Eddystone Crystal Calibrator - UK Vintage Radio Repair and Restoration Discussion Forum

Patrick Dixon · 23rd Jun 2011, 8:45 pm

I'm slowly working through my Eddy 730/1A sorting out the immediate problems.

The current puzzle is the Crystal Calibrator - see circuit.

The valve and resistors are all OK, the circuit and DC voltages are correct, but the waveform at the grid (which is from where a wire is fed into the tuning box) looks rather sawtoothed. The set doesn't seem to pick up any tone.

Is this likely to be the crystal itself? Any other suggestions gratefully received!

MichaelR · 23rd Jun 2011, 9:22 pm

Without appearing to be too basic , are you adjusting the tuning until you can pick up a marker. The cursor could be way off adjustment.

Mike

ms660 · 23rd Jun 2011, 10:23 pm

I do not know about the Eddystone but with some xtal calibrators the valve is driven quite hard in order to generate harmonics up to 30 mhz +, I cannot comment on the waveform you show as I have not scoped an Eddysone calibrator but I seem to remember doing this on other sets years ago and the waveform was not what you would call sinusiodal.
Normally if they are punching out a waveform of soughts and of the correct fundimental frequencey and correct amplitude then you can normally tune through the harmonics on the receiver, if you are wizzing through the tuning they are easily missed + if the local osc is out they are not going to turn up where you expect them.
Hope some of this is usefull to some one.

All the best.

Patrick Dixon · 23rd Jun 2011, 10:48 pm

It's supposed to be a 500KHz marker and it looks about that on the scope. I'm tuning at 500KHz and hearing nothing - maybe just a faint noise, but so slight I could be mistaken.

It's easily possible that the tuning is out, but R5L is about where it should be at 693KHz and 909KHz on the same band, so I expected to be able to find something.

I hadn't thought about the harmonics aspect - it would make more sense for it not to be a sine wave.

ronbryan · 23rd Jun 2011, 10:58 pm

I don't know anything about Eddystone comms receivers, but I wonder if you need the BFO switched on to hear a tone from the marker oscillator - otherwise I presume it would be just an unmodulated carrier. Are you in fact using the BFO?

Ron

MichaelR · 23rd Jun 2011, 11:26 pm

Ron is correct about the bfo , the faint noise you are hearing is the unmodulated harmonic.The waveform is showing a partial square wave.

Where are you connecting your scope probe ?

Mike

Skywave · 24th Jun 2011, 1:31 am

That is the standard Eddystone crystal calibrator that features in many Eddystone valved receivers. It looks like a dynatron oscillator to me: the anode resistor is 270k; the screen resistor is 22k: the screen voltage is much higher than the anode voltage. The waveform you have shown is correct in its waveshape: it is rich in harmonics. This type of oscillator produces such a waveform - that's why that design was chosen.
Since it produces an unmodulated wave, it is necessary for the BFO to be switched ON to hear the harmonics.
I have had little trouble with component failure or ageing of resistors with this oscillator over the years, (however check C87: 0.01 µF & the anode load resistor: 270k), but note that the core of the coil has to be carefully adjusted against a standard freq. for exact calibration. If the core is badly out of adjustment - no oscillation results. But that doesn't appear to be the case here, looking at your waveform.

Al.

Patrick Dixon · 24th Jun 2011, 9:00 am

Quote:

Originally Posted by Skywave

it is necessary for the BFO to be switched ON to hear the harmonics.

Ah, that might be the problem then. The BFO doesn't work at the moment because I swapped the valve into the IF which had a duff one.

I don't really understand what the BFO (or the AF filter) are for really. The BFO (with a working valve) makes a lot of nice whistles and things, and the AF filter seems to put the set into 'underwater' mode.

I'm not all that sure about the Crystal filter either. The max Selectivity position seems to give pretty limited audio bandwidth as it is!

As you can tell, I don't really know much about comms radio!

Skywave · 24th Jun 2011, 10:37 am

You're obviously interested in short-wave reception since you have acquired this receiver. To get the full enjoyment out of short-wave reception that that receiver can produce, I strongly recommend that you obtain one of the many publications from the R.S.G.B.: their Site is on the 'Net. The Radio Communication Handbook is a good general purpose reference book that will answer all the questions you have now - and all those to come. There are also other books that the R.S.G.B. produce that will be of benefit to you.

