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Vintage Radio (domestic) Domestic vintage radio (wireless) receivers only. |
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11th May 2021, 5:55 pm | #1 |
Heptode
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Selga 405 audio problem.
I've already made a post regarding this radio. But thought I would start a dedicated repair thread.
Basically the receiver part of the radio works, I've tapped into the detector using an external amp. I noticed one side of the output transformer was loose so I removed it from the board and seen that the wires had broke off the primary side, So it's useless. I stuck an output transformer from one of those cheapo Chinese kit radios in but still no sound, Not a pop or hiss. The radio is drawing 4mA which would suggest a problem elsewhere, But I'm no expert. Funny thing is the schematic shows the speaker connected to the secondary (two contacts) side of the transformer. But it's actually connected to the primary side (centre tapped side) Speaker is 8 ohms. Cheers, Jay.
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13th May 2021, 2:57 pm | #2 | |
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Re: Selga 405 audio problem.
Quote:
Either the wiring or your observation is wrong. Perhaps someone has had a go at fixing the set before?
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Graham. Forum Moderator Reach for your meter before you reach for your soldering iron. |
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13th May 2021, 3:07 pm | #3 | ||
Heptode
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Re: Selga 405 audio problem.
Quote:
Iv'e noticed the bases of the output transistors have 9 volts on them. Surely that can't be right. A schematic i found online shows the bases as having 9 volts on them but the factory schematic calls for 0.12v Cheers Jay
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13th May 2021, 3:22 pm | #4 |
Dekatron
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Re: Selga 405 audio problem.
The voltages in the schematic I'm looking at are WRT B+ not ground.
Lawrence. |
13th May 2021, 3:34 pm | #5 |
Heptode
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Re: Selga 405 audio problem.
You mean negative voltages ?
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13th May 2021, 3:35 pm | #6 |
Dekatron
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Re: Selga 405 audio problem.
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13th May 2021, 3:47 pm | #7 |
Heptode
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Re: Selga 405 audio problem.
Well the voltages i'm measuring are positive.
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13th May 2021, 4:14 pm | #8 |
Heptode
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Re: Selga 405 audio problem.
Hi Jay,
Enclosing a picture of the PCB. The output transformer is "Tr2" or "Tp2" with the Cyrillic characters, at the bottom left corner of the PCB. It shows the the pin-out is sort of funny, the primary with the center tap is on the side which has only two pins, the primary center tap itself is on the middle of the other side. The secondary are the two outside pins there. Hopefully this makes sense! Regards, Peter |
13th May 2021, 4:33 pm | #9 |
Dekatron
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Re: Selga 405 audio problem.
In the schematic the voltage on the base of the output transistors is shown as 0.12 volts DC WRT B+, which is correct given that the output transistors are PNP, the voltage measurements can indicate -ve or +ve depending which way round the meter probes are connected.
EDIT: Post crossed. Lawrence. |
13th May 2021, 4:45 pm | #10 | |
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Re: Selga 405 audio problem.
Quote:
Lawrence. Last edited by ms660; 13th May 2021 at 4:59 pm. Reason: to clarify |
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13th May 2021, 4:47 pm | #11 | |
Heptode
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Re: Selga 405 audio problem.
Quote:
I think thats right Cheers Jay
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13th May 2021, 4:50 pm | #12 | |
Heptode
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Re: Selga 405 audio problem.
Quote:
Cheers Jay
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13th May 2021, 5:16 pm | #13 |
Dekatron
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Re: Selga 405 audio problem.
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13th May 2021, 5:24 pm | #14 | |
Heptode
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Re: Selga 405 audio problem.
Quote:
Cheers lawrence Jay
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13th May 2021, 5:39 pm | #15 |
Dekatron
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Re: Selga 405 audio problem.
The voltages in the schematic for the output transistors were measured WRT B+ve because the transistors are PNP types, it makes it easier to interpret for those types.
The voltages for the other transistors were measured WRT B-ve because those transistors are NPN, again it makes it easier to interpret for those types. Lawrence. Last edited by ms660; 13th May 2021 at 5:43 pm. Reason: word change |
13th May 2021, 5:53 pm | #16 |
Octode
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Re: Selga 405 audio problem.
I wouldn't worry about voltages until you have fitted the output transformer. You may need to bend up the centre pin and wire it round to ground. A strange way of doing things.
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13th May 2021, 6:17 pm | #17 | |
Heptode
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Re: Selga 405 audio problem.
Quote:
Yeah its a weird arrangement Cheers Jay
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Help!! I've been electrocuted by a glass thing that glows Last edited by Radio Scotland; 13th May 2021 at 6:26 pm. |
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13th May 2021, 6:28 pm | #18 | |
Heptode
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Re: Selga 405 audio problem.
Quote:
Cheers Jay
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13th May 2021, 6:34 pm | #19 | ||
Dekatron
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Re: Selga 405 audio problem.
Quote:
Lawrence. |
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13th May 2021, 8:43 pm | #20 |
Heptode
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Re: Selga 405 audio problem.
The radio is now working. I adapted the cheapo chinese radio kit transformer to match the configuration of the original. Doesn't sound great, But no idea how it should sound, A forum member is sending me a transformer which should be a better impedence match. Once i worked out the crazy transformer configuration the penny dropped. Knew i was missing something obvious.
Not the best quality radio ever, Held together with melted plastic and the tracks lift from the board when the soldering iron is a foot from the PCB Cheers Jay
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