A couple of months ago, I bought a vintage 1956 Bush VHF90 tube AM/FM radio set in an antique shop in Hartlepool, Kiwi Trading.
The VHF90 was in reasonable physical shape, and the band switch was intact (a common problem area on these sets), however it was in unknown operating condition – the power cord didn’t look original, so it seemed that someone had worked on it in the past. The case had some minor cracks which had been repaired with epoxy.
The VHF90 is a reasonably complex mains-powered seven-tube superheterodyne radio receiver, with a channel switch (AM medium wave, and FM VHF) 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
I wanted to take a quick look inside the set before trying to power it up, to see if I could spot any obvious problems beforehand.
The VHF90 is quite an easy radio to disassemble, and only two flathead screws hold the rear cover 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, we get our first look inside the case – we can see that all the tubes are present, and that the mains cable has definitely been replaced in the past, as has the cabling on the speaker and audio output transformer. Someone has used offcuts of mains flex for this – it’s a pet peeve of mine to use standard wiring colours in non-standard applications, for example using an earth cable (yellow/green) to wire up the audio transformer when this chassis isn’t earthed at all, it’s a hot chassis set! Very unprofessional.
It’s possible that the yellow film capacitor on the back of the audio output transformer is not a factory part, also, and that it could have been replaced. The large electrolytic can is still connected, so have any other parts been replaced?
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).
I decided to use my dim-bulb tester (current limitation), variac (voltage limitation), and isolation transformer (mains isolation) to to bring up the radio in a safe manner – that way I could test it to see if there were any problems with the radio before its restoration.
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; a dim-bulb tester allows you to visually tell how much current is being drawn by the device-under-test, and and prevent a dead-short which can damage the device.
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 on both AM medium-wave and VHF FM, and seemed to draw about 60W.
With the rear cover removed, the chassis is held in place with the two front knobs (which just pull off) and two flathead screws – with these removed, it can be taken out through the rear, giving the first proper look at the chassis. In this case, it was reasonably dusty but appeared to be a relatively low-hour set, judging by the lack of wear on the tubes – also, all of the tubes were present and most of the parts seemed to be original.
Along with the new mains cable and speaker and audio output transformer cabling, there were several modifications to the chassis: there were eight axial polypropylene capacitors fitted (yellow/cream Vishay/ERD parts) which seem to have been replacements for several of the original factory paper capacitors, however most of the paper and electrolytic capacitors were still original; there was a lot of scorching in one corner of the underside of the chassis, where a power resistor seemingly once was fitted – this appears to have been moved to the top side of the chassis; the band switch (a common problem in the VHF90) had been modified to swap a damaged set of switches to a spare set of switches.
As the radio seemed to be working, I planned out the restoration as follows:
- Chassis cleaning and inspection.
- Electrolytic and paper/wax capacitor replacement.
- Cabling replacement, including mains cable and rubber cabling.
- 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.
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 rubber cables whose insulation had deteriorated and started to crack away.
The Bush VHF90 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 VHF90 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 six electrolytic capacitors in the QB20:
- 1 x 0.05uF 500V axial electrolytic.
- 1 x 1uF 350V axial electrolytic.
- 1 x 2uF 350V axial electrolytic.
- 1 x 5uF 50Vdc axial electrolytic.
- 2 x 50uF 275Vdc electrolytic (inside the large Dubilier metal can).
Components in the VHF90 are wired point-to-point, and the leads are usually wrapped around one another, which can make them quite difficult to remove even if you’re using a using a desoldering station.
Electrolytic capacitors are polarised, so make sure to install the new parts in the correct orientation as per the originals.
I removed and replaced the parts one-by-one, using cable sheathing where appropriate – I fitted 47nF 630V polypropylene axial part in place of the 0.05uF part, which required removing a vertical component holder (which I replaced the rubber cabling on at the same time), a 2.2uF 450V radial part in place of the 2uF part, a 4.7uF 50V radial part in place of the 5uF part, and two 47uF 400V aluminium axial parts in place of the 50uF parts in the can.
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 the originals all 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 two 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.
I decided to use the existing terminal tie strips on the underside of the chassis to attach the new electrolytic capacitors to, as there was plenty of space available.
Paper/Wax Capacitor Replacement
The VHF90 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 were only six paper/wax capacitors left to replace in the VHF90, all small Hunts parts which may have been overlooked by the previous owner; I also replaced a wax-coated Mica capacitor, as it was in poor physical condition:
- 3 x 0.02uF 150V (axial Hunts) – swapped for 22nF 630V polypropylene axial.
- 3 x 0.04uF 200V (axial Hunts) – swapped for 47nF 630V polypropylene axial.
- 1 x 556pF 1% 800V – swapped for 560pF 1kV ceramic axial.
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.
Cabling Replacement
An important safety consideration when refurbishing vintage radios is the mains cable – the original one on the VHF90 had already been replaced, but I still decided 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.
I also replaced the non-standard speaker and audio output transformer cabling, and all of the remaining old cracked rubber cabling – this was primarily under the tube sockets and to the dial lamps, some of the hottest areas of the chassis.
Case Cleaning & Polishing
The case was in good condition with some minor cracks, but was quite dirty and dull with age – everything was covered in a coarse dust.
I started with a thorough clean of the case interior and exterior 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.
Dial Lamp Replacement
During the course of testing, I also noticed that the dial lamps weren’t working – these are wired in series, so if one is open-circuit, neither would work.
The dial holders push-fit onto the chassis, and can be pulled off for better access.
The original bulbs simply unscrew from the holders – one of them looked and tested OK, the other was badly corroded and tested open-circuit.
I cleaned out and dried the bulb holders, seeing as one of the bulbs was corroded; I then fitted two new dial lamp bulbs (12V 80mA LES E5 screw-base), and they seemed to work.
Radio Reassembly
Reassembling the radio is just the opposite of its disassembly. The chassis is fitted back onto the front case, the front controls refitted, and the rear cover reinstalled.
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 40W, 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.
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