Philips 141U

[paulsherwin]

I stand corrected.

[DonaldStott]

Originally Posted by paulsherwin

I stand corrected.

Glad to hear that you are still standing, I had to go and have a lie down!

Originally Posted by David G4EBT

One option, which might sound a bit OTT, would be an aluminium clad 25-Watt resistor, which would only be 29mm long and 15mm square, mounted on the chassis if space could be found. When not mounted on a heat sink, they're de-rated to 9 Watts.

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25-Watt 270 Ohm (and other values, EG 300, 330) £2.99 post free:

https://www.ebay.co.uk/itm/265460066...Cclp%3A2349624
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As I said earlier, if not bolted to a heatsink (with heatsink compound), the Wattage needs to be de-rated by at least 50%.

Thanks David - I've ordered up some aluminium clad 25-Watt resistors at 270, 300 and 330 Ohms and will experiment to see which value is best to bring down the HT nearer to spec.

I'm assuming that bolting this resistor to the chassis with some heat-sink compound would provide some degree (pardon the pun) of heat dissipation in the absence of a proper heat-sink?


As the set is currently disassembled and the capacitive dropper tag board is on long leads I'm interested to try and track down the DC load current and, if possible, the peak current and RMS value of the current flowing through the rectifier? Not sure if I can even do any of that with my DMM in series.

[paulsherwin]

If you bolt a metal clad resistor to the chassis, the chassis will act as a heatsink and get warm. Heatsink compound isn't really necessary, but will do no harm. Exactly the same amount of heat will be generated as with the 5W resistor, but it will be dissipated over a larger area.

I still feel you are overthinking all this though.

[ms660]

Originally Posted by DonaldStott

As the set is currently disassembled and the capacitive dropper tag board is on long leads I'm interested to try and track down the DC load current and, if possible, the peak current and RMS value of the current flowing through the rectifier? Not sure if I can even do any of that with my DMM in series - for example, what measurement should I expect here:-

Attachment 280370

The meter needs to be a true RMS meter to measure the RMS current flowing through the rectifier.

The DC load current can be measured by connecting the meter (DC Amps) in series with the DC output to the rest of the receiver from the reservoir capacitor.

Ref. my previous posts, there's also at least two other methods of determining the RMS current approx. flowing through the rectifier.

I'll explain those in a bit (I have to care for a patient at home) One method is reasonably simple, the other is also reasonably simple but takes longer than the other.

Lawrence.

[DonaldStott]

Originally Posted by paulsherwin

I still feel you are overthinking all this though.

Please expand as I'm always keen to learn and open to suggestions?

Most of the time it's about practical applications but sometimes it's interesting to look at the theory?

Originally Posted by ms660

The meter needs to be a true RMS meter to measure the RMS current flowing through the rectifier.

I have a Brymen BM785 AC+DC True RMS Multimeter

[paulsherwin]

My point is simply that capacitive droppers have been fitted to this type of radio many, many times over the last few decades, and the component values required are always more or less the same. There's no harm in your digging into the theory and trying various experiments, especially if it interests you and you feel you're learning new stuff, but all that really matters is that the circuit voltages and currents are within tolerance and nothing is being overrun.

[ms660]

Originally Posted by DonaldStott

I have a Brymen BM785 AC+DC True RMS Multimeter

(excuse the delay)

In that case just connect it in series with the anode side of the rectifier.

It would be interesting if you could compare the result to the result obtained if a true RMS meter wasn't available, one method is to use the graph on book page 95 in the link below, all you need to do is measure the mains voltage, the DC load current, the DC voltage across the reservoir capacitor then do so some simple calcs.

First is to determine the peak value of the mains voltage, that's easy, multiply the measured mains voltage by 1.414. Measure the DC voltage across the reservoir capacitor and divide that voltage by the peak mains voltage and multiply the result by 100 that will give you the rectification efficiency (X axis on the graph)

Next the DC load resistance, also easy, divide the DC voltage across the reservoir capacitor by the DC load current.

Next divide the value of the series resistance (the surge limiter) by the DC load resistance and multiply by 100, that gives a figure for the curve on the graph.

Next the power ratio, strike up a line from the rectification efficiency on the X axis until it reaches the curve then read across to the power ratio (Y axis)

Next determine the DC load power, also easy, the DC voltage across the reservoir capacitor multiplied by the DC load current.

Next multiply the DC load power by the power ratio to obtain the power dissipated in the surge limiter resistance then divide that figure by the surge limiter resistance and hit the square root button on the calculator, that will give the RMS value (approx.) of the current flowing through the rectifier and the surge limiter.

Lawrence.

[DonaldStott]

Thanks Lawrence - that's today's Assignment catered for!

Before I start, I connected my Brymen BM785 DMM in series with the anode side of the rectifier on the mA setting and noted the following readings:-

DC : 37.8

AC : 116.5

AC+DC : 133.2

Do these look about right?

The rectifier's surge limiter is R2 so this will be 270 Ohms or thereabouts once I find the correct value that takes the HT nearer to spec?

