Bush DAC90A sound problem
 

Hello All,

When I purchased radio it had no sound. I replaced all recommended parts as per forum advice, still no sound. Starts fine dial lights on just no sound. I have tried different valves from working radios, but no change. Then strangely I put my fingers on the copper band around the frame aerial and radio started playing on a low volume, take fingers off no sound. I am very much a novice regarding radio repair with very limited knowledge, just sort of muddling my way through. So any advice would be much appreciated. link provided below.

Ian

https://vimeo.com/user91185675/revie...004/656c836b7a

Re: Bush dac90a sound problem
 

This sounds like lack of front end gain,possibly anything from cleaning the wave change switch to out of spec components.

Bring it round if you are stuck.

Re: Bush dac90a sound problem
 

Thanks for your reply and kind offer Hamish, hope that is your name and not the dogs.I have cleaned wave change switch and just about everything else that is recommended on this forum.

Re: Bush dac90a sound problem
 

Take care this set has no Mains isolation transformer so the chassis could be live.
Unless you have a test meter and some other stuff you would be well advised to take up the offer of help.
I don’t wish to dampen your enthusiasm, I’m only thinking of your safety.
Cheers
John

Re: Bush dac90a sound problem
 

Yes beware, make certain that it is the mains neutral is connected to the chassis, and not live.
With the mains power cord plugged into the radio but the radio unplugged from the mains wall socket, when the radio is switched on at the volume control, you should be able to measure from the neutral pin of the 13 amp plug through the pilot bulbs to the chassis and the resistance should be considerably less than if you measure from the live pin in the 13 amp plug to chassis.

I find it always best to check a radio from the loudspeaker forwards through the various stages to the aerial.
Have you tested the audio stages to check they are working properly.
If you touch a test meter probe onto the wiper contact of the volume control, with the control turned up you should get a loud buzz in the speaker.

Mike

Re: Bush dac90a sound problem
 

You are very welcome Ian and yes Hamish was my late Boxer dog.

David

Re: Bush DAC90A sound problem
 

Sorry for mix up with names David. Thanks for replys regarding safety. I do understand regarding live chassis, and radio is wired direct to pot. I do have basic knowledge of live chassis radios currently have five dac90a, I know got a bit carried way. I am fine mechanically just not much experience testing. Yes I have basic tools including a multi meter, just early days with my testing. And yes there is a buzz from the speaker.

Thanks again for replys.

Ian.

Re: Bush DAC90A sound problem
 

There will certainly be an audio buzz,though what waxy caps have you replaced? I am afraid on this occasion most are normally leaky plus the hidden one under the tag strip.

Though just work around the UCH (aerial end) towards the audio output checking for out of spec resistors first.

More than 20% which I think most are possibly want changing.

Re: Bush DAC90A sound problem
 

How about some voltage readings, as you have a number of the same models you know pretty well what to expect, of course I presume at least one is working! You say you have tried the valves, in addition to what has already been said you could have a problem with the IF stage. Do you have a sig generator?
Cheers
John

Re: Bush DAC90A sound problem
 

Have changed capacitors including electrolytic, all resistors but one which was well within spec. A number of perished wires, and went on a rapage with servisol. I am going to triple check values of resistors in case I have made any mistakes. Pic of chassis added, may show something I am missing or have done wrong.

Ian

Re: Bush DAC90A sound problem
 

I’ve just nearly finished one of these, and am a beginner to valves like yourself. I strongly recommend you get a Trader service sheet (link top right) and measure some voltages, including valve and HT.

You seem to have replaced the major culprits, but if you did it wholesale it’s possible you’ve made a wiring error. If not, look to the hard-working components you haven’t replaced, like the dropper resistor.

Re: Bush DAC90A sound problem
 

Of course another issue on these is the dust core can drop of it,s thread and detune the IF.

Re: Bush DAC90A sound problem
 

No John have not got a signal generator, I have a family member with one who will check it soon but dont know when with work commitments. And yes all other radios working very well with no major problems to sort out. Have got service sheets, dropper resistor reads fine, output transformer fine. As said in first post radio had no sound before changing components, and were changed one at a time as advised on forum. HT is at between 180-185v so have tried 3 different rectifier valves which gave the small increase to 185v, also different output valve.

Re: Bush DAC90A sound problem
 

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.

Re: Bush DAC90A sound problem
 

Does it work if you turn it upside down? Did you hear a clunk as it turned over?

Re: Bush DAC90A sound problem
 

Sorry the IF cans,that is where the core drops some times.The ally cans 2 of.

Re: Bush DAC90A sound problem
 

No clunk and does not work upside down, cannot hear anything loose.

Re: Bush DAC90A sound problem
 

Hi the IF cans are the square ones along the back of the chassis. if there is a rattle coming from them it could be the common fault of a loose core that can be fixed.
The other cause of this type of problem is related to the AVC line.
This line feeds a DC control voltage to the IF and Fc valves and controls their gain.
It is normally the line on the schematic right at the bottom of the diagram. It is fed with high value resistors (>1 meg) and leaky caps on this line will cause problems out of all proportion to the value of the leak.

Ed

Re: Bush DAC90A sound problem
 

Hi Ed,

No noise at all from IF cans, and all caps and resistors replaced just checked all resistors again for right values and tolerance, all spot on.

Re: Bush DAC90A sound problem
 

I think to get any further you need to measure the voltages on the valves and compare them with what is marked on the service sheet.
If you are not experienced or confident with doing this, bearing in mind it is a "live chassis" set, then seek help from someone local who may be more experienced.

Mike