Wanted output transformer for Bush DAC90A

[superjust]

Hi all.

I am in the process of restoring a Bush DAC90A radio and have discovered an open primary circuit on the output transformer. The radio was in a sorry state with a cracked case and missing valves. A previous owner had also twisted wires round component leads rather than using solder and then wrapped sellotape round them. I've never seen anything as dangerous. It just goes to show that you never know what you will find when you look inside a vintage radio. Anyway if any good person has a spare transformer I will be very grateful. Obviously I will cover costs.

Tony

[Techman]

I would say that 50% of all DAC90A sets that turn up will have an o/c output transformer, it's a weakness with them and very common, so there's more folk out there looking for transformers from scrap sets than there are transformers left that are still working.

I wouldn't waste time with a second hand transformer unless one just happened to be to hand, as most of the working ones will likely be suspect sooner or later. Until recently you could still buy a replacement type brand new for less than twenty quid, but I've not checked lately, however, someone will probably know and point you in the right direction.

The other alternative is to have the original one rewound by one of the transformer rewinders advertising in the 'services offered' section. The other thing to be aware of is what finally finished the transformer off, was it just damp and corrosion causing 'green spot' on the very fine primary winding, or was it the leaking grid coupling capacitor and possibly a bad output valve - you need to investigate these things to save your new transformer from being fried the first time you switch the set on.

Output valves and transformers tend to make these sets rather expensive to repair and unless the set is already in pristine condition, then it's often not worth the expense for a two band AM set with virtually no stations left worth listening to, although when working these sets are a good performer.

[Station X]

Originally Posted by Techman

Until recently you could still buy a replacement type brand new for less than twenty quid, but I've not checked lately, however, someone will probably know and point you in the right direction.

https://uk.rs-online.com/web/p/audio...ormers/2106475

[superjust]

Thanks for your replies. It's probably not worth the cost of a new replacement transformer when as you say very few stations left to listen to. I do have some transformers that could be used and so I'll probably persevere with those.

Tony

[paulsherwin]

You should be able to use any class A valve output transformer that will physically fit, though obviously it may not look great.

[Techman]

I agree. Just experiment with what you have to hand. You can even get away with a small mains transformer fitted backwards - not ideal, but will give you an idea of the set's working performance. The output valves in these sets are often to a greater or lesser degree faulty, with various levels of grid emission and remaining new replacements have become rare and expensive. However, if you do a forum search (well worth doing) you'll find suggestions for modifications to use other valves in this position.

I took the liberty of looking back at some of your previous threads from your forum statistics and I see that o/c output transformer primaries isn't a first for you. With you saying that the case has also been damaged on this set then I personally wouldn't spend money on it, but worth playing with random transformers that you may have kicking around.

The former repairs that you mention come as no surprise. These will be all those people who don't come onto a forum to ask a question, or know how to use the internet or even know how to use a forum even if they did.

[paulsherwin]

It was standard practice to replace UL41s with UL84s in the 60s with a socket change. Obviously that would be frowned on today, but it's worth pursuing if it saves a rough set from the WEEE skip if you have spares in the junk box. Arguably it's a valid period repair.

[Techman]

I quite like a valid period repair.

[Northyorks1]

Hi Tony,

I have a transformer but with a fault one of the pins has snapped off on the speaker connection side, should still be able to connect to it. Its one of the new RS ones had little use and reads ok, 536 ohm and 1.1 ohm. If any use FOC just postage cost.

Ian

[superjust]

I'd just like to than Ian for coming to my rescue by providing me with a suitable transformer free of charge. Thanks again Ian.

Tony

[Sideband]

Only one thing I would advise when using the RS transformer....it's only rated at 40mA and the UL41 is usually running at around that or possibly slightly higher. I would check the anode current before putting the back on the set. If it's close to 40mA, increase the cathode resistor slightly say by 50 or 100 ohms. The slight reduction in output won't be noticeable.

[G6Tanuki]

I have always considered the 40mA thing on those transformers to be a worst case rating when you are using the entire primary winding, so you get both the maximum resistive heating and the greatest DC magnetization to saturate the core.

Using less of the primary turns reduces both effects so you can probably push things a bit more.

[superjust]

Thanks for the advice Sideband.

[superjust]

Hi Ian,

Output transformer arrived safely and now fitted. Anode current drawn is about 30mA and well within tolerance for UL41 valve, so I am leaving the set as it is. No need for increasing cathode resistance. Set still needs a bit of fine tuning which I will save for another time. Thanks everyone for your input.

Tony