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Old 29th Dec 2016, 8:40 pm   #221
ms660
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Default Re: My first valve radio project - Philips 462A

As per post#219, also check that any components/wires on tag 3 of the valve holder are not touching any components/wires on tag 4. Valve holders can leak between the tags by leaking across the valve holders body, worth having a close look.

Lawrence.
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Old 29th Dec 2016, 9:02 pm   #222
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Default Re: My first valve radio project - Philips 462A

Try disconnecting R6 so that the grid floats.

What are the anode and grid voltages then with the valve in?
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Old 29th Dec 2016, 9:05 pm   #223
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Default Re: My first valve radio project - Philips 462A

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As per post#219, also check that any components/wires on tag 3 of the valve holder are not touching any components/wires on tag 4. Valve holders can leak between the tags by leaking across the valve holders body, worth having a close look.

Lawrence.
I've had a really good look and I can see no touching wires/components that connect to pin 3 that could possibly be contacting wires/componets connected to pin 4. The top and bottom view of the valve holder looks clear between pin hole 3 and 4 and I've given the top of the holder a clean. There is infinite resistance measured between pin holes 3 and 4.
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Old 29th Dec 2016, 9:18 pm   #224
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Try disconnecting R6 so that the grid floats.

What are the anode and grid voltages then with the valve in?
Snipped one end of R6 (2.2 Mohms). (remember C19 is still snipped)

Pin 3 = +30V (+V2) and +214V (-V2)
Pin 4 = -400 mV (+V2) and +68mV (-V2)

With the Valve in I get closer to the expected values
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Old 29th Dec 2016, 9:19 pm   #225
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Default Re: My first valve radio project - Philips 462A

"There is infinite resistance measured between pin holes 3 and 4"

That isn't always a valid test for electrical leakage, any breakdown could be happening at a higher voltage than a test meter puts out on ohms range.

That said, if there's still a +ve voltage on tag 4 of the valve holder with the valve removed it should be possible to trace the source of the problem, what I would do next is to disconnect the components from tag 3 in order to isolate that tag then check the voltage on tag 4 again.

Whoops...post crossed.

EDIT: That +ve grid voltage previously measured has almost disappeared since R6 has been disconnected and with the valve removed, a known good C17 should be in circuit, what's the voltage at the junction of R6 and R7? It should be -ve with respect to chassis

Lawrence.

Last edited by ms660; 29th Dec 2016 at 9:31 pm.
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Old 29th Dec 2016, 9:40 pm   #226
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Default Re: My first valve radio project - Philips 462A

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disconnect the components from tag 3 in order to isolate that tag then check the voltage on tag 4 again.

Lawrence.
I have reconnected R6 and C19 back into the circuit for this test.

With valve removed and pin 3 snipped = pin 4 is still positive at +6V

(pin3 has a small +18mV measurement rather than 0)

I'll now resnip R6 for your next suggestion
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Old 29th Dec 2016, 9:49 pm   #227
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Default Re: My first valve radio project - Philips 462A

The bias supply from the -ve end of C25 should be -ve. With V2 removed somewhere along it's route to the triodes grid tag it's going +ve, check the bias voltage at all points along its route from the supply.

Lawrence.
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Old 29th Dec 2016, 10:22 pm   #228
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Default Re: My first valve radio project - Philips 462A

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EDIT: That +ve grid voltage previously measured has almost disappeared since R6 has been disconnected and with the valve removed, a known good C17 should be in circuit, what's the voltage at the junction of R6 and R7? It should be -ve with respect to chassis

Lawrence.
Its positive at R6/R7 junction

Test conditions: R6 + C19 + pin3 all reconnected to original positions.

Pin 3 = 19.5V (+V2); 242V (-V2)
Pin 4 = -65mV (+V2); 4.5V (-V2)
Voltage at R6 - R7 junction (wrt chassis) = +9.5V (+V2); +11V (-V2)
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Old 29th Dec 2016, 10:27 pm   #229
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Default Re: My first valve radio project - Philips 462A

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EDIT: That +ve grid voltage previously measured has almost disappeared since R6 has been disconnected and with the valve removed, a known good C17 should be in circuit, what's the voltage at the junction of R6 and R7? It should be -ve with respect to chassis

Lawrence.
Its positive at R6/R7 junction

Test conditions: R6 + C19 + pin3 all reconnected to original positions.

Pin 3 = 19.5V (+V2); 242V (-V2)
Pin 4 = -65mV (+V2); 4.5V (-V2)
Voltage at R6 - R7 junction (wrt chassis) = +9.5V (+V2); +11V (-V2)
Hang on in....

Voltage on the other end of R7, the end that connects to the junction of R19 and R20 with V2 removed.

Lawrence.
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Old 29th Dec 2016, 10:43 pm   #230
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Default Re: My first valve radio project - Philips 462A

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The bias supply from the -ve end of C25 should be -ve. With V2 removed somewhere along it's route to the triodes grid tag it's going +ve, check the bias voltage at all points along its route from the supply.

Lawrence.
Not completely sure I understand this action. With V2 removed the negative side of C25 is -4.5V. This is connected to the R15 variable resistor (tone control). On the other side of R15 the voltage is -3.5V. This then connects to C21. On the other side of C21 the voltage is +225V. This then connects to V2 triode ANODE (pin 3) rather than the Grid tag (I presume this means control grid pin4). Is this what you wanted me to check Lawrence?

