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1st Feb 2023, 5:27 pm | #1 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Eddystone 840A deafness.
My Eddystone 840A became somewhat deaf; I did the usual checks - voltages on valve-electrodes all showed within-acceptable-tolerances [remembering that the Eddystone manual shows voltages as measured with a 20KOhms-per-volt meter and I was using a DVM which presents a near-as-dammit infinite impedance so the voltages would be expected to be a schmierchen higher].
So - why the deafness? I did the 'traditional' thing of signal-injecting [remembering this is a hot-chassis AC/DC receiver] from the audio output and working backwards to the antenna sockets. Everything looked to be working fine - injecting RF at the grid of the RF amp showed it needed only 1.5 Microvolts to give a healthy output. But to get the same result when squirting RF into the antenna-socket needed more like 3 Millivolts for the same audio output. And the lossiness was the same across all four wavebands. But I noted something a bit strange: Now, because it's an AC/DC radio the two ends of the RF tuned-curcuit primary windings are connected to the A1 and A2 terminals via high-voltage-rated ceramic capacitors. There is a link that can connect A2 to the cabinet/local-ground for use with single-ended longwire type antennas, or the link can be removed and the feeder from a dipole can then be connected to A1 and A2. Strange thing was that the presence or absence of the link made no difference. And there was a great difference in sensitivity depending on whether I injected the signal into A1 or A2 with the link removed. A-Ha! Seems that one of the two ceramic capacitors was open-circuit, and the coupling of the antenna to the first RF tuned-circuit was by way of inter-winding capacitance rather than by anything sensibly inductive! Both capacitors [C1 and C2 in the attached circuit-snippet] now replaced by 0.01uF 1.2KV rated ceramic capacitors [same as I use to couple small transmitter Pi-tank circuits to the anodes of the likes of 5763 PA valves] and full sensitivity is back!! You live and learn.
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I'm the Operator of my Pocket Calculator. -Kraftwerk. |
1st Feb 2023, 9:57 pm | #2 |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,580
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Re: Eddystone 840A deafness.
Interesting! I must check those on mine. It's also rather deaf but as it had been messed around with by Persons Unknown, I found the alignment was quite a bit out. It's still not good so those capacitors are definitely next on the list!
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2nd Feb 2023, 6:07 pm | #3 |
Dekatron
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 14,007
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Re: Eddystone 840A deafness.
I was surprised to find the dead capacitor; having seen reports from someone else on the Forum who rebuilt an Eddystone of similar vintage [though not the same model] and struggled with broken contact-fingers on the wavechange switch, I had traumatic visions of delving into the rather-well-packed and not-at-all-designed-for-accessibility-to-reworkers coil-box.
[I'd been in there a few years back, to fix an original 'it could never have worked on the highest frequency band so how did it ever pass any sort of quality-assurance process?' assembly error in the local-oscillator circuit] In comparison, the capacitors were relatively easy to replace. And while I was in there I replaced both even though only one had failed, on the basis that I now know both are OK and I shouldn't have to delve around in that area again. With both capacitors replaced, the VOLMET stations around 5.5MHz [my night-time listening-of-choice when insomnia strikes] are now much louder!
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I'm the Operator of my Pocket Calculator. -Kraftwerk. |
2nd Feb 2023, 9:04 pm | #4 | |
Dekatron
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,580
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Re: Eddystone 840A deafness.
Quote:
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There are lots of brilliant keyboard players and then there is Rick Wakeman..... |
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3rd Feb 2023, 3:56 pm | #5 |
Pentode
Join Date: Sep 2011
Location: Coventry, West Midlands, UK.
Posts: 210
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Re: Eddystone 840A deafness.
When turned on my 840c for the first time for many years, (after reforming the HT capacitors) I thought it was a bit deaf. But it turned out just to be a lack of transmissions to receive!
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8th Feb 2023, 8:06 pm | #6 |
Dekatron
Join Date: Nov 2011
Location: Surrey, UK.
Posts: 4,400
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Re: Eddystone 840A deafness.
When was it that the "X" and "Y" capacitor designations started to be used? I've seen some pretty ancient capacitors with these markings on them over the years. I suppose that this particular case in what we would now call a "Y" requirement failed in the desired manner though. As far as I can make out, the 670/840 series appear to have been derived from the 710/740-series type receivers but with a goodly sprinkling of insulating washers, bushes, strips and couplings to isolate case from chassis- quite impressive as a diligent adaptation of existing hardware for the contemporary requirement of a DC mains-capable cabin receiver, but I wonder if they would get away with it today in a "general sale" context.
Very true, a sad sign of the times! There's still a few nuggets out there though. |