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Cooking a choke in the oven--Anyone tried this?
I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a
choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#2
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#3
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Is the 866 choke-loaded with this ??
You might try a small C-load . Might just do the trick .... "weisselk" schreef in bericht m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#4
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weisselk wrote:
I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? I'd try wedging toothpicks between the windings and the core. Once you get it quiet, some varnish or such to glue them in place would be good. |
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#5
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weisselk wrote: I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Since it's a potted transformer, there's not much you can do in a conventional sense. I'd say give it a try, at worst it won't improve matters and you'll have to get a new choke (or live with the buzz). It bears noting that in its original application it's unlikely if anyone would have worried about a slight buzz, it would have never been heard by the theatre audience. If it's a choke-input filter (very likely, given the 866 rectifiers), Ronald's suggestion might help. But use only a very small capacitance, say about 0.5 to 1.0 uF. They should be capable of carrying significant AC currents, so I'd suggest a poly capacitor instead of oil-filled or electrolytic. Much more than that and your B+ voltage will rapidly start soaring as you approach a true C-input filter. Cheers, Fred -- +--------------------------------------------+ | Music: http://www3.telus.net/dogstarmusic/ | | Projects: http://dogstar.dantimax.dk | +--------------------------------------------+ |
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#6
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AFAIK during the Thirties no thermoplastic (fusible) polymer was known,
polyethylene was a HiTech stuff used for insulating radar wires and not for shopping bags. I suppose the potting material and the insulating enamel are some sort of phenolic resin (ie. "bakelite"), which is thermosetting: if You heat it up, it does not melt until it gets eventually destroyed by the temperature (above some 450-500 °C). As an alternative, it could be some sort of hard wax or tar: in this case it would easily melt and probably Your idea could work for a while, but I still think that a real jewel like Your amp deserves a brand new, solidly built choke. By the way: thermosetting resins usually have a glassy appearance. Ciao Fabio "weisselk" ha scritto nel messaggio m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#7
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weisselk wrote: I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss Let's assume the bobbin is loose on the core, and it is vibrating. packing the bobbin tight against the core with sheets of cardboard wherever you can get then in is a must. But make sure the gap isn't forced open, and make sure all the bolts are tight. Try to determine what the "potting" compound is. If it is old varnish, then the whole item might be able to be vacuum impregnated with electrical varnish, and baked for 4 hrs at 125C. This may penetrate the existing air spaces in the choke, and all the cardboard packing, and perhaps the paper insulation. It shouldn't hum after such treatment. If the winding is in poor shape, with really loose layers, or if the new applied varnish is incompatible with the existing goop used all those years ago, then rewind the whole choke, which shouldn't be too hard, since plastic bobbins can be bought, and wire, and with patience and a real steady hand, you can wind on a lot of turns in a non layered random winding, using new high temp winding wire, without much tension. Then soak that in a vat of varnish and bake it, maybe get a motor re-winder guy to do that for you with his motor parts. Patrick Turner. |
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#8
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Let's assume the bobbin is loose on the core, and it is vibrating. packing the bobbin tight against the core with sheets of cardboard wherever you can get then in is a must. But make sure the gap isn't forced open, and make sure all the bolts are tight. Try to determine what the "potting" compound is. If it is old varnish, then the whole item might be able to be vacuum impregnated with electrical varnish, and baked for 4 hrs at 125C. This may penetrate the existing air spaces in the choke, and all the cardboard packing, and perhaps the paper insulation. It shouldn't hum after such treatment. If the winding is in poor shape, with really loose layers, or if the new applied varnish is incompatible with the existing goop used all those years ago, then rewind the whole choke, which shouldn't be too hard, since plastic bobbins can be bought, and wire, and with patience and a real steady hand, you can wind on a lot of turns in a non layered random winding, using new high temp winding wire, without much tension. Then soak that in a vat of varnish and bake it, maybe get a motor re-winder guy to do that for you with his motor parts. Patrick Turner. I'm getting into this thread late so I'm sorry if I'm repeating soeone else's idea. If the entire transformer winding is loose on the core you can try this. Get some very thin hardwood shimming and gently tap the wood into the gap between the core and the winding. This should stop the vibration and the wood won't burn. I have done this and have seen it done by others very successfully. -- John "If you dont know where you're going, any road will take you there." -George Harrison |
