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QMX (rev. 2 + diode) dead again...

 

Hello, today I switched on my QMX again, after I had to recharge LiPol battery (3S). It seems RX part is dead again, RF filter sweep shows garbage (random values between -60 and -100 dB). When I change the radio and listen on TS-480, there seems to be normal traffic on the bands. Between my last use of QMX and today it was only stored in a box.
When I switched it on, serial port was not working, I had to do factory reset. The serial port worked some time, then when I was in terminal diagnostics, QMX stopped responding (but the serial port did echo input). When I changed band in diagnostics, it did not change in QMX (I switched to 40 m but frequency remained in 80 m band) and when I pressed T, there was transmission, but diagnostics did not show any data.
What is it? do you now?
Jindra
 

P.S. diagnostics shows battery voltage fluctuating between 12.4 and 12.5 volts. Power and SWR indicators shows expected values.
 

I measured pins 9, 10, 11, 12, 13 of IC402 (band switch):

- pin 9: 2.45 volts (this is the common pin connected to voltage divider R401, R402

- 3.5, 7 and 5.3 MHz, 2.45 volts at pin 13 (Y3)
- 10.1 MHz: 2.41 V at pin 12 (Y2)
- 14 MHz: 2.38 V at pin 11 (Y1)

If I am not wrong, all three voltages should be the same, right? Channel ON resistance of FST3253 should be some 7 ohms and there are no DC connections at pins 10, 11, 12, 13. Does it mean that IC402 is dead (again)?

I also measured voltages at Q508 (2N7002) in RX mode: gate 3.28 V, drain 11.89 V. Does it mean that Q508 is dead, too? I guess, it should be open in RX mode.


j.
Jindra
 

P.S. the internal diode in Q508 is OK
 

Jindra, check the continuity of L508. In RX, Q508 should be turned on, but its source and drain should be approximately ground potential. If L508 is open, you will observe 12V at the drain, and perhaps a volt at L508/Q508 source. The inductor may have been mortally wounded in rework surgery.

Your measurements of bias voltage are fine. I do not see anything to worry about there.

JZ

On Fri, Jan 12, 2024 at 1:42?PM ok4rm <radio.miskovice@...> wrote:

I measured pins 9, 10, 11, 12, 13 of IC402 (band switch):

- pin 9: 2.45 volts (this is the common pin connected to voltage divider R401, R402

- 3.5, 7 and 5.3 MHz, 2.45 volts at pin 13 (Y3)
- 10.1 MHz: 2.41 V at pin 12 (Y2)
- 14 MHz: 2.38 V at pin 11 (Y1)

If I am not wrong, all three voltages should be the same, right? Channel ON resistance of FST3253 should be some 7 ohms and there are no DC connections at pins 10, 11, 12, 13. Does it mean that IC402 is dead (again)?

I also measured voltages at Q508 (2N7002) in RX mode: gate 3.28 V, drain 11.89 V. Does it mean that Q508 is dead, too? I guess, it should be open in RX mode.


j.
Jindra
 

Hi, John,
thanks for the tip. I think you mean L503 (the one between Q508 source and ground). I checked two ways: L503 end-to-end, and Q508 source vs ground (at the power supply connector, there it is easy to reach). Both show 1.8 ohm at 200 ohm range of my multimeter.?
So it looks like replacing Q508 - the diode is OK but the channel seems to be interrupted. It's not even that the drain pin were cold-soldered, the MOSFET is broken...
Another question is how this could happen. An internal "spark" between the gate and the channel that would vaporize part of the channel??
Anyway, everything was fine when the battery voltage was 12 V or below. When I connected almost fully charged battery, the MOSFET failed (and I hope it's only the MOSFET and not something else down the path).
BTW, is there any significant difference between 2N7002 and BSS123?
Jindra
 

Yes, Jindra, good catch on my typo.

It would be useful to see what the pin to pin resistances are for the failed Q508. We might be able to discern what kind of fault occurred.

The biggest and perhaps most relevant difference I see between BSS123 and 2N7002 is Vds, the drain-source breakdown voltage. The BSS123 enjoys a spec of 100V whilst the 2N7002 trails behind at 60V.

This could be the source of? your Q508 mortality.

JZ

On Fri, Jan 12, 2024, 4:48?PM ok4rm <radio.miskovice@...> wrote:
Hi, John,
thanks for the tip. I think you mean L503 (the one between Q508 source and ground). I checked two ways: L503 end-to-end, and Q508 source vs ground (at the power supply connector, there it is easy to reach). Both show 1.8 ohm at 200 ohm range of my multimeter.?
So it looks like replacing Q508 - the diode is OK but the channel seems to be interrupted. It's not even that the drain pin were cold-soldered, the MOSFET is broken...
Another question is how this could happen. An internal "spark" between the gate and the channel that would vaporize part of the channel??
Anyway, everything was fine when the battery voltage was 12 V or below. When I connected almost fully charged battery, the MOSFET failed (and I hope it's only the MOSFET and not something else down the path).
BTW, is there any significant difference between 2N7002 and BSS123?
Jindra

 

SPOT ON!

It must be this... I discovered they actually sell BSS123 in Prague (one and only shop, but TG it sill exists). However, I was too fast and replaced the faulty 2N7002 by another 2N7002. It would be no big deal, unfortunately L503 lost one of its so important terminals. So I ordered 25 pcs?MLZ2012M470WTD25 from Farnell (MOQ is 5 pcs but I ordered more to balance cost of parts with cost of shipping LOL) - these seem to be the only 47 uH 0805-size inductances that are specified above 2 MHz (1000 ohm impedance @ 30 MHz). But of course I will need only one. (So, anybody in E.U. if you need 47 uH SMD RF choke size 0805, send me a Euro and I will send you one in a letter envelope)

73 Jindra


On Fri, Jan 12, 2024 at 11:01 PM, John Z wrote:
Yes, Jindra, good catch on my typo.
?
It would be useful to see what the pin to pin resistances are for the failed Q508. We might be able to discern what kind of fault occurred.
?
The biggest and perhaps most relevant difference I see between BSS123 and 2N7002 is Vds, the drain-source breakdown voltage. The BSS123 enjoys a spec of 100V whilst the 2N7002 trails behind at 60V.
?
This could be the source of? your Q508 mortality.
?
JZ

On Fri, Jan 12, 2024, 4:48?PM ok4rm <radio.miskovice@...> wrote:
Hi, John,
thanks for the tip. I think you mean L503 (the one between Q508 source and ground). I checked two ways: L503 end-to-end, and Q508 source vs ground (at the power supply connector, there it is easy to reach). Both show 1.8 ohm at 200 ohm range of my multimeter.?
So it looks like replacing Q508 - the diode is OK but the channel seems to be interrupted. It's not even that the drain pin were cold-soldered, the MOSFET is broken...
Another question is how this could happen. An internal "spark" between the gate and the channel that would vaporize part of the channel??
Anyway, everything was fine when the battery voltage was 12 V or below. When I connected almost fully charged battery, the MOSFET failed (and I hope it's only the MOSFET and not something else down the path).
BTW, is there any significant difference between 2N7002 and BSS123?
Jindra

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