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texas star 500 dx path of power in amp.

kc8mob

Active Member
Sep 26, 2017
70
67
48
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Regarding this 5 watt power resistor in the bias scheme. How long does it take to get hot?

And while you are testing this and keying it up has your balancing resistor in the output combiner (Near the relay) burned up yet? Doesn't take long.

And you said the resistors associated with the bias and your 10 ohm base resistors have been replaced?

Including the 1.6 or 1.8 ohm resistors by the input transformers?

Also, did you replace all 4 of these finals or just 2 of them?

And what prey tell did you use for replacements?

hfe matched HG transistors or "Toshiba" 2879s from ebay or maybe even legit shibas?

Good internal pics of the amp would help.

The very hot and cold power resistor in the bias scheme is screaming problem problem problem.
 

long night

Active Member
Jan 8, 2020
189
34
38
73
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
Will do. Thanks.
 

long night

Active Member
Jan 8, 2020
189
34
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Please clarify for me.
You are saying no power to rear board.
Are you saying no voltage power to rear board ?
OR,
Are you saying no RF power to rear board ?

A couple of good photos would be helpful.
No voltage power. Not sure what the difference is. If you have no power, you have no voltage or RF power. Correct?
 

Ranch55

Sr. Member
Jan 18, 2016
657
790
103
Fredericksburg, Texas
RF could be getting there to the board, but nothing would be happening if there is not any voltage getting to the transistors.
If I had that amplifier on my bench, first thing I would check for is DC voltage on the outboard end of the output transformer. If voltage is indicated, it should also be on the collectors of the power transistors.
Voltage or not, check the bias resistance on the input transformer end.
 

long night

Active Member
Jan 8, 2020
189
34
38
73
RF could be getting there to the board, but nothing would be happening if there is not any voltage getting to the transistors.
If I had that amplifier on my bench, first thing I would check for is DC voltage on the outboard end of the output transformer. If voltage is indicated, it should also be on the collectors of the power transistors.
Voltage or not, check the bias resistance on the input transformer end.
Hi Ranch55. I am not well schooled in electronics. If you could show me on a diagram where to check the DC voltage on the outboard end of the transformer that would help. Also can I check the power transistors for power on the collectors without removing the transistors? And where to check the bias resistance on the transformer input end? A diagram showing where to check for these things would be very helpful.

Thanks
 

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