Re: Tek 2215


On Tue, 26 Jan 2016, 'Craig Sawyers' c.sawyers@... [TekScopes] wrote:
On Tue, 26 Jan 2016, David @DWH [TekScopes] wrote:

The symptom of attenuated and nonlinear vertical deflection pointed directly at the collector load resistors and ohms measurement from the bottom of the printed circuit board conclusively showed they were bad. They were the first thing I checked because it was easy to do and visual inspection showed that the printed circuit board itself was discolored from high temperature in the area under and around them.

The value for the 51 ohm series resistors is not critical but I wonder how hot they run and about higher parasitic inductance of spiral cut film resistors. Why did Tektronix use carbon composition resistors there?
Tek had a bit of a blind spot when it came to power dissipation in resistors on circuit boards. They invariably mounted them tight to the board. So even if the part was operating inside its dissipation limit, that power specification is always quoted by the resistor manufacturer "in free air". Resistors which are dissipating a decent percentage of their power limit should be mounted on spacers (which can be just a few mm) to ensure good air flow. If you put them tight to the board, the contact is a hot spot which (a) discolours, or in some instances burns the board and (b) compromises the lifetime of the resistor. Seen the same thing is some classic era Fluke gear too.
Maybe mounting such resistors directly to the board was part of the new low-cost process Tektronix was using for the 2213/2215 scopes? All of the boards apart from the preregulator add-on board were made from a single panel which was snapped apart after soldering and all of the components had their leads clenched to hold them into the boards during the wave soldering process.

The only real exception I noted was R911 near the center of the preregulator boards. It is hard to see in the photos, but the original R911 had formed leads to keep it spaced away from the board.

Do you think the Richco CER-6 spacers will be sufficient for R378,R379,R388,R389? With a single spacer under each lead, a Vishay/Dale CCF60340RFKE36 resistor would probably only be 2-3mm away from the board, although the spacers are designed so they can be stacked.

One other issue I noticed which I forgot to mention in my posts to the list last year was the metal bracket that the focus control is mounted to was showing signs of tin whisker growth in all 3 of my 2213 scopes. While I had it and the potentiometer removed so I could replace the 6 1M ohm resistors, I lightly abraded the surface of the bracket to knock the tin whiskers down, but I know if I leave the plating as-is, they will without a doubt eventually return. I am considering using electrolysis to strip the tin plating and then replate the steel bracket with nickel. You can just barely see eruptions in the tin plating on the right side of the bracket in this photo, but up close, 3-5mm long tin whiskers were pretty easy to see.

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