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Old 23rd Nov 2023, 9:12 pm   #1
Cokeoak
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Unhappy Braun SK61 FM issue

Hi there,

I’m looking for some help… any help…

I bought a Braun SK61 recently to have a go at restoring - Turntable works beautifully, LW and MW both getting signals although a little buzzy, like something isn’t grounded. Unfortunately the FM channel is very low in volume and can’t pick up any channels. I wondered if anyone had any idea what this may be. I’ve fixed a few old synthesisers, so I know a little about electronics, but not a lot. Any help would be much appreciated.
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Old 24th Nov 2023, 5:20 am   #2
Kentode
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Default Re: Braun SK61 FM issue

A quick look at the circuit diagram shows an external FM aerial needs to be in place, have you got one connected?
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Old 24th Nov 2023, 11:31 am   #3
Mr 1936
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Default Re: Braun SK61 FM issue

Hi

A quick look shows this to be a 1960's design using valves, probably the last iteration before transistors came along. I think this is one of a series of designs all nicknamed "Snow White's Coffin"

As above, the first thing to try is connecting a cheap "ribbon" style FM antenna to the appropriate 2 pin socket, which will be labelled FM or possibly UKW (= Ultra Short Waves)

AM these days suffers from the twin problems of houses being stuffed full of interference generating electronics and broadcasters closing down transmitters that cost too much to run and maintain, so don't expect much !

If you end up digging deeper, beware of the safety implications of working on valves and their associated high voltages.
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Old 24th Nov 2023, 2:44 pm   #4
Cokeoak
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Default Re Braun SK61 FM issue.

Hi, Thanks so much for reply. no external aerial, but I've ordered one so I'll give that a try. Sorry for my ignorance. Would that relate to the low volume on FM /UKW channel too?

Hi Mr 1936, I will try an ext aerial. I purchased some insulated gloves (1000v) to work on it... and won't be touching it when it's plugged in... just to protect my lack of knowledge.
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Old 27th Nov 2023, 3:49 pm   #5
Cokeoak
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Default Re: Braun SK61 FM issue

Ok - great news! FM now works with an ext antenna! what an idiot i am. I have some repair to do on the wooden side panels and then it’ll be great. Out of curiosity- am i being dumb to assume i can carefully remove the main chassis and the power transformer safely with insulated screwdriver and these insulated rubber gloves? : Ansell ActivArmr E014Y Electrician 1000V Insulating Natural Rubber Latex Gloves. After all the horror stories I’ve read online - i’m being overly cautious and doing my research.

Can’t thank you enough for your help.
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Old 27th Nov 2023, 10:47 pm   #6
Simon Gittins
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Default Re: Braun SK61 FM issue

It's better to be over cautious than the opposite.

The usual approach is to recognise the large capacitors in the power supply which may still be charged when the set is unplugged, then avoid touching them until you have confirmed with a meter that they have had time to discharge.

There are also checks that need to be made as capacitor failure can cause expensive damage.

This section covers the basics, the sections on power supplies and applying power for the first time will be of help:
https://www.vintage-radio.com/repair...ion/index.html
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Old 28th Nov 2023, 10:48 am   #7
Mr 1936
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Default Re: Braun SK61 FM issue

Hi again

Glad to hear that you have FM up and running. If the set is unplugged the charge in the capacitors will normally decay away to a safe level if you leave it for a few minutes, but it's wise to check with a voltmeter that this has happened.

You may read about capacitor replacement being needed on equipment like this, because some types of capacitor absorb moisture over decades and go electrically leaky. Whether this is needed depends on several factors, such as:

Are the capacitors waxed paper inside ? By the 1960s plastic dielectric types were being introduced which are less troublesome.

Are you mending it for someone else and want to guarantee that you won't be revisiting it in 6 months time ?

Would something be damaged if a capacitor failed ? From what I can see of the schematic the output valve control grids have resistors to ground which makes them less susceptible to bias voltage upset if the audio coupling capacitors from the previous stage go leaky.

If you do go ahead with capacitor replacement, I advise changing one at a time and testing in between.
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