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Old 9th Dec 2009, 8:03 pm   #1
dr peppers
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Default rf generator project on this site.

Hi I've made a copy of the sig gen from elsewhere on this site, and cant get it to work.
The rf sections fine, the 1kc oscillator is a pain, with the 22nf cap out of the board and the switch in the external mod position the osc works fine with a 8v p to p 1kc sine wave, as soon as i put the 22nf back in place or switch to internal mod the oscillator stops dead, tried a different transistor, tried different 4n7's cant get it to work on load.
Might make a oscillator out of a 741 or use the 741 to buffer this one.
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Old 9th Dec 2009, 8:14 pm   #2
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Default Re: rf generator project on this site.

sounds like the coupling is too much and is sinking the Osc enough to quench it.
have you got the cct diag?
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Old 9th Dec 2009, 8:40 pm   #3
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Default Re: rf generator project on this site.

Ok I dont think this will be a violation of site rules, it came from here.

Initially my coupling cap was 1uf, but I've now put the correct 0.1u in circuit, no change.

http://www.landyzone.co.uk/photopost...en-circuit.gif
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Old 9th Dec 2009, 9:54 pm   #4
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Default Re: rf generator project on this site.

I don't understand what c13 is suppose to do

Is there an error on the schematic, ?
maybe it should be 22pF

edit:
Actually 22n impedance is about 7K at 1khz so I guess its a low pass filter.
Check C14 and R15 are in circuit and working.

dc

Last edited by dave cox; 9th Dec 2009 at 10:04 pm.
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Old 9th Dec 2009, 10:13 pm   #5
YT2095UK
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Default Re: rf generator project on this site.

nor me, it`s certainly not a Coupling cap, Tr13/14 are set up as a bistable multi-vib, the coupling is via R14, the only thing C13 can do it drag it down, a smaller cap will smooth any Spikes from switching.

looks like a possible mistake.
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Old 9th Dec 2009, 10:33 pm   #6
Brian R Pateman
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Default Re: rf generator project on this site.

Here's a quote from the description of the project;

Quote:
John Shepherd has built this project and made the following comments:

I had to reduce the value of R18 to 180K to get it to oscillate reliably.

Also is the value of C13 right at 22nF - seems a bit high and it attenuates the signal too much on my version.

The audio oscillator is quite particular about the types of capacitor used for C15, 16 and 17. I used ceramic discs - and with these 220K for R18 was fine. Maybe the resistor needs to be adjusted to suit the transistor used?

22nF is the value I used for C13 - it is intended to improve the waveshape and will attenuate the signal a bit.

Has anyone else had the same problems as John?
So it would seem that 22nF works in some circumstances. What type of capacitors have you used in the "critical" positions?

The full information is on Paul's main site here.
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Old 10th Dec 2009, 12:04 am   #7
AlanBeckett
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Default Re: rf generator project on this site.

YT2095UK,
Am I looking at a different circuit? I can't even find a TR13/14!
TR3 and TR4 are Emitter followers to buffer the af oscillator, and R14 provides a bit more isolation.
C13 looks far too big to me.
Alan
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Old 10th Dec 2009, 12:23 am   #8
Denis G4DWC
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Default Re: rf generator project on this site.

Quote:
Originally Posted by YT2095UK View Post
nor me, it`s certainly not a Coupling cap, Tr13/14 are set up as a bistable multi-vib, the coupling is via R14, the only thing C13 can do it drag it down, a smaller cap will smooth any Spikes from switching.

looks like a possible mistake.
Hmmmmm......I don't see a TR13/Tr14 and I don't see a multivibrator

TR5 is a low frequency Phase shift Oscillator with the frequency dependant feedback provided by the three capacitors C15, C16 and C17 along with the associated 27k resistors. R14/C13 is undoubtedly a low pass filter as has been said.
Tr4 is an emitter follower to buffer the oscillator output to the Audio O/P skt and TR3 is a series modulator that modulates the supply to the RF oscillator.

If the oscillator stops when loaded it is probably an indication of insufficient loop gain.
If it were me, as a start, I would probably replace TR5 with a higher gain transistor such as a BC109 or reduce the value of R15 to increase the loop gain.

JMHO
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Old 10th Dec 2009, 2:11 am   #9
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Default Re: rf generator project on this site.

Quote:
Originally Posted by Denis G4DWC View Post
If it were me, as a start, I would probably replace TR5 with a higher gain transistor such as a BC109 or reduce the value of R15 to increase the loop gain.
Or hand select a transistor for high gain. You only need a cheapo DMM hfe tester to do this. If you've got a BC548 at the bottom end of the hfe range, that could indeed explain the problem.

Paul
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Old 10th Dec 2009, 2:57 am   #10
DangerMan
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Default Re: rf generator project on this site.

It seems to me that the above posts are on the right track.
An alternative route to a fix could be by increasing R14 and reducing C13 by the same factor, this will reduce the loading on the audio oscillator. As this filter feeds two emitter followers, they will present a fairly high impedance to the filter anyway, so similar results can be had to the existing design, and in any case, the filter's turnover frequency won't be critical .. (i.e. if you can't use exactly the same factor, it doesn't matter too much)
A 56k resistor plus 4.7 nF for these components might be a good place to start.
Alternatively, you could add a buffer to the oscillator (another follower, same as R12, R13, TR4 feeding the extra R12 directly from TR5 collector and R14 from the extra transistor's emitter). Not so easy to do if you've made a nice pcb though.
TTFN
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Old 10th Dec 2009, 2:14 pm   #11
YT2095UK
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Red face Re: rf generator project on this site.

Quote:
Originally Posted by AlanBeckett View Post
YT2095UK,
Am I looking at a different circuit? I can't even find a TR13/14!
TR3 and TR4 are Emitter followers to buffer the af oscillator, and R14 provides a bit more isolation.
C13 looks far too big to me.
Alan
Quite right, I asked my wife to check and you`re all quite right, Mr Magoo strikes again
it`s been getting progessively worse over the years but happily I decided to bite the bullet, and now have an appointment at the opticians this afternoon, I can`t carry on like this, it`s getting silly now

apollogies to the OP.
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Old 10th Dec 2009, 3:28 pm   #12
dr peppers
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Default Re: rf generator project on this site.

Thanks people.
The gain issue sounds like it could be on the right track.
The tranny I used for the oscillator is new from cpc and came in production packaging, that said the date code goes back to the 80's, so it could well be on the low side.
I'll replace the tranny later when everyones gone home, I have some plastic bc109's.
Just looking at the data sheets for bc458's, theres a big diffo in hfe for the 548a, 548b and 548c, the 548 and 548a being a 100 min, and the c being over 400, obviously the bc548c is a better choice.

Edit: Ok I'm a wally, it clearly states on the parts list that a bc548C is used not a bc548.

Last edited by dr peppers; 10th Dec 2009 at 3:47 pm.
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Old 10th Dec 2009, 8:15 pm   #13
dr peppers
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Default Re: rf generator project on this site.

Thanks for your help people, I have tried the as mentioned mods and the '109, still can get anything reliable, I've spent more than enough time on this project so the circuit now resides in the scrap bin, looking at a couple of gens on ebay, should have gone that route in the first place.
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