Hi all.

I require service information for the above radio, similar to this:

http://www.radiomuseum.org/r/radioltd_ultimate_cygnet_rcp.html

The power transformer is burned out, so I'll need operating voltages as well.

Cheers, Stu

One could try the NZ Radio lot, albeit I have not had any feedback from them yet on a NZ radio & I fixed the radio months ago.

The most common voltage is around 250VDC. So it is a case of, with no feedback,from NZ, possibly the place is closed & their all overwintering here? You may have to reverse engineer it. Pop onto Franks Electron Tube Pages - http://www.tubedata.info/ - & get the valve data sheets. Then add up all the plate & screen currents for 250V & heater current plus dial lights.

That would give some idea of the maximum "B" current that valves draw & you add a % for other leaks (electrolytics). Then there is the sum total of the globes & heaters.

That will give a rough guide to the transformer required.

Marc

Thanks Marc.

The set is a mantel type using Rimlock type valves. Often, small mantels operated with an H.T. of less than 250V, in my experience, hence my question. I have only one transformer in my stock that will fit but I don't know its current rating. It''s a bit smaller than the original. I'll install it once the set has been restored, much work to do. Then I'll measure the temperature rise after running it for a few hours. If too high, I'll take steps to reduce the H.T. and the current draw. I could even put in an EL42 valve in place of the EL41 to reduce current on both the H.T. and L.T., adjusting the bias of course.

Cheers Stuart

EL42 has more than double the heater current. It is probably easier to reduce the HV. That valve 6CK5 / EL41 will take 250V. What is the OP voltage of the transformer that you have.

At the risk of hum (whats the rectifier?) if you loose / reduce the first filter cap capacity & use choke input, that cuts the voltage out of the rectifier.

You may have to do what I have done several times when I knew there was no hope of a circuit, or it had been hacked beyond belief: Reverse engineer. I normally start by putting down the layout with an AutoCAD & then play join the dots. Colour is your friend; Red for B+ Blue screens ...... Unless it has a colour code (some did). Photos before you mess with it, are desirable, even if the set is wired wrongly.

I think I have a transformer out of a Little Nipper in a pile of stuff dumped here. Albeit it could be out of the 449MA chassis that's been hacked? Think is was labelled by whoever took it out.

Hi Marcc.

EL41 Ih = 0.71 A, EL42 Ih = 0.2 A. I've wired in the spare transformer I have, and it all works well with 212 volts anode to cathode on the EL41, so all may be well. I'll measure the transformer temperature increase after a couple of hours operation today or tomorrow. Screen grid voltage on the RF valves is 94 volts, which seems about right. The transformer did measure 250 volts aside but that was with no load. The voltage on the rectifier anodes is 236 volts.

All good so far.

Update. Well that didn't work out! The new transformer is getting too hot. After it has been operating for a couple of hours the internal temperature rise is about 52 degrees, higher that the recommended maximum of 45 degrees.
So I have two options; substitute an EL42 for the EL41 that is in there, saving both heater and anode current, or rewinding the transformer .
The first option is by far the easiest, but offends my sense of originality. The second is just a lot of hard work!

What do others think.

Cheers, Stuart

Simplest solution is increase the cathode bias resistor on the EL41 to 2x existing. Of the back bias resistor if that's what it has.

Hi Stuart, you are flying a bit blind there. What are the actual heater and HT currents?
Wire a 5W WW 100 ohm resistor in series with the HT output and a 0.1ohm in series with the heater. Now you can get a measure of both currents by the voltage drop. Then you can check that the heaters on the valves are drawing the right current and you don't have an output valve drawing 100ma HT !!
Then reduce amps by biasing up one or all valves as Ian suggests.and you have a direct way of reading what you are doing. If the heater volts are a bit high you could ballast that as well and drop the current. Get 10 watts drop and the tranny may be happy.. What we don't know is if the heat is across all the windings or just one.
Perhaps ditch the valve rectifier if it has one and pop in a couple of diodes instead (1000v 3A) rating. You save a heater (5w) draw and get lots more HT and up the efficiency. Then bias the lot off a bit more and save more watts, win win win! If its an octal base rectifier you can always soak the base off a dud just cut the wires to the pinch and hide the diodes in there.
Better to have a great set working a bit non original rather than a smoking original.
Cheers, Fred.

Calculated from voltage measurements in the set, the H.T. current total is 57 mA. The L.T. current adds up to 2.27 A using 0.15 A dial lamps.
Not excessive. Doubling the EL41 bias resistor will reduce the current but also drastically reduce the undistorted power output.
I suspect the transformer I tried has a rating of 50 mA D.C. out and 2 amps for the L.T. That's probably why it ran hot.