Enjoy you radio and the hobby of a SWL: it's a technical hobby and a knowledge of the technicalities involved is required to fully enjoy it.

Al.

HamishBoxer · 24th Jun 2011, 12:08 pm

The BFO is used on all sets for either SSB or CW reception and inserts the missing carrier (Supressed Carrier ) on SSB.

David GM8JET

PS All reses are suspect on Eddystones if i didnt mention before

Patrick Dixon · 24th Jun 2011, 12:30 pm

Thanks. As an EE do I understand the technicalities (or I do when I think about them) - but I'm not familiar with the practicalities. I was actually given this Rx which was found abandoned and unloved in a shed, and I'm enjoying bringing it back to life. But it's difficult to know if things are working as they should be, when you have no experience of what that actually is.

As an ex-private pilot I also understand QFE and QNH, but all the breaker, breaker and 10-4 stuff is a bit beyond me ;-)

I'll have a look at that site ...

neon indicator · 24th Jun 2011, 12:58 pm

Makes CW sound 800 Hz tone if you adjust BFO 800Hz off the carrier. Otherwise you can't really hear morse except as a reduction in background noise when "key down".

It's a very primitive way of listening to SSB, but if the BFO is stable and the main L.O. is stable then it works well. Usually a gentle touch on main tuning and BFO to "clarify" an SSB signal.

The Crystal calibrator will be very clear with BFO working. If you are exactly on frequency, then if BFO is EXACTLY centre of IF you hear nothing. as you move BFO higher OR lower you get 1st a deep motorboat and eventually a tone which get higher as you change.

Having the BFO set to centre frequency of IF and mistuning the main tuning either direction has same effect except when the tone gets high pitched it cuts off suddenly when you reach either edge of the IF bandwidth.

Skywave · 24th Jun 2011, 4:06 pm

Patrick - you have a P.M.

Al. (Skywave) / June 24th. //

23rd Jun 2011, 8:45 pm	#1
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Eddystone Crystal Calibrator I'm slowly working through my Eddy 730/1A sorting out the immediate problems. The current puzzle is the Crystal Calibrator - see circuit. The valve and resistors are all OK, the circuit and DC voltages are correct, but the waveform at the grid (which is from where a wire is fed into the tuning box) looks rather sawtoothed. The set doesn't seem to pick up any tone. Is this likely to be the crystal itself? Any other suggestions gratefully received! Attached Thumbnails

23rd Jun 2011, 9:22 pm	#2
MichaelR Retired Dormant Member Join Date: Mar 2006 Location: Sheffield, South Yorkshire, UK. Posts: 1,587	Re: Eddystone Crystal Calibrator Without appearing to be too basic , are you adjusting the tuning until you can pick up a marker. The cursor could be way off adjustment. Mike

23rd Jun 2011, 10:23 pm	#3
ms660 Dekatron Join Date: Apr 2011 Location: Cornwall, UK. Posts: 13,454	Re: Eddystone Crystal Calibrator I do not know about the Eddystone but with some xtal calibrators the valve is driven quite hard in order to generate harmonics up to 30 mhz +, I cannot comment on the waveform you show as I have not scoped an Eddysone calibrator but I seem to remember doing this on other sets years ago and the waveform was not what you would call sinusiodal. Normally if they are punching out a waveform of soughts and of the correct fundimental frequencey and correct amplitude then you can normally tune through the harmonics on the receiver, if you are wizzing through the tuning they are easily missed + if the local osc is out they are not going to turn up where you expect them. Hope some of this is usefull to some one. All the best.

23rd Jun 2011, 10:48 pm	#4
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Eddystone Crystal Calibrator It's supposed to be a 500KHz marker and it looks about that on the scope. I'm tuning at 500KHz and hearing nothing - maybe just a faint noise, but so slight I could be mistaken. It's easily possible that the tuning is out, but R5L is about where it should be at 693KHz and 909KHz on the same band, so I expected to be able to find something. I hadn't thought about the harmonics aspect - it would make more sense for it not to be a sine wave.

