I don't do many repairs these days, since I no longer buy any more radios, but of course the existing ones break down now and then. So I may as well relate what was done to fix them (or scrap them).

So, first one is a small Philips of unknown model number, with the usual Philips 9-pin valves (6V4, 6M5, 6AN7, 6BH5, 6BD7), with a working clock. Unlike the usual radios with this valve line-up, this one has a ferrite rod antenna.

The dial is missing, the clock surrounds are missing, one knob is missing while the others (on a concentric shaft) are melted.

The only reason I hadn't got rid of it was the fact that the radio part worked well. But recently it started producing hum, with the sound sliding in and out of a morass of hum. I simply assumed it was the main electros and put it aside for a while.

Today was the day to fix it. Firstly measuring the AC on the electros showed they were working perfectly fine (2 volts and 0.1 volts).

Then I noticed a black blob laying on the chassis. It was the remains of the capacitor that goes across the primary of the speaker transformer. Its purpose is to suppress possible transitory spikes when the volume is up high, otherwise things can flash over. I cut it out and the radio was working again. I'm not intending to run the radio at full blast, so I didn't replace the part.

I let the radio run for a while, and there was no trouble. I decided to disconnect the clock, as it was a waste of power, then fixed an issue where the dial pointer got stuck at a certain position, and then put it back together. It passed final testing.

This one?

https://www.radiomuseum.org/r/philipsaus_155.html
https://www.kevinchant.com/uploads/7/1/0/8/7108231/155.pdf

Thanks, yes that appears to be the one. On mine, the "clock overlay" (the white part with the numbers), is missing, as is the badge and dial. There's noticeable rust inside. The result is the ugliest of ugly ducklings.

The only reason I kept it was because the radio works so well. The AGC is both effective and ineffective, oddly enough. It's effective because no matter how much of my house station's power was input to it (external antenna used), it never became distorted, the sound being perfect. However, the volume increased dramatically, which the AGC is supposed to manage as well.

Historically, like buffer caps in vibrator power supplies. The plate bypass cap (plate to ground) Did not have a failure rate it was an attrition rate. Plate bypass was always the most likely to die.

Quick Fix no.2

This radio gets used on a regular basis, but today there was no sound. Giving it a bang produced a few crackles.

It's a STC A5420 (although the diagram just says 5420), with a dark red case and a light grey back - and no knobs.

Valves: 6X4, 12AH8, 6BA6, 6N8, 6BW6. The 6BW6 is a 9-pin version of the 6AQ5.

Touching valves, especially the 6BA6 made the sound come and go, yet the trouble was nowhere near there.

It took quite a while to find a dry joint at the 0.01μF cap that brings the audio out of the detector to the bandswitch.
Once that was fixed, the next thing was that the slightest touch on the antenna wire produced crackles and loss of sensitivity. Although there wasn't anything wrong with the wire that I could see, replacing it fixed the touchiness.

Short wave worked well, but MW seemed to be rather insensitive. It took a while to find another dry joint, on the wire going to the primary of the MW aerial coil. At last the radio was now electrically fine.

The front glass kept sliding down, so I fixed that with a bit of adhesive.

Now the radio is back in service. Short wave picks up a number of stations without needing an external wire.

If the glass was originally held by rubber inside or under a clamp, the automotive rubberised cork works well.

Quick Fix no 3

.. although I'm not 100% satisfied with this "fix"...

A small 4-valve Airzone, with 6AN7, 6N8, 6M5, 6X5GT, Don't know the model number, but it looks like many small radios, with the vertical dial on the right, 2 knobs below, and the speaker on the left.

This sits on a stool next to my bed and often runs all night, tuned to the in-house station. The aerial wire is connected to the chassis in order to keep the signal strength down. The problem is that when tuned to other stations (through the power-line hash) there was a noticeable swish sound when nearly tuned in. It was worse the weaker the station, and no problem at all with the house station.

So, I guessed it's just an alignment problem. Firstly the IF was about 5kHz out, so I adjusted it to 455. Several other adjustments got the sensitivity up quite a bit. However although the swish has been reduced, it's not entirely gone.

The result still works fairly well, as even with the shorted aerial, the Sydney ABC stations are received at good volume.

As an aside, the 6M5 barely gets any warmth, even with the volume up full for a while. The HT was measured at 198 volts.

Because I don't know the model number, I don't have a schematic, so it was all guesswork. Physically, the only thing that keeps the chassis in is the knobs. There's no screw-holes. Also the back is only attached by extra nuts on the mounting of the osc coil. Pretty poor. Perhaps some extra hardware is missing.

This sits on a stool next to my bed and often runs all night

How's your fire insurance? Brad has a story about operating a valve radio like that.

Indeed - back in the day 35 years ago when full restores were considered an option of last resort, due to radios being a lot younger than they are now. Playing a radio unsupervised is one thing I don't recommend. I had a plum pudding go up in smoke one night and the thing that probably saved my bacon was the noise the radio was making as the power transformer nuked itself.

From memory, the only salvageable item was the dial glass and a couple of valves. The rest of the chassis was black and the cabinet stank to the high heavens.

At my last workplace I used to play a Radiolette 500M the same way, though I was with the radio 95% of the time, as it was on a small shelf above my desk. It's transformer gave up in a similar way but because I was there at the time, I was able to save the radio, the demountable office it was in and my job all at the same time.

Not only that but I swapped the transformer over and the radio survives to this day.

Always watch any radio that is plugged in - always. 80 year old transformers can be a bugger of a thing.

Some times you can be lucky and get a set like the Astor HPM feature in the current radio waves. It came from a deceased estate clearing sale.

I knew the person who originally restored it (trained tech). It had been sitting along time and that was most of its issues. Very quick fix.

If that set of yours is a daily driver, or close too. I would be doing a bit of resistor checking around the converter & AGC line. One hopes it has no paper caps. 6K8 had a habit of developing flutter and they put an 8μF cap in the oscillator plate circuit, this might be similar?

How's your fire insurance?

I've had more computers destroy themselves than old radios - that's how rare it happens. I'll often run various radios in the house for hours unsupervised.

Honestly - not really worried.

I fit thermal fuses to old transformers. Strap or glue to the winding as appropriate.
Just remember, you cannot solder to a thermal fuse, even with a heat thief! Use crimps.

Interesting that we mention thermal fuses as the workshop heater fried its Wednesday: Helpful. One gives praise for global warming, as things would have been colder.

I have always considered a, small as possible, connector with two screws in it desirable, if the tubular type fuse is going to blow. I do love the dills that try to solder them in. There are a few of those modern transformers that have them in the wrap, & if it blows, they tend to be E-waste.

Modern equipment made for a price is still vulnerable. Twice the SLA batteries (2 by 12 series for 24V) had one drop a cell, in the UPS. Fortunately it is on the contactor circuit that shuts off all of the peripherals when finished with the computer. What happened both times is that the computer started doing weird things and the charge indicator was falling. So it was rapidly shut off.

When opened up neither battery could be handled for the heat and the batteries had severely distorted. It is quite obvious that there is no current sensor & it just registers "flat battery" and opens up with a sustained charge rate the batteries cant handle. I ended up putting a low temp thermal fuse on its mains supply, in between the batteries. As I suspected it would do it again and it did.