Tektronix 475A repair

[iainwhite]

I picked up a Tek 475A to learn about analog Oscilloscopes as numerous posters recommend doing so (e.g. W2AEW,  Dave's EEVblog #86 video)
I am creating this thread to document my experiences and repairs for myself, and maybe to help other members.

It came with an Instruction manual, an undamaged front plastic cover and two old-ish probemaster 10x probes and a temp probe for the DM-44.
I noticed a couple of issues fairly soon - the ext trigger BNC shroud is bent and the channel B trigger +/- switch is not "clicking" into the detents - just a vague quarter-turn in each direction.
Initially, the scope seemed to work fine and I got a cal signal up on the screen (picture 1).

However, after a day or so (and maybe an hour total time powered-up) I switched it on and there was no display on the screen.

I took the case cover off and proceeded to check the test-points on the power supply board as recommended in the service manual. 
The +5V, +15V, +50V, +110V were all fine and within spec.
The -8V and -15V were both wrong (-0.02V and -22.8V respectively)

I checked the Bridge rectifier CR1462 and found that there was a 120Hz 0-14.5V signal rather than a smoothed half-wave you would expect.
This pointed to a failed filter Cap C1462  (picture 2,3).

[iainwhite]

I desoldered and removed the Cap.  Needed a big D32 chisel point tip and good solder braid (I used Chemtronics SodorWick SW-80 series; MG SuperWick 400 series is also good)   (picture 1)

I purchased a Nichicon LGY-series 5600UF 20% 50V Cap (DigiKey 493-8658-ND) to replace the failed Sprague cap.
I also purchased a 3300UF cap from the same series (DigiKey 493-8651-ND) in case I had trouble with the equivalent cap on the power supply.

I also purchased a selection of Tantalum caps, as I expect I will need to replace some of them.

From DigiKey:
478-4171-ND  CAP TANT 10UF  35V 10% RADIAL
478-4179-ND  CAP TANT 4.7UF 35V 10% RADIAL
478-1877-ND  CAP TANT 22UF  25V 10% RADIAL

From Tayda Electronics:
A-265    1uF 35V Radial Tantalum Capacitor    
A-1609 4.7uF 25V Radial Tantalum Capacitor    
A-422   33uF 16V Radial Tantalum Capacitor    

I ordered an adapter pcb from OshPark to adapt the 2-pin Nichicon Cap to the Tektronix 4 pin layout and some Harwin H2101-01 PC-pins (DigiKey 952-1433-ND) to mount it (picture 2).

[iainwhite]

I made up the mounting board for the new 5600 uF cap and soldered it in place.   
I needed to trim the pins on the top side (under the cap) to provide clearance.   (picture 1)

Tested it OK and ready to solder in to the scope.
 

[PaulAm]

Nice scope.  Generally, if I have to replace one of the filter caps, I replace all of them.  It's not that much more work and it will save you having to do it again in a couple months.  Those caps are a well known failure mode.

The PCB adapter makes for a very nice repair.