Eddystone 840A deafness.

[G6Tanuki]

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.

[Sideband]

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!

[G6Tanuki]

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!

[Sideband]

Originally Posted by G6Tanuki

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]

I sympathise! I had a similar problem and had to replace caps...not easy and involved removing the shaft connecting the switches together......I also had a problem with the highest frequency band where a coil had never been soldered...I doubt it had ever worked.

[G6fylneil]

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!

[turretslug]

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.

Originally Posted by G6fylneil

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!

Very true, a sad sign of the times! There's still a few nuggets out there though.