1959 Kolster Brandes QB20 “Minuet” Bakelite Vacuum Tube Radio Repair & Restoration

A few weeks ago, my mother-in-law spotted a vintage 1959 Kolster Brandes QB20 “Minuet” tube AM/FM radio set in a local charity shop, and – knowing that I have a growing interest in radios – very kindly bought it for me for £10.00 (even though it wasn’t technically for sale, but was being used as window dressing).

I’ve been enjoying vintage radio restoration for some time now (after watching YouTube channels such as the excellent Mr Carlson’s Lab), as I find the electronics fascinating. At the time of writing, this radio is (currently) the second-oldest in my collection, beaten only by my 1956 Philips 353A tube radio.

The QB20 was in quite rough physical shape, and the power cord had had been cut off and pushed through the back of the case, so it was in unknown operating condition – this is probably a good thing, though, as powering up vintage electronics like this without the adequate precautions (especially after a long period of disuse) can lead to potentially catastrophic damage. Everything else looked original, except for the two front knobs for the power/volume control and the band switch, which were generic aftermarket control knobs – whilst these weren’t exactly ugly, they were not in keeping with the styling of the radio.

The QB20 is a reasonably complex mains-powered five-tube superheterodyne radio receiver, with a channel switch (AM medium wave, AM long wave, and FM) and internal AM antenna, and connectors for an external FM antenna.

If you’re interested in how “superhet” radios work, “Transistor Superhet Receivers” by none-other than Sir Clive Sinclair himself is an excellent book.

Radio Disassembly & Inspection

Electronics of this vintage usually contain components that frequently deteriorate or fail with age, and can (at best) cause damage to the tubes and other circuitry if left unchecked – radios from this era therefore need to be inspected and serviced as required before being powered up directly from a mains supply (without current/voltage limitation).

The QB20 is quite an easy radio to disassemble, and only three slotted screws hold the Bakelite rear case on – some sets of this era contain asbestos so must be handled with extreme care, however as far as I am aware this radio does not.

With the rear case removed, the front case and chassis can be taken out through the front, giving the first proper look at the chassis. In this case, it was reasonably dusty but appeared to be a relatively low-hour set – also, all of the tubes were present and (almost) everything seemed to be original.

This chassis is a hybrid, having both an early single-sided printed circuit board and some point-to-point wiring for the bulk capacitors and audio output tube.

I wanted to test out the set before making any modifications, so I could more easily diagnose any existing problems as well as any that might be introduced through the restoration work. Before attempting any power-on, though, I looked over the chassis to check for any immediate or obvious problems.

The tell-tale sign that someone had worked on this set in the past was a blue bodge wire bypassing a wire-wound resistor next to the rectifier – more on this later.

Aside from this bodge wire, the chassis looked original, and – aside from the number of unreliable capacitors littering the board – everything looked OK.

Now that I was reasonably happy that there wasn’t anything obviously wrong, I wanted to power up the set in a controlled manner – I attached a temporary power cord, plugged the radio into my variac, connected that to my isolation transformer, then connected that to the 230V 50Hz mains supply.

A variac allows you to control the output voltage, which is very useful for slowly bringing up old electronics, and allowing components (primarily capacitors) to reform; an isolation transformer is an important safety device which provides galvanic isolation between a device that you are working on and the mains supply, as well as limiting current.

I brought the radio up gradually on the variac to give the capacitors time to reform, and aside from a very scratchy volume control potentiometer, it seemed to work rather well – at least on AM medium-wave, which is all I could seem to get reception for in my shed.

As the radio seemed to be working, I planned out the restoration as follows:

• Chassis cleaning and inspection.
• Electrolytic and paper/wax capacitor replacement.
• Mains cable replacement.
• Case cleaning and polishing.

Chassis Cleaning and Inspection

I took this opportunity to clean up the chassis, in order to get a good look at everything that needed to be worked on – this meant removing it from the plastic case front.

The two front knobs are held on with small grub screws; the tuning capacitor is directly driven from the tuning dial (so no need for tuning cord replacement on this set), and the dial simply pulls off the front; the red indicator is clipped around the tuning shaft.

The chassis is held onto the front panel with five slot-head screws and one hex screw – with these removed, the chassis can be lifted out.

The front speaker is permanently attached to the chassis via four wires (black, blue, orange, and red) which are soldered underneath the electrolytic capacitor can – to remove the chassis fully, these should be desoldered, but be sure to take lots of photos of where to reconnect them when you’re done.

Removing the chassis reveals the underside of the circuit board.

I blew the worst of the dust and muck off with compressed air; then, I cleaned the chassis, components, and tubes using a general degreaser and a microfibre cloth, carefully removing the tubes to do so.

