The AM Forum

Ok, National NC-303. Using an external homebrew T/R switch. All is hooked up as per manual. It is muting ok, but when I un key the transmitter, its like its getting a voltage spike causing a 30db over 9 signal to the receiver, then it will settle back down within a half second or so and all is back to normal. Sometimes it doesn't do it, but 95% of the time it does. It doesn't do it when muting the receiver manually with some clip leads, so i don't think its an issue with the receiver. Im using a DPDT 120vac 10a 8 pin plug in relay, 20ga Teflon wire for all connections. I also built in an RF sampler circuit and an AM de-modulator circuit onto a piece of perfboard inside the case as well. I didn't use any ferrite because all i had was mix 43 and i mostly do 80m AM work so they'd be ineffective. So, my first guess would be RF. I took a voltage reading between the output of the relay that goes to the receiver when normally closed and ground to see if im getting voltage back into the receiver somehow, and nothing. So, #1, its annoying. Especially when im talking to folks who quick key and ill lose the first word or so. And #2, I don't want to mess anything up in the 303....

Any help would be great.

Perry
W3MMR

Sounds like it may be a t/r sequencing issue. If the receiver un-mutes before the transmitter is completely off, the first thing the receiver hears is the last few milliseconds of your transmission.


Don

What are you using for a transmitter? What you describe sounds like an issue i used to have with one of my homebrew rigs. When you would unkey you heard a secons or so of oscillator noise in the receiver, since the oscillator cathode was tied to ground and still operating off of the fading voltage of the power supply after the transmitter was unkeyed. I ended up using a spare set of relay contacts to open up the oscillator cathode ground, which killed oscillation the moment the rig was unkeyed.

Thanks for the replies guys. I should have mentioned the transmitter. Its a Heath-kit DX-100. Ive been in the restoration process for the last few months and its pretty much done and I have the thing honed in pretty good. Replaced all tubes with NOS RCA Black Plate's thru out, Replaced HV caps, Bias Supply Caps, 4-pin mic jack, PTT installed to use a D-104 for keying, has your typical audio mods of a larger gate resistor, negative feedback, coupling caps, auto transformed the mod transformer, and went thru and pretty much re-flowed solder everywhere I could because I was finding lots of the cold and broken solder joints, also almost every ceramic stand-off that had something held to it with a nut, they were loose. It was one thing after another. But anyway, long story short, the transmitter has been pretty much normal from day to day, for the last few weeks, no abnormalities to speak of.

Now when I keyed the rig up for the first time this morning, it didnt do it for the first 4 or 5 keys.. Mind you, these weren't key down, make a transmission, un-key type keys. They were quick keys just to see if it was doing it.

Also, i just disconnected the coax going to the receiver, from the t/r relay and it doesnt do it. If it was oscillator noise, it should still do it correct?

Perry

And another update... Adjusting the Amplifier tuning (plate tuning cap), has an affect on it. I dip the plate, and it will do it, but if i literally bump the knob either way, it will stop. And move it such a small amount that it has barely any effect on the plate current, not even a needles width...

Somewhere in your t/r circuit there should be something that shorts the RF input to the receiver during transmit. I use a Dow-Key relay which accomplishes that function as part of its design. So that's one thing to look into.


Don

Unfortunately, i don't have it shorting the RX input at this time. its just open. Let me look at this for a minute and see if I can accomplish that. I should be able to.

Well i think I figured out whats going on. I noticed that de-tuning the rig slightly would stop it. And that when it was having the issue, the plate current would rise up to about 50mA then drop back down in unison with the S-Meter rising up and down. So it seems as if its oscillating a little bit and the finals need neutralization, which is common on these rigs. So ill do the neutralization mod and see how it works out.

Thanks for the info guys.

Perry

Here's a link to a recent thread on methods of neutralization that may be of help to you. Good luck!

http://amfone.net/Amforum/index.php?topic=44607.0

Thanks! And i see you on the QIX WebSDR chat board in the evenings. I thought that was you lol. We're a crazy bunch aint we? Haha. I was on the DX-100 for a little bit then switch over to the ANAN 200D later. Hope to talk to ya soon.

