Minimod Mk2 parts substitutions?

[Martin G7MRV]

Not wanting to hijack anyone elses build thread, I have a few queries regarding substitutions/values for the Minimod mk2.

I started the build this evening using what I have in stock, which has led me to a few thoughts...

The regulator is 9v, but I believe the circuit will keep going right into the depths of a PP3s discharge, so can we get away with an 8v regulator? I happen to have some but no 9v ones,

Any known issues with changing C1 and C13 from 270pF to a little higher to shift the output lower down the band?

I used 2N3819 JFETs in my dead bug mk1 build, any issues with using them here rather than the BF256?

C12 - I have absolutely nothing in 680nF, nor it seems anything 470nF! Other than searching old PCBs, which I cant currently access, I do have 270nF and 330nF, or 1uF electrolytics. How close to the stated value is required? At a push how would paralleling them affect the response?

Im hoping to complete this build without having to order anything, as i did a big order of components a few weeks ago and it will be a long time before I need more stock.

Great thanks to Peter for the PCB, its a joy to solder!

[Electronpusher0]

Ian (the circuit designer) is best placed to answer most of your questions.

Regarding the 2N2819 question, yes these can be used and were in the original design.
You will have to "jiggle" the leads however as the board is laid out for the BF256, see the attached.

Peter

[Ian - G4JQT]

Hi there.

Yes, 8 volts will be fine. But you can bump up a 3-pin regulator by 0.7V (or more) by adding one or two silicon diodes or even an LED in the ground lead pushed into better conduction by say a 1k resistor from the regulator output to the top of the diodes. But the MiniMod will work happily up to about 14 volts - the LM386N is specified up to 15 volts.

In the article I suggest you can change C1 and C13 up or down 50pF or so to shift the frequency range. Just see that they are decent stable types (especially C1) to reduce temperature drift - as you would in any VFO.

A 2N3819 will usually be fine. I changed to the BF256 because the specification spread of the 2N3819 is quite wide and I built one or two that really weren't too good and that was the cause. As Peter says, check the lead-outs when substituting any transistor.

C12 is non-critical. I see I missed changing it from 680n to 470n on the Mk2 circuit, but it is mentioned in the text. You can parallel up values, but lower than say 330n may reduce the base a little. Going a bit higher than 680n won't do any harm either.

The whole design is fairly tolerant of component values and no one will do any harm just trying what the have.

[Martin G7MRV]

Thankyou Ian, great info. All pretty much as I expected, but always good to check!

I'll go with a 330n for C12 for now and see how it sounds. I doubt losing a little bass will matter when its feeding George Formby numbers to a wartime civilian receiver to entice the punters to buy a brew!

Ive so many 2N3819s I can run them on the tester and select them.

I thought I had 78L08 regulators, but the ones Ive found out first are ZSR800 units. They seem to be pin for pin compatible, so i'll give one a go.