27th Mar 2021, 4:16 pm | #121 |
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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. |
27th Mar 2021, 4:27 pm | #122 | |
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. |
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27th Mar 2021, 5:11 pm | #123 | |
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Re: EKCO U122 - Restore or spares?
Quote:
109Ω
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27th Mar 2021, 5:39 pm | #124 | |
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. |
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27th Mar 2021, 6:08 pm | #125 | |
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Re: EKCO U122 - Restore or spares?
Quote:
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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27th Mar 2021, 6:26 pm | #126 | ||
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. |
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27th Mar 2021, 6:59 pm | #127 |
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Re: EKCO U122 - Restore or spares?
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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27th Mar 2021, 7:26 pm | #128 |
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Re: EKCO U122 - Restore or spares?
At least the bias supply voltage (-ve 4.7 volts) looks about right when it's there.
Lawrence. |
27th Mar 2021, 7:56 pm | #130 |
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Re: EKCO U122 - Restore or spares?
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27th Mar 2021, 8:11 pm | #131 | ||
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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? I will of course investigate and report back.
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27th Mar 2021, 8:19 pm | #132 |
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Re: EKCO U122 - Restore or spares?
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27th Mar 2021, 8:26 pm | #133 |
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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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27th Mar 2021, 9:36 pm | #134 |
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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. |
27th Mar 2021, 11:34 pm | #135 | |
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Re: EKCO U122 - Restore or spares?
Quote:
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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28th Mar 2021, 7:32 am | #136 |
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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. |
28th Mar 2021, 9:54 am | #137 | |
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Re: EKCO U122 - Restore or spares?
Quote:
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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28th Mar 2021, 11:08 am | #138 | ||
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Re: EKCO U122 - Restore or spares?
Quote:
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:
I've still to try the 'gas hob sparking trick' !
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28th Mar 2021, 11:11 am | #139 |
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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 |
28th Mar 2021, 11:24 am | #140 | |
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Re: EKCO U122 - Restore or spares?
Quote:
"ERRATUM: Apologies - of course this should read C26 and R21."
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