UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Amateur and Military Radio

Notices

Vintage Amateur and Military Radio Amateur/military receivers and transmitters, morse, and any other related vintage comms equipment.

Closed Thread
 
Thread Tools
Old 7th Aug 2013, 6:05 pm   #1
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Eddystone 840C deafness

Hello all
A little guidance, please.
Eddystone 840C has lined up beautifully on all bands with sensitivity equally good using the signal generator.
However, with antenna attached sensitivity on bands 4 and 5 is very good.
but on bands 1 and 2 and to a certain extent on band 3 sensitivity is poor.
Has anyone any ideas or perhaps encountered this on a 840c
Regards to all
Ian rose
ian rose is offline  
Old 7th Aug 2013, 8:01 pm   #2
HamishBoxer
Dekatron
 
HamishBoxer's Avatar
 
Join Date: Aug 2007
Location: W.Butterwick, near Doncaster UK.
Posts: 8,923
Default Re: Eddystone 840C deafness

Check all resistors as they are well known for being out of spec on Eddystones. Speaking from mine here.
__________________
G8JET BVWS Archivist and Member V.M.A.R.S
HamishBoxer is offline  
Old 7th Aug 2013, 8:56 pm   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,800
Default Re: Eddystone 840C deafness

If sensitivity was good with a signal generator and bad with an antenna, then it's possible you aligned it with a source impedance different to the antenna, and the antenna is pulling the tuned frequency of the first RF tuned circuit. There used to be lots of discussion over what was the right sort of dummy aerial to use to connect sig gens to radios. Is there an aerial trimmer control on this set?

I'm afraid I can't remember much of the last time I was inside an 840, I've only got an EA12 here.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 7th Aug 2013, 10:34 pm   #4
g4aaw pete
Heptode
 
g4aaw pete's Avatar
 
Join Date: Jan 2008
Location: Birchington Kent, UK.
Posts: 595
Default Re: Eddystone 840C deafness

Hello Ian

I don't currently have a working 840c, but I do have a Mimco 2232. This is an almost identical circuit to the 840.

After reading your post, I switched on mine & can report there was plenty to listen to, right up 28Mhz. Certainly, it isn't as lively as my 680X, but that has an extra RF stage.

Wonder what antenna you're using on the 840?

Regards
Pete
g4aaw pete is online now  
Old 8th Aug 2013, 1:03 am   #5
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Eddystone 840C deafness

Quote:
Originally Posted by ian rose View Post
With antenna attached sensitivity on bands 4 and 5 is very good.
But on bands 1 and 2 - and to a certain extent on band 3 - sensitivity is poor.
Ian Rose
If you think about what you've stated, it is the combination of your aerial and the set working together that is producing weak or zero signals. Since the set returned a good set of performance figures when connected to your signal generator, that throws suspicion on your aerial, and its coupling arrangements to the set.
We need more information about the aerial you are using and those aerial-to-set coupling arrangements.

Al. / Aug. 8, 2013 //
Skywave is offline  
Old 8th Aug 2013, 10:33 am   #6
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Re: Eddystone 840C deafness

Hello Hamish I have checked all R,s and replaced most. Likewise the C,s with the exception of the silvered micas.

Hello David I think you might have a good point there. The set imp at antenna is 75 ohms but my generator is 50 ohms. do you think I will solve that by realigning bands 1 and 2 with the generator loosely coupled with a loop of wire near the ant socket? there is no aerial trimmer on the set.
Hello Pete Yes, indeed, there are plenty of signals on those higher bands.
I have a RA17 and an AR88 here which are on most of the time and they confirm that.
Hello al [skywave] I am using a long wire [about 75 feet straight into the antenna socket, with a bridge in place to earth the other dipole connection as on the circuit schematic] which is producing plenty on Bands 4 and 5.
Thanks all and further comments are most welcome.
Ian
ian rose is offline  
Old 8th Aug 2013, 2:11 pm   #7
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Eddystone 840C deafness

