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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Jim, re “ …. This would put perhaps more then a KV difference ?between one 5 volt winding and the second HV winding sitting just below it ? I wonder where you get the voltages from . ?we can see on the tube voltage chart 760 VDC on the filament ?but nothing about the plates and nothing about the transformer voltage that I can see. ??And I could not understand some reported voltages and I can’t see some in the halli-manual ?but I finally found this: Presumably 880 is a no load voltage so it will run slightly lower depending on operating mode etc After much head scratching it seems that 880 VAC should boil down to something like 760 VDC for a choke loaded ?rectifier so now I “see” 1760 rms volts across the HV terminals and that would be the prime stressor of that insulation, but under your construction idea, which must be close to the actual the 1760 will only take place between the 5V wdg and the nearby HV wdg , as you implied I think. So I am of the same thinking other than your? “ more then a KV” it could be at the end of a layer and the wdg to wdg insulation ?is not wide enough, and there is a creepage path ?around the end and perhaps corona.? Of course the 1760 could just breakdown? the insulation, as most people know the very ends have higher field intensity ?
? ? From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Wednesday, July 17, 2024 6:13 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY ? Rick You are most likely correct, if the power transformer is still good after some 60 odd years, it may not fail, at least in that manner.?? ? Are the two 5 volt windings side by side on the same top layer?? If this is the case then I can see your corona effect between the high voltage winding closest to the core and it's associated 5 volt filament winding on the top most layer.? This would put perhaps more then a KV difference between one 5 volt winding and the second HV winding sitting just below it.? I didn't see this possibility till now; not having seen the actual transformer and how it is wound.? Is all of the insulation between layers paper or Mylar? Regards, Jim ?
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Rick? ??? ?edited /fractured a bit ??????????? Re ??Back to the original "Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY" subject of this thread, I think topics get started before they get properly thought out; this one is not bad but perhaps too specific and not quite what was intended ?I forgot all about? Halden’s?? actual title???? …”transformer failure ?due to rapid? ?STBY-->OFF-->STBY” ?I think that nobody read the word phrase that Halden actually wrote in the title. ?It seems to me? the general? switching? involving ?????the “STBY” position has been grouped into one PROBLEM AREA, so the real failure condition has been diluted into muddy waters, ??EXCEPT THAT? ?THE “STBY” SWITCHING IS ALWAYS INVOLVED ? I would like to remind us all that Halden ??later re-worded? his concern? in??? /g/HallicraftersRadios/message/30429 ?saying partly.. “I still wish to understand how turning the transmitter off without a pause in STBY, or turning it fully off and immediately on again, can cause a voltage spike between these windings that’s substantially in excess of what occurs during operation, and could cause an arc between windings and thus deteriorate the insulation at the arc site.?“…. Etc? ? There are still two situations in that question:? turning “to OFF” and turning “away from OFF” Personally, I think its worth considering any/all the situations. But it is hard to sort them out, as we have seen ?there's isn't any mention of doing this.? Agree ? The only "Caution" is to pause in STBY before turning it off ???????and they don't say why.? ? Agreed on both points long ago ,a n d? …. they don’t say why because they have red faces, and can’t easily fix the problem- only a US ?senate inquiry would extract the true culprit. ? As to some other comments it seems that the switching caution/problems and ?transformer failures were all part of the new stuff in 1959 and not related to age, but things do deteriorate years later also ? ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Rick W4XA
Sent: Thursday, July 18, 2024 4:24 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY ? On Wed, Jul 17, 2024 at 06:30 PM, don Root wrote:
? ? Well, an obvious way could be the same way a "flash-over"? in a tube rectifier kills the Collins 516F-2? transformer.? ? ? ? Back to the original "Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY" subject of this thread, there's isn't any mention of doing this.? The only "Caution" is to pause in STBY before turning it off and they don't say why.? ? I don't think it has much,? or anything to do with the FIL winding shorting to the HV. (which a LOT of HT32/37 transmitters did) ? The only thing different today is the up to 125V AC line voltage VS 115 or 117V of yesteryear AND Hallicrafters always indicated the thing was "rated" for 105-125V ? Maybe anything over about 105V increases failure rates for 60+ year old components in general!! ? ? -- 73/Rick -- don??? va3drl |
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
On Wed, Jul 17, 2024 at 06:30 PM, don Root wrote:
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Well, an obvious way could be the same way a "flash-over"? in a tube rectifier kills the Collins 516F-2? transformer.?
