DSO138 troubleshooting - No AV- (no trace)

[loudspeaker]

I'm sorry, I still ask

in scheme is 1mH/1A, but 1A are too big (1mH / 1A Rdc (max) = 0,426 Ohm)


actually used 1mH / 0,28A? according to size..
is an important internal resistance or no?

[ramunas]

I found another cause for No AV-. My AV- was reading about +3.5v. But all the test voltages were coming out OK all the way up to the collector of Q1. The anomalous readings didn't start until you went to the other side of D1, with a V- reading of only -1.7v. Not enough voltage for the 79L05 to work, so an AV- reading of +3.5V. It turns out that I had the TL084C reversed. With TL084C in proper orientation V- is now -6.5v, and AV- is -4.5V. Still a little bit off, but much closer. I have a new TL084c on order and will continue testing after I know I have a good one in place. The reverse orientation troubleshooting may have damaged it. Perhaps this info will help someone.

[loudspeaker]

fixed! goodbye...

[angel67]

no trace in my screen.

V+=+9.01
+5v=+5.03
+3.3v=+3.2

AV+=+5.03
AV-=-5.03

step1 ok VGEN +0.8
step2 ok Q2b=-1.4
step3 ok Vb1=+8.7 Vc2=+6.6v
step4 ok D1+=-9v D1-=-0.14v
step5 ok V-=-9v

but i have NO TRACE!

any help me?
thanks

[jye1]

How about V1 and V2? Please check them with CPL switch set to GND. They should read 0V and 1.6V respectively.

[angel67]

How about V1 and V2? Please check them with CPL switch set to GND. They should read 0V and 1.6V respectively.

v1=0
v2=-0.62
v3=-2.98
TLVL=+1.34

CPL switch seems to work well (change the mode GND/AC/DC)

¿where I have to check now?
thanks

[miketheb]

Hi All

I also had a problem with no -V

Here are my comments........

The -V supply was 0.7V so I changed Q1 & Q2 and it was OK at -5V.

It then worked OK for 3 or 4 minutes. I am using a 12V supply.

Questions.......

1. Is the circuit oscillating too much? i.e. "taken off" as an uncontrolled oscillation (not the signal fed from the processor which looks OK) So does it need some capacitors to stop unwanted oscillation?

2. Is the problem with Q1 too higher current or too higher voltage? The transistor I replaced with is 2N2907 which is 600mA. So does it need a higher current or voltage transistor?

I emailed JYE and have had no reply

Anyone found a cure?

Regards

Mike

[GM3RVL]

My DSO138 arrived yesterday and I assembled it in a few hours - very good kit with adequate instructions.

The board has an AV- of about +0.5V with a power supply of 9V. All other supply voltages seem correct.

The problem appears to be that there is no VGEN signal from pin 46 of the microcontroller. The voltage on pin 46 is about 3mV and putting a 10k pullup resistor to 3.3V only raises this to 9mV so it is not disconnected from the chip. With power off, the measured resistance from pin 46 to ground is >3Mohm.

The scope powers up with the LCD and seems to operate correctly except that the trace is at the top of the screen. If I attach a -8V supply to TP25, then AV- is -5V and the scope seems to work correctly.

It appears that VGEN is configured as an output and is set to zero.

Any suggestions would be appreciated.

Thanks

Harry

[GM3RVL]

Re my last post earlier today, I have found the problem.

I hadn't realised that VGEN pulse is only present when the LCD is there. After that the problem was found as a faulty Q2 S9014 transistor. Replaced with a BC184 and now have AV- of 5.0V.

It's a useful device and good fun!

I have one remaining problem. The display is quite noisy, particularly with timebase values from 1ms to 0.1ms. The noise pulses (mostly upwards) are very short but can be up to 1 grid line spacing high (1/8 screen height). The noise is not coming from the analogue circuits as I've checked the amplifier output on another scope and it is very clean. In any case the pulses don't expand as you increase timebase speed so they can't be to do with the analogue input. It looks like individual samples are wrong, mostly in a positive direction.

Any suggestions?

Thanks

Harry

[angel67]

How about V1 and V2? Please check them with CPL switch set to GND. They should read 0V and 1.6V respectively.

no trace in my screen.

V+=+9.01
+5v=+5.03
+3.3v=+3.2

AV+=+5.03
AV-=-5.03

step1 ok VGEN +0.8
step2 ok Q2b=-1.4
step3 ok Vb1=+8.7 Vc2=+6.6v
step4 ok D1+=-9v D1-=-0.14v
step5 ok V-=-9v

v1=0
v2=-0.62
v3=-2.98
TLVL=+1.34

CPL switch seems to work well (change the mode GND/AC/DC)
when change switch2 sensibility, the system reboot.

the voltage in v2 is wrong, then where Where do I have to check in order to fix?
thanks in advance