Texas Star DX500 and high SWR

[GnG8d]

I couldn't leave well enough alone.

It's running without that 33uf filter cap .......

2w@14v=90w or 16.53dB
2w@13v=60w or 14.77dB

Even with reducing the feedback it's not showing any signs of oscillation, but that cap deal still bothers me. I think I'll put another at a different location and see what happens before I start messing with trimmers.

 

[RadioGuy772]

Just saw this, I am off to the store for a 1AM run to get an energy drink, have lots of I.T. related stuff to do tonight, I have a couple cloud / virtual servers I am setting up. When I get back, and get them imaging up with their OS, and configs, I will take a few moments to reply.

EDIT: Got a call from another tech that needed help, just got back, and am exhausted. Going to rest. Didn't forget about you. I will follow this up as soon as I have some time.

 

[GnG8d]

I don't know what to think about those exploding filter caps, and I'm running it without one now. I keyed for awhile into a dummy load and then put it inline for a test run, I yacked with 131Mustang for a bit with some long keys and nothing is smoking yet. I did pick up 100w peak or so with the same Cobra29 driving it, but I think my bias relay is going to need a different design. It tested good at DC, but the added RF voltage is making the current limiting resistor hot. I added an inductor that I had on hand and it helped, but I still may go to the NC relay and eliminate L1 all together.

 

[RadioGuy772]

Eisest way is just to remove BOTH chokes. At the point where both 25 ohm resistors connect to the meter lamp, insert your VCC (+13.8 volts) here, switched from the relay. Use the VCC bus on the side of one of the boards for that purpose. On the input of the relay for the 13.8 volts, use a .01uf to ground, keep the leads short, and also add a 100uf electrolytic in parallel with the .01. On the "output" side of the relay it is not really needed, because there *should* be an RF bypass cap at that junction of the two 25 ohm resistors anyways. Make sure that the 25 ohm resistors are good, and also very important is the 1.6 - 1.8 ohm resistors in the divider. You don't really need the 33uf cap there that you are referring to. Personally, I would rather have a 100uf there, 35V electrolytic, and a .047 both together there, from VCC to circuit ground. You can add those if you want.

The only way usually that cap would blow is if you had a huge amount of RF coming back that way. What kind of power out are you getting out of the amplifier for a specified amount of drive and current draw? If the amplifier is "out of tune" then I could see that happening.

 

[GnG8d]

Eisest way is just to remove BOTH chokes.

In a nut shell, I used the old bias supply to fire the newly added bias supply relay. I removed L2 from R30 and used the old switched bias supply between L2/R30 to fire the new relay and fed the transistor bases right from the 13.8vdc rail through the new relay. I put L2 between the new relay and the bias feed point, or I could have replaced it with a jumper ..... I don't think it's hurting anything however. I also replaced the 25ohm and 1.8ohm resistors with new and I'm getting ~185ma of bias current @13vdc and ~215ma @15vdc. I don't have it written down here, but that should put me at ~200ma @14vdc. The only problem I see so far is the RF voltage heating up the limiting resistor that I placed in series with the new relay coil. I added a small inductor/choke and it helped, so some bypass caps should finish it off.

If you remove L1 you lose your switched 13.8vdc source. One way to fix this is to replace the newly added NO relay with a NC relay and then remove L1. Get your 13.8vdc from the anode of D1 which drops to ~1.5vdc when Q1 goes into full conduction and should close the new bias relay, completely separating RF from DC. I think this is what I'm going to end up doing as it kills a couple birds with one shot.

You don't really need the 33uf cap there that you are referring to. Personally, I would rather have a 100uf there, 35V electrolytic, and a .047 both together there, from VCC to circuit ground. You can add those if you want.

The only way usually that cap would blow is if you had a huge amount of RF coming back that way. What kind of power out are you getting out of the amplifier for a specified amount of drive and current draw? If the amplifier is "out of tune" then I could see that happening.

When the 33uf/25v blew, all I had on hand was a 47uf/35v and it blew too. The input tune was off when it was new and I think it's worse now, but I'm going to play with a couple trimmers today sometime and try to tame it a bit, then I'll worry about the output tune.

It's doing 400-450w peak (175-200avg) with a 16-18w peak Cobra29 driving it on 14vdc with a ~100w carrier. The radio and amp are together pulling ~32 amps max in any condition.

So, once I get it going good I'll worry about thermal tracking and/or regulation on the bias. But this is only the second transistor amp I've ever owned so this is as much exercise as it is anything. I never had the lid off of the first solid state amp I had a several years ago and I was hoping this was going to be plug and play too, but it wasn't in the cards I guess.



edit___
I dipped the reflect on the input caps and one section came out at 1200pf and other closer to 1100pf, but i'm still getting 1/2w reflected with 3w in. It keys thru the amp at 4.5w and zero reflect and keys into the amp at 3w and .5w reflected. So it looks like the original high SWR problem is still there, lmao.

 

[GnG8d]

Well, I put the trimmers on the output and as I backed them out the power went up and the current draw went down. The power seemed like it would keep climbing a little, but I stopped when the current draw wouldn't go any lower. It looks like 800pf might even work but I put the 1000pf in to replace the 1200pf. I may have lost a little on peak watts which is fine, but now it acts like it wants to oscillate again. I may have to add a little negative feedback, I don't know. I'm at 100ohm/.01uf now.

So, I added bypass caps parallel to the relay coil and added more filter caps from the 12v rail to ground and I'm done messing with it till next weekend. I'll play it a little tonight and see if it stays together.

