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I've just bought two little Philips radios from the early 1960s. But as with any old piece of electronics, they need some attention before they work properly and above all safely. Tube radios work on pretty high voltages. These specific ones have a chassis referenced to the 220v mains. Others, with proper power supply transformer, may run on voltages up to 275v internally. In this thread i'll post stuff about what i do to revamp these old things, and if anyone needs advice on how to fix their old radio, i guess this is also a good place to ask! Once these work, i might also show how to add bluetooth to old radios while still using the old electronics. Yes, you can just gut the radio but often that's more work than a quick repair, and the results are worse than from the original electronics.  These are the two sets i bought for the princely sum of €10. Granted, they're not in too good a shape. The paint is dirty, pretty badly scratched and will need repainting. Still, they're cute! I took a 'before' picture, but somehow it came out blurry. Just imagine some gashes in the paint, and the black bakelite showing through the white and avocado paint. The seller told me they worked, but they had been in storage for 20 years. Step one of repairing an old radio, is to put the cat in an appropriate box. Otherwise, the cat will walk around on the workbench, and potentially step into unhealthy stuff like bits of solder. Box (with the cat in it) must be placed outside the work space when working with high voltages.  The real step one, is to clean the radio. Nothing is more annoying than getting dirty hands whenever you handle it. For bakelite or painted radios, i use tooth paste. It is highly effective against nicotine/tar stains on smooth surfaces! I prefer it over 'real' cleaning stuff. For wooden radios, just some dishwashing detergent and a damp rag. Step two is to do a visual inspection. We can already see a big electrolytic of the type that has no vent on the top of it. These specific ones have proven to be reliable. Would it have had a vent on the top, i would have taken it out of the circuit. Still, when powering up, it will need current limiting just to be sure. More about that in a later post.  Ah. Yyyyeah. Previous owner made a bad mistake. Computer, enhance!  In the Netherlands, up until the early 1960s, Philips used tar potted paper capacitors. This one here has spewed its guts. Luckily, the leaked stuff is not corrosive. It can, however, contain PCBs. All of these little universal AC/DC sets have a suppresion capacitor, either between line and neutral, or bypassing the rectifier to prevent 'modulation hum'. Every collector should know that without exception, they will blow up. The odd thing is that they're super temperature sensitive. Just for the heck of it, i cut out the capacitor and meggered it (checking DC leakage resistance with a high voltage).  Humm. 8 megaohm. Not great, not terrible. Not enough to make the thing pass so much current that it literally melts and spews out hot wax. Well, these *are* super temperature sensitive. I heated it up from ambient to about 30 degrees. And voila:  Just heating it up 10 degrees makes the leakage resistance drop to 4 megaohms. The radio can easily reach 50 degrees. It is a AC/DC universal set, and because it had to be switchable from 220v to 110v mains voltage (we had both voltages in the 60s!), it has a big loving resistor to drop the heater chain's voltage to the appropriate level. On 110v, it runs dramatically cooler. So yeah. Although it looks 'tolerable' at room temperature, it will eventually warm up. Then it will pass more current. Heat up more. Pass more current and then the inevitable will happen. Deceptive little bastards. Most often, you gotta replace all of them. Luckily, this 1960 radio already uses polystyrene and ceramic capacitors for most purposes. Anyway, more inspection!  The bottom side of the circuit board. There are weird oxidation stains both on the top and on the bottom. A lot of flux has been left uncleaned by the factory. This is fine, but not the milky spot in the middle. These definitely have to be cleaned up because they may start to leak and cause carbon tracking spots, ruining the board. Anyway, that's it for tonight. I'm gonna clean up the board later, and solder in a new suppression capacitor when the kitty's not on the workbench. Solder fumes are bad for her. To be continued! LimaBiker fucked around with this message at 20:12 on Nov 14, 2021 |
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Nov 14, 2021 20:07
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| # ? May 4, 2024 23:52 |
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