The AM Forum

April 3, 2011 - When Good Transformers Go Bad - BC-348 Receiver
I love the BC-348, it was my first tube receiver as a young shortwave listener, so I was understandably delighted to find another some years back. This orphan from World War II has always had a place in my radio collection. Generally, once recapped, the sets are quite trouble-free, and make nice workbench or chair side monitoring receivers. So I guess it's only fitting that something might fail, after all, 70+ years is a long time to go without a problem. So it was that the neat little homebrew power supply inside my pristine BC-348 gave up the ghost Sunday morning, seems the power transformer had a bit of a meltdown. Not sure what was more painful, the part failure, or the horrific smell that seemed filled the ham shack for the day, yikes! A good lesson, never leave tube gear on, no matter how reliable it has been in the past! A few hours, a new transformer, and some time for clean up and this fine old rig will be back in service.

Sorry to see that Bruce, I wonder what is the size of the fuse in that rig? Of course I mean in amperes.

I had a similar situation when I lived in CA with a home brew transmitter. I didn't think anything needed fuses in those days and I let the transmitter on and went out side to work on a car. My wife frantically started yelling out the front door RADIO RADIO, when I went into the ham shack area, you could not see your hand in front of you, there was also a boiling sound inside a transformer no larger than the one shown in your pictures. After that, everything has been fused ;D ;D

Bruce,

Sorry to see this but having seen your workmanship in the past it will soon be as good as new.

Radio gear can generate a lot of smoke in a hurry.  A few years ago one of the cooling fans in my Johnson Desk KW went up in smoke and quickly created a significant stench and haze in the radio room.  With something the size of the desk you can't just pick up the smoking carcass and toss it outside until it quits smoking :)

With fused gear it pays to check to see what fuse is installed.  I am restoring a Halli HT-30 for a friend and I found a 7.5 amp fuse installed which is far too large for this low power transmitter.  I nearly learned this lesson the hard way when I noticed glowing rectifier plates in a CE-20A I was repairing.  One of the 6AG7 tubes had shorted and the previous owner had installed a 20 amp fuse, I caught it before the transformer gave up but it could have been ugly.  The only good thing about the 20 amp fuse is you could say it was the least likely item to fail in the rig.

Fuse, funny thing is it did not blow, 3 amp.
Must have just been cooking long enough not to draw enough current to short out and pop the fuse. Funny thing is the caps look great, so I suspect it was something inside the xformer that created the meltdown. I was on the air with the Collins 30K at the time, thought at first it was the 75A-1 or 310A, thankfully it turned out to be the 348 set.

Must have been outgassing for some time, very nasty staining on the desktop, brown like heavy nicotine, had to really scrub to clean it up, thought it was permanent!

Tnx for the kind words, Rodger, coming from someone of your skills that means a great deal. Starting ether, yep, that wonderful and high flammable stuff, does wonders cleaning. I've got to clean out the chassis, after the smell goes away, and then do a rebuild of the PS.

Just when I thought I had my service backlog cleared up...:-)

Bruce, nobody knows your plight more than me. I turned on my Globe Chump to warm it up for a time on 40M. In the mean time, my son needed to go into town for a trip with his friend. I figured I'd take him in and the Chump would be stable. Imagine my surprise while half-way down the hill my fire pager goes off for a structure fire at my house! Turns out the LVPS tranformer, (with umpteen windings), shorted out and went up in a puff of smoke. Since then, I've replace 3 plate transformers and I just lost the power supply transformer for my NC-183D. But those last ones, I was there for every one so it wouldn't get out of hand.

Check your NC-183D to see whether it has the troublesome .1 cap from one rectifier plate to ground.  A failure here was why my NC-183D came with a replacement transformer.  When that capacitor shorts the transformer goes before the fuse.

Roger Roger leaving old tube gear on 24/7.
I have several powerful audio amps that I don't dare leave on 24/7. Meltdowns happen and it's not very pretty. Usually the iron is gone.
So far my R390A is on 24/7 during the Winter months when MaNature is not striking things nearby.
The good ole 390A gets very cantankerous to be a nice receiver. She takes a part of a day to settle down and be happy again.

Bruce,
Now it's time to switch over to the new and mighty Echophone EC-1. No need for
a transformer just plug it in, make sure the rubbers isolating the external cabinet are not dried out and you are good to go!
 ;)

http://my.opera.com/Frister/albums/show.dml?id=1608251

Frits W1FVB

Then again, if you can't find an Ecophone EC-1, there is always the S-38! 

