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Old 22nd Nov 2020, 10:57 am   #1
DonaldStott
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Default Bush DAC90A Speaker Problems

The current DAC90A on the bench has a loud buzzing from the speaker even with the volume turned down.

This YouTube link is a short video of the problem - the first half is with the volume turned down and then turned up at about 0:28 : -

https://youtu.be/FetMkUEW6rU

Have I got more than just the usual detached speaker suspension here?

I applied a 1.5V DC source to the speaker terminals to move the cone forward and can see daylight: -

Click image for larger version

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No problems in reattaching the suspension as there are many Posts giving good advice on this Forum but just concerned about that loud buzzing when the volume is turned down.

There are no other close by EMF sources such as devices with switched-mode power supplies.
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Old 22nd Nov 2020, 11:49 am   #2
Silicon
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Default Re: Bush DAC90A Speaker Problems

A detached suspension often results in 'coil rubbing'.

If you are unlucky there may also be rust, metal fragments and dirt between the voice coil and pole pieces.

Can you connect this speaker to an another healthy radio to test the sound quality?

Alternatively add a resistor in series with the speaker and test it on your hi-fi.

Have you got a good 2.5 - 4 Ohm speaker that you can connect to the DAC90a?
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Old 22nd Nov 2020, 11:59 am   #3
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Thanks Silicon - I've got a 'spare' speaker from a DAC10 which is 3 ohms so I'll try that?

What would cause the buzzing with the volume turned down?
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Old 22nd Nov 2020, 12:10 pm   #4
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Default Re: Bush DAC90A Speaker Problems

Assuming the smoothers are up to snuff and the usual grid caps have been replaced it might be the UL41 they go internally leaky.

The DAC90A uses an unused pin(4) to anchor R12 (47K)

An improvement can sometimes be made by removing this connection and letting the connection R12, R11, C18 float.

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Old 22nd Nov 2020, 12:34 pm   #5
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Swapped the speaker over for the one from a DAC10 and the buzzing noise has stopped, even at low volume!

So this points toward the speaker being the problem??

@Cobaltblue - thanks Mike, the grid coupling caps were replaced and preliminarily checks carried out before first live switch on, with lamp limiter of course. I'm familiar with most of the DAC90A modifications thanks to kaleee20 and these will be implemented as I progress. I've also got a spare UL41 which I will substitute. The reservoir and smoothing caps in the big can will be tested and replaced if necessary.

I've never had a speaker sound as bad as this and wonder if it's a lost cause? Is it worthwhile even attempting to fix the detached speaker suspension if I have rust and dirt etc. in the voice coil?
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Old 22nd Nov 2020, 1:54 pm   #6
Silicon
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Default Re: Bush DAC90A Speaker Problems

Look after that healthy DAC10 speaker. They have a reputation for going faulty and it is difficult to find a replacement that will fit the restricted space inside a DAC10.

The DAC90a speaker will probably be OK when you glue down the suspension.

If not, old car speakers have the low impedance required and can sometimes have the correct physical dimensions.
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Old 22nd Nov 2020, 2:17 pm   #7
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Quote:
Originally Posted by Silicon View Post
The DAC90a speaker will probably be OK when you glue down the suspension.
Thanks - I'll give it a go and things should be clearer when I get the dust cap off and fabricate some shims.

Most Posts on this subject seem to favour PVA but I'd be a bit concerned about that glue long term?
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Old 22nd Nov 2020, 3:41 pm   #8
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Default Re: Bush DAC90A Speaker Problems

Ok the dust cap is off but it's not going back on - so looking for a supplier of 1" dust caps. Not sure if they are just paper or does it matter?

Got four shims in place ready to glue and leave overnight ...
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Old 22nd Nov 2020, 7:14 pm   #9
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Default Re: Bush DAC90A Speaker Problems

PVA is normally used for repairing splits or tears in the cones. You might need something a bit stronger for the suspension. I've used Evo-Stik (impact adhesive) with great success but any other similar impact adhesive should be OK. No problems after 10 years.
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Old 22nd Nov 2020, 8:04 pm   #10
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Default Re: Bush DAC90A Speaker Problems

Quote:
Originally Posted by Sideband View Post
PVA is normally used for repairing splits or tears in the cones. You might need something a bit stronger for the suspension. I've used Evo-Stik (impact adhesive) with great success but any other similar impact adhesive should be OK. No problems after 10 years.
Thanks Sideband for confirming that an impact adhesive is the way to go - I'm sure I've got a tube of Evo-Stik somewhere, doesn't everyone?

Still can't find any 1 inch dust caps despite searching the usual suspects!
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Old 22nd Nov 2020, 9:22 pm   #11
murphyv310
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Default Re: Bush DAC90A Speaker Problems

I've repaired cones successfully with PVA and strips of kitchen paper.
The suspension when it becomes detached I've always used PVA but I'd say contact adhesive may be a better bet but harder to manipulate as it's a one attempt glue.
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Old 23rd Nov 2020, 12:19 am   #12
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Default Re: Bush DAC90A Speaker Problems

I have used evo-stik timebond for speaker surrounds as it does allow some movement for a while.
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Old 23rd Nov 2020, 10:47 am   #13
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Default Re: Bush DAC90A Speaker Problems

You can glue a piece of felt over the speech coil where the original dust cap was.
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Old 29th Nov 2020, 1:30 pm   #14
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Smallest dust caps I can find are 35mm (1.38inches) - slightly too big but I think these would be acceptable?

Unless of course anyone knows a source for 25mm (1inch) dust caps?

And then of course there is the next question of which glue to use. I would tend to favour PVA while others have used Aleene's Tacky Glue but has anyone used this stuff: -

https://www.ebay.co.uk/itm/Black-505...L/392873719769
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Old 9th Dec 2020, 2:56 pm   #15
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Default Re: Bush DAC90A Speaker Problems

I used acid-free Copydex for my DAC90a cone repair. Weighted down the cone and left for 24 hours. Seems to have been successful.

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Old 10th Dec 2020, 1:54 pm   #16
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Managed to use Evo-Stik to reattach the speaker suspension and all sounds good now.

I used three thin strips of card to centre the voice coil and then used a 1.5V battery with croc clips to hold the suspension in the open position. This allowed me access with a long cocktail stick to apply glue to the detached face of the suspension. Access is a bit tricky but patience will serve you well here. Then it's just a case of reversing the battery polarity and the suspension is held in the closed position. Leave in safe place overnight where it can't be knocked or disturbed - the biggest risk is poorly attached croc clips that could spring off!

Meantime I still haven't found a source for 1 inch diameter dust caps - anyone?
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Old 10th Dec 2020, 2:56 pm   #17
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Default Re: Bush DAC90A Speaker Problems

Quote:
Originally Posted by DonaldStott View Post

What would cause the buzzing with the volume turned down?
Could be a low mains hum, enough to activate the movement of the speaker causing the buzzing.
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Old 10th Dec 2020, 3:04 pm   #18
ThePillenwerfer
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Default Re: Bush DAC90A Speaker Problems

I've replaced dust caps with circles of felt.
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Old 10th Dec 2020, 7:56 pm   #19
DonaldStott
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Default Re: Bush DAC90A Speaker Problems

Quote:
Originally Posted by ThePillenwerfer View Post
I've replaced dust caps with circles of felt.
I'm not obsessive about retaining originality - for me it's always safety first, functionality second and originality last - but if 1 inch dust caps are available then why not ...?

A 1 inch felt circle is of course Plan B.
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