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Old 19th Sep 2019, 8:33 pm   #21
turretslug
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Default Re: Philips BX281U20 valve radio

I don't think that dropper looks too bad at all- compared to some that keep on functioning. You'd be lucky to find a NOS one anyway. Provided that it's electrically OK and it and the ceramic sleeves are not cracked, it'll soldier on. As they run seriously hot, the dust that collects on the rough surface tends to carbonise and make them look grimy over time, but it's only a cosmetic thing.

It's possible that that odd valve socket has some sort of packing plate that adapts Loctal to International Octal-based valves- the pin layout and spacing is the same and the International Octal types have longer, fatter pins that the plate would cater for. It's difficult to tell, but it may be that that socket contact design accommodates both thicknesses of pin type. The late '30s/early '40s saw a variety of valve base types jostling for dominance (not to mention nation states....), Philips radios would have gone from side contact through Loctal and International Octal to miniatures in just a few years and it wouldn't be surprising if some valve types were changed over the production run of a particular set.
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Old 19th Sep 2019, 9:05 pm   #22
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Default Re: Philips BX281U20 valve radio

Thanks guys.
removed whole pulley system obviously left riveted ones in. It all needs a good clean and new cable. The tab for the spring on front of pulley broke so I'll have to make something up. Seems easy enough job he says ha.

Thanks dropper some of the enamel coating has split and couple of the strips bit rusty and the solder joints have gone cold will need re doing. I'll de solder have a good look at it and give it a good clean including those beads as their filthy. I'll remove the air variable capacitor so can get better access to the chassis for a good clean.

That dual 50uf can looks okay underneath no leakage not seen a base like it its large threaded base with large nut securing it. Bit odd can is grounded but one of the positive tags on base has also been grounded as well as feed into the circuit I.e two wires coming of it the other just single wiring hooking into the circuit not come across that before.

Funny old set up this one plus no mains transformer. Plus I came across two which look like fuse resistors. Marked in parts list but ohms readings left blank?

Oh well should be a challenge.
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Old 19th Sep 2019, 11:46 pm   #23
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post


Funny old set up this one.....

Oh well should be a challenge.

It's an old Philips- 'nuff said!
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Old 20th Sep 2019, 12:47 am   #24
Maarten
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Default Re: Philips BX281U20 valve radio

I can confirm it's Belgian made. L, later PL, points to the factory in Leuven, Belgium.
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Old 20th Sep 2019, 9:58 am   #25
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Default Re: Philips BX281U20 valve radio

Ha ha
it's an old Philips like that.

Cheers for info on where made much appreciated.

Gonna be pulling her apart today be good fun. Just need to remember how to put it back together again ha ha.

I'll post pics as think it helps especially if someone wants to restore one at least it's bit of a reference pictorial wise something I could have done with on many occasion. I find pictures speak a thousand words.
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Old 21st Sep 2019, 10:58 am   #26
cdm1christopher
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Default Re: Philips BX281U20 valve radio

Well removed air tuning cap not easy job everything is just so compact but eventually got it of a gives a good clean.
Got main dropper of and again good clean it's still in pretty good shape readings seem okay. Cleaned the ferrite beads as well although they do have heat Mark's.

Now the 50uf dual can. Readings well low one is 14.7uf the other 16.2uf. Now another big job. Its screwed to chassis with a 3/4" screw but again need to remove a dropper resistor which is attached to the cap. The resistor is bolted to chasis and the screw end is you got it in underside of chasis not the other way round. So to get to the screw will need to remove 4 resistors and a capacitor!!! I was hoping I could get two new caps underneath but no room at all.
The other oddity the cap on chasis has 3 metal washers underneath with earth strap connected. All gone rusty so it needs to come out.

I'm hoping I can get two caps small enough to fit in original can. Can is odd size 32mm diameter but a whopping 90mm high. It's 225v so not very high rating.

