DSO138 troubleshooting - No AV- (no trace)

ovkirillov
Posts: 2
Joined: Sun Nov 15, 2015 5:55 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by ovkirillov »

jye1 wrote:Did you check the voltages according to the first post of this thread?
Apparently, yes. The only two points with substantial difference (more than 1V) are TP25 and TP26. My power supply provides 8.7V.
Xfactor
Posts: 1
Joined: Tue Dec 08, 2015 10:05 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by Xfactor »

So my screen seems to be working, it boot and all and the buttons work but there is no trace. I followed the guide in the OP of this thread, everything is fine up to step four, but the voltage on the diode is wrong - about 8,1V on the negative side and less than a volt on the positive side. I have traced the circuit back, and all the voltages up to the Q1 PNP transistor seem to be fine. The base of Q1 sits at 8,1V the emitter at about 8,3 but the collector is at 8,1V (it's connected to the negative side of the diode). So I'm assuming something is wrong with or around the transistor, anyone experienced anything like this. I double checked the components around the transistor and they all seem to be correct.
badmice
Posts: 2
Joined: Wed Dec 16, 2015 9:10 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by badmice »

So this is what happened:
Perfectly followed all steps, up to point A. Use
1) attach probe clips to J1 (done)
2) Touch the red clip with your finger. Do you see signal from your finger? (nope..)

screen shows some stats and a graph and on the topline there is a yellowline (with a thicker part scrolling from left to right)
onscreen: it shows different numbers if i switch the switches but! Vpp 0.00V no idea what it is, doesn't change.


step 1: 0.8v
step 2: Q2 B = -1.38V
step 3:
Q2 C = 6.01V
Q1 B = 7.86V
step 4:
D1+ = -7.77V
D1- = 0.13V
step 5: V- = -7.73V
step 6: AV- = -4.95

extra infopoints:
D2+ (input?) = 8.91V
V1 = 0V
V2 = 3.31 (manual says 1.66V on gnd, but its not...)
+3.3V = 3.24V
+5V = 4.95V
TP22(3.3V) = 3.24
OSC = 2.06V
R38 (l = 6.01V) (r = 7.86V)
R39 (l = -1.39V)

Any clue what went wrong?

edit1: i read on the board the testloop J2 (wich also doesn't show any line on the lcd) says it should be 3.3V 1Khz however the output there is 1.61V

So ... V2 doesn't give the correct value as by manual and J2 doesn't give the correct value as stated on the board.

i've tried to do the https://www.youtube.com/watch?v=MTQEsSRAWRw config steps, but apearently my line is so far maxed out to the top-edge of the screen i can't get it shown in the center of the screen. it just keeps rolling over and over but no \/\/ at all just ________
jye1
Posts: 1226
Joined: Wed Feb 20, 2013 4:58 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by jye1 »

All the measured voltages look good except V2. Please check values of R9 - R11 and soldering of these resistors and U2C pins.

You saw display scrolling from left to right ? or from right to left? When timebase is set 50ms/div or slower scope uses scrolling mode where trace scrolls from right to left. This is for slow signals. To check the test signal it better to set timebase to 0.2ms or so.
badmice
Posts: 2
Joined: Wed Dec 16, 2015 9:10 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by badmice »

Thanks for your reply,
jye1 wrote:All the measured voltages look good except V2. Please check values of R9 - R11 and soldering of these resistors and U2C pins.
R9 l = 0.21v, r = 3.31V
R10 l = -4.95V, r = 3.31V
R11 l = 0.21V, r = 0.0V

by the looks of it all contacts have a little blob of solder flowed on it ... so it apears u2 TL084 chip is soldered correct, any way to measure/check this?
jye1 wrote:You saw display scrolling from left to right ? or from right to left? When timebase is set 50ms/div or slower scope uses scrolling mode where trace scrolls from right to left. This is for slow signals. To check the test signal it better to set timebase to 0.2ms or so.
switches on gnd / 1v / 1x
probe not connected (or touching J2 testloop = same thing)

screen says
Running [--==--]
freq - - - - ..... vmax 0.44V
cycl - - - - ...... vmin 0.44V
pw - - - - ...... vavr 0.44V
duty - - - - ...... vpp 0.00V
................... vrms 0.44V

1V GND 0.2MS auto Z (arrow down)

it apears to go from left to right in the most topline



edit

to be sure i reflowed R11 and all the u2 IC pads and now it works!!!!! :D

edit 2

with screen connected (not tested without) the V2 now is on the 1.56 range (forgot the exact value but it was more to what was expected)
makaron28
Posts: 1
Joined: Wed Dec 30, 2015 6:16 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by makaron28 »

Hi!

