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Old 18th Jan 2017, 12:07 pm   #1
'LIVEWIRE?'
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Default Low Humming noise in Hacker SovereignIII

I haven't yet opened or carried out any checks on this radio, or tried it on batteries, but I suspect the low humm, which is overriden by turning the volume up, but not affected by the tone controls, is caused by faulty electrolytic(s) in the PSU. Would I be correct?
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Old 18th Jan 2017, 7:12 pm   #2
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Default Re: Low Humming noise in Hacker SvereignIII

Hi Livewire,

Is this a mains / battery one? if it is then the first thing to suspect is rectification and smoothing, the next thing I would check is to see if any intermediate stage is being coupled correctly then check the audio stage input and output, you should even be able to trace this with the aid of a scope at different points of the circuit. Hope this helps.

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Old 18th Jan 2017, 9:27 pm   #3
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Default Re: Low Humming noise in Hacker SvereignIII

It is a mains-battery model, Ken, and, since my posting, I've tried it with an external DC supply connected across the Battery Terminals. There's no audible hum at all when powered that way, so, as I suspected, the trouble is in the PSU, which is in a metal box, so the next thing is to open that up and check the rectifier & smoothing capacitor(s) I could, and might scope the PSU DC o/p, though the fault may be obvious once I open up the PSU. All AM stations are about 45kHZ LF of where they should be on the dial, too, but that's another story!
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Old 19th Jan 2017, 1:32 pm   #4
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Default Re: Low Humming noise in Hacker SvereignIII

You would probably have noticed and mentioned the fact if it were, but is the hum by any chance silenced by placing a finger on the top panel? There's an internal screening issue on some Hacker models of this generation, and somewhere back in the Hacker group archives - I think - are empirical notes on where inside the radio to fix a small piece of foil-covered card to provide some necessary screening that seems to have been omitted at the design stage! If touching the top panel has no effect, yes, electrolytics seem the likeliest culprit.

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Old 21st Jan 2017, 6:27 pm   #5
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Default Re: Low Humming noise in Hacker SvereignIII

Im curious where this silver foil goes paul, any ideas?
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Old 21st Jan 2017, 10:48 pm   #6
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Default Re: Low Humming noise in Hacker SovereignIII

I couldn't find any notes, so I've found the Sovereign III I modified. The photo is of the tone control board A/108/6, and trapped between it and the cabinet there's a piece of white card, a little larger than the board such that you can just see its edges. There's foil glued to the cabinet side of the card: obviously the card needs to be such as not to be penetrated completely by any spikes of wire or solder protruding from the board, but not so thick as to cause physical stress. A slight hum when using Sovereign III / Super Sovereign models with their internal mains power supplies, which goes away if the top panel of the set is touched, seems normal, and screening the back of the tone control board in this way has eliminated it completely.

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Old 21st Jan 2017, 11:25 pm   #7
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Default Re: Low Humming noise in Hacker SovereignIII

Interesting will give it a go on the black knight
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Old 22nd Jan 2017, 9:50 am   #8
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Default Re: Low Humming noise in Hacker SovereignIII

The fault in the one I'm repairing is/was in the Power Supply, and consisted of three things (1) Faulty smoothing capacitor (2) loose cap on the 50mA Anti-surge fuse (3) faulty battery/mains c/o switch in mains socket. After dealing with (1) by fitting a new 2200uf Cap., I found that the o/p voltage was still intermittent (due to {2}), and was only about 14v (No Load) though it was nearer 18v at the +ve terminasl of the Smoothing Cap, due to high resistance in the B/M c/o switch. This didn't reliably respond to cleaning, so I'll fit another C7 switched socket saved from a scrap set (New ones seem to be unobtanium, btw)
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Old 22nd Jan 2017, 11:56 am   #9
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Default Re: Low Humming noise in Hacker SovereignIII

Perhaps repeating what others have said here, I've had the same situation with the same set that I've not investigated yet. However, it's fine on batteries but hummy on mains. Suggests the rectification components.
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Old 22nd Jan 2017, 12:14 pm   #10
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Default Re: Low Humming noise in Hacker SovereignIII

The PSU is a simple series pass transistor, with a zener diode between base & Negative(Ground) fed by a bridge rectifier (4 seperate diodesof the 1n400x type), so there are few components to check.
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Old 22nd Jan 2017, 12:29 pm   #11
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Default Re: Low Humming noise in Hacker SovereignIII

Have the diodes got 10n or so caps across each one? The reverse recovery RF hash from 1N400x diodes is notorious.

Is the 0V routing from the bridge to the reservoir and on to the regulator properly done with no common mode ripple current paths?
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Old 22nd Jan 2017, 2:27 pm   #12
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Default Re: Low Humming noise in Hacker SovereignIII

Yes, the caps are included. 100n each. EDIT: 1nF on earlier models.

I've just checked one of mine, and would describe it as hum-free providing you are holding the top plate. When not touching this, as described earlier, there is some low-level mains pickup. Though not objectionable unless you're in a very quiet room.

Interestingly, touching the Hacker badge makes it much, much worse

Happy to take some measurements if needed. But based on post #8, the OP has fixed it.

The smoothing capacitor should be 470uF, not 2200uF. I guess it won't hurt...

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Old 22nd Jan 2017, 4:21 pm   #13
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Default Re: Low Humming noise in Hacker SovereignIII

220uf Axial was all that I had to hand, Mark, and it was easier to fit than a radial. I wouldn't have thought 4 times the (nominal) capacitance would be a problem in a Transistor radio. BTW, there are caps. across each rectifier diode. When I've wired in the replacement Mains socket & M/B switch, the radio should be as good as new, perhaps including the alignment.
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