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Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment. |
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30th Oct 2017, 2:21 pm | #1 |
Triode
Join Date: Feb 2009
Location: Port Elizabeth, Eastern Cape, South Africa
Posts: 21
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FT225 Oscillator
Does anyone have experience or knowledge of the "Local" Oscillator board? This is the one which generates the receiver oscillator from a set of four crystals all one MHZ apart, one per band.
The crystals all work well, as do the outputs from the first tripler. But the output from the second tripler (around 125 MHz) is way down. The transistor concerned, a C784, checks out OK, but the adjustment of the final tuning capacitor is super-critical to get the rest of the circuitry going. If I set it up on the extender board, it doesn't work in its connector in the set. But I'm not sure what voltage output I should be seeing. My scope is a Philips 100MHz PM 3262(?) and and shows about half level at 125MHZ compared to 70MHz from a signal generator Any suggestions? |
30th Oct 2017, 6:31 pm | #2 |
Rest in Peace
Join Date: Feb 2004
Location: Hexham, Northumberland, UK.
Posts: 2,234
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Re: FT225 Oscllator
Using extender boards at RF can be troublesome, particularly when tuning is critical. You may also be getting spurious results by checking a 125MHz signal with a scope that only goes to 100MHz. The other way to check for signals is by using a scanner tuned to the harmonic frequency and compare levels that way. Even better if the scanner has a signal strength meter. I have one of those cheap handheld spectrum analysers and it is worth it's weight in gold for diagnosing faults like this, as you can see immediately if the stage is becoming unstable when tuning up. If tuning has become too critical and sensitive, it could be that a parallel damping resistor has failed, if there is one. I am not familiar with the circuit you have unfortunately.
Alan. |
31st Oct 2017, 4:09 pm | #3 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: FT225 Oscillator
I believe this board will be the same as an FT221 Xtal + Multiplier.
That has a diode detector at the output for use when tuning - connect DC Voltmeter to TP03. Sorry I don't know what a healthy reading would be until home with my own FT221. It is a waste of time trying to tune output stage into the extender board except to prove you can get something. It will all change as soon as it is back in the set. Probably OK for tuning the mid stages though. |
2nd Nov 2017, 3:02 pm | #4 |
Triode
Join Date: Feb 2009
Location: Port Elizabeth, Eastern Cape, South Africa
Posts: 21
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Re: FT225 Oscillator
Thanks for the tips! Yes, I agree that on the extender board things would be different - but when the board is in place the final tweak is absolutely inaccessible!
My TF995 generator goes up to 200Mhz, and I see that the scope gives about 50% reading at 125 MHz.. So, short of someone coming up with a miracle cure ("oh, yes, I had one of those and the cure is........") What I am going to try is injecting a 42MHz (3x 14MHZ) signal into the final tripler and see what voltage is needed to drive it, and produce the 125MHz. I am thinking that the final tweak is probably designed for a greater drive voltage than it is getting. The input and output of the transistor look like nice sine waves at the moment. I'll have look at the other FT2xxx books and see what they say. |
2nd Nov 2017, 3:28 pm | #5 |
Triode
Join Date: Feb 2009
Location: Port Elizabeth, Eastern Cape, South Africa
Posts: 21
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Re: FT225 Oscillator
Right!
Now I have looked at the FT221 version of the circuit, I see that it is almost identical. So, if someone would be kind enough to let me know what sort of voltage can be expected on the output of the Local board, I would be very grateful. ZS2PG Paul |
3rd Nov 2017, 12:12 pm | #6 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: FT225 Oscillator
Now found a sch for a 225 version. They changed it somewhat and now I understand why your final stage is tweaked by capacitor and not an inductor as on my 221s.
They broadbanded it so now you have varicap diodes doing fine adjust on each multiplier stage tuned circuit. That means you need the tune Voltage correct when you adjust. No doubt there was some published procedure for alignment. Worth checking what Voltage you have on Pin 21 - the control line - in different bandswitch positions. Should rise in fairly regular steps with selecting each higher frequency band segment. Most likely your problem is with one/more of D14,15,16. Output test point is now TP02 - still with diode detector though. |
3rd Nov 2017, 5:46 pm | #7 |
Nonode
Join Date: Nov 2015
Location: Welwyn Garden City, Hertfordshire, UK.
Posts: 2,015
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Re: FT225 Oscillator
I have fished out my 2m FT221 and measured this now. I see about 0.1-0.2V depending on which band is selected. (Measured with board in situ so no extender board effects.)
I expected more but resisting the temptation to fiddle. Set works OK so not touching it! Previously tried the same on my 4m FT221 - that failed. The test point diode is removed so it is an RF pick off only on that one. |