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Old 13th Jul 2019, 9:53 am   #1
bobbyd123
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Default Bush DAC10 "IF Transformer fault"

Bush DAC 10
I have now changed all the Wax capacitors under the chassis
Photo attached
Also the over tolerance resistors

Checking the value of The IF Transformers
L10 L12 L13 measured on my AVO8 12 ohms L11 measured 20 ohms

I wonder if this is a problem before I refit the push button unit' as I wont
be able to get at it after. I wonder if there is a bad connection inside the IF Can
Do you think I should remove it and have a look
I have not put any power on the radio so Have no Idea if it works
The seller said it was powered up and crackled (So the plug cut off)

Also I no its too late now but all the wax capacitors replaced with none polarised. I have heard on the internet that they should be checked now I am
Confused! The supplier says they are fine as they are
I no that the electrolyte must be polarised and any others marked on a schematic

I am very grateful for all your help and advice (Thank you)
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Old 13th Jul 2019, 10:28 am   #2
crackle
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Default Re: Bush DAC10 "IF Transformer fault"

If the windings are made of Litz wire then some strands may have corroded.
The Manufacturers list all the IFT windings as 12.5 ohms so it seems there may be an issue with that winding.
Personally I would try re-flowing the solder joints to the can and just try it to see if it works before pulling the IFT apart..

The waxed paper capacitors are non-polarised, they have a marking on one end which indicates the outer foil. But I co not believe it is critical in these radio circuits. So no ir does not matter which way round you have fitted the replacements.
Electrolytic capacitors are polarised and it is critical how you connect them. In this radio there are only the 2 electrolytic smoothing capacitors and they are in the same aluminium can.

Your replacement capacitors and resistors look very neatly done.


Mike

Last edited by crackle; 13th Jul 2019 at 10:34 am.
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Old 13th Jul 2019, 10:47 am   #3
ms660
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Default Re: Bush DAC10 "IF Transformer fault"

The wax capacitors are not polarised types, the only polarised types are the electrolytic capacitors, C25 and C26 (Trader sheet refs.)

The high resistance reading on one of the IFT coils could be due to a poor connection internally or some of the individual strands of the coil winding going open circuit, if it's a case of the latter you might be able to fix that if the breaks are between the actual body of the coil and the transformers connection points, if the break is within the winding the chances of fixing the break are usually slim unless the break is just a few turns in.

IFT's of this type are usually wound with Litz wire, Litz wire is made up of several individual wires that are insulated from each other except where they are soldered together at the transformer terminals, from a resistance point of view Litz wire is a number of equal resistances connected in parallel, if any of the individual strands aren't connected for whatever reason then the resistance of the Litz wire will increase.

EDIT: Post crossed with Mike's.

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Old 13th Jul 2019, 5:23 pm   #4
Graham G3ZVT
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Default Re: Bush DAC10 "IF Transformer fault"

Quote:
Originally Posted by bobbyd123 View Post
Also I no its too late now but all the wax capacitors replaced with none polarised. I have heard on the internet that they should be checked now I am
Confused! The supplier says they are fine as they are
I no that the electrolyte must be polarised and any others marked on a schematic
It used to be common for one side of a tubular capacitor to be marked a band to indicate which side of the capacitor was connected to the outer metal foil.

It's not difficult to identify this on modern caps and mark them yourself, not that I've ever felt the need.

In theory, if this was connected to the more earthy side of the circuit it would pick up less hum and noise, a bit like putting a screening can over a component.

This may have been good practice on audiophile grade hi-fi, but take my word for it, it's not worth the bother on an old valve radio, in fact it could be argued that introducing a bit of hum in one place may actually cancel out some other hum existing in anti-phase somewhere else.
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Old 13th Jul 2019, 7:06 pm   #5
Mike. Watterson
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Default Re: Bush DAC10 "IF Transformer fault"

If it matters on Audiophile gear the layout, PSU and design is poor!

It maybe mattered on one or two HUGE paper caps. Mostly it didn't!
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Old 13th Jul 2019, 7:59 pm   #6
ms660
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Default Re: Bush DAC10 "IF Transformer fault"

The original decoupling capacitors in my old HRO receiver had their outer foils connected to chassis, I'm sure the Hallicrafters I owned did too, did the designers do it for looks or superstition or for another reason....

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Old 13th Jul 2019, 8:32 pm   #7
Mike. Watterson
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Default Re: Bush DAC10 "IF Transformer fault"

If they are big enough caps it does make some difference on gear with wires and 110V to 240V AC.
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Old 13th Jul 2019, 10:33 pm   #8
bobbyd123
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Default Re: Bush DAC10 "IF Transformer fault"

Hello again Thanks for your help
I have removed the IF Can an opened it up!
ms660 you are spot on at least 2 of the strands have broken and come unsoldered
As you say this is the cause of a high reading 20 ohms
I am having a problem trying to join on a small section for re-soldering
In other words repair the end (Trouble is it is so very small) Any Ideas how this might be done (The coil is okay at 12.5 ohms just need to re-fix it)
Thanks again for your help.
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Old 14th Jul 2019, 8:36 am   #9
Mike. Watterson
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Default Re: Bush DAC10 "IF Transformer fault"

I hope you have a magnifying lamp?
You may need some extra wire. It can even be a strand of ordinary wire.
A very hot iron and solder may burn off the insulation at the end.
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Old 14th Jul 2019, 8:53 am   #10
crackle
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Default Re: Bush DAC10 "IF Transformer fault"

Cut the remaining strands to make a neat end, twist the broken strands together. Using a hot iron, about 400 deg, tin the tip and then heat the litz wire with the iron at the same time dabbing the fluxed solder directly onto the wire. Eventually you should be able to melt through the insulation and get a shiny silver end to solder a new piece of wire onto. Use a tinned single strand of wire from a piece of flex to bridge between the broken end of the coil and the solder terminal of the coil former.

Mike
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Old 14th Jul 2019, 8:59 am   #11
Ian - G4JQT
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Default Re: Bush DAC10 "IF Transformer fault"

I use Alusol solder for these awkward un-tinable cases. I has more aggressive flux and allows soldering of fragile wires that you can't clean without damaging them. It's also useful for soldering aluminium tags (it's an aluminium solder) and other normally solderable components where traditional solder just won't 'take' and/or is in a difficult location to scrape clean.

I was wary that it might eat through wires eventually, but in 10 years use I haven't (yet?) noticed a problem. The great thing with it is that once wetted with Alusol normal solder can be used on the same joint.

It's quite pricey, but you don't need it that often so one roll will see me out I'm sure!
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