2nd Jul 2015, 2:26 pm | #101 |
Dekatron
Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,761
|
Re: MiniMod pantry transmitter
I use one of those cheapo (about £10) Chinese multi-tester which test bipolar transistors, N & P Channel FETs, diodes, inductors, capacitors (including ESR), and resistors. You connect any of the leads to any pins and the display identifies which is which. Normally it functions perfectly, but when I tested a BF256C, initially it gave the correct pins per the data sheet: 1/2/3 = D/S/G, then when the mini-mod failed to work and I removed and tested the FET, the tester said 1/2/3 = S/D/G, but if I changed the test leads over, it then said 1/2/3 was D/S/G so the result was ambiguous. As Donald Rumsfeld once famously said 'it's a known unknown'
I still don't know which pin is which, but swapping the source and drain made the mini mod work. I'm clueless about FETs - I'm assuming that I've correctly identified the source, drain and gate in the circuit which I annotated in red text in post 85 above, and the orientation of the BF256 on the component overlay. I console myself with the fact that it works, and works very well! I've attached a pic of my tester. They come as an unboxed unit with an irksome ZIF socket which I removed to enable me to put the unit in a box with a push button test switch and three front panel test sockets. (The FET on test on the meter display isn't a 256C - it's a J310).
__________________
David. BVWS Member. G-QRP Club member 1339. |
19th Aug 2015, 7:36 pm | #102 |
Pentode
Join Date: Jul 2008
Location: Nottingham, UK.
Posts: 151
|
Re: MiniMod pantry transmitter
Just finished building my mini-mod this afternoon and after correcting a couple of wiring faults, it sort-of works. Just plugging it in, I get a signal through on my AM tuner, but it's very distorted. However, I discovered that if I touch the top of L1 lightly, it performs *perfectly* with low distortion and a nice, clear sound.
I thought perhaps it was a dry joint, so I re-flowed it and tried again, with exactly the same result. I don't know whether my finger is adding some capacitance, acting as an antenna, or both, but obviously it's not very convenient to use if I have to keep my finger pressed on it! Can anyone suggest where I've gone wrong with my construction? Cheers. |
19th Aug 2015, 9:42 pm | #103 |
Dekatron
Join Date: Apr 2005
Location: Bewdley, Worcestershire, UK.
Posts: 4,748
|
Re: MiniMod pantry transmitter
When you say "the top" of L1, do you mean the actual top of the can, or the top connection? Have you connected the can of L1 to earth? If not, try that and see what happens.
Pantry transmitters can give rise to some strange effects when used very close to some radios. You may have to experiment with the degree of coupling. You also mentioned "plugging it in" so I wonder if you're connecting the aerial output directly to the AM tuner's input? This would massively overload the front end and is not how the transmitter is supposed to be used. I find that simply putting the insulated aerial wire near the input socket provides ample signal for distortion-free reception.
__________________
Phil Optimist [n]: One who is not in possession of the full facts |
20th Aug 2015, 9:30 am | #104 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,323
|
Re: MiniMod pantry transmitter
Assuming all the right values of components are all in the right places...
With L1 tuned to a clear MW spot near the centre of the dial, L2 should peak the signal with the core at about the same depth as L1. If not, there is the possibility that L2 is not tuned to the frequency generated by L1. C1 and C13 should be the same value or the tuning ranges of L1 and L2 will not match. You only need about 1 metre of wire to act as radiating antenna. This does not normally need to be directly connected to the radio. It's possible the radio is overloading, and touching the can of L1 detunes the Minimod - which it shouldn't if the can and one side of the unused winding is is earthed. Are you sure you have L1 in the right way? The tapping is not at the centre of the coil, but near the earthy end. D1 in the right way? I did have two 2N3819 fets give poor modulation. One reason why I changed to BF256 (which has different pinouts!). What are you using as the audio source? Does it sound the same on other radios? Ian, G4JQT |
20th Aug 2015, 9:42 am | #105 |
Dekatron
Join Date: Aug 2013
Location: Wigan, Greater Manchester, UK.
Posts: 9,433
|
Re: MiniMod pantry transmitter
Confirm the audio input level is not too high and over modulating the MiniMod. The transmission on theses little boxes sounds really good, puts quite a few Medium wave stations to shame on audio quality.
Frank |
20th Aug 2015, 10:26 am | #106 | |
Pentode
Join Date: Jul 2008
Location: Nottingham, UK.
Posts: 151
|
Re: MiniMod pantry transmitter
Quote:
I've tried two different receivers - an old Roberts portable and a modern Pioneer tuner - exactly the same results. My audio source is an MP3 player. The only discrepancy I can find is that I've left the unused winding on L1 unconnected rather than grounded. Could this be the problem? Thanks for all the feedback from everyone. I think it's almost working but unfortunately my 'scope is broken so it's hard to see what's going on. |
|
20th Aug 2015, 4:32 pm | #107 |
Pentode
Join Date: Jul 2008
Location: Nottingham, UK.
Posts: 151
|
Re: MiniMod pantry transmitter
It would help if I hadn't bent C7 so one of it's legs shorted against pin 5 of the amp. Bent it back upright and now it works! Now playing BBC 6-music though an old AM tuner. Yay! Many thanks to Ian for his design and everyone else who has contributed to the thread. I've attached a picture of my (not very elegant but functional) creation, made mostly from bits I already had - hence the odd mix of new and old components. |
20th Aug 2015, 4:52 pm | #108 |
Octode
Join Date: Jun 2004
Location: Reading/Fakenham, UK.
Posts: 1,323
|
Re: MiniMod pantry transmitter
Brilliant! Glad you got it going.
It isn't very critical regarding layout or absolute component values, other than the capacitors mentioned in my original article. However, it's always good practice to place decoupling caps (especially around the audio IC) close to the required decoupling points. The unused winding on L1 won't do any harm left unconnected, but Tony Nailer of Spectrum Communications who got the 10mm range of coils made after Toko stopped supplying, suggested earthing one end helps with reducing temperature-induced capacitance variations between the coils in the can. It's also worth earthing the cans of both L1 and L2. Of course using an L/C tuned circuit to generate the carrier isn't ideal for all sorts of reasons, but in a simple, low power design like this its quite adequate. And any frequency drift will likely be much less that the vintage radio it's tuned to! There's none of the audio compression or audio tailoring required by the 'real' stations, so audio frequency response is determined solely by the receiver's IF bandwidth. Regards, Ian, G4JQT |
20th Aug 2015, 5:31 pm | #109 |
Pentode
Join Date: Jul 2008
Location: Nottingham, UK.
Posts: 151
|
Re: MiniMod pantry transmitter
Thanks, Ian.
If I build a second one (likely) then the decoupling caps will be nearer to where they should be. I didn't really do enough planning of the layout before I started soldering... On the subject of compression, the MP3 player app I'm using (JetAUDIO, £3 download) has some useful controls built in, including a 20-band equaliser and a pre-settable AGC. Tweaking these controls to reduce out-of-band frequencies and compress the dynamic range a bit seems to make a useful difference, although not as good as the pro gear broadcasters use, obviously. However, I've also discovered my cheap Sony handset chucks a load of high frequency noise out of the headphone output so it's not an ideal source. I may experiment with some input filtering as I tend to use modern electronic devices as audio sources. All in all, I'm very pleased with it. If I can build one of these, pretty much anyone with a soldering iron can. It's a very neat, understandable design. |