Help Needed - Identifying a KB Minuet Radio

[TheMightyMadman]

Hi all,

I've been starting to get into vintage radios, moving from vintage computers and other equipment. I recently restored a 1956 Philips 353A:
https://retrorepairsandrefurbs.com/2...r-restoration/

Family members very kindly bought me a KB (Kolster Brandes Ltd.) "Minuet" vacuum tube radio from a charity shop, which was a lovely surprise!

I can't find much information about these. Does anyone know the specific model from the pictures below, please?

From the outside it looks like an OB10, but it also has a "VHF" selection and the internals are different:
https://www.radiomuseum.org/r/kolste..._ob10ob_1.html

The insides look like a QB20 which also has VHF, but the case and dials are different:
[url]http://www.kbmuseum.org.uk/kb_images/qb20/qb20.htm[/ur]

Many thanks,

Adam

[Paul_RK]

As far as I can tell it's a standard QB20. Mike Edwards' note at the KB Museum mentions his having replaced the grey case the set there came in with a red one: I think the two black knobs on your example are non-original and of later manufacture than the set.

Paul

[joebog1]

The black knobs are standard Asian general purpose knobs. I have a large bag of them from Ebay.

Joe

[Cathovisor]

Very much the wrong knobs on that Minuet.

I still think the versions that had a dial light were rather clever.

There are three distinct versions of the Minuet: AM-only, FM-only and AM/FM. As they are small, I have one of each

[TheMightyMadman]

Thanks all for your help, it's much appreciated It's nice to know that it is in fact a QB20.

It is a shame about the knobs, I would quite like to get some originals, or even just some white chicken-head knobs with gold embossing. Can anyone recommend any suppliers for these, please? Are they usually just a standard fitting, or will I need to check? Thank you

The FM portion doesn't seem to work, unfortunately, even with an external dipole antenna attached. I've noticed that R27 (20 Ohm wire-wound fusible resistor) appears burnt out, and has been bypassed with a length of wire.

Does anyone know what kind of power rating a suitable replacement should have, so I can get one on order? The schematic (below) says there's 75mA into the speaker coil plus any AC to ground through R27, so is it just V = IsquaredR? The power rating doesn't seem to be in the service manual.

Does anyone know why R27 would have burnt out, is this a common problem? Could it be due to a leaky C51 or C52 causing heavy HT current draw? Could someone bypassing the fuse have damaged part of the EABC80?

Many thanks,

Adam

[TheMightyMadman]

Originally Posted by TheMightyMadman

Thanks all for your help, it's much appreciated It's nice to know that it is in fact a QB20.

It is a shame about the knobs, I would quite like to get some originals, or even just some white chicken-head knobs with gold embossing. Can anyone recommend any suppliers for these, please? Are they usually just a standard fitting, or will I need to check? Thank you

The FM portion doesn't seem to work, unfortunately, even with an external dipole antenna attached. I've noticed that R27 (20 Ohm wire-wound fusible resistor) appears burnt out, and has been bypassed with a length of wire.

Does anyone know what kind of power rating a suitable replacement should have, so I can get one on order? The schematic (below) says there's 75mA into the speaker coil plus any AC to ground through R27, so is it just V = IsquaredR? The power rating doesn't seem to be in the service manual.

Does anyone know why R27 would have burnt out, is this a common problem? Could it be due to a leaky C51 or C52 causing heavy HT current draw? Could someone bypassing the fuse have damaged part of the EABC80?

Many thanks,

Adam

By the way, I previously attached links to the service manual and screenshots of the service data (with watermarks intact), and these seem to have been removed. Is this against the rules? It will be very difficult for anyone else to help troubleshoot without this information.

[David G4EBT]

Originally Posted by TheMightyMadman

By the way, I previously attached links to the service manual and screenshots of the service data (with watermarks intact), and these seem to have been removed. Is this against the rules? It will be very difficult for anyone else to help troubleshoot without this information.

Rule B 'Posting Messages' states:

8: 'Do not post or request service data, or links to service data, if it is for sale at www.service-data.com. The service data website funds this forum. You may post a small section from published data (such as a section from a circuit diagram or other diagram) to illustrate or help with a discussion'.

[Cathovisor]

Originally Posted by TheMightyMadman

Thanks all for your help, it's much appreciated It's nice to know that it is in fact a QB20.

It is a shame about the knobs, I would quite like to get some originals, or even just some white chicken-head knobs with gold embossing. Can anyone recommend any suppliers for these, please? Are they usually just a standard fitting, or will I need to check? Thank you

The FM portion doesn't seem to work, unfortunately, even with an external dipole antenna attached. I've noticed that R27 (20 Ohm wire-wound fusible resistor) appears burnt out, and has been bypassed with a length of wire.

Does anyone know what kind of power rating a suitable replacement should have, so I can get one on order? The schematic (below) says there's 75mA into the speaker coil plus any AC to ground through R27, so is it just V = IsquaredR? The power rating doesn't seem to be in the service manual.

Does anyone know why R27 would have burnt out, is this a common problem?

