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Old 15th Jan 2023, 8:52 pm   #15
TheMightyMadman
Triode
 
Join Date: Aug 2022
Location: Stockton-on-Tees, Cleveland, UK.
Posts: 39
Default Re: Help Needed - Identifying a KB Minuet Radio

Quote:
Originally Posted by Techman View Post
Quote:
Originally Posted by TheMightyMadman View Post
For R27, I've ordered a 22Ohm 1W fusible resistor to fit as a replacement.
I thought you said it was 20 ohms

I wouldn't have thought 1 watt would be high enough in that position and will just burn out again in a very short time.

The main smoothing electrolytic can is probably fine and replace only if proven to be faulty.

The very low value flat type capacitors are likely to be of a type that are not normally known to be faulty.
Quote:
Originally Posted by Techman View Post
Just to carry on from the above - You've had the set running and it's fine on AM, but no FM. There's no mains hum and the metal can of the main smoothing capacitor remains cool, with no apparent warming after a few minutes of operation, so it's likely to be fine.

No VHF/FM is often down to low emission of the first stage valve (note that it's a 'valve' here in the UK and not a tube, which is an Americanism popularly used by the 'cool' YouTube kids to try to sound 'cooler' than they really are).

I'm not familiar with this set at all and haven't got the circuit in front of me, but regarding that resistor that's burnt, try the one that you've ordered and see how it goes, but I think I would have fitted one of those square white 5 watt wire wound types. Does it actually say in the service sheet that it's a fusible type? See what other folks think before going any further with it.

I think you may end up needing to take voltage readings and then compare them with those stated in your service information, or from online valve data to see the real state of play with the set.
Quote:
Originally Posted by Techman View Post
Just looking at some other questions you asked - Those round black cased capacitors are paper Hunts and will all be faulty in some way. Depending on where they are in the circuit, the fact that they have leakage may or may not matter. If they're in a place that matters, such as coupling to the output valve control grid, it can be an absolute disaster, so replace immediately!

Replacing the 2uf electrolytic with a 2.2uf non-electrolytic type of a higher voltage will be just fine.
Thank you very much for the info

The original R27 was 20 Ohm, and I've ordered a 22 Ohm replacement as it's a more standard value - I don't think the value has much importance as it seems they're just using it as a fuse for the output of the mains transformer (the service manual states "NOTE: the 20 ohm wire wound resistor in series with the metal rectifier is of a type which fuses when excessive H.T. current flows"). I'm actually even starting to think that the 1W rating that I've ordered will be too much, as if the service manual is claiming a 75mA normal draw from the HV, then power dissipation for the resistor itself is 0.075 * 0.075 * 22 = 0.124 W (approx. 1/8 W). I'm tempted to just replace R27 with a standard slow-blow cartridge fuse with an appropriate rating, to be honest - I'll probably measure the normal operating current and select an appropriate rating. When I first got the set, R27 was burnt out and bypassed by a length of wire, and the set worked just fine.

Thank you for the info regarding the lack of VHF/FM, I'll do some more detailed digging once I've replaced all of the potentially problematic parts.

I'll also replace those Hunts capacitors, in that case - thanks.

Noted regarding the replacement of the 2uF electrolytic, also.
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