My first foray into the dangerous world of vintage "upcycling" – Panasonic T100 with Bluetooth.

Hi all, 

I wanted to share some shots and words on a project I completed recently. It was a departure from my usual repair café type work, as I was asked to take an existing (much-loved) radio set and to make it “useful” for the son and grand-daughter of the original owner. They specifically asked that cosmetic changes to the outside were minimised (more on that later). 

The radio in question is a 1965 Panasonic “National” T100 all-transistor radio. As you can see, it was beautifully clean outside (and mostly on the inside too) having been kept under a homemade cotton cover when not in use.

I took the guidance around “minimising the changes” to mean as little intervention as possible, both internally and externally, so with one exception where usability triumphed, all of the changes are relatively easy to reverse. 

The set is battery only, with a dual power setup with 6 1.5v D-cells to power the main set and a completely independent D-Cell that is there purely to power the dial/band lamps. 

D-Cells are not a popular or easily usable power source these days, so part of the modernisation would be to replace these with something more future-proof. After some tests, I determined to use a 3P pack of 2200mAh 18650 Cells, giving 6600mAh at ~3.7V. These would be rechargeable through a simple charging module. Two boost converters would provide the two voltage supplies I would need 9V for the legacy electronics and 5V for the Bluetooth receiver. The cells used are mounted in a spring case to be easily replaceable. I supplied 3 reclaimed cells that I had load tested to be balanced and over 95% of their rated capacity. They should give many years service, but are easily replaced if not.

At this stage, I guess it is worth pointing out that my electronics knowledge and skills are at the “able to diagnose and fix circuits” but not the “invent/create new circuits” level. I look forward to the foreshadowed videos from Mr Carlson’s Lab, which I hope will help me build up my knowledge to be able to design the two power supplies myself. For now, I am using mostly off-the-shelf modular components. 

When the back of the set is opened, we find the battery holder. This was rather corroded by past battery leaks to the point where one of the screws was all but eaten away. I removed the pack and kept it safely to one side; the large space would become home to all the modern invaders. 

The radio had arrived untested and in an unknown state. Before I went too far, I checked the performance. I injected 9VDC into the battery compartment and the radio came to life “sort of”. The radio was alive but not functioning at all well. While AM radio is rapidly dying in the UK, there are still enough stations around that you’d expect to find something. All I could find was a slight disturbance in the static now and then. A quick check of the germanium transistors in the RF stage showed that they were not producing the expected voltages. This made me a little concerned about the rest of the set. I applied a little heat to the RF transistors, which was enough to clear the internal shorts, at least for now. I was able to locate a couple of stations. I sent a message to the owner of the radio and was told that they did not really expect to use the radio as the reception was terrible where they lived and, of course, with the inevitable decline of AM stations. As such, they did not want me to attempt any restoration on the radio unless it affected the ability to use it with the planned “upgrade”. So for now, the old transistors remain. But what about the amplifier and output?

The T100 has a Pickup input, designed to allow you to connect your record player to the radio. This is presented as a 3.5mm jack socket, so I could connect a line-level input and listen to music streamed directly from my phone. This, of course, meant that one option was to do nothing but provide a 9V supply and an external line-in but that was not going to be a neat, portable solution and was quickly ruled out. The music quality was excellent for a radio of that age and the amplifier part of the set showed no signs of deterioration at all. 

I designed and printed a replacement mounting panel that used the existing mounting holes that the old battery box had used. This panel would hold the batteries and the charging circuitry. the Bluetooth receiver needed to be located away from sources interference if possible. As a result it mounts neatly on the side of the new panel, up against the plastic case of the radio. 

The receiver board is a widely available Bluetooth receiver module based on the Jielli Bluetooth chip. These are widely available but take care when buying them. It took me a number of attempts to get a board that was fit for purpose as many of them have a truly awful low quality voice sample for all the Bluetooth pairing prompts. If you can find the right ones, they will have a familiar boop-boop tone, similar to the Windows audio alert when adding a new USB device. From my experience, you cannot even trust the vendor descriptions in many cases. Amazon and eBay are populated by resellers who copy/paste the descriptions and don’t know whether their devices are “voice” or “tone” and often have contradictory text. 

The module is mounted inside the case (blue in the onShape CAD model) 

The case then blots to the “lid” which is fixed to the mounting board. This three-part design will hopefully allow me to adapt the case to other projects in the future where a redesigned mounting board can share the same Bluetooth housing. Nice plan, we’ll see how that fares in reality in the future no doubt.

I was quite pleased with the surgical introduction of the new components.

The audio signal from the Bluetooth module is line-level and thus ideal for attaching to the existing pick-up input. As it happened the pickup input socket had an unused normally closed terminal, when the jackplug is inserted it breaks that connection and passes the signal through a normally open contact pair that bridged by the jack. This neatly allowed the line-out from the BT module to use the normally closed path, leaving the jack socket to function should it be wanted in the future.

The power circuitry was more interesting that I had first expected. Firstly it is important that the bluetooth module can be turned off using the normal control knob, otherwise the drain will leave the battery performance lacking. Moreover, we want to be able to charge the batteries when the set is off, so the charging circuit needs to be independent of the on/off switch. this was relatively easy to achieve by placing the on/off switch just after the battery output stage, before both of the boost converters. The next complication was that in having two separate voltage supplies I had introduced a ground loop hum on the audio. To fix this a simple unregulated 5v DC isolator was used, giving me a cleaner and independent supply to the BT module. 

Finally, the one point that I argued myself back and forth over for long periods. The charging port. The client had already said that they were fine having a USB port, but I was unsure about whether I wanted to break my “all this can be undone” record. There was an option to place the port underneath the battery flap. This would mean no external port, but would also mean that the utility of the set was reduced. You had to have a coin/screwdriver to undo the panel , the panel would be open when charging, etc. etc. Another option was to sacrifice some of the external input/output ports on the thin slot above the battery panel, but as this was pop-rivetted metal there was no easy way to do this without risking a mess. In the end I bit the bullet and installed the port on the back panel. 

The final setup draws just 200mA when running at full volume. I had previously load tested the battery pack at 800mA and it ran for just under 8 hours

The final technical enhancement was to include an NFC tag inside the set (actually near the BT module) that allows a user to pair their phone by holding it to the side of the radio and a final procedural step was to update the schematics add a new page showing the revisions such that any future technician who has the bad luck to look inside this can hopefully understand what I did and no matter what else can rewind and start over if need be.

I think that’s all of the “interesting” parts. Sorry for the long post, I hope some find it interesting. I learned a lot during this exercise, I have no doubt my choices are far from perfect and I look forward to constructive feedback from you all. I look forward to seeing how Paul attacks the same problems and learning from that so that I can keep improving with future projects.

I am already seeing more and more enquiries from people, some ready to “bin” their old sets that are unable to serve their original purpose and I expect this trend to only increase as traditional radio broadcasting services die off and people want to keep hold of these design classics (and as in this case) try to get the next generation to value them too. Offering them choices such as this will save them from landfill.

One day, I hope to explore the possibilities of installing DAB+ modules in a similar fashion, opening up and even better upgrade route that retains the radio functions.

Neil