On Sat, Dec 3, 2022 at 07:03 PM, Mark Phillips wrote:

What about a pair of diodes or even a lamp of some sort?

My R-390(A) has a neon bulb across the antenna input, but of course it is only a receiver.? I have developed the habit of discharging any cable before connecting it to an instrument or radio; briefly shorting inner and outer conductors is sufficient.??
73, Don N2VGU

I'm here just a bit preoccupied...

The long and short of it is... It varies.

Over voltage or over current are likely SWR related.? Assuming nominal
constructions and tuning.

High operating voltage points to the over voltage or current fail case.
Pushing for one more watt!.

The other is ESD, lightening and not a direct hit but enough to induce
high voltages.? ?How high is too high 60V (breakdown voltage for BS170).

One condition all MOSFETs are susceptible to his a case where the
high voltage due to SWR does not exceed the break down voltage but
is in phase with the Gate drive and due to gate to drain capacitance
couples RF back exceeding the gate to source maximum (20V).
This failure can also take out the ACT08.

Generally milling the case will tell you little as often the device melts.

Hint if you blow devices one examine what you are doing.
If you do it more than one stop doing that.

Hi SWR is not alway s a problem but what the actual R,Jx
is more important as a reactive load that match that has
good SWR but not?50 ohms may induce havoc.

Also tuning up on the air using the radio (QCX/QDX) is a
very sketchy practice as invariably you will go through
very bad SWR to get a good one.? I call that
teasing the tail of the dragon.? It bites, and so does
replacing BS170s.

One last item in dry places ESD as in carpet static kills
BS170s as well.? caution when handling.


Allison
------------------
Post online only, please no email.

The only reason that the same questions get asked is maybe that they were not answered to where we understood the answer ? When someone answers that high SWR causes MOSFET failures, I need some more detail. Sure, if the?
antenna is not matched, power is reflected back into the LPF network and the PA for dissipation. This will heat up the PA.....maybe too much and the MOSFET goes into a heat, more heat, component value change, avalanche, more heat, burn out cycle. What about MOSFET Gate capacitance discharge? What about Gate voltage spike reduction by adding a small series Gate resistor. What about a reverse diode across the Gate resistor, What about a series Gate - Source resistor.

73 Kees K5BCQ

On Sat, Dec 3, 2022 at 11:37 PM, Rich Brandt, AE8AA wrote:

Where are all the RF engineers when you need them?

Plenty on here, but pointless if no one listens.

That's why there's barely a week guess by without the same old questions being asked.

Gets boring after a while.
?
--
- 73 de Andy -

Adam via groups.io <qrp-labs@...> wrote:

I hope it didn't kill anything more :) But I ordered three FST3253 (and
four LM4562) to have spares, so if it still doesn't work, I'll see if IC4
is fine and will replace it too.

An update, if someone's interested.

Amplifiers arrived (switches still didn't -- I ordered them from a
different seller, maybe they'll arrive on Monday).

I replaced the amplifiers and now there's a good bias voltage on them
(around 2,5 volts), audio filter sweep looks very nice too.



RF filter sweep shows that the BPF is out of tune, but I'll check it again
when I install the switch IC (I think it can affect that).

So the op amps definitely were damaged.

开云体育

For V/UHF antennas on military aircraft, we used a megohm or more to reduce the discharge. Those antennas used fiberglass radomes and not much could be used for grounding in the blades. ?We used special gaskets that provided contact with the aircraft skin. ?St Elmo’s fire is good example of discharge under certain conditions. ?Sometimes, the discharge is referred to as “P-Static” or precipitation static but never thought it had much to do with moisture. ?Mostly high speed through cold dry air, but the physics geeks didn’t always agree. ?Expect similar things occur on ham antennas subjected to wind. ?

Adding a megohm bleeder resister does seem like a good idea.? Wouldn't it need to go on the antenna somewhere, to keep any static dissipated before you go to connect the antenna connector to the rig's connector?? And it's not at all a sure thing that the antenna's coax shield would represent a good ground anyway, to dissipate charge away.

We need Allison or someone similarly with deep knowledge.? Deeper than mine, anyway!? Where are all the RF engineers when you need them?

If this is likely, what value bleed resistor at the coax socket would remember better than we might ?

