Texas Star DX400 improvements???

[loosecannon]

seeing as how there's a lot of good amplifier discussion going on these days, and since there seem to be quite a few knowledgeable people weighing in on these discussions; i thought it might be cool to use a practical example to illustrate some of the points being made.

the texas star DX400 and DX500 are very popular amps, and i chose the DX400 (4x2290's) because i have one sitting here in front of me.

some of the things being discussed are changing the fuse setup, improving the RF input circuitry, changing the AB bias current, and preventing transistor oscillation.

here is a diagram of this amp, mine is basically identical to the diagram:
http://www.cbtricks.com/Amp/txstar/dx400_dx500/graphics/dx400-dx500_inter_connection_layout.gif

here is a schematic:
http://www.cbtricks.com/Amp/txstar/dx400_dx500/graphics/ts_dx400-dx500_sch.gif

here is a parts list:
Texas Star DX 400-DX 500 Service Manual

sometimes having a specific example to reference can help facilitate the discussion and im hoping this thread has that effect.
LC

 

[dxhound]

It depends on your goals as to what is suggested. First thing I would do, As I am doing now with my 1600, is to independently fuse each pair of transistors. Also upgrade the bias network to keep it stable and in AB. currently they will shift into class C due to poor regulation as the heat increases. Also, not sure on yours, but the incoming negative power lead is daisy chained on my dx1600, I am going to run independent leads to each board from a terminal. If your worried about spectral purity, a bandpass filter on the output wouldnt hurt. Theres alot you can do....Just need to know for what use.

Also, using a keyline is a VERY good mod to do, and get away from RF keying!

 

[9C1Driver]

Get with Peakaboo on what to do. He used to Jokerize Texas Star amplifiers for customers. Why in the world would you want to fuse each pair of finals on a Texas Star 400? For the 400, 500, and 667 adding heavier wire and bypassing the stock fuse setups for a single blade fuse works great.

 

[TonyV225]

I have 1 DX400 2 DX500 and this new 1600 now as far as the 400 and 500 amplifiers what is the deal with these fuse setups?? Are they restricting amperage intake for feeding these amplifiers?? I am just curious Ive never had to pull my amplifiers apart otherthan to ad a new keying trans to one years ago but I didnt really investigate inside because they really have worked well so Im curious as to what the issues with the fuse holders are?? I have seen this mentioned before thats why I ask. I know as far as filtering Ive got that all figured out and all set if I want to use these on more of the HF bands.

 

[loosecannon]

yeah i was hoping peakaboo would jump in here at some point.

i am most interested in learning about my, and other amps.

i am interested in improving the input matching if need be.

improved biasing would be fun, and this amp definitely has the room to mount the TO-220's.

i think a bandpass filter on the output is GREAT idea, and i have built a few chebyshev style filters as stand alone units with SO-239's to go inline with the coax.

what i need to learn about here is how lead length, and mounting location play into it. among other things.


one thing about this amp that struck me as weird was the capacitor across the input transformers (C22 and C31). they are .0015uF which is 1500pF, and thats much more than i am used to seeing there.
heck, the output transformers only have 1200pF across them.
is this something special about 2290's?

also, can the preamp circuit be improved as far as S/N ratio goes?

ok, better stop there, LOL,
LC

 

[9C1Driver]

Tony, as far as the 400 and 500 goes the main improvement is the larger gage wire and then an in-line blade fuse works great. The 667 has a funky twin 30 amp fuse deal that is alittle restrictive. Bypassing this with #8 wire and then a in-line axternal fuse helps them out. About 10-12 inches of cable out the back of the amp into an Anderson style quick-disconnect is the best way I have seen it done. (fuse on the red line) Makes for a nice neat package that can be un-hooked quickly.


Loosecannon, I am sure he will see it and chime in. As I recall he does something to the bias so it goes from passive AB to regulated AB. He might add some caps in certain key locations. Clean up the input output matching, and add a fan to keep things cool. Everyone that has ever had him re-work one was quite happy.

