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Old 4th Apr 2012, 2:51 pm   #1
pepmin 1
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Default Distortion on Bush VTR103.

Hi. I bought a BUSH VTR103 last week. It is working but unfortunately not very well. It also arrived with a broken handle. Darren on this site has very kindly donated one to me and has been very helpful with advice on what might be wrong with the set.

The problem I am having is when I switch the radio on it appears to be fine until I adjust the volume over halfway. This introduces a sort of buzzing and distortion. I can listen to the radio all day on MW and LW as long as the volume is not above halfway and the sound quality is okay. VHF suffers the same symptoms.

Initially when I first tried the radio there were all sorts of noises when tuning; whistling and what I believe to be motorboating? I snipped the three screen leads on the AF116's one at a time but did not hear anything obvious at first, but then all the weird noises stopped for some reason leaving me with just the buzzing and distortion when the volume was raised.

Today I removed each of the AF116's one at a time and replaced them with a known working transistor but, the problem was still there. To be honest I didn't really think that would work as the problem was on all wavebands, but it was easy to try and didn't take long. Voltages seem to be okay as per the trader sheet on these transistors as well. I am thinking it is a capacitor but don't really know. I have no way of checking these. Any other advice would be appreciated.

Thanks,
Colin.

Last edited by pepmin 1; 4th Apr 2012 at 2:56 pm.
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Old 4th Apr 2012, 2:59 pm   #2
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Default Re: Distortion on bush vtr 103.

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Old 4th Apr 2012, 3:47 pm   #3
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Default Re: Distortion on Bush VTR103.

Maybe it's because after halfway the output is overdriven? I find most volume pots are a bit over optimistic, to the point that if I need a new one for a set I will plump for a lower value and make it up to the original value with a series resistor.
 
Old 4th Apr 2012, 4:03 pm   #4
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Default Re: Distortion on Bush VTR103.

If it does it on all wavebands, then i would suspect the O/P stages. I would look at voltages round the Output Stages, make sure nothing has changed value and is the feedback OK. (C53, R22). There are some electrolytics there that may be past their best.

Does anything get hotter than it should and is the speaker OK?

The voltages are on the Bush service sheet, which is available here...

http://www.service-data.com/product.php/673/1118

I'd also look at C57, as it may be leaking and dropping battery volts slightly.

Cheers,

Steve P.
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Old 4th Apr 2012, 4:19 pm   #5
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Default Re: Distortion on Bush VTR103.

Quote:
Originally Posted by merlinmaxwell View Post
Maybe it's because after halfway the output is overdriven? I find most volume pots are a bit over optimistic, to the point that if I need a new one for a set I will plump for a lower value and make it up to the original value with a series resistor.
Colin's problem also embraces whistling and motorboating above half volume, so issues with the volume pot are unlikely to be relevant in this instance. That is, of course, assuming the correct pot is fitted.

My own VTR103 gets used as a testbed for this and that but, even when it returns to original condition, I do find it's rather susceptible to compression distortion at higher volumes. That could, I suppose, be contorted into a volume pot issue but I've never encountered concurrent whistling and motorboating.
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Old 4th Apr 2012, 6:32 pm   #6
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Default Re: Distortion on Bush VTR103.

Thanks for all your advice again guys. I will have another go later on. I have just switched it on and the symptoms have changed yet again. I was listening to Radio 2 at lunch time and with the volume below half way it was okay. The same on LW and MW. Now the motorboating is very pronounced on vhf at all volume levels. The motorboating is only occurring on MW and LW at high volume. Steve I have tried another speaker and as far as I can tell nothing seems to be getting hot. Darren the whistling does not seem to be there at the moment. It was quite sharp a couple of days ago and got loudest just prior to finding a station. The volume pot seems to be original. Colin.
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Old 4th Apr 2012, 7:37 pm   #7
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Default Re: Distortion on Bush VTR103.

The circuit is on sheet 1549.

Motorboating is a form of oscillation which needs feedback to occur. If it happens on all wavebands, then the power supply is suspect. I would change C57, 350uF
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Old 4th Apr 2012, 9:03 pm   #8
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Default Re: Distortion on Bush VTR103.

Have you checked the battery? I have found some of the older transistor vhf sets are fractious when the battery voltage drops below a critical point like when the volume is turned up loud. My Dynatron (Commander?) motorboats when the batteries are getting flat. the squealing may be an unrelated fault. Just a thought!

Rick.
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Old 4th Apr 2012, 9:21 pm   #9
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Default Re: Distortion on Bush VTR103.

Cheers for that Trevor. Rick the batteries are new and the voltage is good on them. On the the subject of electrolytics, where is a good place to buy them. I would like to have some as spares.I live in Inverness and all we have here is Maplins. The electrolytics on this radio are quite large compared with any that I can see on ebay for instance. Do they have to be as big as the originals? I dont want to go down the scatter gun route but it would be nice to have some to try. Any that I have bought in the past have been of ebay. Im sorry if this sounds like a new thread, but it is still in connection with this radio. Colin.
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Old 5th Apr 2012, 10:09 am   #10
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Default Re: Distortion on Bush VTR103.

