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Old 15th Sep 2021, 9:21 am   #1
cammy12345
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Default Bush vintage radio DAC90A.

Recently bought the above radio, and although not working it seems in quite good order.
I suspect the valves are the problem. Some are black and obviously not working. Some seems relatively new. The print on some are partly or wholly unreadable. Can anyone tell me all five valves full details and the best place to purchase them
Thanks Cammy
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Old 15th Sep 2021, 9:27 am   #2
Cobaltblue
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Default Re: Bush vintage radio dac90a

Hi Cammy and welcome to the forum.

The DAC90A is well known on this forum.
First you cannot tell the state of a valve by it's appearance.

The black you describe is the getter and unless its milky white tells you nothing about the valve.

Do you have a Multimeter, do you know how to work safely on live equipment?

The service info (including valve types) is available here: https://www.service-data.com/product.php/603/951/m10603

You could do worse than start here:

https://www.vintage-radio.com/repair...ion/index.html

Cheers

Mike T
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Old 15th Sep 2021, 11:12 am   #3
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Default Re: Bush vintage radio dac90a

Valves are least likely to be your problem....it's a common beginners belief that 'it's the valve'!! Most of this is from the old days of radio repairs where replacing a valve got the set going.....for a while until it failed again simply because the proper cause of the fault hadn't been traced.

If the radio has not been serviced, it will need all the capacitors replaced for a start (these will work out a lot cheaper than a full set of valves). That may well be all that is required. Do not be tempted to plug it in at this stage. If you are an absolute beginner please take note that this set does not use an isolating transformer and the chassis is connected directly to the mains. Extra care has to be exercised when working on these sets.

For an idea of what is involved when working on these sets, please search the forums for DAC90A. There must be hundreds of threads about these...one of the most popular sets ever.
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Old 15th Sep 2021, 5:36 pm   #4
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Default Re: Bush vintage radio dac90a

Quote:
Originally Posted by cammy12345 View Post
Recently bought the above radio, and although not working it seems in quite good order.
I suspect the valves are the problem. Some are black and obviously not working. Some seems relatively new. The print on some are partly or wholly unreadable. Can anyone tell me all five valves full details and the best place to purchase them
Thanks Cammy
Hi Cammy please listen have what has been discussed in post 1.2 3 and please dont be put off & these sets are great to work on . Please can you post some pictures of inside & outside as pictures say 1000 words kind regards Bob
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Old 15th Sep 2021, 6:19 pm   #5
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Default Re: Bush vintage radio dac90a

Quote:
Originally Posted by Sideband View Post
Valves are least likely to be your problem....it's a common beginners belief that 'it's the valve'!! Most of this is from the old days of radio repairs where replacing a valve got the set going.....for a while until it failed again simply because the proper cause of the fault hadn't been traced.

If the radio has not been serviced, it will need all the capacitors replaced for a start (these will work out a lot cheaper than a full set of valves). That may well be all that is required. Do not be tempted to plug it in at this stage. If you are an absolute beginner please take note that this set does not use an isolating transformer and the chassis is connected directly to the mains. Extra care has to be exercised when working on these sets.

For an idea of what is involved when working on these sets, please search the forums for DAC90A. There must be hundreds of threads about these...one of the most popular sets ever.
I would offer an alternative methodology when tackling this set. IMHO changing ALL the capacitors may not be necessary and you may introduce more faults. Mine from 1952 has had one capacitor changed (look up the thread 'what is THAT capacitor') as a precaution and it's still going strong. Also, if it does start working you will not have learned what caused the fault. I would start by checking the output transformer and loudspeaker. Check the valve heaters for continuity and make sure they are making good contact with the bases. Remove the wire springs from the valveholders and throw them away - they can damage the valve pips and cause them to go white.
Please please please take the precautions as previously stated - and enjoy! Keep us all posted.
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Old 15th Sep 2021, 10:00 pm   #6
60 oldjohn
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Default Re: Bush vintage radio DAC90A.

Also, do not attempt to pull / lever the knobs off the set. Slacken the screw in knob through the holes underneath the set, and for side knob it is inside the back of set.

John.
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Old 15th Sep 2021, 10:32 pm   #7
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Default Re: Bush vintage radio DAC90A.

