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Old 4th Oct 2019, 9:35 pm   #61
cdm1christopher
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Default Re: Philips BX281U20 valve radio

Okay to be honest not sure which is the right way round not come across one of these before. I did put new flex and plug on with 3 amp fuse. I've connected it so the live is going to the upper mains input pin and negative the bottom pin. Is that the right way round? I've noticed on the voltage selector you have 3 sine waves at top and below that 3 straight lines is that an indication where live and negative should be? Again usually you just have the figures to choose from.

Mind you come to think of it I tried to wind it up on a variac and even at 220v nothing. But on measuring the bottom pin which is where I have connected -ve was reading the input voltage from the variac whereas from the positive nothing?

It only lit up when I plugged it straight into the mains the variac obviously does not work on this type of unit without a transformer. I suppose it must have something to do with those thermistors not sure.

I don't have a service manual in English so difficult to decipher.
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Old 4th Oct 2019, 10:02 pm   #62
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Default Re: Philips BX281U20 valve radio

This radio will work via a Variac if the Variac is working.

There's only one way you should have the mains connected....Mains Neutral to chassis via the on/off switch and the bias supply resistor R75....Mains Live to the fuse in the receiver, both via the plug/socket at the receiver.

There's no polarity for AC....Just Live (Line) and Neutral.

The sine wave indicates AC....The straight Line with the dotted line below it indicates DC.

Lawrence.
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Old 4th Oct 2019, 10:17 pm   #63
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Default Re: Philips BX281U20 valve radio

Unfortunately without seeing the set it is not possible for me to say whether it should be the top or bottom pin which should be connected to neutral.

It will be necessary for you to trace the wiring to find out which pole ends up on the chassis, bearing in mind that it is connected via a resistor.

This point is most important, if you get it wrong you will end up with the whole lump of metal in front of you being mains live -
with serious danger if it should be touched by someone, especially if they are earthed.

Please be sure of this point before reconnecting to the supply.

Last edited by Croozer; 4th Oct 2019 at 10:36 pm. Reason: Disambiguation
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Old 4th Oct 2019, 10:26 pm   #64
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Default Re: Philips BX281U20 valve radio

Afterthought:
There are other rules for working safely on this class (ac/dc /live chassis) of equipment-and for making this type of equipment safe to use after completing works. It is important that you are familiar with them for your safety and for the safety of anyone else using the set.
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Old 5th Oct 2019, 9:44 am   #65
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Default Re: Philips BX281U20 valve radio

Cheers guys seems to be a rather dangerous design if you ask me. Wonder how many eletrical engineers suffered at the hands of fixing these units.

Variac fine I think it might have been I changed the rectifier valve which forgot to mention then switched on via mains not variac then lit up. Like say was picking up stations just so noisy but unit was cutting out then coming back on. Something shorting I'm sure that would cause that.

I did not realise this unit is polarized on mains I thought it would have been non polar? Nothing on schematic showing this to be polarized.

So it will still work even if the chasis is live? Would this not cause it it to just stop working? Also would I not get a voltage reading off the chasis?

Precautions noted I will have a look at all wiring today.
Lawrence do you mean those thermistors as there are no actual fuses in the unit.

Is the rectifier valve pins secured in base only asking as it is near on impossible to remove once secured in its base you need screwdriver to prize it off!!!! Or wrong octal base fitted or too much solder dropped down pin holes?

Should be a fun day.
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Old 5th Oct 2019, 10:35 am   #66
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Default Re: Philips BX281U20 valve radio

Quote:
Originally Posted by cdm1christopher View Post
Lawrence do you mean those thermistors as there are no actual fuses in the unit.
If there's no provision for a fuse in your receiver, I would have thought that the mains Live would be connected to the other pole of the on/off switch, if it's a double pole switch that's fitted, that would be the normal convention.

Lawrence.
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Old 5th Oct 2019, 12:41 pm   #67
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Default Re: Philips BX281U20 valve radio

Well switched on moved tuning dial dead ha.
Now interesting using external ground chasis was reading AC 28v flipping mains lead round 0v.

Input voltage 245 AC

Two wires one grey one yellow both go to on/off. Yellow connected to R36 then pin 3 rectifier and pin A on voltage selector Now grey goes to R37 and pin e mains selector. This also goes to C1 and C2 -ve so the Yellow seems to be only one which has a ground point along its route. So this I suspect would be -ve point on mains plug.