This thread is very timely for me.  I am cleaning out my extra duplicate rigs.  A friend was looking for an HT-37.  I have 4 of them so I pull one off the shelf and wanted to make sure the transformer was good in it.  Well it was bad and tar was all over the chassis.  I put that one aside for a parts rig.  I pull HT-37 number 2 off the shelf and low and behold that transformer is bad as well.  I have one more to go and I will be up to HT-37 number 3!  I wanted to keep number 4 to have at least one but not sure that one is any good. 
I know the HT-37 transformers are problematic, but I am hoping to find at least 2 good ones out of 4.   What is the issue with these transmitters and their iron? 

Bruce,
Good luck on the clean-up!  Its all part of the lure of these old Boat Anchors.

Stay tuned!

Joe, W3GMS 

Joe,

What I have seen (with the HT-32 and 37) is the 5 volt rectifier winding shorted to ground.  I don't think the insulation was sufficient for the rectified HV that appears on this winding.

My HT-32, 32B, and 37 all have SS replacements for both rectifiers and the 5 volt windings are disconnected and taped.  I am doing the same with the HT-30 that is currently under restoration.

Might pay to run these ancient radios off a variac.

The lines when these where made where quite a bit less...  Would the extra voltage on the primary also increase the heat generated, even just in the magnetizing current?

Ones that where <<marginal>> in the first place might be getting way over-stressed now.

--Shane
KD6VXI

"Fuse, funny thing is it did not blow, 3 amp.
Must have just been cooking long enough not to draw enough current to short out and pop the fuse."

Bruce, on my REtro-75 with add-on VFO kit part of the alignment process requires the VFO board to be disconnected.  Doing this causes 0 drive to the PA and PA dumps all the current into the mod transformer.  Something Dave nor I realized.  In order to align the VFO one had to be fast other wise the mod transformer takes a heavy hit.  I didn't realize this when I was integrating the VFO into the Retro.  I took my time and noticed a curl of smoke coming from the mod transformer. Dave has since fix the alignment procedure.   I have a 1.5 amp fuse for the Retro.  During transmit the Retro pulls about 0.8 amps.  I discovered that about 1.1 amps is enough to cause the mod transformer to take the brunt when doing the alignment but does not blow the fuse.  Fortunately, I didn't damage it, at least I don't think I did. Suffice it to say even though the current draw was low it caused the transformer to get very hot. I suspect it was the same for your rig, just a long protracted current draw that caused it to melt down and die.  Because a transformer has more mass it takes a long time for heat build up to be noticed. By the time one realizes it is overheating it's probably to late and the damage has been done. 

Mikey - the house didn't burn, right, just lots smoke, or did it damage the house? Yikes!

Here's one - A few years back at my biz we had a nice Saab 93 in for service with a bad turbo. The tech replaced the turbo and cleaned the intercooler, but there was still a lot of oil left in the intake. Nothing to do but let it burn its way out, so he let it idle in back of the building...smoking like the proverbial chimney. You might be able to guess the rest...a good Samaritan drove by, saw the hugh smoke cloud from what looked like my building, and called the Wiscasset Fire Department...an interesting conversation ensured.

Frits - I buy one of those EC-1s just to have that ad on my wall, classic!

Bob - Wow, that's quite a message, have seen the something mentioned on the Retro 75 reflector, but did not read it yet, good catch! I just ordered one of those cool VFO board last night, glad I know now what NOT to do. Good job.

Shane and others - all valid points, one of the giveaways with this one, in retrospect, was that the xformer really would buzz, did so since I got the set back in the late 1990s. Never gave it a second thought, but that might have been a warning sign that the laminations were loose. Recall Gary W1ZM, the xformer guy, telling me when Larry and I visited him a few years back that not only are loose laminations an issue, but the windings themselves can vibrate and damage the insulation.

As it is, I consider myself very fortunate, and most thankful that the worse I and the set suffered was some melted potting material...and provided a good lesson and reminder for all.

Bruce said:
Yep. Lots of smoke and a very shaken XYL.  ;)

I can imagine!

Take a gander and the time vs current curve for an AGC standard 3 amp fuse and you will see why it didn't "blow"

http://www.cooperbussmann.com/pdf/b040436b-cbc7-495d-95a6-99fe85202786.pdf

It can pass 4 amps forever. That's 480 VA and likely near that many watts. That will burn up a lot of smoke and stink.

