Causes for output transformer failure?

[Spencervs]

Hello, I have a radio that I'm working on (Ever Ready Sky King). I have recapped it and noticed no sound... so using my limited detective skills, I checked all the valve voltages which seemed ok until I got to the output one which had no voltage and this pin went direct to the output transformer. So I tested that valve in another set that uses them and it was producing sound... so I'm thinking the valve is ok. I then checked the primary winding (connected to the same pin) for resistance and continuity and there was none.

Can I assume a dead transformer? If so what would cause that and is it doomed to happen again?

I read on an article that usually when the coupling capacitor comes to its end, it causes the valve to draw excess current which blows it and the transformer, however the valve works.

Thanks for the help,

Spencer

[Techman]

Hi, it's a very common fault in these battery valve sets and you will find that every other one you get will have an open circuit on the primary of the output transformer, particularly on the vanity case versions. One possibility is the output valve coupling capacitor being leaky has caused the valve to pull too much current down through the transformer, as often happens on mains valve radios. However, I think it's very unlikely to be the reason in these battery sets for several reasons. 1) the sets haven't been used for decades, so the capacitors wouldn't have been leaky when they were last used. 2) The little valve in these sets would probably struggle to pass enough current to kill the transformer anyway. The most likely cause is that years of those big old batteries 'breathing' a slightly corrosive atmosphere inside the confined space within the radio cabinet that gradually corrodes things such as the very fine copper winding of the output transformer primary. This is the reason why (and this applies to ANY radio you intend to work on) you should always check expensive and difficult to get parts before even thinking about randomly replacing capacitors. Never just replace capacitors and cross your fingers and hope, as it's just the wrong way to go about things. Firstly, test things such as the output transformer and then replace only critical components such as the grid coupling capacitor to the output valve as already mentioned - even if you didn't change this it would be alright to run the set for a minute or two just to make an initial assessment. Having said that, the Sky King has so few paper capacitors (is it something like about 6 in all?) that they all have an effect on the performance of the set to some degree, so you generally find that in these particular sets that they do all need replacing. One capacitor that's always seriously bad in these sets is the decoupling electrolytic across the HT, so if you haven't done so already, then this should be replaced as it'll drag the HT down to zero and kill the battery (or power supply), although seeing as it needs a transformer there may not be any point in doing that now. You've had a bit of bad luck picking that type of set for what I suspect is a first repair, but if you follow my advice in future then you won't go far wrong. You could get the transformer rewound at a price and RS used to do a replacement, but I don't know if they still do - worth checking.

[vinrads]

Hi you could try asking in the wanted section , I am always on the lookout for cheap valve battery sets just for the transformers ,I recently won a Philips for parts only on Ebay , but couldn't bare to scrap it .so it ended up repaired , Mick.

[cathoderay57]

The DL96 output valve wants a 13k anode load. RS Components still supply a replacement that would be electrically compatible https://uk.rs-online.com/web/p/audio...rmers/2106475/ and I've attached the spec so you can see if it is small enough to fit the chassis. For a 13k anode load and 3 Ohm speaker you would want pins 1 and 4 connected to HT and anode and pins D and C connected to speaker. There are a few of these transformers about and if suitable you could put a request for one in the wants section in case somebody has a surplus item and maybe willing to sell at < RS price. Cheers, Jerry

[Ed_Dinning]

Hi Spencer, most of the battery OPT's that I've worked on had the dreaded green spot of corrosion on the very thin primary winding.
Battery vapors and the sets often being used in the damp outdoors, then having long periods in storage contribute to this

Ed

[Spencervs]

Ah wonderful, thanks for the advice! As it happens, someone on the forum has sent me a donor radio due to the casing being in poor condition. Hopefully it will contain a working transformer, but we shall see!

In a way I am sort of proud that I was able to diagnose the issue myself as I am just learning, but on the other hand... What a bummer! Just to confirm in this set will the coupling cap be a paper one or a mica/ceramic one? There wasn't a paper one near the valve but plenty of 100pf and such. I also found that the resistors were way over spec in circuit but I didn't remove them first as I read that on valve sets you shouldn't really have to take them out. Is that true? Some are absurd values.

[cathoderay57]

Originally Posted by Spencervs

Some are absurd values.

If you are measuring resistors while still connected in circuit this will usually be the case, on account of parallel electrical paths and capacitors charging up altering the resistance value seen. To get an accurate reading you will need to disconnect one leg. Whether drastic resistance difference matters depends whereabouts in the circuit the component is. Valve circuits are surprisingly tolerant in many cases. Generally I replace capacitors and only check any resistors showing signs of heat stress before powering up. Then I measure voltages compared to those quoted in the Service Sheet, allowing for the fact that is often mentioned here on the Forum that the Service data is usually based on voltages measured using a relatively low impedance Avo meter and therefore in a high impedance circuit the value seen using a DVM is likely to be higher. The output valve coupling cap is listed on Trader Sheet #1275 as C25 (0.001uF). Looking at the data sheet it's impossible to tell what type is used. A close-up image would enable identification. If ceramic then leave it alone; if paper then replace. Jerry

[Cobaltblue]

Battery Valve portable sets have some very high resistor values.

The thing with resistors of this period is the high value ones tend to go higher anyway.

The Sky King ISTR has 5 resistors of 1 MΩ or higher and these are likely to be way out of spec by now.

You can generally measure most of them in circuit providing the caps are good.

Cheers

Mike T

[wd40addict]

Another common problem on battery sets, maybe 1 in 4 that have come my way is open circuit filaments or filament intact, but zero emission. The only explanation I have for this is once the correct batteries became scarce / too expensive Joe public would connect them up to model train transformers or anything else they had to hand including direct to the mains! Hopefully your set will be ok.

