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Old 5th Mar 2023, 6:45 pm   #1
G6Tanuki
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Default Eddystone 840A hum modulation of local oscillator.

With conditions on 28MHz being rather good at the moment this has highlighted an issue with my Eddystone 840A - namely the local-oscillator has horrible hum-modulation which renders it unusable on 28MHz unless you want all stations to sound like they are gargling.

I've checked the HT rail, which doesn't seem to have an excess of hum - and adding 100uF of extra smoothing downstream of the choke made no improvement.

Listening to the local-oscillator using another receiver proves its impurity.

[though it does confirm the way the LO shifts in frequency when the HT line varies significantly - for example when adjusting the RF Gain!]

This is an AC/DC receiver and I am starting to suspect that the humminess is caused by the fact that the UCH42 frequency-changer's heater is at the 'hot' end of the series-heater chain so there's a risk that the electron-flow in the triode part [LO] is being subjected to the electrostatic field from the parts of the filament that protrude from the ends of the cathode-tube.

Somewhere I have a new UCH42 which I will try tomorrow; I will also try adding some capacitors from the ends of the UCH42 heater to ground to ensure that it's not letting the heater be 'live' to RF; I remember some similar things [including isolating chokes] being used with the likes of the ECC85 in VHF/FM receiver front-ends.

Any other suggestions? The LO remains surprisingly pure on the lower-frequency bands where SSB reception is perfrctly OK.
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Old 5th Mar 2023, 6:52 pm   #2
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Default Re: Eddystone 840A hum modulation of local oscillator.

What about adding LF decoupling on the mix/osc cathode against 50Hz leakage h-k.
There is only about 0.05uF there as RF decoupling. Add something much bigger in value and see what happens? Only needs to be rated to 50V or less.
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Old 5th Mar 2023, 6:55 pm   #3
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Default Re: Eddystone 840A hum modulation of local oscillator.

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What about adding LF decoupling on the cathode against 50Hz leakage h-k.
There is only about 0.05uF there as RF decoupling. Add something much bigger in value and see what happens? Only needs to be rated to 50V or less.
A good suggestion! Will experiment tomorrow.

I definitely get the feel that the issue is due to the cathode [or the ends of the heater acting as a pseudocathode] being 'wobbled' by coupling to the heater, which will be sitting around 120V above ground.
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Old 5th Mar 2023, 6:57 pm   #4
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Default Re: Eddystone 840A hum modulation of local oscillator.

I had an 840C with all these issues.

In the end I put a mains transformer in it and went for E series valves. Long gone now.

Hope it works!
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Old 5th Mar 2023, 7:17 pm   #5
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Default Re: Eddystone 840A hum modulation of local oscillator.

Interesting to see I'm not alone in this problem. Will first try the new UCH42, then the extra cathode-decoupling.

I can also try feeding the beast from a well-smoothed DC supply: I've got something here that can do isolated-from-mains 110-127V DC at 5A.

Then there's always the down-converter option: in times-past I listened on 28MHz using a homebrew front-end [EF54 RF amp, EF54 mixer and a similar-based triode whose number escapes me working as third-overtone oscillator using an 8MHz xtal - it was sort-of a reworking of the WWII "RF" units but crystal-controlled] with 28-30MHz translated down to 4-6MHz. Sensitive enough to let me detect 'sky noise' ...

I've got a load of such valves here, along with some SP61. And diecast boxes and the little RF-unit Wingrove&Rogers variable capacitors.

So many Projects, so little time!
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Old 6th Mar 2023, 6:37 pm   #6
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Default Re: Eddystone 840A hum modulation of local oscillator.

Well, a NOS Philips "Miniwatt" UCH42 has not made significant difference to the hum.

Next step [tomorrow] comes experimental additional decoupling-to-ground of the mixer/oscillator cathode.
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Old 6th Mar 2023, 8:24 pm   #7
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Default Re: Eddystone 840A hum modulation of local oscillator.

Not that it helps you, but my S870 also has hum issues, even though I fitted a 240 to 110 isolation transformer inside to replace the dropper, I have hum in the speaker until I get a decent signal in which case it either disappears or I just can not hear it over audio. I put it down to the osc and audio valves being up the heater chain. So I will be interested to hear how you get on.

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Old 6th Mar 2023, 8:39 pm   #8
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Default Re: Eddystone 840A hum modulation of local oscillator.

The HRO gets hum in the speaker from not enough decoupling for the screen of the AF output pentode. (42 or 6V6 )

Very annoying if using low Z headphones from O/P transformer.

