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Hi Nenad,

I tend to disagree with your analysis. The feedback is not primarily intended to keep the base of Q36 at a constant DC level. Within limits Q36 doesn't care. The purpose is to have the bridge + and - bias voltages follow the S-4 input voltage. The only way to change Memory is a suitable AC pulse at the gate of Q36. We concluded already that the avalanche pulse alone is too slow to have effect. The same would be true when the Correction Memory "shakes" the bridge circuit voltages. Moreover, when S901 is Off then the Correction Memory is inoperable. Q960<15> is always conducting then and passing the Offset control voltage to U955 + input. Q954 is always conducting then and passing the forced-constant Corr Mem voltage to U955 - input. There are no pulses at the output of U955 and hence also not at pins 2 &B of S-4.
In the S-2 head some transistors sit in sockets and can easily be removed. Q55 provides the snap-off bias current and can safely be removed. With Q55 removed the S-52 Cert Signal Out was about +2 V and Offset had no effect.
If my analysis is correct then I am still confused about what was really going on in your S-4.
[The S-2 I used was sensitive for pressure at its front and sometimes the Vert Signal Out would also jump to +2 V because of that. In order not to be mislead by this I repeated the test now with the naked S-2 on a sampling head extender. That way I had better control over testing alternatingly with Q55 inserted and removed.]

Albert

Hi, I worked on this PS in a 7904 some years ago. There are two overload conditions, over/under voltage and over current. Either shall turn the supply off. However, I think there is a third condition that will turn the regulator off which is not quite obvious. If you have arcing in the multiplier for example you will get sudden and random load changes and thus disturbances that will destroy the phase of the signal coupled to pins 10 and 11. The phase detector will go nuts and the regulator will turn off, I think.

Can you hear a hissing sound, if so it could indicate arcing?

Meanwhile, there are a couple of tests that you can do but it takes some courage. Short pin 2 (FAULT SENSE) and then pin 13 (ISENSE) to ground. If the behaviour remains the same then shut down is not triggered through these ports.

Carry on
G?ran

On 4/20/19 3:12 PM, [email protected] wrote:

I think the next thing I need to find is a scope mobile cart

I Have model 3 I can send you.

What do you wan to pay?

--
John

These scopes are not a good fit for a hack job if you want extended life. I recommend you get it to an experienced tech that understands these units and can make successful repairs without creating further damage. If you would like to ship unit to me I can help with the evaluation/repairs. Electronixtoolbox@...

When you replaced the bad capacitors, were the originals of the can style
with several ground tabs? Did you restore all those ground connections by
using caps of the same type, or otherwise restore the connections with
jumper wires? In some places in Tek power supplies, the capacitor can is
part of the circuit. Removing a can without replacing it (or installing
jumper wires instead) can result in sections of the board not being
grounded as intended.

Dave Casey

On Sun, 21 Apr 2019 18:04:09 -0700, you wrote:

Steph,

I'm working with only boards A23 and A12. The low voltage regulator board A22 was disconnected from the rest of power supply, and there are no other connections. I pulled out connectors P52,P50 and P54 from A12 board to isolate the problem.
Doing so, the Fault Sense signal at pin 2 of U75 is unconnected, pulled to ground R66/C66 so I assume that this signal is not the culprit.
I've put a dummy load across +17 and -17V (connector P52) , 33 ohm resistor 10 W, I think that it's a reasonable load for the SMPS to start. Or not?
The only voltage really needed to the control IC is the 108V in the rectifier section of A12 board, together with the -17V. These voltages are somewhat normal during the burst mode but, as I stated in my previous message, the tick mode (burst) is very irregular and random, interspersed with single pulses. So no chance to understand what's is happening for now!
Still puzzled.

Ok, perhaps this is a reasonable interpretation.

1) the power supply ramps up voltages
2) if an overload (let's assume that, otherwise it's overvoltage) is
detected, then the power supply shuts down, causing the tick.
3) a good solid short to ground means a dependable state of ticking.

So if the ticks are irregular, then perhaps the fault is irregular.
Something is not shorted completely, but something may be overloading
the supply, just not all the time. You may be looking at something
that's drawing current intermittently.

