Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

开云体育

Jeff, as a guess, your likely insulation breakdown was enough to continue building a carbon track between windings even though the 5v was isolated; and must have eventually destroyed the T-T [turn to turn] insulation on a winding and eventually became a real short. Any T-T short in a transformer ?is fatal . don

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of Jeff AC0C
Sent: Monday, July 1, 2024 5:55 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

Two the fellow's earlier comment on not using the 5V line, when
reworking my 32A, that was the first thing I did - move to SS
specifically to idle the 5V line and avoid the risk of the well reported
short involving the 5V line.? Toward the end of the process, I was
getting ready to finish up alignment and had let the transmitter run
overnight to ensure it had reached stable temp across the chassis.? When
I came down the next morning, the rig was dead.? Painfully confirmed
later that some amount of windings on the primary or secondary had
shorted (there were no detectable winding-to-winding shorts at idle -
and I did not have a spare HVPS available at that time to test higher).?
However, as the variac was ramped up, the current draw of the
disconnected transformer soared.? So the 5V to HV short is apparently
not the only transformer failure mode with this model, unfortunately.

I solved the problem by cannibalizing another HT32.? But assuming Murphy
may like to come see me soon, I bought one of those shiny Dahl
transformers and it's here on the bench ready to run to the rescue when
the current transformer eventually dies.? Of course, with my luck,
simply having bought the Dahl transformer may keep Muphy away allowing
the current transformer to run the rest of my days.? ha ha.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com
?

--
don??? va3drl

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30451

开云体育

If I were wanting to minimize the ?first excursion of current to a choke or transformer, I would want a switch that closes near the plus or minus crest of the incoming voltage, and not one that switches at zero crossings. ??Switching capacitors is another matter as is switching other loads. ?don

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of Paul Christensen
Sent: Monday, July 1, 2024 5:15 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

>”In addition, if you choose the "right" SSR, it will turn-on at "Zero-Crossing" meaning it will activate when the sine-wave passes zero volts (they're designed that way).”

?

Be careful when using zero-crossing (ZC) SSRs to switch extra-large power transformers.? I recently tried ZC type in my Collins 516F-2 and Drake PS-7 power supplies.? In each case, it resulted in pulsing high current surges.? In one instance, the 4A fuse blew open in my Collins power supply during a high-surge event.?

?

The attached Tyco Application Note warns against using ZC type SSRs with highly inductive loads.

?

The problem was fixed by changing to a random-switching type SSR from Crydom.? The AC load is switched from antiparallel SCRs.? Datasheet attached.? I know others have used ZC type SSRs to switch large transformers without any problems. ?I’m only offering this advice out of caution from my own experience.? ?

?

Paul, W9AC

--
don??? va3drl

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30450

Two the fellow's earlier comment on not using the 5V line, when
reworking my 32A, that was the first thing I did - move to SS
specifically to idle the 5V line and avoid the risk of the well reported
short involving the 5V line.? Toward the end of the process, I was
getting ready to finish up alignment and had let the transmitter run
overnight to ensure it had reached stable temp across the chassis.? When
I came down the next morning, the rig was dead.? Painfully confirmed
later that some amount of windings on the primary or secondary had
shorted (there were no detectable winding-to-winding shorts at idle -
and I did not have a spare HVPS available at that time to test higher).?
However, as the variac was ramped up, the current draw of the
disconnected transformer soared.? So the 5V to HV short is apparently
not the only transformer failure mode with this model, unfortunately.

I solved the problem by cannibalizing another HT32.? But assuming Murphy
may like to come see me soon, I bought one of those shiny Dahl
transformers and it's here on the bench ready to run to the rescue when
the current transformer eventually dies.? Of course, with my luck,
simply having bought the Dahl transformer may keep Muphy away allowing
the current transformer to run the rest of my days.? ha ha.

73/jeff/ac0c
alpha-charlie-zero-charlie
www.ac0c.com

Show quoted text

On 7/1/2024 4:15 PM, Paul Christensen wrote:

"In addition, if you choose the "right" SSR, it will turn-on at
"Zero-Crossing" meaning it will activate when the sine-wave passes zero
volts (they're designed that way)."

Be careful when using zero-crossing (ZC) SSRs to switch extra-large power
transformers. I recently tried ZC type in my Collins 516F-2 and Drake PS-7
power supplies. In each case, it resulted in pulsing high current surges.
In one instance, the 4A fuse blew open in my Collins power supply during a
high-surge event.

