Mini-Mod 2021!

[Kentode]

Originally Posted by Ian - G4JQT

I've never known static damage a JFET. IGFETS/MOSFETS maybe but not a humble BF256 or similar.

Thanks Ian, that's reassuring.

I'm still leaning towards using holders though, under 8x magnification the soldering iron looks like a crowbar!

[Stuart R]

Originally Posted by Paul Stenning

Roughly what frequency was yours unadjusted? Useful to know as a starting point

Mine came up at 1062 KHz, before tweaking anything.

I had bought some 60p plastic knitting needles from Home Bargains, whittled one down to fit the IF adjusters in a valve radio, but this was too big. A cocktail stick pared-down with a Stanley knife is easiest and quick!

Happy Listening!

SR

[Ian - G4JQT]

Originally Posted by Stuart R

Mine came up at 1062 KHz, before tweaking anything.

SR

Sounds about right. Just tune to a quiet spot around the middle of medium wave, tweak L1 until the signal appears on the nearby radio and tweak L2 for maximum signal.

The MiniMod does not cover the complete band, but a few hundred kHz (about one third of the band) in the middle. If anyone really needs to have it elsewhere, then increase or decrease both C1 and C13 by 50pF or so.

Ian

[Kentode]

My boards arrived today, very well made. Here's hoping there hasn't been a run on the components from Cricklewood electronics!

[Electronpusher0]

Don't forget Spectrum Electronics have a kit of coils, fet, transistor and IC on ebay.

I am not associated with them but have communicated with them and they assure me that they have plenty of components available.

Peter

[Stuart R]

My Mini-Mod 2021 completes a set of three Pantry Transmitters for use around the house. It also encouraged me to box-up the one I built from David's PCB a few years ago. The third unit is one I've had a few years now, it's the AM-03 that used to be sold by Geoff Hopkins. It worked well, but is no longer made.

• I'll need to re-visit the values of the audio input resistors - I used the 'Tape Out' of my Stereo Tuner to run the AM-03, but this is too much for the Mini-Mod.
  
• The awkward spacing of the Phono sockets on the newer units was in an attempt to mount a 30cm telescopic aerial (donated from a 'Home Bargains' set-top TV aerial). I thought it would make for a neat solution, but the short antenna really limits the range.
  
• A trip to 'Wilkinsons' for some new Speaker wire gives me a nice 2m antenna length and early experiments gave an improved reception when connecting the battery -ve to a nearby radiator. So I've added a Crocodile clip to this earth lead.
  
• I was concerned about the proximity of the PP3 battery to the oscillator coil, but if the battery moves anywhere, it'll be out of the case and the whole thing stops working!
  
• There looks enough space to mount a Bluetooth Module inside - but I'm sure that will release a whole load of misery with noise. I do have some of those handy DC-DC Isolators now, but I think that's one for later.
  
• Due to a 'picking error', CPC sent me a 470pF cap instead of 100pF for C2. Currently using an 82pF Tantalum cap that was purchased for my Magnetic Loop project.

Whilst dismantling the AM-03 to get measurements for the panels, I seem to have killed it. The battery positive lead had come detached and may have contacted something on the PCB. The moral of that story is to use decent connecting wire. I had used some nice looking red/black cable salvaged from an old 'foreign' made car accessory. The conductors inside are extremely thin and break off far too easily.

Thanks again to Peter for sharing-out these PCBs. Anyone else finished theirs yet?

SR

[Steve_Bell]

Here are two of mine, both working well. I've used some different caps/resistors in each, depending on what I had to hand.

One uses a 470nF cap (red) for the audio coupling and the other uses a 680nF (yellow). I couldn't tell the difference.

The LEDs are low current types, taking a fraction of a milliamp. The blue LED draws less than 40uA.

[Stuart R]

Nice ones, Steve. I especially like the retro looking audio cable on the left unit.

I can't remember what my LEDs are rated at. I know they are branded 'Kingbrite' and sold as superbright. I bet they are using most of the available power!

Kind Regards,

SR

[Bazz4CQJ]

I've been trying to find the explanation why Ian decided to leave the 2N3819 for the BF256 but haven't found anything. Can anyone (perhaps Ian) advise? I've got genuine 3819's sitting in the junk box as a well as quite a few 2SK19, which were popular in FM tuners and have served me well as a general purpose J-FET.

