels from 75 miles no problem, but I just wonder if the FM filter would make a difference on some other weak, distant channels. So where is the proper place to install the FM filter?
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post #2 of 124 Old 09-28-2012, 08:10 PM
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What I did is install the signal separator at the base of the antenna where the cable snakes underneath the eaves of the house. At that point, the FM goes via a separate cable to my receiver and the OTA signal goes, via RG-6 to the garage where the drop amp is located. About a 150' run. FM is perfect and OTA is the same. However, the antenna is on the roof at about 30' from ground.
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post #3 of 124 Old 09-28-2012, 08:51 PM
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He's trying to trap out FM, not separate it.

The FM trap belongs between the antenna and the amplifier, nowhere else, if you are trying to prevent FM interference with TV reception. FM interference is most common on VHF channels 4-13. It rarely affects UHF reception.
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post #4 of 124 Old 09-29-2012, 10:36 AM
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Quote:
Originally Posted by ProjectSHO89 View Post

He's trying to trap out FM, not separate it.
The FM trap belongs between the antenna and the amplifier, nowhere else, if you are trying to prevent FM interference with TV reception. FM interference is most common on VHF channels 4-13. It rarely affects UHF reception.
Agreed on the placement of the filter. Utilizing an FM/TV separator would do the same thing. You just terminate the FM port. A filter attenuates the FM band while doing nothing to the surrounding frequencies, while an FM/TV separator pulls the FM band out of the bandwidth and sends it out the FM port.
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CIAO!

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post #5 of 124 Old 09-29-2012, 10:45 AM
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The only TV/FM separators I ever checked out had really weak attenuation of the FM on the TV/thru line, like under 10 dB, so such a device would surely be inferior to any inline 88-108 reject filter.
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post #6 of 124 Old 09-29-2012, 11:42 AM
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Ok. So either one would work in his case, a filter or a separator, with a good filter being the preferred device if he wants to eliminate the FM all together and not shunt it off to a receiver. FWIW, the signal separator works very well for us. None of our VHF HDTV stations are affected. Reception is solid and reliable. Thanks for the clarification of a filter and separator.
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post #7 of 124 Old 09-29-2012, 11:47 AM
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I thought it was called an "FM Trap". biggrin.gif
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post #8 of 124 Old 09-29-2012, 01:06 PM
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Where you put the FM filter (trap) depends on your situation. If your preamp can handle all of the signal it is receiving without clipping, but your tuner can not, it may be better to put the trap after the preamp. Any kind of filter has some insertion loss: the amount it attenuates the signal you want to keep. All preamp have some fixed input noise. The first preamp in the system usually sets the signal to noise ratio of the whole system (unless later amplifiers are really bad). If the preamp can handle the signal without clipping, putting the trap after the preamp will have a better signal to noise ratio. If the signal is causing clipping in the preamp, the trap must be before the preamp.

Also, there are two kinds of FM filter, and the people who sell them seldom tell you which you are getting. One type kills all of the FM, but since channel 6 is really close to FM frequencies, it kills channel 6 too. The other type tries to keep channel 6, but then lets quite a bit of the low end FM. About the only way to tell what you are getting if you don't have a channel 6 in your area is with a network analyzer.

I have FM traps in my antenna lines because the FM signals in my area are 100 times as strong as the TV signals I'm trying to receive (40 dB). The ones I have kill channel 6, but they allow me to receive channel 7 without breakups.
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post #9 of 124 Old 09-29-2012, 01:50 PM
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Quote:
Originally Posted by DAP View Post

I have FM traps in my antenna lines because the FM signals in my area are 100 times as strong as the TV signals I'm trying to receive (40 dB). The ones I have kill channel 6, but they allow me to receive channel 7 without breakups.

