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Old 11th May 2021, 4:55 pm   #1
Radio Scotland
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Default 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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Old 13th May 2021, 1:57 pm   #2
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by Radio Scotland View Post
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.
A loud speaker is always connected to the secondary side of the output transformer.

Either the wiring or your observation is wrong. Perhaps someone has had a go at fixing the set before?
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Old 13th May 2021, 2:07 pm   #3
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Default Re: Selga 405 audio problem.

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Quote:
Originally Posted by Radio Scotland View Post
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.
A loud speaker is always connected to the secondary side of the output transformer.

Either the wiring or your observation is wrong. Perhaps someone has had a go at fixing the set before?
Yeah i'm missing something, The guy i bought it from took it apart hoping it was something obvious he could fix, But he never done any work on it as it looks totally original.

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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Old 13th May 2021, 2:22 pm   #4
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Default Re: Selga 405 audio problem.

The voltages in the schematic I'm looking at are WRT B+ not ground.

Lawrence.
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Old 13th May 2021, 2:34 pm   #5
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Default Re: Selga 405 audio problem.

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Originally Posted by ms660 View Post
The voltages in the schematic I'm looking at are WRT B+ not ground.

Lawrence.
You mean negative voltages ?
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Old 13th May 2021, 2:35 pm   #6
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Default Re: Selga 405 audio problem.

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The voltages in the schematic I'm looking at are WRT B+ not ground.

Lawrence.
You mean negative voltages ?
Yes.
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Old 13th May 2021, 2:47 pm   #7
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Default Re: Selga 405 audio problem.

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Originally Posted by ms660 View Post
Quote:
Originally Posted by Radio Scotland View Post
Quote:
Originally Posted by ms660 View Post
The voltages in the schematic I'm looking at are WRT B+ not ground.

Lawrence.
You mean negative voltages ?
Yes.
Well the voltages i'm measuring are positive.
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Old 13th May 2021, 3:14 pm   #8
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Default 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
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Old 13th May 2021, 3:33 pm   #9
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Default 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.
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Old 13th May 2021, 3:45 pm   #10
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by orbanp1 View Post
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
The center tap of the output transformer on the board layout shows it on the primary side and that it's connected to ground which is correct so far as I can make out.

Lawrence.

Last edited by ms660; 13th May 2021 at 3:59 pm. Reason: to clarify
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Old 13th May 2021, 3:47 pm   #11
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by orbanp1 View Post
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
Cheers peter, Far as i can tell the primary winding has two pins on it, The centre tap runs internally to a centre pin on the secondary which i think is connected to the speaker negative.

I think thats right


Cheers
Jay
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Old 13th May 2021, 3:50 pm   #12
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by ms660 View Post
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.
I'm getting positive 9 volts on all of the output transistor connections, Beginning to suspect a short somewhere.


Cheers
Jay
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Old 13th May 2021, 4:16 pm   #13
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by Radio Scotland View Post
I'm getting positive 9 volts on all of the output transistor connections, Beginning to suspect a short somewhere
If that's WRT to B+ then R26 would also be suspect as well.

Lawrence.
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Old 13th May 2021, 4:24 pm   #14
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by ms660 View Post
Quote:
Originally Posted by Radio Scotland View Post
I'm getting positive 9 volts on all of the output transistor connections, Beginning to suspect a short somewhere
If that's WRT to B+ then R26 would also be suspect as well.

Lawrence.
Just to clarify, With the positive probe on the transistor connections and the negative probe on the battery negative the meter indicates the polarity is correct. If i swap the above connections the meter indicates reverse polarity

Cheers lawrence


Jay
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Old 13th May 2021, 4:39 pm   #15
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Default 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 4:43 pm. Reason: word change
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Old 13th May 2021, 4:53 pm   #16
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Default 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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Old 13th May 2021, 5:17 pm   #17
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Default Re: Selga 405 audio problem.

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Originally Posted by peter_sol View Post
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.
It's all became clear since you posted the above comment peter. I fitted an output transformer from a chinese kit radio which has a standard configuration. Which means its actually fitted back to front. With the speaker connected to the primary. Which would account for the strange voltage readings as the collectors will be floating.. I need to install it the other way round with the centre tap bent outwards like you say and connect it via a wire to ground.


Yeah its a weird arrangement


Cheers
Jay
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Last edited by Radio Scotland; 13th May 2021 at 5:26 pm.
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Old 13th May 2021, 5:28 pm   #18
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by ms660 View Post
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.
Cheers Lawrence. But I think I've finally figured it out, See my above post.


Cheers
Jay
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Old 13th May 2021, 5:34 pm   #19
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Default Re: Selga 405 audio problem.

Quote:
Originally Posted by Radio Scotland View Post
Quote:
Originally Posted by ms660 View Post
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.
Cheers Lawrence. But I think I've finally figured it out, See my above post.


Cheers
Jay
Yes but if your measurement of 9 volts on the emitters was WRT B+ it means 9 volts across R26...

Lawrence.
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Old 13th May 2021, 7:43 pm   #20
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Default 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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