Would appreciate suggestions and comments



http://www.youtube.com/watch?v=vyZJxkvomOw



Keith, Wayzata, MN

Sent from my iPad

Could it be that the "gain" is too low on the B Sweep?



Comments



Keith



From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, January 09, 2015 12:29 PM
To: TekScopes@...
Subject: [TekScopes] Video update 1/9. Tektronix 475A x-y discussion and
triggering/sweep







Would appreciate suggestions and comments

http://www.youtube.com/watch?v=vyZJxkvomOw

Keith, Wayzata, MN

Sent from my iPad

On Jan 4, 2015, at 5:51 PM, 'Leanna Erickson' lle@... [TekScopes]
<TekScopes@...> wrote:

More info

Locked(A Knob): Voltage varied from +2.64 v ----+4.75v

Center position is around 3.7 volts, supposed to be "0" Collector of Q 956
and Q1026 and also the Collector of Q1114(Horiz Amp)

Base of Q956 4.3 volts when it is supposed to be 12.6 volts

Would appreciate suggestions for trouble shooting.

Is it a Sweep problem or a Horizontal Amp problem.

Thanks

Keith in Wayzata, MN

From: TekScopes@... [mailto:TekScopes@...]
Sent: Wednesday, December 31, 2014 3:02 PM
To: TekScopes@...
Subject: RE: [TekScopes] Updae 12/31 Tektronix 475A x-y discussion and
triggering/sweep

Happy New Year.

Looked at the DC Voltage at the Sweep Output using the svc manual
conditions, ie no signal, etc and vary the Horizontal position control.

Results:

Locked(A Knob): Voltage varied from +2.64 v ----+4.75v

Unlocked(A Knob): Voltage varied from -0.242 v ----+1.89v

Any suggestions?

Thanks

Keith in Wayzata, MN

From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, December 26, 2014 9:45 PM
To: TekScopes@...
Subject: RE: [TekScopes] Re: More Info RE: Tektronix 475A x-y discussion and
triggering/sweep

Looked at the waveforms of the A Sweep using another scope.

Waveforms matched pretty much what was shown in the manual.

Keith

From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, December 26, 2014 7:56 PM
To: TekScopes@...
Subject: Re: [TekScopes] Re: More Info RE: Tektronix 475A x-y discussion and
triggering/sweep

This pretty much sums up the problem. I have a rather poor scan of a 475A
service manual which has schematics but it may be incomplete and I have not
gone
through it to clean it up. The 475A oscilloscopes do not seem to be as
common
and I have not had a reason to go through the documentation yet.

On 26 Dec 2014 07:56:31 -0800, you wrote:

Hey Keith,

I'm having trouble finding a service manual with schematics for the 475A

online. All I can find are what looks to be copies of this
<http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Service
<http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Service
%0b%3chttp:/exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Ser
vice%0b%0b%3chttp:/exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475
A%20Ser%0bvice%0b%0b.pdf>
<http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Service
%0b%3chttp:/exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Ser
vice%0b%0b.pdf>
<http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Service
<http://exodus.poly.edu/~kurt/manuals/manuals/Tektronix/TEK%20475A%20Service
%0b%0b.pdf>
%0b.pdf>
.pdf>, which is missing all the good stuff(TM). I think this might be why no
one is piping in yet - whether with opinions, speculation or actual
knowledge:).

If you happen to know of an online scan with schematics, please share it.

Siggi

Could be just wrong adjustment of B-sweep speed: What happens if you pull timebase knob and choose faster B speed?


Raymond

"Could be just wrong adjustment of B-sweep speed:"


What I mean is: Normally, the timebase speed selector makes sure that the B-tim/div <= A-tim/div. The mechanical interlock could have been lost somehow: I've seen the knob slipping on the axis and the axis being loose on the drum (easy to repair with cyanoacrylate glue).


To what extent is the behaviour identical at all timebase settings?
Is the microswitch that is controlled by the "sewing-machine spool" (on timebase axis, just behind the front panel) operating?


Raymond

It looks like the B sweep is quite non-linear, and slows down the farther it runs (the calibrator signal is compressed). Is this true at other speeds as well? Do you have a second scope you can use to look at the sweep ramp?

Thanks for the response



Same response on the end points when B time/div is less than A. The
mechanical interlock still could be a problem.



So behavior appears identical at all timebase settings.



Yes, I have verified that the microswitch does function on the time/div
switch operation, This is SW S-1099.



Now believe the problem may be related to Q 904, 920, or Q922. On the
Collector of Q904, per the voltage check, I have -0.865 when I am supposed
to have +0.5 volts.



Keith.



