2SC transistor in a TV transmitter

[mdanh2002]

Hi,

I am trying to build a simple TV transmitter following the instructions at https://anarchy.translocal.jp/microtv/how_to_advancedTVTX_p1.html (I am well aware that this is for analog VHF TV, not for digital TV)

My only question is that, the diagram seems to use 2SC transistors (2SC1907, 2SC2053, 2SC2001 etc.) I do not have those. I only have the more common 2NXXXX and BCXXX transistors (2N3904, BC547, etc.) Can replace the 2SC transistors with the transistors that I have? They seem to be all NPN transistors and I have seen other design that uses 2N3904.

Thanks in advance.

 

[duffy]

The 2SC2053 has a transition frequency over 1GHz, the 2N3904 is a fraction of that - it just isn't fast enough.

Crossing these is a little difficult. Here's the datasheet on the 2SC2053, notice the transition frequency isn't even listed -
pdf1.alldatasheet.com/datasheet-pdf/view/14/MITSUBISHI/2SC2053/+0_yhDOOSzHypKdtXpvNED+/datasheet.pdf

What I did was used a cross-reference to an NTE number and then found the transition spec on THAT transistor. Once you have enough specs, you can do a "parametric" search on places like Digi-Key, Mouser, etc.

Here's a page of tips that might be helpful - https://www.electro-tech-online.com/custompdfs/2012/04/transubs.pdf
Here's a page of cross-referencing links - https://101science.com/transistor.htm#crossdb

 

[mvs sarma]

2SC2570 s avaialable generally and has higher Ft than 2n3904 etc

 

[panic mode]

that is true, however the circuit itself seem requires choice when building inductors - one has to pick either lower or upper VHF band.

using lower VHF which only operates at or below 100MHz is well within domain of general purpose transistors like 2N3904.

http://anarchy.translocal.jp/radio/micro/howtotx.html

but there is no problem finding higher frequency transistors, for example
DigiKey sells SS9018HBU for $0.18 and they have some 25000 of them in stock

http://search.digikey.com/us/en/products/SS9018HBU/SS9018HBU-ND/1047454

that will make upper VHF band perfectly usable as well

 

[mdanh2002]

Hi all,

Thanks for the tips on getting the equivalent transistors for 2SC2570. I think I will stick to common transistors (2N3904) and operate at low frequency only.

I am currently trying to build the audio section of the circuit (see https://anarchy.translocal.jp/microtv/how_to_advancedTVTX_p2.html). I used a 2N3904 instead. The circuit does not work as expected (e.g. output a 5.5MHz frequency, the PAL B/G audio subcarrier used in Singapore) no matter how I adjust the trimmer capacitor. All I get at the output is 0.05V (and 1.5V just before the 10pF capacitor). Is there a formula for calculating the output frequency? I am currently using a 47uH resistor-like inductor with a 35pF trimmer capacitor.

Also is it correct that the other lead of the trimmer capacitor is connected to ground according to the diagram? It seems strange to me.

 

[panic mode]

f=1/[2*pi*sqrt(L*C)]

where L is in H, C is in F, and f is in Hz.
of course actual capacitance is a little bit higher due other components in circuit.
so your circuit should be at 3.7MHz (when trimmer is on max capacitance) or some 10..11MHz when trimmer is on minimum.

personally i don't like certain things about this article:
- this is titled audio section which is hardly true, this is RF circuit (oscillator / mixer for audio / video)
- there is no real schematic, only layout diagram
- layout is without any dimensional reference (size, orientation etc do matter in RF)
- there is no indication how this stage is to be powered, positive should be at pos. end of 100uF capacitor
- 100uF is essential part of the LC circuit network but electrolytic caps are not good at RF, there should have been ceramic cap in parallel with it.
etc.

this is oscillator that should be adjusted to 4.5MHz, how are you measuring voltages? with oscilloscope?

 

[mdanh2002]

Hi. I tested the circuit by building the audio section alone and using my 50Mhz digital oscilloscope and monitor the on-screen frequency counter. It is random no matter how I adjust the trimmer capacitor. The waveform at the output is apparently the input audio waveform, with reduced magnitude. Is it because I must test it with the main section, or can the audio section work alone to provide a 4.5MHz waveform?

Thanks for any advice.

EDITED: I added pictures of my build (front and back) on a veroboard. I seriously do not think there are any connections issues - this is my 3rd attempt, all with same results.

