FM10 cannot adjust below 95.5Mhz

[Cyfun]

I apologize in advance for the noob question, but I've got this FM10 that I've had kicking around for several years, and I cannot get the thing to broadcast at a frequency lower than 95.5Mhz. As far as I can remember, it's been like that since I made it. I'm not sure if it's supposed to be this way, but when I adjust the L1 frequency adjustment thing, the frequency bottoms out when the screw is in the middle, and if I turn it in or out, the frequency goes up. Any suggestions?

[RobertF]

I'm not familiar with the 10, but if it works but doesn't behave like it should, I'd look for a couple of swapped parts. One is no doubt near the frequency adjustment since that isn't working correctly. Have you verified that everyone is in their correct location? No solder bridges and good shiny solder joints everywhere?

[RDEngineer]

I assume you have the manual but if you don't then you can download it from our site. TO answer your question, C18 and R8 must be changed in order to cover different parts of the band. For 88-91 MHz they are 27pF and 180K, 990-97MHz use 22pF and 220K and for 95-108MHz use 15pF and 470K. This is explained in the manual on page 16.

[Cyfun]

Whoa, thanks for the insanely prompt replies, guys! Yeah, the manual was misplaced long ago. Didn't suspect that it was actually designed like that. Thanks, RDEngineer, I shall rtfm.

[krankshaft]

Also I have personally had problems with 10% ceramic tuning caps.

Sometimes you get a good batch and their tolerence is fine but sometimes they prevent the transmitter from tuning in the appropriate range. To make matters worse ceramic capacitors aren't very temperature stable and can vary slightly in capacitance with temperature.

The repair was replacing them with silver mica caps which are 1% tolerance and it also gave the FM10 some temperature stability.

The ceramic caps themselves are fine and within manufactured tolerances 10% however sometimes their actual value is either too high or too low and affects the tuning range.

[Nemesis]

Originally Posted by RDEngineer

I assume you have the manual but if you don't then you can download it from our site. TO answer your question, C18 and R8 must be changed in order to cover different parts of the band. For 88-91 MHz they are 27pF and 180K, 990-97MHz use 22pF and 220K and for 95-108MHz use 15pF and 470K. This is explained in the manual on page 16.

Isn't it C16 that has to be changed to cover the different parts of the band? Also when was R8 revised to also be changed? I have a fm10a I bought 8 years ago that came with just a 470k resistor and the manual never said to change R8 to the other stated values.
I have never had a problem tuning the FM10 using the 470k resistor. What are the pluses in changing R8 also?

Thank you

[Ramsey Tech Z99]

You need to be careful with reference designations between different revisions. The FM10, FM10A, and FM10C all have different reference designations.

Different variable inductors have been used over time for the RF oscillator circuit due to availability. This means that the capacitor values used for the three tuning bands will be different. It is important that you reference the manual that came with your specific unit. If you no longer have your manual you may contact tech support for help. You will need to provide us with the current capacitor being used and the band you are transmitting in. With that information we will be able to give you the values of the other capacitors.

Changing the value of R8 in the FM10C compensates for the uneven modulation sensitivity of the BA1404 by changing the stereo pilot’s deviation. It keeps the pilot deviation as close to +/- 6.75k kHz as possible