I cleaned all of the tube sockets and controls with contact cleaner; I found that the band selection switch was quite stiff, so I soaked the contacts with standard WD-40 until they freed up, then worked the switch through all of its positions to clean them up; I also cleaned the noisy volume control with contact cleaner.

I then took notes of all of the parts that would need to be replaced, including several electrolytic capacitors, several paper/wax capacitors, and several resistors.

The Kolster Brandes QB20 has a very useful service manual, which includes a parts list and a chassis layout, which can be very helpful during servicing, particularly for identifying potentially problematic components.

Electrolytic Capacitor Replacement

Aluminium electrolytic capacitors are commonly used for filtering, smoothing, and decoupling in both high- and low-voltage electronics. They typically comprise aluminium windings which are coated with a liquid electrolyte, which can dry out over time (negatively affecting the performance of the capacitor, often causing them to fail dead-short), or leak out and cause corrosion to the PCB and surrounding components.

The QB20 is a 1950s radio set, and as such was made with 1950s electrolytic capacitors – these are far inferior to modern equivalents, and coupled with their advanced age, they should ideally be replaced for longevity purposes.

Electrolytic capacitors are polarised, meaning that they must be installed in the correct orientation, otherwise bad things usually happen. It is therefore important to take particular care to ensure that the value, voltage rating, and orientation of the new capacitor are correct – this is more difficult with point-to-point wiring as the correct connection points aren’t obvious and the polarities aren’t marked, so it’s important to take plenty of “before” pictures to refer back to later on.

There are only five electrolytic capacitors in the QB20:

• C45: 40uF 350Vdc electrolytic (inside the large Dubilier metal can).
• C49: 40uF 350Vdc electrolytic (inside the large Dubilier metal can).
• C51: 20uF 350V electrolytic (inside the large Dubilier metal can).
• C43: 2.2uF 150Vdc electrolytic (gold radial capacitor).
• C4: 50uF 12Vdc electrolytic (yellow radial capacitor).

The two axial electrolytic capacitors wired to the mainboard are easy enough to remove and replace, especially when using a desoldering station. Electrolytic capacitors are polarised, so make sure to install them in the correct orientation.

I removed and replaced these one-by-one, using cable sheathing where appropriate – I fitted a, using a 2.2uF 250V metallised film part in place of the gold capacitor, and a 47uF 25V aluminium axial part in place of the yellow capacitor.

The new parts are significantly physically smaller than the originals, which just goes to show how far capacitor technology has advanced over the past 70 years.

It’s also difficult to get exact matching replacements for some of the values – for example, I substituted a standard 47uF capacitor for the non-standard 50uF capacitor. This kind of substitution should be okay for electrolytic capacitors, as they typically have a wide manufacturing tolerance anyway (up to 20%). It is important to use a capacitor with the same or higher voltage rating as the original.

It was definitely worth replacing these, as both of the originals tested as very leaky (electrically) using my ESR meter – they had basically started to turn into resistors over time, and would no longer properly block DC.

As for the three capacitors in the large can, there are several schools of thought on how to replace these: some people remove the capacitors inside the original can and re-stuff it using new capacitors; some people install new capacitors, and remove the can entirely; some people remove the old can from circuit, and install new capacitors.

My preferred method is to remove the can from circuit, and install new capacitors – this means that the radio chassis maintains its stock look from the top, but is easy to service in future. Either way, it is extremely important to remove the original capacitors from the circuit (not just solder the new capacitors across them), as the originals can become physically leaky and affect performance, or fail dead-short and cause radio damage.

Typically, the outside of the can is connected to the negative side of the capacitors inside it, and there is a terminal for the positive side of each capacitor on the bottom of the can.

Space and mounting points are limited on the underside of this chassis, so I decided to use the solder tabs on the original capacitors as mechanical mounting points, by drilling them out and attaching nylon standoffs through the holes.

The new components could then be soldered to screws fitted into the nylon standoffs. In this case, I used two 47uF 400V radial parts, and one 22uF 400V radial part.

Paper/Wax Capacitor Replacement

The QB20 also uses paper/wax capacitors – these antique capacitors are literally made from rolled-up paper and covered in wax, and like the electrolytic capacitors are also prone to failure from advanced age, including electrical leakage or open/short circuits.

These are far inferior to modern equivalents – most notably polypropylene, polyester film, and metallised film capacitors (ceramic capacitors are micro-phonic, so are generally not suitable for use in these circuits) – and like electrolytic capacitors these should ideally be replaced for longevity purposes.