Interesting. As stated above, I just have an old Winegard Signal Separator splitting the FM to my receiver in the living room. Channel 7 comes in just fine with no issues at all in the family room where the HDTV is. Is there even a Channel 6 in our area? I see we both live in San Jose but probably in different areas. FM reception is great but they're being pulled in from an antenna that's about 30' from ground level.
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post #10 of 124 Old 09-29-2012, 08:14 PM
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Quote:
Originally Posted by Ratman View Post

I thought it was called an "FM Trap". biggrin.gif
Not unlike a Speed Trap, which traps those frequencies that go too fast...

CIAO!

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post #11 of 124 Old 09-30-2012, 02:33 PM
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Quote:
Originally Posted by Otto Pylot View Post

Quote:
Originally Posted by DAP View Post

I have FM traps in my antenna lines because the FM signals in my area are 100 times as strong as the TV signals I'm trying to receive (40 dB). The ones I have kill channel 6, but they allow me to receive channel 7 without breakups.

Interesting. As stated above, I just have an old Winegard Signal Separator splitting the FM to my receiver in the living room. Channel 7 comes in just fine with no issues at all in the family room where the HDTV is. Is there even a Channel 6 in our area? I see we both live in San Jose but probably in different areas. FM reception is great but they're being pulled in from an antenna that's about 30' from ground level.

There is a low power analog channel on channel 6 broadcasting from Loma Prieta: KBFK-LP. Most of my tuners are digital only, and the one that is capable of analog does not get this station due to the FM traps I'm using.
I'm on a hillside an 500' altitude with my antenna 30' above ground. My 44 mile line of site to Sutro passes over San Jose, and there are many FM towers between here and there.
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post #12 of 124 Old 10-01-2012, 08:17 AM
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There are two types of broadband FM Traps....one is a full-band (or, full band-stop) filter, that attenuates the entire 88-108 MHz FM band. That one will "slop over" a bit on the ends, attenuating some of TV Channel 6, and some of the aviation frequencies above 108 MHz.

The other , which is often recommended for people trying to receive TV channel 6, starts attenuating around 88 or 89 MHz, but does not offer real attenuation (20 dB is a typical figure) until it gets to about 92 MHz. This type goes with the assumption that FM stations near the low end of the band are usually low-powered "educational" stations, that won't need the full attenuation.
Unfortunately, that stopped being the case years ago....many non-commercial stations are running full power in that part of the band. Also, there are many low-power stations scattered around on cell towers and roof tops, not just on big transmission sites. So, you could have a strong signal from a nearby LP station, affecting the reception.

There are variable traps that allow you to notch out one (often two) FM channels. You could use the 92-108 MHz trap, in series with the variable-frequency trap ("notch filter), and use the notches to attenuate a couple of the strongest FM stations at the low end of the dial.

Ken English, Sr. Engineer, KSL-TV.
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post #13 of 124 Old 10-01-2012, 10:26 AM
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I posted a plot over at DHC last year that compared the old Radio Shack with our filter. Note that the RS number was mistyped and should have been 15-577.


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post #14 of 124 Old 10-01-2012, 11:13 AM
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post #15 of 124 Old 10-01-2012, 11:45 AM
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Quote:
Originally Posted by DAP View Post

There is a low power analog channel on channel 6 broadcasting from Loma Prieta: KBFK-LP. Most of my tuners are digital only, and the one that is capable of analog does not get this station due to the FM traps I'm using.
I'm on a hillside an 500' altitude with my antenna 30' above ground. My 44 mile line of site to Sutro passes over San Jose, and there are many FM towers between here and there.

Hmmm, I'm out in the Evergreen Area with pretty much an unobstructed LOS to Sutro about 50 miles away.
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post #16 of 124 Old 11-09-2013, 03:26 PM
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Sorry to resurrect an old thread, but how exactly should I interpret this graph?

If I'm looking to block FM from interfering with Hi-VHF channels 7 and 12, is there any difference between the Radio Shack and the Antennas Direct FM filter?

http://www.radioshack.com/product/index.jsp?productId=10939219#
http://www.antennasdirect.com/store/FM_band_rejection_filter.html?cjid=6146852

If it helps, here are my TV Fool and FM Fool reports:




Quote:
Originally Posted by ADTech View Post

I posted a plot over at DHC last year that compared the old Radio Shack with our filter. Note that the RS number was mistyped and should have been 15-577.

