Resistors in Hallicrafters radios

[PD3LV]

Hello everyone
I'll introduce myself first: i'm Laurens Visser, i live in Zaandam, The Netherlands. I'm 18 years old and i have a small collection of old radios and hifi equipment. I have been fixing tube radios since my 14th. I can now find and solve most common problems in tube radios.
I have learned morse code using the G4FON koch trainer at 15wpm. In a while i will go for my novice registration (allowing me to work on parts of 40m, 20m, 10m, 2m and 70cm). I'll probably build my first HF CW transmitter myself.

Currently, I'm recapping an old Hallicrafters S-86 general coverage receiver. Most of those paper/wax/paraffin caps go bad (and when measured at 500v, DC resistance varies from 0.5 to 10Mohms for 600v caps).
The power supply electrolytics were bad too.

But what to do with the resistors?
The dutch Philips composite resistors are usually good when there is no high voltage across them, but how about the American Hallicrafters resistors, should i check them or are they still fine?
I have enough experience with dutch radios and resistors, but a Hallicrafters is something exotic for me.

73,
Laurens

[WB5WSV]

Laurens:

Back in the early 80's I bought a WWII vintage oscilloscope and in the process of troubleshooting it I found that all of the 1 Watt composition resistors had essentially opened up, gone to almost infinite resistance. I thought that was odd, and ironic too, because all of the tubes in the scope checked good, exactly the opposite of what I would have expected.

Also, a few years ago I inherited a bunch of electronic components from a friend of mine. Recently I was looking through the parts to find some components I needed and in the process found a number of composition resistors that measured significantly higher than what their color codes indicated. They had not opened up but were at least 20% higher in resistance value. And these were new resistors that had never been installed in anything.

So my recommendation would be if you find a problem with the Haillicrafters then start checking the composition resistors in the appropriate area. Of course you should look at all of them anyway and see if any looked smoked, discolored or burnt. But if they look Okay visually I would not bother with checking them with a VOM unless there is a problem of some kind with the set working properly.

Needless to say, you should not replace composition resistors with wire wound ones unless you are very sure that adding an RF choke in that part of the circuit will not hurt anything.

Good luck!

Robert

[SM0AOM]

First, congratulations to your progress both in Morse telegraphy and in trouble-shooting tube radio equipment.

It can be difficult to say anything general about the long-term properties of resistors.

Modern resistors of the metal-film types are extremely stable, but older resistors, especially the molded carbon-composition types may be prone to ageing drift. Usually they tend to drift upwards in resistance with age.

The rates of change and direction are however dependent on the manufacturing processes used by a particular make.

There are cases reported when composition resistors have increased 40 - 50% in value by just being stored for two or three decades,
and a few 20 kohm resistors in my late 1930's National HRO were almost 40 kohm when measured.

It is advisable to check their values against the values given in the schematic.

73/

Karl-Arne
SM0AOM

[K9STH]

Be VERY careful working on the Hallicrafters S-86. It is an AC/DC receiver (no power transformer) version of the Hallicrafters S-85. Replace the power cord with one that has a ground wire as well as the input AC voltage.

Glen, K9STH

[PD3LV]

@ Glen:
thanks for the warning. I already knew, but thanks anyway. It's better to hear it each day, than to find it out by getting shocked...

This receiver has a problem somewhere, because mains voltage is present on the cabinet depending on how the plug is in the wall outlet.
After finding that out (by measuring, not by getting shocked ;)) i disconnected the internal autotransformer (for 220/110v), i now only use it with an 220/110v transformer with separate windings.


@ the rest:
Thanks for the advise! The receiver has some drifting issues. I'll check the resistors in the oscillator circuits, if the recap hasn't solved the drifting problem.

[K9STH]

Depends on what you call "drifting problems"! Until the receiver has warmed up for at least an hour you can expect it to drift a number of kHz. Even after warm up the stability of the receiver isn't that great. The S-85/S-86 was Hallicrafters kinda "middle of the road" receiver. The bottom receiver was the S-38 series, then the S-53/S-53A, then the S-85/S-86/SX-99, then the SX-100, and finally the SX-101 series.

For its day, the S-85/S-86 were an acceptable quality receiver for amateur radio use. However, when compared to even "entry level" equipment these days, the S-85/S-86 are not very good.

I had an S-85 back in 1960 and got another one to complete the restoration of my 2nd primary station from that period a while back.

Glen, K9STH

[KM1H]

Ive overhauled many hundreds of Hallicrafters and other brands over the years for myself and customers and usually have to replace a few (2-5) to many (25+)resistors. The 5% carbon film are excellent replacements as the originals were 10-20% tolerance. Use the 1W size in place of 1/4 and 1/2W as they are physically comparable to the originals 1/2W and should give extremely long life.

The biggest changes Ive measured were in National NC-183D's and HRO 60's where the screen droppers had increased to in the area of 500K to 2.5 megohms.

Carl

[WB5WSV]

That is an interesting observation, Carl.

Back in the mid-70's my first peice of ham egar was a National NC-155. I loved the radio and still do but decided to make it better by plugging in a solid state replacement for the rectfier tube.

