Micron Circuits

[Dodgy Geezer]

I only seem to come here when I need help! ...

I bought what looked like a good condition Micron PL-7D from fleabay a week ago, and when it arrived it turned out to be a partially-built kit. Without any documentation, of course. It looks like a 27Mhz AM unit.

The tranny box looks fine - sticks assempled but everything unsoldered. And rattling about inside the case were two PCBs - the RF section and the encoder.

The RF board looks nearly finished - just a couple of caps threaded thropugh the holes and bent over but not soldered. There is one hole unused....
The encoder is odd. I had expected a 4017 IC on it, but there isnt one, and there doesn't seem to be any pair of 8 parallel holes where one might go. Instead there are what look like 7 'stages' for the different channels, plus a little bit of extra circuitry. Again, it looks fairly complete for a 4-channel system.

I'm inclined to put it together to see if it works, but I need to understand the circuit, and haven't got a hope of this without a diagram. I have one that PaulJ kindly provided a while ago, but that only shows encoders with 4017 chips. This looks like it's different!

Has anyone got any ideas? or, ideally, a circuit diagram for this PCB?

[Tobe]

It's a very good reason and that's one of the benefit of the forum.
Looks to me to be a pre-IC encoder quite conventional and the unpopulated areas are for 3 additional channels. I don't have much experience with Micron but I'm sure a better answer will come up that will help you.

[Dodgy Geezer]

Thanks, Tobe - this must be quite an early design, then? The transmitter board looks early too - it's not like any Micron circuit that I have..

[AndyS]

The encoder was a multivib and a series of half shot pulse generators. Try here for the information you need:

https://www.norcim.org/

[Dodgy Geezer]

I had tried Norcim before - you are correct that there is one para there describing the cascaded multivibrator circuir which produces the pulses - but the Micron encoder board must do a lot of other things - and so I can't tell whether this board is complete or not - let alone where I put the power and connect the stick pots. What, for instance, is the electronics at the other end of the board to the encoding circuit doing?

I hope that someone must have a circuit diagram around that they can photo from the old days...?

[Pchristy]

Andy is correct - that is a "half-shot" encoder. It comprises a two transistor multivibrator, followed by a string of "half-shots" (half of a multivibrator) to generate the channel pulses. The PL-7D was designed as a 7 channel set, but could be built with fewer channels. I can see circuitry for four half-shots, with space for three more, so this one was built as a 4-channel set.

The extra circuitry at the end is probably to collate all the pulses from the half-shots together into a pulse-position stream to feed to the RF board. This would need to be negative going pulses for an AM system - unless there is an inverter on the RF board.

The sticks are usually connected as the collector load for the transistors, with the wiper connected to the base of the following transistor via a capacitor/resistor bridge. Here is a typical circuit of the kind you have: https://www.norcim.org/p6 (About 1/3 of the way down - the section that starts with "They also developed the first receiver that could count".

For a more detailed explanation, see if your local library can get hold of a copy of Paul Newell's excellent book on the "Theory & Practice of Model Radio Control". Alternatively, you could download the RCM "Classic" articles from the American "RC Modeler" magazine. They started in the April 1968 edition and appear to be downloadable from here: https://rclibrary.co.uk/download_title.asp?ID=1332

The Classic - and its predecessor, the DigiTrio - formed the basis of many homebuilt and kit systems in the late 60s and early 70s. You will probably find that your system has much in common!

Hope this helps!

--
Pete

[Dodgy Geezer]

Pete/Andy,

Thanks - that was really helpful! Funny how words seem to make better sense than diagrams...

I'm now working on the assumption that both these PCBs were (practically?) completed by the first owner, and all that remains is to do the connecting wiring. There are a couple of components still threaded through holes and bent - I will obviously solder those.

The RF board looks easiest - it has +/- and signal cables already connected, and an aerial out line. The only slight query is an isolated hole on the negative track which I can't see any corresponding connection for. I intend to connect a Micron aerial (centre balanced) to this, put in a 27Mhz crystal and power it up to see if I get any radiation. Does this sound like a sensible first step?