23rd Jun 2011, 10:58 pm	#5
ronbryan Octode Join Date: Jan 2003 Location: Guildford, Surrey, UK. Posts: 1,960	Re: Eddystone Crystal Calibrator I don't know anything about Eddystone comms receivers, but I wonder if you need the BFO switched on to hear a tone from the marker oscillator - otherwise I presume it would be just an unmodulated carrier. Are you in fact using the BFO? Ron

23rd Jun 2011, 11:26 pm	#6
MichaelR Retired Dormant Member Join Date: Mar 2006 Location: Sheffield, South Yorkshire, UK. Posts: 1,587	Re: Eddystone Crystal Calibrator Ron is correct about the bfo , the faint noise you are hearing is the unmodulated harmonic.The waveform is showing a partial square wave. Where are you connecting your scope probe ? Mike Last edited by MichaelR; 23rd Jun 2011 at 11:37 pm. Reason: addition

24th Jun 2011, 1:31 am	#7
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Eddystone Crystal Calibrator That is the standard Eddystone crystal calibrator that features in many Eddystone valved receivers. It looks like a dynatron oscillator to me: the anode resistor is 270k; the screen resistor is 22k: the screen voltage is much higher than the anode voltage. The waveform you have shown is correct in its waveshape: it is rich in harmonics. This type of oscillator produces such a waveform - that's why that design was chosen. Since it produces an unmodulated wave, it is necessary for the BFO to be switched ON to hear the harmonics. I have had little trouble with component failure or ageing of resistors with this oscillator over the years, (however check C87: 0.01 µF & the anode load resistor: 270k), but note that the core of the coil has to be carefully adjusted against a standard freq. for exact calibration. If the core is badly out of adjustment - no oscillation results. But that doesn't appear to be the case here, looking at your waveform. Al.

24th Jun 2011, 10:37 am	#9
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Eddystone Crystal Calibrator You're obviously interested in short-wave reception since you have acquired this receiver. To get the full enjoyment out of short-wave reception that that receiver can produce, I strongly recommend that you obtain one of the many publications from the R.S.G.B.: their Site is on the 'Net. The Radio Communication Handbook is a good general purpose reference book that will answer all the questions you have now - and all those to come. There are also other books that the R.S.G.B. produce that will be of benefit to you. Enjoy you radio and the hobby of a SWL: it's a technical hobby and a knowledge of the technicalities involved is required to fully enjoy it. Al.

24th Jun 2011, 12:08 pm	#10
HamishBoxer Dekatron Join Date: Aug 2007 Location: W.Butterwick, near Doncaster UK. Posts: 8,935	Re: Eddystone Crystal Calibrator The BFO is used on all sets for either SSB or CW reception and inserts the missing carrier (Supressed Carrier ) on SSB. David GM8JET PS All reses are suspect on Eddystones if i didnt mention before

24th Jun 2011, 12:30 pm	#11
Patrick Dixon Hexode Join Date: Aug 2007 Location: Bristol, UK. Posts: 480	Re: Eddystone Crystal Calibrator Thanks. As an EE do I understand the technicalities (or I do when I think about them) - but I'm not familiar with the practicalities. I was actually given this Rx which was found abandoned and unloved in a shed, and I'm enjoying bringing it back to life. But it's difficult to know if things are working as they should be, when you have no experience of what that actually is. As an ex-private pilot I also understand QFE and QNH, but all the breaker, breaker and 10-4 stuff is a bit beyond me ;-) I'll have a look at that site ...

24th Jun 2011, 12:58 pm	#12
neon indicator Retired Dormant Member Join Date: Jun 2010 Location: Co. Limerick, Ireland. Posts: 1,183	Re: Eddystone Crystal Calibrator Makes CW sound 800 Hz tone if you adjust BFO 800Hz off the carrier. Otherwise you can't really hear morse except as a reduction in background noise when "key down". It's a very primitive way of listening to SSB, but if the BFO is stable and the main L.O. is stable then it works well. Usually a gentle touch on main tuning and BFO to "clarify" an SSB signal. The Crystal calibrator will be very clear with BFO working. If you are exactly on frequency, then if BFO is EXACTLY centre of IF you hear nothing. as you move BFO higher OR lower you get 1st a deep motorboat and eventually a tone which get higher as you change. Having the BFO set to centre frequency of IF and mistuning the main tuning either direction has same effect except when the tone gets high pitched it cuts off suddenly when you reach either edge of the IF bandwidth.

24th Jun 2011, 4:06 pm	#13
Skywave Rest in Peace Join Date: Jun 2006 Location: Chard, South Somerset, UK. Posts: 7,457	Re: Eddystone Crystal Calibrator Patrick - you have a P.M. Al. (Skywave) / June 24th. //