Re: Tek 2215

 

On Tue, 26 Jan 2016 00:42:40 -0600 (CST), you wrote:

On Mon, 25 Jan 2016, David @DWH [TekScopes] wrote:

The replacements were physically smaller than the original parts. I
thought Tektronix might have selected the originals for good RF
performance but other than size, they are the same series of film
resistors used throughout the rest of the oscilloscope and a transient
response test showed that the new physically smaller replacements
worked fine.

I did not check it before since I did not have more than one resistor
to select from but the Dale MFF series datasheet says "very good high
frequency characteristics" for whatever that is worth:

http://www.33audio.com/enter/data/DaleFilmpart2.pdf
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.

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?

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