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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 27th May 2011, 5:40 pm   #1
inecik38
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Default rectifier and transformator replacement

dear Friends,I have a germany radio, telefunken gavotte9.there is no power transformator inside,and there is a rectifier( AEG B250 C75 N2) but the rectifier is faulty.I have some questions;
-I have a another transformator,the transformator have two 380VAC secondery.can I replacement this transformator?
-I have a another rectifier,SEMİKRON C1500 L 5 B. can I replacement this rectifier?
-last ,there is a 0.15 mikro farad capacitor.for you,must I replacement with 0.1 mikrofarad or 0.22mikrofarad cap?
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Old 27th May 2011, 8:12 pm   #2
inecik38
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Default Re: rectifier and transformator replacement

sorry,transformer secondery is two 350VAC.and rectifier picture.
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Old 27th May 2011, 9:02 pm   #3
'LIVEWIRE?'
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Default Re: rectifier and transformator replacement

The Transformer with two 350VAC secondaries is too high in voltage for the Gavotte.
You need one with secondary 250VAC at most, plus a low voltage secondary of 6.3VAC
at about 2.5 to 3 Amps for the valve heaters and dial lamp(s). If possible, get a circuit
diagram of the radio first. This should list the voltages and currents needed. That rectifier may be suitable, but, what I have done in similar cases is to use four 1N4007
diodes, putting a 100 to 150 ohm resistor in series, to replace a faulty selenium rectifier
such as the B250C75N2. As to the 0.15uf capacitor, I would use an 0.22uf. The voltage
rating should be equal to, or greater than, that of the original. If this is wired across the mains, then a class 'X' capacitor should be used in it's place
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Old 28th May 2011, 4:00 pm   #4
inecik38
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Default Re: rectifier and transformator replacement

I didn't understand that; where will I put a resistor, to after bridge rectifier.between bridge rectifier and smoothing capacitor? and I found a new rectifier but there is no information on that.only there is 7C5 mark, that picture is now.for me 250VAC and 75miliamper,is it O.K.
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Old 28th May 2011, 6:52 pm   #5
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Default Re: rectifier and transformator replacement

I definitely WOULD NOT use that rectifier in your post#4-it is another selenium type, which
may be unused, but is probably 40 or more years old, and could fail soon after fitting.
The 100 to 150 ohm resistor in series with modern silicon rectifiers, such as the 1N4007,
can be wired between the transformer secondary and the rectifier, or between the D.C. output of the new rectifier and the rest of the circuit. I fit this resistor on the input(AC)side of the rectifier
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Old 28th May 2011, 7:15 pm   #6
inecik38
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Default Re: rectifier and transformator replacement

it is O.K.sorry but I am a amateur.
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Old 28th May 2011, 7:41 pm   #7
Herald1360
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Default Re: rectifier and transformator replacement

The sketch is correct for the resistor between the transformer and the rectifier.

The other position suggested would place the resistor between the +ve output from the rectifier and the +ve terminal of the capacitor.

Either position is OK. The resistor simply modifies the forward resistance of the silicon rectifier so that it is similar to the forward resistance of the original selenium unit. This keeps the HT voltage closer to its design value than the higher voltage you would get with just a simple replacement of selenium by silicon.
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