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Old 16th Mar 2014, 8:55 am   #1
ScotsGhillie
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Default Three valve MW/LW 12V car radio.

After much delay, I have got mine working at last! Thanks to some great help from David Higginson, and some modifications, I now have a workable three valve car radio all ready to be fitted to my 1952 Riley.

I followed Howard's thread and replaced the old Plessey electrolytics and the HT suppression caps. I also faithfully copied Howard's really neat 1N4007 silicon bridge conversion of the selenium rectifier. (It certainly fooled one person!). By the way guys, I trust you are all aware that selenium is very nasty stuff and should never be touched with bare skin- "dispose of immediately using environmentally suitable means" - ie in the garbage ASAP. As in Howard's case, I did not touch the integrated twin HT (16uF) smoothing caps even though electrolytic. But more below.

Once on the bench, we checked out the voltages and all seemed reasonable with a nice solid 197V HT feed, but we got nothing but static from the speaker. :-(( This turned out to be a totally duff booster aerial. I think these must be VHF only, because a coat hanger gave a better signal. There was a Hunts capacitor which got changed out, but with the coat hanger and some patience with the aerial trimmer and pre-tuning on LW, I got a reasonable result. (R4 LW on the preset tuning is quite acceptable).

However general MW sensitivity is terrible, loads of static and hum with the AGC maxing out. So I confess I cheated by adding an additional C-L-C (47uF/1mH/47uF) HT smoothing section in series to the output of the bridge - it helped reduce noise but not as much as hoped for.

WD40 worked well to quieten down the noisy volume pot and wave change switch - all nice and silky smooth now.

However I discovered that the valve sockets were loose and a source of noise. Given the rather rough suspension of the Riley, I therefore added some alloy clips to hold the valves steady - not particularly pretty or original, but I can't lay my hands on any of those valve retainer springs and I think they use a different base anyway.

I have ordered another booster aerial which at least claims AM coverage and on it lies all my hopes for a halfway acceptable MW reception.
I note in another thread a comment about poor RF sensitivity and the use of an RF booster - it would be interesting to hear if anyone has a suggestion in this regards if the booster turns out to be a duffer.

Cheers
DG
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Old 16th Mar 2014, 7:32 pm   #2
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Default Re: Three valve MW/LW 12V car radio.

What are the valves? My last experience of this kind of radio had an ECH83 as frequency-changer, an EF97 as IF-stage, a couple of germanium diodes as detector/AGC, and another EF97 as 'space-charge' audio driver to an OC78 and class-A OC28 power-output stage giving a Watt or so of audio.

Intriguingly - its RF stages were permeability-tuned and its IF was around 270KHz. I used it as a "tunable IF" for a crystal-controlled 80/40/20/10--metre converter.
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Old 17th Mar 2014, 4:35 pm   #3
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Default Re: Three valve MW/LW 12V car radio.

ECL82 output, no transistors in this model. Not sure of the rest of valve line up as no longer on the bench.
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Old 17th Mar 2014, 5:51 pm   #4
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Default Re: Three valve MW/LW 12V car radio.

Ah I see - I was imagining it to be a bit later than you were discussing and thought it might be some sort of 'hybrid'.

One thing to consider - the switching behaviour of selenium rectifiers is rather 'soft' - the modern silicon replacements are a lot 'harder' and could now be combining with the rough waveform from the vibrator to generate significant RFI that wouldn't have been there with the selemium rect.

Putting a few hundred pF across each diode, and possibly something like a 100-ohm resistor in series with each diode as well, can go a long way to tame this sort of rf 'hash'. I had to do this when I replaced the age-enfeebled full-wave selenium bridge rectifier with silicon diodes in my ex-military R209 receiver.
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Old 17th Mar 2014, 6:07 pm   #5
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Default Re: Three valve MW/LW 12V car radio.

Thank You,hope DG see,s this post.
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Old 18th Mar 2014, 7:00 am   #6
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Default Re: Three valve MW/LW 12V car radio.

The valve line up is as follows:-

ECH81 heptode & triode RF

EBF89 diode & pentode IF

ECL82 triode & pentode AF

Circuit diagram, voltages and alignment procedure available on two page copy of 1960 Ford Service manual.

I note the point about the silicon switching spikes but I thought my smoothing capacitors and series choke would have cleaned up the HT, anyway will add some pF as soon as I can get to the components shop.

Big issue is lack of MW RF sensitivity....still waiting for latest booster, when it arrives I will update the post.

Cheers
DG
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Old 18th Mar 2014, 11:40 pm   #7
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Default Re: Three valve MW/LW 12V car radio.

Would this by any chance be the, I think, Plessey made radio, that was also badged as a Vauxhall set, and supplied as an optional extra for Velox & Cresta cars in the late 1950s?
(Just curious!)
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Old 21st Mar 2014, 10:07 pm   #8
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Default Re: Three valve MW/LW 12V car radio.

