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Old 25th Nov 2022, 9:36 pm   #1
Ed_Dinning
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Default EKCO AD76 modulation hum

Hi Folks, any thoughts on an AD76 (ac/dc set) with severe modulation hum?

The set has been re-capped and wavechange switches cleaned. all valves appear leakage free. Out of spec resistors have also been changed
The Speaker had previously change for a PM type and the HT dropper has been increased to allow for the across HT field coil being removed.

This set has interstation muting, which appears to work, would removing this help?

Thanks, Ed
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Old 26th Nov 2022, 3:26 pm   #2
cathoderay57
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Default Re: EKCO AD76 modulation hum

Hi Ed. First off I'd check and replace the 0.1uF mains filter caps between live and chassis and neutral and chassis (although I expect you've already done so). These are marked C30 & C31 on the Manufacturer's service sheet. If that doesn't work try a 0.1uF decoupler across the HT line near the mixer/oscillator HT feed point.
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Old 26th Nov 2022, 7:32 pm   #3
Ed_Dinning
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Default Re: EKCO AD76 modulation hum

Hi Thanks for that, already been done with no effect.
I'll re-test the valves for leakage, but they were OK.

Removal of IF and FC top caps removes the signal and the mod hum

Ed
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Old 26th Nov 2022, 7:52 pm   #4
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Default Re: EKCO AD76 modulation hum

Have you tried with a signal generator input at the antenna terminals for a signal so as to eliminate mod hum via the antenna ?

Lawrence.
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Old 26th Nov 2022, 8:08 pm   #5
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Default Re: EKCO AD76 modulation hum

A hundred ohms DC resistance in the form of an inductor is going to have an appreciably higher impedance at 50Hz than a plain 100 Ohm resistor. Any room to squeeze a choke in?

I think the resistor may get the DC voltages on target, but it won't be as good for ripple.

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Old 26th Nov 2022, 8:40 pm   #6
ms660
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Default Re: EKCO AD76 modulation hum

The HT filter choke in the service data I'm looking at is 300 Ohms DCR, is the original choke still in circuit?

Lawrence.
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Old 26th Nov 2022, 8:49 pm   #7
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Default Re: EKCO AD76 modulation hum

In times past the first suggested fix for this would be to take the mains plug out of its socket and turn it through 180 degrees then re plug it.

Can you swap the phase and neutral wires over easily in your situation?? Just to see if it makes any difference??
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Old 26th Nov 2022, 9:59 pm   #8
Ed_Dinning
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Default Re: EKCO AD76 modulation hum

Hi Gents, thanks for the further suggestions. Original choke still in position and additional smoothing used after the additional dropping resistor in HT.
Hum still there on Ae or sig gen. Mains reversal no improvement.
I might go as far as to run an earth ring round the chassis picking up all the chassis connections

Thanks, Ed
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Old 27th Nov 2022, 9:40 pm   #9
Ian - G4JQT
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Default Re: EKCO AD76 modulation hum

When you say "modulation hum" I guess you mean there is only hum on strong stations, not between stations? This is a relatively common problem when listening to mains-powered pantry transmitters. Not sure this is relevant to you though.

Anyway, I put 22nF 600v caps across all the HT electrolytics, then across the rectifiers and/or across the HT secondary. This usually improves things, but not always. I find it's usually worse on AC/DC sets, but not always!

Switching off or unplugging seemingly unrelated items in other parts of the house can pinpoint the problem. My electric blanket (even when turned off) made it worse. But switching it on made modulation hum less noticeable, until a phone charger was used. Modulation hum was noticeable on my Murphy A96, until the Murphy A24 was switched on! USB-charging mains sockets don't help - even if not being used. I had to put X-capacitors across the mains which made a great improvement.

It's all very odd, so if anyone else suffers with this - especially when tuned to a pantry transmitter - just unplug lights, chargers, everything and see what happens. Good luck!
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