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Hi,

during work, all of the sudden my 7854 started to show jitter of the readout
in the horizontal and vertical directions. In stored mode the waveform is
jittery as well, but in scope mode the waveform is rock solid. I checked the
voltages of the main power-supply and aux supply plus the 10V reference on the
display board are all are fine without excessive ripple. The amplitude of the
jitter is about 1/32".
Somehow I can't believe it is a problem in the digital circuitry, first I
thought the DAC reference voltage had a problem, but its fine.
I love this scope and I use it more frequently than any other.
--
Best regards,

Fred S.

Albert,

Thank you for these ideas. I'll check the relationship between TP106 and TP109
when I'm back in the office on Monday. The waveform I see at the base of Q546
is odd: it's a sort of castellated shape with up bits and down bits, but never
appears to go below ground. I can see what look like those 200ns spikes at the
anode of CR116, but they're no longer in evidence at the base of Q546. I
should look more closely for them, though, since they'll be clamped to -0.7V
by Q546's B-E junction.

The big anomaly is that the service manual shows waveform 28 at the collector
of Q546, with 3us-wide pulses heading from -15V to pretty close to 0V. I can't
get anything like that. The best I have ever seen were similar pulses but only
from -15V up to about -13V, clearly not enough to do anything useful!

Chris


On Fri, Nov 30, 2018 at 01:14 PM, Albert Otten wrote:

Hi Chris,

The hold-off multivibrator "looks like it's working". Did you check the
waveforms at TPs around U110? The waveform at TP109 should be the inverted
version of that at TP108 as shown in graph 5. (Note that voltage levels at the
probe tip are 10 times larger than shown in the graph.) But also verify that
TP106 is already low when TP109 goes low. TP106 low helps to bias the base of
Q546 in negative direction. What happens to Q546 base voltage when TP106 goes low?
The time constant associated with C114 and surrounding resistors is 0.2 us or
so. Maybe you missed the short spikes at the base of Q546 for that reason?
Disclaimer: I concluded these things from the diagrams but didn't test it in
my 7T11.

Albert

On Fri, Nov 30, 2018 at 08:53 PM, cmjones01 wrote:

I had it on an extender in the bench today and established that most of it is
working: the trigger circuitry, strobe pulse generator and slow scan all seem
to be doing what they should. However:
- the trace is blanked because the X/sweep output is out of range (it's at about +15V)
- it seems to be out of range because the horizontal memory isn't getting any strobe pulses
- the holdoff multivibrator, which generates said pulses, looks like it's working
- there's a peculiar arrangement of resistors, a diode and a 100pF capacitor
which takes the outputs of the multivibrator (a 7400 chip) and is supposed to
switch on a PNP transistor (Q546 I think) which has its emitter tied to ground
(in equivalent time sampling mode). This is not happening. The transistor's
base never goes below ground so it never switches on.

Looking at the circuit diagram, I can't see how it's supposed to work. I don't
have the manual in front of me right now but does anyone know this part of the
circuit well? My next move is likely to be replacing the 7400 to see if a
different one somehow magics it in to operation.

Hi Chris,

The hold-off multivibrator "looks like it's working". Did you check the
waveforms at TPs around U110? The waveform at TP109 should be the inverted
version of that at TP108 as shown in graph 5. (Note that voltage levels at the
probe tip are 10 times larger than shown in the graph.) But also verify that
TP106 is already low when TP109 goes low. TP106 low helps to bias the base of
Q546 in negative direction. What happens to Q546 base voltage when TP106 goes
low?
The time constant associated with C114 and surrounding resistors is 0.2 us or
so. Maybe you missed the short spikes at the base of Q546 for that reason?
Disclaimer: I concluded these things from the diagrams but didn't test it in
my 7T11.

Albert

On Fri, Nov 30, 2018 at 08:53 PM, cmjones01 wrote:

I had it on an extender in the bench today and established that most of it

is

working: the trigger circuitry, strobe pulse generator and slow scan all

seem

to be doing what they should. However:
- the trace is blanked because the X/sweep output is out of range (it's at
about +15V)
- it seems to be out of range because the horizontal memory isn't getting

any

strobe pulses
- the holdoff multivibrator, which generates said pulses, looks like it's
working
- there's a peculiar arrangement of resistors, a diode and a 100pF capacitor
which takes the outputs of the multivibrator (a 7400 chip) and is supposed

to

switch on a PNP transistor (Q546 I think) which has its emitter tied to

ground

(in equivalent time sampling mode). This is not happening. The transistor's
base never goes below ground so it never switches on.

Looking at the circuit diagram, I can't see how it's supposed to work. I

don't

have the manual in front of me right now but does anyone know this part of

the

circuit well? My next move is likely to be replacing the 7400 to see if a
different one somehow magics it in to operation.

Is there anyone here intimately familiar with the internal workings of the
7T11 plugin? I ask because I'm trying to get one to work. The symptoms are
that there's no trace visible on the screen.

I had it on an extender in the bench today and established that most of it is
working: the trigger circuitry, strobe pulse generator and slow scan all seem
to be doing what they should. However:
- the trace is blanked because the X/sweep output is out of range (it's at
about +15V)
- it seems to be out of range because the horizontal memory isn't getting any
strobe pulses
- the holdoff multivibrator, which generates said pulses, looks like it's
working
- there's a peculiar arrangement of resistors, a diode and a 100pF capacitor
which takes the outputs of the multivibrator (a 7400 chip) and is supposed to
switch on a PNP transistor (Q546 I think) which has its emitter tied to ground
(in equivalent time sampling mode). This is not happening. The transistor's
base never goes below ground so it never switches on.

Looking at the circuit diagram, I can't see how it's supposed to work. I don't
have the manual in front of me right now but does anyone know this part of the
circuit well? My next move is likely to be replacing the 7400 to see if a
different one somehow magics it in to operation.

Thanks
Chris

I seem to remember someone talking about plug-in extenders - were they
for the 7000 series ??

If so are they still available ?

Dave

It is unlikely you will ever know what they did... unless you pull the
attenuator out, remove its covers, and look for yourself.
Pretty easy work with any stereo dissection microscope.

To blow that resistor, your friend must have over voltaged
that input quite a lot. It probably involved the power line.

There is a young mechanical engineer here in the Netherland who have drawn
in 3D and printed for me 3 sets (3 x4 ) of the 335 back feet for me. I
still have 2 sets left. Price is €12.60 for the set off 4 + €1.40 shipping
cost ( as normal letter not registerd ). total € 14,00 which are about
$12.30
For fotos and information contact me at: sp856567@...

From: Harvey White <madyn@...>
Sent: Nov 29, 2018 9:13 PM
To: [email protected]
Subject: Re: [TekScopes] Tektronix P6100 series ground clips

On Thu, 29 Nov 2018 19:38:13 +0100, you wrote:

BTW you can get the standard ground lead with the probe side spring
and crappy alligator clip on Digikey for $2.65:
<>

Thanks, since I'm not using the alligator clip, I don't care about the
quality of the clip.

The one with the marginally less crappy alligator clip can be had for $10:
<>

Ouchie....

BTW: you *can* buy clips that look identical from McMaster-carr.
Problem is that they are not copper, they're 18-8 stainless or zinc
plated steel. Not sure that I want to try soldering to them.

This is assuming that the clips can be had in the right size.

Harvey