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texas star 500 dx path of power in amp.

long night

Active Member
Jan 8, 2020
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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

Well-Known Member
Apr 3, 2011
577
483
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51
SF Bay Area
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.
1651960642964.png

1651960681866.png

1651960739459.png
 

long night

Active Member
Jan 8, 2020
202
36
38
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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

Well-Known Member
Apr 3, 2011
577
483
73
51
SF Bay Area
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.
 

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long night

Active Member
Jan 8, 2020
202
36
38
73
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.
 

long night

Active Member
Jan 8, 2020
202
36
38
73
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?
 

nomadradio

Analog Retentive
Apr 3, 2005
5,325
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Louisville, KY
www.nomadradio.com
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

Well-Known Member
Apr 3, 2011
577
483
73
51
SF Bay Area
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.
 

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