The attached Tyco Application Note warns against using ZC type SSRs with
highly inductive loads.

The problem was fixed by changing to a random-switching type SSR from
Crydom. The AC load is switched from antiparallel SCRs. Datasheet
attached. I know others have used ZC type SSRs to switch large transformers
without any problems. I'm only offering this advice out of caution from my
own experience.

Paul, W9AC

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30449

On Mon, Jul 1, 2024 at 02:15 PM, Paul Christensen wrote:

>”In addition, if you choose the "right" SSR, it will turn-on at "Zero-Crossing" meaning it will activate when the sine-wave passes zero volts (they're designed that way).”

?

Be careful when using zero-crossing (ZC) SSRs to switch extra-large power transformers.? I recently tried ZC type in my Collins 516F-2 and Drake PS-7 power supplies.? In each case, it resulted in pulsing high current surges.? In one instance, the 4A fuse blew open in my Collins power supply during a high-surge event.?

?

The attached Tyco Application Note warns against using ZC type SSRs with highly inductive loads.

?

The problem was fixed by changing to a random-switching type SSR from Crydom.? The AC load is switched from antiparallel SCRs.? Datasheet attached.? I know others have used ZC type SSRs to switch large transformers without any problems. ?I’m only offering this advice out of caution from my own experience.? ?

?

Paul, W9AC

?Hi Paul,

Thanks for that info.....I seem to? remember you mentioning this before and I cannot disagree!

I'm using 2-OPTO22 240A25's in the Loudenboomer power supply which is essentially a complete L-4B (L-4PS) power supply with the exception of using 2 Semtech 5000v diode stacks and 8-330uF 450V? (parallel) caps instead of the original 220uF caps.

I have switched it on and off multiple times trying to get it to "fault" and so far it works flawlessly!

I used OPTO22 120A25 SSR's in the AC-4's and same.? no problems yet.

I suppose I could stick a 1ohm 1% precision resistor in the AC line and directly measure the actual instantaneous? currents with a scope.

OPTO22 does mention what one should do to mitigate this and based on the information on page 17 in their specifications, It's close using the 240A25 for the L-4PS plate transformer and I am well over the requirements using the 120A25 for the AC-4 transformer.

My final plan is to change to the 240A45 in both the Loudenboomer and L-4B power supplies,
--
73/Rick
W4XA
__________________________________
All posts are created using OpenSuSE Leap 15.5 x64 Linux

OPTO22_Solid_State_Relays_data_sheet-5.pdf

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30448

开云体育

>”In addition, if you choose the "right" SSR, it will turn-on at "Zero-Crossing" meaning it will activate when the sine-wave passes zero volts (they're designed that way).”

?

Be careful when using zero-crossing (ZC) SSRs to switch extra-large power transformers.? I recently tried ZC type in my Collins 516F-2 and Drake PS-7 power supplies.? In each case, it resulted in pulsing high current surges.? In one instance, the 4A fuse blew open in my Collins power supply during a high-surge event.?

?

The attached Tyco Application Note warns against using ZC type SSRs with highly inductive loads.

?

The problem was fixed by changing to a random-switching type SSR from Crydom.? The AC load is switched from antiparallel SCRs.? Datasheet attached.? I know others have used ZC type SSRs to switch large transformers without any problems. ?I’m only offering this advice out of caution from my own experience.? ?

?

Paul, W9AC

Crydom SSR.pdf

Solid State Relays - ENG_CS_13C3206_AppNote_0513_Beware_of_Zero-Crossover_Switching_of_Transformers_13c3206r[1].pdf

Re: SX-101 A Drift

#30447

That thing was probably a Dampp-Chaser (with two P's). They were
used in some receivers and in other equipment to keep moisture out. Just
a low power heating element. I don't want to say they are not made any
more because I think I saw one on the web but a search will determine that.