There are eight paper/wax capacitors in the QB20; I also replaced a ceramic capacitor next to the rectifier, as it was in poor physical condition:

• C7 – 0.01uF 350V (axial Hunts) – 10nF 630V polypropylene axial.
• C22 – 0.01uF (10%) 400V (axial Hunts) – 10nF 630V polypropylene axial.
• C26 – 0.05uF 350V (large Plastiseal) – 47nF 630V polypropylene axial.
• C33 – 0.04uF 150V (axial Hunts) – 47nF 630V polypropylene axial.
• C44 – 0.01uF (10%) 400V (axial TOC) – 10nF 630V polypropylene axial.
• C46 – 25pF 750V (underneath audio output tube) – 22pF 1kV ceramic.
• C48 – 0.001uF 350V (underneath electrolytic can) – 1nF 630V polypropylene axial.
• C50 – 25pF 750V (underneath audio output tube) – 22pF 1kV ceramic.
• C52 – 0.03uF 500V (ceramic next to rectifier) – 33nF 630V polypropylene axial.

Some of the paper/wax capacitors are wired point-to-point, and are easy enough to remove and replace – these typically have their leads wrapped around one another for mechanical strength, so this needs to be unpicked while desoldering. The capacitors wired to the mainboard are easy enough to remove, especially when using a desoldering station.

I removed and replaced the parts one-by-one, making sure to keep their leads as short as possible, using the parts listed above. I found it helpful to make annotations on some of my “before” pictures to ensure that the new capacitors were installed correctly. Where necessary, I installed heat-resistant cable sleeve to prevent shorts.

Resistor Replacement

Some vintage resistors are prone to drifting in value over time, and may need to be replaced – the QB20 also has a fusible resistor (R27) on the output of the mains transformer, which is apparently a common failure and burns out as the set ages.

Three resistors had drifted significantly in value: R11 (33 kOhm 1W) measured 44 kOhm, and appeared to have some kind of failing wax coating, so was replaced with a 33 kOhm 1W part; R25 (470 Ohm 0.5W) measured 630 Ohm, so was replaced with a 470 Ohm 1W part; R26 (820 Ohm 10% 0.5W) measured 1.03 kOhm, so was replaced with a 820 Ohm 1W part.

R27 is for surge limiting, and also to protect the mains transformer from high current draw (i.e. a short-circuit somewhere) as it will burn out like a fuse would. In this set, it had failed open-circuit at some point, and someone had bypassed it with a bodge wire – however, overriding a safety feature like this is not a good idea, so I wanted to reinstate it. The datasheet doesn’t specify the power rating, unfortunately – I tried working out the power rating required for a reasonable current draw (i.e. 150mA), but I figured that it would be easier and more service-friendly if I fitted a 20Ohm 6W power resistor in series with a cartridge fuse (i.e. 160mA slow-blow) and holder.

Case Cleaning & Polishing

The Bakelite rear case was in very good condition with no scratches or cracks, but was quite dirty and dull with age; the plastic front case was in quite good condition, but was extremely dirty and covered in a coarse dust.

The speaker is held into the front case with one screw and bracket – this was also filthy, and needed to be cleaned extremely carefully using a dry toothbrush.

I started with a thorough clean of both case pieces using degreaser and a microfibre cloth, to get rid of the worst of the dirt – the front case also needed a lot of time spent with a toothbrush and cotton buds, to get into all of the grilles and crevices.

Then, I polished the rear case piece thoroughly using Brasso wadding until it had a nice, deep shine – it’s important not to cut too deep into the Bakelite, as you can expose the filler material (which sometimes contains asbestos).

Mains Cable Replacement

An important safety consideration when refurbishing vintage radios is the mains cable – the original one on the QB20 had been cut off, so I had little choice but to replace it with a modern moulded cable with 1A fused UK plug (in this case, I just bought a 2m figure-eight lead and cut the end off).

To prevent strain on the mains cable, I fitted it through the rear case and tied a knot in it. I then desoldered the original cable from the power switch terminals, taking care to note the polarity of the live and neutral connections – installing the new cable was then just the reverse of removing the original.

Radio Reassembly

Reassembling the radio is just the opposite of its disassembly.

The chassis is fitted back onto the front case, the speaker reattached, and the front controls refitted; then, the whole assembly can be installed in the rear case. I took this opportunity to replace the non-original front knobs that the set came with, using some high-quality reproduction chicken-head knobs with a similar colour and style to the case.

Radio Testing

For the first power-on following the restoration, I used the dim-bulb tester that I made (which would limit the current drawn by the radio in case of a problem such as short-circuit, hopefully avoiding potential damage) connected to my 300W mains isolation transformer via a mains power meter for safety and consumption monitoring.

When powered up from the limited supply, the dial lamp glowed dimly and the tube heaters seemed to light up okay – the 60W bulb on the dim-bulb tester didn’t light up noticeably and the power meter was showing about 30W, so there didn’t seem to be any immediate issues. I bypassed the bulb, and the radio seemed to work great.

Overall, I’m extremely happy with the outcome of this restoration.