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#9
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Hi
This idea is new to me, dosen't the small C boost B+ noticibly? My RCA MI1228A amplifiers have a choke/ power transformer hum problem, and it would be beautiful if the cure was really so simple as adding a small capacitor thanks Max Ronald wrote: Is the 866 choke-loaded with this ?? You might try a small C-load . Might just do the trick .... "weisselk" schreef in bericht m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#10
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"Max Holubitsky" wrote in message
... Hi This idea is new to me, dosen't the small C boost B+ noticibly? My RCA MI1228A amplifiers have a choke/ power transformer hum problem, and it would be beautiful if the cure was really so simple as adding a small capacitor thanks Max Ronald wrote: Is the 866 choke-loaded with this ?? You might try a small C-load . Might just do the trick .... "weisselk" schreef in bericht m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss A bit of advice that I haven't seen mentioned; and you may have already done this.. If the choke has holes in the corners of the core for mounting bolts, make sure that the bolts are nice and tight. If there are holes but no bolts, get some of suitable length and install them. I've cured many noisy transformers and chokes by simply tightening them. -- Tweetldee Tweetldee at att dot net (Just subsitute the appropriate characters in the address) Never take a laxative and a sleeping pill at the same time!! |
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#11
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Hi Max ,
B+ will be a bit higher , but the AC across the choke is less so there will be less mechanical hum in the choke . If the power tranny hums mechanicaly you have to solve the problem mechanicaly . regards , Ronald "Max Holubitsky" schreef in bericht ... Hi This idea is new to me, dosen't the small C boost B+ noticibly? My RCA MI1228A amplifiers have a choke/ power transformer hum problem, and it would be beautiful if the cure was really so simple as adding a small capacitor thanks Max Ronald wrote: Is the 866 choke-loaded with this ?? You might try a small C-load . Might just do the trick .... "weisselk" schreef in bericht m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#12
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The choke hum is much worse than the power transformer hum. Boosting B+ would be
a problem, because B+ in this unit is already pushing 600V, and it's a choke imput rectifier with a high impedence rectifier tube. If it's a really small cap though, I can see this working - I should really give it a try Max Ronald wrote: Hi Max , B+ will be a bit higher , but the AC across the choke is less so there will be less mechanical hum in the choke . If the power tranny hums mechanicaly you have to solve the problem mechanicaly . regards , Ronald "Max Holubitsky" schreef in bericht ... Hi This idea is new to me, dosen't the small C boost B+ noticibly? My RCA MI1228A amplifiers have a choke/ power transformer hum problem, and it would be beautiful if the cure was really so simple as adding a small capacitor thanks Max Ronald wrote: Is the 866 choke-loaded with this ?? You might try a small C-load . Might just do the trick .... "weisselk" schreef in bericht m... I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? Someone here must have done this before. Thanks in advance, Jonathan M. Weiss |
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#13
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I tried Ronald's suggestion for a buzzy input choke in one of my
amps, but it didn't work as I had hoped. My application is 255vac into an LC filter with 230vdc out @ 150ma. I tried to get by with an inexpensive Hammond frame choke. With .47uf, 1uf, and 2uf input caps there was no change in the AC across the choke (122 vac), output voltage, or buzz. At 5uf, the AC across the choke dropped to 50vac and the buzz disappeared. However, now the output voltage was up to almost 300vdc. I had a regular CLC cap input power supply now. Maybe it will work better with Mr. Weiss's higher voltages? I'm curious to learn how this works out for him. - Paul |
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#14
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#15
