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Old 27th Dec 2016, 7:21 pm   #121
TonyDuell
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Default Re: My first valve radio project - Philips 462A

A shorted turn would indeed get the transformer very hot quite quickly. And I think it would cause a lamp limiter to glow.

It's not something silly like bad/corroded connections on the voltage selector plug arrangement is it?
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Old 27th Dec 2016, 7:29 pm   #122
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Default Re: My first valve radio project - Philips 462A

This all sounds a little peculiar. How confident are you about the accuracy and reliability of your meter on its various AC voltage ranges?

To clarify the expectations, transformer voltages are inter-related by the number of turns of each winding and only catastrophic damage to the transformer will cause them to vary much with respect to one another. As Tony mentions there is a certain amount of voltage drop as load is applied - the ratio between maximum and minimum is called the regulation and it's worse on small transformers than large ones - but you should be seeing results within perhaps 10% of stated figures provided you are feeding in the correct voltage as selected on the dial.

Of course, if there is a mismatch between the supplied voltage and the setting, everything else will be out by approximately the same ratio. E.g. if what is actually reaching the set through the limiter is 3/4 of the selection (say 184V on the 245V setting) then you would expect all the secondary voltages to be roughly 3/4 of their proper voltages. If you feed in 245 on the 245V setting, then a 4V rectifier heater supply should be pretty well on 4V and getting 3V definitely indicates a problem.

Usually, it would be shorted turns or one of the other secondaries overloaded, and hence the thing would be getting toasty or the lamp limiter would be ablaze, which is why it seems odd. However if your measurement is out by 1V, then we might all be looking in the wrong direction, so a double-check of the meter might be worthwhile.
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Old 27th Dec 2016, 7:34 pm   #123
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Default Re: My first valve radio project - Philips 462A

You've explained it rather better than I did, but that is what I was getting at.

I would expect that the voltages would 'track' each other. In other words if one was half of what it should be then all outputs from the transformer would be half of what they should be. As you said, that is determined by the turns ratio of the transformer.

The rectifier heater voltage was half of what you would expect. And yet the high voltage to the rectifier anodes was not half of what the trader sheet said.

What meter is the OP using? Can we trust it?
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Old 27th Dec 2016, 7:48 pm   #124
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by ms660 View Post
Ref post#118, were those voltages measured with all the valves fitted or were they removed?
All valves fitted
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Old 27th Dec 2016, 7:52 pm   #125
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Default Re: My first valve radio project - Philips 462A

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What meter is the OP using? Can we trust it?
As luck would have it I have a brand new multimeter sitting in my draw that I have not used yet. I'll repeat the measurements with the new device (my old one has been knocking about a while so may be inaccurate...
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Old 27th Dec 2016, 8:35 pm   #126
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Default Re: My first valve radio project - Philips 462A

Oh dear, as if this stuff isn't hard enough I think I have been using a faulty multimeter...

I have retested voltage using a new meter at now the Voltage across the heaters (Vh) for all the valves all meet their specs at the 245V dial setting. Slightly higher when I use the 200V setting as you would expect.

AZ31: Vh = 3.8 VAC @245V ; 4.8 VAC @200V
EBL21: Vh = 6.2 VAC @245V ; 7.6 VAC @200V
ECH21: 6.2 VAC @245V ; 7.6 VAC @200V

However this time I am getting NO AC voltage drop at the anode for AZ31 with the new meter (e.g. chassis to pin 4 or chassis to pin 6). The new meter is working because I can measure 240V AC at the mains input.

The old meter is still recording over 300V AC from chassis to anode!!

Interestingly, when I set either voltmeter to DC I measure about -300V from chassis to anode on AZ31.
So perhaps my AZ31 isn't faulty. Its simply not had voltage applied properly. The transformer does appear faulty.
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Old 27th Dec 2016, 8:49 pm   #127
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Default Re: My first valve radio project - Philips 462A

Before you condemn the transformer do a resistance check between the transformers HT winding centre tap tag and chassis, it should measure approx 101 ohms, also measure the resistance across the total HT winding (the other two tags of that winding) and also between the centre tap tag and both of the other two tags of that winding in turn. Power off of course.

