Adding a Digital display to an AR88D

[JanWra1]

After receiving such helpful replies to my earlier inquiry I have a new questions for you:

I like to add a digital display to an AR88D without making any big changes to the old radio and have looked at a small kit (se enclosed file) with these specs:
- Frq: 1 Hz - 50 MHz
- 5 7-segments LED -displays
- a PIC 16F628 microcontrolleer
- add or subtract an offset value from the measured frequent
- Require 5 - 9 V DC
- Price abt £ 10 in Sweden

The question is where to connect it in the AR88D, shall offset be + or -? and will a preamp be necessary?

Jan

[peterpiper]

Hi Jan, I did a similar mod to CR100. Add a 22ohm resistor in series with the cathode of V3 6J5 pin 8 (local oscillator) and connect the counter to the cathode via a 10n capacitor. The counter kit shown has its own pre amp so should work directly. Local osc. runs above signal frequency so I.F has to be subtracted (I think). AR88 IF is 455khz and already programmed in the chip. Very clever kit originally by Wolfgang "Wolf" Büscher, DL4YHF I've built several, the single switch to program it is really clever!

Peter

[Bazz4CQJ]

Are the PIC's in those kits a little sensitive to spikes, static charge or whatever? I built one a while ago and it worked first time, but died after a few minutes. I suspect I may have done something it did not like. Got a replacement (programmed) chip and it's ok again. Prior to using it again, I added a few extra components on to the DC-in line and the RF-in line, intended to to offer a little protection. Additionally, I fixed on an insulating panel so that none of the tracks on the rear of the PCB can be touched by hand.

The addition of DFC to various Rx's (AR88, HRO etc) has been discussed previously on the forum and I recall that buffer amps have often been used.

B

[JanWra1]

Thanks for your inputs

Peter do you mean that the 22 ohm resistor goes between pin 8 and ground and that the 10 nf cap is connected with one leg directly to pin 8?
It looks rather easy, now all I have to do is to buy the AR88 and the counter kit! I am looking at an AR88D and if all goes well it will be mine in 4 weeks.

Jan

[Bazz4CQJ]

Originally Posted by JanWra1

It looks rather easy, now all I have to do is to buy the AR88 and the counter kit! I am looking at an AR88D and if all goes well it will be mine in 4 weeks.

Ah, if you have not already bought the counter, youmight want to look at alternative products. The one you referred to is a bit of a toy and really quite large; unless you have eyesight problems, those large bright red LEDs are not to everyone's taste.

There are numerous, ready-built, very compact DFC's with backlit LCD displays from numerous online suppliers, all of which are probably equally good. You just need to check that the offset function is there is you need that. One I have just ordered (no offset) is this https://www.ebay.co.uk/itm/High-Accu...72.m2749.l2649, chosen mainly for its compact size.

What is Bazz going to use that for...... Clue; no offset required.

B

[G6Tanuki]

When I've done this, rather than significantly disturb or modify the receiver itself I've used a single-JFET source-follower buffer amp [BF244] powered down the coax that connects it to the counter. You can build this on a tiny [centimetre-square] PCB and tuck it into the LO wiring so it picks up the necessary whiff of RF by way of a 'gimmick' capacitor [a few inches of wire].

No mods needed to the receiver, and little chance of significantly shifting the LO (which can happen if you make a direct connection to the LO circuitry).

I'll be doing something similar with the Trio 9R-59 I currently have on the workbench - my plan is to use the Ambit "DFM7" LCD prescaler/counter kit I bought 30 years back but never used!

[Bazz4CQJ]

Originally Posted by G6Tanuki

When I've done this, rather than significantly disturb or modify the receiver itself I've used a single-JFET source-follower buffer amp [BF244] powered down the coax that connects it to the counter. You can build this on a tiny [centimetre-square] PCB and tuck it into the LO wiring so it picks up the necessary whiff of RF by way of a 'gimmick' capacitor [a few inches of wire]

Is that already described in detail on the forum? I know there have been a few buffer amps suggested over the years and it does really seem the way to go. I've got DFC's ready to fit to my HRO and DX-160, but, they've been on the To-do list for quite a while .

