Halli S-108

Discussion in 'Amplitude Modulation' started by AC0OB, Mar 20, 2019.

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  1. AC0OB

    AC0OB Subscriber QRZ Page

    After re-tubeing and a tuneup I found a resistor that was heating up and not supplying enough current to the 6SK7 RF stage. Even with this resistor going high value, it still had good sensitivity for a single conversion receiver.

    In tracing down this component of course I looked closely at the bandswitches. The bandswitches have a "copious" amount and I mean a "copious" amount of brown grease on them.

    I have a Hallicrafters HT-40 transmitter and it has none of this stuff on the any of it's switches.

    Was this a norm for Hallicrafters or was this something a previous owner applied to it?

    Addendum: I also have an HT-37 I have been working on and it does not have this brown goo on any of the switches.


    Last edited: Mar 20, 2019
  2. K4KYV

    K4KYV Subscriber QRZ Page

    In a lot of older equipment, carbon composition resistors have drifted in value far beyond tolerance, sometimes doubling in resistance or more. But often as not, substantial changes in one or two resistors makes no audible difference in performance.
  3. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    I have worked on a fair number of the Hallicrafters general coverage receivers and have not run into "grease" on any rotary switch contact.

    The S-108 is just a "re-boxed" S-85 with a slide-rule bandspread dial instead of the rotary dial on the S-85.

    The R.F. amplifier stage should be a 6SG7 and not a 6SK7. The 6SK7 tubes are in the I.F. stages. A 6SK7 will function in the R.F. stage but is far from optimum. Some people "soup up" these receivers replacing the 6SG7 with a 6AC7 or even a 717A. Both tubes are lower in noise figure than the 6SG7 with the 717A being the lowest in noise.

    The 717A was used in VHF and even UHF receivers during World War II.

    Glen, K9STH
  4. AC0OB

    AC0OB Subscriber QRZ Page

    You are correct about my typo, as the RF stage is the 6SG7 with two 6SK7's as IF amps.

    I may try a 6AC7, as it appears I would also see some increased stage gain since the transconductance is 5000 uMhos higher than the 6SG7.

    BTW, I think this receiver has the best audio of any of my tube receivers.

    I have taken some of those large cotton swabs and have removed most of the brown goo.

    I will use some circuit wash to do a final cleaning with a towel underneath to catch the stuff that runs off.

    Thanks for the info regarding the 6AC7. :cool:

  5. W4KJG

    W4KJG Subscriber QRZ Page

    Pheel -- I've owned a lot of Hallicrafters equipment in the last 60+ years. I can only guess that some previous owner thought that lubricating the switch, with some unknown substance, was a good idea.

    One thing I do know is that regular use can sure keep tube radios and even light bulbs working without much degradation for well beyond their planned life expectancy.

    I have a Hallicrafters S-118 that has only been powered down for short periods of time since it was purchased new in about 1963. It was the WWV receiver in the calibration lab for an RF company I worked for. It was powered on when it was purchased. When we moved into a new facility and closed the metrology lab I got the radio in a silent auction in the early 1980s. It was powered down for it's ride to my home. I've had it powered on since I got it, except during power outages or when I've moved from one residence to another. To the best of my knowledge, the back cover and case have never been taken off. At least in the nearly forty years I've owned it, it has never failed. I continue to use it as my AM radio and as WWV/CHU radio in my work shop.

    About a year ago I gave up my beloved Hammarlund HQ-100AC. It too had been powered on almost continuously since I became the second owner in about 1963-64. It never needed repair. The Q-multiplier was about 0.5 kHz off when I gave it up. Everything else was well within the original specs.

    While I was in college, one of my many jobs was daily checking of a couple of AM transmitter sites at sundown antenna switch-over time. One of them was KDAL AM at 610 kHz. I was told to never turn off the light in the main transmitter room. It had been on since about 1935. The bulb was the same type as we used on the antenna towers. I worked there from 1966 to 1969. The light was still on when I left after getting drafted as of May 29, 1969. I can't remember doing any repairs to the old RCA transmitter or the test equipment during that time. They to were constantly powered on.

    I have a few other boat anchors that I have not used in many years. As I slowly restore them I find that I have to first remove all the grease from the tuning sections. I have to clean the switches and potentiometers. I have to replace all of the paper capacitors. I just plan on replacing the electrolytics. And, I frequently have to replace a number of the carbon resistors. Just as a general rule I also replace all of the #40-series bulbs with LEDs.
    AC0OB and KD0DQZ like this.
  6. AC0OB

    AC0OB Subscriber QRZ Page

    Glen's suggestion for the 6AC7 was right on. I was somewhat skeptical that any major RF gains could be had, and only minor ones, but replacing the 6SG7 with the 6AC7 showed an AGC voltage increase of 250 mV ( -1 volt to -1.250 volts) with the same input signal as measured on Band 2.

