Re: Tek 2215

Tothwolf
 

On Tue, 26 Jan 2016, David @DWH [TekScopes] wrote:
On Tue, 26 Jan 2016 00:42:40 -0600 (CST), you wrote:

I believe I found them. They are actually the same Vishay/Dale CCF series I was going to use for R398 and R399, although it appears I inadvertently ordered 15 ohm instead of 51 ohm for those two (good thing I only purchased 10), so I guess those are going on the next parts order.

R378,R379,R388,R389 340 ohm 1% 1/2W metal film Dale MFF1226G340R0F

Replace with Vishay/Dale CCF60340RFKE36

R398,R399 51 ohm 5% 1/2W carbon comp Allen-Bradley EB5105

Replace with Vishay/Dale CCF6051R1FKE36 (51.1ohm, but close enough ;)
There is probably no reason to preemptively replace them if close inspection shows no damage. On the one I repaired, the resistors were visibly discolored but the cracks could not be seen with the CRT in place until they were removed.
I was mainly thinking if I space them away from the boards, there will be less heat to discolor the boards in B0228xx and B0229xx, and will also help prevent any further heat damage to B0233xx.

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.
I could certainly measure them with a thermocouple, however since the board in B0233xx is showing signs discoloration/heat damage just around R378,R379,R388,R389, that alone probably means those resistors are running hot enough where they shouldn't be in direct contact with the board.

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?
Given that one of the scopes had one carbon film and one carbon comp fitted for R398 and R399 from the factory, it may be that the carbon comp resistors were just readily available 1/2W resistors. I came across this with other 1/4W and 1/2W resistors in these scopes where carbon film and carbon comp seemed to be used almost interchangeably. The downside to the carbon comp parts of course, is that many of them have drifted out of spec, which is why I've replaced quite a few with metal oxide or carbon film parts.

Join TekScopes@groups.io to automatically receive all group messages.