It does have a real time clock, so that explains the 32.xxKHz Xtal.

I like this. Surely that isn't brass?

 

 

Can't believe my iron died part way through it! I can actually, it's a cheap one.
I couldn't solder the daughter board back.

 

So I tied the tip from my 30 Watt iron around the tip of my 60 Watt iron with copper,
and am back in business

It seems one of the two grey wires are input to the frequency counter.
If I short them, the maximum freq for the current range (band) is displayed.
The brown wire in the ribbon directly drives the "tuned" LED - lights when a station is tuned.

It appears the band select switch (on the radio) also selects the range
for the frequency counter, so one line in the ribbon cable must do that.
There must also be another for power to the freq counter,
even though it's a buttons on the LCD board control power.
...and another for AM/FM LED select.

It does look like you could provide a different input to the frequency counter.

 

Would you mind some further explanation of the interface between the
radio and the freq counter with a FET MrChips?

I'm right to go with it now. I wonder if it could be optically or transformer isolated
at low freq with a bit more electronics, half of which, powered by the radio.
To start with, I'm sure I'll just follow advice to get it working.
I know the off the shelf unit won't subtract IF, I will replace the freq counter later.

And why a Realistic DX-xxx? Is it something you wanted, and couldn't afford at the time,
so you got it later, or is it just that good, and why?

You can tap off the signal from the local oscillator (LO) using a FET.
You need to know the intermediate frequency (IF) being used in the radio.
Then you have to add/subtract (depending of the radio design) the IF to/from the LO frequency.

If you are building your own frequency counter there is a way to add/subtract the IF.

Here is a photo of my DX radio with a homemade frequency readout showing the IF offset that is preset into the counter.
I can choose to add or subtract the IF offset.

 

So that's this one.. experimental radio I don't mind wrecking, IF is 455kHz.

As expected, noise when connected... got to be isolated.

 

The FET signal buffer was placed inside the radio. I have to dig up the circuit.

The Realistic DX-150B from Radio Shack was affordable and had all the features one would want in a basic communications receiver.

• continuous coverage from 550kHz to 30MHz
• fine tuning
• antenna matching
• RF gain control
• AGC (Automatic Gain Control)
• Noise limiter
• Signal Strength meter
• SSB (Single Side Band)
• BFO (Beat Frequency Oscillator)
• headphones output
• separate speaker to eliminate feedback

It was designed to be used with a transmitter, disabled while transmitting.

The only thing missing was the digital frequency readout.

 

Ok, thanks
In the long run, maybe it would pay to use optic cable, so the thing shares no connection.
I think I'd just have to make a buffer and led powered by the radio.

 

Can't you make a sniffer antenna, to get the freq instead of a direct connection? If your freq counter does not have enough gain you will need a small one-transistor RF amp.

 

Can't you make a sniffer antenna, to get the freq instead of a direct connection? If your freq counter does not have enough gain you will need a small one-transistor RF amp.

I have had that suggested (elsewhere).
The frequency counter ends up being the interrupt pin of a pic,
so the interface has to work with that.
I have also learned that there are small packaged opto-isolators.
It's not that the freq counter has to be physically far away.

I'll do some Googling for a "sniffer antenna" before coming back to bug you about it

This freq counter is that sensitive, at no attenuation, I can do things just by
touching the insulated lead etc... but haven't tested that with a pic ever.

The idea is I make a custom freq counter for the valve radio,
then use stepper motors to turn it's original tuning gang capacitor.
I say stepper "motors" because one would have to be geared for fine tuning
when the dial approaches the correct frequency.

Once the thing is connected properly and it becomes a microcontroller project,
it's kinda back in my domain, and I should be ok from there.

 

Can't you make a sniffer antenna, to get the freq instead of a direct connection? If your freq counter does not have enough gain you will need a small one-transistor RF amp.

I used a BC547 which buffers the signal, and then the emitter still provides a good signal for the freq counter,
but it doesn't light an LED, then I tried sending that output to a 7404.
I would have thought that would be fast enough.
It seems I can make the LED on all the time or off all the time.
Tried two inverters with no luck. I don't think it matters if the clock is inverted.

I think the inverter is just leaving the LED on or off permanently.

 

I suppose if at first you don't succeed, use another transistor.
Now I suppose it's off to get some opto-couplers and photo transistors.

 

Looking good.

 

Looking good.

When I can receive it, and safely find the LO in the valve reciever, it's home free

This must happen!
It's not tuning to a station by itself that I want to see, but mechanically
correcting itself at run time to fix natural drift.. that I must see.
It will never settle at even a single decimal place, so it will need to accept
a tolerance before moving any stepper motors, or it will try to fix itself constantly.

 

It took an amazing amount of stuffing around with optocouplers and discreet
phototransistors to figure out that once you're into the thousands of kHz,
you're actually into the MHz, and the hardware might not be fast enough.

In the end, the solution was simply off the shelf, and absolutely unmodified!
red LED in one end, square 7404 buffered signal out the other.
It must just be that the phototransistor is fast enough.

 

Can't you make a sniffer antenna, to get the freq instead of a direct connection? If your freq counter does not have enough gain you will need a small one-transistor RF amp.

Lol, after all of that, A ferrite rod antenna placed near the tuning gang,
or near any IF transformer provides a better signal than anything I've made
and it will probably be able to be hidden under the chassis.
I guess I still get to use the little 2 transistor amplifier

I also see how a magic eye simulation can receive input. the voltage increases if the radio is tuned to a station.

 

That will be a cool project, if you can get it to successfully "tweak" the mechanical control with a motor in response to the digitally measured frequency.

Have you checked out the HAM and antique radio sites to see if it has been done before? Conceptually it's simple but I don't remember ever seeing it done in practice on an antique radio.

 

That will be a cool project, if you can get it to successfully "tweak" the mechanical control with a motor in response to the digitally measured frequency.

Have you checked out the HAM and antique radio sites to see if it has been done before? Conceptually it's simple but I don't remember ever seeing it done in practice on an antique radio.

Conceptually is the qualifying word there!
Once you need some custom gears for stepper motors, etc.. I'm glad I have help there.

I haven't seen it before.
I have seen a method called "huff puff" which is an improvement to tuner stability with a few modern components,
and also another where the tuner is digitally synthesised.
In the latter case, I think the original radio tuner is out of the picture.

I have to reject both of those.. my motor won't have any job to do if the tuner is made more stable.

 

So that's all good, what needs to happen to be able to read it?
I can connect the ferrite antenna directly to a freq counter or scope ok,
but my two transistor amplifier does nothing with it.

I tried a signal diode in between, and it messes up the signal on a scope.

 

Lol, now it can tune all the way down to zero kHz on it's AM band
Obviously stops working at the extreme low end, but the freq display still works.
359 kHz is some robot at an airbase near me that constantly announces weather info.