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Old 18th Apr 2019, 2:18 pm   #1
mike1 ryan1
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Default Bush DAC90A only receives 2 stations

Hi. I have a dac90a that I have re-capped and replaced quite a few resistors that were away from spec. Turning it on it will only pic up 2 stations fairly close to one another on the scale around the middle and nothing on the other position on the wave change switch. The wave change switch is turned towards the front of the set when it works, sorry I don't know what that position is. The stations are loud and clear, all 2 of them.
Mike
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Old 18th Apr 2019, 3:33 pm   #2
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Default Re: Bush DAC90A only receives 2 stations

I'm going to have a go at checking the alignment of the set and was just wondering if there is any kind of lubricant that can be applied to run down into the cores before I attempt to move them. I have the service data, a signal generator and frequency counter and an oscilloscope to check for the peak settings. I've always avoided doing the procedure in case I break a core or make it worse but can't put it off forever.
Mike
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Old 18th Apr 2019, 3:48 pm   #3
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Default Re: Bush DAC90A only receives 2 stations

I would not try realigning unless it is obvious that the "twiddler" has been there and moved the cores.
You have a fault if only one waveband works and you can only get 2 on the other.
Which 2 do you get? If its radio 4 you're on Long wave, there should be lots of stations still on Medium wave.
Fix the fault first.
Did you change the silver mica caps in the aerial and oscillator coil circuits, the flat thin ones?
If so you will have upset the front end, these caps seldom need replacing.
Blanket recapping causes problems like this, you should get it working first then do one at a time.
Check all your work its probable that you have introduced a fault.
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Old 18th Apr 2019, 4:22 pm   #4
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Default Re: Bush DAC90A only receives 2 stations

Quote:
Originally Posted by mike1 ryan1 View Post
Hi. I have a dac90a that I have re-capped and replaced quite a few resistors that were away from spec. Turning it on it will only pic up 2 stations fairly close to one another on the scale around the middle and nothing on the other position on the wave change switch. The wave change switch is turned towards the front of the set when it works, sorry I don't know what that position is. The stations are loud and clear, all 2 of them.
Mike
On LW you should be able to hear Radio 4, and RTE from Ireland but not as strong as R4. (You should be able to tell that it's RTE from the Irish accents, or sometimes speaking in Gaelic). Those may well be the two stations you are hearing. You may also hear two French stations if only quite weak, given that the DAC90A has an internal aerial. At least if it's working well on one waveband, that shows that the frequency changer, IF, detector and AF stages are all working, so it's a 'front end' fault on one waveband. (You should get ten or 12 stations on MW).

As Sam says, if you hear nothing on MW, don't attempt to fiddle with the alignment as you'll probably do more harm than good. You need to check the continuity of the wave-change switch, coils and associated 'front end' wiring and compare it with the circuit to trace any wiring errors or open circuits. Hopefully, the only caps you've changed as the waxy paper ones - not any in the tuned circuits.

Good luck with it.
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Old 18th Apr 2019, 4:43 pm   #5
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Default Re: Bush DAC90A only receives 2 stations

My 90A has Long Wave with the switch pointing to the rear and Medium Wave switch forward.

I guess one of 'your' stations is BBC Radio 5 on 330 Metres (909kHz) that is central on the dial, plus a local? That is if the calibration is anything like correct...
As you have a signal generator why not use that to provide a modulated signal on Long Wave and 'chase' it through the set? Do Not connect the generator directly to the Bush, loop 2 or 3 feet of wire, from the RF out, close to the bush aerial. Wave change switches often cause problems on these sets.

Alan
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Old 18th Apr 2019, 4:46 pm   #6
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Default Re: Bush DAC90A only receives 2 stations

I had a fault on a DAC90A only working on one waveband (MW) it transpired one of the sockets on UCH42 valve holder was broken and not making good contact with the valve pin - great fun transplanting a new 'contact' from a donor chassis! The forward position on the waveband control is medium wave as Alan in #5 says.
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Old 18th Apr 2019, 9:57 pm   #7
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Default Re: Bush DAC90A only receives 2 stations

Thanks for your replies. I only changed the wax capacitors and only the resistors that were out of spec. The only reason why I was thinking of doing the alignment procedure was that all 4 of the if adjusters are fully wound out which I thought might be a bit suspect. However i shall endeavour to dig desperate. TBH I planned on carefully noting where everything was before starting, and making sure to note things down so I can return everything to where it currently is having a go at the procedure for experience as much as anything. Tracing a signal through the set seems to be a better first step though so I will try that first.
Mike
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Old 18th Apr 2019, 10:03 pm   #8
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Default Re: Bush DAC90A only receives 2 stations

Quote:
Thanks for your replies. I only changed the wax capacitors and only the resistors that were out of spec.
The silver mica caps are wax - did you change them?
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Old 18th Apr 2019, 10:38 pm   #9
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Default Re: Bush DAC90A only receives 2 stations

Quote:
- great fun transplanting a new 'contact' from a donor chassis!
Quite often there are "spare/unused" pins in the rectifier socket which can be borrowed. Very often it will be the same type of socket too!
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Old 18th Apr 2019, 11:14 pm   #10
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Default Re: Bush DAC90A only receives 2 stations

