Swedish DUX U 373 mains transformerless radio.

[erikossu]

I got this radio some 20 years ago. It is in nice shape and was working many years with me before it just did not turn on one day. It has tubes wired in series so I thought that when one goes all will be off. Since it was difficult for me to test the tubes 10 years ago it just sat on the shelf until now.
I have had some experience with tubes over the years so I decided to take this one on again.
It uses tubes UM80 UCC85 UCH81 UBF80 UF89 UABC80 UL84.
To my dismay this unit is without mains transformer. Instead it has a rather large coil, multi-part resistor and a selene rectifier. So the anode is taken strait from mains 220V and rectified with selene SR 250 Y 130. I know I should not manipulate it with bare hands under power.
It appears that the reason to die was one scale light bulb burned out that is part of the heater ring. Now when I replaced it it came to life. However what worries me is the coil that has suffered from some burn some time in the past. Also the capacitor across the power 0.02uF was melted(replaced now). And we have more like 250V in house here, so I do not want to run this under these circumstances for longer period.
I think that the selene rectifier should be replaced, but this is the 2 diode type so no normal bridge will be possible. I need help with this(I read you need to add a resistor on +DC line). Also I would like to get this coil thing fixed somehow.
But it is almost impossible to find anything on this radio model. Maybe someone has a schematic/explanation on transformerless radio or something similar. It is made in Sweden by Philips.

['LIVEWIRE?']

I do not know this particular radio, but all AC/DC Valve radios work on the same principle, and that valve line-up is fairly standard. The selenium rectifier can be replaced with a silicon diode such as the 1N4007, with a series resistor of around 470* ohms -the exact value depends upon the HT current consumption, which I think would be between 40 & 60mA. A 3 or 5 watt resistor should be used. I once removed the contents from a selenium rectifier and mounted the resistor and diode inside it. That way the rectifier could be screwed back to the chassis in the original position. Be aware that radios of this type have a 'live' chassis i.e. one side of the mains, which should be the Neutral side, is connected to the chassis. * The resistor should be chosen so that the resulting HT + is the same as it was with a working selenium rectifier.

[erikossu]

This schematic will show what is inside this rectifier. Should I use the 2 diodes the same way and add resistor to the +DC?

[TrevorG3VLF]

The coil just looks as if it has been insulated with pitch and should be OK.

What is wrong with the rectifier? It only has two connections like a 1N4007, why do you wish to use two diodes?

[erikossu]

There is third that goes to chassis. Since chassis is hot (neutral) I think it cannot be ignored.
Something has caused melting of wax and tar. These selenium rectifiers make me nervous.

[erikossu]

One more thing: the radio needs 2 scale-light bulbs 19V 0.1A. These are not easy to find. I could get 24V 100mA. Our mains is 240-250V instead of 220V - could I use these instead.

[ukcol]

This Philips radiogram uses the same valve line up as your radio and was made for Sweden.

https://www.radiomuseum.org/r/philips_fs_751_uf.html

Does the front of the radio chassis in the second picture look like the one in your set?

No schematic though.

As has been pointed out your contact cooled selenium rectifier is a single diode half wave type. The markings mean that it is rated at 250v 130mA. Also as has been mentioned, only replace it if it is proved faulty. The common symptoms are low HT or complete breakdown with a horrible smell. The usual procedure when replacing it with a 1N4007 (or similar) and a wire wound resistor in series, is to experiment with resistor values of a few hundred ohms until you get the correct HT voltage.

The coil is probably a mains RF filter and working fine.

[paulsherwin]

You are correct, 24V 2.2W automotive bulbs (as used in trucks) will usually work reasonably well if the original bulbs are difficult to find.

[erikossu]

Thank you!
I like what you say. So I leave the rectifier alone - it works fine for now and shall not worry about the coil. And replace bulbs with 24V ones. The only thing I may do is wire mains so that neutral will go to chassis always using modern 3 prong plug.
My set looks like DUX V373.

[ms660]

Originally Posted by erikossu

There is third that goes to chassis. Since chassis is hot (neutral) I think it cannot be ignored.
Something has caused melting of wax and tar. These selenium rectifiers make me nervous.

That schematic is from an amateur radio transmitter, so far as I can make out it shows two SR250y130's half wave rectifiers, your radio will only have one, so one IN4007 plus resistor as suggested by others will do if it needs replacing.

EDIT: Post crossed.

Lawrence.

[julie_m]

From that diagram, it looks as though the set is using a capacitive voltage dropper. If so, the second diode is necessary to provide a discharge path for the dropper capacitor during the "trough" of the mains cycle, when the HT rectifier is not conducting. It can actually have a lower PIV, since the set itself is limiting the reverse voltage across it. (A very similar circuit using a Zener diode in that position appeared in many consumer electronic devices before the advent of cheap switched-mode supplies, to power circuits that did not need to be isolated from the mains -- thermostats, ignition sequence controllers, IR remote receivers and the like. Also in disco light controllers.)

In a set with a resistive dropper, a diode in that position would mean the dropper resistor was passing current on both halves of the cycle -- and in the "reverse" direction compared to what the set wanted, it would have the full mains voltage across it. So it would get more than twice as hot as it should.

[ms660]

It looks like a resistor on the left, but it's a secondary transformer winding:

http://www.oh3abn.net/pdft/5litraa.pdf

Lawrence.

[emeritus]

What current does it draw from the mains? From the text accompanying Lawrence's circuit diagram, (if it is the one for the set) it seems to be rated at 50W with 220V mains, meaning it draws about 230 mA. Applying Ohm's law, this means that a series resistor of about 75 Ohms is needed to drop the excess 20V for use on 240V. If I have done the sums right, this is approximately the resistance of two of the above-mentioned automotive 24V 2.2W bulbs connected in parallel, which could be used as a mains dropper if no suitably rated power resistor is to hand.

[julie_m]

Ah, so that diagram isn't for this set. That makes sense now!

[erikossu]

My radio is working rather happy with new 24V bulbs. Everything else I left is it was for now. It does not hum really so all is well. In another radio from the same era made in Finland I had to replace el.cap(same production as in DUX) that was leaking and causing awful noise. This one looks pretty healthy though.
There is one more thing I would like to take on. This radio has FM band and like in old days it goes up until 101,.... Most of the stations I like to listen are over that range. I remember in the past(40 years ago) I was able to tweak the range somewhat turning a ferrite in certain coils. However I do not want to do this by trying - I have ruined few things using this "technique" sometimes. So maybe there is good way to identify the right coil. I do not mind to lose the lower range - there is nothing there.

[erikossu]

Some pictures.
On the last one you see the FM block. Maybe it is too much to ask without schematic but witch of these 2(red dot) coils to turn and what the tall screw(green dot) is for?

[erikossu]

Hmm, I think I spoke too soon about the hum. The set has developed one that does not react to the volume control. And gets pretty annoying when stuff heats up. It has 2 big can el.caps 100uF and 40uF.
What is a good procedure to try to eliminate hum in hot chassis and series heater(I can not take tubes out one at the time) radios?

[TrevorG3VLF]

Tack a good capacitor across the existing smoothers to find the faulty one or look at the HT line with a scope. Use an isolating transformer.
You could look at the HT ripple by measuring the AC voltage through a capacitor, say 0.1uF.

[stuarth]

Do you have a multimeter?

If so, you need to check the capacitor feeding the grid of the UL84. If it fails (goes electrically leaky), it can increase the current in the UL84 and cause damage. Your increased hum could be caused by this.

Look up “that capacitor” on this forum.

Stuart