A couple of months ago, I bought a vintage 1958 Marconi T69DA tube AM/FM radio set in an antique shop in Marske-by-the-Sea, Vintage by the Sea.
The T69DA was in good physical shape, however it was in unknown operating condition.
The T69DA is a reasonably simple mains-powered six-tube superheterodyne radio receiver, with a channel switch (AM medium wave, and FM VHF) and internal AM bar antenna and FM dipole 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.
I planned out the restoration as follows:
- Chassis cleaning and inspection.
- Mains cable replacement.
- Electrolytic and paper/wax capacitor replacement.
- Case cleaning and polishing.
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 T69DA is quite an easy radio to disassemble, and only six 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 – the interior looks fairly original, albeit very dusty, however one of the valves was missing and it looks like the speaker cabling had been replaced at some point in the past.
Chassis Cleaning and Inspection
This is a series-string set, meaning that the heaters on each of the valves are connected and driven in series – if a valve is missing or has an open-circuit heater, the set will not work, so I needed to replace the missing valve before testing the set out.
Looking at the tube chart on the inside of the case, or the service manual, the missing tube was a UCH81 triode oscillator – I bought a new-old-stock replacement part on eBay.
I also removed the chassis from the case to give it a look over before attempting power-on, as well as a good clean, as it was absolutely filthy – the chassis is held into the case by the four front knobs (which simply pull off) and four flat-head screws at its front, as well as the three wires to the speaker transformer and two wires to the FM antenna connector.
This is a fairly unusual 1960s radio set as it features a printed circuit board (PCB) – valve radios from this era are typically wired point-to-point.
I pulled all of the tubes from their sockets, blasted off the worst of the dust with an air compressor, cleaned the remaining dirt off with degreaser and a toothbrush, then cleaned all of the tube sockets out with contact cleaner.
The tuning capacitor was also completely seized up, so I lubricated it with WD-40 and exercised it in each direction until it freed up and moved smoothly; I also cleaned the power switch, volume control, and tone control with contact cleaner.
An important safety consideration when refurbishing vintage radios is the mains cable – the original one on the T69DA was in reasonable condition and probably wasn’t original, 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). I desoldered the original cable from the power switch terminals, taking note of the polarity of the live and neutral connections, then soldered in the new cable.
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 reattached the speaker and brought the radio up gradually on the variac to give the capacitors time to reform, and whilst the dial lamps lit up and the heaters on all the tubes started to glow, there didn’t seem to be any noticeable audio output.
I didn’t see anything obviously wrong, so carefully started tapping components to see if there were any obvious cold solder joints – sure enough, one of the main dropper resistors would cause the radio to start and stop outputting audio when tapped.
I therefore reflowed the solder joints on the large power resistors, electrolytic capacitors, and tube sockets – this takes time, particularly with the power resistors which sink a lot of heat from your soldering iron before their legs properly start to take solder.
With this done, the radio now seemed to work OK.
I then took notes of all of the parts that would need to be replaced, including several electrolytic capacitors and several paper/wax capacitors.
The Marconi T69DA 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 T69DA 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 T69DA:
- 1 x 4uF 100V axial electrolytic.
- 1 x 10uF 25Vdc axial electrolytic.
- 1 x 32uF 275Vdc electrolytic (inside the large Plessey metal can).
- 2 x 40uF 275Vdc electrolytic (inside the large Plessey metal can).
Electrolytic capacitors are polarised, so make sure to install the new parts in the correct orientation as per the originals – I typically remove and replace parts one-by-one, then double-check their capacity, voltage rating, and orientation afterwards.
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 standard 47uF capacitors for the non-standard 40uF capacitors, 33uF for the 32uF, and 4.7uF for the 4uF. 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 new part 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 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 underneath – 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 damage.
Unfortunately, due to the PCB in the T69DA there is limited room underneath the large can to fit replacement parts, so I removed the can and mounted them on top instead.
Paper/Wax Capacitor Replacement
The T69DA 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 microphonic, 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 three paper/wax capacitors left to replace in the T69DA:
- 1 x 0.02uF 150V (axial Hunts) – swapped for 22nF 630V polypropylene axial.
- 1 x 0.03uF 200V (axial Hunts) – swapped for 33nF 630V polypropylene axial.
- 1 x 0.04uF 200V (axial Hunts) – swapped for 47nF 630V polypropylene 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.
Case Cleaning & Polishing
The case was in good condition, but was quite dirty and dull with age.
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.
The case on the T69DA is a painted bakelite, and although this cleaned up well despite minor marks, it was still quite dull – I decided to use a microfibre cloth and some car bodywork polish to polish it up, and it came up quite beautifully.
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.