Lawrence

Last edited by ms660; 27th Dec 2016 at 9:03 pm.
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Old 27th Dec 2016, 8:57 pm   #128
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Default Re: My first valve radio project - Philips 462A

As a very quick test, with the set unplugged from the mains and the AZ31 removed, measure the following resistances with your meter :

Between pin 4 and pin 6 of the AZ31 holder (this is the resistance of the HT secondary winding of the transformer)

Between chassis and pin 4 of the AZ31 holder (this is half of the HT secondary winding in series with the bias resistor network)

Between chassis and pin 6 of the AZ31 holder (ditto, with the other half of the HT secondary).
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Old 27th Dec 2016, 9:02 pm   #129
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Default Re: My first valve radio project - Philips 462A

Ah, now I understand your '2-3V' readings etc. A multimeter is only accurate to a certain percentage of the maximum value it can display on a particular range, plus or minus a few counts. If you were trying to measure the heater voltage using the 750V AC range on the clamp meter, even if it saw exactly 4V it might legitimately display anything from maybe 2V to 6V which is not precise enough here. That range would be fine for measuring 230V as an indication between 228 and 232 volts would be plenty accurate for most purposes. You must always choose a range that corresponds to the voltage you want to measure, to achieve proper accuracy. In this case the 'electrician's' type of meter didn't have the range you needed.
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Old 27th Dec 2016, 9:48 pm   #130
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Default Re: My first valve radio project - Philips 462A

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Ah, now I understand your '2-3V' readings etc. A multimeter is only accurate to a certain percentage of the maximum value it can display on a particular range, plus or minus a few counts. If you were trying to measure the heater voltage using the 750V AC range on the clamp meter, even if it saw exactly 4V it might legitimately display anything from maybe 2V to 6V which is not precise enough here. That range would be fine for measuring 230V as an indication between 228 and 232 volts would be plenty accurate for most purposes. You must always choose a range that corresponds to the voltage you want to measure, to achieve proper accuracy. In this case the 'electrician's' type of meter didn't have the range you needed.
Yes, I was pushing it accuracy wise with my old meter - but why did it record an erroneous VAC reading from anode to chassis? This was around 300V so well within range...
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Old 27th Dec 2016, 9:54 pm   #131
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Default Re: My first valve radio project - Philips 462A

Looks like you now have your meter problems sorted.

The HT voltage will be DC and is measured from chassis to pin 2 or 8 of the AZ31 rectifier.
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Old 27th Dec 2016, 9:58 pm   #132
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Default Re: My first valve radio project - Philips 462A

Quote:
Originally Posted by TonyDuell View Post
As a very quick test, with the set unplugged from the mains and the AZ31 removed, measure the following resistances with your meter
Between pin 4 and pin 6 of the AZ31 holder (this is the resistance of the HT secondary winding of the transformer) 391 ohms


Between chassis and pin 4 of the AZ31 holder (this is half of the HT secondary winding in series with the bias resistor network) INFINITE RESISTANCE

Between chassis and pin 6 of the AZ31 holder (ditto, with the other half of the HT secondary). INFINITE RESISTANCE
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Old 27th Dec 2016, 10:11 pm   #133
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Default Re: My first valve radio project - Philips 462A

I've got so interested I've printed he Trader Sheet out! Not the easiest set to start with. I can see why Philips Radio's aren't loved by everyone
Les [p119] the Trader Sheet does make a point of describing the Mains Tx as wound in three isolated sections with seven leads.

Hang on in there Nicola. I suspect it's taking longer because it's often more difficult to describe than show something directly.
I'm sure Graham and the team will get you there and it's an interesting read.
These things are often like a crossword puzzle in the end. At least you are going to stay safe-especially with no HT voltage so far but don't get complacent.

