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Old 3rd Apr 2014, 11:56 am   #1
Wendymott
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Default Eddystone EC10

Hi peeps. I bought an Eddy EC10 for re engineering, into a more "modern" line up, such as silicon transistors etc, removal of old elect caps and carbon composition resistors, all is well, except the mechanical tuning gang is very microphonic and is a liability. Thus I bought 3 x 35 - 350pf varicap diodes, thinking it was going to be relatively easy to retro fit the VC diodes. Not so...On the HF range of 14 - 30 Mhz, the local oscillator stops at about 24 Mhz, above that it works ok thus 24 - 30 Mhz. I have tried re biasing the osc transistor, which does improve slightly but not down to 14 Mhz, the other ranges are also limited, but not as greatly. Anyone any ideas ?? Please...
I know its not strictly a Vintage subject in the purest form, but its certainly interesting.
Regards
Wendy G8BZY

Just to let you know....replacing the germanium devices plus the carbon resistors, certainly improved the noise figure...you don't know it is switched on until a signal is tuned.... sensitivity is better than 5uV on any band.

Last edited by Wendymott; 3rd Apr 2014 at 11:58 am. Reason: Additional data
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Old 3rd Apr 2014, 3:52 pm   #2
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Default Re: Eddystone EC10

At a guess, the mechanical capacitor would go much lower than 35pf. Adding a series capacitor might help, but would the combination then go high enough?

Begs the suggestion of an EC10 with memories, though!
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Old 3rd Apr 2014, 4:01 pm   #3
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Default Re: Eddystone EC10

When you say "the LO stops at around 24MHz" do you mean it won't actually tune to lower frequencies, or the local-oscillator stops oscillating at these lower frequencies?

If the latter, remember that varicap diodes generally have a lower "Q" than traditional variable-capacitors - so your osc transistor may need a bit more gain to overcome the losses incurred when switching to variable voltage tuning.
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Old 3rd Apr 2014, 7:38 pm   #4
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Default Re: Eddystone EC10

As said, Varactors have significantly lower Q (=Higher losses) than mechanical variable capacitors. High capacitance diodes are worse than lower capacitance types and wide tuning ratio ones (still not as wide as the old mechanical ones) are worse still.

On top of this Q degrades at lower tuning voltages = lower frequencies.

So oscillation stopping at the bottom end is a common problem.

You also are very sensitive to noise on the tuning line.

David
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Old 3rd Apr 2014, 8:01 pm   #5
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Default Re: Eddystone EC10

Could the shaft wiping fngers be corroded and a source of microphony, or maybe there's a bit too much (hopefully adjustable) endfloat? I'd certainly be inclined to try quite hard to solve whatever was causing it before resorting to varicaps. Even if it's (i.e. with varicaps) made to work at all frequencies on all bands presumably there will be scale correlation headaches?
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Old 3rd Apr 2014, 10:14 pm   #6
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Default Re: Eddystone EC10

Oooh, this is interesting, but it would have been a good idea to call the thread something like "Re-engineering the EC10", because the EC10 itself is rather less interesting and I for one nearly skipped the thread.

Re the original tuning gang, is microphony a known issue with the EC10, or is it just something particular to this set? I wonder what the options might be for getting the gang out and cleaning it and perhaps using something 'rubbery' on the mounting points. I would not be keen to start looking at end-float until other possibilities had been crossed off the list.

Re replacing the Ge transistors, did you stick with bipolar at the front or was a FET a possibilty?

I am quietly (in a very dark closet), 're-engineering' a very interesting Realistic DX-160. Mine has a BFO tuning capacitor that seems to have been in contact with a lump hammer at some time, and for reasons explained on another thread, I'm toying with idea of running that with varicap, but at ~455kHz, I assume that Q is probably not an issue.

Have you got a link to an EC10 service manual Wendy; would be useful to be able to see the subject in question

B
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Old 3rd Apr 2014, 11:11 pm   #7
Wendymott
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Default Re: Eddystone EC10

Hi Guys. Thanks a million for the interest............. I rather guessed it was "Q" rather than anything else that was the problem, but until somebody else suggested it......I was adrift...Bill, according to the Eddystone details the tuning cap goes from 35pf to around 365pf. David..... the spectrum analyser says the oscillation stops at around 24 Mhz...If you look at the schematic, there is a series 400pf cap on all the sections on that range, and a 330pf on the range below, etc etc, thus to negate the large value ... The tuning voltage would have been well decoupled and regulated,. I infact added the MK2 mod of a "bandspread" varicap to the LO, and with the, well cleaned supply it works a charm.
Bazz....... The tuning cap is already rubber mounted, but with trhe speaker in the same metal cabinet, serious feed back can ensue. The solution was to padd the speaker with foam rubber and also a shield around the tuning cap.
Turret slug...yes I had that sorted out, if the Vari caps would have worked full range..
I had cleaned and lubed the mechanism as one of the first jobs.
Just to finish off, I replaced all the OC171's with a BF970...changing some bias components, if necessary for maximum smoke.......the AGC action still seems to be relatively ok. Bazz I found the EC10 data on the Eddystone users group...
Dunno if that URL breaks the rules.. hope not..
Anyways... thanks guys....... the EC10 is now a lot more use than it was, and well worth the effort......I ...MAY change it to a + supply at some time in the future.. All the best
Wendy G8BZY
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Old 3rd Apr 2014, 11:39 pm   #8
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Default Re: Eddystone EC10

I've not got the circuit in front of me, but in general terms you'll find the oscillator is an amplifier tapped into a tank, and if you increase the loop gain a bit by changing the tapping ratio it may well start working.... the tap may be capacitive or inductive.

Another method is to try changing the bias current of the oscillator device. J fets make good oscillators, bipolars are OK, and mosfets are not so good - to make sweeping generalisations.

David
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Old 4th Apr 2014, 12:29 am   #9
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Default Re: Eddystone EC10

EC10: http://www.service-data.com/section.php/1947/1/ec10
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Old 5th Apr 2014, 1:35 pm   #10
Wendymott
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Default Re: Eddystone EC10

Hyas David.
I had already changed the bias of the oscillator .. it improved the LF coverage slightly but not to make a significant difference. I also take your reference to J Fets, its certainly an idea to look at........... hmmmmmmm. As it is at the moment the performance is much improved, so I may keep it on the back burner...I am now developing software to control a synthesiser for VHF use, if anyone has an interest in PIC software, maybe another thread could be opened.
Regards
Wendy G8BZY
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