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Author Topic: 2 Meter Amp Repair - Burning Resistor  (Read 5767 times)
AD7C
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Posts: 81




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« on: February 10, 2013, 11:54:04 AM »

I was given a TPL 2M 5-8w in 120w out FM amp.  TPL PA3-1AE-2.  The amp had a bad COR sense circuit.  The 2n2222 transistor was dead so I replaced it.  Schematic here: http://www.ad7c.com/schematics/tpl_hi-b.pdf

New Issue: 5 watts in, the amp keys, and I get about 6 watts out.

I have checked through the amp for obviously blown components but I can see nothing.  What I can notice is that when the amp is keyed there are two 10ohm 1/2w resistors near the final stage that instantly heat up and one is turning dark (burning).  Not burnt yet as I don't key down for long (3-4 seconds)... but obviously a sign of an issue.

In the schematic (page marked -25-) the resistors in question are R1 and R3.  R1 is getting a lot hotter than R3.  Anybody have any ideas?  I have lifted the base and collector from Q4 and the transistor tests OK.  I wanted to post this before I start going through all the other transistors as removing them from the circuit is a pain. 

73,

Rich
AD7C




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W9GB
Member

Posts: 2622




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« Reply #1 on: February 10, 2013, 01:13:58 PM »

Quote from: AD7C
5 watts in, the amp keys, and I get about 6 watts out.
Unless you are in a bypass mode, then very likely you have damaged or dead RF transistors.

Quote from: AD7C
In the schematic (page marked -25-) the resistors in question are R1 and R3.  R1 is getting a lot hotter than R3.  Anybody have any ideas?  I have lifted the base and collector from Q4 and the transistor tests OK.
Page 8 has the BLOCK Diagram.
Page 25 has the correct schematic diagram for your TPL model.


LOOK and READ the schematic, What is the purpose of the R1, R2, and R3 resistors??
Do they appear to BALANCE the inputs to Q2, Q3, and Q4 ?

Quote from: AD7C
I have lifted the base and collector from Q4 and the transistor tests OKAY.
You have NOT tested Q2 and Q3.

Based on your VISUAL observation of R1 and R3, which transistor do you think is DEAD or DAMAGED ??
Why didn't you mention or observe R2 ?
Quote from: AD7C
... removing RF transistors from the circuit (PC board) is a pain.
YES, that is why the e-bench techs get the big $. 
« Last Edit: February 10, 2013, 01:30:04 PM by W9GB » Logged
AD7C
Member

Posts: 81




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« Reply #2 on: February 10, 2013, 01:32:10 PM »

Yes.  R1, R2, and R3 are used to balance the base of Q2, Q3, and Q4.

I felt that Q4 was the culprit, hence the reason that I lifted the emitter and base to test it.  R1 was right next to it and at first I thought Q4 was bad because it got hot quickly.  I believe most transistors of this type fail 'shorted'.  However, upon closer inspection it was just R1 heating it up.  When it tested good.. I am stuck.  Amplifiers are not my forte so troubleshooting this type of equipment is difficult for me.

If you believe you know the transistor or part that is the likely candidate I would appreciate the input. 

AD7C
Rich
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AD7C
Member

Posts: 81




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« Reply #3 on: February 10, 2013, 01:57:00 PM »

Thought about what R1, R2, and R3 does some more.  Took a look at what had to die to make more current flow through R1.  Lifted Q3.  Tested bad.  Bingo. 

Rich
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W9GB
Member

Posts: 2622




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« Reply #4 on: February 11, 2013, 03:04:01 PM »

Quote from: AD7C
Thought about what R1, R2, and R3 does some more.  Took a look at what had to die to make more current flow through R1.  Lifted Q3.  Tested bad.  Bingo.  
That is what I wanted you to do -- THINK through your observations of abnormal operation, compared to normal.
Troubleshooting is 80% thinking and only 20% actually doing repair work at the electronics bench.

For transistors, the 2 common failures are SHORTS or OPENS.  
You have to consider both, and you normally test or diagnose for both possibilities.
==
Now you have the procurement process for replacement parts.

RF Parts (California)
http://www.rfparts.com/

Communication Concepts, Inc. (Ohio) - Select RF Transistors
http://www.communication-concepts.com/index.php/components.html
« Last Edit: February 11, 2013, 03:08:38 PM by W9GB » Logged
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