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19th Nov 2021, 12:05 pm | #1 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Wobbulator for FM Alignment
Morning all. I have a thread going concerning an Armstrong Jubilee MK2 receiver here https://www.vintage-radio.net/forum/...d.php?t=185310. I have reached the point where, to improve the FM alignment, I need a wobbulator. I have located the article by Raymond Haigh in Radio Bygones No.82, April/May 2003 and I have also read this closed thread https://www.vintage-radio.net/forum/...d.php?t=148303 that indicates without the Toko coils, which are unobtainium, the project is a non-starter. Well I only need to cover the 10.7 MHz range therefore I only need the pink coil KXNK3767EK. Data lookup shows this to be 1.2uH with Q of 80. Is there any reason why the Spectrum coil 1u2HL (0.7-1.4 uH, Q of 69) wouldn't be suitable? I still have to obtain a varicap diode KV1236 or equivalent, and find a means of getting a small quantity PCB produced from the artwork so advice on those topics also welcomed. Cheers, Jerry
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19th Nov 2021, 2:16 pm | #2 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: Wobbulator for FM Alignment
I'm going off this idea somewhat. I've discovered that my scope (a Tektronix T912) doesn't appear to have a sawtooth timebase output, and I don't fancy much digging about inside it to extract the signal. Therefore that would mean I'd also have to build a sweep generator to drive the wobbulator and simultaneously trigger the scope, presumably.
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19th Nov 2021, 4:30 pm | #3 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
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Re: Wobbulator for FM Alignment
The ramp generator could be as simple as a 555 plus a few simple parts so not too complex especially as you’re doing the more difficult bit.
I gave up with analogue wobulators so built a sweeper using a dds board and arduino, all inside a biscuit tin for screening. The parts were then reused in another project. Ken |
19th Nov 2021, 4:43 pm | #4 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: Wobbulator for FM Alignment
Thanks Ken, fair point. I've been offered the loan of a Marconi TF995B2 by a local Forum member so I'm collecting that tomorrow. It provides a decent modulated FM signal which will be an improvement over what I've been doing up to now. Whether I can also use the sync output of the sig gen to drive the X input of the scope and plot the IF curve remains to be seen. Cheers, Jerry
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20th Nov 2021, 12:48 am | #5 |
Heptode
Join Date: Feb 2006
Location: Marlborough, Wiltshire, UK.
Posts: 915
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Re: Wobbulator for FM Alignment
Hi Jerry, I guess just use the sig gen sync output to trigger the scopes sweep (so connect to ext trig input) then adjust scope timebase to sweep just a little faster than the sig gen does. Good luck.
Ken |
20th Nov 2021, 12:09 pm | #6 |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,296
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Re: Wobbulator for FM Alignment
Have a look at this thread, I have found substitutes for the coils and have a design for the timebase.
https://www.vintage-radio.net/forum/...d.php?t=172685 I am thinking of designing a pcb for the combined wobbulator and timebase. I will open a separate post. Peter |
21st Nov 2021, 3:37 pm | #7 |
Nonode
Join Date: Oct 2017
Location: Bristol, UK.
Posts: 2,384
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Re: Wobbulator for FM Alignment
In the light of the above, would mods please close this thread. Thanks, Jerry
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