[Spencervs]

Guys your answers are amazing, thank you so much! So I think I may have to make a request in the wanted section but does anyone have a spare transformer? If not can anyone advise on what specs I should be looking for. I know I can get a new one for £20 but would rather use up a spare junked but working part.

I noticed a 2.2m ohm resistor measuring in at 3m ohm. Is that normal?

[Techman]

Originally Posted by vinrads

Hi you could try asking in the wanted section , I am always on the lookout for cheap valve battery sets just for the transformers ,I recently won a Philips for parts only on Ebay , but couldn't bare to scrap it .so it ended up repaired , Mick.

You're not the only one. I also need several of these very small transformers for similar sets - me and a hundred other folk, I suspect!

In reality I'm unlikely to bother with them and just have them for their ornamental/interest value. You have to ask yourself what you're going to do with the set when it's done, they're AM MW/LW only, so will you be listening to radio 4 all day? They're a little power hungry and making up batteries is relatively expensive considering life expectancy - running them from a power supply defeats the object of having them in the first place.

The reality is that these sets can be had for virtually nothing, so finding similar sets for spares is relatively easy, but I guarantee that a 'spares' set is always going to be too good to break for parts, so you'll still end up looking for a transformer - hopefully the transformer in your scrap set will be ok.

Thinking about it, I think it's correct that the grid coupling capacitor is a ceramic type in most of these radios. I seem to remember that there's a paper one across the output transformer primary that leaks badly, reducing the audio to a crackly nothing, but it doesn't damage the transformer in that particular position - luckily.

It's true regarding the valve heaters being blown in these sets, due to folk trying other transformers and 9 volt batteries etc.

Another thing to check is the number of pins on particularly the output valve - you may find one missing due to it being still stuck in the base socket.

Several years ago there was someone on here with a whole load of these radios of the mainly the vanity case types that he was trying to sell. He sold some of the working ones, but in the end was struggling to give what was left away and came up with that usual threat of taking the whole lot to the tip to be recycled. Still there were no takers for the last few, so I guess they were all dumped, never heard any more about it, but these things happen, sad to say.

[wd40addict]

It's typical for old carbon resistors to drift high, but that is quite a lot. Radio will probably still function anyway, but I'd change it whilst you're in there.

The output transformers were already failing back in the day. I have a Pye P114BQ which has been repaired with an RS replacement.

[Spencervs]

Interesting, thanks for the info! Sounds like I fell into a little trap here and my choices are either fingers crossed for a transformer or just bite the bullet and buy a new one. Problem is I doubt it's worth the money buying a transformer new for this radio.

You mention the coupling capacitor is probably a ceramic type. How to I identify which it is? It is the nearest to the output valve? How does one know and as it is ceramic, should I bother to test it? I also noticed there is a ceramic one inbetween the primary terminals. Could this be suspect?

[Techman]

Originally Posted by Spencervs

You mention the coupling capacitor is probably a ceramic type. How to I identify which it is? It is the nearest to the output valve? How does one know and as it is ceramic, should I bother to test it? I also noticed there is a ceramic one inbetween the primary terminals. Could this be suspect?

You need to look at the circuit diagram - it helps.

Get to know valves and how they function.

The coupling capacitor will go to the control grid (g1) of the output valve. No need to replace a high quality ceramic type.

Post a picture!

[Station X]

Just checked service sheet. The control grid of the DL96 is on pin 6.

I'm surprised that the OP had to ask the question as the position of the capacitor concerned (C21 Manufacturer's sheet) is clearly shown on the under chassis layout.

[Techman]

Originally Posted by Spencervs

In a way I am sort of proud that I was able to diagnose the issue myself as I am just learning.

I agree, and so you should be!

You did well considering your level of experience. Most people in your position would have been going round in circles with that one, so it actually proves that you have logical thinking and you'll probably make a good radio engineer.

[Station X]

Which is why I'm surprised that the OP couldn't identify C21.

I wonder whether he has a service sheet?

https://www.service-data.com/product...02/2406/m17902

[ms660]

When measuring the output valves g1 voltage on it's pin, you need to be aware what the loading effect of the meter will have, as an example a meter with a 1meg input resistance connected between the grid pin and chassis could cause the valves maximum allowable cathode current to be exceeded.

Lawrence.

[Spencervs]

Originally Posted by Techman

I agree, and so you should be!

You did well considering your level of experience. Most people in your position would have been going round in circles with that one, so it actually proves that you have logical thinking and you'll probably make a good radio engineer.

Thank you so much for the kind words! I have been reading everyone's posts on here over the years and also watching videos on youtube about diagnosis and such. I don't have any special equipment except an AVO and a DDM and I have a lot to learn but I feel that I am definitely in better stead than I was a few years ago! My process was this an feel free to add in anything:

I decided after I changed the capacitors as pictured (hopefully is not to shoddy!) and noticed the sound was still dead that I should check continuity in the transformer windings just in case. There was of course nothing for primary so I figured I would check the valve voltage as specified on the service sheet with the assumption that I could isolate the issue better. I did this knowing that the valves on this set are fragile as well being directly heated cathodes (is that right?) and much more tiny than on the mains sets. The results were ok for all except the output DL96 pin 2 which was dead. I decided this wasn't the valve's fault as pin 3 ad the correct voltage and worked in another set. So, following that, I checked the resistors and came to the conclusion that the transformer itself must be at fault.

Sorry about the silly question re: coupling cap as I had seen C21 on the paperwork and in the set but was surprised that it was ceramic as I thought they were normally paper... again, this is my learning curve I fear!

[Spencervs]

woops forgot to add the photos! Well here you go, finger on c21, the transformer in question and my new capacitors.