Yes that is it. If you zoom in you can see the bulge I mentioned. The black thing is a rubber cap on the top of the earphone jack which is now removed. I will re install the white cap and work out how to bridge the missing earphone socket. hopefully I can get back to at least getting stations at the volume pot.

Yep that's it. If you zoom in you can see the bulge I was on about. The black thing is a rubber washer at the top of the earphone socket which is now removed. I have just finished hand drawing an almost complete schematic from the tiny damaged one inside the back cover. I don't think it will help me much but I did find a component which had the symbol of a resistor surrounded by an elongated circle.
Marked T05 100. Could this be a thermister as suggested by Robbbert? Have to admit I do not know what a thermister is or what it does so I have some reading ahead of me. I have replaced the white cap and re soldered my earlier work. Happy to say the radio is working again as before. That is a nice clear audio output from the volume pot. When I get a little further along the circuit the loud hum appears. So now I will try to figure out how to bypass the earphone jack and then test the audio transformer and speaker.

What are you powering it with? If its a DC battery there should not be hum. An AC plug pack, or similar, can be dirty and cause hum, or your fingers in the circuit, inject hum.

If it has Germanium Transistors in the output Thermistors were not uncommon as that variety of transistors had a bad habit of thermal runaway.

That probably has a 2.5mm audio socket, of three wires. One is the body (ground) the other two can likely shorted together, off the socket.

Within that transistorised stuff, unlike tubes, Electrolytic caps were often used as coupling caps, between stages. In car radios in particular, they they "dried out" & lost capacity, killing the volume. I have replaced a lot of them. I would suggest replacing every electrolytic in it.

Note from photos their orientation to get the polarity right.

Although, re electrolytics in transistor radios, I have a 1957 vintage Oz Philips radio (the "handbag") that has all its original yellow plastic sleeved Ducon electros and still works perfectly, with great fidelity and impressive sensitivity. It has one of the best AGC performances I've heard in any radio with a dial full of local and distant stations all coming in at the same volume.

Apart from replacing the 9 volt battery plug with a 6xAA battery holder, nothing has ever been done to this radio.

No PCB in this set! Point to point wired and the transistors sit in little grommets in the metal chassis.

OK after a bit of reading on the interweb I am pretty sure now that Robbbert was correct. The white cap does in fact seem to be a thermistor. So it is back in and will be left alone from here in. As for the hum I keep on about. It is not really a hum. Much more like. In fact I would say exactly like the tone I get when I hook up my home made signal generator. The sig gen is a very basic unit built from plans. Neither the voltage or the frequency is adjustable. I think it puts out a 1K signal and that tone is very similar to what I am hearing in the later stage of the radio. Except of course there is no signal gen connected to the radio.
I managed to bypass the missing earphone jack but the speaker was still dead. I found that one of the wires attaching the speaker cone to the terminals was corroded away and broken. With the set running I put the signal tracer across the speaker leads and voila. I got that loud tone through the speaker. This is progress I think. So I hooked up a small spare 8 ohm speaker I had laying around and turned the radio on again. Nothing. Dead as a dodo. I checked for signal at the volume pot and the whole radio was dead. Long story short(ish). The on/off switch slash volume pot gave up the ghost. Last time I tried to pull one of these things apart it did not end well. But bugger it I have to try and I had a slightly smaller but quite similar spare in a little transistor radio kit that I bought a few years ago as a source for parts. The original pot is proving difficult to remove. Lots of solder on the switch terminals to get off and it seems to be rivetted to the board as well. I have had enough for today so will give it a rest for a day or two and get back at it after that. It's a bit of a shame really as the pot itself has quite a bit of brass in it and seems well made. I think the root cause was the wiper section which seems to be either worn or corroded away and not the switch itself as it is all brass. I had been getting by jiggling the volume control until it worked but finally it died.
If I get past this hurdle I am going to replace the caps as advised and I would like to replace that very crusty audio transformer as well. It is actually marked on the schematic as "90:8 ohm". I have had a look online and the only similar center tapped audio trans I can find seem to be 1K:8 ohm. Would they be a workable replacement for mine? I cannot find anything at all that is 90:8 ohm and sadly I do not know what the 90 even refers to. I am guessing it may be 900 Ohms? Any thoughts?