Quote:
Originally Posted by 60 oldjohn View Post
Also, do not attempt to pull / lever the knobs off the set. Slacken the screw in knob through the holes underneath the set, and for side knob it is inside the back of set.
Good advice unless the set is from fairly late in production, in which case there aren't any retaining screws for the front knobs and they do pull off. Rotate one of them while shining a torch through the nearest hole underneath and you'll soon see whether or not a screw is used.

Paul

Last edited by Paul_RK; 15th Sep 2021 at 10:46 pm.
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Old 15th Sep 2021, 11:51 pm   #8
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Default Re: Bush vintage radio dac90a

Quote:
Originally Posted by Bob_Moss View Post
Quote:
Originally Posted by Sideband View Post
it will need all the capacitors replaced for a start
I would offer an alternative methodology when tackling this set. IMHO changing ALL the capacitors may not be necessary and you may introduce more faults. Mine from 1952 has had one capacitor changed
I agree regarding NOT changing all the capacitors - my example also has only the grid coupling capacitor replaced and it performs just fine. The only other thing is the small wax capacitor on top of the chassis across the incoming mains should be completely disconnected if it hasn't already exploded and blown its end out and that's about it - it can be replaced with a special 'X' type, but the set will work without it.

I'm very shocked and surprised at Sideband advising replacing ALL the capacitors - I would have thought he'd be the last person to suggest such a thing to an obvious total novice. We've already got a thread running that I spotted earlier where it sounds like the person has replaced everything including probably the silver mica flat types and has now got squealing and howling and a non-working radio!

The only other thing that I don't agree with is regarding removing and throwing away the springs on the valve holders. I say just carefully lever them up out of the way of the pip and rest them on the metalwork of the skirt when removing the valve, then on replacing the valve just carefully let them back down into the slot. People need to learn to be proper radio engineers and not just remove and dispose of original parts just because they don't know how to deal with them properly - if the radio is ever sent through the post, then you'll wish those springs were in place when you find all the valves floating around and smashed inside the cabinet...OK, so you're going to say that the whole radio is unlikely to survive the postal system if it gets chucked around enough to knock all the valves out of their sockets, so why worry about the springs being missing and you could be right
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Old Yesterday, 12:29 am   #9
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Default Re: Bush vintage radio DAC90A.

Welcome to the Forum. A lifetimes experience in 8 posts already cammy! Sideband clearly outlined the "It's the valve isn't it? urban myth." The idea of automatically replacing all the caps as a matter of course is not a myth. It seems to have originated in American Hi Fi circles but nobody really knows as you can see from threads on that subject. This approach was unknown until relatively recently. It probably spread via the internet and is only really useful if the restorer is either extremely experienced generally or within one speciality [I'm not] or seeking to replace everything, which will be an arduous task and very likely to lead to greater confusion, as said. I'm sure Sideband is fully qualified to take the risk but it's not a good idea otherwise. Guess work is not the way forward overall.
I wonder how you came to believe that valve problems can be diagnosed externally? Maybe you replaced one or two darkened ones and the set worked so you thought that was all there is to itv

Dave W

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Old Yesterday, 12:58 am   #10
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Default Re: Bush vintage radio DAC90A.

Saying replace ALL the capacitors is a very dangerous thing to say to a novice. I can only think that Sideband wasn't thinking when he said that - I know I've probably said things without thinking and caused possible confusion.

ALL the capacitors includes the silver mica types that'll destroy the alignment if they're disturbed for replacement and will need specialised test equipment (which the OP won't have or even be able to afford) to put things right. It also includes the main smoothing can which virtually never fail on this model of radio.

What he should have said was ALL the round, wax paper types that are mainly on the tag board. They will all be faulty as regards having leakage, but it's only really the one feeding the output valve control grid at the end of the board that's critical and should ALWAYS be replaced.

There's a screen grid decoupler on that tag board and just for the hell of it on mine I removed all the solder from one end connection and carefully unwound the wire from the tag and lifted the end of the capacitor away. It measured leaky and I'd previously checked the screen grid voltage while it was still in circuit and it was in the ball-park of what it should be. I tacked a new replacement capacitor in place and it made about half a volt difference to the screen voltage and no noticeable difference to performance, so I unsoldered it and carefully wound the wire of the original capacitor back on its tag and re-soldered the connection.