 

[GnG8d]

The only way usually that cap would blow is if you had a huge amount of RF coming back that way.

I just ordered a new relay and was thinking of this. The second time this cap blew there was no RF present, I keyed it manually. The first time I was operating it normally and it tripped my 70a astron. The second time I keyed it manually and it didn't trip the ps.

Anyway, I'll dig into it again this weekend maybe. I still think the input can be tuned better if all traces of instability are gone.

 

[GnG8d]

This is starting to make sense. The bean counters told the engineers they had 2 pole relays to work with. Engineering told accounting it wouldn't work. Accounting told engineering to make it work.

So engineering added chokes, big wire wound resistors, heavy negative feedback and called it good.

Sound familiar to anyone?

I'm not sure if anyone is following this anymore, but the 2 pole relay is just not going to work. Once you remove L1 you lose all switched DC power. I thought I could go with a NC relay and use the voltage drop in the keying circuit to fire the new relay, but the coil in the new relay loaded the keying relay too heavily and caused issues there.

It occurred to me while redoing the bias that a NC relay wasn't safe anyway .......... I'd rather not have bias current than not be able to turn the bias current off if the relay or the circuit failed.

Soooo, now it looks like the only way to separate the DC bias current from the RF path, and remove both L1 and L2, is to use some perf board and build what amounts to a separate RF sensing circuit, or replace the 2 pole relay with a 3 pole relay.

I really, really didn't want to take this board up and spend all this time on an antiquated design using obsolete devices, what I really wanted was to just plug this thing in and play.

 

[GnG8d]

For anyone who may still be following this, disregard everything I said about relays. Just parallel the new coil with the old coil at D1, I completely had my head up my ass and tried to use a point on the 12v rail closest to the new coil at the bias feedpoint. It won't work.

But now I think I have created a perfect oscillator. L1 and L2 are gone and the bias feed is directly off of the 12v rail with no "intentional" RF riding DC. I went from 100ohm/.01uf on the negative feedback to 75ohm/.1uf and it's still oscillating badly.

So I'm going to drink a beer and stare at it awhile and see if that fixes it

 

[gary31525]

359, I just read this whole thread, thank you for all your information! I really appreciate it and I'm following it.

 

[Bow]

Very cool thread... I'm following it, as I have a DX-500V...

Thank you everyone for sharing their information

 

[2RT307]

Very cool thread... I'm following it, as I have a DX-500V...

Thank you everyone for sharing their information

When I started this thread, I knew of others that had the same issue, and had discussed it with Mad Scientist. Didn't know the thread would take off like this, but I'm glad to see others benefitting from the information being shared. One of many reasons that I REALLY like this forum.

 

[Shockwave]

We should remember that the basic TS design has several problems that contribute to self oscillations and make a few tests prior to doing any modifications attempting to correct the problem. The bias is very problematic especially if the original chokes have burnt up. However, we shouldn't just assume it's the problem. It's a good idea to disconnect the bias from the positive voltage feed and simply ground that wire to test the amp in class C.

Keep in mind just because the problem may go away, it is not a 100% indication that RF getting into the DC bias is your problem. Just having bias increases the small signal gain and possibility of self oscillations even when the bias is perfect. The thing this test will tell you for certain is if the problem is NOT in the bias if you find it doesn't clear up when you run it in class C. If the amp still oscillates and drives the output SWR up considerably, you can abandon the bias as the source of the problem in that amp.

Some of the other problems include poor RF grounding on the board due to the positive ground DC isolation through the grounded bypass caps. If it's only being used in a negative ground vehicle, bring the RF ground to DC or chassis ground in several places covering at least all of the locations where existing bypass caps are going from RF to chassis ground.

Radios Shack sells an assortment package of wire eye crimp on connectors. The small red ones are useful here. Pull the red plastic off the end by heating it with your soldering iron. Cut and shape the wire end so you can put the eye under the screws that hold the transistor down and solder the end to the grounded emitter traces to reduce inductance across RE prime.

The next issue is no isolation of the RF fields created by the input and output RF transformers. We often see people solder a copper ground strap flat down, over the top of the transistors. If we simply stand that strap on its edge vertically and cut it to the height of the transformers, we can improve the RF grounding and shielding between the input and output simultaneously.

Whenever we have a driver and final stage being fed off the same DC line we have to pay particular attention to proper decoupling of that DC line. That little ferrite bead the DC wire passes through once on its way to each output transformer is lacking to say the least. Use a 1/4 or 1/2 inch # 43 combiner core and put 3 or 4 turns of # 12 or 14 wire through it so the RF has a place to drop in front of the bypass caps. That keeps RF from the final stage from working its way back through the DC line and getting back into the drivers DC feed where it can cause undesired feedback and self oscillation.

 

[RadioGuy772]

Yes, yes, yes, and perfect. I agree 100% with Shockwave, and personally wish people would listen to his advice. From everything I know, he is spot on. (not that I am some sort of superior qualified to judge, I mean this with the utmost respect)

 

[HeavyAssault]

After reading this information I decided to take the DX500 out. In doing this I was able to clean it up, dust it out and check for anything that seemed "unusual". I found one component on the board that seemed to draw a question. Other than that I think all is in good working order. As conditions permit I have no trouble getting out.

My obsersvations lead me to the reason for my post. My fuses are soldered to the power wires as pictured above using 35a fuses. Now I'm confused as to how the power in feeds the amp. What does the smaller thin wire on the rear of the fuse holder supply power to on the board?

I'm quite a novice to the electroincs world so please bear with me. Thanks.