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Old 24th Mar 2021, 9:57 am   #21
Paul Stenning
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Default Re: Mini-Mod 2021!

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

No knitting needles or files here. I'll look around for something in soft plastic of the right sort of shape to adapt with a knife (looking at the pocket clip on a BIC pen cap).
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Old 24th Mar 2021, 10:23 am   #22
60 oldjohn
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Default Re: Mini-Mod 2021!

Don't worry if you do not have the 1k5 resistor. The LED indicator has a 1.5k resistor this only takes a little over 4ma, I have used 5k6 and the LED draws 1ma this obviously the LED will be dimmer but was ok down to 5v.

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Old 24th Mar 2021, 10:27 am   #23
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Default Re: Mini-Mod 2021!

Quote:
Originally Posted by Paul Stenning View Post
Roughly what frequency was yours unadjusted? Useful to know as a starting point.

No knitting needles or files here. I'll look around for something in soft plastic of the right sort of shape to adapt with a knife (looking at the pocket clip on a BIC pen cap).
It was adjusted by Ian before I received the MiniMod.

Would a wooden skewer from the kitchen work suitably shaped?
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Old 24th Mar 2021, 10:41 am   #24
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Default Re: Mini-Mod 2021!

Toothpick or similar bit of wood with the end shaped into a chisel point to fit, works well.
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Old 24th Mar 2021, 11:13 am   #25
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Default Re: Mini-Mod 2021!

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What have people fabricated for adjusting the inductor cores? I know it needs to be non-metallic or at least non-ferrous.
In my article I suggest wooden cocktail sticks (or match sticks but they may be too soft) cut to a wedge.

Having read my article again, I didn't make a couple of matters completely clear about the 90uH inductors and their windings. If you're using Electronpusher's boards this is all taken care of, but if you're using your own layout this info might help.

It's a bit late after 10 years - and presumably most constructors know this already - but just in case:

L1 (Printed on side 90UOL)
The two pins opposite the three pins are for the secondary/link winding. They are not used in L1. They can be ignored, although Tony Nailer at Spectrum Communications suggested earthing just one side of the unused link winding will improve stability slightly. Electronpusher's boards do this.
The centre pin is not a centre tap, but about 25% from the end that is connected to earth. The centre pin (to Q1 source/C3) is about 0.9 ohms from the earthed pin and about 2.6 ohms to the end that connects to C1/C2.

L2 (Also printed on side 90UOL)
The side with the three pins is connected across C13 and the antenna. The centre pin in not connected to anything. The side with just two pins, each pin goes to R8 and Q2 collector respectively.

More info on the 90uH (90uOL) coils is found in the table SPECTRUM 10mm RF/IF COILS on this page of Spectrum's website:
http://www.spectrumcomms.co.uk/Components.htm

Note: Spectrum Communications have a range of similar coils with an 'H' replacing the 'O' in the suffix (e.g. 90UHL) which are not suitable. I have spoken to Tony at Spectrum and he only supplies the 90UOL for the MiniMod.

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Old 25th Mar 2021, 11:30 am   #26
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Default Re: Mini-Mod 2021!

I've got my kit from Spectrum (2 coils, LM386,BC548, BF256B), other parts on order, and looking forward to receiving the PCB.

I note from a previous thread
https://www.vintage-radio.net/forum/...=149604&page=4
that David G4EBT had to swap source and drain connections on the BF256.

It would be useful if someone who has also recently had the Spectrum kit and has successfully completed the MiniMod could confirm whether the FET as supplied by Spectrum needs to have its connections swapped.

TIA
Les.

Last edited by bileaflet_valve; 25th Mar 2021 at 11:30 am. Reason: Added David's callsign
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Old 25th Mar 2021, 7:36 pm   #27
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Default Re: Mini-Mod 2021!

^I've not long finished building my MM and the FET supplied by Spectrum DID need to have its connections swapped to fit on the PCB.

What adjustments need to be done to the coils? Is there any instructions for this?
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Old 25th Mar 2021, 7:46 pm   #28
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Default Re: Mini-Mod 2021!

I'm hoping my FET from some other eBay seller doesn't need the leads swapping as I have only mounted it a couple of mm above the PCB so there probably won't be enough lead length. I do have one more but that one arrived loose in an envelope with no antistatic packing so I have no idea if it is OK or not. It's a job for the weekend.
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Old 25th Mar 2021, 8:12 pm   #29
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Default Re: Mini-Mod 2021!

