ADC getting very hot and ... bye bye ...

[CyberSeb]

Hello everybody!

What a cute unit. I really love the design of it and appreciate its
functions. However, I am experiencing some problems with mine:

Just by randomness (I can't figure out a specific rule) the ADC gets
incredibly hot (within in 1 second or so), the display contents vanish
and the display backlight gets dimmer. This may happen after the start-
screen disappears (about 3 seconds) or after minutes of proper
operation. If the ADC is hot, the device fail just after it is powered
up again.

The ADC seems to be working, though.

I could imagine, that the analog portion of the circuit starts to
oscillate and produces a high load onto the TLC5510. Maybe the TCL5510
oscillates on its own or is defective, I don't know.

While testing and fuzzing around I have managed to corrupt the EEPROM
(I was able to restore it) and to burn (!!!) Q1. It smoked. No idea
what happened. I had replaced it with a generic PNP transistor as
suggested in the troubleshooting guide.

The overheating problem existed from the beginning, however. Now the
device is working again, but the random overheating still remains. In
addition, I can see random spikes, even if the DSO ist set to GND.
Maybe the ADC is faulty and a bit is flipping ... :-(

I already have:

- Resoldered all SMD parts
- Replaced the capacitors around Q1 and Q2
- Checked AV-, AV+, VGEN - they seem to be OK (at least, when the AVR
is running)
- Checked for soldering joints / broken soldering point - seem to be
OK
- Mounted 100pF capacitors on the L8705 to suppress oscillation

What I have noticed: at first I thought that I could trigger the
problem, if I set V.POS to the most upper position of the screen and
input any signal (finger on tip contact). But I am not sure about
this.

Is there anybody has some suggestions? I would highly appreciate it. I
can't find any nice alternative to the JYE DSO, so I would really love
to get it running properly.

Thanks and greetings from Germany
Sebastian

[JYE Tek]

Dear Sebastian, please change the value of C32 to something smaller like 47uF or 22uF and see if the oscillation goes away. We got similar cases reported and were solved this way. Please post your result here. Thank you.

 

 

2010/2/5, CyberSeb <cyber...@googlemail.com>:

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[CyberSeb]

Hi!

Thanks a lot for your quick support!

I have replaced C32 with a 22 uF capacitor as you have suggested. It
really helped - the unit seems to be running just fine now for hours.
The ADC apparently has survived the tortures without damage: the
"random" spikes are gone as well.

Please keep up the good work, I'm looking into your future products.
I'm still amazed by the nice design, I really love the PCB "case",
what a clever and economic idea. I think this can be considered as
art. :-)

Many thanks,
Sebastian