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Old 27th Mar 2021, 4:16 pm   #121
paulsherwin
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Default Re: EKCO U122 - Restore or spares?

Have you seen this thread? https://www.vintage-radio.net/forum/...d.php?t=111365

Unfortunately many UL41s in old radios are getting towards the end of their lives. A bad UL41 is often the reason a radio was relegated to the loft or shed in the first place.
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Old 27th Mar 2021, 4:27 pm   #122
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Default Re: EKCO U122 - Restore or spares?

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Originally Posted by DonaldStott View Post
Fell at the first hurdle ...!

Connected my DMM to the chassis and to the junction of -ve C23, -ve C28, R7 and the switched Neutral i.e. the Neutral tag on the dual electrolytic can.

Got an initial reading of -ve 0.45V - heard a couple of clicks from the set and the voltage dropped to -ve O.08V !
With the power disconnected, measure the resistance between the chassis and the switched Neutral at the switch.

Lawrence.
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Old 27th Mar 2021, 5:11 pm   #123
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Default Re: EKCO U122 - Restore or spares?

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Thanks Paul - that's my Saturday night entertainment sorted!

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Originally Posted by ms660 View Post
With the power disconnected, measure the resistance between the chassis and the switched Neutral at the switch.
109Ω
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Old 27th Mar 2021, 5:39 pm   #124
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Default Re: EKCO U122 - Restore or spares?

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Quote:
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With the power disconnected, measure the resistance between the chassis and the switched Neutral at the switch.
109Ω
That sounds about right, with the valves normal operating current flowing under no signal conditions then approx. 4.6 volts across R6+R7 according to the service info.

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Old 27th Mar 2021, 6:08 pm   #125
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Default Re: EKCO U122 - Restore or spares?

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That sounds about right, with the valves normal operating current flowing under no signal conditions then approx. 4.6 volts across R6+R7 according to the service info.
R6 (39Ω) + R7 (68Ω) = 107Ω

You may remember this Thread: -

https://www.vintage-radio.net/forum/...d.php?t=175422

I was beginning to think that I had a dodgy on/off switch!
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Old 27th Mar 2021, 6:26 pm   #126
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Default Re: EKCO U122 - Restore or spares?

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Quote:
Originally Posted by ms660 View Post
That sounds about right, with the valves normal operating current flowing under no signal conditions then approx. 4.6 volts across R6+R7 according to the service info.
R6 (39Ω) + R7 (68Ω) = 107Ω

You may remember this Thread: -

https://www.vintage-radio.net/forum/...d.php?t=175422

I was beginning to think that I had a dodgy on/off switch!
I don't follow.

Lawrence.
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Old 27th Mar 2021, 6:59 pm   #127
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Default Re: EKCO U122 - Restore or spares?

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I don't follow.

Lawrence.
Moving on quickly ...

With the very 'hummy' UL41 still in place and the Control Grid bias creeping just above -ve 3.0V I'm now measuring -ve 4.7V at the Neutral tag on the dual electrolytic can.

Readings are not stable and drop dramatically when the set clicks or crackles?

Of course when I ground the Control Grid to the chassis the hum disappears.
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Old 27th Mar 2021, 7:26 pm   #128
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Default Re: EKCO U122 - Restore or spares?

At least the bias supply voltage (-ve 4.7 volts) looks about right when it's there.

Lawrence.
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Old 27th Mar 2021, 7:45 pm   #129
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Default Re: EKCO U122 - Restore or spares?

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At least the bias supply voltage (-ve 4.7 volts) looks about right when it's there.
But why does it keep disappearing - is the noise from the set coming from the UL41?
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Old 27th Mar 2021, 7:56 pm   #130
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Default Re: EKCO U122 - Restore or spares?

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Quote:
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At least the bias supply voltage (-ve 4.7 volts) looks about right when it's there.
But why does it keep disappearing - is the noise from the set coming from the UL41?
Perhaps you could tell us.

Lawrence.
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Old 27th Mar 2021, 8:11 pm   #131
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Default Re: EKCO U122 - Restore or spares?

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Thanks Paul - that's my Saturday night entertainment sorted!
Or so I thought - my quick glance at the linked Thread assumed it was instructions of how to do the 'gas hob sparking trick' but it seems as if I need: -
  • A traditional valve-radio mains-transformer with a 250-0-250V secondary winding or similar, and a 6.3V heater winding.
  • A current-limited 12-volt power supply capable of giving 4 or 5 Amps.
  • A B8 valve-base.
  • Some lengths of wire.
I have plenty of wire but that's about it - I'll be cheaper looking for a UL41!

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Perhaps you could tell us.

Lawrence.
Perhaps I should have written - is it likely that the noise from the set is coming from the UL41? I will of course investigate and report back.
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Old 27th Mar 2021, 8:19 pm   #132
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Default Re: EKCO U122 - Restore or spares?

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Perhaps I should have written - is it likely that the noise from the set is coming from the UL41?
My answer would be the same.

Lawrence.
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Old 27th Mar 2021, 8:26 pm   #133
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Default Re: EKCO U122 - Restore or spares?

