My thinking is that a 50 mHz scope can't be expected to see 100 mHz.

Also, I don't understand the meaning of your last sentence. Are you getting a clean carrier when no modulation is applied?

Dave

Thanks for the prompt reply Dave. No, I am not getting a clean carrier even with no modulation applied. By that I mean I am not plugging anything into the audio input jack of the transmitter. By the way, I presently have connected just a plain white wire from the antenna pick up coil (that's located at the front of the enclosure) to the hobby whip antenna (that's located in the rear of the enclosure) as per a member's suggestion. In addition, I made the termination of the 47ohm resistor & the 1000pF cap at 3.5 turns from the RF coil bottom, and not 4 turns (if that makes any appreciable difference). The 0.4uF RF coil that I am using is a 10 turn coil as yours.

As far as the 50 MHz oscillscope, I was hoping at least to see a "streak" or a "ghost" of signal output on the scope, but of course it's not assured. You could be right on that score.

Moreover, the only FM radio that I had which was able to receive the transmitted signal was the Califone AM/FM/CD boom box. I was unable to receive anything on my 1956 Telefunken AM/FM/SW radio and my 1951 Philco AM/FM/Phono console radio.

It would depend on how sharp the scopes rolloff is but, just out of curiousity, I tried my 20 MHz Hitachi and, with everything gained up, could 'tell', barely, that 100 Mhz was there.

For FM my scope is a 300 MHz Tek 485 (albeit with 100 MHz passive probes) but any direct probe connection kills my, granted different, osc. After all, a typical 10x passive probe is 20 pF and that ain't chicken feed at 100 MHz. Besides affecting tank resonance, probe impedance is dramatically lower and your "10 Meg" probe may have an effective impedance under 1k at 100 MHz.

I probe 'air' near the tank coil. That doesn't give me circuit voltages but at least I can tell if it's running and at what frequency.

Thanks for the handy tip Flipperhome. That's something to remember.

I am still confused about this statement. Does this mean that your transmitter is blanketing the entire band and messing up the normal reception of FM stations?

I question the use of a coil wound around a torroid. I would think it should have far fewer turns than yours seems to have.

Dave

http://www.surplussales.com/Inductors/I ... ils-3.html

Hi Dave. What I mean is that I can barely pick up the carrier signal on the FM band of the Califone AM/FM boom box at times. When I do, the signal seems to be very weak. At other times, I can't pick up the carrier at all. I have the Califone boom box located about 5 ft away from the transmitter.

I think that the first thing that I will do now is to check the 6AU6 and the two 6C4 tubes out on the Conar tube tester. I did not check out those tubes previously. Then I will do some stabbing around in there.

I purchased the 0.4uH RF 10 turn coil from SurplusSales.com (as per the above link). The Cat. No. for the coil was (ICH)TOR.4. I cannot afirm that it is indeed a 0.4uH coil as my meter will not register such a small value. I did purchase three of those particular coils from SurplusSales.com, and they all look the same.

I might also temporarily disconnect part of the air variable cap from the circuit too, and tack in a small variable cap in its place .

It's also possible that I might try tacking in a somewhat different resistance than the present 10K grid resistor, and see what happens. Thanks Dave.

How did you conclude that a .4 uH toroid coil was the correct type of coil to use. How did you decide where to place the B+ tap?

I'm not saying a toroidal coil can't be used in this circuit...just wondering. My instinct stays your coil may have too many turns.

Dave

Dave said: How did you conclude that a .4 uH toroid coil was the correct type of coil to use. How did you decide where to place the B+ tap?

I'm not saying a toroidal coil can't be used in this circuit...just wondering. My instinct stays your coil may have too many turns.

Thanks for the reply Dave: I didn't see why a 0.4uH toroid couldn't be used in this application. As far as the B+ tap, I placed it more or less as to where your's was positioned. You may be correct in that the coil may have too many turns. As I stated previously, the value of the coil was too low for my meter to measure it. Well, I suppose I can always try something else other than the toroid. Thanks.