Show quoted text

On 7/1/2024 11:21 AM, Alan Swinger wrote:

I had a SX-101A a few yrs back. In the process of restoring it I found
that there was a large "Heating" resistor on the underside of the
chasis, the purpose of which is to keep the oscillator at a temp so it
does not drift. It was inop and I could not find a direct replacement.
So, I found smaller heating resistors that I mounted on a copper strip
which I cut to fit the old resistor standoff mounts. It worked all OK,
and no drift! Hope that helps. - alan K9MBQ

_._,_._,_

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

Re: SX-101 A Drift

#30446

About the only way to check drift on AM is to listen to the local
oscillator on another receiver. Since the other receiver may not be
stable the LO should be beat against a stable oscillator, perhaps a
crystal calibrator. Otherwise how are you to tell if the LO is drifting?
With no BFO there is only gross drift such as will put the station out
of tune. Because the BFO operates at a frequency not commonly covered by
receivers it will not be possible to hear it against a stable oscillator.
BFOs do drift but since they usually operate at low frequencies, the
drift is usually small compared to the local oscillator. OTOH, on my
HQ-129-X I can tell the BFO drifts in comparison to the crystal flter.
If I tune a signal exactly to the peak of the crystal filter at its
narrowest (about 50 Hz) and zero beat it, after some time, especially if
the receiver is cold, the BFO will change. This drift is quite large
compared to what could be accounted for by drift in the filter crystal.
Its plain thermal drift. If I adjust the BFO after the set has heated up
for an hour there is no longer any drift. The HQ-129-X despite being
something of an antique is fairly stable.

Show quoted text

On 7/1/2024 10:19 AM, thoyer via groups.io wrote:

If you mostly operate CW (as do I), did you check the "drift" on AM?
Could be the BFO?

Tom
W3TA

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30445
Edited

On Mon, Jul 1, 2024 at 11:31 AM, HF wrote:

A few minutes after posting my question, I was thinking about what happens when a transformer turned on and came up with an idea:
If the transformer is turned on in the middle of a power line cycle, the initial current flow would be higher than it would be if turn-on occurred at the zero crossing.? The transformer's magnetic field is at zero and suddenly there's a high voltage on the primary.? Could that current cause a faster change in the magnetic field than would occur at subsequent cycles?? A faster change in the magnetic field would cause a voltage spike at the secondary.
Halden VE7UTS

Halden,

You're quite right!

You have hit on something that is thought to be a cause of certain ON/OFF switch failures in the L-4, L-4B, L-7 amplifiers and the Drake T-4X/TR-4/6 transceivers and others.

Have you ever heard the fairly loud "SNAP" or seen the spark that occurs *sometimes* when turning on any light wall-switch or appliance or plugging/un-plugging a power cord from something that is already switched on?

It doesn't always happen. but when it does with plugging/un-plugging? you actually see the spark (and/or hear it) !!

That's because you are doing it at or near the exact moment at the sine-wave peak (+/-)? voltage which for 120AC is about 170V (peak) and for 240V the peak is around 340V
https://www.allaboutcircuits.com/tools/peak-voltage-calculator/

It's no wonder that switch (or relay)? contacts eventually "burn" enough to render them inoperable (and in the case of the L-4 and L-4B (and maybe the L-7)? having "un-obtanium" switches,? catastrophic failure)

One way around this is to use an electro-mechanical? relay which just transfers the contact burning to the presumably replaceable relay contacts. (it does nothing for the "stress" on the other components)?? The original relay in my HT45 Loudenboomer had nearly unusable burnt/melted contacts from many years of ON/OFF switching.

On the other hand, using a solid-state relay eliminates the stress on the switch contacts because the switch is now switching potentially only a "few" mA of either low voltage? DC or 120/240 AC (if using an AC controllable SSR) .? In addition, if you choose the "right" SSR, it will turn-on at "Zero-Crossing" meaning it will activate when the sine-wave passes zero volts (they're designed that way)

I did this in my HT-45 Loudenboomer power supply with the Drake L-4B plate transformer (2-240V, 25A AC solid-state relays in the 240V line)? using a similar circuit to the Command Technologies,? Commander 2500 below.

Commander 2500 manual excerpt:

I also did 2 of my 4 Drake AC-4 power supplies (I'm planning to do the other 2)
(Photo below is before wiring it up)

Prior to installing this SSR, the transformer would sometimes make a "thump" at turn on.? With the SSR in place, it never makes a noise @ turn-on.

I was planning to do the same with my Collins 516-F2 but the excellent Electronic Specialties rebuild kit includes an electro-mechanical relay that takes the stress away from the nearly unobtainable power switch integral with the mic-gain control on a KWM-2/2A.? I suppose the relay will eventually "burn" and if it does, I'll wire in an SSR there too!.