The linked thread describes how to connect the UL41 pins, amongst other things. The internal shorts are cleared by applying a high voltage spike - it doesn't matter if it comes from a spark igniter or a transformer.
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Old 27th Mar 2021, 9:36 pm   #134
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Default Re: EKCO U122 - Restore or spares?

So here's the question. Why does the UL41 sound ok in the bush and hums in the ekco?

Last edited by Gabe001; 27th Mar 2021 at 9:45 pm.
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Old 27th Mar 2021, 11:34 pm   #135
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Default Re: EKCO U122 - Restore or spares?

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So here's the question. Why does the UL41 sound ok in the bush and hums in the ekco?
Perhaps you could tell us?

I'm now in the market for a new(ish) UL41 if only to prove to myself that this is the problem.
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Old 28th Mar 2021, 7:32 am   #136
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Default Re: EKCO U122 - Restore or spares?

I don't know Donald. My hypothesis is that the lack of cathode resistors unmasks internal shorts/leaks to cathode more than in sets where this resistor is present. I suppose in times where ul41s were plentiful and cheap this wouldn't be a major issue, but poor you those valves aren't cheap now.
Maybe Paul and Lawrence have a better explanation.
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Old 28th Mar 2021, 9:54 am   #137
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Default Re: EKCO U122 - Restore or spares?

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So here's the question. Why does the UL41 sound ok in the bush and hums in the ekco?
Certain pins within the UL41 are marked n.c (no connection) and it was common for radio manufacturers to use the unused (n.c) pins on the valveholder as convenient connecting points for other components. I think pin 4 on the UL41 is just such a pin. When the valve starts to age, deposits build up on the inside of the glass at the base and over time forms a partly conductive path (leakage) to other pins. Whatever might be connected to pin 4 (otherwise unused) can then become linked to (say) the cathode via leakage which can cause hum.

Using the same valve in another set may well not hum if pin 4 is not used as an anchor point for other components.

The 'spark' trick mentioned earlier is intended to burn the deposits away.
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Old 28th Mar 2021, 11:08 am   #138
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Default Re: EKCO U122 - Restore or spares?

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I don't know Donald. My hypothesis is that the lack of cathode resistors unmasks internal shorts/leaks to cathode more than in sets where this resistor is present. I suppose in times where ul41s were plentiful and cheap this wouldn't be a major issue, but poor you those valves aren't cheap now.
Maybe Paul and Lawrence have a better explanation.
Thanks Gabe - my question to you was slightly tongue-in-cheek hence the but your explanation seems reasonable to me.

The cathode of V4 (UL41) in the EKCO U122 is connected back to the chassis, no cathode bias resistor. If we also consider what Lawrence highlighted again in Post #109 "Bias voltage for the Mixer, IF and output valves is developed across two resistors in series that are connected between the chassis (HT-ve) and the switched Neutral" then any shorts/leaks in the UL41 will be very apparent?

The cathode of V4 (UL41) in the Bush DAC10 (and the DAC90A) has an 150Ω cathode bias resistor which may help to ameliorate this problem but I'm just guessing? It wouldn't have cost EKCO that much to bung in a few cathode bias resistors, a few shillings at most, but no doubt that adds up over the course of a particular models production run?

Would a simple fix be to bodge-in a V4 150Ω cathode bias resistor in the U122 or is that too simplistic?

Quote:
Originally Posted by Sideband View Post
Certain pins within the UL41 are marked n.c (no connection) and it was common for radio manufacturers to use the unused (n.c) pins on the valveholder as convenient connecting points for other components. I think pin 4 on the UL41 is just such a pin. When the valve starts to age, deposits build up on the inside of the glass at the base and over time forms a partly conductive path (leakage) to other pins. Whatever might be connected to pin 4 (otherwise unused) can then become linked to (say) the cathode via leakage which can cause hum.

Using the same valve in another set may well not hum if pin 4 is not used as an anchor point for other components.

The 'spark' trick mentioned earlier is intended to burn the deposits away.
Thanks Sideband - back in Post #38 I noted that " ... V4 Pin 4 is used as an anchor tag linking C22 and R18. We've all done the DAC90A UL41 Pin 4 Mods so I'm assuming the same would apply here?"

I've still to try the 'gas hob sparking trick' !
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Old 28th Mar 2021, 11:11 am   #139
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Default Re: EKCO U122 - Restore or spares?

Pin 4 on the UL41 valve base (marked ic in the valve data) is used as a connection point for R21 and C26 (Trader) in the feedback network.

EDIT: Post crossed.

There's no need to bodge the bias.

Lawrence.

Last edited by ms660; 28th Mar 2021 at 11:18 am. Reason: extra info
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Old 28th Mar 2021, 11:24 am   #140
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Default Re: EKCO U122 - Restore or spares?

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Pin 4 on the UL41 valve base (marked ic in the valve data) is used as a connection point for R21 and C26 (Trader) in the feedback network.
Forgot my own ERRATUM in Post #41: -

"ERRATUM: Apologies - of course this should read C26 and R21."
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