So, assuming that you're getting something failing, then the question
is how to find it....

One thing you can do is to look at the voltage at each output of the
supply for an overvoltage. I'd look at the shutdown signal, something
somewhere may be shorting out, or trying to, and then the voltage at
each supply might be rising, or trying to rise. That this is a
repeating situation is to your advantage, since it's producing voltage
waveforms you may be able to measure.

Doing so will probably require you to analyze the exact mechanism of
the overvoltage/overcurrent and go from there.

I seem to remember some threads on a similar supply.

Harvey

Max

Steph,

I'm working with only boards A23 and A12. The low voltage regulator board A22 was disconnected from the rest of power supply, and there are no other connections. I pulled out connectors P52,P50 and P54 from A12 board to isolate the problem.
Doing so, the Fault Sense signal at pin 2 of U75 is unconnected, pulled to ground R66/C66 so I assume that this signal is not the culprit.
I've put a dummy load across +17 and -17V (connector P52) , 33 ohm resistor 10 W, I think that it's a reasonable load for the SMPS to start. Or not?
The only voltage really needed to the control IC is the 108V in the rectifier section of A12 board, together with the -17V. These voltages are somewhat normal during the burst mode but, as I stated in my previous message, the tick mode (burst) is very irregular and random, interspersed with single pulses. So no chance to understand what's is happening for now!
Still puzzled.

Max

Is another test-bed link needed re Fault Sense signal P83-4? Suggest trace the signal path. [It has a convoluted journey through Main Interface (sheet 3 where renamed "HV sense"), then through Z-axis/Focus PCB (sheet 13) and finally back to T14 power transformer.]

Steph

Seems there are some Fab Labs, Hacker Space's or Maker Space's in the area that may be able to assist. Maybe is a place to visit and see who can assist.

I'm sure there are SME's in the area... though I'd guess with California pricing... everyone is busy working to afford living there.

Kindest Regards

I've been working and studying on this beast for three days but with no results, except a big
headache.

Any ideas and/or suggestions?

Situation normal with the Tek switchers AKA "High efficiency power supply". There are three that just
sit here having defeated all attempts at fixing.

Craig

I got this 7854 to repair.
Main symptom:it blows the 4A main fuse. After several tests and measurements, the culprit was found in one of the two leveling capacitors (C16), 950uF 200V with a serious leakage.
I replaced both capacitors C16 and C17 for safety. The SMPS chassis sits on my workbench, fully extracted from the mainframe.

I put a dummy load across +15 and -15 V (one resistor 33 ohm 10W), insert a jumper on P72 on LV board (thermal switch) to avoid a false FAULT sense condition and power up the SMPS.
The fuse does not blows. Good. The security indicator neon lamp is steady on.
I got the somewhat expected tick-tick from the SMPS. Damn...
I already read the various topics on this forum regarding the infamous 7xxx SMPS and all the possible documentation and/or links about that.
So I proceed with measurements and investigation.

With my surprise, the tick-tick is not as usual (one tick/burst every 500 ms or so) but is very irregular
With my oscilloscope I see short random pulses followed by one, two sometimes even three or more bursts (pin 9 of U75, base drive control).
On pin 1 of U75 I see a signal that grows towards + 0.6V but in a very irregular manner (in correspondence with the very short pulses) and then reach 0.6 / 0.7V when (in a random manner) the normal burst begins.
At this point I am totally puzzled because due to this random behavior I cannot understand how the correct signals should be.

To be sure, I've disconnected completely the LV regulator board and I've put the dummy load directly across the +17/-17V.
I checked all the voltage levels on the low-voltage rectifier section: during the burst they reach more or less the nominal value, so there are no short circuits or lossy capacitors. The +108V are present.

The fault sense pin shows an irregular signal of 50mV or less so I don't think that's the cause.
The over-voltage stop seems O.K. in the sense that the emitter of Q45 is LESS positive than its base. The zener diode VR45 is correctly regulating to 120V.
I've checked with my analog VOM (ICE 680R) all the bjts. All of them seem OK. The programmable UJT Q30 is OK also.
Checked the power transistors Q34 and Q40: they are good.
Checked for leaky capacitors OK
Checked all the diodes OK

I've been working and studying on this beast for three days but with no results, except a big headache.

