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Old 13th Mar 2009, 12:55 pm   #1
'LIVEWIRE?'
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Default Probs. with Blaupunkt Bremen 'W' Car radio

THis mid/late 1960s Car radio - a 'simple' MW/LW receiver, typical of it's time, is very unstable - as soon as the volume is advanced more than about 25% above minimum, the radio oscillates. The circuit is quite conventional for the time, using AF126 & AF139 transistors(2 of each) in the RF/IF Stages, and 2 x AC153(AC128), plus 2 x AD148 in a transformer coupled Amp. It is built on 3 boards, connected together via pluggable leads, and is switchable for both 6 & 12v, and -Ve/+Ve earth, using slide out reversible tabs for this purpose(Yes I've checked for dry joints, bad connections, etc, but cannot find the cause of this problem!) Can anyone help, please?
NB I don't have a manual for this model(request out on the 'Info. & service data wanted' section).
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Old 13th Mar 2009, 2:04 pm   #2
Lee de Forest
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

Check de-coupling capacitors.
I've just built a valve set from spare parts that had the same problem. I did not fit the tone control to start with because I wanted to see if I could get the basic set working. The set worked O.K. amplifying audio input from a CD but when switched to radio in there was high frequency oscillation above 25% volume. Fitting the tone control capaciitors and pot removed the problem. I assume the capacitors filtered/de-coupled the high frequency from the voltage supply to the input stage of the radio.
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Old 13th Mar 2009, 7:12 pm   #3
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

Lee, the Bremen 'W' has switched tone correction, and the oscillation persists whether or not the tone correction is selected, so I'm still searching for the cause, but thanks for your input on this subject.
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Old 13th Mar 2009, 8:02 pm   #4
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

There will be other Capacitors that do decoupling. It could be any one.

Unless it can be made to come and go with an insulated prod.

Cheers,

Steve P.
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Old 14th Mar 2009, 12:37 pm   #5
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

Steve, the instability is pretty much permanent above ca. 25% volume setting. I have discovered an intermittent problem with a screened lead goinh from the RF/IF PCB to the tuner, but, if replacing/resoldering that doesn't cure the fault, it begins to look like a recap job. The radio is about 43 years old after all!
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Old 14th Mar 2009, 1:22 pm   #6
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

A 'scope would be invaluable in such cases to see where the unwanted oscillations are coming from. Just touching a probe to each decoupling cap' in turn until you see them.
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Old 14th Mar 2009, 11:06 pm   #7
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Default Re: Probs. with Blaupunkt Bremen 'W' Car radio

David, Unfortunately, as I mentioned in another recent thread, my 'scope is out of action, and because of the amount of repair jobs I've undertaken, I've not had the time to open it up and repair it. That's one of those 'when I get aound to it' jobs!!
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