All went well, until one day when I went to do some work in the kitchen and since I wanted to be able to hear the AM radio, and did not want to turn off the NC-155, I flipped the "Standby" switch. And left it that way for a while.

Then I smelled burning. I ran to the NC-155, flipped the standby switch to On, and the radio worked fine. But I opened it up and found that one resistor was smoked. It was the resistor that fed power to the VFO, and so it remained active with the switch in Standby. Sitting there with the higher voltege on it, exacerbated by the Standby condition, had smoked the resistor. The funny thing was that compared to the value in the schematic it was exactly 10 times too high in resistance. You could no longer read the color code on the resistor becasue of the overheat. And the set still worked fine.

Now did the resistor increase in value from overheat by exactly 10 times or is the schematic wrong and the value is correct?

Anyway, I put the old rectifier tube back in.

[PD3LV]

The standby switch on my S-86 doesn't reduce power consumption, it has the same effect as turning down the volume...
I'll let it warm up for an hour, then i'll check if it still drifts too much.

[N6MKC]

Just to throw in my 2 cents on this, I just finished rejuvenating a Hallicrafters S-40B, circa 1950. I replaced a total of 14 resistors that were way out of tolerance. The winner of the highest out of tolerance award were two 22kΩ 2 watt resistors in parallel to form an 11kΩ resistance across one of the filter capacitors. One tested out of circuit as 16MΩ, and the other about 5MΩ. In regards to schematic values, I noticed on a couple of occasions what appeared to be factory-installed substitutions. For example, the 2 22kΩ resistors I mentioned above looked to be factory-installed, yet the schematic showed a single 12kΩ 4-watt resistor. Ironically, the radio worked fairly well with all of these failed resistors. Just goes to show how forgiving tube electronics can be.

As for carbon resistor failures, my understanding is that 50+ years of expansion/contraction due to heat generated in the radio causes the internal structure of the carbon in the resistors to crack apart, thus increasing resistance.

[KM1H]

Id do the math and check those 2 resistors again.

From your post on another forum it seems that changing the resistors after some prompting were more helpful than indicated above.


Okay, I replaced a total of 14 out of tolerance resistors. The winner of the highest out of tolerance award were two 22kΩ 2 watt resistors in parallel to form an 11kΩ resistance across one of the filter capacitors. One tested out of circuit as 16MΩ, and the other about 5MΩ.

Reception on the 4th band was markedly improved after doing this, but still not 100%. Like you guys suggested, the radio would definitely benefit from a proper alignment.

[N6MKC]

Id do the math and check those 2 resistors again.

From your post on another forum it seems that changing the resistors after some prompting were more helpful than indicated above.

I'm not following your meaning, Carl. What math did I miss? Don't 2 22kΩ resistors in parallel make an 11kΩ total resistance?

Yes, the folks over at the antiqueradios.com forum suggested checking resistors, because this radio was deaf on one band after recapping. It worked fairly well on the first 3 bands, even before recapping. As I indicated, replacing the out of tolerance resistors greatly improved the weak fourth band. But it still isn't perfect.

[KM1H]

I am referring to the path the B+ takes after it has to go thru about 5 megohms instead of 12K. I dont see how the radio works fine on 3 bands that way if the resistance readings are correct.

Carl

[N6MKC]

Yeah, you're right...that doesn't make sense at all. Okay, I saved the replaced resistors...let me go re-measure them....

Well I'll be darned. Okay, first 22kΩ resistor measured completely open.

http://www.martinihenry.com/temp/open.jpg


The next one measured 622kΩ.


http://www.martinihenry.com/temp/623k.jpg

I'm trying to think what I could have done wrong. I measured these in the circuit, but with one leg cut on each. I made certain I wasn't touching a path across both leads with my hands when I saw the meter registering above a megohm. The leads were waxy, but I thought I scraped through them sufficiently to make good contact. Perhaps not? How strange. Not sure how I goofed, but clearly I did.

In any event, that one resistor is nearly 30X what it should be. Wouldn't that alone have been enough to significantly squelch the B+ line to the point where the radio shouldn't have functioned at all?


I am referring to the path the B+ takes after it has to go thru about 5 megohms instead of 12K. I dont see how the radio works fine on 3 bands that way if the resistance readings are correct.

Carl

[KM1H]

That leg of the B+ is only going to the 6SA7 so the current is quite low. Looking at the voltage chart you can calculate the drop across the resistor or just replace it temporarily and measure at the 6SA7 socket.

Carl

[WD0GOF]

The most overlooked spec for resistors is the working voltage and the over voltage spec. Most 1/2 watt film resistors are rated at 500wv and a short term o.v. of 1000v. Resistors in the 1/4 and 1/8 watt range have 300 and 200wv specs. Particularly when working with boat anchors it is easy to exceed these ratings. As Carl stated in his last reply if you stick with 1/2 watt resistors 99% of the time you are safe. Just be aware of where you are sticking them. Hi voltage RF is particularly hard on metal film resistors. The metal film has traces etched in its surface to achieve proper resistance. These gaps will arc over if the ratings are exceeded. Metal film are said to be low noise and it is said that they great for low voltage applications in preamps and receiver RF and IF stages. When I look at the noise specs and the boat anchor applications I really can't see much differance so I stick with carbon film for everything.

73, Walt