Hold on ... I've seen the path to the grid too...

C25 also goes via R20 to R7 to R6 and then onto the grid tag (pin4). The voltage on the R20 side of R7 is -1.6V. On the other side of R7 it's +8Vs This is where is goes from neg to pos.

Last edited by indigo.girl; 29th Dec 2016 at 10:56 pm.
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Old 29th Dec 2016, 10:57 pm   #231
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Default Re: My first valve radio project - Philips 462A

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The bias supply from the -ve end of C25 should be -ve. With V2 removed somewhere along it's route to the triodes grid tag it's going +ve, check the bias voltage at all points along its route from the supply.

Lawrence.
Not completely sure I understand this action. With V2 removed the negative side of C25 is -4.5V. This is connected to the R15 variable resistor. On the other side of R15 the voltage is -3.5V. This then connects to C21. On the other side of C21 the voltage is +225V. This then connects to V2 triode ANODE (pin 3) rather than the Grid tag (I presume this means control grid pin4). Is this what you wanted me to check Lawrence?

Hold on ... I've seen the path to the grid too...
That tells us that the grid bias supply at source is being produced as expected polarity wise and that it changes polarity somewhere between its source and the grid side of R7, to be absolutely sure that it's -ve when connected to R7's bottom end as viewed in the schematic measure the voltage at the actual end of R7, that's the bottom end looking at the schematic, in order to get to the bottom of this we have to eliminate all possibilities, It will happen I'm sure.

EDIT: Yes C21 does connect to the triodes anode and not the triodes grid.

Lawrence.
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Old 29th Dec 2016, 11:08 pm   #232
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Default Re: My first valve radio project - Philips 462A

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to be absolutely sure that it's -ve when connected to R7's bottom end as viewed in the schematic measure the voltage at the actual end of R7, that's the bottom end looking at the schematic, in order to get to the bottom of this we have to eliminate all possibilities, It will happen I'm sure.

Lawrence.
Yes, at every bit of bare wire between R20 and the bottom of R7 the voltage is -1.6V.
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Old 29th Dec 2016, 11:22 pm   #233
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Default Re: My first valve radio project - Philips 462A

Is that with everything connected apart from the top end of R6?
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Old 29th Dec 2016, 11:26 pm   #234
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Default Re: My first valve radio project - Philips 462A

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to be absolutely sure that it's -ve when connected to R7's bottom end as viewed in the schematic measure the voltage at the actual end of R7, that's the bottom end looking at the schematic, in order to get to the bottom of this we have to eliminate all possibilities, It will happen I'm sure.

Lawrence.
Yes, at every bit of bare wire between R20 and the bottom of R7 the voltage is -1.6V.
Ok, can your other test meter measure resistances above 2M ohm? If not I would replace R7 (2.2M) before you go any further, If it's gone very high or open circuit it would leave the grid tag floating, anything that's floating can pick up stray voltages from a high impedance source and sometimes a high impedance digital meter will register them.

Lawrence.
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Old 29th Dec 2016, 11:28 pm   #235
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Default Re: My first valve radio project - Philips 462A

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Is that with everything connected apart from the top end of R6?
No, R6 is connected.
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Old 29th Dec 2016, 11:31 pm   #236
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Default Re: My first valve radio project - Philips 462A

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Ok, can your other test meter measure resistances above 2M ohm? If not I would replace R7 (2.2M) before you go any further, If it's gone very high or open circuit it would leave the grid tag floating, anything that's floating can pick up stray voltages from a high impedance source and sometimes a high impedance digital meter will register them.

Lawrence.
Just to confirm; you mean R6 here don't you?

And yes my other meter does do higher resistance measurements up to 20 or 2000Mohms. I'll try it now.

Correction: My other meter needs an extra attachment for high resistance measurements - so no I can't measure it. So its possible its gone very high resistance (I presume not completely open circuit else snipping it out would have given no different effect but it did).

I'll have to see what I have knocking around to see if I can replace it tonight....

Last edited by indigo.girl; 29th Dec 2016 at 11:52 pm.
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Old 29th Dec 2016, 11:48 pm   #237
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Default Re: My first valve radio project - Philips 462A

My apologies, I had annotated the schematic I have with the values of R6 and R7 but I wrote them in the wrong positions

Lawrence.
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Old 29th Dec 2016, 11:53 pm   #238
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Default Re: My first valve radio project - Philips 462A

If your testmeter is limited to 2Mohm, put a known value in parallel (470K, 1M or why?), take measurement and then calculate correct value for your 2.2Mohm suspect. formula: 1/Rx=(1/R1+1/R2).
Les
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Old 30th Dec 2016, 12:01 am   #239
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Default Re: My first valve radio project - Philips 462A

Keep at it Lawrence and Nicola, its getting quite exciting.
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Old 30th Dec 2016, 12:04 am   #240
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Keep at it Lawrence and Nicola, its getting quite exciting.
Yeah, luv it, trouble today is we ran out of firewood last night so been cutting and hauling timber from down the woods for a few hours today...My pillow beckons, anyways enough of that.

Lawrence.
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