Loewe Radio Zwergsuper Opta 3516GW

[bobskie]

I can see this is going to be a bit of a challenge
Would this resistor be able to be changed to run on 240v more safely?

Cameron

[ms660]

"Would this resistor be able to be changed to run on 240v more safely?"

You can choose a dropper resistor for 240 volt operation no problem.

Looking at the schematic and out of interest I can't see any extra dropper switched in to the rectifiers anode when selecting 220 volt operation or is there something else going on?

http://85.144.220.212/nvhr/Opta_3516GW.pdf

Lawrence.

[boxdoctor]

Hi Bobskie,
If you end up retaining the fitted metal rectifier, it should be mounted horizontally, not vertically as it is at present.
This will allow the air to circulate freely between the fins,and enable them to do the job of keeping the rectifier junctions cool, which is what they are intended to do. Cheers, Tony

[turretslug]

Originally Posted by music-centre

Must be a rare set - the dial is unusual, hardly any UK stations shown but its got Welsh regional on MW! - what would the London shown on LW next to Droitwich be?

"Pressburg" is also an interesting historical snapshot- dial markings can be one of the most appealing things about old radios, plenty of historical, cultural, even aspirational appeal. We hear plenty about the world-wide reach of the internet- in those days, a radio that could apparently "reach out" to far-away places must have seemed amazing- though, in this case, the markings are more provincial in aspect and I suspect that most radios spent much of their time on just a handful of stations anyway.

[bluepilot]

Originally Posted by ms660

Looking at the schematic and out of interest I can't see any extra dropper switched in to the rectifiers anode when selecting 220 volt operation or is there something else going on?

As far as I can see, the UCH11, UBF11 and UCL11 heaters are connected in series. For 220V operation this chain is connected in series with the UY11 heater. For 120V operation the chain is connected in parallel with the UY11. (So what I said in post 14 isn't completely correct, only for 120V operation.)

[ms660]

What happens to the HT, eg HT voltage when on 110v and HT voltage when on 220v?

Also half of the field coil is shorted out when 110v is selected, I've not seen that arrangement before.

Lawrence.

[G6Tanuki]

It's interesting that on the Radiomuseum site the entry for the UY11

http://www.radiomuseum.org/tubes/tube_uy11.html

shows a plug-in solid-state replacement which includes a dropper-resistor to mimic the behaviour of the heater!

This makes me suspect that the original vacuum-tube version of the UY11 may have had some underlying 'issues' which mitigated against continuing production/use as a replacement and so made it worthwhile for a manufacturer to go to the effort of designing/marketing the solid-state substitute.

[bluepilot]

It's interesting that on the Radiomuseum site the entry for the UY11

http://www.radiomuseum.org/tubes/tube_uy11.html

shows a plug-in solid-state replacement which includes a dropper-resistor to mimic the behaviour of the heater!

For those who can't understand German, I note that there is an amusing pun there that I hadn't heard before. German for rectifier is Gleichrichter. The selenium equivalent is described as a "Gleich-riecht-er" meaning it immediately stinks, which being selenium it could well do.

[turretslug]

With that high voltage, low current heater, I wonder if the ends outside the cathode cylinder were prone to "flare" and early failure? I'm intrigued by the component apparently in parallel with the black finned rectifier- it looks like a gas spike-quencher (unlikely here), or possibly a ceramic capacitor, I've also seen early solid-state rectifiers in this kind of package, also some wire-wound resistors....

Ideally, the rectifier should indeed be horizontally mounted but I suspect that space is at a premium here, it's probably not passing that much current in this "standard" domestic superhet circuit. Probably a "standard" servicing fix that worked in a great many sets.

The HT would have been low on 110V- but the DAC90A works OK with signal valves running at around 100V as built, also Eddystone cabin sets like the 670, 840 were made that worked "adequately" with low HT from 110V. Perhaps shorting half of the field coil served two purposes- getting a few more volts HT (with less HT draw at lower voltage, perhaps lower amplitude ripple needed less smoothing) and a weaker field meant less likelihood of clipping damage.

[ms660]

How would reducing the resistance of the field winding increase the HT?

Lawrence.

[turretslug]

Is it parallel, rather than series field?- I hadn't checked the circuit, I'm always wary of clicking links!

[ms660]

It's parallel, across the HT.

The link's ok.

Lawrence.

[turretslug]

Thanks Lawrence! So keeping the current up, but reducing the number of turns and adding damping- the plot thickens....

Colin

[bobskie]

Here is a question, if I were to keep the metal rectifier in place for now would it cause any damage if it has failed? There is so much wrong with the radio that maybe it's best just to try it with this setup.

[PJL]

My guess is the field coil is not as simple as drawn and it takes twice the current when set to 120V so the field strength is constant. The resistance of the series choke would then drop the HT due to the higher current demand.

The most likely fault is high forward resistance but you can test the forward voltage and reverse leakage current. You would need to check the heater chain current in any case.

[TrevorG3VLF]

Originally Posted by bobskie

Here is a question, if I were to keep the metal rectifier in place for now would it cause any damage if it has failed? There is so much wrong with the radio that maybe it's best just to try it with this setup.

The few metal rectifiers that I have encountered have gone high resistance in the forward direction, giving a low voltage out. They have been OK in the backward direction. It was therefore possible to leave them in place and use the tags for a parallel diode.

It is a good plan to get the set working as it was before changing anything.

[bobskie]

That's good to know, I'll start working on it soon and keep an eye on eBay.de but if I don't get anything at least I can test it.

Cameron

[bobskie]

I made a start on this today, replaced a couple of dodgy looking caps but I'm struggling to find the one in the space seen in the first picture on the circuit diagram, what does one of those back plugs look like on the diagram? The bulb circuit also seems to have deviated from the diagram though I could be mistaken. What is the component in picture two? A resistor? Also I can't locate the wave change switch on the schematic, where is it? This radio has definitely been changed quite a lot from the schematic I think so it will be a job getting it all back to specification especially for someone with little experience like me, but it's a good challenge. Thanks for the help,

Cameron

[G6Tanuki]

The component in your second picture is either a power-resistor or possibly a 'thermistor'.

Thermistors are resistors whose resistance varies with temperature - they are usually used to control the switch-on current-surge in 'series-connected' valve heater circuits. The thermistor begins cold with a high resistance [to limit the current surge] but as the current-flow heats the thermistor its resistance falls away and progressively allows the full voltage to be applied to the valve heaters.

Without seeing a full circuit/schematic I can't tell you if this is a resistor or a thermistor.

[ms660]

The wave change switch on the schematic I'm looking at comprises of contact numbers 1 through to 22.

Lawrence.