EEVblog #803 - HP1740A Analog Oscilloscope

[bktemp]

let me guess, bad main switch contact?

It sounds obvious because all rails drop at the same time, but I think it is unlikely: The voltage drop and therefore the power dissipation would be high enough to melt or burn all the plastic around the contacts. Dave would have smelled that.

My guess is something on the 15V rail draws too much current.

[nfmax]

Well the obvious thing that might make the 15V rail go wrong would be A16R26 (and or the other resistors in the chain, A16R25 & A16R27). Or it might be bad contacts on the connections to Q4, which is a connector, not soldered.

[Bickenheuser]

Dave,
Sometime ago I had a HP 3457a that would take off and do very weird stuff, error messages. All of the power supplies dropping about 50%. Sometimes a light smell of electrical burning. Turned out to be the 120/220 AC line switch . The 3457a is in very good nic , must have sat somewhere for most of its life.
 I have never seen contacts so black from corrosion. If You get back to that scope, Give it a shot. Also that scope is known for bad board to board connections. 

[tautech]

Link to the repair:
https://www.eevblog.com/forum/blog/eevblog-804-hp1740a-oscilloscope-repair-part-2/

[drussell]

The voltages on the twitter photo were all down by roughly the same percentage; there wasn't one that was extremely out of line with the rest.  I would start looking for something in common to all the rails, like on the primary side.

Intermittent candidates: 120/240V selector switch, power switch, connector, bad solder joint.

You're forgetting that in these kinds of equipment the regulators are typically all referenced off of one reference supply.  The reason they drop in unison is not because the voltage coming in drops and all of the outputs track it, it is because the voltage they (the regulators) are referenced to drops.  This could be due to a supply side drop but it would have to be affecting the master regulator before any of the others in order fot them all to drop in unison....

I've seen this posted before but I find it very unlikely that all secondary voltages will drop the same percentage if you lower the primary voltage. I fail to see that if 4 different regulators are going out of regulation they will drop the same %.

Yes...

[tautech]

You're forgetting that in these kinds of equipment the regulators are typically all referenced off of one reference supply.  The reason they drop in unison is not because the voltage coming in drops and all of the outputs track it, it is because the voltage they (the regulators) are referenced to drops.  This could be due to a supply side drop but it would have to be affecting the master regulator before any of the others in order fot them all to drop in unison....

That's what many of us suspected, but it wasn't the case. 
Watch Pt 2

[MarkL]

You're forgetting that in these kinds of equipment the regulators are typically all referenced off of one reference supply.  The reason they drop in unison is not because the voltage coming in drops and all of the outputs track it, it is because the voltage they (the regulators) are referenced to drops.  This could be due to a supply side drop but it would have to be affecting the master regulator before any of the others in order fot them all to drop in unison....

Not necessarily.  True that if the master drops they all drop, but another cause could be that they all reached their drop-out voltage at the approximately the same primary voltage.  Or, consider that if the other regulators reached their drop-out before the master, they're going to track the input voltage.  Then it doesn't matter what the master is doing at that point and it could drop out on its own and then it too would also track the input voltage.

And besides, I didn't forget about a reference supply.  I said it here:

https://www.eevblog.com/forum/blog/eevblog-803-hp1740a-analog-oscilloscope/msg770825/#msg770825

[EEVblog]

Someone tracked down the guy who tested and signed off on that scope!

Yes, I was an electronic technician at HP, Colorado Springs, Colorado. I worked on the HP1740A oscilloscope line from 1980-1983. In 1983, I went to HP Germany with an engineer to transfer the new 1980B oscilloscope production line to HP Colorado (production remained at both Germany and USA throughout the 1980's). I was only an electronic technician at HP from 1979~1987 then moved on to Quality Assurance, managing a production line, then moved into buying componants and scheduling production builds. This was all done at HP's Colorado Springs manufacturing and R&D sites. I ended my career at HP/Agilent after 24 years in 2003 when we spent time in Singapore transfering production lines from Colorado Springs to Asia. The video sure did bring back some great memories!
Thanks,
Rob Schillinger

[WN1X]

Very cool! 

[ale500]

I showed my wife the part where Dave shows the manual and repeated Dave's comment on how that was p0rn for electronics people... she looked at me kind of funny 

[AF6LJ]

I showed my wife the part where Dave shows the manual and repeated Dave's comment on how that was p0rn for electronics people... she looked at me kind of funny 

Some women don't get it....
Then....
Some of us Do.