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Old 13th Oct 2019, 1:49 pm   #101
Maarten
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Default Re: Philips BX281U20 valve radio

Again about the measuring of heater voltages: that's only a rough indication and not worth spending several posts back and forth on that could have been spent on how to go about measuring the current (using ohms law and the heater series resistor for example). In series heater circuits the only way to determine whether valves are heated in their allowed operating range, is to measure current. Philips very clearly specified so in their data sheets and it makes sense since that's the parameter production is tightly controlled for in these types.

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Old 13th Oct 2019, 1:52 pm   #102
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post
Well another check again ew.
although visually and measuring wise seem perfectly fine.
It may be worth noting that from your measurements to date, the valves are bring significantly overrun.

One of the frequency changer valves has 26V across the heaters instead of 20V - more than a 25 per cent too high. Like wise the output valve has 10 V more than it should across its heater and the rectifier appears high by 5V .

This is only measurement of the heater circuit, but over running old valves by this much will not do them any favours, especially if thry are subjected to prolonged running .We have yet to understand or measure the HT circuit voltages but it would be reasonable to expect these to be high as well owing to the set's design voltage being lower than your mains voltage.

It would perhaps be worth getting the heater voltages right before exposing replacement valves to the same treatment.
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Old 13th Oct 2019, 2:31 pm   #103
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Default Re: Philips BX281U20 valve radio

Cheers Marrten I do apologise for the continued subject but unfortunately I'm not an electrical engineer so when you start using measuring current, converting to ohms, finding wattage, working out amps etc bit beyond my intellect. Throw stats at me fine. So for those of you that have had years of experience working through measurements etc. using physics it may seem simple to yourselves but not for me I'm afraid. Hence I come on here to gain knowledge and sound advice which is always appreciated and it can become repetitive. It's bit like driving for the first time there's alot to take in but eventually it becomes second nature but when your stuck behind a learner it can become frustrating as we forgot we all started in the same way.
I think changing out the resistors adding some more new wiring hopefully I'll get some better readings. But as suggested since it's 220v and it's getting 245 volts as already suggested the readings will be high. So it will probably mean adding in a shunt to bring the voltage down which I think was suggested across volume on/off switch. But I will have to try and work out using ohms law what resistance is required and wattage plus heat dissipation and whether a diode will also need to be incorporated into the circuit.
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Old 13th Oct 2019, 4:21 pm   #104
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Default Re: Philips BX281U20 valve radio

Maarten - i agree it would be desireable to get heater current rather than voltages- i was just looking to get a quick and easy indication by direct measurement rather than complicating matters with a resistor measurement, a voltage measurement and an ohms law calculation.

To find out the current by direct measurement it would be necessay to break into the heater circuit and insert the meter in series with the heaters.

But from ohms law we know V =IR
We then need to find a resistor which carries only the current in the heater circuit.

We would then measure the actual rresistance of that resistor (with the set switched off)

We would then measure the voltage drop across that resistor (with the set switched on and warmed up and stable) .

We would then have V and R. We would work out I by dividing V by R.

From the diagram, such a resistor would be R39.
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Old 13th Oct 2019, 5:52 pm   #105
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Default Re: Philips BX281U20 valve radio

Another way to do it: Meter on AC volts, across heater pins of valve. Set on and warmed up.
Record voltage V.
Switch off power to set.
Switch meter to ohms.
Record resistance R. (Quickly! -- it will change as the valve cools.)
Calculate current using I = V / R.
Disconnect meter (or at least, switch back to volts) before reapplying power to the set.
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Old 13th Oct 2019, 11:04 pm   #106
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Default Re: Philips BX281U20 valve radio

Just a quick note to tidy up a couple of loose ends.

To bring the 220V set to a condition for operation on your mains (245v?) It will be necessary to add a resistor. This should come after the on off switch and be in series with the existing dropper.

(It should not be in shunt (parallel) across the on off switch)

To calculate the resistance required we need to know the volts to be dropped as discussed before (20 -25V ). We also need to know the total current consumption of the set ie. The heaters plus the HT.

We know the heater current is 100mA

For the HT current, we can either (i) measure it using ohms law. We know all and only the HT current flows through R1, so by measuring the resistance of R1 and the voltage drop across it,we can calculate the current from I =V/R

Or (ii) using the table on p3 of the service sheet we can add up the anode and screen grid currents of all of the valves.

By adding the heater current and the HT current we have total current consumption.

By dividing the volts to be dropped by the total current we get the value of the resistor necessary.