 

[mdanh2002]

Any suggestions?

 

[mvs sarma]

why use 9V while design is done at 12 to 13.8 V?
the audio output would be seen as frequency change in out put and perhaps not as level change as Audio is FM

 

[panic mode]

if you look at oscillators used in mini FM transmitters, you will see they look nearly the same. one difference is that your circuit is supposed to operate at 20x lower frequency so it should be easier to get stability while all the issues like keeping things short etc are less critical. your pictures are fuzzy and i cannot get good look but there is nothing obvious jumping out. what i would like to see is capacitor value for example. one thing you can try is removing (or just desolder one side) of 1000pF capacitor that is between Emitter and GND. at 4.5MHz that capacitor is low impedance and killing feedback, making oscillations difficult if not impossible.

for example see
**broken link removed**

the second transistor (portion of circuit on the right) looks quite similar.
the emitter resistor is 470 ohm, base resistors are 15k and 10k (vs 27k and 10k, check voltage!).
base is grounded through 2.2n capacitor (10nF capacitor in your case, that's expected since lower freq.)
feedback from collector to emitter is 3.3pF (you have 1000pF which is expected due lower frequency)
there is coil in collector circuit (same in your case, just different value)
the only big difference is that none of these circuits have capacitor from emitter to gnd (and there are hundreds of these circuits) but your circuit does have capacitor for some reason - and fairly large too (impedance only 35 Ohm at 4.5MHz).

EDIT:

Sarma makes good point, make sure there is no input signal to this stage while measuring frequency (should have been obvious).

 

[mvs sarma]

The schematic is now made using eagle and pdf attached to enable discussion.

Ps: attachment is replaced. sorry for mirrored one.

 

[mdanh2002]

Hi mvs sarma. Thanks for the schematics althought it is mirrored and I need to flip it horizontally to view it. I found the layout diagram to be weird and confusing. With a schematics, it's better.

Hi panic mode, thanks for the detail explanation on the circuit and the differences with other circuits. I finally made it work by:

+ Removing the 1000pF from the emitter
+ Places a 100nF capacitor in parallel with the 100uF decoupling capacitor.

This is also why I believe attempting random circuits from the net without at least checking for errors/understanding some troubleshooting steps may be time consuming. I spend a lot of hours checking for connection errors on the stripboard.

Thanks again for the suggestions. I will work on the main section and post back if any problems.

 

[panic mode]

no problem, good luck with rest of the project

 

[mvs sarma]

i had replaced the pdf , please dowload.
it appears portrait as you see, but in fact a landscape. i downloaded and cross checked.

 

[mdanh2002]

@mvs sarma: thanks for the updated schematics. I will continue with the entire circuit when I have time to.

Just for the sake of curiosity, is it feasible to build a TV transmitter with audio by using 2 independent transmitters, one for video and the other is for FM-audio tuned to the audio channel? I got the idea from https://en.wikipedia.org/wiki/Split_sound_system and tried it last night, using a video-only TV transmitter circuit and another FM transmitter I built previously. I replaced the trimmer in the FM transmitter with a variable capacitor from a radio that has a plastic handle so that I can tune it without causing disturbance. It was very difficult to get the FM transmitter to tune into exactly 5.5MHz (PAL B/G) from the video frequency, but I finally got it. However, although the audio can be heard on the TV with video after some adjustment, there is always a background white noise. This happens on all the TVs that I have.

The FM transmitter audio, however, can be heard clearly with no background noises on my radio.

Any ideas what cause this?

 

[mvs sarma]

while , in principle, nothing is impossible, it is always that the audio carrier is 5.5MHz wrt vision carrier. under PAL B system that both our nationa use. Thus it is prudent to use combined one. there are modules of this type freely available as used in erstwhile Video casstette recorders.thus it wont be difficult to see one of such schematics

for any design , you need filters to prevent harmonics that make the signal and white noise etc present on output. i recollect that modulation input level matter white noise. so try to have a pot in the video in section

generally thesecoils used for oscillators of audio and video, should be stable with no flexibility.
i hopw we can use 5.5 MHz ceramic filter as osc element with modulation superimposed.
this obviates need for too much of adjustment. I enjoy your working around fm audio and am video circuits.
Sure that in the begining they appear hardnut, as you progress things would be to your satisfaction . all the best.