#problem #magicsmoke #smoke #problem #magicsmoke #smoke


alex.savochkin@...
 

Hello!

I need a bit of advice on QCX repair. The initial state of QCX was correctly assembled and working device. After sudden unlucky switching power wires, I've got a single row of white blocks on the screen. I told: "Okay, I know where the troubleshooting manual is", so I figured out that my MC is probably dead. Voltage measurements showed the following:
  • LCD PIN 1-3, 16: as shown in the reference table
  • LCD PIN 4: 0.12 V
  • LCD PIN 5: 0 V
  • LCD PIN 7-14: 5 V
  • LCD PIN 15: 4.79 V
  • CLK PIN 1, 2: 0 V
  • T1 PIN 5, 8: 2.5 V
  • The voltage on the analog receiving path (IC5-9) seems to be correct.
  • No voltage at all on Si5351 since D2 has become an insulator (have no idea why) although D1 is alive.
Current right after the catastrophe was not measured exactly but seems to be very high according to the temperature of IC11 7805 — as far as I can figure out it was at least as hot as 80 C. Now total current is about 180 mA or a bit less. IC4 was quite hot when the entire transceiver consumed abnormal current but now has normal temperature.
The resistance between VCC and GND pins of IC2 (MC) is 98 Ohms in both directions which do not look normal.

So, my questions are:
  1. Is it correct that IC2 is dead and has to be replaced?
  2. What else should I check to figure out the reason for the relatively high current consumption?
  3. Should I replace also IC1 (Si5351) and IC4 (FST3253) together with MC and D2 diode?
  4. What reason can cause breakdown od D2 diode and MC?
Of course, I can simply replace already detected defective parts but I'd like to be sure that new ones will not be smoked right after switching power on. If there are any ideas I'll be grateful for that.

73!
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX


Ronald Taylor
 

  1. Is it correct that IC2 is dead and has to be replaced? Not necessarily. Remove IC2 from the socket and see if the 98 ohms is still measured on the socket at either pin 7 or pin 22. If the 98 ohms is still present, then it’s somewhere else on the 5 volt rail. If the load goes away, then replace the MCU chip.
  2.   3.   Should I replace also IC1 (Si5351) and IC4 (FST3253) together with MC and D2 diode? Since IC1 is disconnected from the 5 volt rail with D2 open, it is no longer contributing anything to the loading either. Don’t replace it yet and since the FST3253 wasn’t hot I would wait to replace that one as well. 
You reported IC3 being very hot. It could indeed be the device that’s causing the 98 ohm load on Vcc. Start there. See if you can isolate it from the 5 volt rail. May have to remove it unless you want to cut a board trace.If that is the source of the loading, then replace it first and go from there. 

I don’t know why D2 would have failed in this scenario, but certainly replace it in order to get voltage (5v - two diode drops) to IC1. 

I’m a little surprised to hear that reversed voltage caused a problem. D3 is supposed to prevent that. 

Good luck … Ron

On Sun, May 17, 2020 at 9:13 AM <alex.savochkin@...> wrote:
Hello!

I need a bit of advice on QCX repair. The initial state of QCX was correctly assembled and working device. After sudden unlucky switching power wires, I've got a single row of white blocks on the screen. I told: "Okay, I know where the troubleshooting manual is", so I figured out that my MC is probably dead. Voltage measurements showed the following:
  • LCD PIN 1-3, 16: as shown in the reference table
  • LCD PIN 4: 0.12 V
  • LCD PIN 5: 0 V
  • LCD PIN 7-14: 5 V
  • LCD PIN 15: 4.79 V
  • CLK PIN 1, 2: 0 V
  • T1 PIN 5, 8: 2.5 V
  • The voltage on the analog receiving path (IC5-9) seems to be correct.
  • No voltage at all on Si5351 since D2 has become an insulator (have no idea why) although D1 is alive.
Current right after the catastrophe was not measured exactly but seems to be very high according to the temperature of IC11 7805 — as far as I can figure out it was at least as hot as 80 C. Now total current is about 180 mA or a bit less. IC4 was quite hot when the entire transceiver consumed abnormal current but now has normal temperature.
The resistance between VCC and GND pins of IC2 (MC) is 98 Ohms in both directions which do not look normal.

So, my questions are:
  1. Is it correct that IC2 is dead and has to be replaced?
  2. What else should I check to figure out the reason for the relatively high current consumption?
  3. Should I replace also IC1 (Si5351) and IC4 (FST3253) together with MC and D2 diode?
  4. What reason can cause breakdown od D2 diode and MC?
Of course, I can simply replace already detected defective parts but I'd like to be sure that new ones will not be smoked right after switching power on. If there are any ideas I'll be grateful for that.

73!
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX


alex.savochkin@...
 

Ron, thank you for advice. :)
You reported IC3 being very hot.
No, the hot one was IC4, which is SOIC-16 FST3253. Not sure if I could disconnect it from 5V in an easy way so I'll think that it works at the moment. If not, I'll find it out using the oscilloscope. And yes, it was the device that caused the 98-ohm load on VCC. But looking at the schematic diagram I don't think that IC4 actually has a 98-ohm resistance between VCC and GND. This resistance is probably living somewhere else. So I'll replace D2 and see what will happen. I don't like the fact that I didn't see clock oscillation on MCU using the oscilloscope but okay, I've already ordered a new MCU.
I’m a little surprised to hear that reversed voltage caused a problem. D3 is supposed to prevent that.
Of course, D3 prevented reversed voltage. The voltage was not reversed, but something wrong happened when I reconnected a "+" wire to a power supply in the correct polarity.
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX


alex.savochkin@...
 

Progress report.

D2 has been replaced, Si5351 now is under voltage but still not oscillating as well as MC. This makes me assume that MC should be replaced too. I will inform this group about any achievements.
--
Alex R2ATO
SKCC: 21618. RCWC: 1275
QTH: KO85OX