Faulty Juma TRX2

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Faulty Juma TRX2

Postby M0ZHN » 15 Jul 2020 15:30

Good evening,

My TRX2 has been sitting on the shelf for some time, I finally picked it up with a view to update the software. I switched it on and everything was working fine, I was enjoying listening to 20m band. Next day I switched it back on but the fuse in the Juma blew. I replaced the fuse today and powered up, it came briefly to life only to release a puff of smoke that appeared to be coming from the optical encoder on the control board. I cannot see any visible damage or signs of fault so my question is, how do I go about troubleshooting my Juma to find the fault please?

Any help is much appreciated.

73 Elia,
M0ZHN
M0ZHN
 
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Re: Faulty Juma TRX2

Postby M0ZHN » 16 Jul 2020 09:42

The fault was traced to C14 on the Control DDS Board which had developed a short. After replacing it the radio turns on straight away without pressing the power button, also cannot switch it off with the power button any more. Could it be that Q1 and/or Q2 have gone faulty as well? How to check without removing them from the pcb?

Thanks,
Elia M0ZHN
M0ZHN
 
Posts: 4
Joined: 03 Feb 2013 18:47
Location: England

Re: Faulty Juma TRX2

Postby 5B4AIY » 17 Jul 2020 10:36

Hi, Elia,

Let's talk about how the power switch of the TRX-2 works. In the OFF state there is +12V applied to pin 2 of Q2. Q2 is off at this point. When the PWR switch is pressed, two things occur, the first is that +12V is applied via D1 to the gate of Q1, turning it ON. This pulls the gate of Q2 to ground, turning it ON, and power is applied through Q2 to the rest of the transceiver. The microprocessor powers up, and after the initial boot sequence it outputs a +5V signal, PWR-ON, from pin 59 to the gate of Q1 latching it ON, and holding Q2 ON as well. The second thing the PWR switch does is, after the completion of the boot sequence pressing the PWR switch sends a +5V signal to the PWR-SW pin 60 of the microprocessor and this is used to lock the VFO, cancel an operation, and if present for long enough, to force a shut-down.

In your case, disconnect the power supply, remove the top cover, and with a multi-meter on the OHMS range with the + lead connected to pin 2 of Q2 and the - lead connected to pin 3 (These pins are accessible via J4). If Q2 is OK this should measure a very high resistance, otherwise, if Q2 has gone short-circuit, then it will measure a low resistance, and you will have to replace Q2. It is also possible that Q1 has failed short-circuit, in which case this would also give you the same symptoms, i.e, immediate power up when applying power to the transceiver. You can check for this by again using your multi-meter on the OHMS range, and measuring the resistance between the gate of Q2 and ground. If Q1 is OK, you should measure a high resistance, if Q1 is shorted, then you will see a very low resistance, which would turn Q2 on leading to the symptoms you describe. Either of these failures will certainly result in the transceiver being unable to be turned OFF by the PWR button.

My bet is that Q2 is shorted. It is very likely that with C14 a short-circuit although this would blow the main fuse, this takes time, and Q2 could easily be damaged by the high-current surge, leading to a failure.

If you need any other assistance, let me know.

73, Adrian, 5B4AIY
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Re: Faulty Juma TRX2

Postby M0ZHN » 17 Jul 2020 11:49

Hi Adrian,

Thank you for the assistance, yes you are absolutely correct, Q2 is gone. I checked as per your email and it is a short. I don't have a similar transistor to hand so will order a replacement and see how goes. The radio is working at the moment so hopefully no other damage has been done.

Once repaired I will attempt loading your software :D

Thanks again,
73 Elia, M0ZHN
M0ZHN
 
Posts: 4
Joined: 03 Feb 2013 18:47
Location: England

Re: Faulty Juma TRX2

Postby 5B4AIY » 18 Jul 2020 11:21

Hi, Elia,

Yes, that was the most likely candidate. The problem arises because of the use of tantalum electrolytic capacitors when connected directly to a low-impedance current source such as a power supply. These capacitors have very low effective series resistance, and hence when subject to a switch-on transient, especially after having been idle for a long time, they are very likely to fail short-circuit. If the power supply you were using has a large value capacitor at the output, then there is often more than sufficient energy available to cause C14 to fail, and the resultant surge current could well be large enough to damage Q2. From its data sheet, its absolute maximum pulsed current rating is 8A, and its avalanche current rating is 5A. These currents are readily obtainable from power supply electrolytic capacitors. Always with the advantage of hindsight, it would probably have been better to have used an aluminium electrolytic for C14, but that's always the case, so it's no reflection on the original designers.

Anyway, glad to have been of some assistance, hope you get it fixed shortly, so you can try my firmware.

73, Adrian, 5B4AIY
5B4AIY
 
Posts: 188
Joined: 13 Nov 2011 09:22
Location: Cyprus


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