Quote:
Originally Posted by ian rose View Post
The set impedance at antenna is 75 ohms but my generator is 50 ohms. Do you think I will solve the problem by realigning bands 1 and 2 with the generator loosely coupled with a loop of wire near the antenna socket?
Ian
It might do, but I doubt if it will make that much difference. I still reckon the trouble lies with your aerial, etc. Try interposing an AMU between the aerial and the set. If you don't have an AMU, it's simple to built one that will be suitable. And also make sure that your have a good R.F. earth running to the set as well.
Having said all that, when you re-aligned this set, are you really sure that you didn't align the front-end tuned ccts. to the image frequencies? On many Eddystone radios, that is a remarkably easy mistake to make .

Al. / Aug. 8, '13. //
Skywave is offline  
Old 8th Aug 2013, 3:50 pm   #8
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Eddystone 840C deafness

Could be worth checking that you have actual DC continuity through the antenna coupling-windings on all bands: I've seen more than one radio where the primary of the antenna tuned-circuit has been 'fried' and is open-circuit - the radio usually still works a bit by virtue of the stray capacitive coupling between the remains of the primary winding and the tuned secondary, but is deaf as a deaf-thing.
G6Tanuki is offline  
Old 10th Aug 2013, 12:33 pm   #9
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Re: Eddystone 840C deafness

Hello Al
I have re-aligned bands 1 and 2 with generator loosely coupled to ant input resulting in a slight improvement. You were correct --- Generator frequency was being pulled by the set with direct conn of generator. My counter indicated a pull of around 3mhz.
Am next to check continuity of all coil windings on bands 1 and 2 as suggested by Tanuki.
ian rose is offline  
Old 10th Aug 2013, 3:30 pm   #10
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Unhappy Re: Eddystone 840C deafness

Goodness me! - So you're saying that you found that the generator freq. was being de-tuned by the high degree of coupling between it and the receiver ? And the extent of that de-tuning was as much as 3 MHz? Doesn't say much for that sig. gen. does it? What is the make and model of that sig. gen., please?

Al. / Aug.10, 2013 //
Skywave is offline  
Old 10th Aug 2013, 4:43 pm   #11
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Re: Eddystone 840C deafness

Hello al
Your surprise prompted me to re-examine and I found that at 14 and 28 the frequency was unaffected by the set [no pulling at all!]do not know what happened before.
Nevertheless I still achieved a minor improvement with loose coupling.
Generator is Marconi Tf995B/5. counter is Insteck SFG2104.
I am now engaged on examining all continuities around those bands and the operation of the wavechange wafers [for the 4th time!] Lower bands [4 and 5] remain v good.
cheers Ian
ian rose is offline  
Old 10th Aug 2013, 11:03 pm   #12
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Eddystone 840C deafness

O.K. Ian, so now you're making some headway: that's good, of course.

But I am still confused about some of the things you have reported.
In post 9 you said that the frequency 'pull' was 3 mhz. I took that to mean 3 MHz, not 3 milli-Hertz: 3 MHz seems much more likely. In that case, I assume that the freq. counter was displaying a freq. that was 3 MHz 'out', compared to the dial setting on the Marconi sig. gen. Now depending on your physical coupling arrangements of the sig. gen., the counter and the radio, it could have been that the amount of signal actually reaching the counter was insufficient for it to make an accurate measurement. That seems far more likely than the freq. of the sig. gen. was being 'pulled' off frequency. Then there is the question of what type of modulation - if any - you were using on the signal from the generator. Some counters get awfully confused if the modulation is A.M. and with a modulation depth exceeding about 30%.

I'll be very surprised in you find any O/C primaries on any waveband, since (a) that is a very rare fault, in my experience, and (b) you did say earlier that the measurements made with the sig. gen. returned 'sensible figures' - or words to that effect - which, in turn, caused me to make my comments about your aerial arrangements, etc. - which I still think are a possible contributor to your 'lack of signals' problem. And yes, dirty contacts on the wavechange switch are always a potential source of trouble on Eddystone radios - but, fortunately, usually easily fixed.

