Hitachi VC-5430 Oscilloscope troubleshooting help (FIXED)

[Kaferlago]

Hello everybody,

I'm revamping my electronic repair hobby, I realized I have quite a lot of old stuff around me that I can fix and save fro the landfill, so i invested in a new soldering station, ESR meter and some other tools.
It then come to my mind that I still had in the office a 20y old 30MHz portable digital oscilloscope I left aside for 10+ Years.
So what is more appropriate to start the landfill saving hobby with a tool I can use for the future projects?

Now comes the product description: the Hitachi VC-5430 is a digital portable 12V or internal battery powered dual traces 30MHz 30MS/s oscilloscope. It does not sound much but 20y ago that thing was really cool.
I mainly used to troubleshoot automotive components, mine is dated 1994 on the mainboard, it is still in pristine condition but developed a strange behaviour during it's inactivity.

Fault description: The waveform is not clean, on both channel (grounded inputs) a dip is present, it's amplitute des not change with V/div and the frequency is not stable. Or maybe the noise comes in after the trigger... mmmm...

However I imemdiately replaced all power supply trough hole electrolitic apacitors, which all turned ot to be ok with the ESR meter. The only one bad was the 1F 5V memory supercap.
Problem is still there, I checked the voltages ot of the DC-DC and those look ok with the DVM, the scope works so no surprise the voltages are within tolerances.
I downloaded a service manual for the VC-5430 but that cover till 1993, my 1994 mainboard PEG-14 is not covered.

I surely need to troubleshoot more, I was wondering since this problem has a visual clue, if any of you may have an idea of what to look for.


Photos of the problem here:
https://www.dropbox.com/s/cr94t0p3wm42wtv/2015-02-17%2022.43.59.jpg?dl=0


My next to do list is:
1. check SMD elcap with ESR meter
2. check power rails with my working oscilloscope at work
3. check A/D Voltage
4. scope the analog input signal path

Any suggestion is welcome!

[tautech]

Welcome to the forum.

Your image shows ~ 1/2 div @ 1V/div noise. Does this apply to all vertical ranges?

Could replacement of the memory supercap have resulted in lost calibration?

Ripple is the biggest worry on the PSU rails, we'll wait for your findings.

[ARF]

The problem could be in a capacitor. I bought used osclilloscope and fixed this bug in it too.
Now I don't remember exact position but I can check it in my oscilloscope if you need.
I can advise you to replace capacitors to tantalum SMD body A,B.

http://infotex58.ru/forum/index.php?topic=921.0 - Russian forum where you can download manual, firmware, resolve problems.

[Kaferlago]

Quick update

@tautech:
- noise is always 1/2div in all vertical ranges.
- the scope has a self calibration feature, it goes through the cycle every time is disconnected from power from a long time. When the memory cap was bad it would recelibrate any power off/on cycle.

@ARF.
- thank you for the tantalum suggestion, I already ordered aluminum electrolytics. Will do next time.

I've scoped the rails and I didn't find anything obviously wrong, just the -8V was a touch higher at -7.6V but no ripple.

Then I picked my new MESR-100 ESR meter and started to measure the little caps in circuit, they were almost all around .8ohm, the new ones .1ohm. So I decided to pull one and found that leaked black stuff at the bottom causing little corrosion at the solder mask level. ineed to check closely the pads and traces before placing thenew components.

 I've pulled all of them now and damaged 3 pads, not a big deal fix is easy. How do you remove the SMD EL caps? I tried the wiggle technique but ended up using two irons. I felt way more confortable with the latter.
I was thinking to buy a SMD hot tweezer iron but I think it may not always have cleareance to be effecctive.
 
I just ordered the new capacitors, next week I should be able to  put them back.

[Kaferlago]

Update.

I've replaced all the electrolitytic caps and turned on, Auto Calibration failed.
Pressed PWR + MODE to skip the Calibration and found CH1 with little DC offset while CH2 had so much DC offset that the trace was out of the screen at any V/div.
I found -V missing on the first CH2 input signal opamp negative voltage pin, fault was caused by one of the new cap shorted, (maybe bad soldering issue).

Replaced the cap and CH2 returned on the screen.

Now both CHs had the same little offset that slightly changed with V/div.

I then performed a Full Calibration, Calibration Passed and as result DC was compensated for all Attenuator (V/div) ranges .
I believe little offset is normal if calibration is cleared and registers set to zero.

After full cal I checked all values in the Service Menu registers and found them all within spec range.

It now works!

Beauty

Below the test signal coming from the cheap ESR meter "MESR-100V2" almost sine wave:



 

[tautech]

 
We always like a happy ending. 
Well done and thanks for sharing, hopefully it will help others.

[ADent]

This was incredibly helpful! Just bought one, and googled to see what it looks like when running, found this thread.
After some fiddling around I got it to start, and it even started to charge the battery, and ran on it a short while when one of my power cables came loose!

And of course mine shows the exact same problem as yours! Which electrolytic capacitors did you change? _everyone_ ? both smd and through hole?

When opening it the "motherboard" is visible and has some caps, how about the (presumably) signal PCB in parallell with the motherboard? The display-PCB?

Best regards
Mikael

[ADent]

Ok, I had to figure it out myself. I opened it up and found my way to the bottom PCB (everything mentioned below applies only to that PCB).
Changed all the 47uF 16V capacitors since some of them had leaked (I almost think one of them was mounted in reverse?)
But this did not help with the problem. I then followed the link to the russian forum a bit up in this thread and it suggested I
change C471 22uF 6V. I changed it and it might have helped a little bit but the problem persisted.

I then found the service manual with all the schematics and I started to measure the voltages with my other oscilloscope
and found that 5V-ref had the same kind of disturbance as the VC-5430 showed. Followed it backwards via the schematics
and came to C612 a 47uF 6V capacitor. I changed that one and all the other 47uF 6V capacitors. Made a recalibration on page 6 in the menu
and now it works fine!

In case anyone else gets this problem.

Best regards
Mikael