The excess energy is dissapated as heat. We can work out the rate at which that happens using another equation - P (power)= IV

This will give us the absolute minimum wattage of resistor necessary. To be safe add 50pc.
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Old 14th Oct 2019, 9:31 am   #107
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Default Re: Philips BX281U20 valve radio

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Originally Posted by Croozer View Post
To bring the 220V set to a condition for operation on your mains (245v?) It will be necessary to add a resistor. This should come after the on off switch and be in series with the existing dropper.

(It should not be in shunt (parallel) across the on off switch)

To calculate the resistance required we need to know the volts to be dropped as discussed before (20 -25V ). We also need to know the total current consumption of the set ie. The heaters plus the HT.

We know the heater current is 100mA

For the HT current, we can either (i) measure it using ohms law. We know all and only the HT current flows through R1, so by measuring the resistance of R1 and the voltage drop across it,we can calculate the current from I =V/R
All the HT current doesn't flow through R1....

Then there's the ripple current....

Lawrence.
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Old 14th Oct 2019, 10:37 am   #108
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Default Re: Philips BX281U20 valve radio

Just for the record, in case it's not clear, in the calculations above, V=voltage, I=current and R=resistance.

(I just wondered for the first time "Why I?".... apparently it came from M Ampère from the phrase "intensité du courant".)


Anyway, just to be clear, you can measure the heater current by measuring the voltage across R39 and dividing by its resistance. (Should be 190 ohms, but check with the set off and disconnected from the supply). The supply paths for the rectifier feed and heater feed are split and only heater current flows through R39.


In round figures, you need to lose about another 25V in the heater chain series resistors which at the 100mA heater current will need 250 ohms dissipating 2.5W. Simplest fix is to add a 220 ohm 5 or 6 watt wirewouind resistor in series with R39. That'll be "near enough" in engineering terms!


The higher HT will not be a problem unless the voltage rating of any of the capacitors subjected to HT related voltages is exceeded. If you do need to reduce it, extra resistance in series with R38 can be used. Value would need some experiment, but start around 100 ohms with a 2 or 3 watt rating.
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Old 14th Oct 2019, 11:35 am   #109
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Default Re: Philips BX281U20 valve radio

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All the HT current doesn't flow through R1...
Thanks Lawrence.
Indeed it does not. I altogether missed the line running off the bottom of the diagram to the output valve anode and drawing the heaviest current of the lot.

If possible its always worth doing the sum in more than one way to catch errors.

PS Chris - Ive been re-reading previous posts with an eye to what you said about learning and preassumptions we make when we post (eg. apologies i should have said I =current), esp. Post 93 when you posted voltage measurements.

On re reading that post it occurs to me that it may be worth mentioning that a voltage isn't a property or value which one point or pin has by itself. A voltage is another way of saying 'potential difference ' and a difference is a property -like distance -which can only exist between two points.

As a current flows through a conductor (eg a heater or a resistor) a voltage (or potential difference) occurs between one end of the conductor and the other, so a voltage has the property of size and the property of the two points between which it exists (like distance).

The greater the current, and the greater the resistance, the greater the voltage difference between each end of the conductor. (This is ohms law V=I xR)

Apologies if this is 'old ground' or unnecessary, but i am conscious of taking knowledge for granted.
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Old 14th Oct 2019, 11:51 am   #110
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post
Cheers Marrten I do apologise for the continued subject
No need to apologise, my message was directed to those who were trying to help you by directing you how to measure the heater voltages, of each individual tube none the less, creating more work for you. In a series heater string, those voltages can be a few volt off under normal conditions depending on manufacturing tolerances, so measuring them will in this case only give a rough indication.

To be clear: the only way to find out if - and by how much, the heaters are overrun is by simply measuring the voltage across R39 and then taking it from there, using ohms law.
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Old 14th Oct 2019, 12:15 pm   #111
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Default Re: Philips BX281U20 valve radio

Measuring the heater voltage was valid because in one instance in the OP's measurements no voltage was recorded across one of the UCH21's heaters (post#93)

As a fault example a short circuit across one of the heaters (due to whatever reason) would not give a voltage indication across that heater and would give a higher voltage indication than expected across the other heaters.

Lawrence.
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Old 14th Oct 2019, 2:07 pm   #112
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Default Re: Philips BX281U20 valve radio

Guys I think someone has been playing around with the UCH21 at the end of the line.
R32 IS 10K which is connected to C that goes onto R40 and also pin 4 on the ubl21. This is located on pin 3 of UCH21 on the schematic. Now on the unit the UCH21 is connected to pin 7 not 3? Now pin 4 is connected to pin 8 the heater on the ubl21!!

Took out resistor R32 and its reading around 8meg!!!! On the resistor is printed 10k now remember it's supposed to be on pin 3 and it's on pin 7. There are no resistors apart from R35 that are in the meg range that high.

