
Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. 

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17th Oct 2019, 3:25 pm  #121 
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 9,060

Re: Philips BX281U20 valve radio
As I understand it from previous posts your mains voltage is around 245 volts and the receiver is set for 220 volts, if that's the case you need to drop the difference which is 25 volts, you can do that by connecting a resistor in series with R37.
Both the heater current and the rectifier current will flow through the resistor, use the heater current given in the valve data sheets and add that to the rectifier current, for a ball park figure for the rectifier current assume that it's 2.2 times greater than the DC load current. The heater current given in the valve data sheets is 0.1 amps, the DC load current can be worked out by adding up the anode and screen grid currents given in the table in the manual or alternatively it can be found using Ohms law by dividing the voltage given across C75 in the manual by the resistance of R75 which equates to 8.2 (volts) divided by 121 (ohms) which equals approx. 0.068 (amps) Multiplying that by 2.2 equals approx. 0.150 amps, adding that to a heater current of 0.1 amps equals 0.25 amps.... Now do the Ohms law for the ball park resistor value which is 25 (volts) divided by 0.25 (amps) which equals 100 (ohms) The power dissipated by the resistor will be the voltage across it multiplied by the current flowing through it....25 (volts) multiplied by 0.25 (amps) which equals 6.25 (watts) so use one rated at 10 watts for a happy resistor life. Lawrence. 
17th Oct 2019, 5:16 pm  #122 
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 14,394

Re: Philips BX281U20 valve radio
I thought I'd covered this without needing to fret about common heater and half wave pulses here:
https://www.vintageradio.net/forum/...&postcount=108
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17th Oct 2019, 6:17 pm  #123 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Cheers guys thanks for working out the math bit lost on that one I'll have a look in one of me books there all full of calculations but related to amps not radios. Okay so Lawrence saying in series with R37 but Herald saying R39! So which is best option R37 seems logical whereas R39 is slightly up the chain where the voltage has already dropped. Or does it not matter?
I'll have to fit that resistor underneath somewhere not sure where quite tight in there. No room for it ontop side. So I did not need the heater volts as they where always going to be high. So not sure why I was being asked for them? Ha. So question is why is pin 8 on the last UCH21 going to negative on pot and negative to C75? Whereas all others are in series? Must be a reason. Cheers again Chris 
18th Oct 2019, 12:01 am  #124 
Dekatron
Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 14,394

Re: Philips BX281U20 valve radio
I picked R39 because it's in series with the heaters only, so working out how to drop 25V or so was easy with only the 100mA rms heater current to consisder. And a slightly high HT is unlikely to cause any problems. If it does, it could be dropped by increasing the value of R38 by trial and error.....
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18th Oct 2019, 9:53 am  #125 
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 9,060

Re: Philips BX281U20 valve radio
I did the ball park calcs for a dropper resistor in series with R37 not necessarily to maintain the main HT at the specified value in the manual but to keep the rectifiers maximum hot switching current within the limit set by the manufacturer.
Lawrence. 
18th Oct 2019, 10:35 am  #126 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Okay guys I think I'd leave the thermistor side as you also have ferrite beads on dropper that end and don't want to mess around with them.
Be easier I think from other end. I'll have a play around today as I have quite a few resistors. I was thinking of increasing R1 by a bout 300 ohms as C1 & C2 bit high. That should bring voltage down or not? I also think due to the design it is what it is and unless I put a mains transformer in there somewhere the voltages are always going to be bit high. Be interesting see how voltage will be affected. Cheers Chris 
18th Oct 2019, 1:02 pm  #127  
Pentode
Join Date: Jul 2016
Location: Edinburgh, Scotland, UK.
Posts: 105

Re: Philips BX281U20 valve radio
Quote:
Some time ago at the start of the thread you asked about running a set marked 220V on UK mains. By measuring the heater voltages you established (i) that the set had not been modified for modern UK operation and (ii)that the heaters were being significantly overrun more than double the accepted +/10pc for voltage. A valid point was also made that in series heater strings like this it is the current which is the primary parameter and instructions were given several times by me and others on how to measure this without needing to break the heater chain. Instructions were also given on how to add a series resistor of appropriate value. Herald and Lawrence have both gone through the calculations with actual figures so that you know the value of the component necessary. One (post 108) provided an option to deal only with the overrun heaters, (and,as an option, the HT circuit if necessary) the other (post 121) provided an option to deal with dropping the voltage for the whole current requirements of the set. It may be worth re reading both of those posts as they both provide solutions with resistor values necessary to make the heater voltages (and HT voltages if necessary ) correct for this set without adding a transformer, and without guessing at component vales.  xx Throughout this we have been trying to explain 1. ohms law ( see my post 109) V=IR 2. Power (P) (measured in Watts) P=IV These two equations form the basis of electricity and I'm going to have another go at trying to show how you can use them as I am determined that we should not have gone through 120 odd posts without you having the chance to master ohms law. The datasheet says the radio consumes nominally 40W at 220V. We know the house mains are 245V. Without doing any measuring at all can we work out the value of the series resistor necessary to operate the radio on the house mains? We need to choose a resistor which will drop 25V across it. To work out R (from V=IR ) we need both the voltage drop across it V (25V) and the current flowing through it (I) The resistor is will be in series with the radio and the current (I) is the same at any point in a series circuit so if we can find the current drawn by the radio we can do the s necessary to find R (because we will have two of the three terms in the equation) For the radio, we don't have I but we do have P (40w) we can rearrange P=IV as I=P/V so I (the current drawn by the radio)= 40/220 = 0.181A we know that we want to drop 25V (245220) across the reisistor and the current flowing through the dropping resistor should be 0.181A So from ohms law rearrange V=IR for R : R=V/I R= 25/0.181=138 ohms  pretty close to Lawrence's result. Note that the above calculation is a gross simplification and 'quick and dirty' ;eg. the 40W figure quoted by the manufacturer is nominal only for actual component values, use the sums and methdology which have been done in previous posts. These two sums were intended to illustrate ohms law, to show how calculations can be done so that guessing is unnecessary and how by tackling the same problem in different ways you can 'prove' your answer is in the right ballpark. 