--
73/Rick
W4XA
__________________________________
All posts are created using OpenSuSE Leap 15.5 x64 Linux

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30444

My homebuilt HT-37 uses diode boards in place of the vacuum tube rectifiers so that is definitely an option. My overall supply is not the original Hallicrafters design though. Uses a large TV transformer from the junkbox and the "economy power supply" circuit to get HV and low B+. Also eliminated the swinging choke needed for the tube circuit.

Ed, WA9GQK

On Monday, July 1, 2024 at 03:25:52 PM CDT, HF via groups.io <incorridge@...> wrote:

It's actually not necessary to restore this transformer.? I just need to not use the 5V winding.? That might be as easy as stuffing a couple diodes into the socket where the HV rectifier used to be.? I could probably hook the 5V winding in series with the primary to reduce the output voltage a bit.
Cheers
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30443

It's actually not necessary to restore this transformer.? I just need to not use the 5V winding.? That might be as easy as stuffing a couple diodes into the socket where the HV rectifier used to be.? I could probably hook the 5V winding in series with the primary to reduce the output voltage a bit.
Cheers
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30442
Edited

?oops ? ? edit? all. ?? dumb computer handler , I deleted that first stuff
now...
motors rely on air FLOW for convection cooling so don't block intended air passages with anything.

?these transformers have only internal conduction heat transfer so it is another story IMO.
hope this edit works , don

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of edward schumacher
Sent: Monday, July 1, 2024 3:23 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

Just a thought here. VPI'ing the entire transformer will reduce its heat dissipating capability.

?

I once painted an electrical insulating varnish obtained at work on the exposed windings of an old window fan motor. In short order, it overheated and died. No good deed goes unpunished! It needed the old thinner wire coating so the air motion would also cool the motor.

?

Perhaps a judicious painting of insulating varnish just around the area of failure would work if it can be accessed. Don't know if Glyptol is still made but we also used to use ordinary nail polish even on the HV leads in TV sets.

?

73 ... Ed, WA9GQK

?

On Monday, July 1, 2024 at 01:43:19 PM CDT, Jim Whartenby via groups.io <old_radio@...> wrote:

?

Cut a plastic milk jug that is just a bit bigger then the transformer so that the transformer can be completely covered.? Under vacuum for 10 minutes should be good enough.

Jim

?

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

?

On Monday, July 1, 2024 at 01:31:53 PM CDT, HF via groups.io <incorridge@...> wrote:

?

Hi Jim,
Thanks for the tip on how to make a vacuum chamber!? That sounds quite easy.? There are plenty of industrial vacuum pumps out on ebay that might pump it down faster.? Or, if one has an air compressor with a fitting on its inlet, it might be put into service as a vacuum pump.?
I'm fortunate to have a vacuum oven I bought at a laboratory surplus sale 25 years ago.? But the pressure cooker would be even better than that because if there is any leaking liquid, it'll just accumulate in the bottom of the pot where it can be easily cleaned up.? On the other hand, the vacuum oven has a window, enabling spectators to observe the proceedings.
But how would one get the varnish in?? Immersion?? Would marine spar varnish work?
Halden VE7UTS

?

--
don??? va3drl

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30441

Just a thought here. VPI'ing the entire transformer will reduce its heat dissipating capability.

I once painted an electrical insulating varnish obtained at work on the exposed windings of an old window fan motor. In short order, it overheated and died. No good deed goes unpunished! It needed the old thinner wire coating so the air motion would also cool the motor.

Perhaps a judicious painting of insulating varnish just around the area of failure would work if it can be accessed. Don't know if Glyptol is still made but we also used to use ordinary nail polish even on the HV leads in TV sets.

73 ... Ed, WA9GQK

On Monday, July 1, 2024 at 01:43:19 PM CDT, Jim Whartenby via groups.io <old_radio@...> wrote:

Cut a plastic milk jug that is just a bit bigger then the transformer so that the transformer can be completely covered.? Under vacuum for 10 minutes should be good enough.

