Re: Puzzle of the day: 465b will not trigger on channel 2 SUCCESS!


Recheck the voltages on Q1746.  E-B-C . If you really have +15.02 on the emitter and +8.92 on the base, that transistor has an open B-E junction.

----- Original Message -----
Sent: Friday, May 17, 2013 11:34 PM
Subject: [TekScopes] Re: Puzzle of the day: 465b will not trigger on channel 2 SUCCESS!


I checked those transistors as you requested... here is a link to the schematic with my voltages:
465 schematic 2

Q1845 is showing +15.02 at the base not +9.5.   I tried swapping Q1845 with Q1045 from CH1 but still no trigger on CH2!

Hold on!
I was just about to post this when I thought I would try swapping Q1746 with Q1150 from CH1... and yes I get a steady trace on CH2!! You were spot on!
The transistor is a SPS246. Do you think it will be part of a matched pair or can I just use any equivalent device?

I appreciate all the time and effort you and Tom put into help me solve this. I have learnt a lot and it has fueled me to go and read up more to understand more about the scope.  Do you know of any good books that would help?

I'm sure you need a rest from all my questions.. thank you again David.


--- In TekScopes@..., David wrote:
> The 465B schematics have been bothering me for a while because the
> block diagram I have shows the vertical output amplifier on schematic
> 4 while the schematics I have include it on schematic 5 and have no
> schematic 4. I finally just compiled them and printed out a set.
> Going by the voltages Tektronix gives, the collector current of Q1938
> should be 4.24 milliamps. Your values show 6.45 milliamps which is
> consistent with the problem of the voltage at point 16 being too high
> and causing Q1833 to go into cutoff.
> The voltages at Q1938 look good so the problem is north of there
> around differential buffer Q1746 and Q1954 or differential
> transconductance amplifier Q1845 and Q1948.
> I would measure the voltages marked on the schematic around those four
> transistors.

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