Update:
I remembered I had an old chassis from a Tecnico 1140. I calculated that its L.T. requirements were 2.44 amps, and also, the core size is about the same as that in the original, so I have put it in. The H.T. voltage has increased to about 228 volts across the EL41 so not excessive. It still runs hot though! I measured a temperature rise of fifty degrees but this was in the morning when the ambient temperature went up by at least 5 degrees over the test period. It was a hot winter's day here on the Gold Coast! I figure it will be OK. I'll make a temperature measurement over part of the day when the ambient temperature doesn't increase.

I may yet take steps to decrease the current drain if I still feel the transformer is too hot.

Thanks for your inputs.

Cheers, Stuart

Hi Stuart, yep 57 ma and 2.27A sounds in the ball park for a 5 valve size of set.
Not sure what class of wire or insulation your trannies would have but I guess enamel wire and varnish paper.
50 deg rise so total of 70 or so would be about right for a varnished unit but a bit much for an old wax dipped thing.
Much more tougher if your ambient is above derating point say 30 upwards, no wonder the wax used to drip out of some of the old units!

The mention of temp rise reminded me about the 'old timers' way of calibrating temperature when I was an apprentice and enamel wire ruled the roost. "Just spit on it son, if the spit evaporates slowly it's ok, but if the spit dances you got a problem!"
The other more "scientific" way was the "how long can you hold you hand on it?" test. Continuous was minimal temp rise and good forever, 10 down to 5 seconds was working hard but safe and if you left skin behind you better shut it down pronto!
Not quite what I learnt in Measurements classes!!
Of course all that went out the window with the poly types of wires and motors and transformers needed a bit more than a 100 degC mercury thermometer to check working temp. I wound up using a IR gun or probe on site or in the test room, reading up to 199 deg despite being called a softy by the mob!
I'm way out of current practice and would not be surprised with units having 150 deg rise, very scary.
Cheers, Fred.

There are a couple of things here. What have you not replaced?

If you still have old waxed paper caps in it, and old electrolytic caps: Stop now. The old electrolytic caps will have lost form and can present as a short, or draw excessive current. I have seen an example this year, of what happens when one goes bang!

As noted you can get a good approximation of current draw by adding up the screen & plate currents on the valve data sheets. Same with heaters. If they are old what voltage rating is on the filter caps.

Note if the OP valve has a cathode resistor? Then valve & its resistance, that can be a clue to voltage. Look for a "Back bias" resistor if there is no cathode resistor. It will go CT to ground & there can be more than one in the train. Often it is part of a big long tapped "Candohm" resistor.

Hi Marcc. All paper and electrolytic caps replaced, I always do before even thinking about applying power. All resistors checked and replaced where necessary, only 2 resistors out in this case. Valves and all windings checked also. Power cord replaced, clamped and chassis earthed in the approved manor also. I have to do this, as the set will be tagged for resale for an antique dealer for whom I have been restoring sets for 25 years.

Update: I measured the internal temperature rise again, this time taking into account ambient temperature change. Result; 47.5 degrees rise. This is not too much more than the 45 degree rise given in the Radiotron Designers Handbook. In that book, they allow that hot spots may be considerably higher, but I measure the temperature rise of the primary winding, which, being the inner winding, would be the hottest. I measure by resistance change. The transformer does not smell at all, so I think this should be fine.

As for the radio itself, it is a good performer. I got between 2.5 and 4.5 uV sensitivity after careful alignment, and it doesn't sound too bad either. This is despite some of the valves being a bit tired, reading between 70 and 90% on the AVO.

Thanks Fred, Marcc and Ian for your inputs.

Cheers, Stuart

Hi Stuart, funny how I forget things, R1/R2 was exactly how I checked temp rise in alternators ( 1 to 200kva) while doing development work.
I could brake the test bed to a stop and graph the resistance for a short time of the armature and field windings to take into account the surge from loss of cooling. You are quite right as with a transformer you can log all the windings and get an idea how your gauge sizes of each went. I had just plain forgotten! Too long away from the test scene as we went out of production in the 1970's thanks to dumb governments. Funny thing with the trannies I have wound lately I have logged the ambient and loaded resistances but never bothered to calculate the temp rise. I just feel them after an hour or wave the IR thermometer over them as well as each resistor and especially the electros! Any electro with more that a couple of degree rise I am suspicious of. When I reform old can electros I measure the leakage milliamps and check the temp rise, any heat and they go in the bin.
Cheers, Fred.

Yes Fred, it is a handy method. I usually use it after rewinding a power transformer to check. For those unfamiliar, use the following formula:

Delta T = (R hot - R cold)/(R cold x 0.00393)

The recommended maximum delta T is 45 degrees, although I think more modern transformers can withstand more. I remember reading somewhere that Precedent TV transformers were designed to run hotter than most others, but they used materials that could withstand the higher temperatures reliably. This meant that they could use a smaller sized transformer, saving on weight and materials.

Cheers.