DIGITECH QC-1922 Oscilloscope Schematic

[MrRadiotron]

Hi all,

I bought a DIGITECH QC-1922 Oscilloscope a few years ago from JayCar that developed a fault about a year ago.

of course it's out of warranty and JayCar don't sell it anymore.

I was hoping some one here had the schematic for it and that they would be kind enough to share it with me.

One guy a few years ago got the schematic from JayCar here: http://www.electronicspoint.com/digitech-qc-1922-dual-trace-oscilloscope-t185758.html

He emailed JayCar and had the schematic within an hour.

JayCar won't even answer my emails and the people I get on the phone keep fobbing me off.

Th QC-1922 is not a good oscilloscope, Dave Jones even commented on it's terrible build quality once: http://www.electronicspoint.com/jaycar-qc1922-cro-pics-t134520.html

but it's what I got, and I would like to fix it if I can.

any help is appreciated!

Thanks.

[vk6zgo]

What kind of fault is it?

[MrRadiotron]

the beam sits in the middle of the screen not moving.

twiddling the horizontal offset knob does nothing, but the vertical offset knob deflects the beam up and down like it should.

I opened it up a while ago and couldn't see any thing wrong, no shorts or bulged caps.

got deeper last night and found a fuse hidden under the transformer!!!!

I replaced the fuse and powered it and the fuse blew straight away.

the fuse seems to be across a 18V rail, the fuse is a 250V ~1A one. so something is funny there.

The high voltage rails seem fine, and of course the beam is visible so that's ok.

[c4757p]

Well, not to sound insulting here, but... have you checked that it's not in X-Y mode?

[duskglow]

Check the caps anyway.  I had a lecroy that exhibited the same problem - turned out some of the caps had exploded.

[MrRadiotron]

Well, not to sound insulting here, but... have you checked that it's not in X-Y mode?

That was a very mean thing to say, it made me question my sanity all day until I got home and double checked.....

Check the caps anyway.  I had a lecroy that exhibited the same problem - turned out some of the caps had exploded.

Good tip, I'll start checking them when I find some time.

[harveyradar]

I think this might be the schematic you require?

regards
Harvey

[c4757p]

Well, not to sound insulting here, but... have you checked that it's not in X-Y mode?

That was a very mean thing to say, it made me question my sanity all day until I got home and double checked.....

Sorry!

[MrRadiotron]

I think this might be the schematic you require?

regards
Harvey

Amazing! thanks so much harveyradar!

Out of interest, where did you come by this schematic? If I'm not being rude that is.

[harveyradar]

RE: Out of interest, where did you come by this schematic?


Checking deep into my trashed emails - AMAZINGLY IT WAS FROM JAYCAR!

I assume I was repairing one at the time - it's my job

Harvey

[MrRadiotron]

well harveyradar you're a life saver.

I've already repaired the fault. An internal cable was put on backwards! (wouldn't have seen it without the schematic)

so either I did that a some point, like a wally.

or my suspicions are confirmed that this was a demonstrator or a returned unit. the striped screw heads and threads I saw gave me clues,
but I remember when I got it there was grease on one of the front switches as well as a few odd scratches.

Thanks again harveyradar!

[Jbliss]

Hey i have this scope and i am having the same problem i found the hidden fuse and i replace it but the fuse keeps blowing and i getting no trace

[Jbliss]

please HELP 

[stoochie]

FIX (maybe)

I hope joining the discussion this late isn't too frowned upon, I've been lurking here for a little since I got my own QC1922 a few months ago and had the exact same problem with it where the dot would only move vertically and wouldn't move horizontally to display time. I just got my CRO working today and made an account to comment my news here.

The rail hidden under the transformer (F701 on the diagram) kept blowing the fuse, so I got frustrated and packed my CRO away. Opened it again today after a few months and measured the rail with a multi-meter; initially I got 16.1V dc and 2.23A (this is why the fuse blew).

Measuring it a second, third and fourth time right after the first showed 0.67A ("odd", I thought), so I inserted a new 1A fuse and hey presto, it worked!

My noob theory is that if it's left off for too long it'll draw a bunch more current than it should when it's switched on. It might just need a "soft start" if you haven't used your QC1922 for a while. You could maybe do a soft start with something like a variable resistor in series with the fuse.

Hope this helps!