Forgotten project - Sudden GQRP 40m receiver

[ORAWA01]

This kit had been in storage over 10 years now, and it is half finished.

Recently I found it in the cupboard, and worked on it again to finish it off. When it is finally finished, it was powered on with 9V pp9 battery, but no sound. I hear some faint hissing sound, but no 40m reception.

When I move the wire antenna onto the tuning cap O output, it suddenly starts receiving, but not the ham band 40m, but the MW Radio 5.

When I try to tune around, it just keep receiving Radio 5 MW, no changes.

I thought it could be something wrong with the Mixer Oscillator SA602AN. I took it out from the chip seat, and checked all the legs, and they are straight and true, and put it back to the seat firmly. But still no joy.

Just faint hissing sound on its own, and loud Radio 5 when the tuning cap has been touched with the wire antenna.
Not even sure if these days, the ham band 40m is so quiet, and it is just the way it is, or this receiver has some problems.

When I switched on, my Yaesu FRG7, and tuned around 40m, there was nothing apart from some static noise this afternoon.

Any clues on this problem? Thanks.

[G6Tanuki]

It's a direct-conversion receiver, which means that the local-oscillator runs at the signal-frequency.

Do you have another receiver that covers 7MHz? If so, set it up for receiving CW or SSB on 7MHz then tune your 'sudden' across the band - at some point, if the LO is working, you should hear a whistle.

[ORAWA01]

Yup, tried it with another receiver, but no response from the Sudden.

When I touch C15 cap, it also receives MW, and when touched C2 cap. it seems receives SW - there was Radio Romania, not sure what freq. though.

But on its own, it is almost silent with faint hiss.

[Nuvistor]

As suggested in post #2, the oscillator needs to work, presume you have tuned either side of the 40 m band in case the osc is way off tune.
Apologies if you know this but a DC receiver uses the LO to mix with the wanted signal to produce an AF output. So to receive CW the LO would be set approx 1KHz away from the wanted signal to produce a 1Khz CW tone.

[ORAWA01]

In the instruction pp.5, it says that a built in capacitor in the LO(2U6LC), must be broken off with a small screw driver, before attaching it to the board.

It does not explain why.

I didn't break the built in capacitor in the LO coil, and soldered it on. Could it be problem?

[Nuvistor]

Quite possible that the osc is a long way off 7Mhz or not oscillating if the cap should have been removed.
Can you post the instructions and circuit or a link to them, I have some instructions but I think there were a few versions of kit made and there is no mention of removing a cap.
The coil in this version is a 2U6L, perhaps the C suffix is the capacitor in question.

[Terry_VK5TM]

The V2.4 instructions from 2010 mention removing the capacitor in L3.

If you haven't broken it out, then your local oscillator is way off frequency which is partly why you are not receiving anything on 40m. The other reason is at times the 40m band can be very quiet with no activity.

As for why you receive other signals when you put your finger on either of those components, your body capacity is shifting the tuning point of the Rx.

[ORAWA01]

Yes, sounds like it. I was trying to break it off as the instruction with a screwdriver, but it wasn't breaking off clean. So I just left it at that, but maybe more than likely the coil has been damaged during the endeavour.

The coil is also now totally unscrewed down from the casing, and resting on the circuit board.

The local oscillator seems fault.

For MW reception when touching other caps and connectors, it was so loud and later it was coming out with 3 - 4 stations all together.

I wonder if some other parts (such as crystals or DDS VFO?) could be used as a replacement for the 2U6LC oscillator.

[Radio Wrangler]

Or you could wind another coil. Something on an iron dust toroid, and add a trimmer capacitor to adjust the scaling. With a DC receiver,there's no tracking to worry about.

David

[Nuvistor]

It’s a very simple receiver, may as well keep it that way. If you can’t source another coil then David’s suggestion of winding your own is very worthwhile.

[MrBungle]

This is a common design. The GQRP one specifies the IF cans for this which annoys me as they are unstable things. You can get a new can (without a capacitor!) from Spectrum Communications for not much money.

You can use a T37-6 toroid, capacitor and trimmer in parallel instead too. Tends to be a lot more stable. I used one here on my homebrew version of this class of receiver:

https://www.vintage-radio.net/forum/...d.php?t=139481

I actually used this receiver earlier this year (now with added audio filtering!) and a homebrew transmitter using a 2n2222/MRF237 combo to make a CW contact! Wasn’t exactly easy though.

[ORAWA01]

Thank you for the suggestion, the link and ideas. Yes, I will go for the toroid coil and trimmer option, especially when it offers more stability.

[ORAWA01]

I cut the LC3 out from the board, and injected 7Mhz - 7.2Mhz sine wave from the Function Generator onto the input point of the LC3.

It then received the 7Mhz OK. There was CW contest going on.

[Nuvistor]

Should be straightforward to sort out now you know the rest of the radio works ok. I used toroids when building DC receivers, they work very well.

[ORAWA01]

Yes, I will make a toroid coil for it. Need to get some toroids and enamel wire.

But meanwhile I am using a function generator to inject DDS freq. into the receiver. Yes, the rest of the receiver seems OK.

https://www.youtube.com/watch?v=jYkTHQJEBHk

[dsergeant]

For information, if you are a GQRP member, the latest SPRAT, 181, contains an article on an updated version of the Sudden, the Super Sudden, by G1TEX. Doesn't discuss the local oscillator and its transformer per se but does discuss replacing it with an Arduino driven synthesiser.

73 Dave G3YMC

[MrBungle]

That’s what I use for my stuff now. I really like the engineering purity of an analogue VFO but the effort required to source all the bits and do the mechanical work and get it to stay put is similar to sticking an arduino and si5351 together and writing the software from scratch. Also has three channels output so you can use it to generate out of phase signals for direct conversion phasing receivers (in progress!) and drive a transmitter chain for CW.

[ORAWA01]

I am not a GQRP member, but thinking of joining in one of these days.

I tried the Sudden receiver with a coil from the Dip Meter last night, and it worked too. It sounded like a regen receiver.
And when injected with the sine wave of 7Mhz from Function Generator, it sounded more like a digital receiver.
But Arduino driven synthesiser VFO would be very nice.

Thank you for your info, Dave. 73

Jay

[MrBungle]

It’s definitely worth it. £6 a year and you get access to their shop full of cheap bits and a quarterly magazine full of glorious ideas

Found a picture of my current VFO creation. Mostly from aliexpress:

[Radio Wrangler]

The G-QRP club is a very friendly bunch. Completely free from all the politicking the RSGB seems to suffer from. I've been a member for about 36 years and know most of the active people. £6 is a no-brainer, you get a lot of value for it.

You'll find a G-QRP club stand at the Braehead rally every year, and I think at Galashiels.

David GM4ZNX