From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, January 09, 2015 12:47 PM
To: TekScopes@...
Subject: Re: [TekScopes] Video update 1/9. Tektronix 475A x-y discussion and
triggering/sweep





"Could be just wrong adjustment of B-sweep speed:"


What I mean is: Normally, the timebase speed selector makes sure that the
B-tim/div <= A-tim/div. The mechanical interlock could have been lost
somehow: I've seen the knob slipping on the axis and the axis being loose on
the drum (easy to repair with cyanoacrylate glue).


To what extent is the behaviour identical at all timebase settings?
Is the microswitch that is controlled by the "sewing-machine spool" (on
timebase axis, just behind the front panel) operating?


Raymond

Thanks,



Yes, I have a plethora of other scopes..



Believe it could be a problem with either Q904, 920, or 920.



On the collector of Q904, I have -0.865 volts when I should have +0.5 volts.



I have another Q920 coming, a JFET, pn 151-1025, 2N4416 coming, should have
it on Saturday. This is part of the miller intergrator.



Keith



From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, January 09, 2015 1:29 PM
To: TekScopes@...
Subject: RE: [TekScopes] Video update 1/9. Tektronix 475A x-y discussion and
triggering/sweep





It looks like the B sweep is quite non-linear, and slows down the farther it
runs (the calibrator signal is compressed). Is this true at other speeds as
well? Do you have a second scope you can use to look at the sweep ramp?

On the collector of Q904, I have -0.865 volts when I should have +0.5 volts.

In the 465 and 485 scopes the A and B sweeps are identical. If the 475A is the same, you can try switching components from one to the other, one by by one, and see which one causes the problem.

Thanks,



But now Murphy came calling as I lost all triggering, so have to back up
several steps!



Keith



From: TekScopes@... [mailto:TekScopes@...]
Sent: Friday, January 09, 2015 3:09 PM
To: TekScopes@...
Subject: RE: [TekScopes] Video update 1/9. Tektronix 475A x-y discussion and
triggering/sweep

On the collector of Q904, I have -0.865 volts when I should have +0.5

volts.


In the 465 and 485 scopes the A and B sweeps are identical. If the 475A is
the same, you can try switching components from one to the other, one by by
one, and see which one causes the problem.

Hey Keith,

maybe you can scan or snap a picture of the sweep gen schematic and post in photos. That'd help, as I don't think there are any good (free) schematics available for folks here to refer to.


Good luck,
Siggi

Yes, I have a plethora of other scopes..
Believe it could be a problem with either Q904, 920, or 920.

Sweet - it should be easy to sort this with a second scope. If you look at the shape and extents of your sweep, you should be able to characterize the failure (once you get your trigger back).


I have to say I love the way most of the test points on my 485 are brought out to a handy eyelet. Perfect for hooking a probe on.
Also they're all labeled, in copper, as there's no silk screen on the PCB IIRC.

Yes, I have a plethora of other scopes.. > Believe it could be a problem with either Q904, 920, or 920.

So I couldn't resist, I went and bought the 475A service manual from Artek Manuals. Turns out the 475A is quite different from e.g. the 465/485.
Instead of two sweep generators, it has three of them. One is the A sweep, and the remaining two are used for the B sweep, which has a "slow" one for below .5us, and a fast one for .5us and above.


Q904, 920, 922 belong to the A sweep generator, which does seem to be behaving roughly as it should. I'm guessing the voltage you report at the gate of the Q920 JFET is normal, and that there's an error in the service manual in that the sign on the voltage ought to be negative.
This voltage should be defined by the BE drop of Q922 and Q920's gate-to-source voltage. For this voltage to be at +0.6, the JFET would have to have a pinch-off-voltage of zero, but I think more commonly they're in the neighborhood of a negative volt or two, which could come out to roughly -0.6 gate voltage.
This voltage should also be invariant throughout the sweep - e.g. if you look at it with a scope, it should be as near to DC as makes no difference.
You'll also see that the gate voltage on the fast sweep gen at Q1010 is annotated as -0.6V, which is more in line with what I'd expect.


Now, the B sweep is generated from one of the other sweep generators, depending on the sweep speed. The setup is more complicated than for the 465/485 sweep gens I'm more familiar with due to the MIX mode.
In this mode, it appears the starting voltage of the B sweep is not constant. During the first part of the A sweep, the start voltage of both of the B sweep generator keep pace with the A sweep as it's running. When the B sweep then starts, it starts from the A sweep voltage at the time the delay comparator triggers.


If the B sweep is also non-linear above 0.5us - which is where the fast sweep gen cuts over from the slow one - then the problem is likely in the surrounding circutry, and not in the miller generators themselves.


If it's linear above that threshold, then I'd look toward the slow sweep gen, which is Q974, Q980, Q984 et al.


Good luck,
I hope you get your triggers back ASAP,
Siggi