Adding some 10uF there improves it quite a bit.
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Old 7th Mar 2023, 3:24 am   #9
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Default Re: Eddystone 840A hum modulation of local oscillator.

The '840A isn't a printed circuit built set is it?

If not, I would alter the heater wiring such that V1 and V2 are at the chassis–end, with a series resistor of 150 Ω 5 W in series with V1 heater to drop the 1.5 V difference between the UAF42 and UCH42 heaters, then V3 and V4 in parallel, reconnect V6 where V2 was wired with another 150 Ω 5 W resistor to drop the 1.5 V needed to replace the UAF42 where the UCH42 heater was – me thinks the BFO valve probably would work where V2 was originally connected in series with the UY41 heater – the original wiring was done such at the UCH42 plus the UY41 in series gives 45 V, the heater voltage of a UL41 valve!

Unless you still listen to SSB a lot, you probably wouldn't use the BFO all that much!

Failing that, why not try half–wave rectifying the heater current with a 1N4007 diode, using a surge limiter of 22 Ω 5 W in series with the diode, with a 100 μ 400 V reservoir capacitor at the top of the heater chain, followed by a 100 Ω 25 W aluminium–clad resistor in series to drop the extra 20–30 V given by rectifying 115 V.a.c?

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Old 7th Mar 2023, 3:53 am   #10
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Default Re: Eddystone 840A hum modulation of local oscillator.

. . .If you're still stuck, or it's not practical to try rewiring the order of the heaters, I'd add a small 6.3 V transformer to supply all the heaters in parallel, replacing the valves with two ECH42s, three EAF42s, one EL41 and an EZ40 with its two anodes strapped across for half–wave rectification – this would allow one side of all heaters to be earthed, and the heater/cathode insulation of the EZ40 is rated to withstand the full rectified output when the maximum allowable r.m.s. voltage is used on its anodes, so a separate 6.3 V winding for the EZ40 is unnecessary!

Chris Williams

PS!

The dropping resistors in series with the UAF42 heaters in my previous suggestion should have been stated as 15 Ω 2 W, NOT 150 Ω 5 W – senior moment time !
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Old 7th Mar 2023, 3:25 pm   #11
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Default Re: Eddystone 840A hum modulation of local oscillator.

PPS!

If you decide to go down the "E" series valves route, then if you can find a 6 V 3 A transformer with a dual–primary or 115 V tap on the primary, you can eliminate the mains dropper resistor altogether by feeding the anodes of the EZ40 from the 115 I primary tapping!

If you want to go down the isolated–chassis route, I think a transformer from a scrapped/parts mule 1950s a.m./f.m. "woody" (it would need to be an "upright mounting" type) will satisfactorily power the 840A, if you feed the EL41 anode from the EZ40 cathode direct, use two smoothing resistors to give you h.t. Lines of 110 V and 170/190 V, with the 170/190V line feeding the screen–grid of the EL41 and the EAF42 a.f. amplifier anode circuit, and the 110 V line for the other valves.

If you can get a transformer that's good for 80 mA from it's h.t. secondary then you can add a 108C1 neon regulator to feed the Local Oscillator of V2 and the B.F.O. V6, this modification would give you a virtually silent background and much more stable reception!

If you'd like me to clarify what I've said with diagrams, PM me your email address!

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Old 7th Mar 2023, 6:13 pm   #12
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Default Re: Eddystone 840A hum modulation of local oscillator.

Some interesting suggestions. I don't really want to go the whole-hog and replace the valves for 6.3V ones [if I did that there would be some EF184s and ECH81s in there, probably an ECH81 as a combined BFO/product-detector in the hole where the BFO valve currently sits, semiconductors for the HT rectifiers and AM/AGC detectors, and an ECL80 as the audio preamp/output].

Like I said, next step will be enhanced decoupling of the UCH42 cathode, then adding a bifilar-wound choke to its heater so the heater is not 'live' to RF. Maybe repositioning it in the heater-chain so it's not at the 'hot' end.

I want this receiver to be primarily good for listening to amateur SSB transmissions.

Some effort to stabilise the HT to the LO stage might be on the agenda; either a chain of Zeners [I have a bandolier of 5.1V ones here] feeding something like a MJE340 pass-transistor, or I wonder if anyone has ever used the two-terminal IC regulators [TAA350?] that were used to stabilise the 33V Varicap-tuning-rail on 1970s TVs?
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Old 7th Mar 2023, 6:33 pm   #13
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Default Re: Eddystone 840A hum modulation of local oscillator.