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Last edited by dave walsh; 27th Dec 2016 at 10:18 pm.
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Old 27th Dec 2016, 10:17 pm   #134
TonyDuell
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Default Re: My first valve radio project - Philips 462A

To me this sounds like the HT secondary winding is fine. It's not open-circuit (that's what the first test shows). Most of the time the 'centre tap' is actually a junction between the 2 halves of the winding, made at the transformer solder tag, so there's no wire in the transformer that could break to disconnect the tap and leave the entire winding continuous.

But it would appear that there is no connection between the centre tap and the chasiss through the bias network. This would explain why you see no output from the rectifier. In fact the rectifier could be fine, C26 (Trader Sheet) could be charging, but because there is no circuit from the centre tap back to the chassis, a meter check between rectifier cathode and chassis will show no voltage.

Anyway, I would check R19 and R20 (Trader Sheet) next.
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Old 27th Dec 2016, 10:34 pm   #135
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by ms660 View Post
Before you condemn the transformer do a resistance check between the transformers HT winding centre tap tag and chassis, it should measure approx 101 ohms, also measure the resistance across the total HT winding (the other two tags of that winding) and also between the centre tap tag and both of the other two tags of that winding in turn. Power off of course.
I've measured all the resistances between the tags and have labelled up the diagram found in the manufacturers service sheet. The voltage disc was set to 245V. The resistance between all the windings between the mains input is about 45 ohms.

I can't measure any continuity between any tag and the chassis (all infinite resistance.
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Old 27th Dec 2016, 10:42 pm   #136
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Default Re: My first valve radio project - Philips 462A

Have a look at the fourth picture in post #85.

You need to reconnect the white wire you snipped, it's the centre tap of the HT windings.
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Old 27th Dec 2016, 10:42 pm   #137
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Default Re: My first valve radio project - Philips 462A

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To me this sounds like the HT secondary winding is fine. It's not open-circuit (that's what the first test shows). Most of the time the 'centre tap' is actually a junction between the 2 halves of the winding, made at the transformer solder tag, so there's no wire in the transformer that could break to disconnect the tap and leave the entire winding continuous.

But it would appear that there is no connection between the centre tap and the chasiss through the bias network. This would explain why you see no output from the rectifier. In fact the rectifier could be fine, C26 (Trader Sheet) could be charging, but because there is no circuit from the centre tap back to the chassis, a meter check between rectifier cathode and chassis will show no voltage.

Anyway, I would check R19 and R20 (Trader Sheet) next.
I checked R19 and R20 earlier (these were R3 and R2 on the manufacturers sheet). These resistors are on spec at 33 and 68 ohms.

On my last post I put up a the diagram showing the transformer winding from the manufacturers sheet (there isn't much detail on the traders sheet). What do you mean by the 'centre tag'? Which is it on the diagram?
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Old 27th Dec 2016, 10:45 pm   #138
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Default Re: My first valve radio project - Philips 462A

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Originally Posted by G8KBG Tony View Post
Have a look at the fourth picture in post #85.

You need to reconnect the white wire you snipped, it's the centre tap of the HT windings.


Here's the photo plus another close up

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I cut that wire to remove C28 temporarily (the mains RF bypass cap) - have I incorrectly disconnected a crucial part of the transformer?!?!

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Old 27th Dec 2016, 10:59 pm   #139
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Default Re: My first valve radio project - Philips 462A

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Have a look at the fourth picture in post #85.

You need to reconnect the white wire you snipped, it's the centre tap of the HT windings.
That wire only connects to the C28 capacitor thats snipped at the other end too - should I connect it to chassis?
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Old 27th Dec 2016, 11:05 pm   #140
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Default Re: My first valve radio project - Philips 462A

No. The second picture in post #139 is not the end of C28 I'm referring to.

Reconnect the (off) white wire you cut from the other end.
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