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Old 13th Aug 2019, 8:57 pm   #1
bill knox
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Default Chapman radio

Following on from this thread: https://www.vintage-radio.net/forum/...d.php?t=157847

Hi

I have made up a small audio amplifier on the chassis as there was a hole for a B9A valveholder, I used an ECL86, it all works OK but it appears that I am getting a very high frequency oscillation, not sure if it is the IF breaking through or that the circuitry is oscillating.

What I would like to know is, if I put a couple of ferrite beads on the audio input connection, how effective would they be.

Any answers on a stamp would be grateful.

Bill

Last edited by AC/HL; 14th Aug 2019 at 12:19 pm. Reason: Continuation
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Old 13th Aug 2019, 10:36 pm   #2
Herald1360
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Default Re: Chapman radio

Nothing to lose by trying but you might do better with a couple of grid stoppers (10k or so) on the control grids.

Is there any decoupling in the HT feed to the new circuitry?

Is its grounding layout sanitary?
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Old 13th Aug 2019, 11:36 pm   #3
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Default Re: Chapman radio

The BBC RC1/1 was a Chapman S6 (the S6BS without bandpsread, otherwise the same circuit) with the addition of a power supply, a line driver amplifier and a carrier alarm by the BBC. In case it is of any value, here is an excerpt from the schematic showing the power supply and line driver including decoupling arrangements for the latter.


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Old 19th Aug 2019, 8:59 pm   #4
bill knox
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Default Re: Chapman radio

Hi all

I finally found out what the problem is with the audio amplifier that I built.

It is the output transformer radiating back into the amplifier, I have managed to remove most of the oscillation by fitting a 10nF capacitor across the primary winding, there is still a slight oscillation but nowhere near what was there before, as the oscillation was at quite a high frequency I thought maybe even a lower capacity capacitor.

Any views please

Bill
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Old 20th Aug 2019, 10:33 am   #5
Herald1360
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Default Re: Chapman radio

Maybe reversing the primary connections on the OP TX would help?

Is there any negative feedback loop in your ECL86 amplifier?
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Old 20th Aug 2019, 9:01 pm   #6
bill knox
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Default Re: Chapman radio

HI

Yes, there is NFB installed that was originally around the wrong way but that is OK now, what would happen if I reverse the primary winding and I also assume that if I did that I would also have to reverse the secondary winding, or I would introduce positive feedback.

Bill
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Old 20th Aug 2019, 9:17 pm   #7
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Default Re: Chapman radio

OK, you're already ahead of me there!


It might just be worth trying reversing both or at least checking to see if the anode end of the primary is closer to the core. Is the amplifier stable without the feedback?


With the triode and pentode in the same bottle there's a higher potential for layout/grounding problems, though the common cathode in the ECL80 is probably the trickiest to tame!

You could try wrapping copper foil tape around the outside of the windings and core (NOT round the windings through the core, that would be a shorted turn!) and grounding that.
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Old 20th Aug 2019, 10:55 pm   #8
bill knox
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Default Re: Chapman radio

Hi

Also what I should have said, is that due to lack of space I had very limited space to fit the O/P transformer, it is mounted EXTREMLY close to the valve base connections and it is mounted in the only space available

Bill
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Old 20th Aug 2019, 11:24 pm   #9
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Default Re: Chapman radio

There may be a significant amount of residual IF getting into the output stage and being radiated- it's not unusual in the typical two-stage AF amplifier stage in radios like this (I assume that the ECL86 is being used in this fashion) for there to be a capacitor in the low hundreds of pF between the 1st AF stage anode and ground to filter IF that has got through this stage out of the subsequent AF output stage. Sometimes, there's also a stopper of perhaps 10-22k in series with the AF output stage control grid.

Apologies if you've already been through this!
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Old 21st Aug 2019, 8:36 pm   #10
bill knox
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Default Re: Chapman radio

Hi Turretslug

I did think about IF breakthrough and so I removed the 2 IF amplifiers and it made no difference I then also removed detector/AVC valve an EBF80 and that made no difference either, that is why I came back to AF instability in the audio stage, as I said I have put a .01uF across the primary winding and that has put paid to about 95% of the problem, I just wondered if there may be an optimum value of capacitor that would do a better job

Bill
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Old 21st Aug 2019, 9:30 pm   #11
Herald1360
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Default Re: Chapman radio

That size capacitor is going to hit the treble response noticeably I would think. It can't really be a cure, just palliative treatment.
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