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Hi Martin,
I measured the IF signal at the TR8 collector (finally I get maybe a bit of a grasp how a FM tuner is working...;-). I get a ~5.8V 10.8MHz signal when tuned into a local station (signal strength: ~4 on the meter). Then after the filter (#101) at point D this signal vanishes...all that is remaining is a very weak 30mV 10.8MHz signal. This explains why there is nothing going on at the base of TR9. Does this mean the filter has a problem? Thanks!
Rudy
Either that or something is loading down the voltage at the right side of the filter.
A filter of this type normally has a loss of around -6dB(half voltage)
I would try a new filter.But just for testing almost any old 10.7 Mhz CF will do.Even a cap can work as a temporary solution.
I think the circuit is o.k. around the filter, i.e. I would like to try replacing it. Since I do not have a filter laying around, the capacitor sounds appealing...;-). Are you suggesting to put a cap between the filter in/outputs and GND to form a 10.7MHz RC lowpass with R97 and R98? C=1/(2piRf)=20pF? I know, I probably just know enough about RF to be 'dangerous'...;-). Thanks much in advance for advice!
Rudy
PS.: With regard to replacement filter choice, I found this datasheet for Murata 10.7MHz IF filters: http://www.jameco.com/Jameco/Products/ProdDS/1914365.pdf
Are these adequate? What bandwidth and "center frequency/tolerance" should I choose?
One more: The service manual also makes this cryptic statement:
Is this related to 1972 component tolerances, or to be taken seriously? Thanks!!
Excellent advice!! I put a 5 pF cap across the suspected filter, and things got much better! The S-lights now work, and also the volume is much stronger now. The stereo light works, too. The question remains, which is the correct replacement filter, and should one exchange all three? Thanks for guiding me through this!
Rudy
The question remains, which is the correct replacement filter, and should one exchange all three?
If you don't have the equipment & skills for a complete RF/IF alignment, replace as little as possible. I would expect modern components to have close enough tolerances so that you don't necessarily need to touch anything else.
Yes...excellent advice! But I definitely would like to replace the filter. It seems there is not much to align in the IF section (they are actually proud of this engineering feat in the manual...;-)...The signal reduction across my emergency capacitor is definitely not like the Martin prescribed -6dB more like -15dB. Unfortunately the manual does not give any spec on the 3dB Bandwidth of the filter and the center frequency/tolerance parameters given for 'modern' IF filters (http://www.jameco.com/Jameco/Products/ProdDS/1914365.pdf). Any advice with regard to selection, or whatever part has worked as replacement in the past would be very welcome! Thanks much in advance!
Rudy
I presume the filters are Murata.What exactly does it say on the filter and what colour is the dot(if any)?With these data it should be possible to find out both bandwidth and center frequency.
In those days it was not possible to make CFs so that all resonated exactly on 10,7Mhz.That's why they are divided into groups with the same center frequency and a corresponding colour dot.Red dot is spot on 10,7 if I remember correctly.
I have catalogs on Murata filters.
And yes,all three must come from the same group.
So-find an exact replacement for the non-working(NOS or new),or replace all three.
They have red dots. The info I found on Murata filters groups them by capital letters, i.e. the 'dot color code' is not transparent to me right now. So, any advice from your catalogs would be much appreciated!!
Can one say that the necessary minimum bandwidth of the filters depends on the frequency precision/stability of the TR3 oscillator that is used to generate the IF carrier f(RF)-F(TR3)? If TR3 is off by some margin then the IF frequency could be filtered out if the bandwidth is too aggressive?
Thanks much for your input!
Rudy
Hi
If it helps I have a red set of filters x 3 ( I think red ) that work the amp had a issue if you want them they are from a later model but look the same
PM if of any help Regards
Again-what does it say on the filters?
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