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Originally Posted by G6Tanuki View Post
I want this receiver to be primarily good for listening to amateur SSB transmissions.
Well, the sunspot numbers are climbing and we've already had some walloping good openings on 15m and 10m, so the 'big fun' activity is moving up onto the higher bands, where the 840 will come under most stress for stability and LO hum. It's rather marginal on selectivity for SSB, anyway and that affects all bands when they're crowded. And then there's image rejection, or lack thereof, on the higher bands.

Perhaps you'd have had a better start with an EA12 or something of that ilk?

David
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Old 7th Mar 2023, 7:20 pm   #14
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Default Re: Eddystone 840A hum modulation of local oscillator.

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Quote:
Originally Posted by G6Tanuki View Post
I want this receiver to be primarily good for listening to amateur SSB transmissions.
Well, the sunspot numbers are climbing and we've already had some walloping good openings on 15m and 10m, so the 'big fun' activity is moving up onto the higher bands, where the 840 will come under most stress for stability and LO hum. It's rather marginal on selectivity for SSB, anyway and that affects all bands when they're crowded. And then there's image rejection, or lack thereof, on the higher bands.

Perhaps you'd have had a better start with an EA12 or something of that ilk?

David
Yes, the current solar-cycle is looking up. And I have a range of 'other' HF receivers [AR88D, RACAL RA217D/MA323, Eddystone S.750] for serious HF listening... and a FT897D or a PRC320 if I want to call the guys in the US.

But I'd really like to get this 840A working well-enough - it's my 'bedside radio' which I use for random tuning-around when I'm unable to fall into the arms of Hypnos or Morpheus.
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Old 7th Mar 2023, 8:32 pm   #15
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Default Re: Eddystone 840A hum modulation of local oscillator.

Mmmm, the 840 sounds like it will need an IF filter if you want to listen to SSB comfortably. The Hammarlund design?

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Old 7th Mar 2023, 9:09 pm   #16
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Default Re: Eddystone 840A hum modulation of local oscillator.

I think G6T would like to leave it alone apart from getting rid of the LO hum.
And so did I in his position.

I modified my 840C because I was curious. It was what I had and I wanted to see if it could be improved. To begin it didn't need better filters for 10m use, firstly it needed not to hum on SSB.
Then maybe to get rid of the images which picked up CB and after that the microphony which caused it to howl around when set loud.

Did I mess it up? Yes - very probably. And when I was done it was no longer an 840C.
That was reflected in the price I let it go for.

To find a minor mod which removes the hum would be very rewarding without turning it into something else. That's why I was pleased with adding screen grid decoupling on the OP Valve of an HRO to remove AF hum. I must have added something as a screen resistor since there was nothing in the original circuit. I will have to investigate what that was. Too long ago.

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Old 7th Mar 2023, 10:00 pm   #17
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Default Re: Eddystone 840A hum modulation of local oscillator.

Perhaps try running it on a DC supply ?
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Old 7th Mar 2023, 10:22 pm   #18
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Default Re: Eddystone 840A hum modulation of local oscillator.

Is there space for a wee 100ish volt rudimentary dc supply?
I once made one for a 670, using a small toroidal transformer mounted above the deck, pass transistor on the lumpy side before the reservoir capacitor, which kept things a bit cooler than the conventional layout...(I can tell someone's going to shoot the idea down, but sometimes things just work!) It didn't take up all that much room, specially with the dropper (which in this instance was completely wrecked) out of the way.

Cheers,

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Old 8th Mar 2023, 2:30 pm   #19
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Default Re: Eddystone 840A hum modulation of local oscillator.

Adding 100uF from the cathode of the frequency-changer to chassis has helped significantly.

Next experiment will be to run it off DC; I have a 220-110V autotransformer here, a nice big 400V-rated bridge-rectifier, and 1000uF of 450V electrolytics...

Yes I accept the 840A is nothing really more than a broadcast-radio-with-a-BFO-in-a-steel-case, I'm not trying to make something super-selectiveor perfectly-sensitive [I've got plenty of radios that meet those needs]. I just want it as a bedside radio for tuning-around when Insomnia strikes!
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Old 8th Mar 2023, 3:26 pm   #20
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Default Re: Eddystone 840A hum modulation of local oscillator.

That's good news. I wish I had tried that on my 840C. Maybe a perfectly smooth HT will sort it.
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