What I would say is that all those half dozen or so round wax paper capacitors on that tag board WILL be leaky to some degree and it could be that if the radio has spent many years in a damp shed or similar place, then the absorption of moisture and leakage of all of them could be enough to compromise the performance of the radio, so it wouldn't be too bad a thing to replace them as well as that critical one at the end, but it's easy to make mistakes when replacing a group of components in one go.

What you need to do, seeing as it sounds like the radio has already been powered up, is to disconnect that wax cap by the on/off switch and on top of the chassis (if it's still there) and then replace just the grid coupling capacitor. If the set still doesn't work, then it's time to do proper fault finding by taking voltage readings as a first start.

Be very careful around high voltages and remember that one side of the mains goes straight to the chassis metalwork on these particular sets!
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Old Yesterday, 1:04 am   #11
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Default Re: Bush vintage radio DAC90A.

Hey Dave, are you trying to get your post count up? Double post!

I agree with what you say!
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Old Yesterday, 9:46 am   #12
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Default Re: Bush vintage radio DAC90A.

Blimey, quick to condemn, don't panic Mr Mainwaring...

I'm sure Sideband meant the usual capacitors.

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Old Yesterday, 10:15 am   #13
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Default Re: Bush vintage radio DAC90A.

What Sideband meant and what the OP may have understood him to have meant are two different things.

A bit of clarification (not condemnation) has done no harm.

Previous threads suggest that some sets never work again after changing ALL the capacitors.
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Old Yesterday, 10:30 am   #14
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Default Re: Bush vintage radio DAC90A.

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Previous threads suggest that some sets never work again after changing ALL the capacitors.
Is that because the repairer gave up?

Lawrence.
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Old Yesterday, 10:38 am   #15
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Default Re: Bush vintage radio DAC90A.

I expects so. What I regard as proper fault finding seems to be going out of the window. You just replace all the capacitors and out of spec resistors, give the post a squirt of Servisol 10 and magically it all works.

Who needs a test meter or other test gear?

Let's wait for a response from the OP.
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Old Yesterday, 2:47 pm   #16
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Default Re: Bush vintage radio DAC90A.

Yes I hope cammy comes back on this. Unfortunately some posters are never heard from again. I usually spot when I've doubled up Techman but it was late I've too much to say as it is but unfortunately less is usually more so I'm afraid the many posts that I've done already signifies very little

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Old Yesterday, 3:43 pm   #17
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Default Re: Bush vintage radio DAC90A.

I now need to buy loads of valves and replace them in every set for good measure, it will only cost hundreds to do so but everyone knows that all valves found in every set are defective, it's a light bulb after all.
The above statement is random crap I made up, we all know how reliable valves can be, apart from the UL41 but the rest are fine.
The point I'm making is, why do so many people think a valve works like a bulb? The mind boggles.
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Old Yesterday, 4:28 pm   #18
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Default Re: Bush vintage radio DAC90A.

No matter what experienced advice is given, guys usually go there own way with the obvious disasters. I have always been a 'get it working regardless of how badly' before mass changing components.

You cannot become a radio repair guy with one recapping of a DAC90.

Never has so much been written about a 5 valve AC/DC superhet.
Just take care and new guys on the block, please change one component at a time checking the results as you go.
You need a service manual and a test meter and know how to use it. A very cheap meter is all that is required for radio repairs. Aim to get a first squeak out of it and the rest should be easy. J.
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Old Yesterday, 5:14 pm   #19
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Default Re: Bush vintage radio DAC90A.

There is/was a basic capacitor kit available on eBay and elsewhere for the DAC90A with instructions, I can't think of a better start for restoring one. Only the capacitors likely to fail are included, not all as the RF mica jobs are super reliable.
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Old Yesterday, 5:21 pm   #20
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Default Re: Bush vintage radio DAC90A.

Maybe my approach is wrong but it goes something like this, test valves first, test the speaker TX primary, change any waxy things for good measure, reform the electrolytic can, check the output cathode resistor.
If the set works after all that work, leave it alone, alignment is only performed as a last resort, this had to be done on the octal version DAC90, it was seriously misaligned but now works perfectly.
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