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k7mdo
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« on: August 31, 2016, 12:19:35 AM »

I have been through quite a few '50's and '60's radio receiver restorations over the last couple of years.  Most have gone really well and it has been a great deal of entertainment!

One of the most recent re-caps and alignments has been a pristine Hallicrafters sx-110.   It came to me in its original Hallicrafters shipping box along with speaker, also in original box!

The problem is that even with no apparent operational issues its transformer runs pretty darn warm... almost too hot to touch after half an hour or so.

The tubes (all original Hallicrafters logo tubes) test good, the circuit voltages test near perfect... any ideas?

I hate to leave it this way.

Tom
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KA2DZT
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« Reply #1 on: August 31, 2016, 01:00:43 AM »

Line voltage may be too high for the xfmr, try running it on 115 volts.  Use a Variac if you have one.  I'm assuming the set works OK.  Could be a leaky input filter cap.
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k7mdo
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« Reply #2 on: August 31, 2016, 06:59:57 AM »

I should have mentioned that it is being run at 110 VAC on the bench.  Also, though the new filter caps were not tested, they have been replaced.

In an attempt to "lighten" the load I also replaced the #47 pilot lights with #44's.  Saves near an ampere at 6.3 volts as there were four of them. LED's would be next step.

I'm kind of thinking a tube like the rectifier might have not tested bad but still "leaks" a lot or ??

Tom




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KA2DZT
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« Reply #3 on: August 31, 2016, 09:31:19 AM »

I should have mentioned that it is being run at 110 VAC on the bench.  Also, though the new filter caps were not tested, they have been replaced.

In an attempt to "lighten" the load I also replaced the #47 pilot lights with #44's.  Saves near an ampere at 6.3 volts as there were four of them. LED's would be next step.

I'm kind of thinking a tube like the rectifier might have not tested bad but still "leaks" a lot or ??

Tom






Tom

IRRC  type #44 bulbs draw more current than type #47 bulbs.   Type #47 bulbs draw 150 ma, type #44 bulbs draw 250ma.   Xfmrs in those radios will get warm but shouldn't be hot.  Run the radio on whatever line voltage it's rated for.  What type rectifier is in the set?

Fred
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WD5JKO
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« Reply #4 on: August 31, 2016, 10:43:55 AM »


Perhaps this "hot" transformer issue is "normal" for the sx-110? Perhaps some old timers can chime in?

If it were me, I'd pull all the tubes out to do a test. Also pull out the light bulbs. This unloads the high voltage and filament windings. Then use a clamp on ammeter to measure the primary current, and vary the primary voltage with a variac. If the transformer core saturates, you will see a large nonlinear surge in current as the voltage is increased, most likely as you go above 120v. On the other hand, if there is a shorted turn, the primary current will be higher than normal across the range, and then at some point go non linear and high as the voltage gets too high.

My hunch is that the secondary HV winding has a lot of dc resistance, and this alone will cause a fair amount of I^2*R heating. Add to that the possibility of a shorted turn, and core saturation above 120v, and you have one hot brick.

The DC load current could also be too high. The transformer should run cooler when the receiver is tuned to a strong AM station due to AVC action (less RF & IF DC pull with a strong signal). Also the audio output tube bias might be off, and this could cause excessive class A DC current draw.

lots to consider...

Jim
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k7mdo
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« Reply #5 on: August 31, 2016, 12:28:15 PM »

Sorry about the pilot light mix-up 44 for 47.... that comes from writing the email in bed at 3 AM from memory.... yes, I have the low current ones substituted....  the rectifier is a 5Y3...

I am going to plug in an AC ammeter this morning to see what the current flow really is in the line....  there is a rating on the set in watts and I can verify if it is going over that value.

This radio has had very little "on" time in its life as can maybe attested to by the fact that all of the tubes are original and it still functions well as a receiver.  Nothing in the underside looks "stressed" from the heat so one wonders if "hot" is "normal" or not.

Pulling the tubes and measuring primary current flow will also be tested... hopefully today as the XYL is away for the morning.

Tom
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WD5JKO
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« Reply #6 on: August 31, 2016, 02:34:32 PM »

Tom,

   Having something like a Kill-A-Watt meter, then you can also read power factor. An unloaded transformer will have an excitation current that is primarily reactive current. Any heat from copper loss, or core loss is real, and that will raise the power factor. Another way of looking at it, a perfect transformer has no loss since the excitation current is completely reactive. Having 110vac at 1 amp might be 110VA but it is not 110 watts unless the power factor is 1.

Kill-A-Watt:
https://www.amazon.com/P3-P4400-Electricity-Usage-Monitor/dp/B00009MDBU?psc=1&SubscriptionId=AKIAILSHYYTFIVPWUY6Q&tag=duckduckgo-d-20&linkCode=xm2&camp=2025&creative=165953&creativeASIN=B00009MDBU

Jim
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k7mdo
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« Reply #7 on: August 31, 2016, 05:32:44 PM »

Jim, as it turns out I have no way of making an accurate measure of the power consumption of the set...  I thought I remembered having an AC ammeter in the "warehouse" but could not find it.

