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Texas Star DX500 and high SWR

I *AM NOT* advertising, or teying to sell anything. Let me start off by saying that. I am only here to share information. When we used to do this mod to the amplifiers, we would charge 80.00 for the 4 or 5 transistor amplifiers, although on the 667 we really would push customers to just allow us to bypass the driver. If they insisted, we would still do it up, and only charge the extra 3.00 in parts to rework the driver. The issue is that as the amplifier stands, the driver shares the bias supply with one of the push pull pairs on the "finals". This is a no-no.

The 2 transistor amplifiers, we would charge half, 40.00.

Once again, not advertising, just throwing out the numbers of what we used to charge to maybe give you a fair starting point.

EDIT: Was just thinking, what about, (I have been contemplating it for years) a step by step guide for amplifiers, export radios, with things like how to fix issues we just discussed, installing aftermarket filters like collins, etc into export radios, changing the RF amps and IF amps to dual gate mosfets, changing audio frequency response of transmitters and receivers, etc? Kind of like the older "super duper secret screwdriver hack job" books, but instead written more like a Lou Franklin book, like the Getting Started In Electronics.... only with general sections that apply general theory on the modifications, so with the knowledge it can be applied to most devices, and then also different volumes that cover step by step instructions for popular chassis radios? Even some ham radios could be covered, for instance flaws in the FT-100 / 100D, Icom 706MKIIG, come to mind with some things the users could do, if they wanted to attempt it. But they would be REAL mods, not just cut a part, or turn this. Maybe another volume dedicated to just radio theory, another on antenna myths, etc. It could really go a long way and help the community and get good information out there. Plus, everyone feels good when they accomplish something themselves. I certainly don't want to stray or hijack this thread, so I will get back on subject, but even this little conversation back and forth shows me that there are educated people on RF theory, and also operators that do desire to runs things properly, but have a lack of resources to do so.
 
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Thanks for the info Shockwave and Radioguy and everyone else who replied here, I understand you are not advertising, I dont understand alot of what you guys say but I appreciate you guys time and effort to explain. I have 2 667V's, one of which does what is mentioned in this thread and I am sure the other will do it also in a matter of time.

I dont have the extra cash right now to fix it but it is good to know a ball park figure for the future.

Thanks, Greg
 
Keep in mind adding things inline with the antenna like a TVI filter will change the reactance of the load that the amp sees. This is the most common thing to cause an unstable amplifier to shift its operating conditions. Be it from stable to oscillating or vice versa. Harmonics are almost never the cause of high VSWR because even in the worst case single ended solid state amp, harmonics never reach more than a few percent of total output power. Oscillations on the other hand can produce substantial power on undesired frequencies.

This post caught my attention.

I just tried using a new TS500 that I recently purchased and it's acting really strange. If I set the carrier at 75-80w on the antenna or dummy load it doesn't change. But then if I drive it into the final amp that carrier drops by 50%. That's backed up by less current draw and the final amp is still doing 13-14dB. I have a dual line section between driver and final, plus final and antenna so I can see it all dynamically.

The final amp has input tuning and I can find a nice sweep so it doesn't appear the air variable is being maxed or exceeded. At no time do I see big reflect numbers or any reflect spikes.

At first I was thinking input/output tuning, but it should still load up the same. Now I'm thinking oscillation even though reflect isn't spiking.

One thing I think I witnessed was the amp changing carrier power output and current draw on it's own with no input signal. That also looks like oscillation as in "on the verge".

I thought about switching the DEI's for Toshiba's but maybe this relay deal will get it acting better.
 
Seems the thread went cold for awhile.

I'm planning on swapping the DEI's for the Toshiba's and isolating the bias supply on it's own relay ........... soon, lol. Hopefully before the weather turns full spring. After spring/summer gets here I rarely turn a radio on, I don't even play all that often in the winter honestly.
 
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I'm planning on swapping the DEI's for the Toshiba's and isolating the bias supply on it's own relay ........... soon, lol. Hopefully before the weather turns full spring. After spring/summer gets here I rarely turn a radio on, I don't even play all that often in the winter honestly.

I hear ya,

I'm still planning on trying the retrofit as well. I just haven't had the time to play radio much.
 
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Blew that filter cap right off the board?
Yep, hit the top of the case so hard hit crushed the can.

I knew it tried to oscillate into a dummy load or antenna, but it worked as a driver a couple times ...... the difference in reactance must have settled it down. Anyway, it was a crappy rainy/windy day today and I thought I'd try it (should have sent it back), and it took off on the first key. It blew that cap and the splitter/combiner resistors look like they got hot, but I don't see anything else obvious. I guess it's time to get a parts list together :whistle:
 
Well I found myself in a bit of a quandry. I thought if these DIEs were still good (which I think they are), that I'd just make the bias changes and leave them in there for testing and swap out the Toshibas later. However, it appears that they've made a few small changes in the feedback circuit, bias circuit and output tuning when checked against the published drawing.

The 1.8ohm resistor in the bias voltage divider is 1.6ohm.
The 39ohm/.15uf resistor/cap in the feedback circuit is 56ohm/.1uf.
And the 1000pf tuning cap is 1200pf.

So, I thought about using all of the suggestions here with bias and feedback circuits with the DEI transistors and now it looks like I may as well order everthing to swap the Toshibas in. I kind of wondered if they just dropped these in as replacements or if they did some fine tuning, and it appears they've made some changes. It seems like they should have left the 39ohm/.15uf combo in the feedback circuit and this thing might have ran.
 
The 1.6ohm is really no big deal, a bit less bias, but will hardly be noticeable on the air, and also can help stabilize the thing up. I would go back to 1000 pf that can help drop current, and also you will only notice a small drop if any in output. On the negative feedback, it all depends, I have seen some amplifiers that need more, some less. Depends on the gain of the devices and particular circuit. If you re-work the bias circuit, you don't need as much so you can increase the value here. They seem to use a lot of neg feedback because of the crazy bias circuit.
 
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I didn't forget about this, but Mouser kinda jacked my order up so I've only gotten the DC bias supply isolated from the RF path. If I were to do it again, I'd have ordered a NC relay instead of a NO relay to eliminate L1 as well. I didn't study it enough before I ordered the relay, but it should be sufficent leaving L1 in and firing the relay at R30.
 
Unless someone has an idea off the top of their head, I'm shelving this for a rainy day.

The DIE transistors checked good, but I put the Toshibas in anyway. I redid the feedback and the bias circuits and it blew C40 again. This time with no RF applied, I just manually keyed it and it blew like a Lady Finger after several seconds. I was wathcing the imporartant (expensive) components with my mini-temp gun when it popped. I've been all over these boards and I don't see any hot components or wayward solder.
 

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