There is also pretty much like a solid state amp relay switch. On switch on after 5 seconds the lamp comes on and valves start to warm up. So it must be those thermistors obviously when optimum temperature reached they warm up then allow larger current through as resistance drops. I think that's how these work.

Any how I have a dead in the water radio voltage getting to switch that's it. I did notice on the switch that both power sources are bridged but neither connection is giving continuity across the bridges the wipers are bit green and grubby might just desolder clean up see what happens. Change those thermistors and just whack in a couple of 10 watt 220r resistors see what happens. I think these drop to around that rating once warmed up.

Worth a try?

Hope I've not fried the valves I could get me taylor out and check them.

So bit of bummer but hey ho.
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Old 5th Oct 2019, 1:25 pm   #68
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Default Re: Philips BX281U20 valve radio

I may have solved the problem the wipers on mains switch are not making contact to the inner wipers just cleaned one up and good continuity now as nothing before so could be why it was coming on and of intermittently minimal current getting through. I'll clean the other see if that's problem.
Sorry for going on guys.
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Old 5th Oct 2019, 1:51 pm   #69
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Default Re: Philips BX281U20 valve radio

A fuse is shown on the circuit diagram at Z1 before the mains switch. What appears to be a fuse holder is shown on figure 9 on the service sheet connected to wire numbered 15.

It is vital to connect this unit correctly to the mains, if you have measured your mains voltage at 245V, then one orientation of the mains connector will put the whole chassis at or close to 245V above earth.

There are other precautions you need to take, (eg. Work with one hand in your back pocket) but their successful implementation depends on understanding what you are doing.
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Old 5th Oct 2019, 3:23 pm   #70
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Default Re: Philips BX281U20 valve radio

Yes I can see that there seems to be some kind of bakerlite plate attached to the metal screen with which looks like it may have had a 0.3amp fuse connected. Which is incredibly low rating! I have attached pic which shows where this plate might have gone. It is also appears to be all situated around the dropper resistor and mains selector. So maybe because it was set at 220v perminantly the fuse was omitted. Just a thought! and it was there if switched to 125v. As it shows the pole switch next to R37 and appears that you can break the fused circuit when selecting a voltage rating. As it shows the switch not in line with the fuse both are facing upward away from the fuse.

The mains input wiring on this set looks like it has been the same since manufactured as there seems to be no evidence of it being desoldered pic attached.

I think it's only draws max 50 watt. So I've installed a 3 amp fuse although I think you could get away with a 1 amp.

If non polarized would the chassis become 245v live? and what's best way of measuring whether the chasis is holding that voltage? Be very quick way for correcting the right orientation of the plug without shocking results. Bad joke I know.

Anyhow it should work now as all contacts nice and clean with exceptional continuity. I'll use variac.
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Old 5th Oct 2019, 3:27 pm   #71
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Default Re: Philips BX281U20 valve radio

put wrong pics on oh dear.
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Old 5th Oct 2019, 5:15 pm   #72
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Default Re: Philips BX281U20 valve radio

Quote:
Originally Posted by cdm1christopher View Post

If non polarized would the chassis become 245v live? and what's best way of measuring whether the chasis is holding that voltage? .
This is an important point because it relates to safety. It needs understanding rather than simply following instructions so I will try to explain this in a different way.

There is no positive or negative pole on the AC mains electricity serving your house. One side of the mains is connected to the earth, either at your fuse box or at the electricity substation. This is the neutral side.

The other side is the live side.

240V separates live and neutral.

In a live chassis radio, one side of the mains is connected to the dropper resistor, valve heaters and rectifier.

The other side is connected to the chassis.

If it happens that you connect the live side of the mains to the chassis, the chassis will be at mains voltage above earth and dangerous.

It is therefore necessary to make sure that the chassis is connected to the neutral side of the mains so that the chassis is at or close to earth voltage to avoid a shock hazard.

This is a vital point for safety. There are various permutations of danger which can arise (live spindles/speaker frets, broken neutrals etc.) as a specific result of live chassis sets like this. Much has been written on how to mitigate the dangers, but successful mitigation depends on understanding the theory.

