Home |
Search |
Today's Posts |
|
#1
|
|||
|
|||
Bringing up old valve radios slowly?
On Thu, 1 May 2014, Rambo wrote:
On Wed, 30 Apr 2014 21:24:41 -0700, "coffelt2" wrote: Garth, I'm no expert, by any means, but have dabbled in old stuff for quite a few years. I am always concerned that even at the first application of voltage, something might be actually shorted, and not just needing "reforming". Good idea if one can devise some way to measure, even roughly, how much current is being drawn from the Variac. Some (me included) have inserted a low wattage incandescent lamp in series with the applied voltage. Better, probably would be an AC ammeter. Of course one never knows just what current is normal, but a nearly dead short should be reasonably easy to detect. Start off very slowly, feeling (without touching) around for warmer than normal parts, keeping nose carefully tuned for any odors not present when the stinking thing was still cold and unpowered. Keep fire extinguisher and telephone handy. Good Luck! Old Chief Lynn, W7LTQ "gareth" wrote in message ... Having now borrowed my friend's Variac, what is seen as good practice for ramping up old valve radios to reform the capacitors? Start off at, say, 50VAC and then increase by 50VAC every 1/2 hour until 240V is reached? Gareth, Ive done this in the past and its not for the fainthearted! Check Check and check again that the chassis is not live! Then if you have an ESR meter check for very low resistance capacitors. An Esr meter is quite essential when dealing with vintage gear. Most electrolytics will be leaky as hell. An ESR meter won't be useful here. Those only really became common with the arrival of switching supplies, where electrolytics are being fed at a very high frequency, and the circuits are so complicated that it may not immediately be obvious what's wrong (and you may not see any output until the bad capacitor is replaced, since they are generally feedback systems). They are also useful because theoretically they can test capacitors in circuit, really useful when you have endless electrolytics in the circuit (something that only happens in solid state devices since they are low impedance/high current devices). There will be only a handful of electrolytics in an old radio, and the major one will be in the power supply. But since they are only seeing 60 or at best 120Hz (or 50/100Hz in some countries), they don't need to be really low ESR. If they don't filter properly, you'll hear hum in the radio. There will be a few other electrolytics as coupling capacitors and cathode bypass capacitors in the audio stage(s), but not really anywhere else. A shorted electrolytic as a cathode bypass will mean higher gain in that stage, and maybe the tube running too hot. It might mean needing to replace that tube, but lots of tubes have lived after being stressed. Coupling electroltytics, and suddenly I realize those aren't likely to be seen in tube gear since a small value capacitor will pass the needed low audio frequencies with the high impedances of the tube circuitry. But if one of those was an audio coupling capacitor, it would mean putting voltage on the grid of the next tube. Bad capacitors can be a problem in old tube gear, but we're talking the common types, low value radio bypass capacitors at 0.1uF or lower. That's a whole different matter, since it's not because of a need to get high capacitance into a small package (as in electrolytics, where the electrolytic can dissipate over time or use), but because at one point, capacitors weren't made so well. But a shorted bypass capacitor will show up, and probably quite well with an ohmmeter, since otherwise the circuitry is high impedance. It won't tell you which one, but it will give you warning. On the other hand, once one is starting to replace bypass capacitors, it's not much harder to do them all (hence no need to figure out which is bad), and given the work to access some areas of some equipment, it's foolhardy to not do them all since the work in opening up the rig is harder than replacing the capacitors. Michael The pp is quite right in suggesting a current indicator/ limiter in the form of a bulb in series use one about the same or higher than the rating of the set. |
Reply |
Thread Tools | Search this Thread |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Forum | |||
Digital advancements, slowly but surely ... | Policy | |||
Wilderness Sierra output power slowly increases? | Homebrew | |||
Bringing a radio into Israel | Shortwave | |||
Avo Valve tester FS | Boatanchors | |||
Avo Valve tester FS | Swap |