Michael 2E0IHW

I'll add another thought to my earlier comment.? I've had no personal experience with it, but I've heard that an antenna can gradually pick up a static charge which can zap your finals at the instant you connect your antenna.? This is probably more likely with a long wire antenna, with some wind blowing.? To avoid it you only have to briefly short the center pin of your coax connector to the threaded ring / shield.? The hard part is to remember to do it.

Antenna static would be just another way to subject your final amplifier transistors to electrostatic discharge.? The thing is, the transistor might look perfect from the outside, and leave you puzzling as to why and how it failed.

It's easy to guess What Went Wrong if your component is visibly damaged, maybe with a smokin' hole punctured, or worse, burn marks.? Its those cases where it still looks intact that make you wonder.

Buy a few spares.? I've got 60 BS170s on hand and haven't needed to replace any yet.? "Carry an umbrella to prevent rain" kind of thinking!

Thanks guys, I’ll use a common mode choke on feed line and?snap some toroids on key cable and see if that works.

Rich,

FWIW, I've had problems with my internal keyer also.? I've also added 6 ferrite beads to my keyer cable with some improvement but not yet complete success.

RF feedback is a common problem with QRP unbalanced antennas (EFHW, long wire, even trapped verticals) that is aggravated by poor or non-existent grounds with these rigs.

I've always been able to use my straight key, however.

All the best.

Jack

I had a similar problem and it appeared to be rf on the keyer cord. Some snap on toroids solved it for me. Rf from the tuner I expect.?

Joshua,
Very interested in this topic.? I've had great success with the QDX and FT8 on 20m but am looking to branch out into other digital modes and remembered the fun of packet from many moons ago.? Thanks for the info.? I may try to emulate your setup and test it out.
Duane

--
73, Oleh, I2/UY2ZA

I have not seen this particular problem but my QCX is much happier with a common mode choke. I use it with a random wire or a qrpguys tribander. My random wire uses a 49:1 unun and a common mode choke at the feed point and another common mode choke at the radio. The wire is hidden wrapping in the eves of my workshop. It’s about 47’ of wire. I use a tuner with it. It’s decent enough to test equipment but not useful for most communications.?
--
Colin - K6JTH?

Hi Guys, wondering if someone had this same experience and could share a solution.

This am I was fortunate enough to have some time to go out and play POTA. I decided to use my QCX mini as I have never used it at a POTA site, but rather only in the shack and “backyard” portable. ?The mini was built by Hans et al and is prob less than one year old. ?At the park, I had my 40 m hamstick attached to a hood mount and connected the feed line to a ZM2 tuner and then to my QCX. I tuned up no problem and nearly extinguished the LED on the tuner suggesting a good match. ?I sent a few QRL’s to make sure the freq was clear then I use the memory keyer and sent a couple CQs. After I got a reply, I tried to send the caller’s call sign back to him but my keyer was locked up. There was nothing I could do to get the radio to send via paddles. I hit the memory keyer again and it sent, but no response to the paddles.

I had to power down the rig and “reboot” to get it to work with the paddles. After another memory keyer CQ, same thing w/ the paddles. ?Luckily I had a back up radio to use and finished the activation. Has anybody had a similar experience? ?I wonder if RF got into the radio and screwed something up. I’m thinking I may throw a common mode choke on the feedline and see if that helps. FWIW, I’ve been using the same hamstick antenna setup w/ my KX2 and Penntek radios without issue. Any suggestions esp from someone who had a similar issue would be greatly appreciated.

Rich

KQ9L

"I strongly recommend the LiFePO4 chemistry. It has great properties"

I agree with that.? Great batteries. Light weight compared to lead acid, no need to charge regularly and very safe. They are supposed to be the safest batteries around and won't catch fire. .? I use them on a mobility scooter and they are very good and better performance than the original lead acid batteries. For standing around not being used for a while it's best to have around 60 to 70% charge in them, not 100%. That's it. Lead acid batteries require a regular charge or float charge or they go bad. Maybe need to check the LifePo4 once a year.

Admittedly they are more expensive than lead acid but they will last far, far longer and unless the lead acid is regularly maintained they will have a pretty short life span.

Reg???????????????????????????? G4NFR

Kees

As others are conveying, transistors are zapped with too much instantaneous voltage, too much current or getting too hot while operating. Placing an imperfect load on it can greatly raise voltage. These longer duration steady on waveforms make things warmer. And those humans who want every last watt of radiation get into the mix.?

Curt