I have spent the morning with the set upside down, on, and monitored for temperature rise.  A couple of design characteristics are not helping the transformer.  there are two large wattage dropping resistors in the power supply section, one is 12 watts and the other is close.  These get very hot and when I look around for venting of their heat it must go up around the power transformer.  The transformer is installed in a "lifted off" position which provides about 1/8" of gap completely around it.  All of the convection heat from those dropping resistors must pass through that gap!  And, as one might expect, the transformer does not get quite as hot in the inverted position where the heat does not surround it.  However, it still gets healthy warm after about 15 minutes though not "burn your fingers" warm.

There is possibly one other issue that may somehow contribute to the warming....  I have closely watched the AVC voltage (-2.5 or so DC) while tuning around the broadcast band and it is definitely proportional to station strength.... but on doing so I have noticed that there is a significant hum that backgrounds really strong close AM stations while there is no apparent hum on other bands or weak stations.  The hum is definitely associated with signal strength....  

I did test all of the tubes again this morning for shorts but I didn't pull the 6H6 as it was "inconvenient" to get at....  

Any other simple ideas to look at?

Thanks so far for the thoughts, Tom

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KA2DZT
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« Reply #8 on: August 31, 2016, 07:57:42 PM »

Does this set have a filter choke in the power supply??
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k7mdo
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« Reply #9 on: August 31, 2016, 08:02:14 PM »

No filter choke.... caps and resistors only.

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kb2vxa
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« Reply #10 on: September 01, 2016, 09:51:39 AM »

I've had my share of boat anchor receivers with transformers running hot, hot enough for the ones in metal cans impregnated with wax or tar to leak gobs on the bottom covers. I'd say this is pretty much par for the course, but what I won't say is not making an effort to reduce normal heating won't pay off in the long run.
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73 de Warren KB2VXA
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w4bfs
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« Reply #11 on: September 02, 2016, 09:24:45 AM »

yup ...agree with Warren... solid state the supply and use rectifier winding to buck the primary .... helps cool things off
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KA2DZT
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« Reply #12 on: September 02, 2016, 10:44:47 AM »

I looked at schematic for that receiver.  The set is rated to run on as much as 120vac.  Best way to reduce heating is to solid state the rectifier and then add a series resistor off the diodes to the first filter cap.  Instead of the resistor, adding a small choke off the diodes to the first cap is the best way.  This converts the power supply from cap input to choke input.  This reduces the VA load on the xfmr.  The set will lose a little B+ voltage but it will be increased by running the set on the higher line voltage.  In addition, the heat and VA load from the 5Y3 is removed.

I have a NC-183D that ran so hot you could fry eggs on it.  I completely redesigned the power supply.  Solid stated the rectifier, removed the stock choke,  added a much better input choke and a second choke.  Ended up with a LCLC filter.  I used the 5U4 socket to add a large VR tube to regulate the screen voltage to the IF and RF sections.  This required not using the stock screen resistors and using 2K decoupling resistors and caps for each stage.

The set worked better and ran much cooler.   The power xfmr also ran much cooler, before the xfmr ran very hot.  The NC-183D is known for blowing power xfmrs. That is the main reason I made all these modifications.  Sometimes leaving everything exactly stock is not always best.

Fred
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k7mdo
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« Reply #13 on: September 02, 2016, 02:14:17 PM »

I have considered some of the suggestions on solid state and bucking the transformer.... kind of hoping there was something "wrong" with the set as it is "untouched" by ham or anyone else since new and thought someone might place a higher value on it at a hamfair whereas "mod"ing it up will almost certainly degrade the value to the "crazy" collector who demands "pristine".... 

I find the set to be so-so even as a general coverage receiver but..... 

On an SX-111 I got a few months ago I found it had a replacement transformer in it that was insufficient.... so I "added" a separate filament transformer and that cooled the whole thing down a bunch....  however, I always hate to use one of my collection of filament transformers even if I don't have a present need for one...

I put it away last night still original and it its original Hallicrafters shipping boxes...  hopefully someone comes along and wants it as is.

73, Tom   
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KC4VWU
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« Reply #14 on: September 02, 2016, 07:33:50 PM »

Seems I've read somewhere that Hallicrafters were a little skimpy in the iron department on a lot of their stuff. But then, I had a 75A2 for quite a few years that ran fine. Out of the blue, one night while cruising around the bands I noticed the unmistakable "smell of death". It's sort of an acrid stench that one immediately recognizes and sure enough, when I felt the transformer, it was quite warmer than expected. Knowing that it was on it's last leg, I continued to use it for about 3-4 more months until one night it just quietly checked out.

I have noticed that most receivers run decently well on the variac cranked down to 100-105 VAC. One possible problem with that would be cathode poisoning due to low heater voltage, and I'd suppose adjusting the input voltage to keep it close to 6.0 VAC would be wise.
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