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RdM wrote: (weisselk) writes: : I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a : choke that is mechanically vibrating and buzzing. The choke reads : fine, so I am assuming that the potting compound shifted over the : decades. Since I had to replace the big oil caps, I would guess that : the extra current surging into the choke melted the compound : insulating the thing. Here is the question: Would placing the choke in : a 200 farenheit oven, very carefully monitored, until the compound : liquified, solve the problem of the mechanical hum? : : Someone here must have done this before. : : Thanks in advance, : : Jonathan M. Weiss Thoughts ... I've come across at least one old HV oil-filled choke (in isolation, ie loose on a shelf) that had signs of slight leakage and audibly sloshed when moved or shaken ... I didn't buy it but wonder if it might have buzzed due to low oil level ... you mention having to replace oil caps. Was there oil leakage, or did they measure bad? See where I'm going ... Could the choke be, or have been, oil-filled? (Obviously an oven won't help if it was.-) (again, just thoughts;-) A choke with a loose bobbin on the core will vibrate and make noise if its submerged wholly or partially in oil. The winding is trying to move, like an armature of a motor that cannot move. There are serious forces inside transformers and chokes with AC across them. The wanna tear themselves apart, but we don't wanna let them. If the potting compound had melted (how long would that take under near short conditions, and would the wiring have held out?) and re-solidified, why would re-melting it and letting it cool again help? [Not saying it wouldn't ... maybe it got hot enough near the windings to boil, create voids on cooling, or something, which could reseal...] If the choke is over heated, maybe due to a failing tube, and a big extra current draw, the windings are stressed and whatever can move, will move. Perhaps you can get at the inside somehow to see what is in there? Drill a small resealable hole if it's a pressed sort too hard to undo? I wonder what was in use then - oil, pitch, wax? Someone might know. Very hard to achieve, so a new choke is the best solution. Why agonise over a bloomin choke? chop off the windings with a chisel, remove the core and rewind it if its a good enough core, and use a fresh bobbin. Bob's your uncle. No more trouble. Finally, perhaps thick shock/grommet mounting away from the chassis, especially if steel, may help, if external field interaction is a cause, or if it is between windings and core, bracing the core with shims and repotting. It might be magnetostriction in the core alone. I'm not sure if tightening the stack if you can get to it would help. Perhaps a combo of repotting, remounting, and acoustic deadening? I potted a large 1.1 kW mains transformer which I wound many years ago, because even though the B was only 0.85 Tesla, it still hummed a bit, and I find C-cores sometimes do that. I made sure all the leads had temperature rated glass fibre sleeving, and then I made a galvanised steel can, with the trannie sitting in the can upside down, just with terminal and fixing lugs flush woth the can top, then I poured molten roof pitch around the whole item, and after it'd cooled down, there was NO more hum. But I vanished the transformer well before potting; this is essential to get right before you pot the item. I must have used maybe a litre or two of roof pitch. The pot forms a nice iron shield around the tranny. I don't like oiler caps or chokes in oil. If ever they leak, you have a mess. Patrick Turner. (0.00c, no charge;-)) Ross M |
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#16
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: cause, or if it is between windings and core, bracing the |COIL| with : shims and repotting. It might be magnetostriction in the core alone. Some chokes just buzz. As Fred said, "it would have never been heard by the theatre audience." http://hyperphysics.phy-astr.gsu.edu...magstrict.html |
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#17
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On Tue, 28 Oct 2003 15:41:40 +0000, weisselk wrote:
I have a late 1930's RCA theater (845pp and 866 rectifiers) amp with a choke that is mechanically vibrating and buzzing. The choke reads fine, so I am assuming that the potting compound shifted over the decades. Since I had to replace the big oil caps, I would guess that the extra current surging into the choke melted the compound insulating the thing. Here is the question: Would placing the choke in a 200 farenheit oven, very carefully monitored, until the compound liquified, solve the problem of the mechanical hum? I've depotted the tar out of old potted radio transformers before and I can tell you that if you try this in a home oven, it'll probably be the last time your wife lets you in the kitchen. Better try getting a junk grill out of a dumpster somewhere, build a low charcoal pile and keep a fire extiguisher handy just in case. It's worth a shot. If worse comes to worse you can put a new choke in the can and pot it with modern material. -- Ned Carlson Triode Electronics Chicago,IL USA www.triodeelectronics.com |
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