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

On Monday, July 1, 2024 at 01:31:53 PM CDT, HF via groups.io <incorridge@...> wrote:

Hi Jim,
Thanks for the tip on how to make a vacuum chamber!? That sounds quite easy.? There are plenty of industrial vacuum pumps out on ebay that might pump it down faster.? Or, if one has an air compressor with a fitting on its inlet, it might be put into service as a vacuum pump.?
I'm fortunate to have a vacuum oven I bought at a laboratory surplus sale 25 years ago.? But the pressure cooker would be even better than that because if there is any leaking liquid, it'll just accumulate in the bottom of the pot where it can be easily cleaned up.? On the other hand, the vacuum oven has a window, enabling spectators to observe the proceedings.
But how would one get the varnish in?? Immersion?? Would marine spar varnish work?
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30440

The only problem is that the circuit is inductive so the voltage and current are out of phase with each other, with the voltage leading the current.? Remember "ELI the ICE man"?? At the voltage zero crossing, current is near a maximum along with the magnetic field.? The changing magnetic field generates a counter EMF in the transformer windings to oppose a change in current.? In my mind, arcing is a voltage thing so current plays a minor role.? But I have been wrong many times in the past!? <grin>

Regards,

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

On Monday, July 1, 2024 at 01:31:22 PM CDT, HF via groups.io <incorridge@...> wrote:

A few minutes after posting my question, I was thinking about what happens when a transformer turned on and came up with an idea:
If the transformer is turned on in the middle of a power line cycle, the initial current flow would be higher than it would be if turn-on occurred at the zero crossing.? The transformer's magnetic field is at zero and suddenly there's a high voltage on the primary.? Could that current cause a faster change in the magnetic field than would occur at subsequent cycles?? A faster change in the magnetic field would cause a voltage spike at the secondary.
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30439

Cut a plastic milk jug that is just a bit bigger then the transformer so that the transformer can be completely covered.? Under vacuum for 10 minutes should be good enough.

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

On Monday, July 1, 2024 at 01:31:53 PM CDT, HF via groups.io <incorridge@...> wrote:

Hi Jim,
Thanks for the tip on how to make a vacuum chamber!? That sounds quite easy.? There are plenty of industrial vacuum pumps out on ebay that might pump it down faster.? Or, if one has an air compressor with a fitting on its inlet, it might be put into service as a vacuum pump.?
I'm fortunate to have a vacuum oven I bought at a laboratory surplus sale 25 years ago.? But the pressure cooker would be even better than that because if there is any leaking liquid, it'll just accumulate in the bottom of the pot where it can be easily cleaned up.? On the other hand, the vacuum oven has a window, enabling spectators to observe the proceedings.
But how would one get the varnish in?? Immersion?? Would marine spar varnish work?
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30438

开云体育

Jim I think you mentioned the VPI before.? It likely is a good idea ?for a new transformer, but as the carbon track increases, there is a point of no return, or no restoration. Once the ?path is bad enough to ?be noticed, it might be too late. It’s worth a try in the interest of saving the baby. ,,?? don

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of Jim Whartenby via groups.io
Sent: Monday, July 1, 2024 2:01 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

Perhaps vacuum impregnation of varnish will prevent the arc and salvage the transformer?? I have made vacuum chambers from old pressure cookers.? Quite easy to do only requiring a new gasket, fittings and a vacuum gauge.? An old pressure cooker can be had for $20 or so and the gasket, fittings and gauge is another $20.? I use a hobby vacuum / pressure pump since that seems to be sufficient.? In my experience, a vacuum works better then an oven in removing moisture from electronics.

Regards,

Jim

?

--
don??? va3drl

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30437

Hi Jim,
Thanks for the tip on how to make a vacuum chamber!? That sounds quite easy.? There are plenty of industrial vacuum pumps out on ebay that might pump it down faster.? Or, if one has an air compressor with a fitting on its inlet, it might be put into service as a vacuum pump.?
I'm fortunate to have a vacuum oven I bought at a laboratory surplus sale 25 years ago.? But the pressure cooker would be even better than that because if there is any leaking liquid, it'll just accumulate in the bottom of the pot where it can be easily cleaned up.? On the other hand, the vacuum oven has a window, enabling spectators to observe the proceedings.
But how would one get the varnish in?? Immersion?? Would marine spar varnish work?
Halden VE7UTS