The other cap electrolytic bypass is odd 100uf only reading 24.6uf and it's 12.5volt not come across that voltage rating before. Plus not marked +/- I'm assuming the black end of cap is negative as it's going to chasis ground.

Still just thought I'd fill you in bit long winded.

If anyone restored one of these bit help would be great.

Cheese
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Old 21st Sep 2019, 11:13 am   #27
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Default Re: Philips BX281U20 valve radio

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Originally Posted by cdm1christopher View Post
The other cap electrolytic bypass is odd 100uf only reading 24.6uf and it's 12.5volt not come across that voltage rating before. Plus not marked +/- I'm assuming the black end of cap is negative as it's going to chasis ground
The black end that's connected to chassis will be +ve, the other end is connected to the bias supply which is -ve, the bias voltage is developed across R75 which has the current for all the valves flowing through it.

Lawrence.
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Old 21st Sep 2019, 12:44 pm   #28
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Default Re: Philips BX281U20 valve radio

Thanks Lawrence I take it this is because there is no mains transformer?

Oh lucky me found two 47uf 250v caps in me tub plus right size to fit in can. Readings on this can now out of circuit are well low they are now reading 1.7uf and 4.9uf. Any ideas why two wires at base attached to can? See pic.
I know can -ve but not seen wires attached to side of can before.

Well time to clean her out.
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Old 21st Sep 2019, 1:00 pm   #29
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Default Re: Philips BX281U20 valve radio

Look at the schematic...the -ve of the dual capacitor in the large can is not connected to the chassis, the can should be isolated from the chassis and should connect to the bias resistor etc and mains Neutral via the on/off switch.

The method of obtaining the bias that way was used in live chassis receivers and mains transformer receivers.

Lawrence.
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Old 21st Sep 2019, 1:25 pm   #30
cdm1christopher
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Default Re: Philips BX281U20 valve radio

Yeah just looked at can setup pic attached.
The bakerlite ring goes to chassis the metal ring with solder tag goes to the base of can where the two wires are coming from side of can. The tag is then soldered to capacitor ontop of chasis and a yellow wire which goes off to a resistor under chasis which then via a black cable goes to the mains voltage selector. So the -ve on the new caps need to go onto that tag? Have I got that right? Want to make sure.

Cheers Lawrence
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Old 21st Sep 2019, 1:37 pm   #31
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Default Re: Philips BX281U20 valve radio

Yes, the wires that are attached to the large ring tag should be connected to the -v's of the replacement capacitors.

Lawrence.
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Old 21st Sep 2019, 2:16 pm   #32
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Default Re: Philips BX281U20 valve radio

Just a thought - has the set been connected to the mains lately? If not, you may find that the capacitances will come back up if you apply a low-ish (ca. 20V) DC voltage across each section - charging and discharging each section in turn a couple of times. Remember also the values on the can are nominal only and the set will probly operate on wide tollerances, (16 mFd was common for resevoir capacitances) as long as they aren't actually leaking and you could avoid a replacement.

The point Laurence makes about the 100mFd being positive-to-chassis ie "upside down' is interesting. If you check the diagram you will see that there are none of the typical cathode resistors to raise valve cathodes above chassis (and grid) potential. Rather its the cathodes that are at deck voltage and a -ve feed is provided to make the grids negative - hence the 'upside down' condenser - a bit like an old-fashioned 'grid bias' circuit in a battery set.
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Old 21st Sep 2019, 2:43 pm   #33
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Default Re: Philips BX281U20 valve radio

Please disregard previous post. Superceded by events.
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Old 21st Sep 2019, 4:22 pm   #34
cdm1christopher
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Default Re: Philips BX281U20 valve radio

Cheers guys.
oh dear I thought I had it all wrong taken me 20 minutes to work out it's right. The diagram keeps throwing me as was not sure where junctions where plus linking up 13 14 15 etc.From top chassis to base then looking at schematic.
Plus putting +ve to chassis throws me a bit ew. Still good learning curve. Plus the no mains transformer!!!