 

[TonyV225]

I often was boggl;ed by why the hell they used such thin wire in making these amplifier's. That stuff is hardly enough to feed these amplifiers (if there being run hard) I have the standard 10 guage or whatever it is and use the Orange Anderson connectors the power source leads on the other end are 8 gauge on my 400 and 1 of my 500's

I dont run these hard and figure if ran at the 400 at 200 watts and the 500 at 300 watts they should be ok I know that better 8 guage would be a better choice going into the amp to the board for better power flow I may just order several feet of 8 guage black and red wire and do the updates for the sake of doing it and making it right.

 

[dxhound]

The reason for fusing each pair of transistors is in case one set or one transistor shorts,the fuse will blow and prevent the others from being grounded as well.

 

[loosecannon]

the first thing im noticing is that C3, a ceramic disc cap which is the first cap the input coax sees, shows to be a 1000pF cap in the schematics, but in my amp its a 2000pF cap.

this raises a couple of questions.

is there any difference as far as performance goes between a 1000pF and a 2000pF cap here?
if so, which is better?

the next question is would a silver mica cap work better than a ceramic disc cap here or would it even matter?

thanks for any input.
more questions coming.
LC

 

[9C1Driver]

The reason for fusing each pair of transistors is in case one set or one transistor shorts,the fuse will blow and prevent the others from being grounded as well.

I don't really see where that would be realistic with a TS400 or 500 but if it makes you feel better go for it I guess.

 

[BOOTY MONSTER]

this guy spent 23 years on the bench for texas star and hes doing his own thing now . i asked him about upgrading my texas star 500v when i had it and he suggest bypassing the fuses and upgrading the power lines to 8 gauge . heres a quote where he introduced himself on sparkys store forum .

~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~`

"notice techs and hobbiests,
I am Mike from Texas Star.
23 years at their bench and free lancing now.
I offer free tech support on the internet.
e-mail questions to TexasStarRepair@cox.net
I do like a digital picture of top of amp so I can see modifications.
check out my troubleshooting page
at Home Page
this is kept real basic

also check out Texas Star Amp Repair on youtube

I am going to try to give out some basics
so as to save you from frying your new replacement transistors after completing a repair. Always check the bias. It only takes 1 minute. "

 

[dudmuck]

the first thing im noticing is that C3, a ceramic disc cap which is the first cap the input coax sees, shows to be a 1000pF cap in the schematics, but in my amp its a 2000pF cap.

C3 is a DC blocking capacitor for the RF input.
It is intended to be an open circuit to DC, but at 27MHz it should be of low loss.
1000pF is about 6 ohms at this RF.
but 2000pF is 3ohms, and might give slightly better input vswr or less loss.

You may also notice C1 is 15pF, which is used to couple a small amount of RF to turn on transistor which keys relay. 15pF is like 400ohms to 27MHz, so only a small amount of energy will pass across it.

 

[loosecannon]

thanks for the reply dudmuck,

if this is just a DC blocking cap, i guess a ceramic disc cap is as good as any.

the first thing im going to do is bypass the fuse arrangement, but i have a couple of questions about that.

the positive lead splits off at the fuse connectors and one lead goes to one board, and the other goes to the other board, however only one of these leads is attached to a "button diode". why is that?

second, the fuses in my amp are each 25 amp fuses, for a total of 50 amps.
is this really enough? or should i fuse it with a higher amperage fuse when i get rid of the old fuse holders?


oh, and booty, thanks for reminding me of mike. i will contact him and see what he has to say.
LC

 

[dudmuck]

the positive lead splits off at the fuse connectors and one lead goes to one board, and the other goes to the other board, however only one of these leads is attached to a "button diode". why is that?

I suppose the button diode you refer to is MR750.
This is for reverse polarity protection.
The idea is that the diode D3 is supposed to conduct the large current if you connect DC backwards. This is only a 6A diode, but for the short time it takes to blow fuses, the fuses should blow up first. If the diode blows first, then the whole amp goes up in smoke.

If you want to put higher current fuses in there, then you should upgrade the D3 diode also if you still want reverse polarity safety. Or you could parallel more big diodes onto D3 if you have them as a cheap insurance policy.

 

[loosecannon]

dont know why i thought it would need a diode for both boards, brain fart i guess. LOL

reverse polarity protection is always a nice thing.
any reason i couldnt just use a rectifier diode instead?

as far as the amperage rating for the diode goes, could the diode short before the fuse goes and cause the transistors to blow?

what would your recommendations be for diode and fuse rating?
LC