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Here is the gamma match video I mentioned in the thread I deleted asking if I should do it (decided to regardless), so here it is.
Edit: considering a part 2 in the near future where we deviate from recommended values and see what effects it has on the bandwidth and capacitor voltage (since...
1208a is in the cbtricks stuff under the 138XLR
Edit: swapping attachment for link to the file on this site since there is no need to take up server space with duplicates...
https://www.worldwidedx.com/cbt/radios/cobra/138xlr/graphics/cobra_138xlr_139xlr_tb1208a.pdf
Why not pop the cover and monitor the temperature of the transistors at the designed frequency and then bump it up to 468 and monitor again. It it starts getting hot compared to the baseline temp curve, quit. One thing is for sure, if it shouldn't be done, there will be a mismatch at the...
It sounds like the VCO tuning range might be a bit off. I think the VCO tuning slug is L304, but just in case I am wrong, don't change it. Since it is struggling with higher frequencies, I would guess the slug needs to be backed out a bit.
One way to change a tuning inductor value without...
Schematic from CBTricks. I don't see any AB bias circuitry in this schematic, just a switch that adds some delay to the relay. Unless I have the wrong schematic, that's class C correction: class B.
Edit: you can tell by the transistor's bases. They are tied together. Class AB has...
There may be an issue with coax sniffer probes (H-field probes) in that that they are made of 50Ω coax and the tiny current they pick up produces tiny voltages a FC will not see and is likely only going to work with something like a spectrum analyzer with a very sensitive front end. Most FC's...
The brand don't matter, but trying to align a radio with an alligator type test lead on your frequency counter might not go as planned. They don't come with them because they would only work for audio and low impedance test points. Most radio oscillators will go way off frequency or die...
Ground plane antennas usually have built in ways of adjusting the match and are easily tuned to 50Ω, while a dipole made of plain 102" whips is unlikely to be 50Ω on its own without additional matching, especially near another dipole. You must first ensure that the feed points of the dipoles...
And that just made me realize something....
Being the thickness of the oxide layer IS voltage dependent, and it is that thickness which sets the capacitance, you might find that after a while, when the dielectric deteriorates to the thickness of the voltage you are operating it at, the...
I asked that very question here a while back and didn't get an answer.
In that post, I though that with a higher voltage rating, the ESR would be higher, but I now know that its the other way around. Higher voltage caps have a lower ESR. From what I found on EEVBlog, it sounds like some...
I believe that it is sometimes mounted on the transistor screw so it is the same temperature as the transistor. As the transistor heats up, it's Vbe drops a little. If the bias does not change in response, the transistor conducts more and the operating point changes. If the diode is on the...
I think those small rechargeable hand warmers are a bad idea too. My dad gave me one that can be set to 110-130°F and with it set to 110°F and set down for a minute on the table, within minutes it swelled up enough to hear the sound of crunching plastic when squeezed. If a smart phone on a car...
Not sure why you want a 1:1 balun. The feed point of a split element driven on a yagi is not 50Ω. I also think that style balun uses common-mode current and messes with the antenna pattern. There are better options.
When you say you got real close to 50Ω with an SWR of 1.25, was that into...
Awesome! I didn't know they had put the original site here. I knew that some of the info was incorporated under the dxRadioTech tab, but this is the first time I saw (edit: what looked like upon first glance) the whole site here. I noticed the graphics are gone. Not sure if it is a work in...
I'm scared of even the little ones. I was just given a box of stuff from a relative and it contained some of those little matchbook sized plastic-wrapped lithium drone batteries. I put them in the steel shed. With how often the cat knocks stuff off my desk, all it would take is the dog to...
I got a side mic 148GTL free from a friend that said he used to do radio work for a local company. He gave it to me with some other scrap radios. Anyhow, like you mentioned above, the bias pots were cranked all the way up. Tracked it down to a bad crystal filter. Trying to make a missing...
Had that advice been followed, you would not have made an easy buck on it and impressed the locals. We need screwdriver techs, otherwise most radios would last a very long time and people that fix radios for a living would be doing nothing but recaps. Besides, the last guy had to try learning...
I thought "up" was ch19, middle was ch9 and down was NORmal mode. Never seen one where "up" was the NOR position. Is your radio installed upside down, or do you really like ch19?
The NOR position provides the ground for the channel knob. Taking the switch off the NOR position removes the...
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