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Back to the original "Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY" subject of this thread, there's isn't any mention of doing this.? The only "Caution" is to pause in STBY before turning it off and they don't say why.?
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I don't think it has much,? or anything to do with the FIL winding shorting to the HV. (which a LOT of HT32/37 transmitters did)
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The only thing different today is the up to 125V AC line voltage VS 115 or 117V of yesteryear AND Hallicrafters always indicated the thing was "rated" for 105-125V
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Maybe anything over about 105V increases failure rates for 60+ year old components in general!!
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Rick and jim ?Re You are most likely correct, if the power transformer is still good after some 60 odd years, it may not fail, at least in that manner.?? How many ways do you think there are to smoke the HT-37 transformer? Just one? ? I am a professional knob-speed-twisting-tester and can prove you wrong--- on your rig. The original owners read the bible before plugging it in,? but then some others don’t, just plug’er in, turn the knobs and find out what happens.. that is what they are there for!!? …. CW via smoke signals! From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Wednesday, July 17, 2024 6:13 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY ? Rick You are most likely correct, if the power transformer is still good after some 60 odd years, it may not fail, at least in that manner.?? ? Are the two 5 volt windings side by side on the same top layer?? If this is the case then I can see your corona effect between the high voltage winding closest to the core and it's associated 5 volt filament winding on the top most layer.? This would put perhaps more then a KV difference between one 5 volt winding and the second HV winding sitting just below it.? I didn't see this possibility till now; not having seen the actual transformer and how it is wound.? Is all of the insulation between layers paper or Mylar? Regards, Jim
-- don??? va3drl |
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Re: HT-44/SX-117 Sidetone?
I used one of these for a sidetone oscillator for a SR 150.? C$7.25 | DC 9-12V XR2206 Function Signal Generator DIY Kit Sine/Triangle/Square Output 1Hz -1MHz Signal Generator? Adjustable module I used the sine wave function and coupled it to the speaker with an electrolytic capacitor. I used a double pole relay, one pole to key the oscillator to the speaker and one to isolate the grid block from the key.? 73 Don ve3ids? On Wed., Jul. 17, 2024, 7:25 p.m. Floyd - K8AC via , <floydsense=[email protected]> wrote:
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
On Wed, Jul 17, 2024 at 03:23 PM, Jim Whartenby wrote:
Can't say.
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I've never done a forensic analysis of one of these transformers.? The one in my HT32B (5V winding is taped off) has been that way ever since I got it.? It's still "shorted" to the HV winding but it doesn't seem to hurt anything.? Everything works.? No guarantee it won't "zap" itself someday though!? I'll likely be plunking down the $$ to get another one from Hammond.??
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I like the HT32B so much? that I might get one anyway and just change it out to prevent mayhem in the future!
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
Rick You are most likely correct, if the power transformer is still good after some 60 odd years, it may not fail, at least in that manner.?? Are the two 5 volt windings side by side on the same top layer?? If this is the case then I can see your corona effect between the high voltage winding closest to the core and it's associated 5 volt filament winding on the top most layer.? This would put perhaps more then a KV difference between one 5 volt winding and the second HV winding sitting just below it.? I didn't see this possibility till now; not having seen the actual transformer and how it is wound.? Is all of the insulation between layers paper or Mylar? Regards, Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Wednesday, July 17, 2024 at 03:44:10 PM CDT, Rick W4XA <myr748@...> wrote:
On Wed, Jul 17, 2024 at 12:12 PM, Jim Whartenby wrote:
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I would venture to guess that if an HT32 or HT37 transformer has NOT had a FIL-->HV short after only 60 years or so,??? If probably never will.? I would submit that a HIPOT test wouldn't be required.? If it didn't short @ 700+ volts DC, it probably won't at a line voltage peak of only 170V .