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post #17 of 124 Old 11-10-2013, 12:49 AM
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Ch7 is 174-180 MHz, so 2nd Harmonics of 87-90 MHz in FM Band can cause interference....so A-D FM Filter would be Better, although it also attenuates Ch6 a bit.....but that's not an issue for you.

Ch12 is 204-210 MHz, so 2nd harmonics of 102-105 MHz in FM Band can cause interference....so EITHER will work.

PS: Here's a LEGIBLE copy of ADTech's chart (Center is 100 MHz with 5 MHz per Horizontal Division):
http://www.digitalhome.ca/forum/showpost.php?p=1255180

FYI: The above R-S Filter was REPLACED with the RS 15-024 part number....do we have a chart for the new one????
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post #18 of 124 Old 11-10-2013, 09:07 AM
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In your situation, either would likely be an appropriate choice unless you are using a 2-69 channel antenna. Are you having issues with 7 or 9 now?
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post #19 of 124 Old 11-10-2013, 05:36 PM
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Wow, what a huge difference a $5 FM Trap from Radio Shack made! It seems being just three miles away from a large group of FM transmitters was really hurting my reception.

Just out of curiosity, I tested two antennas, the Antennas Direct ClearStream 2V and the RCA ANT751R in three combinations: just the antenna, using a $5 Radio Shack 15-024 FM Trap, and using a $25 RCA TVPRAMP1R Preamplifier with the FM Trap enabled.

Here's the results:



This was tested using a Hauppage WinTV-HVR 950Q USB card as the tuner connected to 30' of vanilla RG6.

The numbers in the cells are the SNR as measured by the "Digital Signal Quality" application Hauppage includes which ranges from 0 to 27.

Cells colored red are stations that were found during a channel scan but wouldn't come in, and those colored yellow came in just fine but under the maximum SNR of 27.

Blank cells meant I was unable to lock onto that channel during a channel scan, even though TV Fool said it might be possible.

It's interesting how especially with the RCA ANT751R there were a huge chunk of stations that weren't even viewable without a FM Trap. Also interesting how the ClearStream 2V only really struggled with VHF 12 but had no problems with that channel once the FM trap was installed.

So, based on these results, would anything help out with the VHF 12 or UHF 36 or 51, the only three channels that aren't coming in at the top SNR of 27? Or with UHF 27 or UHF 42, the two channels I never located?

A different type of FM Trap? A different antenna? Something else?
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post #20 of 124 Old 11-11-2013, 04:26 AM
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Since you are well over the minimum threshold of 15-16 dB SNR (assuming that is what is being reported), I wouldn't waste time "chasing numbers" just for their own sake.

I'd keep the C2V and FM trap and get rid of the two RCA items. Unless you need longer cables or splits, the amp doesn't appear to be needed for basic reception on your current tuner.

KAXT is likely getting covered up by either adjacent channel interference from KCSM or from co-channel interference from KPIX's Mt Vaca translator which might be getting a strong reflection off the hill in front of you.
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post #21 of 124 Old 12-16-2014, 07:45 AM
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<bump>

This came up on a related Google search I was doing and I see there was a question about the 15-0024's response. I bought one back in 2012 or so and tested it last year. Click on the thumbnail below to see how it performed.

Caveats: Single sample, etc...

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post #22 of 124 Old 12-16-2014, 10:39 AM
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So RS-15-0024 has more than 27 dB Loss midband and 34 dB Loss on 108 MHz, BUT NOT MUCH LOSS on the lower part of the FM Band, due to the need to PASS Ch6 with minimal Loss (STILL looks to be about 5 dB on Ch6 Audio Carrier for those stations repurposing Ch6 into FM Band service).

OTOH, A-D FM Filter provides more than 24 dB Loss over MOST of the FM Band (including ALL of Ch6), but with only about 4 dB Loss on 108 MHz.

