08-04-2011, 10:29 PM
Whether or not to change waxy caps on sight is a personal judgement, and it depends on one's approach to either restoration v conservation, which are not interchangeable terms, but two entirely different things. 'Conservation' means doing as little as possible beyond making a set safe - replacing perished wiring, or components which are known to be hazardous. In that, we must include 'that cap' which couples the IF to the grid of the output stage, and the filter cap fitted across the mains input, which should at least be removed, and if not replaced won't pose a problem, but if it is replaced, should be an X rated cap, which is self healing. Also, on AC/DC receivers, caps which isolate the aerial should likewise be x rated.
'Restoration' or 'renovation' in my view means putting the radio back into 'factory fresh' condition - as good as when it left the assembly line. that means all waxies need to be removed, and any resistors which are outside their original specified tolerance - generally +/- 20% should be replaced. Any wiring that's the least bit dubious, I replace with heat resistant silicone rubber flex in the same colours as the original. I don't personally stuff old caps with new ones. They're out if sight, and had the radio have failed during its working lifetime a radio repairer would have used whatever was current stock. If other prefer to stuff old ones, then I think it's important to note this or at some furture time, it may be assumed that they're original and thus should be snipped out and replaced.
To properly test a capacitor, it must be tested under the conditions under which it operates. To test non-electrolytic caps for leakage, that means applying their maximum working voltage, with an Ammeter is series to check what leakage current is flowing. Some capacitor leakage analysers of old used a neon, which will strike intermittently if the cap is leaky. it's not such a complicated matter to build a leakage checker and a useful circuit and discussion can be found at this link:
http://www.angelfire.com/electronic/funw..._caps.html
Hope that's of interest.
In another post I'll mention the problem of high 'effective series resistance' (ESR) of electrolytic caps, which is more or less the opposite to the problem of of high leakage.
David
'Restoration' or 'renovation' in my view means putting the radio back into 'factory fresh' condition - as good as when it left the assembly line. that means all waxies need to be removed, and any resistors which are outside their original specified tolerance - generally +/- 20% should be replaced. Any wiring that's the least bit dubious, I replace with heat resistant silicone rubber flex in the same colours as the original. I don't personally stuff old caps with new ones. They're out if sight, and had the radio have failed during its working lifetime a radio repairer would have used whatever was current stock. If other prefer to stuff old ones, then I think it's important to note this or at some furture time, it may be assumed that they're original and thus should be snipped out and replaced.
To properly test a capacitor, it must be tested under the conditions under which it operates. To test non-electrolytic caps for leakage, that means applying their maximum working voltage, with an Ammeter is series to check what leakage current is flowing. Some capacitor leakage analysers of old used a neon, which will strike intermittently if the cap is leaky. it's not such a complicated matter to build a leakage checker and a useful circuit and discussion can be found at this link:
http://www.angelfire.com/electronic/funw..._caps.html
Hope that's of interest.
In another post I'll mention the problem of high 'effective series resistance' (ESR) of electrolytic caps, which is more or less the opposite to the problem of of high leakage.
David
