Help with Darius's Channel 1 Modulator

[Philips210]

Hi everyone.

I have finally got around to building Darius's Channel 1 modulator and was wondering if anyone here can help with a few questions regarding the circuit
?
I appreciate that this may have been covered before but can't seem to find the information that I'm looking for.

1) I assume the oscillator coils are five turns on a 6mm former spaced about 1mm apart initially. Obviously then adjust to obtain the correct frequency, wire gauge being 20 SWG ECW?

2) There is a parallel LCR network connecting the +5v supply to the mixer stages, I am unsure about the coil details here. From the original photographs it looks to be around four turns but the diameter looks greater than the oscillator coils.

3) I am not sure but does the 18pF capacitor associated with Q64, originally shown to be connected to the chassis, infact go to the emitter of Q64?

4) Also does the 18pF capacitor associated with Q63, originally shown to be connected to the chassis, infact go to the emitter of Q63?

5) Would BF199 transistors be suitable replacements for the BF496?

6) On the separate bandpass filter, ( London, 41.25MHz to 46.25MHz) are the coils six turns on a 5mm former 20 SWG ECW? The turn spacings look to be nominally 1mm.

Finally, in this bandpass filter, are the stage coupling capacitor values 8.2pF? The reason I ask this is I have seen 10pF caps in one of the photos.

Any help on this or an up to date circuit diagram would be greatly appreciated.

Regards
Symon.

[HamishBoxer]

Re BF496 equivs are BF225,314,502,503 and 505.Not BF199 .

Cheers David

[Philips210]

Originally Posted by HamishBoxer

Re BF496 equivs are BF225,314,502,503 and 505.Not BF199 .

Cheers David

Thanks David, I think I may have some BF225s around somewhere.

Regards
Symon.

[Sean Williams]

Ive built one of these.....

Oscillator coils sound about right, think I used 8mm dia, and 18swg wire - it really isnt that critical - squeezing and stretching the coil gives massive tuning range.

I do not recall deviating from Darius' original design - it produced oodles of RF!

As for the BPFs....

Mine was built to that design, and were nowhere near right - I ended up using an 8mm mandrel. and 5pf coupling caps, using trimmers to adjust the filter sections while checking the response on a Spectrum Analyser.

Good Luck, it is a repeatable design, but will need one or two tweaks once built to make it right, especially if substituting components.

Sean

[Nickthedentist]

Darius himself occasionally looks in here... his username is "Oldeurope"

[Philips210]

Originally Posted by Sean Williams

Ive built one of these.....

Oscillator coils sound about right, think I used 8mm dia, and 18swg wire - it really isnt that critical - squeezing and stretching the coil gives massive tuning range.

I do not recall deviating from Darius' original design - it produced oodles of RF!

As for the BPFs....

Mine was built to that design, and were nowhere near right - I ended up using an 8mm mandrel. and 5pf coupling caps, using trimmers to adjust the filter sections while checking the response on a Spectrum Analyser.

Good Luck, it is a repeatable design, but will need one or two tweaks once built to make it right, especially if substituting components.

Sean

Hello Sean.

That's reassuring about the oscillator coils being tunable over a fairly wide range so at least that part of the coil winding isn't too critical, only the setting up is. I do have access to a spectrum analyser so it should't be too difficult (hopefully) to set the oscillators frequency.
The band pass filter looks to be more of a challenge. Incidentally, how critical is the coil tapping position on the input and output parts of the BPF?
Did you include the LCR filter in the supply feed to the mixer stages or is it not necessary?
Thanks for your advice.

Regards
Symon.

[peter_scott]

Originally Posted by Philips210

1) I assume the oscillator coils are five turns on a 6mm former spaced about 1mm apart initially. Obviously then adjust to obtain the correct frequency, wire gauge being 20 SWG ECW?

Just so

2) There is a parallel LCR network connecting the +5v supply to the mixer stages, I am unsure about the coil details here. From the original photographs it looks to be around four turns but the diameter looks greater than the oscillator coils.

This should be centred over the 41.5 to 45 MHz range. Looking at mine I have 7 turns whereas my oscillators have 6 and 5 turns respectively but best to suck it and see in your own layout.

3) I am not sure but does the 18pF capacitor associated with Q64, originally shown to be connected to the chassis, infact go to the emitter of Q64?

I think my 18p goes to ground.

4) Also does the 18pF capacitor associated with Q63, originally shown to be connected to the chassis, infact go to the emitter of Q63?

Again mine goes to ground.

