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Old 17th Oct 2019, 8:25 pm   #1
JUSTDAMO
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Default Bush DAC90A - power but no sound

Hello

Having acquired the above radio, it does look dusty inside, possibly not been used for a while so I have put a 3 amp fuse in and powered the radio up. The dial light came on but dimmed a little straight away, all valves appear to be lit but there is no sound from the speaker whatsoever.

So although I have little experience, I'm armed with a test meter and the willingness to learn.

Just wondering where I should start to sort this problem out?

Thanks
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Old 17th Oct 2019, 8:56 pm   #2
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Default Re: Bush DAC90A - power but no sound

If you haven't already done so get yourself a copy of the service sheet.

A completely dead set is indicative of no HT supply, an open circuit output transformer winding, or an open circuit speaker.

I suggest that you measure the resistance of the primary winding of the output transformer, as you can do this without needing to concern yourself with safety considerations. Do this test with the set unplugged from the mains.

You could also check the resistance of R17 (Manufacturer's sheet).
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Old 17th Oct 2019, 8:59 pm   #3
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Default Re: Bush DAC90A - power but no sound

Valuable guidance here https://www.vintage-radio.com/repair...and-tests.html good luck
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Old 17th Oct 2019, 9:37 pm   #4
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Default Re: Bush DAC90A - power but no sound

The output transformer primaries on these do fail open circuit. I am currently repairing a 90A in which that has happened. Forum member Ed Dinning can rewind the transformer for 30. There are also new output transformers available from various sources, but before 'jumping in with both feet!' do check the DC resistance (should be about 400 ohms from memory), check the speaker(disconnect one lead to measure the DC resistance), and other points as mentioned by Graham (station X)
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Old 17th Oct 2019, 9:54 pm   #5
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Default Re: Bush DAC90A - power but no sound

If you check the primary of the output transformer, make sure you use the low ohms setting on the meter. With luck you should measure about 500 ohms give or take 50 ohms or so. If it's much above 600 ohms you MAY have a problem. If there is no reading then you DO have a problem but it is not insurmountable. Using the low ohms range across the primary should produce a crackle from the speaker IF the transformer is good. As has been said, Ed Dinning does an excellent job of rewinding these.
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Old 21st Oct 2019, 10:22 am   #6
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Default Re: Bush DAC90A - power but no sound

ok will check all this info and get back to you thanks!!
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Old 21st Oct 2019, 2:13 pm   #7
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Default Re: Bush DAC90A - power but no sound

Hi,

I am not sure if I am doing this quite right but I will try and explain what I have done via the picture

I had the tester on ohms and it was set to a min (20)

measuring the two metal parts in which the speaker wires are attached to it comes up as 00.8 - 010

but measuring the other two metal parts opposite, they don't read anything

I think the R17 appears ok... having done a few readings.
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Old 21st Oct 2019, 2:17 pm   #8
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Default Re: Bush DAC90A - power but no sound

If you look at the tags you'll see that two of them have the tone correction capacitor wired across them. If the resitance between these two tags is infinite/open circuit/no reading the primary winding has failed.

However to be certain of this you'll have to switch to a higher resistance scale on your meter, say 1k, as the meter may be indicating that the reading is out of range.
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Old 21st Oct 2019, 2:29 pm   #9
JUSTDAMO
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Default Re: Bush DAC90A - power but no sound

Yeh I checked the wires and tags attached to the capacitor on top and there is no reading......

is this a rewinding job then / replacement?
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Old 21st Oct 2019, 2:39 pm   #10
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Default Re: Bush DAC90A - power but no sound

That would confirm that the output transformer primary is open circuit. As I think I mentioned earlier, new output transformers are available, or the existing one can be rewound by Ed Dinning.
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Old 21st Oct 2019, 2:42 pm   #11
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Default Re: Bush DAC90A - power but no sound

Slow down a bit! As I said a no reading like OL on a DMM may just indicate an out of range reading. Repeat the test on a higher resistance range or ranges.

It looks like you were using the 20 ohm range and the ouput transformer's primary winding resistace should be around 600 ohms.
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Old 21st Oct 2019, 4:45 pm   #12
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Default Re: Bush DAC90A - power but no sound

That would confirm that the output transformer primary is open circuit. As I think I mentioned earlier, new output transformers are available, or the existing one can be rewound by Ed Dinning.
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Old 21st Oct 2019, 7:59 pm   #13
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Default Re: Bush DAC90A - power but no sound

I double checked and have but the setting on (2m) it flashed up a number 1.114 then went straight back to 1 (thats the default) I tried this a few times and again it will pop up a random high number and then fall back to the default.
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Old 21st Oct 2019, 8:12 pm   #14
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Default Re: Bush DAC90A - power but no sound

I'll hazard a guess that you're using the "2M" two Megohm range rather than "2m"?

The initial reading of 1.114 Megohm will be down to capacitors charging. Once charged the reading reverts to "1" denoting infinite resistance.