I ran across an advert for the MCM 33-341 "FM Trap"...which is in fact a TRUE FULL-BAND FILTER....and VERY inexpensive. Spec claims are 20 dB Loss on BOTH Band Edges (88 + 108 MHz) and 30 dB Loss on 93 MHz. Needless to say, it's going to ALSO severely attenuate Ch6:
http://www.mcmelectronics.com/product/33-341
Unfortunately, no Specs on Out-Of-Band Loss, so unknown what impact is on Ch2-6 and Ch7+, but since they advertise it is for CATV systems, it's probably not a problem. Perhaps someone can run a test and generate a Frequency Response Curve......

Last edited by holl_ands; 12-16-2014 at 10:44 AM.
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post #23 of 124 Old 12-16-2014, 11:36 AM
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MCM FM Traps

Quote:
Originally Posted by holl_ands View Post
...I ran across an advert for the MCM 33-341 "FM Trap"...
http://www.mcmelectronics.com/product/33-341
The attached images are some measurements on a few of these filters.

This first image shows FM band attenuation of 11 different filters. Filter #5 (bold orange line) was from a previous batch purchased a couple of years ago. The others, including 5a, are from a purchase made during this year.

The attenuation is mostly more than 35 dB across the band, with greater attenuation near 102 MHz.



The 2nd image below shows the loss within the VHF-Hi band. Loss is pretty good (less than about 0.3 dB).



The 3rd image below shows return loss, again about as good as as one could expect. Given the design configuration of the trap (3 poles) and desired FM attenuation, one could not expect better return loss (particularly from #5 , not shown, it was near 20 dB return loss).



The PCB was relaid out between the purchase of the two batches of traps in which #5 was a member.. The newer PCB is smaller probably to reduce cost slightly.

No attempt has yet been made to measure the loss at UHF, but it maybe expected to be reasonably low. This expectation is based on the high quality of the coils used in the trap.



The above image shows the inside and circuit configuration: A series-tuned center section and parallel-resonant sections on each end (input and output).
.
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Last edited by tripelo; 12-16-2014 at 11:45 AM. Reason: fix typo
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post #24 of 124 Old 12-16-2014, 04:23 PM
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Thanks for the info...so MCM 33-341 "FM Trap" has MORE than 32 dB Loss, except at Band Edges SOME units only provided 27 dB (88 MHz) and 29 dB (108 MHz).....so it is TRULY a FULL FM Band Filter, unlike the other choices...and has no effect on Ch7-13...and probably no effect on other channels either....other than obviously severely degrading (or killing) Ch6.

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post #25 of 124 Old 12-16-2014, 05:35 PM
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Nothing you do to the FM band (88-108MHz) will affect channel 7 as it's up at 174-180MHz. Channel 6 is at 82-88MHz, so unless it's a brickwall filter, any filter you install for the FM band will affect channel 6.

I used to listen to channel 6 on the radio in my car.

CIAO!

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post #26 of 124 Old 12-16-2014, 06:47 PM
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Quote:
Nothing you do to the FM band (88-108MHz) will affect channel 7 as it's up at 174-180MHz.
Sure it can. 2nd harmonic of 87-90 MHz falls into channel 7. If there are certain combinations of frequencies, 3rd order intermods can fall into VHF channels including ch 7.
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post #27 of 124 Old 12-16-2014, 06:58 PM
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Most FM reject filters are not full band filters. Most of them roll off starting at about 93MHz. They are commonly designed that way because cities that had channel 6 analog TV transmitters in them generally didn't have any strong FM radio transmitters in the low end of the band.