Thanks

B

[Ian - G4JQT]

Originally Posted by Bazz4CQJ

I've been trying to find the explanation why Ian decided to leave the 2N3819 for the BF256 but haven't found anything. Can anyone (perhaps Ian) advise? I've got genuine 3819's sitting in the junk box as a well as quite a few 2SK19, which were popular in FM tuners and have served me well as a general purpose J-FET.

Thanks

B

I explained it in one of the threads, but can't remember which one now.

The reason I changed the 2N3819 for a BF256 was because after making about 30, I had one or two that gave poor modulation. I tracked this down to insufficient drive to Q2. It seems the performance characteristics of the 2N3819 are quite wide, so some just weren't up to the job. I expect J310s would do but I haven't tried them.

I also found that changing Q2's base resistor from 10k to about 3k3 also slightly improved things using the BF256.

Like most circuits the design is generally fairly tolerant of component values (+/- 20% shouldn't be a problem) except perhaps C1 and C13. Any transistor equivalents should work, but be very mindful of the lead configurations!

I did find that mica or good quality ceramic capacitors used at C1, C2 and C13 gave a bit more output than polystyrene ones. (Cheap disc ceramics used for C1 or C2 might introduce temperature-dependent frequency drift.) See this post: https://www.vintage-radio.net/forum/...&postcount=150

Also remember to earth one side of the unused winding of L1 to aid frequency stability. The boards supplied by Peter take care of that.
After the ones I made and sent out, I wonder how many have been built in total?

For anyone not familiar with the simple MiniMod initial setting up, I attach again the instructions I sent out with the ones I built.

Ian

PS: I'm glad LEDs now have low enough current not to have any significant battery drain. I think in 2011 when I published the design such LEDs were a bit pricey so I didn't bother.

[Gabe001]

Quick question. Does the indicator led require any resistor in series or are they simply 'plug and play' using this PCB? Or does it depend on the LED?

[Paul Stenning]

A series resistor is needed or you will destroy the LED.

R9 on the PCB is the LED series resistor and would be set to suit the LED being used and desired brightness. Around half the LED rated current is a good starting point; for a 2mA LED I would try a 6K8 resistor (assuming 2V for the LED and 7V across the resistor) to give around 1mA as a starting point.

Higher resistor and thus lower LED current is better for battery life, as long as the LED is bright enough to act as a useful indicator.

[Gabe001]

Thanks paul

[Bazz4CQJ]

Originally Posted by Ian - G4JQT

I explained it in one of the threads, but can't remember which one now.
The reason I changed the 2N3819 for a BF256 was because after making about 30, I had one or two that gave poor modulation. I tracked this down to insufficient drive to Q2. It seems the performance characteristics of the 2N3819 are quite wide, so some just weren't up to the job. I expect J310s would do but I haven't tried them.

Thanks for that info Ian; I did look for previous comment, but just couldn't find it. I've just ordered the Spectrum kit, so will be using the 256.

B

[coopzone]

Hi everyone,

I've just built number 1 of the 2 pcb's i got. Works a treat.

However be aware. Check your JFET. The one's i have are by Texas Instruments, NOS. They are part number BF256B made around 1983 i think. But the D and S pins are reversed, see bad photo.

So watch out.

Derek

[SeanMcGee]

Hi guys, I’ve mostly built up a board now and am hoping for your advice. I bought a jack just like the attached, but confused on how to wire it to the board, where would each wire run to?

Thank you

[SeanMcGee]

I forgot the photo!

[Electronpusher0]

The audio input on the pcbs can be either a miniature 3 way socket or by vero pins or wires direct to the Board.

For the socket shown connect the longer terminal to one of the 0V points.
For the shorter terminals connect one to the point marked L and one to the point marked R.
It does not matter which as the Left and Right signals are added to form mono.

Peter

[David G4EBT]

Though it doesn't really matter in this instance as the stereo signals are combined into mono on the Mini-Mod, if ever you're not sure which jack socket solder tag is which, the easiest way is to plug in a jack plug (if you have one) and check the continuity of each socket terminal and tip, ring and sleeve terminals of the jack plug. As depicted in the attached pic, the tip is the left+, the ring is the right+, and the sleeve is the shield.

Hope that might help.

[60 oldjohn]

This is how I connected my Minimod. Note the fishing line acts as a strain relief just fitted through hole twice. The yellow wire is the sleeve.

John.