Can I test this semiconductor in circuit?

[Chris Wilson]

I managed to feed 1kW of 137kHz RF up the RX antenna socket of my Kenwood TS-590S (please don't ask..... ) I cast it aside as the damage seemed terminal, but having a fairly quiet day today I decided to do some probing. I believe the damage is limited to the RX/TX RF board and relatively localized. Some damage to surge absorption components, a 150nF SMC inductor, a few HVC131 Pin Diodes (gee, they are seriously TINY), and a 0.01 chip cap that's literally disappeared taking a plated through hole with it.

I am a bit stuck about how to test a semiconductor in the BPF switching section though. I am paranoid about damaging the PCB so do NOT want to remove SMC components without VERY good reason as I am not very good at it! Could someone tell me if there's enough info in the attached excerpt from the schematics to say if I can test Q132 (highlighted in blue and a switching semiconductor for the BPF that was in use at the time of the "accident") in circuit, or what's the minimum to remove to so do please?

Thanks. If more of the schematic is needed please ask!


If the schematic excerpt doesn't load it's also at http://www.chriswilson.tv/test.jpg

[james_s]

I think I would replace any obviously damaged components and then try it out and see if it works. Often times parts can be tested in circuit but with this sort of damage it's hard to say.

[German_EE]

The device highlighted is a 'digital transistor' and these are used a lot in Japanese transceivers. Basically it's a darlington pair with some resistors added.

Testing should  be reasonably easy, with a logic 1 on the input the output should be grounded, it might be an idea to test the input for shorts first.

Good luck with your repair.

[SeanB]

Testing those diodes and transistors in circuit is simply looking at them with a DMM in diode test mode, they will for most diodes be either open or junction voltage in circuit, or for the digital transistors check there is no CE short, the built in base resistor protects it quite well. That will exonerate most of them, and the remaining iffy ones ( measures leaky in one direction and lower than diode in the other) it is a lot quicker to replace them, then test the removed one before tossing it. Check the small resistors as well, the ones connected to the collectors of those digital transistors are fusible, and can easily be either open circuit or high in value, just a quick in circuit resistance check and if it is nominal value or slightly lower it is fine, but if higher in value it is cooked. Same with the inductors, they should be lowish resistance at DC ( probably under 10R, but depends on the actual inductor, check manufacturers data sheet for typical DC resistance values for the specific series and value to be sure) and the capacitors should of course measure as very high resistance.

[Chris Wilson]

Thanks Sean. I now believe these devices are OK, time will tell. Things are moving along but i have a question re the dual diodes D102 and D103 to the right, in this schematic excerpt. They are across two relay coils and with the board un-powered they show about 2.6V each in both directions using the diode test function of my multimeter. Can I say that is OK or not? I could remove the relays to check them, but would rather not. Thanks again!

http://www.chriswilson.tv/dual-diodes.jpg

[james_s]

It sounds like those diodes are open circuit and you're just reading the voltage drop of the relay coil. Forward drop of a diode should be in the range of 0.6V or less, they're just snubbers to protect the drive electronics from inductive spikes produced by the relay coils.

[Chris Wilson]

Sorry, I was going by memory, the diode test shows 0.21 and 0.23 volts across them both ways, respectively, not over 2 Volts...

[SeanB]

Well, 0.23V simply means you have a 250R or so coil in the relays, and a pretty good chance they are actually fine, as diodes typically fail short in that size with limited energy, before blowing open circuit. Relays themselves could be cooked contact wise, but that is easy to check later when powering the board and measuring the contacts close and open as commanded by the coils.

[james_s]

That's probably ok then, it sounds like the DC resistance of the relay coil is what you're actually measuring. Diodes typically fail shorted anyway so if you have a drop that's higher than 0V and the diode is not a charred crater then they are probably fine. It would be difficult to kill those anyway since they're in parallel with a relay coil.

[Chris Wilson]

Thanks Sean. I now believe these devices are OK, time will tell. Things are moving along but i have a question re the dual diodes D102 and D103 to the right, in this schematic excerpt. They are across two relay coils and with the board un-powered they show about 2.6V each in both directions using the diode test function of my multimeter. Can I say that is OK or not? I could remove the relays to check them, but would rather not. Thanks again!

http://www.chriswilson.tv/dual-diodes.jpg

I have finished testing and Sean, you were SPOT ON! Three bands have no real RX or TX. The same three have collector resistor values of 7k rather than 68 Ohms.  Two of the same 3 bands have the 33 Ohm resistors up at 49 Ohms and 42 Ohms. The digi transistors themselves are all switching fine, so they and the shift registers are OK.

One band that actually seemed to work OK has a 68 Ohm resistor up at 71 Ohms. These all measured on an in spec Philips / Fluke LCR meter, so I am confident the differences are real. It uses a Kelvin tweezer thing as well. I changed some failed PIN diodes (with great difficulty, they are the smallest components on this board), an inductor and three chip ceramic caps, and some input protection diodes. I just need to ascertain the size of these resistors, physically, as the 33 Ohm are quite a bit smaller than the 68 Ohm ones. Length wise the 68 Ohms seem about 3.5 mm overall including the "feet" and the 33 Ohm ones about 2.5 mm inclusive.

With any luck this might live again, many thanks to all.


EDIT: I have managed to decode the Kenwood part numbers and ascertained the size and wattage and tolerance of these resistors and RS Components are sending some out today. Getting there, slowly, I hope