Re: New member introduction for Joe Butter

Fabio Trevisan
 

Hello Joe,
I have a 464 which had problems with the HV circuitry when it got to me.
The H.V inverter of the 464 is VERY, VERY similar to yours (it's basically
the same circuit).
In the process of solving my primary problem (which was in the H.V.
transformer... not necessarily your case) I got my original tek part
151-0140-00, a "selected" 2N3055 burned.

What brought me to writing to you is only to emphasize FURTHER (others
already pointed it out) that the 2n3055 (Q1418) in this circuit is REALLY
critical.
When my original got burnt, from about 5 or 6 different 2n3055 I tried, the
circuit only worked with one of them, and still, I needed to change the
value of a resistor, to lower a little bit its base drive current.
The one that worked on my case was the MJ15015 (Which David Hess mentioned
earlier and which he added to his list because of my case).
I tried the 2n3772 and it oscillated at an harmonic of the transformer
characteristic frequency (I think 3rd or 5th harmonic, don't remember now).
Curiousby itslef because exactly the 2n3772 has a low Ft of 200KHz, the
lower of all.
I even thought it was a fake, but I opened it up later on and it was
clearly an original part (mine was from ST).
I tried to buy from a local auction site a vintage RCA 2n3055, dated
something around 1969 (markings where too faded away). I`m not sure it's a
Hometaxial part as the original tek part is, but this one didn't oscillate
whatsoever.
I tried modern 2N3055s from different brands and some wouldn't oscillate at
all, and others would oscillate so wildly that the regulator couldn't hold
it and the H.V. went all the way to the roof.

Add to this the uncertainty around any old or used 2N3055s because of fakes
flooding the market and you have a difficult part to source or even to
assure the one you got inside the scope is up to the task.

If your Q1418 is not an original Tek part, or if it tests bad, avoid
yourself the trouble and get an original part (Qservice in Greece or Sphere
in Canada are the 2 reputable sources that everybody in this group know
that comes to my mind).
If they don't have one with the Tek part-number in it, it will at least be
from the right "family" (Hometaxial) or yet, at least known to work on your
scope.
I don't bear any relation to either of them sellers, besides their
reputation on this group.

There are already too much variables to the troubleshooting of this
circuit, to be unsure about the transistor, if it would or would not work.

I wish you luck.

Rgrds,

Fabio





2017-11-01 16:02 GMT-02:00 joseph@... [TekScopes] <
TekScopes@...>:



pdxareaid,

Thanks for the offer and information. I think it might be a little much
for me right now but I do have some interest. This scope (Tek 465) requires
a little understanding on the circuitry and quite possibly a little luck to
troubleshoot. My interest in the simulation was to keep myself from
throwing parts at it (ex. capacitors, multiplier, other) and understand the
circuit before messing with it. This scope has no diagnostics, a
rudimentary service manual, and appears to have a condition that may not be
a typical issue for the scope. The regulator for the oscillator appears to
have a very weak drive signal and the frequency is off (25 kHz instead of
50 kHz) and I'm trying to figure out why. Most folks have the opposite
situation that drives the oscillator too hard and it blows the fuse or the
multiplier has failed and it blows the fuse.

When the restorer diodes went bad on your Tek 465B, what was the failure
mode? No display at all/Partial display/unusable display/blowing
fuses/oscillator frequency not correct/other??????

Thanks again for the offer.

Joe


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