We have a VHF/UHF amplifier (quite an expensive one) that the builders covered with glass fibre in a loft. The air temp in the loft was 48C so God knows what it was inside the amp, but it was dead and it burnt the fingers of the person who discovered it. Thing is, it has now been on soak test in a 30C environment and it is working fine. Can we now trust this amp, or do we scrap it?
Bill
If it's working now, why would you consider scrapping it? Because it might fail in the future? Is its absolute reliability really that vital?
if it all works it ought to be ok. Some bits will have had their lives shortened, but probably not by enough to kill it in a decade. Ultimately it's all a play the odds game with any electronics.
NT
If it survived a soak test, why would you scrap it? What I would do though is to not put it back under the glass fibre. It needs to be somewhere where it doesn't have the heat it generates trapped so that it cooks.
Jim
In article , Bill Wright scribeth thus
Change any electrolytic capacitors in it and i expect it will be fine for years to come!...
Chances are that the resistors got too hot and shifted out of range upsetting the bias on the amp transistors. Cool it down and they return maybe not to the original value but at least within the operational window.
As suggested have a close look at any electrolytics in the power supply - tants you can ignore - and replace them if they look at all out of shape.
A fault could cause a lot of people to lose TV reception for a while.
Bill
Any electrolytic capacitors will have a curtailed life probably, and if it's critical (or just a b***** to get at) change them anyway.
Andrew
Look at what others have said, re electrolytics etc., but I would hope that a "quite expensive" unit would have some self-protection circuitry in the event of overheating. Not that that would prevent damage altogether, but I'd hope it might back off the power - possibly to zero
- in the event of overheating. You often get such protection on amplifiers that provides _some_ degree of self-protection in the event of a heatsink becoming dislodged or omitted. It'd be worth having a look at its datasheet, if you can still find it, to see if it has anything like that.
Simply change the voltage regulator to be on the safe side. Brian
Then I understand your concern.
It's a low power amplifier, it'll be fine. Concerns about electrolytic capacitors are likely unfounded, but if you can, take a look inside and make sure there's nothing obviously bulging or leaking.
FWIW, most of the electronics I design runs at above 180'C (gas mark 4).
Cheers
You prolly spec a better grade of cap;!
More likely a better grade of silicon to be operating *above* the absolute limit of 175 deg C, typical of most silicon based semiconductor technology. Incidentally, that's the temperature where the device dissipation would typically be de-rated to zero.
The caps look fine and the DC rail has no ripple.
Bill
Semiconductors can stand a surprising amount of thermal abuse before they fail - chances are it shutdown because one or other of the single chip regulators exceeded 125C internally and removed power.
I have known similar but cruder amplifiers that had lightly charred paper under them when I found it which alarmed the hell out of me.
If it works OK on a bench soak test then it is probably fine but the capacitors will have had some of their working life reduced. You could always hold a spare amplifier in reserve on premises if you are worried about imminent failure but there is probably plenty of life left in it.
Bolting it to a piece of aluminium plate as extra heatsinking might help in this unusually hot weather.
Yes, I remember burning my finger on a BC108 that I'd mis-biased; it seemed to be undamaged. As I've said, it probably was indeed a self-protection cutoff - though Martin's suggestion that it might have been specific to a regulator chip might mean that, although the overall amp. therefore had such self-protection, it doesn't actually claim to have it in its datasheet. []
Especially if you can bend it so that it holds the amp. so it sticks above the insulation. And maybe put "do not cover" stickers on it (for what little good they'd do - but they might protect _you_). You also might want to ensure - if you can - that it's mounted away from the rafters anyway: I assume they're wood, and you wouldn't want to risk it being the cause of a fire.
Prolly OK
+10 That is almost certainly the explanation that fits the facts best.
I've done that on a model tiger moth - constnat servo movments and rather poor heatsinking and the 5v regulator shut down
All servos went to full travel but the motor shut off.
Could you put a small (computer) fan in it for a little forced ventilation?
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