75A4 Help Request

[W5COA]

Howdy the forum,

I recently purchased a 75A4, and it mostly works, except that sometimes on 80m, it would not turn on and pass a signal. Switching to 160m and back to 80m would get it going.

I felt like the HF crystal oscillator might be the problem, so I consulted the manual. Chapter 9 says to connect a VTVM through a 1meg resistor to pin 2 of V3, and adjust L12 for maximum output.

What do they mean by maximum output? The tube measurement chart shows pin 2 having -2.7V bias. I can take the bias all over the place with L12.

- Am I supposed to adjust the L's for -2.7VDC, or something else? I will appreciate any and all help.

73, Jim W5COA

[WQ9E]

Jim,

When you peak the output using the coil you should find it drops off quickly on one side of peak and more slowly on the other.  I would adjust it just slightly past the peak in the direction where it drops off slowly.  This should provide plenty of output along with most reliable oscillator starting.

[k4kyv]

I'd say adjust for maximum grid bias.  The grid rectifies the rf and  generates the bias across the grid resistor. I don't have my 75A-4 manual here in front of me, but as I recall, that's the procedure.

It may be a flaky fixed mica that is wired across the crystal oscillator plate coil to resonate the tuned circuit. Mine had a similar problem on 20m; I could peak the oscillator with the tuning slug, but next time the thing would be dead again.  I even ordered a new osc crystal from International Crystals, and it still did the same thing.  I tried replacing the mica cap, and that fixed the problem.  Those little red mica caps that are used all over the A4 are notorious for flaking out.  Sometimes the ones used  for plate blocking caps become leaky and deposit a positive grid bias on the following stage, which screws things up royally. I think I have replaced at least a half dozen of those caps since I acquired the A4 in the early 80s.

[Roy K8VWX]

          Right on Don - Have had the same trouble thinking it was the crystal and found out it was the red capacitor at fault. I was telling someone that Art Collins must have had some poor quality capacitors when they made the 75A4s but he said HELL they are 40 years old or more. See if your rice boxes are still working at that age.  73 

[KM1H]

I believe the TS-520 is pushing 40 and most seem to be still running fine with just a HV electrolytic replacement..and a few tubes.

Ive had my 75A4 since 65 and there are no plans to get rid of it either but it has needed a lot of TLC over those years as component quality was far from the best.

Carl

[k4kyv]

See if your rice boxes are still working at that age.

I believe the TS-520 is pushing 40 and most seem to be still running fine with just a HV electrolytic replacement..and a few tubes.

Those older riceboxes made mostly with discrete components can probably be kept running as long as the older tube stuff. The exact transistors may not be available in the future for replacement, but it isn't that hard to find a different transistor or FET with similar enough characteristics to work.  Sometimes one becomes available that works better.  The problem lies more with newer rigs that are full of IC chips, some of which may be proprietary by the manufacturer. By the time one fails (or gets wiped out by the pulse from a lightning surge), there is a good possibility that the chip will be "discontinued" with no  similar replacement available anywhere for any price. I have heard hams complain that replacement parts were no longer available for their rigs that were only five years old. I am afraid that some of the proud owners of top-of-the-line plastic appliances may soon be holding $10,000 door-stops.

But there is one problem regarding the early SS rigs with replaceable discrete components, particularly receivers: maybe they can be made to last for ever, but performance was often compromised in the quest to achieve "solid state". A good example is the FT-101E series. The receiver section is a piece of crap compared to the better tube type receivers of the same era or that just preceded its release on the market. Early solid state receivers did not have the dynamic range and noise figures of good hollow state design, and it took them years, if not decades, to achieve performance equal to that of the best tube design.

Maybe the day will come when newly manufactured discrete transistors will be unobtanium just like tubes are to-day, but you won't likely see vendors at hamfests and ads on-line advertising "discontinued transistors" the way we see to-day with tubes, and I doubt if after-market manufacturers will pop up in China as they have with tubes.

[W2XR]

Don,

Many germanium transistors from the early pre-silicon days are becoming somewhat difficult to source, and are priced accordingly.

I have a TMC SPU-2 Speech Processor Unit that I use with my GPT-750D-2; it uses all germanium semiconductors (except for the silicon rectifiers in the PSU section). I have been looking to keep a few of each of these germanium transistors on the shelf as spares, as these devices will never be made again, and when they are gone, they are gone. And some of the transistors used in this unit appear to be quite scarce now.

73,

Bruce

[k4kyv]

I read somewhere recently that the ubiquitous 1N34 has become almost impossible to find.  I have a couple n.o.s. in original boxes in my parts collection.  As I understand it, the germanium diode requires less forward bias to make it conduct, than a typical 600PIV 1A silicon diode, so it makes a more sensitive diode detector. Many of the SWR and RF power meter projects in the ARRL Handbook, QST and HR magazine recommended the 1N34.

Maybe the hot-carrier diode would make an acceptable (or perhaps better substitute). Or have these been "discontinued" as well?

The Schottky diode (named after German physicist Walter H. Schottky; also known as hot carrier diode) is a semiconductor diode with a low forward voltage drop and a very fast switching action. The cat's-whisker detectors used in the early days of wireless can be considered primitive Schottky diodes.

