Sorry...I do have a day job...;-). It says SFC 10.7MA on one side and MURATA X (or I) on the other. Below are a couple pics of the filter. I downloaded the current Murata filter catalog (http://www.murata.com/products/catalog/pdf/p50e.pdf#SFELF). If the current naming convention is the same as in '71, then this may mean the filter has a 50kHz band width and the center frequency is exactly 10.7MHz (see page 2 "CERAFIL for FM"). Thanks!
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Beomaster 3000-2 Tuner: S (arrow)-Lamps and Stereo Indicator Not working
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For some reason I couldn't open your link.
Unfortunately my catalogs start with the year 1975,and by then the SFC types were gone and the new smaller SFD types had come.
But I found this interesting pdf:
http://html.alldatasheet.com/html-pdf/193810/ABRACON/ASFC10.7MA/297/1/ASFC10.7MA.html
Could this be applied to old Murata filters?
If so,your filters are 280kHz which seems OK since there are three- the real bandwidth becomes narrower when cascading filters.
According to my catalogs you could also use the later SFE10.7MA15-Z,or SFE10.7MA5.Both are 280kHz.
The color codes:
I think you are right: A 280kHz band width filter is what fits. I verified this by extracting one of the two working filters and measuring its frequency characteristics. This is the circuit I used. Both my waveform generator and spectrum analyzer have 50 Ohm out/inputs, i.e. I used 270 Ohm resistors for R1/2 to match the 330Ohm impedance of the filter (it apparently has 330 Ohm - see radiomuseum.org: http://www.radiomuseum.org/tubes/tube_sfc-107ma.html):
In the flesh:
The measured spectrum (x-axis goes from 10.2 to 11.2 MHz, i.e. it is 0.1 MHz/decade; y-axis from 0 to -50 dBm):
We can see here that the filter is actually not exactly centered at 10.7 MHz but rather at about 10.77 MHz. The -3dB width of the spectrum ranges from about 10.65 to 10.92 MHz, i.e. the width is about 270kHz. This suggests that the filter is of the 280kHz variety.
I found in the Murata filter sheet their CERAFIL "Standard Lead Type SFELF Series". They come in 180, 230, and 280 kHz widths:
And their frequency response:
It appears the FA type has a pretty similar response like the SFC 10.7MA. I will obtain a few and report back after I put them in!
I got the new filters (SFELF10M7FA00 from Digikey). They have the 280kHz band pass. The first thing I did was to measure their frequency response and compare them with the old filter. This is what I got:
The old filter is shown in black, and the new one is in red. It is obvious that they do not have the same center frequency. The new one is much better centered at 10.7 MHz. I was worried that using these new filters would result in an increased loss in the IF stage. To test whether the shifted pass window would affect the gain in the IF stage, I first replaced the first two filters between the initial two amplification stages (TR5 and TR6). Before the exchange I measured the 10.7 MHz amplitude at the collector of TR5 (point B) and after passing through the filters at the base of TR6. When tuned into a strong station (>4 on the meter) then the TR5 collector signal was about 250 mV and after the filters I measured about 80 mV.
After exchanging the two filters I measured pretty much the same values. This gave me confidence that replacing the third (broken) filter should work. I did, and the radio plays now loud and clear. Also the S-lamps are now much more responsive...no surprise considering that they get their signal after this filter. The amplitude at point D after the filter measures now about 2.5 V compared to only about 50-70 mV before, i.e. a significant improvement in signal strength resulted from exchanging the filter.
However, a new phenomenon occurred: Now the stereo light is always on, even between stations. I measured the 38kHz subcarrier signal at the collector of TR21:
This photo shows the signal when a strong station is tuned in (it is a bit stronger than what the manual suggests - 12.4 vs. 10V peak-to-peak voltage):
This is what I measured in-between stations (only noise in the speakers, meter at ~2)
The signal fluctuates, but s still pretty strong. TR22, which seems to control the stereo light, is always on no matter if a station is tuned or not.
When I pull the antenna, then the stereo light does go off (strong stations are still audible at a very low level, but the meter only shows a level of approx.1).
I wonder: Are today's stations just too close/strong? I looked at the relevant adjustment steps in the "adjustment plan" of the manual, but it seems the decoder is in pretty good alignment...the subcarrier is too strong if anything.
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