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Old 30th Dec 2016, 12:18 am   #241
indigo.girl
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by MotorBikeLes View Post
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
Great tip Les I'll try that in the morning and see what R6 really is
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Old 30th Dec 2016, 12:19 am   #242
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Default Re: My first valve radio project - Philips 462A

A trick you could try to measure R7 even with a meter that can only read up to 2MΩ would be to put a resistor less than 2MΩ in parallel with it, and measure the resistance of the combination (which obviously will be less than 2MΩ). When resistors are connected in parallel, it is the inverses of the resistances that add up; i.e. 1 / R = 1 / R1 + 1 / R2, where R1 and R2 are the individual resistances and R is the combined resistance. If you substitute your known resistance for R1 and the combined parallel resistance for R, you can rearrange the equation to work out the actual value of R7.

For instance, if R7 really is 2.2MΩ, and you connect 1 MΩ in parallel with it, the combined resistance will be 1 / (1 / 1000000 + 1 / 2200000) = 687500 Ω, but a standard digital multimeter with a "1999" display will only be able to show this as 687 kΩ or 688 kΩ. (To work this out using a calculator, enter:
Code:
1 [exp] 6 [1/x] + 2.2 [exp] 6 [1/x] = [1/x]
[exp] is the "exponential notation" button, 6 is from 10 to the power 6 which is 1 000 000, [1/x] may be a button in its own right or a sequence using the INV button.)

EDIT: Post crossed with Les. Great minds think alike ..... Also, I'm with Mike (crackle), this is proper edge-of-the-seat stuff!
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Old 30th Dec 2016, 12:37 am   #243
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Default Re: My first valve radio project - Philips 462A

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I'll have to see what I have knocking around to see if I can replace it tonight....
OK - so here my bodge job to get as near as I can to the resistance I need....

I have taken my 1Mohm variable resistor that I was using half open for R7 and am now using it fully open to replace R6. i.e. R6 is now 1M ohm (lower than its spec of 2.2M but in the ball park).

To get something close to R7 of 470K ohms I have connected together 6 x 47Kohm resistors in series (my entire stock) making a total R7 of 282Kohms.

So with V2 IN I have measured the Voltage across all the stages from C25 to control grid pin 4 of V2 like before.

Interestingly my multi-resistor set up has allowed me to track the bias voltage changing from neg to positive as I move from C25 to pin4 at lots of positions along R7. The voltage at the C25 end of R7 starts negative and then increases at each step along R7 until it becomes +ve finishing at +2.3V (where is meets R6). The diagram attached explains it better...

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Bottom line is though that even with a known albeit lower resistance at R6 we are still not getting that much neg voltage at pin 4

Anyhow - that's me done too for tonight - I'm off to bed to dream of resistors no doubt

Mike and Julie - glad you are enjoying it - me too I think I should start taking bets for what the eventual fault will be once we find it!


Last edited by indigo.girl; 30th Dec 2016 at 12:44 am.
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Old 30th Dec 2016, 12:54 am   #244
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Default Re: My first valve radio project - Philips 462A

Is that -0.2V on pin 4?
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Old 30th Dec 2016, 9:38 am   #245
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Default Re: My first valve radio project - Philips 462A

I've been staring at the drawing in post #243 for ages and to me it makes no sense at all. It doesn't even fit the scenario of some kind of contact between HT positive and the control grid.

All I can suggest is getting rid of the pot substituting a 2.2M resistor and the series string of resistors substituting a 470k resistor. Fit a couple of resistors of the correct value.

It doesn't take much current through a 1M resistor to cause a substantial volt drop across it, so stray pick up may be the problem.
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Old 30th Dec 2016, 10:12 am   #246
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Default Re: My first valve radio project - Philips 462A

Could it be crumbling insulation on the wiring? My set had lots of this.
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Old 30th Dec 2016, 10:20 am   #247
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Default Re: My first valve radio project - Philips 462A

If the voltages in the picture in post#243 were taken with V2 fitted then the +ve voltage is being introduced at the junction of R6 and R7 so far as I can make out, the reason that the +ve voltage is lower on the grid is because the grid and cathode are acting as a conducting diode, the grid being the anode of that diode, that diodes internal resistance will be comparatively low. So far it all points to a leakage somewhere unless we're missing something, the most common cause for that is valve holder leakage or tag strip leakage, the voltages shown in the picture seem to suggest that it isn't valve holder leakage.