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But enough Collins 516-F2 power supply tube rectifiers have internally "flashed" and prompted a LOT of people to consider using solid state rectifiers. to make me at least a little wary any up to 60 year old 5U4, 5V4 , 5R4 and quite frankly most all vacuum diodes.???
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Certainly not all of them fail and they're easily replaceable.? Transformers OTOH are well,? replaceable too but they seem to cost a tad more!
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I am of course not suggesting the solid state rectifier swap is a complete cure-all.? But it is a good alternative if done right.
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ymmv
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
On Wed, Jul 17, 2024 at 12:12 PM, Jim Whartenby wrote:
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I would venture to guess that if an HT32 or HT37 transformer has NOT had a FIL-->HV short after only 60 years or so,??? If probably never will.? I would submit that a HIPOT test wouldn't be required.? If it didn't short @ 700+ volts DC, it probably won't at a line voltage peak of only 170V .
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But enough Collins 516-F2 power supply tube rectifiers have internally "flashed" and prompted a LOT of people to consider using solid state rectifiers. to make me at least a little wary any up to 60 year old 5U4, 5V4 , 5R4 and quite frankly most all vacuum diodes.???
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Certainly not all of them fail and they're easily replaceable.? Transformers OTOH are well,? replaceable too but they seem to cost a tad more!
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I am of course not suggesting the solid state rectifier swap is a complete cure-all.? But it is a good alternative if done right.
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ymmv
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
Rick I'll keep my remarks centered on the HT-37 since that is the topic at hand. You said: "The voltage drop afforded with certain tube rectifiers is definitely a factor for low B+ voltage levels.? It's the main problem in the Johnson Valiant.? (Low B+ should be around 300V and many measure a little less than 400V)" Both of the power supplies in the HT-37 are not regulated so their exact output voltage depends entirely on whatever the line voltage happens to be at the time of measurement.? Thankfully the HT-37 manual gives the voltage range it was designed to operate with:? 105 to 125 VAC.? The mean voltage of this range is the old standby, 115 VAC.? This line voltage standard was introduced in 1927 at the dawn of AC powered radios! The voltage chart in the manual, Figure 13, shows a typical heater voltage of 6.3 volts and a typical B+ of 300 VDC.? The B+ on the 5V4 cathode is listed as 335 VDC at the input to the pi filter.? So some 35 volts is lost in the power supply filter.? It is interesting that the schematic shows the 5V4 as a directly heated cathode but in reality, the 5V4 is indirectly heated.? Because of the added mass to the cathode, it takes longer to come up to conduction temperature which is a crude soft start feature. Safe to say that the 6.3 heater voltage shown was measured at a line voltage of 115 VAC.? So both the heater and B+ voltages will change as the line voltage goes from one limit to the other.? This means that the heater voltage varies over the range of 5.7 volts to 6.9 volts.? B+ also varies from 270 VDC to 330 VDC. You said: "An advantage of freeing up a 5V filament winding is that one can then use it to "buck" the primary so as to accommodate increased AC line voltage.? (I don't have enough auto-transformers [VARIAC's] to go around!)" If the power transformer used in the HT-37 is prone to a hard short circuit between the 5 volt heater supply and the associated high voltage winding, then the use of the 5 volt winding to buck the line voltage is a solution which could do real damage.? One would have to do a high voltage test for leakage resistance between the two windings and then hope that the two windings will remain isolated when the 5 volt heater winding is then used to buck the AC line. You said: "If the "low" b+ is still too high then an FET "HV" regulator can be a pretty good option." I feel that voltage regulation is a solution in search of a problem.? As stated earlier, the B+ range is line voltage dependent so the range for the low B+ is 270 VDC to 330 VDC.? If your B+ measurement is within this range, all is good. You said: "The? voltage drop with HV rectifiers such as the 866A (according to the data sheet) isn't much and doesn't seem to be as much of a? problem.? But with the 5U4 and other low voltage "diode" valves, it's significant." Let's look at the 5V4 specs in the GE tube manual. see:? The graph labeled "Average Plate Characteristics", at the top of page 2, shows how the internal resistance of the 5V4 changes with the change in current