Quote:
Originally Posted by G6Tanuki View Post
One thing to consider - the switching behaviour of selenium rectifiers is rather 'soft' - the modern silicon replacements are a lot 'harder' and could now be combining with the rough waveform from the vibrator to generate significant RFI that wouldn't have been there with the selemium rect.
You could try FRED's or BYV96E.
To improve on them put 0.1 disc ceramic caps across each of them..

Tubeglow.

Last edited by Tubeglow; 21st Mar 2014 at 10:18 pm.
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Old 26th Mar 2014, 9:34 am   #9
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Default Re: Three valve MW/LW 12V car radio.

I also had this model in a 1960 Ford Zodiac MK2. The volume control with it's long threaded mounting spindle was available from 'ENFO' [Engineering division Ford Motor Company] in 1968 when I owned the car. J.
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Old 26th Mar 2014, 2:41 pm   #10
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Default Re: Three valve MW/LW 12V car radio.

Whats a FRED?
 
Old 26th Mar 2014, 7:11 pm   #11
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Default Re: Three valve MW/LW 12V car radio.

Quote:
Originally Posted by merlinmaxwell View Post
Whats a FRED?
Hexfred diode.
(Ultrafast soft recovery)

http://www.ixys.com/Documents/AppNotes/IXAN0044.pdf

Hope this helps.

This can help as well,

I used it in a B+ inverter circuit to remove the hash.
Between PSU and circuit, or between caps in a PSU after an inverter.

Just for interest.

Tubeglow.
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Old 27th Mar 2014, 11:52 am   #12
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Default Re: Three valve MW/LW 12V car radio.

Thinking about it afterwards, is it possibly Fast Recovery Electronic Diode? (FRED)
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Old 27th Mar 2014, 1:25 pm   #13
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Default Re: Three valve MW/LW 12V car radio.

Nearly.

Fast Recovery Epitaxial Diode.
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Old 27th Mar 2014, 1:36 pm   #14
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Default Re: Three valve MW/LW 12V car radio.

Thank You.

Had not read the pdf spec sheet.
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Old 2nd Apr 2014, 6:18 am   #15
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Default Re: Three valve MW/LW 12V car radio.

Well, the AM aerial booster arrived and does appear to work to the extent that the signal is better and the set is less sensitive to aerial trimming.

HOWEVER!!! The signal to noise ratio is still not really acceptable - having done a trial in the car, I can't see my passengers tolerating it.

I note all of the suggestions above and have worked on the HT, adding disk caps (but not the FREDs) and also a series dropping resistor to bring the voltages within spec. (HT was too lively - 192V instead of 168v). I also tested for stray RF using another AM radio aerial held over the open chassis - there is nothing untowards.

I have tweaked the RF inductors and trimmers per the service manual, albeit using Radio 4 instead of a sig-gen as source. This had made a small improvement.

I am now stumped, I don't want to start meddling with IF tuning (ferrites are tight anyway).

I am sure something has gone off spec. - any tips ?

Cheers
DG
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Old 2nd Apr 2014, 7:32 pm   #16
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Default Re: Three valve MW/LW 12V car radio.

The increase in signal/noise ratio will certainly increase with an amp.
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Old 8th Apr 2014, 11:47 am   #17
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Default Re: Three valve MW/LW 12V car radio.

As a matter of interest, the sensitivity figures quoted by the makers are as follows:-
MW <30uv for 0.5 watt o/p - LW <200uv for 0.5 watt o/p. Max Power o/p 1.7 watts @1kHz with 13v DC i/p. Incidentally I've just notice a 'typo' in Specifications. The ECL82 is described as (quote) Audio Amplifier and Output Rectifier !!
The coverage is described as 542-1600kHz MW, and one LW pre-set, adjustable between 160 & 245kHz (perhaps I should have used Kc/s, which was the term used back in the 50s!)
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Old 8th Apr 2014, 9:48 pm   #18
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Default Re: Three valve MW/LW 12V car radio.

So, if I read that correctly, if I can get an intelligible signal on LW, I should have lots of choice on MW, but it is not the case.
The MW sensitivity is rubbish - just one station can be made out (worse still it is endless Scottish football commentary - just how sad is that? )
Re-checking the circuit suggests that very few components could be implicated...
DG
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Old 9th Apr 2014, 8:41 am   #19
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Default Re: Three valve MW/LW 12V car radio.

I don't know what MW (local) stations you have in the Paisley area, DG, but I would have thought that R. Scotland on 810kHz would be a strong signal there, along with possibly Absolute R., Radio 5 Live & Talksport. Here in N. Oxfordshire the latter 3 are usually strong signals on 1215, 693, & 1053kHz respectively.
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Old 16th Apr 2014, 12:10 pm   #20
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Default Re: Three valve MW/LW 12V car radio.

For those who may be interested, attached are scans of the chassis and specifications of this radio
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