Standard fuses protect against short circuits and are near useless for overloads.

N4LTA said:
Interestingly, while I was troubleshooting my NC-183D and its bad transformer, I put the Fluke 85 in line to read in-rush current. It got up to 8A for about 3 seconds before the 3A slo-blo fuse went.  :P

Sound like it would be good practice to cut back on the fuse size on these old radios. If it uses a 3 amp fuse, put in a two amp fuse. If it holds great, if not, go to 2.5 amps.

Rodger,

Well your right on!  After going through (3) HT-37's yesterday, the ones with the bad xmfr's had the 5V winding shorted to the HV winding.  The one that survived had the 5R4 pulled and a home brew solid state plug in installed with the filament winding disconnected and taped off. 

It's good to know where the transformer weakness is!

Thanks,
Joe, W3GMS

Correction,,,,I meant to say to shorted to gnd, not shorted to the HV winding.
Joe, W3GMS

Joe your HT-37 syndrome resembles  a chronic issue with the LV transformer in the 32V series Collins transmitters.

The best readout I've ever gotten as to the cause was from Jess Price, the guy who rewound my original LV iron about 40 years ago.  He said it looked like the windings, during manufacture, were wound too tightly, and that over a period of time the thermal expansion and contraction of the layers simply led to a failure of the aging insulation, which showed signs of deformity from compression beyond what would be typical.

Curiously, his observation took place when the transmitters were only 20 years old or so.  Yet, even that early autopsy may have been the best we get. I haven't found a better reason to explain why mine and so many other examples of the transmitter have suffered the same failure.

One of the frustrating aspects is that this failure seems to be random, from factors beyond what we usually keep track of, and not as the result of any mistreatment by the operator.

Hi Paul,

Yes, I have heard about the transformer issues with the 32V series transmitters.  That interesting on what the transformer re-winder told you and the theory for the failure.  In addition to that possibility, back in "those days" they used some rather poor insulation between the various windings of a multi-voltage transformers.  I really prefer separate iron for the various voltages as compared to HV and LV along with bias and filament windings all being together.  That is certainly something to keep in mind when home brewing gear.  The Heathkit Apache and others had just a HV transformer and nothing else mixed in with it.  From my experience its rare to find bad iron on those rigs unless they are really pushed in service. 

Of coarse Peter Dahl did great work with his replacement transformers since he knew how to build them to last.   I have not had any experience with the firm that bought his business.  I know that Gary, WZ1M does transformer rewinding and has a lot of happy customers.  I could do it if I had to, but I would rather send one out and pay the price! 

Joe, W3GMS   

Jeff at Harbach has done a good job trying to keep Pete's reputation and company alive.

HOWEVER, it's NOT Pete Dahl winding them!  It's MagCap.

They are NOT the same.  I know of 1 person who after getting a new MagCap built to his spec's ended up replacing it with a used Dahl.

They work, and are probably one of the better xformers out there (and even rarer, cater to hams), but it is NOT the quality of Pete's, nor are you able to pick up the phone and call a competent engineer like Pete...  Someone who's been there and done that... 

It would be like someone taking over Walt's "Reflections", and their homework before doing so was reading a copy of "Reflections".  Yeah, it can be done, but others might be better.

--Shane
KD6VXI

Thanks Shane for sharing your comments about "life after Peter Dahl".  I had pretty much heard the same story.  Just as you have indicated, 2 factors come into play and obviously the design knowledge is important but years of practical experience along with the design knowledge is hard to beat.
Joe, W3GMS

I lost the low B+ voltage power transformer in a Viking II back around 1980 in a manner similar to what Bruce described for his converted BC-348.  Powered up the Viking II and the SB301 I was running with it in the basement shack to warm them up one hot summer night and then when out to do some yard work before the sun went down.  Went back in about an hour later to a stinky black cloud of smoke hovering  about a foot down from the ceiling throughout the whole basement.  As I recall that Viking had both sides of the AC line fused at the plug, and neither fuse was taken out when the transformer cooked itself.

Hi Chris,

Many times not enough primary current flows to blow the fuse.  It depends on the failure mode within the transformer.  I think it is good practice to use some secondary fusing in case a filter cap or something else downstream starts to pull excess current to slowly overload the transformer but not to the point where it pops the primary fuse.
 
It sounds like your DX-100 XMFR was a slow burn! 

This stuff is pretty old and its bound to happen.  Our best friend may be the transformer re-winder!

Regards,
Joe, W3GMS