through the rectifier.? This internal resistance is obviously non-linear.? At plate currents below 50 mA, the internal resistance is at or slightly above 240 ohms.? At 120 mA, the internal resistance is now 167 ohms.? At 220 mA, the internal resistance is 136 ohms and so on.? At the rated current of 175 mA, averaged over a full cycle of the AC line voltage, the internal resistance is 143 ohms.? You should accept that all of this data was published in the age of slide rules.? These are all approximations. The peak maximum one-time current is 3.5 amps, the steady state peak current is 525 mA.? What this means is that the charging current for the filter capacitor plus the power supply load has to be limited to these values.? In order to comply with this restriction, an additional resistance is inserted in series with each rectifier plate.? The total resistance is specified as 100 ohms in the 5V4 spec and this resistance is made up of all the series resistances in the power transformer primary and secondary and noting the turns ratio of the power transformer plus what else is needed to make a total of 100 ohms per plate.? For a explanation of this process, see: Radiotron Designers Handbook, 4th Edition, page 99, paragraph (iii) (a) Rectification test. Power diodes. Silicon diode rectifiers do not have this low of a current restriction.? The low current voltage drop across a typical SS rectifier is about 0.6 volts.? At one amp, this voltage drop is about 1 volt.? So the internal resistance of a SS rectifier is typically less then half an ohm.? So going solid state eliminates the power loss seen in vacuum tube rectifiers.? This power loss is the sum of the heater power of 10 watts plus the power loss internal to the rectifier ( I^2 X R) or about another 4.5 watts at rated current.? Going solid state eliminates some 15 watts of heat.? Of course eliminating this heat raises B+ by about 25 volts.? There is no free lunch! What effect does this increase in B+ have on the equipment?? Not much, IMHO.? Generally, most vacuum tubes are pentodes so there is little to no change in plate current since plate current is controlled by grid bias and not by plate voltage.? The triodes used in the HT-37 all appear to all use a cathode resistor which self biases the grid to hold plate current relatively constant.? As an example, if the plate current is 10 mA and B+ increases by 25 volts, the tube will dissipate about an additional 1/4 watt.? The heater power of a 12AT7 is 6.3v @ 0.3 amps for 1.9 watts per tube.? For a 6AH6, it is 6.3v @ 0.45 amps for 2.8 watts per tube.? The increase in plate dissipation caused by using a solid state rectifier is swamped by each tube's heater power. Regards, JimLogic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, July 16, 2024 at 05:29:32 PM CDT, Rick W4XA <myr748@...> wrote:
On Tue, Jul 16, 2024 at 01:43 PM, Jim Whartenby wrote:
?
The voltage drop afforded with certain tube rectifiers is definitely a factor for low B+ voltage levels.? It's the main problem in the Johnson Valiant.? (Low B+ should be around 300V and many measure a little less than 400V)
?
An advantage of freeing up a 5V filament winding is that one can then use it to "buck" the primary so as to accommodate increased AC line voltage.? (I don't have enough auto-transformers [VARIAC's] to go around!)
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If the "low" b+ is still too high then an FET "HV" regulator can be a pretty good option.??
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The? voltage drop with HV rectifiers such as the 866A (according to the data sheet) isn't much and doesn't seem to be as much of a? problem.? But with the 5U4 and other low voltage "diode" valves, it's significant.?
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73/Rick
W4XA __________________________________ All posts are created using OpenSuSE Leap 15.5 x64 Linux |
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Thanks Paul, I didn’t know? what?? “PW Dahl-designed Hammond transformer” meant. ??Now I see ………? ? Peter W Dahl Transformers PWDAHL Series Magnetics for Amateur Radio Products This listing of transformers and chokes is directly from the Peter W. Dahl files. ? ? this link might be helpful to others don From: [email protected] [mailto:[email protected]] On Behalf Of Paul Christensen
Sent: Tuesday, July 16, 2024 9:22 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY ? Just me but I dislike SS rectifier replacements for several reasons.? To deal with the HV to filament winding problem, a new PW Dahl-designed Hammond transformer was recently ordered for my HT-37. ?Although there’s no proof, I must believe that Dahl knew how to design a transformer with adequate winding isolation. ? Paul, W9AC
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
I have no proof of this but I think the original transformers were simply poorly constructed, meaning the original supplier (Stancor ,or other manufacturer of the time) just didn't do a very good job insulating the filament winding.?