Now pin 1 is going onto pin 5 on the other UCH21? I'm sure someone has rewired it as none of that makes sense and is also why I'm getting no heater readings on the last UCH21 pins 1 & 8 as no connected as heaters.

Any ideas.
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Old 14th Oct 2019, 2:23 pm   #113
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Default Re: Philips BX281U20 valve radio

Now pin 5 is supposed to go to pin 5 on the other UCH21 no it's going to on/off pot plus linked to R36, -ve on smoothing caps and R75a/b and R 35. Do you think someone was using a different valve and re wired accordingly? Can also explain why voltages are wrong for sure.
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Old 14th Oct 2019, 3:38 pm   #114
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Default Re: Philips BX281U20 valve radio

Okay I think I may have found what's happening the numbers on octal base do not match up with the orientation of the spigot. I'm assuming working from right to left from spigot that would be number 1? Except on here its 5. But 1 is connected to Pot and resistors as it should be using spigot orientation and Pin 4 would be pin 8 on base. So numbers are all wrong not sure if that's possible? But I'm sure rule of thumb is to right of spigot is pin 1?

Cheers Chris
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Old 14th Oct 2019, 3:53 pm   #115
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Default Re: Philips BX281U20 valve radio

Looking at the underneath of the valve holder as shown in the schematic pin 1 is to the left of the spigot key and pin 8 is to the right of the spigot key.

If in doubt always refer to the valve data:

http://www.r-type.org/pdfs/uch21.pdf

As viewed from underneath.

Lawrence.

Last edited by ms660; 14th Oct 2019 at 3:56 pm. Reason: Link
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Old 14th Oct 2019, 4:27 pm   #116
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Default Re: Philips BX281U20 valve radio

Cheers Lawrence but numbers definitely don't tally underneath with spigot orientation that's why I was getting no readings on the heaters touching the wrong pins as numbered underneath. Still I figured it out mystery solved. Quite pleased with myself ha. Learning quite alot working on this little unit obviously with the help of you guys.

Cheers Chris
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Old 14th Oct 2019, 4:32 pm   #117
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Default Re: Philips BX281U20 valve radio

Heaters for the valves are pins 1&8, 1&8 are either side of the spigot key.

The orientation of B2 valve holder is different to the orientation of B3 & B5 valve holders.....Always go by the spigot key.....like in the manual and the valve data...

Lawrence.

Last edited by ms660; 14th Oct 2019 at 4:49 pm.
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Old 14th Oct 2019, 6:39 pm   #118
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Default Re: Philips BX281U20 valve radio

Yeah that's what I mean the numbers on octal base are all incorrect. Either side of the spigot underneath is numbered 3&4 when it should be 1&8. So I was taking heater readings on pins 3&4 hence no reading.
Pic attached show you what I mean.

Cheers Chris
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Old 14th Oct 2019, 7:24 pm   #119
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Default Re: Philips BX281U20 valve radio

Quote:
Originally Posted by ms660 View Post
Measuring the heater voltage was valid because in one instance in the OP's measurements no voltage was recorded across one of the UCH21's heaters (post#93)
Ah, yes, sorry I missed that. A non-working heater circuit would warrant individual measurements. Either resistance with the set switched off (safer but hard to find partial shorts) or voltage with the set switched on. My main point still stands, though: to evaluate or modify the circuit for a higher mains voltage, the current remains the most important parameter.
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Old 17th Oct 2019, 11:48 am   #120
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Default Re: Philips BX281U20 valve radio

Okay guys been feeling bit better today not been well for couple days so re took readings bit laborious but I think there pretty near the mark.
Rectifier:
Chasis ground readings pin1 = 116vac pin8 = 173vac
Reading across both pins as to be expected = 57vac

UBL21
Chasis ground readings pin1= 25v pin8= 90vac
Reading across both pins =65vac

1ST UCH21
Ground chasis readings pin1 = 116vac pin8= 90vac
Readings across both pins = 26v

2nd UCH21 Note pin 8 is connected to -ve on on/off switch
Ground chasis readings pin1 = 25vac Pin 2 = -18 dc
Readings across both pins 25vac to be expected as pin 8 as said is connected to negative.

I think then
Rectifier should be 50vac so 7vac higher
UBL21 should be 55vac so so 10vac higher
1st UCH21 should be 20vac so 6vac higher
2nd UCH21 as above is 5vac higher but not sure how to calculate in the negative 18vdc on pin 8?

Is it worth increasing R1 to 1.5k as opposed to 1.2k as is? This goes to pin 7 on rectifier reading 226vdc.
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