Needing a bit of help

[walterjn]

Amp is a Texas Star DX350 factory 3 "pill". I am not sure what is going on with it right now, however it has blown the same electrolytic cap twice within a year. I'm wondering if I need to replace the keying transistor. Pics will be posted soon. I will also include the schematics for reference...

 

[walterjn]

Board as it sits

 

[walterjn]

Here's a closeup of where that cap was. I've left the leads so as to be able to find the location easily.

 

[walterjn]

Board layout

 

[walterjn]

Trace layout this shows that it was a factory design.

 

[walterjn]

Finally here's the schematics to this little bugger.

 

[KneeBiter]

Alot of stuff looks way over heated. Choke is burnt. Caps are dark.

 

[999]

Alot of stuff looks way over heated. Choke is burnt. Caps are dark.

Agreed. That thing looks like it has been run EXTREEMLY hard. A lot of stuff if well and cooked in there.
I would question the values on all of those brown caps and both chokes are cooked.
Also 2879's have been swapped into this where SRF3222's used to live. I'd be concerned that the caps were the wrong values for the 2879's before they got cooked.
This thing was designed to be used with a very low power radio. Nothing modern short of a very stock 29, and probably that would be too much, would allow this to live very long.
That being said I would have this gone through completely from stem to stern if you want to keep running it. The board will probably need to come out for a thorough inspection. TS amps have a tendency to blow traces on the bottom of the board when they get pushed too hard.

 

[walterjn]

Agreed. That thing looks like it has been run EXTREEMLY hard. A lot of stuff if well and cooked in there.
I would question the values on all of those brown caps and both chokes are cooked.
Also 2879's have been swapped into this where SRF3222's used to live. I'd be concerned that the caps were the wrong values for the 2879's before they got cooked.
This thing was designed to be used with a very low power radio. Nothing modern short of a very stock 29, and probably that would be too much, would allow this to live very long.
That being said I would have this gone through completely from stem to stern if you want to keep running it. The board will probably need to come out for a thorough inspection. TS amps have a tendency to blow traces on the bottom of the board when they get pushed too hard.

I run my Grant XL at 2 watt am dead key. And never had a problem until a couple of years ago when that cap blew. I would not be surprised to learn that this amp is 30-40 years old.

 

[999]

I run my Grant XL at 2 watt am dead key. And never had a problem until a couple of years ago when that cap blew. I would not be surprised to learn that this amp is 30-40 years old.

It's definitely from the early 80's.
I'd suspect that 2 watts is way too much for that amp. I dead key ¼ watt into my 667, I suspect that would be close to perfect for that amp.

 

[walterjn]

It's definitely from the early 80's.
I'd suspect that 2 watts is way too much for that amp. I dead key ¼ watt into my 667, I suspect that would be close to perfect for that amp.

I adjusted the dead key until it keyed the amp 9/10 times, then measured the radio output.
I would love to have it key at 1/4 watt.

 

[loosecannon]

honestly Walter the transistors you have in that amp are worth more than the amp itself.

2 watts is too much deadkey for that amp. a 1 driving 2 is just a bad design from the start and IDK what texas start was thinking when they made these.

either way, you only want to put 1 watt into that amp max. and you don't want to swing up past 10 or so. Sure the amp will handle 20 watts of swing but its going to drive the snot out of it.

Texas star amps really don't like any voltage source that is more than 14 volts.
is there any chance the amp is getting more voltage than this?
LC

EDIT to add that it looks like there is a burn spot on that red power wire that is really close to a case mounting screw location. if that red power wire was to touch that mounting screw, it would surely blow that cap if it's across the power leads.

also because of the way texas stars are built, there is always a chance of a solder ball rolling under the PC board while you are working and causing a short that you won't find when you pull the PC board up. also component leads on the bottom of the PC board need to be kept very short.

the keying transistor would not cause that cap to blow. if you are having trouble keying the amp, i would look at replacing the relay as they can wear out.

 

[walterjn]

honestly Walter the transistors you have in that amp are worth more than the amp itself.

2 watts is too much deadkey for that amp. a 1 driving 2 is just a bad design from the start and IDK what texas start was thinking when they made these.

either way, you only want to put 1 watt into that amp max. and you don't want to swing up past 10 or so. Sure the amp will handle 20 watts of swing but its going to drive the snot out of it.

Texas star amps really don't like any voltage source that is more than 14 volts.
is there any chance the amp is getting more voltage than this?
LC

EDIT to add that it looks like there is a burn spot on that red power wire that is really close to a case mounting screw location. if that red power wire was to touch that mounting screw, it would surely blow that cap if it's across the power leads.

also because of the way texas stars are built, there is always a chance of a solder ball rolling under the PC board while you are working and causing a short that you won't find when you pull the PC board up. also component leads on the bottom of the PC board need to be kept very short.

the keying transistor would not cause that cap to blow. if you are having trouble keying the amp, i would look at replacing the relay as they can wear out.

I would love to keep this thing as it has been a good amp for me. I shall address the power wire issue, and find a new relay and cap.
I don't believe that it gets more than 14 volts, if it does it shouldn't be much more. I do keep the leads as short as possible, and I'll make sure there are no loose solder balls running around inside.

 

[nomadradio]

The capacitor that went critical is C28. It serves to filter audio noises from leaking into the amplifier through the power input.

Has no involvement with the relay or keying circuit, as pointed out above.

The capacitance value is not critical. Schemo shows 22 uf. Any capacitance equal or over that is probably fine. Just make sure the voltage rating is 25 or higher.

And how the amplifier will behave without it? Depends on the source of power. A well-filtered DC power supply with short power wires would make C28 unnecessary. Texas Star included it to compensate for long power leads that are too skinny, and properly filter audio noises.

The 5-Watt ceramic bias resistor at the lower-right corner of the pic is dated 1978. The amplifier is probably newer. But not all that much.


73

 

[walterjn]

The 5-Watt ceramic bias resistor at the lower-right corner of the pic is dated 1978. The amplifier is probably newer. But not all that much.

So, I can figure that this thing is, let's say, about 42 years old with little to no issues until a few years ago... I think I will see about rebuilding it, or at least replacing the older caps that are looking bad, the relay (providing I can find one that doesn't cost a small fortune), and see about keeping it on the air for another 40 years. I like the little oddity that it is.
Yeah, I figure that I'm fighting a losing battle, after a few years, right now I'm good with that.
I guess I gotta have a project to spend $$ on and keep me out of the bars...