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Old 23rd Oct 2020, 7:58 am   #1
falcon123
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Default Hallicrafters SX-43 FM drift.

This is my last issue with this set (I promise) although I'm not sure anything can be done about it. the FM drifts badly on this set until it warms up and that can take over an hour when it's in its case in an airconditioned room. Out in the shop on its end with the underside of the chassis in open air it's much worse. I've read "they all do that" but somehow I'm unconvinced that 2 Mc/s was acceptable in 1947,but maybe it was. There's a temperature compensating capacitor, C41, buried next to the first wafer of the band switch. It's a strange affair that looks like a ceramic/mica padder, but it doesn't appear to have an adjustment and I really can't see the very top of it without a dental mirror which I don't have. After chasing the p/n,44A158, in other Hallicrafters sets like the s-76 and s-40 I discovered its value was 3pf. It's connected to pin 3 of the 7F8 tube and has 195 VDC on it. The cap goes to chassis ground. I've limited experience so I'm not really sure it has anything to do with it what-so-ever. but it's the only temperature controlled device that's called out in the drawings and looks suspicious. The distracter is that AM seems quite stable. Anyone have a clue if this could be the issue? Thought suggestions are all welcome.
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Old 23rd Oct 2020, 10:00 am   #2
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Default Re: Hallicrafters SX-43 FM drift.

2MHz drift seems a lot, a decade later a well known UK manufacturer of domestic valved FM receivers had one of their receivers tested for drift and the maximum drift for the first 30 minutes was 13kHz and after 30 minutes was almost flat line.

Lawrence.
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Old 23rd Oct 2020, 12:07 pm   #3
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Default Re: Hallicrafters SX-43 FM drift.

Mine drifts too as you know - but it equates to a slight turn of the dial for me. It can go for about 10 minutes before I need to slightly adjust the dial. It's like I am chasing the signal. My 1957 Zenith FM radio I listen to all day 5-6 days a week in my office never drifts. I only have to touch the adjustment knob when I accidentally bump it.
The SX-43 FM drifts to the point it is disruptive for listening to while I work as I am getting up every 10 minutes of so to adjust it. The other bands (AM, long wave, SW) are all stable and pretty much stay put reasonably as well as my other fleet of SW radios.

The FM drift on my SX-43 officially is on my bucket list to solve.
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Old 23rd Oct 2020, 1:28 pm   #4
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Default Re: Hallicrafters SX-43 FM drift.

Have you tried heating the temperature controlled resistor, with say a hair dryer to make sure it's working and if so which way it corrects the drift?
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Old 23rd Oct 2020, 2:36 pm   #5
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Default Re: Hallicrafters SX-43 FM drift.

As silly as it sounds I did not know the sets I am working with had such a resistor in it (a thermistor?).

What might be that resistor in the SX-43?
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Old 23rd Oct 2020, 3:25 pm   #6
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Default Re: Hallicrafters SX-43 FM drift.

Quote:
Originally Posted by mrrstrat View Post
Mine drifts too as you know - but it equates to a slight turn of the dial for me. It can go for about 10 minutes before I need to slightly adjust the dial. It's like I am chasing the signal. My 1957 Zenith FM radio I listen to all day 5-6 days a week in my office never drifts. I only have to touch the adjustment knob when I accidentally bump it.
The SX-43 FM drifts to the point it is disruptive for listening to while I work as I am getting up every 10 minutes of so to adjust it. The other bands (AM, long wave, SW) are all stable and pretty much stay put reasonably as well as my other fleet of SW radios.

The FM drift on my SX-43 officially is on my bucket list to solve.
It seems that all early FM sets drifted! The Zenith set probably has AFC. I have several different Zenith set and they all seem to be stable.
Dave, USradcoll1, too many sets of all makes.
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Old 23rd Oct 2020, 4:21 pm   #7
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Default Re: Hallicrafters SX-43 FM drift.

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Originally Posted by mrrstrat View Post
As silly as it sounds I did not know the sets I am working with had such a resistor in it (a thermistor?).

What might be that resistor in the SX-43?

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Have you tried heating theC with say a hair dryer to make sure it's working and if so which way it corrects the drift?
Sorry I should have said "Temperature Compensating Capacitor". UK members may remember Oxley TempaTrimmers.
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Old 23rd Oct 2020, 4:56 pm   #8
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Default Re: Hallicrafters SX-43 FM drift.

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UK members may remember Oxley TempaTrimmers.
I sure do.

Lawrence.

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Old 23rd Oct 2020, 5:59 pm   #9
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Default Re: Hallicrafters SX-43 FM drift.

Fondly!

I created a thermistor/varactor network which could give a variable tempco as an alternative. What's more it can be adjusted in one go at each of two temperatures - saving on the to-and-fro messing around. It's in the ARRL handbook (1995 onwards years)

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Old 23rd Oct 2020, 6:20 pm   #10
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Default Re: Hallicrafters SX-43 FM drift.

The oscillator and mixer valves (a double triode) derive their HT from the reservoir capacitor.
There are then several R C stages to provide decoupling.

Have you ever monitored the HT voltage at C52 over the first hour after switch-on to see if it changes?
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Old 23rd Oct 2020, 6:56 pm   #11
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Default Re: Hallicrafters SX-43 FM drift.

Like C53 (the infamous 'lightning crash' cap), C52 is located in a very buried part of the radio. There are threads devoted to just getting to and removing C53 (let alone replacement). Those caps are entombed within the unmovable parts under the 7F8 valve.

(Note: C52 and C53 here are from the Hallicrafters nomenclature and not the popular Sams Photofact for this set)

Quote:
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The oscillator and mixer valves (a double triode) derive their HT from the reservoir capacitor.
There are then several R C stages to provide decoupling.