Quote:
Originally Posted by pepmin 1 View Post
Cheers for that Trevor. Rick the batteries are new and the voltage is good on them. On the the subject of electrolytics, where is a good place to buy them. I would like to have some as spares.I live in Inverness and all we have here is Maplins. The electrolytics on this radio are quite large compared with any that I can see on ebay for instance. Do they have to be as big as the originals? I dont want to go down the scatter gun route but it would be nice to have some to try. Any that I have bought in the past have been of ebay. Im sorry if this sounds like a new thread, but it is still in connection with this radio. Colin.
Just about all modern electrolytic capacitors(and for that matter other components)are smaller than those of 40 or more years ago. This doesn't matter. Personally I replace failed components with the nearest available suitable modern types. Others prefer to maintain original appearance by hiding new capacitors inside the cases of old ones. Maplin are the only national company of which I know that has branches in most larger towns, though there are (a few?)independent components suppliers with retail shops.
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Old 5th Apr 2012, 1:53 pm   #11
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Default Re: Distortion on Bush VTR103.

Udate on the radio. Had a look through my box of bits and found a used 470 UF radial electrolytic. I unsoldered one end of C57 -350 UF and piggy backed that in place. The radio is now working great. I realise that this capacitor is of the wrong value but it was for test purposes only. There is still room for more improvement but I am happy with the way it is performing at the moment. Thanks to everyone who suggested I try that capacitor and to everyone else for their advice. This is certainly the place to come if you want your radio working again. I need to invest in a multimeter that checks capacitance I think. I have cleaned the cabinet up with T CUT metal polish and it has come up great. All I need now is an original aerial, and when cleaning the cabinet I noticed part of the S in the bush logo is broken. Also the chrome has worn off. I may put a call out on here for those two things. I need to source an electrolytic now, and I can put the radio back together again. I dont fancy putting in a Maplins radial although I totally appreciate what Livewire is saying about modern replacements. I will try and find a suitable modern axial. Thanks again. Colin.

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Old 5th Apr 2012, 7:44 pm   #12
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Default Re: Distortion on Bush VTR103.

Colin, I'd be inclined to retain that 470uf*, or preferably fit a new one. Most electrolytics have a wide tolerance + &- of their nominal value, and, in any case a new 350uf will be (almost?)impossible to source, although the nearest modern preferred value (330uf, 63v) is available fromwww.cricklewoodelectronics.com by mail order. (*Ensure that the working voltage of any permanent replacement is equal to, or greater than, that of the original)

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Old 5th Apr 2012, 9:51 pm   #13
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Default Re: Distortion on Bush VTR103.

Thanks for that information livewire. I knew that these old capacitors had a + and - tolerance but was not sure how much. I was struggling to find a new 350 uf axial. But I did see a few 470 uf ,s. Cheers for the link to Cricklewoods I will have a look on there. Colin.
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Old 5th Apr 2012, 10:31 pm   #14
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Default Re: Distortion on Bush VTR103.

C57 350uF is straight across the battery supply after the switch. Within reason it can have as much capacitance as you like- certainly 470uF will be fine. No need to fit anything closer to 350uF.
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Old 6th Apr 2012, 5:20 am   #15
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Default Re: Distortion on Bush VTR103.

Thanks Chris. Thats good to know.
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Old 6th Apr 2012, 6:12 pm   #16
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Default Re: Distortion on Bush VTR103.

Could someone tell me how to check the quiescent current on this radio?

Thanks,
Colin.
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Old 6th Apr 2012, 8:01 pm   #17
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Default Re: Distortion on Bush VTR103.

There's a 10R resistor in each output device emitter lead. Measure volts across each and divide by 10.

There's no way to adjust it, though, short of resistor changes. If the two currents are mismatched, first check the resistors are indeed 10R, if still unbalanced, the output devices are probably mismatched, which will increase distortion. Without a stock of OC81s to try against each other there's not a lot could be done about it. Pretty much any gain matched silicon PNP could be substituted with a tweak to the bias- change R34 150R to somewhere in 390-470R range?
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Old 6th Apr 2012, 9:48 pm   #18
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Default Re: Distortion on Bush VTR103.

Thanks Chris.
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Old 6th Apr 2012, 10:13 pm   #19
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Default Re: Distortion on Bush VTR103.

I have had another look at the diagram, C57, 350uF, smooths the input voltage. There is also C44, 350uF, which smooths the earlier stages of the set.
Replacing one of these seems to improve things, I would change the other as well to be sure.
Any ripple in the earlier stages will give a voltage on VT7 base.
Previously I suggested using 1000uf (easy to get) but this was removed from my post.
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Old 7th Apr 2012, 4:22 am   #20
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Default Re: Distortion on Bush VTR103.

Thanks for that advice Trevor. I think that is a good idea. Colin.
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