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Old 26th Mar 2021, 2:45 pm   #101
DonaldStott
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Default 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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Old 26th Mar 2021, 3:02 pm   #102
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Default Re: EKCO U122 - Restore or spares?

I've left the DMM attached for about 15-20 minutes and watched the Control Grid voltage gradually creep up (or down?) to -ve 0.89V.
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Old 26th Mar 2021, 3:06 pm   #103
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Default Re: EKCO U122 - Restore or spares?

Quote:
Originally Posted by DonaldStott View Post
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.
Then according to that you have no grid bias voltage to speak of, if that were to be the case then your UL41 and anything in it's path will be much stressed, you wouldn't have connected the switched Neutral to chassis or otherwise have a short from the switched Neutral line to chassis by any chance? If not then troubleshoot in the normal way, the bias voltage across the series combination of R6 and R7 should be approx. 4.6 volts -ve WRT chassis and at the bias tap approx. 1.7 volts -ve WRT chassis.

Lawrence.
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Old 26th Mar 2021, 3:06 pm   #104
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Default Re: EKCO U122 - Restore or spares?

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I've left the DMM attached for about 15-20 minutes and watched the Control Grid voltage gradually creep up (or down?) to -ve 0.89V.
I wouldn't worry too much about that if it's working normally and the other circuit voltages are close to where they should be.

UL41s aren't the most reliable valves in the world
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Old 26th Mar 2021, 3:08 pm   #105
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Default Re: EKCO U122 - Restore or spares?

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I wouldn't worry too much about that if it's working normally and the other circuit voltages are close to where they should be.

UL41s aren't the most reliable valves in the world
Have you looked at the schematic?

Lawrence.
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Old 26th Mar 2021, 3:13 pm   #106
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Default 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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Old 26th Mar 2021, 3:15 pm   #107
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Default Re: EKCO U122 - Restore or spares?

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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.
Have you looked at the schematic?

Lawrence.
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Old 26th Mar 2021, 3:18 pm   #108
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Default Re: EKCO U122 - Restore or spares?

No, I don't have it to hand. Is there anything unusual about it?
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Old 26th Mar 2021, 3:22 pm   #109
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Default Re: EKCO U122 - Restore or spares?

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No, I don't have it to hand. Is there anything unusual about it?
No cathode bias resistors are used, all amplifying valves cathodes are connected directly to chassis (HT-ve)

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.

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Old 26th Mar 2021, 3:25 pm   #110
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Default Re: EKCO U122 - Restore or spares?

Ok, thanks for that clarification. I wonder why they did it that way.
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Old 26th Mar 2021, 3:32 pm   #111
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Default Re: EKCO U122 - Restore or spares?

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Ok, thanks for that clarification. I wonder why they did it that way.
A few birds with one stone is my guess, two resistors instead of three, three cathode bypass capacitors dispensed with, AGC delay provided with the minimum of component additions.

Lawrence.

Last edited by ms660; 26th Mar 2021 at 3:36 pm. Reason: correction
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Old 26th Mar 2021, 3:32 pm   #112
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Default 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
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Old 26th Mar 2021, 3:45 pm   #113
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Default Re: EKCO U122 - Restore or spares?

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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
Yes, from Post#40:

Quote:
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Quote:
Originally Posted by DonaldStott View Post
For me the issue is not whether or not the V 5 Pin 4 anchor tag should be retained, it's more to with understanding that the Neutral tag of the AC mains input is connected to V4 Pin 6 (Control Grid) albeit via R22 in series?
No problem.....Remember I said that the bias voltages are developed across R6 and R7, remember that almost all the valves electrode currents flow through those two resistors, that's why the bias for g1 of V4 is taken from the switched Neutral side of the on/off switch and it's junction with R7 etc via the 1meg resistor R22 because at that point the voltage is -ve WRT the chassis, and therefore the cathode, which is what's needed for grid bias, grid -ve WRT the cathode. If g1 of V4 was connected directly to the bias resistor R7 then the signal at g1 would in effect be shorted out by R6 and R7 which only have a total resistance of 107 Ohms.

Lawrence.
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Old 26th Mar 2021, 4:12 pm   #114
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Default 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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Old 26th Mar 2021, 5:34 pm   #115
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Default Re: EKCO U122 - Restore or spares?

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Then according to that you have no grid bias voltage to speak of, if that were to be the case then your UL41 and anything in it's path will be much stressed, you wouldn't have connected the switched Neutral to chassis or otherwise have a short from the switched Neutral line to chassis by any chance? If not then troubleshoot in the normal way, the bias voltage across the series combination of R6 and R7 should be approx. 4.6 volts -ve WRT chassis and at the bias tap approx. 1.7 volts -ve WRT chassis.
I have three connections from the switched Neutral - one goes to the lamps, one goes to the Neutral tag on the dual electrolytic can and one goes to Pin 6 of V4 via R22.

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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Old 26th Mar 2021, 5:50 pm   #116
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Default 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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Old 26th Mar 2021, 6:04 pm   #117
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Default Re: EKCO U122 - Restore or spares?

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Not sure where I should be measuring??
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.

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Old 27th Mar 2021, 1:42 pm   #118
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Default 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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Old 27th Mar 2021, 2:42 pm   #119
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Default 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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Old 27th Mar 2021, 4:03 pm   #120
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Default 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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