5) Would BF199 transistors be suitable replacements for the BF496?

You've so much gain bandwidth in hand I'm sure BF199 would be usable.

6) On the separate bandpass filter, ( London, 41.25MHz to 46.25MHz) are the coils six turns on a 5mm former 20 SWG ECW? The turn spacings look to be nominally 1mm.

Finally, in this bandpass filter, are the stage coupling capacitor values 8.2pF? The reason I ask this is I have seen 10pF caps in one of the photos.

Sorry, I didn't include the bandpass in mine.

Peter

[Philips210]

Hi Peter.

Thank you for clarifying these points, I have more confidence now to proceed with the construction of the modulator.


Regards
Symon.

[Philips210]

Hi again.

I have nearly finished building the channel 1 modulator and was wondering about the 47uF bipolar electrolytic capacitor in the video section. The bipolar types that I have come across are physically too large for my circuit. I think these are more intended for loudspeaker cross-over networks. I do remember seeing smallish radial lead BP electrolytics a while back but they aren't so easy to find now. I had thought about connecting two ordinary 100uF (polarised electrolytics) in series -ve to -ve to obtain a 50uF BP type. The question is would this be in order? If this is acceptable then what would be a suitable voltage rating for the individual capacitors or should I try and obtain the correct BP cap instead?
I have also finished building the two PCBs for Darius's Standards converter and hope to test this out in the next month or so if I can spare the time that is!!

Regards
Symon.

Regards
Symon

[peter_scott]

Hi Symon,

I think Darius is being a little over-cautious with that capacitor. I just used a polarised component. I haven't measured it with a signal present but mine is sitting with about 0.5 volts of forward bias on it. I think mine is 25 volt rated.

Others may disagree but in my experience liquid tantalum caps can't survive with any reverse bias but solid tantalums and aluminium electrolytics can tolerate small negative bias, especially if this is just small signals sitting on a positive bias.

Peter

[Philips210]

Hi Peter.

Interesting point. I really wondered how critical it was to use a BP electrolytic. I was only concerned about the reverse polarity situation leading to excessive leakage current.

Thanks for your reply.

Symon.

[Philips210]

Originally Posted by Nickthedentist

Darius himself occasionally looks in here... his username is "Oldeurope"

Hi Nick.

I have not seen any postings from Darius (Oldeurope) for quite some time now and also noticed that he is not present in the members list. I wondered if he is still on this forum.

Regards
Symon.

[Sean Williams]

Darius was last here 28/12/09

HTH
Sean

[yagosaga]

Hello, I have just telephoned with Darius. When he tries to log in and to reply, he get the message "you don't have permission ..." and so on. Means, he is able to log in but not to write. IMHO there seems to be a fault in the forum software, or is it another reason?

The questions discussed here are treated in his documentation on

http://fernsehmuseum.net/uvb/uvb_e.htm

at the bottom of the page. You find also an email adress there to send a private message to Darius for further questions.

Is it possible that someone can look for the reasons why he can log in but to to write or reply?

Kind regards,
Eckhard

[Station X]

We are aware that Darius/Oldeurope is having problems, but have been unable to contact him because emails sent to his forum email address "bounce".

[yagosaga]

You find his email adress at the bottom of

http://fernsehmuseum.net/uvb/uvb_e.htm

Try this one, please.

[Darren-UK]

We have sent an email to Darius via what we think is his correct address. We now just await confirmation from him so we can do a reset allowing him to access the forums again.

[oldeurope]

Originally Posted by Philips210

Hi again.

... was wondering about the 47uF bipolar electrolytic capacitor in the video section. The bipolar types that I have come across are physically too large for my circuit. ...
Regards
Symon

Hi Symon,
Try RS ordercode 5211766 for example.

Please read the text

"General discription for the PNP and NPN transconductance modulators"

in the link yagosaga gave you. Please note the hyperlinks.
If you have still have questions I am happy to help you.

I am happy to be able to write here again. I thought I am not welcome any more.
I wasn't able to sent you PMs and wasn't able to see your email adress, sorry.

Kind regards,
Darius

[Philips210]

Hi Darius.

Good to know that you are back on the forum.

Thanks for the help re the bipolar cap.
I have now fully constructed the channel 1 modulator and hope to test it soon.
I will post some pictures of the completed unit when time permits.

Regards
Symon.

[Philips210]

Originally Posted by yagosaga

You find his email adress at the bottom of

http://fernsehmuseum.net/uvb/uvb_e.htm

Try this one, please.

Thanks yagosaga for this link.

Regards
Symon.