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A great many of them had this problem and a great many of those were "fixed" by just pulling the rectifier tube and replacing it with either individual diodes or a module.? Mine has the module that just plugs into the original tube socket.? It's been that way as long as I have had it (probably 15 years or so) with no more problems.
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The corona issue might have been a separate problem,? but it appears that whatever they did to "fix" that didn't fix the poor insulation.
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In my opinion, the best final fix is to get the high voltage DC off that winding.?
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I suspect PW Dahl did know about the "problem" and more than adequately addressed insulating the winding so the Hammond transformers are also probably? just fine.?????
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Now, having said that, if I were to replace my transformer, I would leave the Si diodes in there and if the voltage ended up a little too high, I would use an FET regulator.
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On Tue, Jul 16, 2024 at 06:22 PM, Paul Christensen wrote:
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Just me but I dislike SS rectifier replacements for several reasons.? To deal with the HV to filament winding problem, a new PW Dahl-designed Hammond transformer was recently ordered for my HT-37. ?Although there’s no proof, I must believe that Dahl knew how to design a transformer with adequate winding isolation. ? Paul, W9AC ? From: [email protected] <[email protected]> On Behalf Of Rick W4XA
Sent: Tuesday, July 16, 2024 9:06 PM To: [email protected] Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY ? My plan would be to replace the tube rectifiers with diodes anyway.? ? -- 73/Rick |
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
My plan would be to replace the tube rectifiers with diodes anyway.?
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
Tube Rectifier Compatibility Chart attached.
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
On Tue, Jul 16, 2024 at 01:43 PM, Jim Whartenby wrote:
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The voltage drop afforded with certain tube rectifiers is definitely a factor for low B+ voltage levels.? It's the main problem in the Johnson Valiant.? (Low B+ should be around 300V and many measure a little less than 400V)
?
An advantage of freeing up a 5V filament winding is that one can then use it to "buck" the primary so as to accommodate increased AC line voltage.? (I don't have enough auto-transformers [VARIAC's] to go around!)
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If the "low" b+ is still too high then an FET "HV" regulator can be a pretty good option.??
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The? voltage drop with HV rectifiers such as the 866A (according to the data sheet) isn't much and doesn't seem to be as much of a? problem.? But with the 5U4 and other low voltage "diode" valves, it's significant.?
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
开云体育Rick , Re ??Disc ceramic caps shorting would seem rare but you never really know what a 60+ year old part will do! ? the judicious use of fuses even buried inside can mitigate a small failure from escalating to a major disaster . may tektronix 555 has fuses soldered in for that reason. ?? ?I was lucky when the heat and aroma ?arose? when my S-47 was upside down in the sick bay. ? And it happens on 6 months old stuff too don again?? Now what about adding? micro smoke detectors in your HT exhaust ? ? From: [email protected] [mailto:[email protected]] On Behalf Of Rick W4XA ?? ……………… Disc ceramic caps shorting would seem rare but you never really know what a 60+ year old part will do! ? -- don??? va3drl |
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
On Tue, Jul 16, 2024 at 01:09 PM, don Root wrote:
Yeah....No kidding!
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I even took it a step further and replaced the original HT-45 plate transformer (SEC voltage 6750VAC? center-tapped) with the Drake L-4PS transformer that requires a voltage doubling circuit (identical to the L-4/L-7 amps)
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There wasn't really anything wrong with the old plate transformer,? I just wanted to eliminate what I thought would be the next fault point:? The swinging choke in the HV line (and bleeder required minimum current necessary to make it 'swing')
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The original manufacturers of the old components that our old radios are using never expected these parts to still be in use 60-70 years later!?
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Even simple things like disc ceramic caps can (and do)? fail!
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In my HT-44, I had a screen bypass capacitor (C95 or 96 rated at 500V)? short to ground that caused? the 6DQ5 screen dropping resistor (R77) to quickly overheat..... with 250+ volts on it, that's roughly 35W!? I was pretty quick on the? switch but it was pretty obvious which resistor was R77!!!
