Help Needed - Identifying a KB Minuet Radio

[TheMightyMadman]

I'm almost finished with the initial rebuild on the QB20 - I'm just waiting on a 2.2uF 630V polyester film capacitor to arrive, then I'll put everything back together for testing.



In total, the parts replaced so far are: the mains cord, five electrolytic capacitors (C43, C45, C47, C49, C51), nine paper/misc capacitors (C7, C22, C26, C33, C44, C46, C48, C50, C52), and four resistors (R11, R25, R26, R27).

The electrolytics in the large can are out-of-circuit, and I've used standoffs to secure the new parts in place.

[Techman]

It's unusual to find a fusible resistor in a set like that. I suppose the manufacturer thought it was a good idea at the time and seemed like a good protective safety feature, but what it actually did was cause a reliability issue. Normally in that position in most sets, you find either no resistor, or a 'watty' surge limiter type. This is one position where ohms law doesn't really work when it comes to reliability. Whatever the calculation comes to I usually double it, and then even double it again. The higher the power rating of the resistor, the cooler it will run and the longer it will last. Unfortunately you can't do that with this set as the resistor needs to run right at its working limit so that it will fail if the current through it exceeds that which is specified. Fitting a higher power rated resistor may mean that it won't fail so quickly if there's a fault.

If this were my set then I'd be fitting a 5 watt wire wound type and be done with it. However, a word of warning, from the 1970s onwards this type of component would have been regarded as a safety critical device and it would have strictly speaking been illegal to replace it with a standard part. Back when your set was made this may not have been thought of so much, particularly as most other sets didn't have this component fitted anyway, but I need to tell you this just in case you go with the watty wire wound and a fault causes it to burn your house down!

[ms660]

Using the info in the manufactures service manual I did a calc. for the approximate power dissipated in a 22 Ohm surge limiter resistor for a DC load current of 75mA.

The result was 0.64 Watt, that equates to an RMS current to DC current ratio of 2.26 which given the receivers DC load current would be in the ball park.

Lawrence.

[TheMightyMadman]

Originally Posted by Techman

It's unusual to find a fusible resistor in a set like that. I suppose the manufacturer thought it was a good idea at the time and seemed like a good protective safety feature, but what it actually did was cause a reliability issue. Normally in that position in most sets, you find either no resistor, or a 'watty' surge limiter type. This is one position where ohms law doesn't really work when it comes to reliability. Whatever the calculation comes to I usually double it, and then even double it again. The higher the power rating of the resistor, the cooler it will run and the longer it will last. Unfortunately you can't do that with this set as the resistor needs to run right at its working limit so that it will fail if the current through it exceeds that which is specified. Fitting a higher power rated resistor may mean that it won't fail so quickly if there's a fault.

If this were my set then I'd be fitting a 5 watt wire wound type and be done with it. However, a word of warning, from the 1970s onwards this type of component would have been regarded as a safety critical device and it would have strictly speaking been illegal to replace it with a standard part. Back when your set was made this may not have been thought of so much, particularly as most other sets didn't have this component fitted anyway, but I need to tell you this just in case you go with the watty wire wound and a fault causes it to burn your house down!

Originally Posted by ms660

Using the info in the manufactures service manual I did a calc. for the approximate power dissipated in a 22 Ohm surge limiter resistor for a DC load current of 75mA.

The result was 0.64 Watt, that equates to an RMS current to DC current ratio of 2.26 which given the receivers DC load current would be in the ball park.

Lawrence.

Thanks both I've decided that instead of a fusible resistor, I'm going to go with a suitable (i.e. 160mA, 200mA, 250mA, etc) 5x20mm cartridge fuse and holder, inline with a 20R 6W wire-wound resistor - that way I don't have to worry about R27 burning out, I can easily control the maximum current out of the secondary of the mains transformer, and I can easily replace the fuse if necessary.

[TheMightyMadman]

Hi all,

I've finished the restoration on the KB QB20 Thank you for your help.



For R27, I decided to install a large 6W power resistor in series with a cartridge fuse and holder, instead of using a fusible resistor.



All of the electrolytic and paper/wax capacitors have been replaced, along with some resistors that had drifted in value.

[TheMightyMadman]

Hi all,

I've done a quick write-up for the restoration, see below if interested
https://retrorepairsandrefurbs.com/2...r-restoration/

Many thanks,

Adam

[Rich Woods]

Nice Job Adam, & what a nice set.

[TheMightyMadman]

Originally Posted by Rich Woods

Nice Job Adam, & what a nice set.

Thank you 😊