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Old 30th Oct 2018, 11:52 pm   #21
Guineafowl
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Default Re: Bush DAC90A sound problem

Quote:
Originally Posted by Northyorks1 View Post
David and John are both mentioning potential issues with the IF stage, bit beyond me. Is the IF stage the output of audio. Sorry for limited knowledge, but thats why I am here.
IF stands for Intermediate Frequency. The radio converts any signal you tune it to to this frequency, which is 465 kHz for your DAC90A. This allows a very focused, optimised handling of the signal which would otherwise be very difficult, given the very wide range of frequencies available to the aerial. Look up ‘superheterodyne’ for more info.

The IF stages are the two square cans with the valve inbetween.
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Old 30th Oct 2018, 11:53 pm   #22
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Default Re: Bush DAC90A sound problem

Thanks for advice Mike, will get some voltages checked.
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Old 30th Oct 2018, 11:57 pm   #23
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Default Re: Bush DAC90A sound problem

Check the condition of the valveholders, if the McMurdo ones are fitted, the forked pins can fracture right inside and leave the pin disconnected.

I assume all the valves light ok, the type of valves used in the DAC90A can lose their vacuum but leave the heater chain intact.
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Old 31st Oct 2018, 2:22 am   #24
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Default Re: Bush DAC90A sound problem

All valves look ok from top view all lighting up, but when you view from underneath chassis there is no orange glow from valve 2 and 5.
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Old 6th Dec 2018, 5:19 pm   #25
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Default Re: Bush DAC90A sound problem

I have been following this with interest I too have an audio problem the volume is very low but when I place my hand on the copper aerial band up comes the volume, usual things looked at and all the usual components replaced checked and rechecked and all the area around V4 UL41 rechecked and I am now at a loss anyone got any ideas?
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Old 6th Dec 2018, 5:56 pm   #26
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Default Re: Bush DAC90A sound problem

Quote:
anyone got any ideas?
Are the stations roughly where you expect them on the dial?

Sounds like front-end alignment particularly the two trimmers C23 & C24
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Old 6th Dec 2018, 8:05 pm   #27
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Default Re: Bush DAC90A sound problem

The stations BBC 5 live on MW and Radio 4 on LW seem to be in the right place
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Old 6th Dec 2018, 9:15 pm   #28
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Default Re: Bush DAC90A sound problem

Get an ohm-meter and with the set switched off, measure the resistance of the windings of the IF transformers. If you find they are 12 ohms or more, you have coil rot in the IF transformers. Common fault in Cornwall. It's the damp. It only takes one rotten coil to ditch the gain as you have here. You can take a replacement IFT from pretty much anything from an AC11/DAC11 onwards through AC31, AC34, AC41 and all the DAC equivalents. None of them seem to be as prone to coil rot as the DAC90A. A good coil is down to 5 or 6 ohms resistance. Between 6 ohms and 12 ohms is a kind of no-mans land where it might work, but it will mean you have a pretty poor performance.
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Old 6th Dec 2018, 11:01 pm   #29
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Default Re: Bush DAC90A sound problem

Which pins to take a reading?
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Old 7th Dec 2018, 1:14 pm   #30
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Default Re: Bush DAC90A sound problem

Looking at the IFT from underneath there is a bakelite piece projecting down under the chassis and the pins are anchored onto this. Two stick out one side, two or three the other. The pins are paired up one side and the other. One pair one side primary, one pair the other side secondary. On the second IFT between the UF41 and the UBC41 the secondary is tapped in what seems to be the middle of the winding. This pin is the centre pin of where there are three in a row, but the tapping comes out on the other side of the bakelite bit from the pair of pins which are the ends of the secondary. In other words the centre tap of the secondary lies between the pins for the connections to the primary. Why? Makes for an easier layout under chassis. Confusing much?
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Old 7th Dec 2018, 1:40 pm   #31
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Default Re: Bush DAC90A sound problem

IFT1 6ohms IFT2 6ohms so on the money
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Old 7th Dec 2018, 2:40 pm   #32
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Default Re: Bush DAC90A sound problem