Lawrence.
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Old 30th Dec 2016, 10:22 am   #248
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Default Re: My first valve radio project - Philips 462A

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Could it be crumbling insulation on the wiring? My set had lots of this.
Could be, but there isn't any insulated wiring in the chain of 47k resistors, yet the voltage reverses polarity within one of them.

I wonder what the input impedance of Nicola's meter is?
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Old 30th Dec 2016, 10:28 am   #249
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Default Re: My first valve radio project - Philips 462A

I guess there must be a tag at the junction or R6, R7 and C17. Isolating it might provide a clue.

In fact R6 and R7 are physically some distance from C17, so perhaps there's a wire with insulation problems between these points?
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Old 30th Dec 2016, 10:35 am   #250
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Default Re: My first valve radio project - Philips 462A

A picture of that junction area might give a clue?

Lawrence.
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Old 30th Dec 2016, 10:48 am   #251
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Default Re: My first valve radio project - Philips 462A

Another observation if I've got it right:

In the picture of the manufacturers chassis layout posted way on back, R6 (trader) connects to R7 (trader) on what looks like a tag strip, the tag next to it has the local oscillators anode voltage on it....leakage possibly?

Lawrence.
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Old 30th Dec 2016, 10:54 am   #252
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Default Re: My first valve radio project - Philips 462A

What post number is that Lawrence?
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Old 30th Dec 2016, 10:59 am   #253
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Default Re: My first valve radio project - Philips 462A

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What post number is that Lawrence?
Apologies Graham I should have said that it's post #12, 2nd picture.

Lawrence.
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Old 30th Dec 2016, 11:14 am   #254
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Default Re: My first valve radio project - Philips 462A

Got it.

In manufacturers speak we're talking R10, R11 and C29. Plenty of tags and screened wires involved.
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Old 30th Dec 2016, 11:21 am   #255
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Default Re: My first valve radio project - Philips 462A

Yes, that's the one I was on about, all worth checking to eliminate/narrow down.

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Old 30th Dec 2016, 11:29 am   #256
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Default Re: My first valve radio project - Philips 462A

1st refs are manufacturers:

The tag above C29 (C17 Trader) carries the main HT feed.

The tag below C29 (C17 Trader) carries the mixers screen voltage.

The tag above R10/R11 (R6/R7 Trader) junction carries the local oscillator's anode voltage.

All possible suspects, shouldn't be too difficult to isolate.

Lawrence.
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Old 30th Dec 2016, 11:44 am   #257
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Default Re: My first valve radio project - Philips 462A

Hi Nicola.
At the moment you don't know if you have a fault or are simply suffering from lack of gain
Check C25 if not already by bridging it to see if volume becomes louder.
Adding a preamp would enable the volume control to be turned down more to prove it.
why not use the IF amp valve as a preamp.
Leave the screen grid as is, disconnect the anode and put a resistor between anode and HT line 100k or 220K or whatever you have
Disconnect cathode and put a resistor between it and Chassis 1 or 2.2K should do.
use a cap 0.01 or so from anode to the new gram input you have created. The grid should have 500K or 1 meg or and feed your signal into that.
Feel free to improvise with whatever components you have to hand.
I am sure someone will draw it out for me.

Good luck
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Old 30th Dec 2016, 11:51 am   #258
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Default Re: My first valve radio project - Philips 462A

According to the voltages posted thus far, there is a fault.

Lawrence.
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Old 30th Dec 2016, 12:11 pm   #259
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Default Re: My first valve radio project - Philips 462A

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Is that -0.2V on pin 4?
Yup
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Old 30th Dec 2016, 12:19 pm   #260
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Default Re: My first valve radio project - Philips 462A

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Could it be crumbling insulation on the wiring? My set had lots of this.
The plastic covering over the wiring looks generally OK but there is a very thick type of wiring I've not seen before. This wire has a central core covered by some kind of resin, which then has a long strip of metal wound around it and is connected to ground. The whole lot is encased in a plastic covering. I suspect this is a way to shield the central wire in some way. The resin separating the central core and the outer wire casing is very crumbly and in a couple of instances I've had to unwind the outer metal slightly to ensure the central core remains clear of it when I solder something to it. There are also some instances where in a middle section of the cable the plastic covering has been removed to reveal the outer metal winding and this has been solder directly to chassis. This ground it and hold it in place. If some of the resin had crumbled away underneath it is possible that when being soldered direct to chassis some of the solder may have made its way through to the centre core too.

Do these shielded wires ever cause trouble?
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