Re: New member and owner of a dead 2225


 

On 14 Nov 2017 07:18:45 +0000, you wrote:

I've seen those videos. Dave's tear down is really good though my wife makes me wear headphones for any of his videos - she's a non-fan. I kind of get her point...
Crikey! Dave's Australian speech makes for entertaining listening but
I can understand how it would get old after a while. The same thing
happens to me if I binge watch Doctor Who episodes.

After following the circuits on the schematic, I think the 38V/67.5V feeds are probably OK because there doesn't seem to be anything particularly sensitive there. Two 100n 50V ceramics are worth looking at, one on the X-Axis Amp and one in the vertical output amp. Everything else feeds though resistors. Though the lowest is 330 so bears looking at a bit more.
Damage to the vertical CRT amplifier transistors is possible.
Depending on the bias conditions which are not real clear with the
power supply malfunctioning, the 2N3866s could have a specified
maximum breakdown voltage between Vceo of 30 volts and Vcbo of 55
volts. Luckily they should be easy to replace if damaged.

I am surprised that the switching preregulator did not include an SCR
crowbar circuit to protect against a shorted switching transistor but
maybe Tektronix determined that it was not necessary.

On the caps, I've found what look like decent replacements - reasonably low ESR, same or better tolerance and all are 105C. However, not a single lead pitch matches. I can make them fit by bending the leads in an L to make them fit the original pitch. I assume that there is no issue with that. So, not worried except for C900 - the big 2200 uF 80V on the mains input board. The only matches are snap-in caps with fat, stiff, stubby leads and the wrong terminal pitch. (10mm vs a measured 12mm on the one that's there now). I guess I can make a little adapter but it will be a bit ugly. If I was industrious, I'd 3D print a foot for it to keep it from moving around - though it will probably be hot glued instead.
Usually capacitors with the correct lead pitch can be found. Higher
voltage parts can be selected if necessary and this may even be
desirable for improved reliability and operating life.

The big bulk input capacitor is one of the aluminum electrolytics
which is likely to last the longest so maybe leaving it in place would
be best unless testing shows that it has failed. The reason for this
is that it was originally selected based on capacitance instead of ESR
or ripple current rating which will be more than enough and it is the
last one which impacts operating life.

Another question. There are at least 9 10 uF axial electrolytics in various places. I should replace them as well? I'm not wild about removing the attenuator/timebase board, though it's probably easier than it looks.
They will not have caused the failure but replacing them may lead to
better performance and perhaps less noise in the displayed trace. On
the other hand, replacing parts unnecessarily risks causing damage.

Once I get my ESR meter (due this Friday), I'll hopefully find a smoking gun.
Unfortunately an ESR meter will consider a shorted capacitor good.
They are useful and convenient for in-circuit measurements but not
always easy to interpret.

Soon your 2225 will be operating with peak efficiency and Bob's your
uncle!

You should hear some of the San Dimas slang I picked up when I worked
there which was not featured in the Bill and Ted movies.

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