No sorry just the ones on the main tagboard
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Old 18th Apr 2019, 11:17 pm   #11
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Default Re: Bush DAC90A only receives 2 stations

As for introducing a fault I understand that is where you would look first but I have been very careful to check and recheck everything I have done. It was completely silent to start with so at least it's going in the right direction.
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Old 19th Apr 2019, 1:40 am   #12
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Default Re: Bush DAC90A only receives 2 stations

Does the fact all 4 cores are fully out suggest it's worth checking.
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Old 19th Apr 2019, 7:32 am   #13
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Default Re: Bush DAC90A only receives 2 stations

Quote:
Originally Posted by mike1 ryan1 View Post
Hi. I have a dac90a that I have re-capped and replaced quite a few resistors that were away from spec. Turning it on it will only pic up 2 stations fairly close to one another on the scale around the middle and nothing on the other position on the wave change switch. The wave change switch is turned towards the front of the set when it works, sorry I don't know what that position is. The stations are loud and clear, all 2 of them.
Mike
Hi Mike,
Have you cleaned the wave change switch and how was it working before you did the work and did you change one at a time and test each time?
(Sorry just re-read your note about being silent before you started)


Quote:
Originally Posted by mike1 ryan1 View Post
I'm going to have a go at checking the alignment of the set and was just wondering if there is any kind of lubricant that can be applied to run down into the cores before I attempt to move them. I have the service data, a signal generator and frequency counter and an oscilloscope to check for the peak settings. I've always avoided doing the procedure in case I break a core or make it worse but can't put it off forever.
Mike
If you have rechecked your work, cleaned the switch, checked the valves and wiring I would then take Voltage readings and compare these with the service sheet before doing any realignment.

But if all is well why not have a go, make sure you have the right equipment and follow the instructions on the service sheet.

And don’t assume if you’re listening to R4 you’re on long wave, I made that mistake when doing my first realignment! In the North East, Carlisle and Belfast (there are others) R4 is also on MW so you could be picking up one of those.

And to go back to your question: I don’t know about lubricating the cores but I would be interested to know what everyone else thinks.

Have fun!

John

Last edited by stitch1; 19th Apr 2019 at 7:35 am. Reason: Not paying attention
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Old 19th Apr 2019, 8:01 pm   #14
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Default Re: Bush DAC90A only receives 2 stations

Thanks John. Unfortunately no radio 4, I think radio 5 maybe and local radio Cumbria.
Mike
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Old 20th Apr 2019, 9:52 am   #15
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Question Re: Bush DAC90A only receives 2 stations

I do realise that this is a DAC90A, but is this model prone to the problem that arises in the DAC90, whereby the cores in the I.F. transformers tend to drop out of the coil formers?
I had that once on a DAC90 - and the symptoms were same as described in this thread.

Just a thought.

Al.
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Old 20th Apr 2019, 1:53 pm   #16
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Default Re: Bush DAC90A only receives 2 stations

Hi Mike, if all of the core slugs are fully out then it sounds like they have been twiddled. The only other most likely cause of frequency shift are (referring to component numbers on Trader Sheet No. 1161) C7 and C8. If these are either ceramic or silver mica it is unlikely that they will have drifted in capacitance, but not impossible. Already suggested, but if you haven't done so already, a good blast of switch cleaner onto the wavechange contacts would be worthwhile. Cheers, Jerry
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Old 20th Apr 2019, 6:41 pm   #17
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Default Re: Bush DAC90A only receives 2 stations

Quote:
Originally Posted by Skywave View Post
I do realise that this is a DAC90A, but is this model prone to the problem that arises in the DAC90, whereby the cores in the I.F. transformers tend to drop out of the coil formers?


Just a thought.

Al.
....and a very good thought Al. Not sure if the cores in the 90A suffer with this but an easy check would be to see if anything changes with the chassis upside down or upright.
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Old 21st Apr 2019, 1:17 pm   #18
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Default Re: Bush DAC90A only receives 2 stations

Further to Al comments, have you tried rocking chassis back and for, sometimes the cores will be Brocken and you can hear them clunk?
You can tell if the IF is faulty by loosely coupling you sig gen with setting on min output, and if possible do a comparison with a known working receiver.
You will soon get the feel for a faulty IF.
Cheers
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Old 22nd Apr 2019, 3:31 pm   #19
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Default Re: Bush DAC90A only receives 2 stations

Ok so I tried the alignment procedure and was encouraged by the adjustment of the second if. There was an obvious change and the peak signal was easy to reach, however, when I moved to adjust the 1st ift no difference was found wherever they were adjusted, also where it was possible to hear the tone coming through on the 2nd ift there was no tone on the 1st. Having disconnected everything more stations are tuneable and louder and there is one on the other band. I will investigate the state of the cores in the 1st ift and see what I can see.
Mike
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Old 22nd Apr 2019, 6:48 pm   #20
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Question Re: Bush DAC90A only receives 2 stations

Presumably, you used your signal generator to make those adjustments. To what part of the circuit did you connect that sig. gen. and what method did you use to connect it to that point the radio?
Both of those can be significant in obtaining the correct alignment of an I.F. amplifier.

Al.
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