Thought I might try my hand at repairing a small transistor radio. So I bought a Philips 90RL125 on that auction site just now. So far I have been unable to locate a schematic for it. If anyone has one or knows where I can find one I would love to hear from you.

A lot of those things had circuits within, otherwise they can be a rare beast. Some of the newer ones were IC and that poses an issue if it proves faulty.

The cold hard facts are, that if its too modern and the fault is not simple its e-waste. Many are similar to an autodyne.

Fixing procedures follow a similar plot as the methodology used in tube radio. An early printed circuit is prone to cracking; and that incudes joints. as are some horrid little axial caps around 3/8" by 1/2"crack & short. I have a Tandy MMS_10, that generates heat in the output and quite often cracks joints. It gets a lot of work.

Electrolytics often used for coupling do dry out and lose capacity causing low volume.

Circuit board soldered joints for external power supply and earphones are prone to cracking. Was a regular fix on a Sangean ATS 800.

Thanks Marcc and STC. Hopefully when it arrives it will have a schematic inside back cover. It is early 70's so I am hoping it's not too modern. Funnily though I had a possible cantidate kicking around my shed that I had recovered from a kerbside trash pile a few years ago. When I started thinking about transistors I started on it. As soon as I pulled the back off I noticed about three rather large IC's. It was working on FM but not AM. Those integrated circuits totally threw me so I buttoned it back up and put it aside. Now patiently waiting for the new victim to arrive.

I should have said external power supply and earphones jacks.

The new (to me) radio arrived in the post today. Philips 90RL125 It was only $17.50 delivered so not a huge risk. But the old saying you gets what you pay for certainly rang true in this case. The iron core on one of the small transformers has swollen with rust and there is corrosion everywhere. Both ends of the battery terminals are completely gone and there is rust evident throughout the set. I think it may have been underwater for a time. Never mind. At least there was a tiny schematic stuck inside the case. I am going to try and coax it off and enlarge it somehow.
In the meantime. After a bit of a cleanup I rigged up a power supply (4.5VDC) and turned it on. The set was dead as expected but probing around with a voltmeter proved the switch good and I was getting from zero to around 300 millivolts across the volume control pot as I turned it up and down. I don't really understand if that is significant or not so I hooked up the signal tracer and had a dig around with that. My tracer is just that. I mean it is not a combined tracer/generator.The strange thing is that I was getting a fairly loud tone at various points of the circuit. I was hoping to hear some static and maybe even a station or two at the volume pot wiper as I moved the tuner through it's range and probed around. No such luck. Instead I got a tone that was quite obviously being amplified at some points and quieter at others. Where is this tone coming from? It isn't frequency dependant. I mean it is there regardless of how I tune the radio. Am I hearing the local oscillator? Thats about the only thing I can think of. Any guidance would be appreciated.

With a lot of those things, it stops, its abandoned and then the batteries left in it decompose & do massive damage. I would strongly expect that transformer in an older set to be the audio output, even that its transistor. A lot of them had Push Pull outputs with transformers. Corrosion may indicate that its cactus.

I would photograph the circuit, perhaps scan it? What you are doing is high risk.

The crap on it is likely shorting stuff, or has eaten it.

Photos would be desirable.

Pics sent to Brad for upload. Meanwhile can anyone tell me what that tone I am hearing with the signal tracer might be?

You need an oscilloscope, or a "trained ear" to ID the signal, Square Waves, sound mellow like a reed instrument, Sawtooth ragged, Sine waves more like a string instrument.

Then you need to understand how they are generated. Square waves are more the multivibrator where the feed back loop is on and off switching, Sawtooth, charge / discharge and Sine just plain feedback within, normally one tube, or transistor.

If you can hear it, its more probable to be in the audio, albeit on odd occasions, heterodyning between two RF signals.

For me, a rust pile means straight to the bin.

Good luck with fixing it though.

Been using a Realistic 12-715 transistor radio to listen to music from my house station while working in the backyard. The 9 volt battery seems to last forever.

We await the photos before assessment and the passing of judgment. It may turn out to be an interesting exercise for someone new.

I forgot to mention that with probes & such; The ideal is to have a shed like mine which acts as a "Faraday Cage" and the Generators of RFI like lights, the wireless NBN, mobile phone plugged into the computer & to close to the engine plus one XP computer, can be dealt with. The bench light closest to the action is Halogen.

RF riding on the mains is also dealt with.

Photos uploaded.

That looks like a project for shango066: https://www.youtube.com/@shango066/videos

I have made some progress on the transistor set. The big blue cap alongside the audio transformer in the pics looked to be bulging quite a lot and as it was easy to get at I decided to change it out. Just in case I fired up the radio and checked the volume pot with my signal tracer. To my great surprise I got a lovely clear local station. The speaker was still dead but at least I have a working set as far as the volume control. Now might be a good time to check that crusty audio transformer I thought. So I did a resistance test and I think it is good in spite of the rusty plates. I got 7.5ohms across the primary and 3.7 ohms from the center tap to each end with no shorts anywhere. The secondary measured 1.3 ohms. I really have no idea what resistance to expect but the figures kinda made sense to me. Am I mistaken?
I remember reading somewhere recently that a common failure point in small transistor sets is the earphone jack. That could possibly explain the dead speaker so I had a dig around and discovered the earphone socket was broken. I removed it as gently as I could but it came apart as I desoldered it. I don't think I will easily find a replacement. It is marked Hosiden Japan and a quick search didn't bring up anything that looked right. Never mind. I can bypass it for the time being and press on. Unfortunately I haven't been able to figure out how to bypass this socket yet but I will continue trying to work that out. I have a bigger problem at the moment. Flushed with my good fortune on replacing the Blue cap I found a tiny white cap. At least I think it is a cap. It looks exactly like a small ceramic cap except it's white. It had a lump on it's side which I thought was indicating a failure. So after quite a bit of de soldering and wrestling the part out I think the bulge is just a dab of paint. In any case I measured it seeing I had it out and my component tester thinks it is a 95 Ohm resistor. So I think it is probably a cap and probably faulty. I just hope little white cap looking bits are not something that everyone except me knows about. There are no markings on it other than two small concentric circles with two parallel lines inside the outer circle and touching the inner circle. Hopefully someone will advise me on this. Without a schematic I have no idea what value it should be or even if it is a cap or some other component. I intend to re install it tomorrow as the set is no longer working without it and a few other parts that are still in place but de soldered. I am finding it so hard to see exactly which lump of solder hides the component lead on the other side of the board so I have de soldered a few wronguns. I feel like I am going down the path of no return and will soon reach a point where I cannot put everything back without bridging something or incorrectly placing a part. I guess you cannot buy experience and at least this set is a good candidate for my fumbling efforts.
Despite all the grumbling I am actually enjoying working on a small transistor radio. I could't believe my luck when I heard a station after replacing that cap. Maybe it will live again after all.
Thanks Brad for uploading the pics.

The only white thing I can see in the photo is between a transformer and that black round thing. It might be a thermister, if it measures 95 ohms you could try a 100 ohm resistor - or just put the thermister back in if it's intact.

I'd probably replace all the electros around the audio stage as they appear to have been affected by water.