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The HT44 sits on a desk and my wife came in the room to ask me something and standing right at the the thing If she hadn't noticed (the smell) right away, it would have surely caused some fireworks under the tube!
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Disc ceramic caps shorting would seem rare but you never really know what a 60+ year old part will do!
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Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY
Comments is BLUE Jim Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy
On Tuesday, July 16, 2024 at 01:56:22 PM CDT, Rick W4XA <myr748@...> wrote:
On Tue, Jul 16, 2024 at 09:15 AM, Jim Whartenby wrote:
You're right.? The additional load due to the filament current is pretty much insignificant Not sure about this, see below:? ?
HV choke input power supplies? used tube type rectifiers in a full wave CT configuration and placed high voltage on the either the separate fil-transformer or integral filament winding in the main power transformer. Yes, I understand? ?
?Those filament transformer windings had to be designed to withstand very high operating voltages.? ?Yes ?
It appears that most (or all) of the (HT32/37)? main power transformers failed due to HV breakdown in teh filament winding.?
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I have failures here.? The separate? filament transformer in my Hallicrafters HT45 (pair of 866A rectifiers) faulted HV to filament.?
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In my HT32B, the filament winding in the main power transformer failed due to an HV short to the filament winding. What this indicates is that the filament winding has made contact with a wire in the HV winding layer just beneath it. Assuming a 1/2 volt per turn, the 5 volt winding will have 10 turns which is most likely in a single layer.? As long as only one connection was made to the HV by the filament winding, no harm done when going solid state.? But what mechanism is preventing a second internal short between the HV winding and the filament winding? ?
The problem is not heat in any integral HV filament winding, it's HV breakdown and it was so frequent in the
HT-32/37 series that they tried (it appears unsuccessfully) to "fix" it by redesigning the transformer.? This is the rub.? One end of the filament winding is connected to the HV winding at the rectifier socket.? The other filament winding goes to the other filament pin on the rectifier socket.? Lets say that the differences in wire thickness between filament winding and HV winding is 10X.? So the 5 volt winding over lays a 50 volt winding.? Is 50 volts enough to punch through the insulation of the two respective windings? ?
According to the tech service note on Walts site:
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If this indeed happened in 1959, then some of the HT37's and maybe all of the HT32A/B's should have gotten this updated transformer.
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My HT32B certainly would have,? but it still had the HV/FIL short. Then the failure mechanism is different from what is assumed.? On other words, it is not corona. ?
None of these failures would have been due to transformer over-heat unless the heat caused the "corona" condition.? I would think that's unlikely. Heat will cause minute movement of the windings as they expand and contract so this possibility should not be discounted.? All that matters is how many heat cycles have taken place over those 65 years since the HT was new. ?
It was an insulation breakdown (or the actual "Corona" condition)? that they tried to fix,? but it didn't fix it in my HT32B.
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The only sure way to fix it is to remove the high voltage from the filament winding. (Either by installing a separate filament transformer or solid state diodes ) Agreed, but either method eliminates the differential expansion between the filament and HV windings.? The filament winding is the prime mover with 10 watts of driving force.? We would have to calculate how much power is in the HV winding layer just below the filament winding but it is most likely much lower. ?
There is no way I would apply 700-900v DC to a filament winding in a 60-70 year old transformer now or in the case of the? HT-45,? Would I apply? 3000+ VDC to any old filament transformer. Well, if it is a separate transformer, the above mentioned movement does not exist between the two windings. ?
The smartest thing anyone can do in these old radios is to get the high voltage off the filament transformers or windings in main power transformers. This is interesting as an aside.? This is perhaps the only time I can recall where one is advocating the change to SS and ignoring the lower voltage drop of SS rectifiers vs the tube rectifier voltage drop.? This has been an on going debate on many reflectors.? <grin>? That said, I believe that you are right! ?
So changing to solid state diodes absolutely fixes the problem by eliminating the fault path. Well, we really don't know what the fault path is, we are only assuming that we do.? It will be interesting to see if the newest replacement transformers have the same problem down the line.? If any of us live that long! Regards, Jim ?
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73/Rick
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don Root
Rick W7IMM
Don VE3IDS
Floyd - K8AC
Jim Whartenby
Paul Christensen