You may not need to replace the electros. Looks like they are outside the "capacitor plague" era.

Flexible wires on speakers can be replaced. One strand of 1.27 pitch ribbon cable is ideal.

I would leave the transformer in place and just give it a soak with CRC or similar to stabilise it. It ain't broke, so don't fix it!

Yep. I think you are right Ian. I will leave the transformer alone and concentrate on finding the source of that awful and loud what I think is about a 1 kHz tone/hum. I was powering from a 4.5v volt power supply so I switched to 3 aa batteries in series but the tone is still there. I do think I will replace the electro caps if I have spares. I hope it doesn't turn out to be a transistor as I just looked for replacements for the two germaniums in the audio section and they cost more for one than I paid for the whole radio.
On a more positive note I did manage to instal the spare volume/switch pot and the set is working again up to the volume pot. At least I have not gone backwards (yet).

The era of the bad electrolytic caps, has not really gone away. I am a moderator on an American forum and they still happen. There are a few out there making crapacitors.

I reached the point sometime ago, where I built the Rodney Champness designed Electrolytic Reformer onto the Bench HV power supply. It also has a Tube based regulated supply: This add on has proven seriously useful. That build was because, with especially 450V electrolytics & others, as you did not know how long they had been stored.

That has lead me to testing every electrolytic before use, rather than it fail in service: Which some allegedly new ones do. By default you can leakage test Non-polarised caps with it, however I prefer an insulation tester, or the VCT's Neon Flash tester.

Rather than fry things; This unit can power the HT in a tube set slowly (Tubes in or out) to assess leakage as part of assessment; or alternatively it often gets used to power the HT in a refurbished set, to see if the HT will hold, and saves cooking a transformer as the LR-8 in the regulator will lock up around 20mA .

I'll let you into a secret.

Any PNP transistor in a transistor radio can be replaced with a BC557

Any NPN transistor in a transistor radio can be replaced with a BC547

These cost a few cents each and work better than the germanium RF transistors ever did.

Admittedly, replacing germaniums with silicon in the output stage will result in a little crossover distortion. If this bothers you, the thermistor can be replaced with a 1N4148 diode (forward biassed of course).

I would note that I and others have been finding quite a few oscillator transistors, failing.

Thanks Ian. Such a co incidence that you shared that tip. I was getting quite frustrated at my lack of progress with this set. After a few painstaking hours I finally got the tiny schematic from inside the back cover re drawn so that I could read it. I did have to guess at a letter or two. Especially in the transistor ID's at the top of the schematic. But by and large I think my drawing is accurate. So armed with the drawing I started cross checking. It became apparent that someone has been in here before. The transistor lineup was not as per schematic. Instead of 2 ED1501A's in Q1 and Q2 there is a pair of 2SC1390's (they are marked). Q3 seems to be original so I am assuming it's an ED1501 but it is not marked so I don't really know. Seeing that the set works quite well when I take a signal from the volume pot I am assuming that these changes did not cause any problems. Q4 and 5 are supposed to be ED1401 but they appear very different from each other. Q4 is square and angular just like Q1-Q3 transistors. Q5 however looks like a little plum pudding with a small flat on the side. I am guessing that may be an aid to identifying the pins as there are no markings on it and I am also guessing that this one may be the original ED1401. In any case I took a gamble and replaced it with a BC548 which I think is very close to the one you had suggested. My reading suggested the BC548 was a very good match for an ED1401. From my limited experience I think Q4 and Q5 are audio amplifier stages but I am not confident at all that this is correct. I am thinking that I should change Q4 as well so that they match but I haven't done that yet.
All these transistors appear to be silicon. The last two transistors are supposed to be a pair of 2SD72's which are germanium transistors. They are two identical round cans and quite tall for a transistor so I am assuming they are as per schematic but as usual there are no markings on either of them. Is it unusual to have a mix of Si and germanium transistors in a set?
I will upload my schematic in case anyone is interested. So far I have changed all the electrolytic caps. Replaced the volume pot and switch , bridged the earphone jack and replaced one transistor. Other than an improvement in volume I have had no effect on the existing loud tone slash oscillation problem that I hear in the circuit after the volume pot. Nagging away in the background my thoughts are that those two audio output transistors are most likely the culprits (being germanium) but I don't want to change them out without understanding the root cause. One because they are expensive and hard to get but also because I won't learn much if I just swap out parts without knowing why. At the moment I am stumped. Any thoughts on where to from here will be appreciated.

