Repairing a Tek2213a, things that make you go ARGHHH and Ockham's Razor..

[wkb]

I thought I might share this with you, as I found it both instructive and very, very frustrating  .  It also reminded me of Ockham's Razor 

Remember I had this Tek 2213a which only had half a horizontal timebase, so only the right hand side of the timebase line was visible?  The problem was intermittent at first, after some tapping around it turned into a solid fault.  Hmm..  So I spent a couple of $$ to get the service docs of this particular model. 

Started measuring voltages as well as waveforms (I have 2 other scopes, one Rigol DS1052E and a Tek2465CTS).  Hmmm...  Lets see what horizontal deflection voltages exist at the tube, on MP22 and MP23, seem to a bit out of the range the service docs describe.  Would that be the problem? 

Hmmm... lets measure somewhat further upstream, MP24 and MP25.  Bit of a yucky signal, but can this be the problem??  Some poking later I found the timebase' Miller circuit produced really nice clean sawtooths, as described in the service docs.  So that part is A-OK.

Endless poking and measuring later I was none the wiser.  As a sort of last resort I unsoldered one-at-a-time the 100ohm series resistors that connect the H amplifier to the tube.  Huh?  Unsoldering one left me with a single dot on the screen, unsoldering the other gave me the half-timebase line.  Obviously one of the deflection plates did not play.  Don't have a broken tube, no? 
 
Well, no.

So what WAS the problem?  Well, the tube's H plates are connected via rather stiff wires to the PCB.  The tube's connections are hiding on the underside of the tube, a couple of cm above the main PCB.  Nothing you can observe visually.  I finally discovered these wires have little plugs on them that slot over the electrode's pins. I had assumed they were soldered onto the tube, assumption being the mother of all f* of course.  So one of the plugs became unseated, that was all.  The stiff wire used kept it nicely close to the tube's pin, hiding inside the tube's shielding cover.

Plugged it in and things worked immediately..

Guess Tekfan will be ROFL by now 

Anyway, I had given this scope to a friend of mine, at least it works now!

What does this teach us/me?  Just don't assume anything, test it.  In retrospect, with 20/20 hindsight always easy to say that of course.

[tekfan]

I know what you mean. Nothing more frustrating than trying to find a connector that isn't properly seated.

I usually completely disassemble oscilloscopes for cleaning and when I put them back together it usually doesn't work (it worked before disassembly). After checking all the power supplies are present (including the HV) then the real head scratching begins. After about an hour of troubleshooting I realise that I forgot to plug in the CRT socket.

[wkb]

I know what you mean. Nothing more frustrating than trying to find a connector that isn't properly seated.

I usually completely disassemble oscilloscopes for cleaning and when I put them back together it usually doesn't work (it worked before disassembly). After checking all the power supplies are present (including the HV) then the real head scratching begins. After about an hour of troubleshooting I realise that I forgot to plug in the CRT socket.

   Do you also always have 1 spare screw?

My Tek was one short when I disassembled it (it has seen quite some repair in its days from the looks of it, it came from a major service site of Digital Equipment Corporation, RIP).  So I put in a new one in the course of me fixing it.  And, surprise surprise, I have an extra screw left.  Different size though..

[tekfan]

Inevitably yes. I always end up with one or more too many screws. In many cases there is just a screw rattling around inside the scope and I don't know where it came from. In the smaller Tek 7000 series I actually deliberately leave out one screw which causes stress on a transistor and chassis.

I've got a small bag of imperial screws (leftovers from other test equipment) so I don't really worry about missing screws.