Have you ever monitored the HT voltage at C52 over the first hour after switch-on to see if it changes?
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Old 23rd Oct 2020, 11:09 pm   #12
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Default Re: Hallicrafters SX-43 FM drift.

you were reading my mind. i thought of the hair dryer trick last night.with no test equipment used,i heated the corner where this cap is located. using my cheap and questionable accuracy chinese laser thermometer i managed to heat the cap to 100F while the chassis registered 82F. the set had sat all night unplugged.i turned it on and by the luck of the draw the tuning was spot on. it appears im on to something.the cap is in parallel to the low cap tuning gang to ground as well as pin 3 of the 7f8 when in the fm mode. UNLESS im reading the switch sequence in the schematic wrong. so to my mind this cap is gaining capacitance as it cools shifting the fm tuning up. 105mc becomes 107. it will be reasonably difficult just to remove it. it would be a bit more conclusive if i had the part removed where i could heat it and measure it. unfortunately ive no data on how it was intended to behave. i dont really want to strip this to bits unless ive got at least an idea and the required parts to repair it. supply lines are extremely long here to texas or canada.
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Old 23rd Oct 2020, 11:13 pm   #13
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Default Re: Hallicrafters SX-43 FM drift.

i dont know how c52 gets in here,but the cap in question is C41 p/n 44A158
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Old 24th Oct 2020, 7:39 am   #14
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Default Re: Hallicrafters SX-43 FM drift.

Some receivers use a stabilised HT supply for the local oscillator valve.

The intention is to reduce frequency changes from various causes.

C52 is not a tuning capacitor or a temperature compensated capacitor.
It is the HT supply capacitor nearest the oscillator valve.

In theory, it is a good place to measure the HT voltage.
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Old 24th Oct 2020, 10:32 am   #15
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Default Re: Hallicrafters SX-43 FM drift.

the voltage on c52 from cold then for an hour is 229 vdc measured with a fluke 79III
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Old 24th Oct 2020, 11:17 am   #16
falcon123
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Default Re: Hallicrafters SX-43 FM drift.

radio wrangler, can you post or provide a link to your design?
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Old 25th Oct 2020, 7:20 am   #17
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Default Re: Hallicrafters SX-43 FM drift.

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radio wrangler, can you post or provide a link to your design?
It's in the ARRL handbook, starting in 1995 and repeated in subsequent years. It's in the Oscillators and Synthesisers chapter, sometimes titled 'AC/RF Sources'.

I've seen chapters from the book appear on the internet for a while before they're taken down for breach of copyright, so I can't point to any at the moment. If you know any radio amateurs it's quite likely that some of them have paper copies of the book.

It consists of a stable voltage supply (resistor into a temperature compensated zener diode) I used an 8.2v diode from the 1N823A family.

This feeds the slider of a pot which is added into a bridge of four resistors. The top resistor on one side and the bottom resistor on the other are thermistors (both the same type)

A second pot is connected across the horizontal axis of the bridge and its slider goes to bias a varactor diode in your oscillator.

At room temperature you use the top pot to balance the bridge. Wind the centre pot to-and-fro between end stops. The oscillator frequency will shift. Adjust the top pot until the frequency shift is zero. The bridge is now balanced at this temperature, and there is no voltage difference across the second pot.

Now, note the oscillator frequency. Don't touch the tuning of the oscillator, but change the temperature to the highest you want to compensate to. Give it time to settle. Now, adjust the second pot to return the frequency to the value you noted.

You now have temperature drift nulled at two spot temperatures, and it will give a good approximation between these temperatures and for some temperature change beyond them.

The bridge and two pots trick allows you to adjust it without lots of trial and error.

I didn't give values of components, they can be chosen to suit whatever thermistors you can get, but thermistors around 10k Ohms at room temperature would be suitable, 10k fixed resistors, a 5k pot at the top of the bridge, and a 100k pot in the second position.

I could draw it in a moment with paper and pencil, but I don't have a schematic drawing package on this computer.

David
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Old 25th Oct 2020, 9:10 am   #18
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Default Re: Hallicrafters SX-43 FM drift.

This is the circuit
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Old 30th Oct 2020, 6:32 am   #19
falcon123
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Default Re: Hallicrafters SX-43 FM drift.

i really dont know what to say about this,perhaps my query was premature. ive been waiting for a dial string and repalacement dial to arrive. as the c41 is buried behind all that and ive nothing to replace c41 with anyway i didnt want to take this apart. but i HAVE been playing it every day on its side and out of the case and have discovered that the drift seems to vary a bit some days. yesterday and today it was only 800@kc instead of +2 mc.. i can only guess as to why. humidity,temperature variation or just something inconsistent with c41 or SOMETHING ELSE.whatever it is i haven't been able to detect it heating various parts of the set when cold with a hair dryer. temp compensated ceramics are not made any more but i did manage to find a limited selection. its impossible to look for something to replace the original part as the while the value of c41 is known.3pf,i was not able to discover its % of compensation. the only thing i can do i guess is try a npo 500v 3pf and see what happens.i suppose the worst outcome is that it remains as it is. it indeed be NICE to know how the set was performing as new in 1949!!
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Old 5th Nov 2020, 12:53 pm   #20
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Default Re: Hallicrafters SX-43 FM drift.

This must be the limited stock you spoke of: https://www.surplussales.com/CAPACIT...MPDOGBONE.HTML

Our shop has 10s of thousands of NOS caps, I have seen these before. I'll look for a 3pF just to see if we even have one of them.

Do you think a 3.3pf value would work for C41?
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