Its sorta old hat, but its not obsolete & being involved in newsletters over decades. Photocopying quite often involved and still sometimes does, a real digital now, camera with tripod (its not dead) and taking a photo of it, which can then be fed to an image editor.

Agree, try scanning an old photo with a stippled surface. A camera does a better job.

Do not change any transistors! They will almost certainly not be the problem.
The squeaking and squawking you're getting will probably be an O/C electrolytic. Try tacking a replacement on the back to check.

If you want to check a transistor, there is no need to remove it from the board. Measure voltages on B, E and C. For an NPN silicon transistor, the base should be 0.6 volts more positive than the emitter.

For a PNP, it's 0.6 volts more negative.

If you get more than this, the B-E junction is O/C.

If less, the transistor isn't being biassed on - maybe a bad resistor or a broken trace on the PCB.

For germanium transistors, the voltage is about 0.2 volts.

Transistors are easy if you know this.

Do not replace parts by guessing! You'll waste a lot of time and likely lift tracks on the paper-phenolic PCB.

I'll paste the response I tried to make when the site crashed!

Those Hong Kong made radios had transistors substituted depending on what was available at a good price. The "ED" type numbers are manufacturer's specials. These were often parts that fell outside the characteristics of mainstream part numbers in the grading process and were offered to set makers by semiconductor fabs at a reduced price with a special part number.

Sometimes (as in the case of Philips) the "manufacturer's" part number could reveal info about the particular part, as in the later "OC9xxx" series of small signal and low power silicon transistors. Example - an OC9464 is an NPN, 600mA, 40 volt part. An OC9288 is PNP, 800mA, 80 volts.

Transistor radios of that era are really not fussy about the transistors you use, (as long as it was the same polarity) because the early germanium transistors had huge characteristic spreads and designs evolved to handle this.

Mix of germanium and silicon? It all came down to cost and what was available at the time. Germanium continued to be used for audio output transistors for a number of years (e.g Philips AC127 / AC128 and AD161 / AD162 complementary pairs), until such time as it became cheaper to make higher power silicon parts.

As I had already drawn the schematic I photographed it and sent to Brad for upload. Now waiting for him to do his stuff. I have replaced Q4 with a BC548 to match Q5. Sorry Ian. I did that before your latest post. There was a slight increase in volume but there is now some distortion. I had no luck in repairing the original speaker as the wire had corroded away right to where it disappears into the cone. So I hooked up a slightly smaller speaker. The speaker is still dead but suddenly there is no oscillation /howl any more. Removing the speaker from the circuit makes the howl re appear.
Could the lack of a load on the output transistors be causing them to go into oscillation?
I had already replaced all the electrolytic caps so I am pretty sure thay are not the problem. However there is one cap that runs between the voltage supply lines to the base of Q4 and Q5 which I think are audio amplifier transistors. It is 1UF and is listed on the schematic as a polarised cap. It is a tall,skinny green coloured cap with the 1μF marking and a plus sign I guess designating polarity. I will take it out and test it. Can I use an electro to replace this cap if it's bad?
Next step for me will be to take the voltage readings suggested by Ian. That will be interesting. Thanks Ian it's things like this that I need to learn. I will post my results asap.