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


jeffbathwater1
 


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.

Dave


--- 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.


 

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.
 
 
Tom
 

----- 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.

Dave


--- 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.


jeffbathwater1
 

Hi Tom,
I checked the transistor with a DMM on diode (which I think I read in the tek manual you shouldn't do... oops) and an analogue meter and I cannot get any sign of life at all. I think all the connections are open.

I see that Sphere has a few matched pairs of these. It looks like an equivalent transistor for the SPS246 is a 2N4258.

I really appreciate all your help!

Dave

--- In TekScopes@yahoogroups.com, "Tom Miller" <tmiller11147@...> wrote:

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.


 

I looked the transistors up earlier. Q1746 and Q1954 have an Ft of
900 MHz, Ccb of 3 pf, and are similar to a 2N4258 or PN4258.

Any PNP RF transistor like an MPSH81 should work but you can use
2N3906 transistors temporarily. Matching is not needed because
adjusting R1835 will trim out any offset.

On Sat, 18 May 2013 03:34:55 -0000, "jeffbathwater1"
<jeffbathwater@gmail.com> wrote:


I checked those transistors as you requested... here is a link to the
schematic with my voltages:
465 schematic 2 <http://www.horusdevelopment.com/tmp/465bSchematic2.jpg>

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.

Dave


jeffbathwater1
 

You are a genius!
I just happen to have a 2N3906 in my junk box... I now have two rock steady traces! Thank You!
I will track down the proper replacement but for now it looks like this will get me going.

What a neat group!

Dave

--- In TekScopes@yahoogroups.com, David <davidwhess@...> wrote:

I looked the transistors up earlier. Q1746 and Q1954 have an Ft of
900 MHz, Ccb of 3 pf, and are similar to a 2N4258 or PN4258.

Any PNP RF transistor like an MPSH81 should work but you can use
2N3906 transistors temporarily. Matching is not needed because
adjusting R1835 will trim out any offset.


 

The 2N3906 transistors will be too slow and have too much input
capacitance which may affect the bandwidth and transient response but
you may not even notice without a good test signal.

On Sat, 18 May 2013 04:51:19 -0000, "jeffbathwater1"
<jeffbathwater@gmail.com> wrote:

You are a genius!
I just happen to have a 2N3906 in my junk box... I now have two rock steady traces! Thank You!
I will track down the proper replacement but for now it looks like this will get me going.

What a neat group!

Dave

--- In TekScopes@yahoogroups.com, David <davidwhess@...> wrote:

I looked the transistors up earlier. Q1746 and Q1954 have an Ft of
900 MHz, Ccb of 3 pf, and are similar to a 2N4258 or PN4258.

Any PNP RF transistor like an MPSH81 should work but you can use
2N3906 transistors temporarily. Matching is not needed because
adjusting R1835 will trim out any offset.


Robert Simpson
 

Congratulations on sticking with it. Yes, this is a great group who have helped me many times.
Bob

--- In TekScopes@yahoogroups.com, "jeffbathwater1" <jeffbathwater@...> wrote:

You are a genius!
I just happen to have a 2N3906 in my junk box... I now have two rock steady traces! Thank You!
I will track down the proper replacement but for now it looks like this will get me going.

What a neat group!

Dave

--- In TekScopes@yahoogroups.com, David <davidwhess@> wrote:

I looked the transistors up earlier. Q1746 and Q1954 have an Ft of
900 MHz, Ccb of 3 pf, and are similar to a 2N4258 or PN4258.

Any PNP RF transistor like an MPSH81 should work but you can use
2N3906 transistors temporarily. Matching is not needed because
adjusting R1835 will trim out any offset.


jeffbathwater1
 

I tracked down the correct transistor for Q1746 and replaced the
temporary 2N3906 and I still get my stable CH1 and CH 2 when the trigger
source is set to CH1 or CH2 (with the calibration signal going into both
channels).
I do have one more problem though and I thought it was the 2N3906 that
was causing it so I didn't pursue it. When the trigger source is set to
Normal, The trigger light flashes and I don't have a stable trigger. If
I set the trigger mode coupling to LF REJ, I get 2 stable traces again.
(AC, HF REJ or DC do not get a stable trace).
If I set the trigger mode coupling to AC and the Vert Mode to Chop I
also get stable traces.....
Do you think I have more fault finding to do on the vertical Preamp
board?