Perry,

I was first exposed to the AM community more than 40 years ago. Its been a part of my life ever since. New calls come, old calls go. But there's something about it that never changes. I wouldn't presume to give that something a name. What ever it is, its been around a good long time — with no end in sight.

Carrier on!


Don

I was exposed as a child and i've been around it my entire life. My father, W2SDR, formally WA3JBT, has been an AM operator for 50 years. So radio has been a big part of my life. Im only 32. Im one of the youngest AM operators out there. But we have 1 gu, KC3FKR, who is I think 20 or 21, who has a home brew 813 the he built, he is a part of his University's radio club where him and his mates are converting a broadcast transmitter, make and model slip me at the moment, and we also have W2BTK, 33, who is running a highly modified Johnson Viking 2 thats externally modulated, all done by him, and hes in the process of building a dual 810 rig. So here in the Northeast, the AM community is alive and well!!

Perry

P,

Welcome to the board.

I looked at your qrzed page and there is the UAN.  Didn't expect to see Jeff there.




KLC

I couldnt help but use that picture lol. Im sure you've seen the web page. Classic.

Neutralization is done. No more oscillation, no more keying issue. Thanks for the help guys.

Whats the UAN??

WA3UAN

https://www.qsl.net/wa3skq/wa3uan_page_02.html

Can you post the link for the chat board? I use Steve's receiver very often

I am a bit curious about this DX 100 issue,  I wonder why it needed to be neutralized?  There were hundreds of them that did not require to be neutralized and I am using a radio that has a 6146 driver that does not require it.  If proper construction procedures were followed Pentodes normally do not have a problem.

I wonder if there is something in the building that is wrong leading to the problem?

Janis, AB2RA, wrote a nice article on this subject:
https://wireless-girl.com/Projects/AMTransmitters/Neutralization.html

Interesting in that I wonder if the transmitter in question has a B variant in it with other problems?  Even so I have had several DX 100 transmitters over the years and never experienced the problem he had in any of them that are properly constructed.    In looking at the quoted article, I noticed several exceptions to the general rule of neutralizing.

I wonder if all voltages are correct or, sometime in the past life, it has had a component or two replaced with something that caused the problem.  Maybe one or both finals have some kind of problem.

Its at the bottom of the page on his SDR

I think that neutralization doesn't need to be perfect, just close enough. With variations in builds, tolerances and aging of components, and on different bands, one might sometimes find that the neutralization isn't close enough.

In the instant case, a problem surfaced occasionally. The modification that was done allows for tuning the neutralization into the "close enough" range. The manufacturer included this modification on later units after the problem arose many years ago. So, I guess the engineering prototype worked fine. And most of the first fielded units worked fine, but some didn't. The mod fixed it.

Perhaps if a complete RF cage was used on the final stage it would have been fine. Maybe a different layout would have worked better. As it is, a short piece of wire is all that's needed. But there are still some DX-100's out there that didn't get the memo. ;D


Don

It did it with another set of finals as well. Well actually 2 other sets, all 3 sets being NOS. All tubes are NOS RCA Blackplate tubes as well. Screen voltages are good, bias is good, plate voltages are good, filament voltages are good, etc. Im pretty sure it has been recapped at come point, has K-Cap brand caps throughout, which my other DX-100 has older looking, more original caps. I could be wrong though. Most resistors are original. I personally replaced the HV caps and bias supply caps along with all caps and resistors in the audio section when doing audio mods. But from everything Ive been able to research, its just the nature of some of these rigs. Why it was doing it, i dont know exactly, but its not doing it now and it tunes up much easier when loading up the rig now, the grid current stays steady as opposed to changing and having to go back and re peak the grid, go back and forth and back and forth. So it was definitely an improvement all the way round.

Oh and I forgot to mention, my parts DX-100 has had the neutralization mod done to it previously as well and if you look in the picture i attached, you can see another hole that someone has drilled in the back panel, directly in line with the lug on the other side of the "Driver Tuning Cap". So i guess someone tried this in the past? I dunno.