Is there a need to factor in the resistance of the thermistor R5 - Hot 220 Ohms?

[ms660]

Originally Posted by DonaldStott

Thanks Lawrence - that's today's Assignment catered for!

Before I start, I connected my Brymen BM785 DMM in series with the anode side of the rectifier on the mA setting and noted the following readings:-

DC : 37.8

AC : 116.5

AC+DC : 133.2

Do these look about right?

The rectifier's surge limiter is R2 so this will be 270 Ohms or thereabouts once I find the correct value that takes the HT nearer to spec?

Is there a need to factor in the resistance of the thermistor R5 - Hot 220 Ohms?

1) It depends what the DC load current, DC reservoir voltage and the mains voltage is.

Note the DC load current for calculation purposes (for a comparison) is the current flowing between the reservoir capacitor and the rest of the receiver, not the current flowing between the cathode of the rectifier and the reservoir capacitor.

2) It all depends what the mains voltage was when the voltage measurements shown in the manual were taken (the mains voltage isn't given so far as I can see in the info I have) The schematic shows the voltage selector set to the lower position so it's a bit of a judgement call, the value is only critical in as much that it shouldn't be lower than what's needed to protect the rectifier and not low enough to cause the HT voltage to over run any of the valves in the circuit as designed.

3) No.

So the AC mains voltage in, the DC load current and DC voltage across the reservoir capacitor and we can compare...

Lawrence.

[DonaldStott]

Originally Posted by ms660

So the AC mains voltage in, the DC load current and DC voltage across the reservoir capacitor and we can compare...

Thanks Lawrence - I'm at the very limits of my understanding of such matters so I may have to bail out soon!

AC mains voltage in = 247.4V

DC voltage across the reservoir capacitor = 227.1V

DC load current = ??

[ms660]

Originally Posted by DonaldStott

Thanks Lawrence - I'm at the very limits of my understanding of such matters so I may have to bail out soon!

DC load current = ??

Looking at the schematic connect your meter (set to measure DC current) in series between C1 and the HT feed to R1 and S19.

Meter +ve to C1

Meter -ve to the HT feed to R1 and S19.

No signal, minimum volume.

Lawrence.

[DonaldStott]

Originally Posted by ms660

Looking at the schematic connect your meter (set to measure DC current) in series between C1 and the HT feed to R1 and S19.

Does 12.6mA make any sense?

[ms660]

Originally Posted by DonaldStott

Does 12.6mA make any sense?

If the receiver was working prior to that measurement and given your other measurements then no, make sure the anode current of the sound output valve is included.

Lawrence.

[DonaldStott]

Originally Posted by ms660

If the receiver was working prior to that measurement and given your other measurements then no, make sure the anode current of the sound output valve is included.

Lawrence.

The receiver is working well and as far as I as concerned the only remaining issue is what resistance value and Wattage of the anode load resistor (R2) to the rectifier is required to lower the DC HT voltage from 217V to nearer the spec of 196V.

The complex calculations (for me) of how to calculate the RMS value of the current flowing through the rectifier and the surge limiter have me completely befuddled as none of the measurements I am getting seem to be helpful? I am therefore obliged to draw a line at this point, thank Lawrence profusely for his patience and perseverance and move on ...

I will report back once the various values of metal clad resistor arrive and through trial and error should hopefully bring the HT down to nearer spec.

[ms660]

Originally Posted by DonaldStott

The complex calculations (for me) of how to calculate the RMS value of the current flowing through the rectifier and the surge limiter have me completely befuddled as none of the measurements I am getting seem to be helpful?

No problem, but it's only the DC load current that was required, you had all the other measurements.

By the way, the calcs. are nothing more complicated than Ohms Law calcs. and certainly less complicated than the capacitor dropper value you managed to calculate from the formula I posted.

Measuring DC current in a circuit is also straight forward.

But if that's where you're at then that's where you are.

Suck it and see for the surge limiter resistor value if you want to match the voltages in the manual but whatever value you use then using the method I mentioned earlier for the wattage rating will result in less localized heat and less general stress to nearby components/hardware etc. it's what's recommended in most circles.

Lawrence.

[Katar83]

Just out of curiosity, does anyone fit small fans into their radios. If I had space in the case and it was getting hot with everything else up to spec, that's first thing I'd fit in probably. PC fans come in all sorts of sizes and lots of ~12V points to connect them to? They are so quiet these days too.

[paulsherwin]

You can fit a fan, but most people feel it's a step too far in terms of preserving originality.

[Gabe001]

I haven't read the full thread, but have we effectively traded one hot resistor for another?

[paulsherwin]

The argument upthread was that the 240 ohm 5W resistor was being run close to it's rating. You're right though, all 240 ohm resistors will dissipate the same amount of heat - that's just basic physics.

[ms660]

Originally Posted by DonaldStott

I'm interested to try and track down the DC load current and, if possible, the peak current and RMS value of the current flowing through the rectifier? Not sure if I can even do any of that with my DMM in series - for example, what measurement should I expect here:-

Attachment 280370

Not current with the meter showing AC Volts...

Lawrence.