Once upon a time I knew where there was a plentiful supply of the correct knobs: Dulwich. Sadly gone now.

The first time I put my hands in one of these it'd be the early 80s and that same resistor had expired. Never did find out why...

[TheMightyMadman]

Originally Posted by David G4EBT

Rule B 'Posting Messages' states:

8: 'Do not post or request service data, or links to service data, if it is for sale at www.service-data.com. The service data website funds this forum. You may post a small section from published data (such as a section from a circuit diagram or other diagram) to illustrate or help with a discussion'.

Ah, I see - thank you for clarifying, I must have forgotten about that one. In that case, my sincere apologies.

[TheMightyMadman]

Originally Posted by Cathovisor

Once upon a time I knew where there was a plentiful supply of the correct knobs: Dulwich. Sadly gone now.

The first time I put my hands in one of these it'd be the early 80s and that same resistor had expired. Never did find out why...

Thank you

[TheMightyMadman]

Hi all,

I've been working on identifying which capacitors need replacing in the QB20.
I think I've found all of the obvious electrolytic and paper capacitors, but I have some questions, please see below.

#1: this square Erie capacitor, is this ceramic, and will therefore be OK?


#2: there are two 25pF 750V capacitors underneath the audio output tube, which are rectangular, flat, and coated in wax. Are these paper capacitors, or else, do they need to be replaced? Would 22pF 1kV ceramics be a suitable replacement?


#3: there are a few small, cylindrical, black or white capacitors dotted around the board. What kind are these? Do they need to be replaced?


#4: one resistor is a type I'd not seen before, it appears to be wax coated and is not in good shape. If the bands are right it's supposed to be a 33k but it's measuring 44k out-of-circuit. Would a 33k 1W metal film resistor be a suitable replacement?


#5: C43 was a 2uF 150V electrolytic, would it be OK to swap this out for a 2.2uF 630V polypropylene capacitor?

For R27, I've ordered a 22Ohm 1W fusible resistor to fit as a replacement.

I've also ordered a couple of sets of different knobs, to try some out and see which ones look and fit the best.

Thanks in advance

[Techman]

Originally Posted by TheMightyMadman

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.

[Techman]

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.

[Techman]

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.

[TheMightyMadman]

Originally Posted by Techman

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.

Originally Posted by Techman

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.

Originally Posted by Techman

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.

[joebog1]

I think you will find that 20 ohms or 22 ohms might also set the bias current for the output valve.
I would double check that by the circuit diagram. If you cannot deduce that yourself, it might be correct to scan just that part of the circuit and post it here for help/discussion.

Joe

[Cathovisor]

Originally Posted by joebog1

I think you will find that 20 ohms or 22 ohms might also set the bias current for the output valve.

No, it's merely a fusible resistor in the anode of the rectifier, so it acts as both fuse and surge limiter. 22Ω will be fine in that position.

[TheMightyMadman]

Originally Posted by joebog1

I think you will find that 20 ohms or 22 ohms might also set the bias current for the output valve.
I would double check that by the circuit diagram. If you cannot deduce that yourself, it might be correct to scan just that part of the circuit and post it here for help/discussion.

Joe

Originally Posted by Cathovisor

No, it's merely a fusible resistor in the anode of the rectifier, so it acts as both fuse and surge limiter. 22Ω will be fine in that position.

Thanks both

I'm going to try the following 0.25W 22R 250V fusible resistor, which should allow up to approx. 107mA from the secondary of the mains transformer (which should only be about 75mA during normal operation according to the service manual).
https://www.ebay.co.uk/itm/134127518196

If this runs too hot or burns out, I'll bump up the power rating to 0.33W (122 mA) or 0.5W (150mA) as needs be.

[ms660]

Originally Posted by TheMightyMadman

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).

The current flowing through the surge limiter resistor will be considerably more than the DC current supplied to the load, that's due to the nature of the rectifier circuit (capacitor input filter) eg: the rectifier is directly connected to a large value capacitor (the reservoir capacitor) as a result of that the current flowing through the rectifier and the resistor will be in the form of a high peak current pulse of relatively short duration, the RMS value of that current will far exceed the DC current supplied to the rest of the receiver.

A good rule of thumb is to double the value of the HT current and use that to calculate the power dissipated in the resistor.

Lawrence.

[TheMightyMadman]

Originally Posted by ms660

The current flowing through the surge limiter resistor will be considerably more than the DC current supplied to the load, that's due to the nature of the rectifier circuit (capacitor input filter) eg: the rectifier is directly connected to a large value capacitor (the reservoir capacitor) as a result of that the current flowing through the rectifier and the resistor will be in the form of a high peak current pulse of relatively short duration, the RMS value of that current will far exceed the DC current supplied to the rest of the receiver.

A good rule of thumb is to double the value of the HT current and use that to calculate the power dissipated in the resistor.

Lawrence.

Thanks Lawrence In that case, I'll bump up to a 0.5W resistor for R27.

I've bought some original-style chicken-head knobs for the front controls, and they look far better than the ones that were on there before!
Everything has cleaned up quite nicely.

https://www.ebay.co.uk/itm/155130820342