The oscillator section of the "New Band" FM transmitter that I built some weeks ago works fine, and I did observe something in the operation that might explain the audio distortion that I was recently experiencing with this device.

It seems that the patchcord (with one 1/8" phone plug on each end) which was connected between the headphone jack of my Califone AM/FM/CD boom box and the audio input of the "New Band" FM transmitter always had to be physically laid out in a certain way. If not, there would either be no transmitted audio or the audio would be distorted. Even if the patchcord was laid out in a certain fashion (usually consisting of a half loop somewhere), I would still lose either the audio, or the audio would become distorted after just a few minutes of operation.

When I attempted to connect a shielded RCA cable (with 1/8" phone jacks on each end) between the Califone boom box and the audio input of the transmitter, I would always receive no transmitted audio.

The transmitting power did jump up significantly (while delivering some decent audio) whenever I would both lay out the patchcord in a certain position, and then put two adjacent fingers of my hand over the patchcord (w/o touching the patchcord) while also moving my body in a certain way.

Moreover, if I touched the patchcord while the transmitter was in operation, the transmitted audio would immediately become mute. I could imagine that all this is probably due to the high frequencies involved with this transmitter, but I'm not sure at this point what I could do to correct this.

Sounds like the preamp stage is being driven into cut-off by the strong RF signal from the oscillator. I noticed that you built your circuit using perf-board mounted in a plastic case. I had assumed that you used a metal case. Good grounding and shielding is quite important when dealing with intense 100 mhz frequencies. I believe someone else mentioned wire length and layout in the other thread where your transmitter was being discussed. At this point, you might try connecting a 100 PF mica cap from pin 1 (grid) to pin 7 (cathode) of the 6AU6 to see if you can bypass RF that may be swamping the tube's grid, keeping the leads as short as possible.

Dave

Thanks for the very helpful information Dave. I'll try that. I also noticed that I have the .02uF cap lead (that's leading to the 6AU6 control grid) located somewhat close to the antenna pick up coil lead, and that could have some bearing too. Yes, the RF from the oscillator is incredibly strong. There's certainly no doubt about that.

Pictured is the oscillator & audio section of the "New Band" FM transmitter that I built.

Yes, indeed, the antenna lead should be kept away from the 6AU6 socket as far as possible.

Dave

I may build one of these, but what I really want is a device which will receive several channels of new FM, then output all of them on old FM. Say nice channels selected for the inputs, and nice outputs - 42, 43, 44, 45, 46, 47, 48, 49, and 50. Yes, it would be possible to build separate single-channel units, but a combo device would be nicer for use with the old radios. I would not expect anyone would want to do it with tubes, as it would then also be a space heater (nice receivers, nine transmitters).

I'm aware of the adapter which uses harmonics, and has the stations scattered on the old FM dial, but it would be quite difficult to implement on some radios which use band switching to share front-end compo, and it would ne good to have the stations in the proper order on the dial.

You could build a wideband coverter but the entire new FM band will not fit into the spectrum of the old band. I think the converters offered in the late '40's and perhaps '50's had a switch to select which portion of the new band you wanted.

Dave

The one I saw had three bands.

Did the stations tune in reverse order on the old band set?

Scott

I never used it and, so, an't say much about it's operation. All I remember is there was a switch for the three bands.

Why would you think it tuned in reverse order?

It would depend upon whether the local oscillator runs above or below the present FM band. If it's above, then do the math: L.O. - 108 < L.O. - 88, meaning the higher up on the new dial the lower it'll be on the old. If it runs below the FM band, you'd likely have at least one setting where the L.O. frequency is actually running IN the old band.

Scott Todd

Oh, I see what you mean.
I never had a pre-war FM tuner so the issue never came up but they had some novel ideas back then.
Here's one with no (additional) LO. http://somerset.net/arm/reprints/one_diode_fm_converter.gif