Much simplified drawing attached for assistance.
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Old 5th Oct 2019, 5:26 pm   #73
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Default Re: Philips BX281U20 valve radio

PS I have shown the rectifier as a silicon diode in the drawing. In your case it is a valve but I thought we could do without the complications of heater wiring and insulation,but the principle is the same.
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Old 5th Oct 2019, 6:31 pm   #74
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Default Re: Philips BX281U20 valve radio

Ah nice one I can see where your coming from. Looking at your drawing I definitely have it the right way round. Really appreciate your input and your concerns on the inherent dangers of working with a set like this. As this is the first time I've encountered such a circuit design. But I've learnt something new and hope to continue too with members like yourself Lawrence etc. and as always it's very much appreciated.

Yes I must agree that it's taking me a while to figure out valve designs and how they work which is an education in itself let alone the rest of the components which seem simple enough but are not. But I have my trusted books and tend to do repetitive reading as well as putting it into practice on real circuits. Although funny enough I don't have book that covers this circuit design. One without a mains transformer!!!

I have her on the variac at the moment and she has lit up so what I thought was going to be a complicated trail of elimination just ended up being incredibly dirty contants on mains/volume on/off switch!!!! But I'm slowly winding her up to give things a chance to charge and settle. Hopefully the resistors are fine. Plus I managed to find schematic again with the help of Lawrence which gives the voltage readings around the circuit.

Many thanks Chris
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Old 5th Oct 2019, 7:33 pm   #75
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Default Re: Philips BX281U20 valve radio

Happy to help. When I worked on the railways our Health and Safety Manager's mantra was 'education is the best mitigation'. Sometimes it just takes a different perspective to understand something. Best wishes with the rest of the restoration.
C.
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Old 5th Oct 2019, 9:35 pm   #76
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Default Re: Philips BX281U20 valve radio

Your receiver is designed so that under normal working conditions there's a DC potential between Neutral and chassis, that's due to resistor R75 being connected between the said, the potential of the chassis with respect to Neutral is +ve, none of the heater current flows through R75, only the dc load current through the valves flows through it, the voltage across it (8.2 volts quoted in the manual) is used for the bias supply for some of the valves control grids, the bias supply being -ve with respect to chassis.

Due to the bias supply being -ve with respect to chassis is the reason why the electrolytic capacitor (C75) that's connected across R75 has its +ve side connected to chassis.

Often the first easy give away in a schematic where this type of biasing arrangement is used is the fact that the cathode of the output valve is connected directly to chassis.

Lawrence.

Last edited by ms660; 5th Oct 2019 at 9:46 pm.
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Old 6th Oct 2019, 9:34 am   #77
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Default Re: Philips BX281U20 valve radio

Okay I'll keep that in mind when taking readings today.
I had her running yesterday and no visible problems apparent no burning smell etc. I tend to always have a good sniff ha.
The valves brightness seems fine. Tuning in okay although my basic aerial will need connecting to it. Should pick up more stations. I might have to put bit of insulation tape over one of the RF coil solder tags as it's about 1mm away from fins when tunning giving a static charge.

Pic of her lit up not great pic though.

All left to do is put new dial cord on front end the new inner cord all done.

Quite loud for small unit.

Cheers Chris
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Old 6th Oct 2019, 11:23 am   #78
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Default Re: Philips BX281U20 valve radio

Lawrence do you mean R36 as this is connected to C75 and C109 on output valve UBL21?
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Old 6th Oct 2019, 11:46 am   #79
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Default Re: Philips BX281U20 valve radio

Since it's a series heated set, you should measure heater current, not voltage which was suggested on the previous page. Heater voltages in series valves have been known to be off a few volts while the current measured spot on.
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Old 6th Oct 2019, 12:33 pm   #80
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Default Re: Philips BX281U20 valve radio

Thanks for that I've noticed that I'll scroll through later today.

Again completely confused here which is the right way round on mains plug after re checking again.

Both connections on the on/off go in a loop as such.
Now the yellow cable briefly follows this route from mains in. Basically goes 2 ways 1st is linked to the -ve on the 100uf cap the +ve which goes to ground. 2nd route goes to R37 on dropper then onto C110 which is connected to -ve on the 50uf dual can then on to pin 3 of the rectifier the anode. Am I right that this is the positive connection on the mains input plug?
Or since everything is going to -ve should it be neutral on mains input plug?

I know we have been over this but I just need to double check.

Many thanks Chris
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