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^I've not long finished building my MM and the FET supplied by Spectrum DID need to have its connections swapped to fit on the PCB.

What adjustments need to be done to the coils? Is there any instructions for this?
Here attached below is the original BVWS Bulletin article. It is also posted in various other MiniMod threads, but I'll post it again here. It explains how to set the inductors and antenna length. The circuit has evolved a little.

1) Q1 now BF256 and the new boards have the GSD in the correct place - at least for the BF256s I have.
2) Q2 is any general-purpose equivalent to a BC109. The BC548 is a suitable substitute, but check the orientation of the EBC leads.
3) R2 now 3k3 (3.3k ohms)
4) C12 680n is not as easy to source as 470n, which will be OK as an alternative.
5) The LED is an addition. I didn't use one in my original design as it increased the total current consumption by about 50%. But new high brightness/low current LEDs make it a useful mod.

Also attached are the instructions I sent out with the MiniMods when I was making them myself.
Attached Files
File Type: pdf The_Bulletin_2011_MiniMod2.pdf (777.5 KB, 158 views)
File Type: pdf MiniMod instructions.pdf (127.9 KB, 141 views)

Last edited by Ian - G4JQT; 25th Mar 2021 at 8:19 pm.
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Old 25th Mar 2021, 9:15 pm   #30
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Default Re: Mini-Mod 2021!

Quote:
Originally Posted by bileaflet_valve View Post
I've got my kit from Spectrum (2 coils, LM386,BC548, BF256B), other parts on order, and looking forward to receiving the PCB.

I note from a previous thread
https://www.vintage-radio.net/forum/...=149604&page=4
that David G4EBT had to swap source and drain connections on the BF256.

It would be useful if someone who has also recently had the Spectrum kit and has successfully completed the MiniMod could confirm whether the FET as supplied by Spectrum needs to have its connections swapped.

TIA
Les.
Since I had that problem, Iíve built several mini-mods with BF256Bs supplied by Spectrum, and I know other forum members who have done so too, with complete success and no problems with the pinouts.

Hope that helps.
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Old 25th Mar 2021, 9:42 pm   #31
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Default Re: Mini-Mod 2021!

I sourced my BF256B from another ebay seller and it worked first time with no pinout problems.

Peter
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Old 25th Mar 2021, 9:44 pm   #32
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Default Re: Mini-Mod 2021!

No problems with Spectrum-supplied kit BF256B - did not have to swap leads. I inserted into the PCB as per the silk screen and it just worked.
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Old 25th Mar 2021, 10:07 pm   #33
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Default Re: Mini-Mod 2021!

Sounds promising then.
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Old 26th Mar 2021, 9:58 am   #34
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Default Re: Mini-Mod 2021!

Thanks for the replies to my query (post 26). I'll try it the "right" way (as per PCB) first!
Les.
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Old 26th Mar 2021, 11:13 am   #35
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Default Re: Mini-Mod 2021!

Has anyone used a Dil plug in holder and/or a plug in mount for the lm386 and the bf256 fet?

When I built my Velleman sig-gen l was more comfortable with these as l knew that I wouldn't fry things.
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Old 26th Mar 2021, 2:19 pm   #36
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Default Re: Mini-Mod 2021!

I think I may have killed my FET with static. I had difficulty removing it due to the perfect fit of the legs in the holes. With the replacement I lightly sanded the legs with wet and dry paper to take the sharpness off the corners of the square legs of the FET.

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Old 26th Mar 2021, 2:34 pm   #37
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Default Re: Mini-Mod 2021!

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I think I may have killed my FET with static. I had difficulty removing it due to the perfect fit of the legs in the holes. With the replacement I lightly sanded the legs with wet and dry paper to take the sharpness off the corners of the square legs of the FET.

John.
Thanks John, does that mean I need to earth myself and the PCB before I go near the Fet?
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Old 26th Mar 2021, 3:20 pm   #38
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Default Re: Mini-Mod 2021!

Here's a question. If one doesn't have an antstatic wrist strap, would tying and shorting all the legs of the FET while handling it reduce the chances of it getting accidentally damaged ?
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Old 26th Mar 2021, 4:54 pm   #39
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Default Re: Mini-Mod 2021!

I did not use a wrist band for either FETs. My soldering iron is always earthed, it will not work without an earth. Others will know better than me of any precautions that need to be taken.

John.
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Old 26th Mar 2021, 5:15 pm   #40
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Default Re: Mini-Mod 2021!

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