18th Oct 2019, 5:56 pm  #128 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Okay yes I understand how those calculations are done it's just remebering ohms law E over I×R. E voltage, I current amps and R.
So just moving around depending on what your looking for in this case voltage drop. This part Lawrence has written I do not fully understand and at present I'm still trying to interpret abbreviations used on valve spec sheets as well as trying to understand the graphs. Both the heater current and the rectifier current will flow through the resistor, use the heater current given in the valve data sheets and add that to the rectifier current, for a ball park figure for the rectifier current assume that it's 2.2 times greater than the DC load current. The heater current given in the valve data sheets is 0.1 amps, the DC load current can be worked out by adding up the anode and screen grid currents given in the table in the manual or alternatively it can be found using Ohms law by dividing the voltage given across C75 in the manual by the resistance of R75 which equates to 8.2 (volts) divided by 121 (ohms) which equals approx. 0.068 (amps) Multiplying that by 2.2 equals approx. 0.150 amps, adding that to a heater current of 0.1 amps equals 0.25 amps... Bit lost on DC current side but sure repetitive reading will help work that out. Now I was lost on the R75 121 ohms now I assume since R75 is 2 resistors in parallel that would mean R1xR2 ÷ R1+R2 so 59400 ohms ÷ 490 ohms = 121 ohms. Is that correct? Now since putting in the new resistor in series C75 has dropped to 7 volts bang on. So 0.05 amps or I x 2.2 = 0.12 + 0.1 = 0.22 amps. So think I understand that math. Cheers guys really appreciate your time and the 100 or so posts I will need to print off the important ones so makes it easier to reference. I've noticed a drop of 45v by adding in a 180 ohm resistor. Not checked heater currents will do that bit later. Cheers guys 
18th Oct 2019, 7:29 pm  #129 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Wahey guys am I chuffed or what thanks a million for your input.
You won't believe this but as said I put 180r in series with R37 as suggested by Lawrence and every heater is bang on not a volt out. Rectifier 50v UBL21 55v UCH21 pair 20v. Taken 4 measurements over an hour. I have changed around 5 resistors as they where starting to deteriorate and where well out of spec. Brilliant guys I'm over the moon I can now refit the unit back in its cabinet tidy up wiring etc and done it does sound nice and clean now. Superrrrrrrrrrrr All best Chris 
19th Oct 2019, 4:26 pm  #130 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Well it took several hours to try and get the tunning dial cord secured that is not an easy job at all if you are thinking of removing it for reference. Very easy to remove but trying to get tension on refiting have a think first!!!!
Just a picture for those who need to get the voltage down using 10 watt resistor at the end of R37 theres a hole in chassis which allows you to drop it through partially so no need to drill holes or try and find room for it underneath. I'll post some pics of it all complete then job done guys. 
22nd Oct 2019, 1:21 pm  #131 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Well all done except if someone lives in Kent and can re set the cord on pulley dial be helpful. Quite happy to pay. I've tried over 2 days and just cannot get the cable to stay on pulley with immense frustration.
Again many thanks for all your help and especially the ohms law and being able to get the voltage down so the valves are now running perfectly. All in all it's cost me including the price of the radio but not including labour 80 quid. I think that's cheap considering condition as theres one on ebay at present for 69 quid missing 4 valves back and bottom panel missing no dial huge crack in cabinet rust bucket it's basically a mess so going by that I think I've done well. I've also cleaned up the case which looks nice. Pics posted. Again if anyone can do the dial let me know. All best Chris 
22nd Oct 2019, 1:22 pm  #132 
Hexode
Join Date: Apr 2019
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Posts: 461

Re: Philips BX281U20 valve radio
last 2 pics

22nd Oct 2019, 1:24 pm  #133 
Hexode
Join Date: Apr 2019
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Re: Philips BX281U20 valve radio
pic of case forgot

31st Oct 2019, 11:56 pm  #134 
Hexode
Join Date: Apr 2019
Location: Birchington, Kent, UK.
Posts: 461

Re: Philips BX281U20 valve radio
Thanks for your help everyone all finished hopefully some decent info for others wanting to restore the Philips BX281U20 valve radio.
Cheers Chris Post can be closed 