texas star 500 dx path of power in amp.

[long night]

Hi. My texas star 500 dx has power to the front 2 pills, but no power to the back 2 pills. All power transistors and suspect resistors have been replaced. Any ideas why there is no power to the back 2 pills?

Thanks for your help.

 

[999]

If you still have the stock power wire configuration with the fuse holders, power is directed to each board individually from the fuse holders. The rear board is through a bare wire that solders to the top of the protection diode and then to the rear board on the rear trace.
The front board gets power from the red wire on the right trace.

 

[long night]

Hi 999. Yes I still have the stock set up. I can see power at the front board when checking, but can not see power at the rear board. Also the front wire wound resister on the right side of the board gets very hot. I replaced it with a new resistor of same value and it too gets very hot. Do not know what causes this condition.

Thanks

 

[999]

This is crude, but you should get the idea.
Power for the rear board travels through the small wire from the fuse holders, touching the protection diode, and then to the rear trace on the board. From there it goes through the wire with the chokes to the trace where the output transformer is soldered. Power then flows through the transformer to the other side where it is soldered to the traces that the collectors are soldered to. That is where the transistors get there power.
The blue circle is the diode, the green is the chokes, and red is the path of power.

 

[AudioShockwav]

I know this is obvious , but has the fuse holder failed?
I have seen them fail before, just check to see if power is getting through the fuses.

73
Jeff

 

[Greg T]

Just my opinion, but I would start with the power wire upgrade and tossing the fuses away, power each board separately with 8ga or 10ga wire and then go from there.

 

[Shadetree Mechanic]

The ground wire that goes from one board to the other is very small, might be a problem.

 

[long night]

I know this is obvious , but has the fuse holder failed?
I have seen them fail before, just check to see if power is getting through the fuses.

73
Jeff

Fuse holders and fuses are good!

 

[long night]

This is crude, but you should get the idea.
Power for the rear board travels through the small wire from the fuse holders, touching the protection diode, and then to the rear trace on the board. From there it goes through the wire with the chokes to the trace where the output transformer is soldered. Power then flows through the transformer to the other side where it is soldered to the traces that the collectors are soldered to. That is where the transistors get there power.
The blue circle is the diode, the green is the chokes, and red is the path of power.

Thank you this information and drawing. Much appreciated. I will use this to try and pin the issue down. Any idea why the 25 ohm 5 watt wire wound resister on the left front side of the amp gets so hot? the one on the right side does not.

Thanks.

 

[999]

I know this is obvious , but has the fuse holder failed?
I have seen them fail before, just check to see if power is getting through the fuses.

73
Jeff

If one had failed I think that would just cause the other fuse to pop from over current as the two holders are still connected to each other.

 

[AudioShockwav]

If one had failed I think that would just cause the other fuse to pop from over current as the two holders are still connected to each other.

Correct.

73
Jeff

 

[long night]

If one had failed I think that would just cause the other fuse to pop from over current as the two holders are still connected to each other.

It is the wire wound resistor that is getting hot.. Any ideas as to why?

 

[999]

It is the wire wound resistor that is getting hot.. Any ideas as to why?

I couldn't tell ya.

 

[nomadradio]

The 5-Watt resistor gets hot because it has power applied to it when the relay is keyed. Texas Star scrimped on the size of that resistor. 5 Watts is almost big enough, IMHO. A 10-Watt resistor would not run as hot. Or be as cheap. The one in the rear stays cold because no power is reaching it.

The fuseholder for the back pair of transistors has a wire that leads down to the rear pc board. Take the meter probe and see that this wire has power, and then follow the foil trace where it's soldered and see how far down the probe still shows power present.

Really sounds like a damaged foil trace on the rear pc board.

73

 

[999]

The 5-Watt resistor gets hot because it has power applied to it when the relay is keyed. Texas Star scrimped on the size of that resistor. 5 Watts is almost big enough, IMHO. A 10-Watt resistor would not run as hot. Or be as cheap. The one in the rear stays cold because no power is reaching it.

The fuseholder for the back pair of transistors has a wire that leads down to the rear pc board. Take the meter probe and see that this wire has power, and then follow the foil trace where it's soldered and see how far down the probe still shows power present.

Really sounds like a damaged foil trace on the rear pc board.

73

The fuse holders are tied together and feed both boards.