Oops, shorted out my HP 1220A Oscilloscope

[adrianarcher]

I was trying to check voltages in my oscilloscope because the voltage rails were reading 10%ish off, and I shorted the 95V rail transistor base (I think) to ground (I think it has a grounded heatsink).

It still is powering up, but only shows a dot on the trace now, and all the voltage test points are really low.

Do you think I should try replacing the transistor? I’m a little reluctant to start taking boards out.

I am obviously by no means an expert and I can’t decide if I should just sell it as not working. I only paid $50 for it.

[lordvader88]

Man you're lucky, wish I could find a scope to work on for $50

Hell I wish I could find ANYTHING to work on for free, that's not a blackbox like a hardrive or motherboard, or impossible to source chips, etc.

I live in Newfoundland, no one fixes anything. Soon I want to try some CRT TV repairs before they are all gone too. But even the free ones people give away usually work, and I'd have to through a glass of salty water on it to do mystery damage.


In the meantime I do have scopes to repair. I needs a new transformer or PSU I'd like to make. Another might have problems, very very soon I'll be trying that 1 again. My 2 working scopes could use a tune up. 1 is a black box of chips tho.

[coromonadalix]

@adrianarcher    you'll have to do something the transistor may have gone kaput

Some links
http://www.hpl.hp.com/hpjournal/pdfs/IssuePDFs/1974-02.pdf     shows some diagrams and working priciples

the service manual from Hp is complete rubbish
http://bee.mif.pg.gda.pl/ciasteczkowypotwor/HP/1220A.pdf

and a thread here
https://www.eevblog.com/forum/repair/hp-1220a-help-with-repair/

[adrianarcher]

Thanks for those links!

I actually have the original manual that came with it (I think I bought it from the original owner) so if someone wants a scan just ask.

I was bummed when I broke it, but I am getting more excited to try to fix it. If not I’ll donate to my local tool library and let them fix it.

[james_s]

Before you blindly start replacing stuff, test the transistor, you can use the diode check function on your multimeter. You've got one or more shorted semiconductors somewhere in there almost certainly.

[adrianarcher]

I'm cleaning my basement and decided this is not a project I want to tackle.

Does anyone in Canada want this at shipping cost only? You could have it in the USA too, but the shipping will be much more expensive.

Thanks,
Adrian

[Stray Electron]

  Smart move IMO.  The 1220s are very old and they were cheap (for HP) scopes to begin with. If I were you I'd look for something like an HP 54501 or an HP 54510 on E-bay. They're both reasonably cheap and they're pretty light weight so they shouldn't cost much to ship.  They also have a built in self test and if they pass the self-test then they should be good.  So there's no excuse for a seller to sell them as "Untested".

[Dolphin34]

Hi saw your post about your scope. Sorry to hear about the damage. (Sort of sorry, as you will see). I have the same scope. Did you manage to repair it, or is it toast? The reason I ask, is that I need a replacement volts/division knob for mine. Thanks Doug