Thanks...
Dave

--- In TekScopes@yahoogroups.com, "jeffbathwater1" wrote:

You are a genius!
I just happen to have a 2N3906 in my junk box... I now have two rock
steady traces! Thank You!
I will track down the proper replacement but for now it looks like
this will get me going.

What a neat group!


jeffbathwater1
 

I did a bit of digging around the normal trigger pickoff. I noticed that (what the block diagram calls the delay line driver next to the pickoff) transistor Q1426 is a NE472 not 151-0472-00 or NE41632 equiv. Q1428 is the correct Tek one, 151-0472-00.
Is this causing the bad trigger problem?
The voltage on the collector of Q1426 is -1.5v not -1.2 as on the schematic but I guess that is down to the wrong transistor.

Thanks,
Dave

--- In TekScopes@yahoogroups.com, "jeffbathwater1" <jeffbathwater@...> wrote:

I tracked down the correct transistor for Q1746 and replaced the
temporary 2N3906 and I still get my stable CH1 and CH 2 when the trigger
source is set to CH1 or CH2 (with the calibration signal going into both
channels).
I do have one more problem though and I thought it was the 2N3906 that
was causing it so I didn't pursue it. When the trigger source is set to
Normal, The trigger light flashes and I don't have a stable trigger. If
I set the trigger mode coupling to LF REJ, I get 2 stable traces again.
(AC, HF REJ or DC do not get a stable trace).
If I set the trigger mode coupling to AC and the Vert Mode to Chop I
also get stable traces.....
Do you think I have more fault finding to do on the vertical Preamp
board?

Thanks...
Dave


 

Dave,

The LF trigger mode is working correctly.
When you try to trigger on 2 alternating traces, the signal going to the trigger circuit includes a lower freq square wave, which is a composite of the trace position of the 2 signals.
This lower freq square wave gets differentiated by the AC coupling to the trigger circuit.
So, the trigger sees a differentiated spike with the vertical signals riding on the differentiated slopes.
Note that the trigger instability improves when you position the 2 traces closer together.
This is because the "square wave" component of the trigger signal is being reduced in amplitude.
Trigger off either of the signals & the problem will go away.
You could also try DC coupling but you may still have the problem.

"Chop" mode usually does not display this problem because the chop rate is much faster than the Alt rate.
So, it doesn't get differentiated by the AC coupling to the trigger circuit.

The purpose of LF Reject triggering is to address this specific problem.
You will not find LF trigger mode on a single-trace scope.

 
HankC, Boston
WA1HOS


jeffbathwater1
 

Thank you Hank for your excellent explanation. The traces do indeed
stabilize if I bring them together. I was just expecting a stable
display with the same signal going into both channels. Looks like I can
close it up and call her done!

Appreciated,
Dave

--- In TekScopes@yahoogroups.com, HankC wrote:

Dave,

The LF trigger mode is working correctly.
When you try to trigger on 2 alternating traces, the signal going to
the trigger circuit includes a lower freq square wave, which is a
composite of the trace position of the 2 signals.
This lower freq square wave gets differentiated by the AC coupling to
the trigger circuit.
So, the trigger sees a differentiated spike with the vertical signals
riding on the differentiated slopes.
Note that the trigger instability improves when you position the 2
traces closer together.
This is because the "square wave" component of the trigger signal is
being reduced in amplitude.
Trigger off either of the signals & the problem will go away.
You could also try DC coupling but you may still have the problem.

"Chop" mode usually does not display this problem because the chop
rate is much faster than the Alt rate.
So, it doesn't get differentiated by the AC coupling to the trigger
circuit.

The purpose of LF Reject triggering is to address this specific
problem.
You will not findLF trigger mode on a single-trace scope.



HankC, Boston
WA1HOS