Keep at it - you'll get there eventually. The 840C is not an unduly complicated set.

Al. / Aug. 10, 2013 //
Skywave is offline  
Old 11th Aug 2013, 3:53 pm   #13
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Re: Eddystone 840C deafness

Hello Al
Thank you for your further comments.
I did report a large frequency pull on post 9. I think that has now been eradicated since now that I am loosely coupling the gen to the counter I am able to give the counter a substantial signal to work with. 10mv on the gens adjustment. The counter is directly coupled to the gen.
So, I think 'pulling' is a thing of the past. Mod is AM at 1Khz [the gen is not producing 400cs at present]. The gen is very stable.
Aerial is long wire [75feet or so] and good earth is provided.
The same aerial gives good results on the RA17 and the AR88.
I have now discovered that a contact on the wafer of switch S1D has been badly bent. However it is the contact that earths the mixer coils for band 1 when other bands are in use so has no effect on this fault I am investigating. Nevertheless I think I am going to have to remove the switch spindle in order to ease that wafer over so I can inspect the side that is hidden from view in the set for damage.
I have had it out before in order to replace Cs and Rs which are hidden away beneath the switch. When I first acquired the set I had to repair the wafer of switch S1a which had been fractured [araldite] I also had to rebuild the V1 section because even the valveholder was missing! So whilst I am 'in there' I will have another good look around and take some readings.
I have also taken on board your comments re possible alignment with images at the front end. Am checking this.
Thank you for your encouragement and optimism!
Ian
ian rose is offline  
Old 11th Aug 2013, 11:52 pm   #14
Skywave
Rest in Peace
 
Join Date: Jun 2006
Location: Chard, South Somerset, UK.
Posts: 7,457
Arrow Re: Eddystone 840C deafness

Ian - it looks as though you are now very close to solving your reported problem: the faulty / damaged w/c switch.

I now understand your arrangement for the counter: directly coupled to the generator: fine. As for the "3 MHz out" - who knows? Operator error? Anyway, that's a thing of the past now: time to forget it and move on.

As for a the w/c switch having a coil short-circuiting connection, that's quite usual. The idea is that lower freq. coils have a s/c placed across them when higher freq. coils are selected. The idea is by doing that, unwanted resonances that might occur on unused o/c coils then do not occur. Such unwanted resonances will desensitize reception when that higher freq. cct. is in use. You may have been aware of all that, sure, but I just thought I'd mention it in case you didn't.

Since you get good reception on your other two sets with your 75' wire, I think we can rule our aerial trouble. But I would still recommend building an AMU to install between your aerial and then to the set you have selected for reception. The AMU will probably require a re-tune as you go from set to set when receiving the same freq.

Accidentally mis-aligning the front end ccts. to the image freq.: as you are probably aware, this is most easily done on the higher freq. bands, especially one that tunes to 30 MHz and using a 465 kHz IF. Even if the IF is something like 1620 kHz, it's still easy to get it wrong (talks from experience! ) A check for mis-alignment with that now follows: if the local osc. is on the 'high side', (usual case for comms. receivers), the image will be on the 'high side' too. Here's an example. You set the generator to 28 MHz and peak up the R.F stage and mixer trimmers. Now re-tune the generator to 28 MHz + 465Khz + 465 kHz (= 28,930 kHz). If you can 'hear' the generator - and at a reduced level - then you haven't aligned on the image. (Note: that arithmetic applies for a 465 kHz IF. If the IF is other than 465 kHz, just 'plug in' the different values).

Get that switch sorted out, do a re-align. (may not be necessary) and then come back and tell us what you then have: I anticipate a success story at that point! Good luck.

Al. / Aug. 11, '13 //
Skywave is offline  
Old 12th Aug 2013, 9:29 am   #15
ian rose
Pentode
 
Join Date: Aug 2010
Location: Isle of Wight, UK.
Posts: 217
Default Re: Eddystone 840C deafness

Will co, Skip!
ian rose is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 11:01 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.