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30436

A few minutes after posting my question, I was thinking about what happens when a transformer turned on and came up with an idea:
If the transformer is turned on in the middle of a power line cycle, the initial current flow would be higher than it would be if turn-on occurred at the zero crossing.? The transformer's magnetic field is at zero and suddenly there's a high voltage on the primary.? Could that current cause a faster change in the magnetic field than would occur at subsequent cycles?? A faster change in the magnetic field would cause a voltage spike at the secondary.
Halden VE7UTS

Re: SX-101 A Drift

#30435

I had a SX-101A a few yrs back. In the process of restoring it I found that there was a large "Heating" resistor on the underside of the chasis, the purpose of which is to keep the oscillator at a temp so it does not drift. It was inop and I could not find a direct replacement. So, I found smaller heating resistors that I mounted on a copper strip which I cut to fit the old resistor standoff mounts. It worked all OK, and no drift! Hope that helps. - alan K9MBQ

Show quoted text

-----Original Message-----
From: <[email protected]>
Sent: Jul 1, 2024 11:55 AM
To: [email protected] <[email protected]>
Subject: Re: [HallicraftersRadios] SX-101 A Drift

?

Thanks for asking!? You questions made me look at this problem critically!? I only work CW on 40 and 20, so my remarks will be confined to that area of operation!

The drift is not as bad on 20 as 40, but still there.? It is a "slow" drift!? I can bring it back using either the tuning control, or pitch adjustment.? I have wonder if the "drift" was in the audio circuits, and not in the RF portion. Is such a thing possible? When I put the 100 kc marker on, it still drifts.

It's a "slow" drift.? Sometimes it kind of "jumps" but not too often.? The pitch will go up and down.? I'll have to pay more attention to the change in frequency!? Its on and old Hallicrafter SX-101 with a slid-rule type dial.? Not a lot of accuracy like on a digital read out!

Thank you for you thought!? If you have any more, please let me know!

?

Also many thanks to others who have offered ideas!? I will address each on!? MANY thanks to all!

?

George KN3PAT

?

On Sunday, June 30, 2024 at 09:15:08 PM EDT, Richard Knoppow <1oldlens1@...> wrote:

?

What is the nature of the drift? That might be a clue. Is it steady
in one direction or does it drift back and forth. Does it jump (jumping
is frequently due to bad silver mica caps). Can you get to the
components with a heat gun or freeze mist? Real freeze mist is hard to
obtain these days but turning a can of compressed air upside down will
work quite well. Heating and cooling might help to isolate what is
causing the problem. Another trick is to use a Variac to vary the line
voltage, does the drift follow the voltage? usually, there will be some
variation but the amount might give you a clue.

On 6/30/2024 2:38 PM, Anthony W. DePrato wrote:

if it drifts still after 6 hrs? i would look for a bad mica cap in the
vfo also.
73 Tony wa4jqs

--
Richard Knoppow
Los Angeles
WB6KBL
SKCC 19998

?

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30434

开云体育

Halden Good work on doing testing and evaluation, and failure analysis.

In my words, the transformer is not shorted, but the insulation has broken down, so arcing can take place at ?voltages well below the designed voltage.

I don’t know the operation of the HT-37 or its relatives, but when the topic began I felt that? a quick On-Off?? is the Achilles heel of a quick On-Off-ON; so there is no need to turn it back on to add some more over-stress. It seems that inrush is not part of the problem.

I tended to think that the dangling voltage on the caps was likely involved. This thought might be bogus, but when you switch it off the AC power input no longer controls the core flux excursions, and voltages, so what does in that first cycle after turn-off? ?As you mentioned, the cap voltage is suspicious, but my mental gymnastics are in a bog, so I’m stalled out about the HV and 5V voltage excursions at that critical instant ?. ?I also wonder if there is enough stored energy in RF ?tuned circuits to kick back. I want to know the answer too!.. frustrating..Don

?

Show quoted text

From: [email protected] [mailto:[email protected]] On Behalf Of HF via groups.io
Sent: Monday, July 1, 2024 12:33 PM
To: [email protected]
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

Thanks, Randy, for letting me evaluate the transformer from your failed HT-37!? I found that all windings are isolated from each other by at least 100 megohms, as measured by a Keithley 2100 DMM.? However, when applying 2000 volts from a Bertan HV power supply (5 mA limit), I found arcing between the HV winding and the filament winding marked “YEL” on the schematic.? These wires are pale yellow.? The other 5V winding, “BRN” now has pale red or orangish insulation.? It seems possible that these were once brown.? All other winding combinations withstood the 2000 volts.? Not wishing to destroy insulation elsewhere in the transformer, I did not test the other windings with more than 2000 V.

