26th Mar 2021, 2:45 pm | #101 |
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Re: EKCO U122 - Restore or spares?
I'm using my usual Toolzone EL060 DMM which has an Input Impedance of 10meg on all Ranges.
Measurements are between Pin 6 and Chassis - not sure how I could have a low impedance when the measurement was taken. Control Grid (Pin 6) to Cathode (Pin 7) has the same voltage reading.
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26th Mar 2021, 3:06 pm | #103 | |
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. |
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26th Mar 2021, 3:06 pm | #104 | |
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Re: EKCO U122 - Restore or spares?
Quote:
UL41s aren't the most reliable valves in the world |
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26th Mar 2021, 3:08 pm | #105 |
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Re: EKCO U122 - Restore or spares?
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26th Mar 2021, 3:13 pm | #106 |
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Re: EKCO U122 - Restore or spares?
It's normal to get slightly odd voltage readings from output valve control grids, both because the impedances are very high and the valves are prone to small internal leaks. If the bias really was way off the cathode voltage would be completely wrong.
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26th Mar 2021, 3:15 pm | #107 | |
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. |
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26th Mar 2021, 3:18 pm | #108 |
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Re: EKCO U122 - Restore or spares?
No, I don't have it to hand. Is there anything unusual about it?
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26th Mar 2021, 3:22 pm | #109 | |
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Re: EKCO U122 - Restore or spares?
Quote:
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. Lawrence. |
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26th Mar 2021, 3:25 pm | #110 |
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Re: EKCO U122 - Restore or spares?
Ok, thanks for that clarification. I wonder why they did it that way.
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26th Mar 2021, 3:32 pm | #111 | |
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Re: EKCO U122 - Restore or spares?
Quote:
Lawrence. Last edited by ms660; 26th Mar 2021 at 3:36 pm. Reason: correction |
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26th Mar 2021, 3:32 pm | #112 |
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Re: EKCO U122 - Restore or spares?
I got caught out as well, but it's all explained on page 2. For anyone reading this thread, my understanding is that here are 2 bias resistors between neutral and chassis (total 107ohms) that for a set consuming approx 40mA should produce a bias of -4v without the need for cathode resistors. The cathodes are connected to chassis directly. Happy to be corrected if wrong.
I suspect the 3rd ul41 valve may also be internally leaky, although less so than the previous one |
26th Mar 2021, 3:45 pm | #113 | |||
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Re: EKCO U122 - Restore or spares?
Quote:
Quote:
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26th Mar 2021, 4:12 pm | #114 |
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Re: EKCO U122 - Restore or spares?
Thanks Lawrence, you're a star. I'm curious to see how this restoration pans out.
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26th Mar 2021, 5:34 pm | #115 | |
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Re: EKCO U122 - Restore or spares?
Quote:
The two parallel resistors that comprise the lamp shunt are connected at one end to the Neutral tag on the dual electrolytic can and to the lamps at the other end - both lamps are illuminating correctly. R7 is also connected to the Neutral tag on the dual electrolytic can and is in series with R6. The connection from R6 runs to a chassis tag and then on to C27 which in turn is connected to Pin 2 of V4. From the circuit diagram, all the above seems to be in order (?) although I am close to the limits of my technical understanding as to where any fault could lie? Not sure where I should be measuring??
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26th Mar 2021, 5:50 pm | #116 |
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Re: EKCO U122 - Restore or spares?
I would start with the values and voltage drop across across r6 and r7, and perhaps a comparison with bias of V3 (junction of c20/r12 to chassis).others more experienced may have better ideas
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26th Mar 2021, 6:04 pm | #117 |
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Re: EKCO U122 - Restore or spares?
First establish what the bias source voltage is, that's the voltage between the chassis (HT-ve) and the junction of R7, R8, C23 -ve, C28 -ve etc (the switched Neutral rail)
Then the bias voltage at g1 of the UL41 measured between chassis (HT-ve) and pin 6 (g1) of the UL41. EDIT: Post crossed. Lawrence. |
27th Mar 2021, 1:42 pm | #118 |
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Re: EKCO U122 - Restore or spares?
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 !
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27th Mar 2021, 2:42 pm | #119 |
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Re: EKCO U122 - Restore or spares?
Curiouser and curiouser ...
Pulled my last UL41 from a known working DAC10 and tried that - after several minutes the Control Grid bias had risen to -ve 3.0V but I have a deafening hum from the set! All stations on LW and MW are tuneable.
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27th Mar 2021, 4:03 pm | #120 |
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Re: EKCO U122 - Restore or spares?
Would be great to get a stable UL41 or equivalent fitted if such a thing exists?
I know there are alternatives but they are also scarce, expensive or involve additional circuitry/components, something I don't want to contemplate at this stage. As I've nothing to lose with the duff UL41s I have I'm considering trying the gas hob sparking trick if someone can advise on how exactly the valve should be connected e.g. which pin(s)??
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