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Attempting vintage receiver repair - Please Help!

KrisBlueNZ

Sadly passed away in 2015
Nov 28, 2011
8,393
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Nov 28, 2011
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8,393
Yes, the part with the coloured dots is an inductor. See the "L201" marking on the PCB underneath? "L" means inductor.

The parts in the other photo are almost certainly inductors too. Check the marking on the PCB. Their values are given in µH so "103" is 10,000 µH or 10 mH. The "J" is a tolerance indication, meaning 5%.

As for what to replace...

Electrolytic capacitors, yes I would replace them en masse. They dry out. And there's not a huge number of them in there. As Steve says, use an equal or higher voltage rating. Modern electrolytics will be quite a bit smaller for the same specifications. If you have the option, go for Japanese brands - Nichicon, United Chemi-con (UCC), Rubycon, Panasonic, or a European brand, instead of cheap Chinese brands.

Other capacitors, probably not, unless there's some visible damage. If you suspect a capacitor, desolder one lead and check it for leakage using a multimeter on resistance range. It should read OL, i.e. open circuit. Also check capacitance if you can borrow a multimeter with that capability.

Inductors, no. Suitable replacements could be hard to find anyway.

Transistors... I'd be inclined to replace them all, but that could cause problems for the radio-frequency circuits because various parameters will change. But the transistors on the amplifier board would be fair game. I suggest you wait until we find out what's wrong with it though. If none of the transistors has failed yet, you might be better to let sleeping dogs lie.

Diodes I would treat the same as transistors. Wait and see whether any of them have failed.

For the resistors, I'm not sure. Replacing every resistor in the thing would take ages, but those cheap old carbon ones can just fail. You could try to measure them to see how many are out of tolerance, but you can't reliably measure them in-circuit, and removing them is likely to make their resistance change, if not actually break them. So it's tricky.

You may find that high-value resistors (e.g. 100 kilohms and higher) are all out-of-spec whereas lower-value ones are generally OK, unless they dissipate significant power and get warm.

I don't have much experience with old carbon resistors and how they fail, but I Googled failure of carbon composition resistors and found this interesting post on a thread at http://www.eham.net/ehamforum/smf/index.php?topic=35646.0:

Carbon composition resistors have two primary modes of failure.

The leads embedded in the carbon corrode from moisture and develop a high resistance interface, and this causes an "open" or very high resistance. The aging rate depends upon the humidity level, storage or operating temperature, and materials used in the resistor. It is generally an effect taking a decade or longer of sitting unused.

High temperaures cause the carbon to bond tighter. This reduces the resistance of the resistor. This is the tyical failure mode when a carbon composition is operated at elevated temperatures, like in a power application. This is often the source of failures in resistors used as voltage dividers, bleeders, or power loads. The resistor ages down in value, passes more current, gets hotter, ages down more, gets even hotter....eventually the resistor can "short". This was the cause of fires in TV sets by Sylvania in the 70's, and why parasitic suppressors on high power tubes go off value over time.

This is also why carbons should never be used in voltage equalizing or current or voltage safety critical applications.

As for your resistors, there is no way to tell except check them. You would primarily be interested in the first major failure mode if they in storage. Unless you stored them in a refigerator.

W8JI (Tom)
That's just one guy's experience though. You can try Googling relevant keywords yourself to see what others say.

Re wholesale replacement of components, I suggest you tell your friend that it's just a possibility at this stage until we know more about the condition of those resistors and semiconductors.
 
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