Any ideas and/or suggestions?

Max

Thank a lot for the input will try eevblog

On Sun, 21 Apr 2019 05:33:05 -0700, you wrote:

On Wed, Apr 17, 2019 at 07:55 AM, Harvey White wrote:

On Wed, 17 Apr 2019 07:06:19 -0700, you wrote:

I have been given this tool. I have a Tek 2246A and a Hitachi scope already.

What can I do with this that I can't with the other scopes? I have never done
anything with video nor intend to. Also, the trace is hiding deep lower left
of CRT. Can't get it out of there.

If you're doing anything with baseband video, such as CCTV cameras for
surveillance systems, video tape recorders, etc., then you'll want
this.

It looks at very specific things on the video waveform with an ease
that is hard to duplicate on most oscilloscopes unless they have some
very specific TV signal processing/filters built in.

Having said that, this is the old standard 450 or so line resolution,
analog video, 75 ohms, 1.4 v p-p video that uses either BNC, SO-239,
or RCA jacks (on consumer equipment).

Generally not found as much as you'd think.

If you never intend to do anything with video, then it won't be of
much use to you. As has been mentioned, it's not a general purpose
scope.

Harvey

Both my existing scopes have a TV trigger setting. Since this is a specialized tool, I get the idea that a general use scope won't be likely to duplicate what it can do.

I might do some security camera surveillance in the future with my business. What value would this have for that? Would my existing scopes fill that need? I am determining if I should offer this for free to the group.

There's a difference here.

The TV sync settings on a "normal" scope allow jitter free (or at
least easier) triggering on a TV waveform.

To set up a video system, there are often VITS (Vertical Interval Test
Signals), which you could sometimes see by rolling the picture down or
underscanning it on one of the old broadcast TV signals.

Literally, you'd want to look at line N out of 525 to find the signal.
There were also standards for the height of the vertical/horizontal
blanking, sync, color burst, and so on. The video monitor had
controls, trigger circuits and logic inside to make that kind of
waveform observation easy.

For instance, on a conventional scope, looking at line N involves two
time bases and a digital delay counting horizontal sync pulses. The
waveform monitor has this as a switch setting.

Most of your existing scopes would do reasonably well in diagnosing
CCTV stuff, because you're not messing with the things that this
waveform monitor is designed to do. In the earlier days of TV, all
these parameters were adjustments and set up by the chief engineer
during maintenance.

With today's CCTV stuff, all of this is pretty much fixed in chip
design, not made to be adjustable, and you're more looking for absence
of signal or gross distortions than not.

*I* would keep it (and I have a similar Hitachi monitor and a vector
scope), but just because I know where a few bodies are buried.

However, if you ever maintain, want to maintain, or think you might
ever work on VHS or Betamax, any of the older video technology up to
but not including DVDs (although the composite output may be
interesting), then I'd definitely keep it.

Harvey

NielsenTelecom

On Sat, 20 Apr 2019 23:07:50 -0700, you wrote:

On Sat, Apr 20, 2019 at 07:39 PM, Harvey White wrote:

the 2467B may be
different,

Hi All:
I remember something about an limiter circuit to limit the anode current, when driving the CRT hard (displaying a bright trace). That circuit was supposed to keep the maximum brightness to a safe level and to protect the MCP from aging too fast.
Don't remember any CRT timer... or warnings about limiting brightness... could be an option in the software?
Also, if you run the self tests, you can see how many times it has been powered up, and I think... the total number of powered on hours. (the lower the better, for CRT life.)

Since the 2467B is mostly software driven, and the 710x series is
hardware without a bit of software, I can see the mechanisms being
quite different. The idea, though, is much the same. Not at all sure
that the 67B will have the same problem, but I'd be suspicious and
take it easy until I knew definitively if there was a problem.

Trying to minimize the time that the CRT is set for an extremely
bright horizontal line and left on for hours.... Seen that one.


Harvey

Best regards and wishes.