B

[Biggles]

Anything that uses LEDs which are multiplexed could be a source of interference and affect radio reception. It may be better to use something with an LCD display where the switching will be "softer"and less likely to spew square waves everywhere with their associated harmonics. Of course it depends on the design of the counter, but just something to be aware of. Some careful screening may be all that is required.
Alan.

[G6Tanuki]

Originally Posted by Bazz4CQJ

Is that already described in detail on the forum?

Not that I remember - so here goes!

Look at the circuit here: it's the basic source-follower design.

Typical values: C1 0.01uF, R1 1MOhm, R2 1KOhm, C2 0.01uF FET BF244.

To power it down the coax: wire a 2.5mH RF choke from the "output" end of C2 to the Drain of the FET, and add 0.1uF from the drain of the FET to ground (so the drain is decoupled to RF).

Coax runs from the "output" terminals [centre core to C2] to the frequency meter.

At the frequency-meter end you connect the centre of the coax through another series 0.01uF to the input of the counter, then use another 2.5mH RF choke with one end connected to +12V and the other end connected to the junction of the coax centre and the 0.01uF - this essentially makes the centre of the coa 'live' to +12V which then travels down the centre wire to power the ampliifer.

A few inches of insulated wire from the input terminal on C1, poked into the area of the LO valve, will 'sniff' the RF. It's then transformed to a low impedance by the action of the source-follower, so it can feed down the low-impedance coax without loss.

[peterpiper]

Hi Jan, yes, disconnect pin 8 of the 6j5 and add a 22ohm to ground. connect a (about) 10n cap between pin 8 and the input of the meter. I like to power things like this from the cathode of the o/p valve but annoyingly this cathode is grounded and G1 is fed from a negative source. You could half wave rectify the 6.3v heater supply and feed a 7905 regulator, I think the red PCB already has one on board. the current consumption is less than 20Ma. When I did the CR100 I scoped the oscillator amplitude to check if the o/p changed, it didn't. calibration changed by about the same amount as the CR100 drifted (not much) The meter module is obviously static sensitive so will need to boxed, I like Hammond/Eddystone boxes. If it isn't screened the RX will pick up all sorts of interference from the PIC. Heres one I made earlier, Its destined to sit on the chan 1 output of a Kikusui scope Peter

https://www.vintage-radio.net/forum/...1&d=1516649395


https://www.vintage-radio.net/forum/...1&d=1516649395

[JanWra1]

Thanks guys,
All inputs very helpful, reverting regarding the result in due time.

Jan

[Bazz4CQJ]

[QUOTE=G6Tanuki;1010331]

Originally Posted by Bazz4CQJ

Not that I remember - so here goes! Look at the circuit here: it's the basic source-follower design.

Thanks for that; I might have been expecting something more complex, but to 30MHz . I do recall the late Bob Green posted a circuit for use with the HRO which I think used 3 or 4 transistors, but all all BJT's. My recollection is that, with an un-modified HRO, the RF from the LO could probably be sniffed quite easily from the cable from the set to the PSU, or the PSU mains lead. Mum and Dad's downstairs TV could certainly detect it until I shovelled in some extra bypass caps.

B

[G6Tanuki]

You don't really need *gain* in this situation - a wire dangled near the LO and connected to near-enough an infinite impedance (well, the 1M gate-resistor and a pF or so of input capacitance from the FET) will 'see' a volt or so of RF.

The purpose of the circuit is essentially to stop that volt or so being lost in the 100pF of centre-to-screen capacitance of the few feet of low-impedance coax between the AR88 LO and the frequency-counter's input.

I've used a similar circuit [admittedly with two FETs in what I guess you could call the FET equivalent of the old "Super-Alpha" bipolar configuration] to feed a LCD frequency-counter as a 'wideband' work-out-what-frequency-the-planes-are-using detector for taking to airshows: once I got the frequencies I shoved them into my scanner.

Yes know what you mean about the LO leakage of HROs: allegedly some of the Radio Amateurs assigned to one of the WWII "Y" intercept-stations ran an informal between-buildings CW net using the HRO local-oscillator-leakage that fed back through the antenna distribution system as their 'transmitters'.