I hope I am not hijacking this thread it may help the original poster and others who are following when I touch the MW terminal on the plastic aerial former up comes MW but a bit raspy as described Dalek like, on LW sound is there but a tad weak, I have rechecked the connections from the aerial to the set and cleaned the wave band switch. So I am not convinced the UL41 is behaving properly and though reading the correct measurement I am not convinced about the audio transformer, two other sets I have successfully brought back to life needed this replaced and just for good measure I have another 470k pot so I will replace that . I will try and keep you posted.
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Old 10th Dec 2018, 1:38 pm   #33
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Default Re: Bush DAC90A sound problem

Hi, cernem1alt,

No you are not hijacking this thread, any extra input is very wecome.

Cheers Ian.
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Old 15th Dec 2018, 6:39 pm   #34
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Default Re: Bush DAC90A sound problem

OK so I have a new Audio Transformer and a UL41 when I touch the aerial loop the set bursts into life all be it with a hum, remove my hand and the set goes quiet,,, any ideas? is it IFT1 and or IFT2?

All caps and resistors changed and a new pot
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Old 15th Dec 2018, 7:07 pm   #35
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Default Re: Bush DAC90A sound problem

Possibly something open circuit in the front end unlikely IF,s.Check coils etc Trust wave change switch is ok?
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Old Yesterday, 9:37 pm   #36
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Default Re: Bush DAC90A sound problem

closer inspection of the wave change switch revealed it is not very healthy, the top piece of the board came away in my hand, I have bodged it with a super glue and cardboard fix also rechecked a connection to the switch to the chassis was iffy, now I have Longwave but no Medium Wave just a great deal of buzz on MW and no audio, well if it is there it is very raspy so I am hedging my bets on the switch, these are like hens teeth to find so I will sleep on it a while perhaps someone will come up with another alternative switch remedy. But progress made.
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Old Today, 2:40 am   #37
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Default Re: Bush DAC90A sound problem

I am going down the Lorlin switch replacement route, which may be helpful to others, I believe it may have been covered before somewhere on here perhaps in success stories but I cant find it so I will hope someone can point me in the correct direction,

What Lorlin switch to get? I am thinking 2 pole 6 way? and has anyone done it in a Bush DAC 90A and have some wiring info, at first glance it looks daunting but not with time and patience will it be impossible

Link to Lorlin data sheet https://4donline.ihs.com/images/VipM...03722030-1.pdf

Last edited by cernem1alt; Today at 2:47 am.
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Old Today, 2:58 am   #38
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Default Re: Bush DAC90A sound problem

You only need a 2 way switch surely.

There is a you tube

https://www.youtube.com/watch?v=8NLIpFAmy64

that may help.

I think a normal 4 pole 2 way will fit but not done it.
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Old Today, 3:11 am   #39
cernem1alt
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Default Re: Bush DAC90A sound problem

Thanks Boater Sam I got the description of the switch muddled yes 2 way, I have watched that video before however I think it looks too easy in the video and more hassle in reality, I would think as you have only 5 wires to hook up and one of those go to the chassis from a mid point on the switch it should, (famous last words) be possible, it is finding the correct pole and slim enough switch so 6 pole 2 way should be the way to go.
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Old Today, 11:11 am   #40
David G4EBT
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Default Re: Bush DAC90A sound problem

It's fiddly to replace a DAC90A switch, but straightforward enough.

The video in post 38 only shows how to modify an old switch - enough of a faff itself - but why bother when for £1.50 or so, you can by a new Lorlin 2-pole six way switch which will fit in nicely, and remove the surplus tags? I'm not sure of the origin of the two diagrams I've attached below, but all credit to the originator. I'm surprised it's not more widely known, given that the wave-change switch on the DAC90A is a weak point.

The second pic makes very clear which of the five wires go where - how hard can that be?

Lorlin switch:

https://cpc.farnell.com/lorlin/ck103...26%20Solenoids

Hope that helps.
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