When current flows through a diode there is a small voltage drop across the diode terminals. A normal silicon diode has a voltage drop between 0.6–1.7 volts,[1] while a Schottky diode voltage drop is between approximately 0.15–0.45 volts. This lower voltage drop can provide higher switching speed and better system efficiency... The most important difference between p-n and Schottky diode is reverse recovery time, when the diode switches from non-conducting to conducting state and vice versa. Where in a p-n diode the reverse recovery time can be in the order of hundreds of nanoseconds and less than 100 ns for fast diodes, Schottky diodes do not have a recovery time, as there is nothing to recover from (i.e. no charge carrier depletion region at the junction). The switching time is ~100 ps for the small signal diodes, and up to tens of nanoseconds for special high-capacity power diodes. With p-n junction switching, there is also a reverse recovery current, which in high-power semiconductors brings increased EMI noise. With Schottky diodes switching essentially instantly with only slight capacitive loading, this is much less of a concern.

http://en.wikipedia.org/wiki/Schottky_diode

[Todd, KA1KAQ]

Had the same issue with one of mine, it either wouldn't receive or took several minutes before it would kick in on 80, sometimes 40 as well. Tweaking the associated trimmers did the trick. I chalked it up to aging crystals, but all of these sets and their components are approaching 60 years old, having come out in 1955. No doubt they used the best components available at the time, not concerned that anyone would still be using them as front line gear a half century later.

But there is one problem regarding the early SS rigs with replaceable discrete components, particularly receivers: maybe they can be made to last for ever, but performance was often compromised in the quest to achieve "solid state". A good example is the FT-101E series. The receiver section is a piece of crap compared to the better tube type receivers of the same era or that just preceded its release on the market. Early solid state receivers did not have the dynamic range and noise figures of good hollow state design, and it took them years, if not decades, to achieve performance equal to that of the best tube design.

Ain't that the truth! Had a chance to do a side-by-side comparison with the FT-101EE I had alongside a KWM-2A that was ten years older, with a switchbox between them. The difference was like night and day. The older KWM-2A blew the Yaesu away. Had a hard time comprehending how much I'd been missing! To its advantage, the 101 was an all-in-one box with a handle on the side and 160 coverage and capable of AM. It also had a few other bells and whistles. Was a great beginner's rig in the early 80s, but it sure was numb on the receive side.

[KM1H]

Germanium transistors are like paper caps, they get leaky with age, even in a NOS wrapper.  Not all but quite a few, I have that problem regularly with HRO-500 and other early SS repairs. Once NTE quits support thats the end Im afraid.

Ive already made changes to my four TS-830's to improve NF and signal handling using readily available FET's and bipolar silicon. These are used mostly with VHF thru microwave transverters and one is used on the amplifier service bench. The 6146's dont give me any backtalk when an amp decides to do a light show.

Last I heard Rat Shack had plenty of 1N34A's; dont know as I still have some stock.

The Schottky diodes have about a .1V lower threshold than germanium and I use them (1N5711's) in lots of SS radios since the mid 80's with the TS-930, to replace the germaniums in ring product detectors. Less distortion also. The same goes for balanced modulators.

Another source of IMD is in diode switched front ends and there I use PIN's. What was expensive in the 80's is pennies today.

Carl

[ke7trp]

I have done the same comparison to the m2a.  However, If you swap out to the hotter and newer Nfets in the 101,(cheap and easy plug in swap) and then do a proper alignment, alot of that difference goes away. Hard to beat that old collins rig though!

C

Had the same issue with one of mine, it either wouldn't receive or took several minutes before it would kick in on 80, sometimes 40 as well. Tweaking the associated trimmers did the trick. I chalked it up to aging crystals, but all of these sets and their components are approaching 60 years old, having come out in 1955. No doubt they used the best components available at the time, not concerned that anyone would still be using them as front line gear a half century later.

But there is one problem regarding the early SS rigs with replaceable discrete components, particularly receivers: maybe they can be made to last for ever, but performance was often compromised in the quest to achieve "solid state". A good example is the FT-101E series. The receiver section is a piece of crap compared to the better tube type receivers of the same era or that just preceded its release on the market. Early solid state receivers did not have the dynamic range and noise figures of good hollow state design, and it took them years, if not decades, to achieve performance equal to that of the best tube design.

Ain't that the truth! Had a chance to do a side-by-side comparison with the FT-101EE I had alongside a KWM-2A that was ten years older, with a switchbox between them. The difference was like night and day. The older KWM-2A blew the Yaesu away. Had a hard time comprehending how much I'd been missing! To its advantage, the 101 was an all-in-one box with a handle on the side and 160 coverage and capable of AM. It also had a few other bells and whistles. Was a great beginner's rig in the early 80s, but it sure was numb on the receive side.

[W5COA]

Thanks to all that responded, and I think I have it fixed.

I tried deoxing the bandswitch, tuning the slug, replacing the 12AT7, to no avail. I then replaced the mica cap, and it has worked fine for over a week now.

I checked the old cap for pF and leakage, and it checks ok, so it must just get tired sometimes.

Again, thanks for the help.

Now to tackle the Valiant arcing problem, wherever that is happening.

73, Jim W5COA