If someone find themselves in an inconvenient situation where they absolutely have to pass a digital channel 6 but because of their proximity to even a weak, lower FM frequency transmitter that they have to trap it, they are best served with a 93-108 band reject filter coupled with a single fixed FM notch filter. Microwave Filter company has single frequency, cylindrical notch filters tuned to each and every FM frequency on-the-shelf and last I knew, sold them at reasonable prices. i think they had a rejection dept of about 50dB.
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post #28 of 124 Old 12-16-2014, 11:44 PM
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I was looking for a frequency response test that would verify that there are no unexpected "surprises" in the Out-Of-Band performance, e.g. Ch2-5 and Ch7-51. Although I don't ANTICIPATE any problems, it is ALSO easy to have TOO much stray capacitance inside the filter box that MIGHT affect some of the channels above the FM Band....you just never know until it's been verified that it is NOT a problem. Same thing can be said about any of the other FM Filters. Yes, I'm a stickler for DETAILS.....and VERIFICATION.....

Choice of FM Filter pretty much boils down to whether you want to RECEIVE Ch6 (use RS-15-0024) or if you don't CARE about Ch6 (use A-D...or even better use MCM).

I've looked at dozens of FM Fool Reports when commenting on various posts....those pesky FM Stations are scattered all over the country, rather than just being concentrated in a few locations like TV Stations. So some are more likely to be VERY CLOSE to your location, rather than on some distant mountain top.

A Single Channel Notch Filter (such as in some Preamps) typically attenuates ONLY ONE FM Station in the LOW end of the FM Band to protect Ch6...and low band FM stations frequently have lower power. A Notch Filter does very little to suppress the REST of the FM BAND, which can generate Second Harmonics throughout the Hi-VHF Band (2x88=176 MHz=Ch7 and 2*88=216 MHz=Ch13) as well as lower level 5th Harmonics in UHF Band which can be an issue if the FM signals are strong enough and/or the Preamp is highly susceptible to "Overload" (like the CM7777).

If all you want to do is keep FM signals from degrading Ch6, perhaps a Notch Filter will do the job....but if you want to keep STRONG FM signals from degrading Ch7-13 and possibly some UHF frequencies, esp. if using a Preamp, you'll want an inexpensive FM Filter that attenuates as much of the FM Band as possible.

A Single Channel Filter approach would end up spending a LOT more money for MULTIPLE (3? 4? 5? more?) Single Channel FM Filters, all in Series, each increasing the Out-Of-Band Insertion Loss. And they are ALSO going to attenuate 1-3 MHz on either side of each Notch Filter, degrading whatever signal is left at an attached FM Receiver, if you had any ideas of sharing the TV Antenna's downlead Coax. Which is yet another reason for using a Separate FM Antenna with a Separate Downlead....with NO Preamp....it doesn't "need" it....and a Preamp can really mess up the FM signals due to Intermodulation Distortion within and external from the FM Band.

Last edited by holl_ands; 12-16-2014 at 11:59 PM.
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post #29 of 124 Old 12-17-2014, 08:33 AM
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Mike,

Can you read off the capacitor values for C1, C2, & C3?


That FM-88 on MCM's website indicates it's on clearance. Once their inventory is gone, that will likely be it for that device. I can't find anyone else selling it for a reasonable price.

The four devices discussed (RS 15-0577, 15-0024, AD FMFLT, and FM-88) are almost identical in regards to their internal construction and layout. The attached schematic is one I laid out a while back in Orcad of our FM filter. The cap designated as C4 isn't in ours, it's in the RS15-0024 (I was experimenting with the simulation). The only visible difference I can see besides C4 that is how the coils have been adjusted (knifed). Coil values were found by disassembling a sample filter and measuring inductance of each piece with an LC meter.
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post #30 of 124 Old 12-17-2014, 09:49 AM
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MCM FM Trap Schematic

Quote:
Originally Posted by ADTech View Post
...Can you read off the capacitor values for C1, C2, & C3?
C1 & C2 are 30 pF (parallel tuned, in & output)
C3 is 12 pF (series tuned central resonator)

Below is a image of a schematic that fairly closely replicates (in circuit software simulation) the performance of the MCM units.

Capacitor values tweaked to be within tolerance and some strays.





Quote:
The four devices discussed (RS 15-0577, 15-0024, AD FMFLT, and FM-88) ...
Similar findings here on a few RS Trap units, and some other different branded although nearly identical traps.
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