I measured AC voltage and winding resistance on the transformer which I’ll post later.

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.? If the transmitter is turned on before the HV B+ bleeds off (RC is 3 seconds) from having been on just before, then the 5V winding could still be at about 1000 V relative to the center of the HV secondary.? If the rectifier filament has cooled enough such that the tube is no longer conducting, then each end of the HV winding will alternately reach about -790 volts, for a total of about 1800 V that the insulation must withstand.? During operation (but not transmitting), the HV winding is loaded by the bleeder resistors via the rectifier, drawing 3.3 mA.? Each side of the HV secondary has 113 ohms of resistance, dropping less than a volt in the winding.? So that’s not it; ~1800 V appears during standby operation, too.

What voltage condition occurs during turn-on with charged HV capacitors that is not also present when the transmitter is on?? I understand that heavy inrush current is typically present at the primary, but I'm not aware of that current causing a voltage spike at the secondary.
Or, what voltage situation is present at turn-off that’s different when turning off from STBY compared to turning off from MOX?

Cheers

Halden VE7UTS

--
don??? va3drl

Re: HT-37 transformer failure due to rapid STBY-->OFF-->STBY

#30433

开云体育

I have saved a bunch of EF Johnson transformers.? They short like this.? I took one apart and found that the short is at the end bell and not deep in the windings.? ? I slid drinking straws over the wires with heat shrink to insulate them.? Then pumped it full of RTV.? ?Fixed.? They always short from plate/Fil line at that same spot.? Wonder if this one is the same?

C

Show quoted text

From: [email protected] <[email protected]> on behalf of Jim Whartenby via groups.io <old_radio@...>
Sent: Monday, July 1, 2024 11:00 AM
To: [email protected] <[email protected]>
Subject: Re: [HallicraftersRadios] HT-37 transformer failure due to rapid STBY-->OFF-->STBY

?

Perhaps vacuum impregnation of varnish will prevent the arc and salvage the transformer?? I have made vacuum chambers from old pressure cookers.? Quite easy to do only requiring a new gasket, fittings and a vacuum gauge.? An old pressure cooker can be had for $20 or so and the gasket, fittings and gauge is another $20.? I use a hobby vacuum / pressure pump since that seems to be sufficient.? In my experience, a vacuum works better then an oven in removing moisture from electronics.

Regards,

Jim

Logic: Method used to arrive at the wrong conclusion, with confidence.? Murphy

On Monday, July 1, 2024 at 11:43:43 AM CDT, HF via groups.io <incorridge@...> wrote:

Thanks, Randy, for letting me evaluate the transformer from your failed HT-37!? I found that all windings are isolated from each other by at least 100 megohms, as measured by a Keithley 2100 DMM.? However, when applying 2000 volts from a Bertan HV power supply (5 mA limit), I found arcing between the HV winding and the filament winding marked “YEL” on the schematic.? These wires are pale yellow.? The other 5V winding, “BRN” now has pale red or orangish insulation.? It seems possible that these were once brown.? All other winding combinations withstood the 2000 volts.? Not wishing to destroy insulation elsewhere in the transformer, I did not test the other windings with more than 2000 V.

I measured AC voltage and winding resistance on the transformer which I’ll post later.

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.? If the transmitter is turned on before the HV B+ bleeds off (RC is 3 seconds) from having been on just before, then the 5V winding could still be at about 1000 V relative to the center of the HV secondary.? If the rectifier filament has cooled enough such that the tube is no longer conducting, then each end of the HV winding will alternately reach about -790 volts, for a total of about 1800 V that the insulation must withstand.? During operation (but not transmitting), the HV winding is loaded by the bleeder resistors via the rectifier, drawing 3.3 mA.? Each side of the HV secondary has 113 ohms of resistance, dropping less than a volt in the winding.? So that’s not it; ~1800 V appears during standby operation, too.

What voltage condition occurs during turn-on with charged HV capacitors that is not also present when the transmitter is on?? I understand that heavy inrush current is typically present at the primary, but I'm not aware of that current causing a voltage spike at the secondary.
Or, what voltage situation is present at turn-off that’s different when turning off from STBY compared to turning off from MOX?

Cheers

Halden VE7UTS