I'm afraid the output transformer's had it.
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Old 21st Oct 2019, 8:24 pm   #15
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Default Re: Bush DAC90A - power but no sound

it does say 2M yes.... oh well, I was hoping it would be my first project.

Thanks for all the posts
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Old 21st Oct 2019, 8:35 pm   #16
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Default Re: Bush DAC90A - power but no sound

Without wishing to patronise, you really should become familiar with using a testmeter. Test instruments (of any kind) are only useful if the user knows how to use them. Best thing is to measure some resistors first and become familiar with how the meter reads the values. Low values, high values, intermediate values and check how the value of the resistor is read and which range to use for best accuracy.

Saying that 'the number 1.114 came up on the 2M range' clearly doesn't mean anything to you. The meter is in fact telling you that it (briefly) measured a resistance of 1.114 megohms (since you were on the 2 Megohm range) and then showed infinity (open circuit). Considering the resistance of the primary should only be about 500 ohms you should have been on a much lower setting resistance range (probably the lowest but depends on your meter). The meter would then probably have just read infinity (open) in which case the fault would have been more apparent.

I think we've established that the transformer primary is open and the reason for your no sound fault. It's a very common problem. DO NOT run the set with the transformer in this condition....it won't do the output valve (UL41) any good at all and these valves are now getting very difficult to find and also expensive. Next step is to either find a good transformer or have the existing one rewound by Ed Dinning. Ed will charge for his service (I've just had a transformer rewound by him) but it will be worth the effort.

While you are waiting for a transformer (either a replacement or a rewind), put the time to good use and get used to your meter, how it works and what the readings are telling you. If you are not sure....ask! Also you MUST replace the output valve coupling capacitor...it's probably one of the reasons why the transformer failed.

If you want to proceed with a rewind, I can PM you Ed's details.

Enjoy the hobby! We all started somewhere......
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Old 21st Oct 2019, 8:40 pm   #17
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Default Re: Bush DAC90A - power but no sound

I understand the lowest range of the OP's meter is 20R, but that looks odd to me.

What make and model of meter are you using JUSTDAMO? A picture of it would help.
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Old 21st Oct 2019, 9:48 pm   #18
JUSTDAMO
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Default Re: Bush DAC90A - power but no sound

Thats sound advice and all noted.

this is the meter I have.

Quote:
Originally Posted by Sideband View Post
Without wishing to patronise, you really should become familiar with using a testmeter. Test instruments (of any kind) are only useful if the user knows how to use them. Best thing is to measure some resistors first and become familiar with how the meter reads the values. Low values, high values, intermediate values and check how the value of the resistor is read and which range to use for best accuracy.

Saying that 'the number 1.114 came up on the 2M range' clearly doesn't mean anything to you. The meter is in fact telling you that it (briefly) measured a resistance of 1.114 megohms (since you were on the 2 Megohm range) and then showed infinity (open circuit). Considering the resistance of the primary should only be about 500 ohms you should have been on a much lower setting resistance range (probably the lowest but depends on your meter). The meter would then probably have just read infinity (open) in which case the fault would have been more apparent.

I think we've established that the transformer primary is open and the reason for your no sound fault. It's a very common problem. DO NOT run the set with the transformer in this condition....it won't do the output valve (UL41) any good at all and these valves are now getting very difficult to find and also expensive. Next step is to either find a good transformer or have the existing one rewound by Ed Dinning. Ed will charge for his service (I've just had a transformer rewound by him) but it will be worth the effort.

While you are waiting for a transformer (either a replacement or a rewind), put the time to good use and get used to your meter, how it works and what the readings are telling you. If you are not sure....ask! Also you MUST replace the output valve coupling capacitor...it's probably one of the reasons why the transformer failed.

If you want to proceed with a rewind, I can PM you Ed's details.

Enjoy the hobby! We all started somewhere......
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Old 21st Oct 2019, 10:20 pm   #19
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Default Re: Bush DAC90A - power but no sound

That's too small for me to see clearly. Looks like there's a 2K (2000 ohms) resistance range. That's the one to use for checking your output transformer primary winding.

I think the ranges are

2M, 2,000,000 Ohm
200K, 200,000 Ohms
20K, 20,000 ohms
2K, 2,000 ohms
200, 200 ohms
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Old 22nd Oct 2019, 10:15 am   #20
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Default Re: Bush DAC90A - power but no sound

Good Advice there, Graham. Speaking of DAC90A's-is there ay reason why the output transformer shouldn't be mounted with it's primary side closest th the tagboard, and secondary side facing the front of the chassis? To me that is more logical than the other way round as shown in some illustrations, and it's how I plan to mount the O/P Tx on a 90a on which I'm working. I've checked and there's no likeliehood of any short circuit between adjacent components. In addition is there any convention as to whether the 'Start' or 'Finish' of the primary winding should go to HT+ ? AS no Feedback or 'Hum-Bucking' tap is used I would have said that the way the primary is wired in circuit is of no conswquence.

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