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Old 20th Jan 2021, 8:40 pm   #1
poppydog
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Default Ekco A222 Resistor Query

Father Christmas brought me a lovely Ekco A222 but I need some advice on some work its had done on it in the past.

Someone has changed the mains resistor droppers with something else and its caused me some confusion and concern.

I have not powered this thing up yet as I’m waiting for some suitably sized 10nf caps to arrive as a replacement for the grid coupling cap.

In its current state the dropper for the heater (R23) is 1.161k but should be 930R. The resistor to the anode of v4 (R22) measures 175R and the filter resistor (R21) across C27 and C28 has been soldered straight across the terminals on the dual can., this is only a 600R.

How well is this thing going to work with these modifications? I don’t want to be chasing faults that are caused by these modifications. By the look of the chassis with its 6 inches of dust on top, this thing hasn’t been powered up for a while.

Please see pics below.

Regards
Poppydog
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Old 20th Jan 2021, 8:42 pm   #2
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Default Re: Ekco A222 Resistor Query

Is this A222 or U222, the A series are usually AC only the U AC/DC?
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Old 20th Jan 2021, 8:43 pm   #3
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Default Re: Ekco A222 Resistor Query

Its the A222 Frank.
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Old 20th Jan 2021, 8:50 pm   #4
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Default Re: Ekco A222 Resistor Query

The RS "Universal" dropper is not that rare; if anything given that these days the supposedly-240V mains can go above 250V [it regularly does round here] a bit more resistance in the heater-dropper is not necessarily a bad thing.

The under-chassis smoothing resistor across the 2 smoothing-capacitor sections - yes it's a replacement but I wouldn't really fret about the precise value.

Measure voltages when the radio's fed from your actual mains-supply, and if they're within 10% don't fret.

[Remember though that the voltages listed in many old maintenance-documents assumed you were using a test-meter with a rather low Ohms-per-Volt rating; these days if you're using a modern DVM with a Megohm or more of input-resistance you should expect to see a bit higher voltage-on-test]
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Old 20th Jan 2021, 9:32 pm   #5
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Default Re: Ekco A222 Resistor Query

The datasheet suggests that the dropper resistance for 230-250V should be 1230R (930+300) with a 150R surge limiter to the rectifier. The values you have of 1161R and 175R will be just fine, they're within 10% of nominal anyway. 600R instead of 680R for the smoothing resistor will just run a bit cooler without any noticeable effect on operation.

Looking at the soldering on the 600R it looks like a workmanlike job and the Radiospares dropper is a good fit for purpose replacement.
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Old 20th Jan 2021, 9:37 pm   #6
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Default Re: Ekco A222 Resistor Query

If the original type valves and scale bulb are fitted then it looks as though it's for approx. 235v mains in so far as I can make out.

Those rat turd capacitors are not to be trusted.

Lawrence.

Last edited by ms660; 20th Jan 2021 at 9:44 pm. Reason: additional info
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Old 20th Jan 2021, 10:18 pm   #7
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by Al (astral highway) View Post
10nF is way out for a grid-coupling cap.
No it isn't, look at the circuit impedance.

Lawrence.

Last edited by ms660; 20th Jan 2021 at 10:26 pm. Reason: letter removed
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Old 20th Jan 2021, 10:32 pm   #8
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Al (astral highway) View Post
10nF is way out for a grid-coupling cap.
No it isn't, look at the circuit impedance.

.
My bad - wrong glasses ! I see c21 is indeed .01uF
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Old 20th Jan 2021, 10:43 pm   #9
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by Al (astral highway) View Post
Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Al (astral highway) View Post
10nF is way out for a grid-coupling cap.
No it isn't, look at the circuit impedance.

.
My bad - wrong glasses ! I see c21 is indeed .01uF
Which you said was 10nF and it's reactance at mid frequencies was too high.

Lawrence.
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Old 20th Jan 2021, 11:12 pm   #10
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Default Re: Ekco A222 Resistor Query

[QUOTE=ms660;1332875]
Quote:
Originally Posted by Al (astral highway) View Post

Which you said was 10nF and it's reactance at mid frequencies was too high.

.
Hey Lawrence, not quite - Poppydog says it is 10nF, third para of the original post.