On Wed, Apr 17, 2019 at 07:55 AM, Harvey White wrote:

On Wed, 17 Apr 2019 07:06:19 -0700, you wrote:

I have been given this tool. I have a Tek 2246A and a Hitachi scope already.

What can I do with this that I can't with the other scopes? I have never done
anything with video nor intend to. Also, the trace is hiding deep lower left
of CRT. Can't get it out of there.

If you're doing anything with baseband video, such as CCTV cameras for
surveillance systems, video tape recorders, etc., then you'll want
this.

It looks at very specific things on the video waveform with an ease
that is hard to duplicate on most oscilloscopes unless they have some
very specific TV signal processing/filters built in.

Having said that, this is the old standard 450 or so line resolution,
analog video, 75 ohms, 1.4 v p-p video that uses either BNC, SO-239,
or RCA jacks (on consumer equipment).

Generally not found as much as you'd think.

If you never intend to do anything with video, then it won't be of
much use to you. As has been mentioned, it's not a general purpose
scope.

Harvey

Both my existing scopes have a TV trigger setting. Since this is a specialized tool, I get the idea that a general use scope won't be likely to duplicate what it can do.

I might do some security camera surveillance in the future with my business. What value would this have for that? Would my existing scopes fill that need? I am determining if I should offer this for free to the group.

NielsenTelecom

The 2467B will go into a screen saver mode after about 5 minutes
of inactivity (eg. no knob motion). It also will move the display
readout positions every so often to prevent burn-in.

The counters for power on and number of operating hours are usually
reset every time the scope is calibrated. They are only there to
give an idea of how the scope was used since its last calibration.

-Chuck Harris

Roy Thistle wrote:

The 2467B and 7104 most assuredly do *not* have the same CRT!

There is a good 6 inches of difference. They both have a micro
channel plate (MCP) image intensifier, but they are not the
same CRT.

By the time the 2467 came along, many of the difficulties with
the 7104 CRT had been mitigated. The 2467 has a more sophisticated
timer mechanism that shuts the CRT down after 5 or 10 minutes of
inactivity. It also moves the display information around in the
upper and lower graticule area to help distribute any burn-in
that will occur.

So, even if you don't pay any attention to the MCP weaknesses,
the 2467 will give you a pretty long life.

Unfortunately, tektronix did not change the control program much
between the 2465B and the 2467B, and the calibration routine does
serious damage to the MCP screen every time it is done. And, they
change the intensity, every time the buttons on the right side of
the CRT are used, to an excessively bright value.

-Chuck Harris

Harvey White wrote:

Did I mention that the TD is extremely delicate. It can't withstand ANY voltage on the output. If
you
think a series capacitor will protect it, think again.

I believe that the last list price for that TD from Tek was $800. And that was decades ago.

The only other place that diode is used, is in the 7T11 sampling timebase.

So yes - read the manual.

Although the 284 is certainly an interesting beast (I have one) and provides other useful outputs in
addition to the 70ps fast rise, if you want a truly practical pulser that gives half the rise time (so
about 30-40ps) this is the thing to buy
. Gets its
power from a USB port. Absolutely perfect for measuring the performance of scopes and plugins up to
10GHz. And you get a calibration certificate too.

However in 1968, when the 284 was introduced, it was a typically Tektronix design tour de force. And
cost $525 (so about $10k at today's prices!)

Craig

On Sat, Apr 20, 2019 at 10:05 PM, vivek jayaraj wrote:

whom to pm.. to ask for help

If you haven't tried the EEV Blog Forum, you can post there too.
There are some threads about the 2467B on that forum too.
Best regards and wishes.
Roy

On Sat, Apr 20, 2019 at 07:39 PM, Harvey White wrote:

the 2467B may be
different,

Hi All:
I remember something about an limiter circuit to limit the anode current, when driving the CRT hard (displaying a bright trace). That circuit was supposed to keep the maximum brightness to a safe level and to protect the MCP from aging too fast.
Don't remember any CRT timer... or warnings about limiting brightness... could be an option in the software?
Also, if you run the self tests, you can see how many times it has been powered up, and I think... the total number of powered on hours. (the lower the better, for CRT life.)
Best regards and wishes.