I just want to tell you about the problems I had and the solutions I applied.

At first the scope was showing a trace always on top of the screen, regardless of the coupling (AC/DC). Moreover, the analog negative voltage (A-) was not -5V but rather around positive 0.68V.

I carefully checked all the elements which might have been causing this problem and I found that L2 coil had a very high resistance, so I assumed it was broken as the coils like these have this value very small (usually less than 10Ω).

The only coil I had which could withstand a current up to 500mA had an unknown inductance but at least it had around 2Ω and had a similar appearance ;) . I soldered the new coil and the scope started to work. I checked the analog negative voltage and my voltage meter was reading -5V.

But then I found out that Q1 transistor started to build up heat very rapidly. Luckily I turned everything off before it boiled.

To prevent the Q1 from blowing up I did what user palo0406 suggested - I exchanged two elements:
  • - R40 for a 2.2kΩ resistor,
    - C23 for a 470pF capacitor.
Those values were selected quite randomly but I thing the key factor was to lower the capacitance.

Now the scope works great, Q1 gets warm but not hot what is fine.

I wish a good luck everyone who had the similar problem.
supertallarin
Posts: 1
Joined: Fri Jan 08, 2016 10:50 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by supertallarin »

Hi all, i'm having the ame issue, no trace and wrong AV-, it's about 0.66V. So I followed the steps listed before:
Power = 8.7V
1)0.8V
2)-1.8V
3)Vc2 = 7.16V
Bv1 = 7.13V
which are a bit off of what it expected but may be okay
4)D1(-) = 0.002V
D1(+) = 0.166V
From step 4(included) onwards everything goes wrong, V- = 0.167V, AV- = 0.66V. V1 = 2.773V isntev of 0V and V2 = 3.28V insted of 1.66V
I checked D1, C24, L2 and they looked good to me aswell as Q1, maybe it's damaged on the inside but I dont't really know how to check that, any ideas? All welds appear well but I will double check them, diodes and capacitores polarities too.
How should i test chokes, diodes and transistores (all of them already soldered) to find out if they are damaged and need a replacement?
walter49
Posts: 1
Joined: Mon Jan 11, 2016 2:35 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by walter49 »

hi, my DSO138 all at once stopped running and i see no trace. I mesurad and found these values:
AV = 4.18V
V+ = 9,3V
V on R4 =0
VbQ2 = 0
R39 = 9962 ohm
R40 = 470 ohm
Vcb (and Vbe (Q2) = 0,69 V
Vb1 = 8,52 V
Vc2 = 8,52V
What can you tell me? Some component is broken and i can change it or i must buy another kit and begin it again?
Burney
Posts: 1
Joined: Wed Jan 20, 2016 10:02 pm

Re: DSO138 troubleshooting - No AV- (no trace)

Post by Burney »

Hi there,
my DSo138 is starting up fine, but the trace is missing. I went through the desribed steps and all looked good until I measured D1. The voltage after D1 (before was ok, I guess --> 0.05) was about 3/4 of the expected value. I think, as a result, most voltages in the analogue section are off.

Here are all the measures desribed in the manual (bold/red values are off):

TP22 3.32
V1 3.62
V2 4.01
AV- 0.64
AV+ 6.05
+3.3 3.32
+5 6.13
V+ 8.47
D1 0.05
R38 6.35/8.26
D2 9.29
R39 -1.37
R40 0.83
OSC 2.51
V- -6.12
L1 -6.24

Any idea, how I sould proceed? I would very much appreciate it!

Cheers,
Burney
Edison
Posts: 1
Joined: Mon Feb 15, 2016 9:07 am

Re: DSO138 troubleshooting - No AV- (no trace)

Post by Edison »

Hi, i've this case:
V1=0v V2= 3v AV-=-4 V+=8,45
1) R40 0,81
2) Q2b -1,4
3) Vb1 8,25 Vc2 6,3
4) D1 have right 0,19 left -4,9
5) V- (TP25) -4,9
6) AV- (TP26) -4
You can help me?
L2 (1mH/1A) have up 0 and down 0,15 !!!!
C24- = -4,9
Thank's
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