Can we leave it there? I made a mistake and retracted it, so let’s not clog up the thread .
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Old 21st Jan 2021, 3:25 pm   #11
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Default Re: Ekco A222 Resistor Query

Thanks for all the replies. It seems these changes are going to be ok judging by the replies I've had.
Lawrence's reply regarding it being wired for 235v has got me confused yet again, any chance you could explain why you think that is please Lawrence??


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Old 21st Jan 2021, 3:40 pm   #12
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by poppydog View Post
Thanks for all the replies. It seems these changes are going to be ok judging by the replies I've had.
Lawrence's reply regarding it being wired for 235v has got me confused yet again, any chance you could explain why you think that is please Lawrence??
Adding all the resistance values printed on the mains dropper for the heater circuit comes to 1,150 Ohms if I've read them correctly.....120+120+60+200+650.....the heaters are 0.1 Amps, so the voltage drop across 1,150 Ohms will be 1,150*0.1 which comes to 115 volts.

Adding up the voltages of valve heaters and the voltage of the scale bulb comes
to 119.6 volts approx. (approx because the scale bulb is shunted by a resistor and a thermistor).....14+17+45+31+12.6......119.6+115 (rounded up) comes to 235, so 235 volts.

Lawrence.
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Old 21st Jan 2021, 4:04 pm   #13
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by poppydog View Post
Thanks for all the replies. It seems these changes are going to be ok judging by the replies I've had.
Lawrence's reply regarding it being wired for 235v has got me confused yet again, any chance you could explain why you think that is please Lawrence??
Adding all the resistance values printed on the mains dropper for the heater circuit comes to 1,150 Ohms if I've read them correctly.....120+120+60+200+650.....the heaters are 0.1 Amps, so the voltage drop across 1,150 Ohms will be 1,150*0.1 which comes to 115 volts.

Adding up the voltages of valve heaters and the voltage of the scale bulb comes
to 119.6 volts approx. (approx because the scale bulb is shunted by a resistor and a thermistor).....14+17+45+31+12.6......119.6+115 (rounded up) comes to 235, so 235 volts.

Lawrence.

Well, I can't argue with that reply. I will try to work out how I can remedy this, I was going to temporarily tak in what 10nf I have here at the moment and run it up on the lamp limiter but I'd better hang on.

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Old 21st Jan 2021, 4:20 pm   #14
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Default Re: Ekco A222 Resistor Query

Determine what mains voltage you want it to operate on then select the total resistance for the nearest.

Lawrence.
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Old 22nd Jan 2021, 5:10 pm   #15
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Default Re: Ekco A222 Resistor Query

In the pic in post 1 of the mains dropper you can see the lower 2 tags are bridged, I have removed the wire link.
If I insert a 5w 68r ww resistor between those 2 tags, that gives me a total of around 1.211k at the heater tag. This is the best thing so far I can come up with to be able to safely run this thing at todays varying mains voltages and perhaps the neatest. Is this acceptable?

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Old 22nd Jan 2021, 5:22 pm   #16
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Default Re: Ekco A222 Resistor Query

Don't overthink this - the valve heaters will be fine being slightly overrun, and 235V may be a good fit for your mains voltage anyway.
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Old 22nd Jan 2021, 5:32 pm   #17
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Default Re: Ekco A222 Resistor Query

The other alternative without fitting any extra resistor would be to open that link across the 200 Ohm section and short one of the 120 Ohm sections out.

But like Paul says, it's not super critical.

Lawrence.
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Old 22nd Jan 2021, 5:51 pm   #18
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by ms660 View Post
The other alternative without fitting any extra resistor would be to open that link across the 200 Ohm section and short one of the 120 Ohm sections out.

But like Paul says, it's not super critical.

Lawrence.
And that would give me 1.233k, almost perfect.
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Old 22nd Jan 2021, 5:54 pm   #19
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by Nuvistor View Post
Is this A222 or U222, the A series are usually AC only the U AC/DC?
It's an AC/DC radio chassis, but there's also a synchronous electric clock present which makes it AC only.
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Old 22nd Jan 2021, 5:59 pm   #20
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Default Re: Ekco A222 Resistor Query

Quote:
Originally Posted by paulsherwin View Post
Quote:
Originally Posted by Nuvistor View Post
Is this A222 or U222, the A series are usually AC only the U AC/DC?
It's an AC/DC radio chassis, but there's also a synchronous electric clock present which makes it AC only.
Looking at the state of it, I've got a bit to do on the clock yet. I've not even tried it out to see if it works.

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