The encoder is going to be more complex. I have identified + and - tracks, but these do not have an obvious hole for a pin connector - just a copper lan. The same thing is the case for my guess at where the stick pot lines ought to go. And I then have no idea of where the encoded output should be....

I have one question about the half-shot encoder. The diagram on Norcim page 6 shows 5 pots, with the first connecting to one of the multivibrator transistors. The total set of channel inputs shown is therefore 5, and that is matched by the encoder board I have - a multivibrator and 4 channel transistors. The fifth pot, matching the fifth transistor, has no output for its wiper of course, because that would require another transistor. Does this mean that I only connect 4 pot wipers, to the multi and the first three transistors?

I will try to make up a better drawing to show what is happening...


P.S. - I suppose I should mention that the box that I bought contained what I believe to be the remanents of a kit. It had some general paperwork, a completed receiver, an assembled Tx box, the two PCBs I have mentioned, and a bag of oddments which look like the charge unit - including a transformer. There are a few resistors and a diode in this bag, 3 of the resistors do not match the charger circuit diagram which was included. They may be random, or they may indicate that incorrect resistors have been installed somewhere on the PCBs...!! I really need a circuit diagram...

[Pchristy]

The easiest way to think of a half-shot encoder is by imagining a domino chain, where knocking the first one over makes the rest fall down in sequence. The multivibrator kicks the first half-shot, which then triggers the second, etc, etc...

The size of the kick determines how fast the domino falls. We control the size of the kick with the stick pot. So, consider the diagram I pointed you at from the norcim page:

Q1 and Q2 form the mutlivibrator, which runs at about 50Hz (20mS period). This provides the frame rate of the system, and the sync pulse is derived from its output (A).

The collector load resistor for Q2 is replaced by the Ch1 stick pot, which governs the size of the "kick" given to the first half-shot - Q3. The length of the pulse leaving Q3 at point B is derived from this kick. Similarly, the collector load of Q3 has been replace by the Ch2 stick pot, which provides the kick to Q4, and so on down the line.

So your encoder appears to be a 4 channel one, closely related to the one illustrated above.

The various pulses all need to be collected and shaped to provide the necessary "spike off" (PPM) signal to the RF board, and that is the purpose of the various diodes and last two transistors that you see on your board.

One end of all the pots is connected to the positive supply. The other end of each pot is connected to the collector of its appropriate transistor. The wiper connects to the base junction of the next transistor in line, via a capacitor, which provides part of the time constant governing the pulse length.

On most kit built systems, the only adjustment available was to adjust the wiper position in the stick unit to get the right centre (1.5 mS). The throw was taken care of by the mechanics of the stick and choice of capacitor value, and could be a bit variable. More sophisticated circuits added a trimmer pot in series with the base bias resistor (the one going from the base to the positive rail). You could then adjust the throw and centre on each channel independently, but this meant going round in circles many times until you got it right, and was usually left out of simple kit systems.

Reversing throws always means re-centreing the channel, as the stick pot is never at the centre of its travel to get the pulse neutral right.

So basically, your encoder board appears pretty much complete. Some of the extra bits you have may be for extra channels to be added later, if required.

Do have a look at the documentation I pointed you to. It explains it much clearer than I can in a post like this, but hopefully this will get you started!

Best of luck!

--
Pete

[Dodgy Geezer]

Pete,

Ah, it's the synch pulse! Now I follow it.

I have read through the documentation - but I find your explanations much easier to understand. If I can't sort out where tbe final frame is coming out, I suppose I could power the board up and put a scope probe onto some likely places until I find a pulse train...

Thanks for all the help you have been...

[AndyS]

I have had a search through my garage where I have a Micron PL6D that I built from kits. The tx is now on 2.4GHz with a diy Frsky rf module and a Phil G encoder. I do have some of the documentation. I recall that drawings showing component placing rather than ccts was supplied. I do have some notes on the PL6D and 7, which I have taken pics of. Here there are, may be of some help.