Bit late now can all cleaned out and prept for new caps. To be honest on opening up the wires on two connections were corroded and hanging on by a whisker and the electrolytic was as dry as a bone just fell out once tar removed from base.

I did think about that re caps bringing up voltages but the set looks like it's prob not been used for 20 years just stored in its case so might as well change them while it's apart. Don't want the oh leave it alone put it all back together only to have probs then taking it all apart again. Done that once don't wanna do that again ew.

Any ideas on decent tuning dial cord. I've got some waxed leather stitching cord pretty robust stuff but suppose wax on it will collect dust just thought of that!

I think it's been stored in garage or loft as there are spots of corrosion.

Still just like to say really appreciate your help.
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Old 22nd Sep 2019, 1:28 pm   #35
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Default Re: Philips BX281U20 valve radio

Nice new cap pic attached.
Any ideas on these two resistors there on spec sheet but have no denomination.
I think maybe fused resistors?

Cheers
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Old 22nd Sep 2019, 1:40 pm   #36
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Default Re: Philips BX281U20 valve radio

The replacement capacitors that would conveniently fit in the can with lots of room to spare, seem way too small to handle the ripple current which can be up to a few hundred milliamps. Before going through quite some length to mount them and then after some hours of usage replace them again, look up their datasheets.

Given the dimensions of the can, I would be looking for two snap in capacitors which would fit snugly with some shrink sleeving for insulation (the factory sleeve isn't qualified as insulation.
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Old 22nd Sep 2019, 2:43 pm   #37
cdm1christopher
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Default Re: Philips BX281U20 valve radio

These are fine 105C 920mA 10,000 hour life span I don't buy cheap capacitors as I think it's a waste of time as you said be replacing them every 5 minutes. These are not that small but their only 250v so size will be different to the 450v I have which would not have fitted. I did check but thanks for input.
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Old 22nd Sep 2019, 2:47 pm   #38
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Default Re: Philips BX281U20 valve radio

Quote:
Originally Posted by cdm1christopher View Post
Any ideas on these two resistors there on spec sheet but have no denomination.
I think maybe fused resistors?
They might be thermistors, remove the scale lamp and measure the cold resistance of each one and post the results.

Lawrence.
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Old 22nd Sep 2019, 7:30 pm   #39
Maarten
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Default Re: Philips BX281U20 valve radio

Quote:
Originally Posted by cdm1christopher View Post
These are fine 105C 920mA 10,000 hour life span I don't buy cheap capacitors as I think it's a waste of time as you said be replacing them every 5 minutes. These are not that small but their only 250v so size will be different to the 450v I have which would not have fitted. I did check but thanks for input.
920mA (at 120Hz presumably) is quite good! I was indeed fooled by the 250V WV.
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Old 22nd Sep 2019, 9:27 pm   #40
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Default Re: Philips BX281U20 valve radio

Like Marten, i was surprised by a ripple current of nearly a whole amp in such a small space. This is a live chassis set and so has a half wave rectifier, the capacitor has to work twice as hard as full wave - its a configuration that works the component very hard (the data sheets don't quote below 50 hz!) , so i had a quick check on the datasheet i used last time i bought bits. Of course it is unlikely to be for the same component -or even the same maker as yours, but the ballpark numbers may make a second check worthwhile.

For 250V it gave 930mA for a 220mFd
and 330mA for a 47mFd.

These were at 120Hz, so would reduce to 230 mA at 50 Hz for a 47mFd.

There is a formula to work out ripple current which i can't remember, but which will be unnecessary if the spec of your capacitor is 920mA.

But it's an easy table to miss-read, and 920mA is very close to 930mA (which is why i mention this) and a quick re-check now may save difficulty later.

As Marten says, you can expect a half wave configuration like this to impose a significant ripple, and associated stress!
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