    BTW, if you ever have an intermittent drop in signal after warm-up, think seriously about replacing C16, the 390 pF El-Menco mica capacitor in the oscillator (Converter) circuit.

    At first I thought I might have audio capacitors C45 or C47 failing, but scoping at the junction of R14-C41 confirmed the 455 kc signal dropping about 200 mV and then slowing rising back up.

    I really, really, really, really, really despise those old rectangular mica's. :mad:

  7. K4KYV

    K4KYV Subscriber QRZ Page

    YMMV. I kept my old HRO with the 2.5v tubes running almost all the time. I would shut it down when I left the house for a weekend or extended period, but when I was here to keep an eye on it, it ran 24/7. The tubes seemed to last for ever.

    I found it different with my 75A-4 and my T-360 PTO. Running 24/7 the tubes would wear out about in less than a year. The same with my outboard audio amplifier with 6V6s in the output. So I started shutting them off overnight and turning back on an hour or so before operating the station, and they seem to last a lot longer.

    Something I have thought of, but never tried yet, would be to keep the switches turned on, but power up slowly with a variac, and after use, power them back down slowly. Maybe use a geared down reversible motor to run the variac up and down, timed to take about 10-15 minutes to cycle each way. At least with light bulbs it's the sudden expansion and contraction of the filament during the on/off cycle that shortens their life. But running a tungsten-filament light bulb 24/7, it will eventually fail as the filament material evaporates. You can see the residue on the inside of the glass envelope of an old light bulb, even before it burns out. With tubes, it's the coating on the filament or cathode that affects emission. The filaments of most tubes don't get hot enough to evaporate the way light bulbs do.

    Some old style carbon filament light bulbs are said to have kept burning non-stop for decades and still be going strong.

    Unfortunately, many of those were destroyed during the mid 50s when it was discovered they sometimes oscillated in the VHF range and generated TVI. Broadcast industry interests initiated a nationwide advertising campaign to collect carbon filament light bulbs from the public and crush them so they would "never interfere with another TV again". I used to read about it in Radio and Television News, Popular Mechanics and similar rags. They offered to swap the customer a brand new tungten-filament bulb (with life expectancy of about 6 months) in exchange for an old carbon filament bulb that had perhaps been in use ever since the house was wired for electricity at the turn of the 20th century. They had to have been running for decades, since they quit making carbon-filament light bulbs sometimes in the 1920s I believe.
  8. K5UJ

    K5UJ Ham Member QRZ Page

    If being on all the time is what wears out miniature tubes, I'm not sure slow on and off is the solution. If the 2.5 v. and octals last longer running all the time with no variac used, then I suspect the filament composition is the root cause of the problem with miniature tubes. Somewhere I have a handbook that explains the different filament alloys. It might be a RCA tube manual; I can't remember.

    I prefer to follow K4KYV's suggestion and put in 12 v. lamps. I'm not super anal about everything original but LEDs in an old black wrinkle set is too much hi hi.

    Same here. No time to spend on each one figuring out if it is a mica or micamold, or if it is any good...they all get the heave ho.
  9. K4KYV

    K4KYV Subscriber QRZ Page

    Particularly the little red ones in Collins equipment like the 75A-4. They tend to become leaky over the years, and where used for plate-to-grid coupling caps they totally spoil the performance of the equipment by screwing up the grid bias on the following stage. I changed out a handful of those in my A-4.

    In rare cases when I have a vintage piece that I'm really anal about keeping original, I at least check them for capacitance and leakage using my ECG solid state capacitance meter, my Heathkit capacitor checker, and now, my HI-pot tester set to the working voltage of the capacitor. I have a cigar box full of ones pulled out of equipment or salvaged from parted out equipment, so if I find a bad one and don't want to replace with a modern cap, I'll look through my old ones hoping to find a good replacement.

    The unique looking Sangamo Type A transmitting micas in 1920s to late 30s equipment test about 40% bad as well, usually leaky or the capacitance is way off. Less so, the tan phenolic WWII and post-war ones, too. I can usually find a replacement for those that tests good in my junk box. Don't know where one would find a new more modern equivalent for those. I prefer not to go the ceramic capacitor route if I can avoid it. If the old transmitting micas pass the leakage test, I can usually tolerate a ± 20% or so error in capacitance, unless it's in a critical resonant circuit.
  10. K9STH

    K9STH Platinum Subscriber Volunteer Moderator Platinum Subscriber QRZ Page

    Especially with antique / vintage equipment, that I am going to enter into various shows and competitions, I can be anal to make sure that the original parts, at least original parts "stuffed" with modern components, are in place. But, for anything else, I have no qualms about replacing with modern components.

    I may leave things like canned capacitors, bathtub capacitors, etc., in place with modern components below. Or, especially with bathtub capacitors, I remove them and then put sheet metal covering the holes. The idea is to get the unit to work without the possibility of an old component failing and taking out a lot more parts with it.

    Glen, K9STH

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