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2445A CH1 attenuator

Fred Schumacher
 

Hi All,
I got a 2445A Tek scope with a CH 1 problem. Applying a square wave to the input of this channel gives me a square wave on my display with an overshoot on the trailing edge, which isn’t present on all other channels.
Switching from 1 mV/div. To 50 mV presents a perfect square wave but switching to a higher attenuation the mentioned problem is present.
Connecting the CN1 attenuator output to the input of the CH2 pre amplifier and connecting the CH2 attenuator output to the CH 1 pre amplifier input gives me the mentioned problem on my CH2 display.
Also I interchanged the connecting rc network between the attenuator and pre amplifier. Checking the internal resistors of both attenuators showed no difference between them. The attenuator showed correct attenuation.
This mean the problem is in the CH 1 attenuator.
So my question is, is it possible to repair this unit or do I have to replace it as a unit.
Part number of this attenuator is 119-2342-01.
Thanks for your advice.


Re: 7A16A high frequency compensation.

unclebanjoman
 

On Mon, May 16, 2022 at 09:12 AM, Ozan wrote:


On Sun, May 15, 2022 at 11:49 PM, unclebanjoman wrote:

Do you put the attenuator at source side or at the plugin side?
Yeah, I always use this signal path: PG506 -> 80 cm coax -> x10 50 ohm attenuator -> 50 ohm feed-thru termination -> BNC of 7A16A

Does removing Q150 still clean up the waveforms? If you keep Q150 in place and
remove U350 does it change anything? This experiment is in case somehow
amplifier stage is loading Q150 output and it is not able to function as
source follower. I think you swapped U350 but there could be a fault in the
components around U350. The problem with this theory is final settled
amplitude is correct.
O.K. I will try with mooore calm this evening.

Otherwise cam switches of the attenuator (e.g. if one of the attenuator
sections is not completely isolated) and attenuator ground connections are
possible suspects. However, it doesn't explain behavior with Q150 removed.
I'm thinking the same. One test at a time and I will report to you ASAP.

Max


Re: 7A16A high frequency compensation.

Ozan
 

On Sun, May 15, 2022 at 11:49 PM, unclebanjoman wrote:


Thinking of, you should note that the input signal is clean with no
aberrations when the input selector is in the GND position. In this case the
input signal is grounded via the series combination R102/R104 (1 Mohm + 56
ohm) and all things are fine.
Positioning the selector in the DC (or AC, has the same effect) things changes
completely. Seems that some strange thing happens inside the input stage...
while theoretically should see always 1 Mohm.... or not?
Because of C132 high frequencies (or faster edges) will see a lower impedance. However, it should still be relatively higher impedance than the 50-ohm source.

Do you put the attenuator at source side or at the plugin side?

Does removing Q150 still clean up the waveforms? If you keep Q150 in place and remove U350 does it change anything? This experiment is in case somehow amplifier stage is loading Q150 output and it is not able to function as source follower. I think you swapped U350 but there could be a fault in the components around U350. The problem with this theory is final settled amplitude is correct.

Otherwise cam switches of the attenuator (e.g. if one of the attenuator sections is not completely isolated) and attenuator ground connections are possible suspects. However, it doesn't explain behavior with Q150 removed.


Ozan


Re: 7A16A high frequency compensation.

unclebanjoman
 

Thinking of, you should note that the input signal is clean with no aberrations when the input selector is in the GND position. In this case the input signal is grounded via the series combination R102/R104 (1 Mohm + 56 ohm) and all things are fine.
Positioning the selector in the DC (or AC, has the same effect) things changes completely. Seems that some strange thing happens inside the input stage... while theoretically should see always 1 Mohm.... or not?

Max


Re: 7A16A high frequency compensation.

unclebanjoman
 

I use the same cables and terminations with my 7A16 and 7A19 with no problems. I've checked and tried different feed-thru terminations and cables, with no difference.
The same cable and termination (they are labeled and distinguishable) I used to calbrate this 7A16 about one year ago.

The problem had probably been dormant for some time. I realized it a cople of weeks ago while I was making measurements on an RF oscillator output using the offending 7A16A.
As the frequency changed between 100MHz and 200 MHz, there was a point where the amplitude of the generated sinewave decreased and then rose again, something that did not happen to me with the 7A19.
Suspecting something anomalous in the bandwidth in the 7A16A, a quick check with the PG506 revealed (to my regret) the problem I am now trying to solve.

The slow start of PG506 is due to the fact that, when i took that photo, its output signal was very low. For very low signals my PG506 always has a slow start, that decresases with increasing amplitude. In fact I usually use a x10 attenuator to circumvent this idiosyncrasy when calibrating the 5/10 mV range. Which I didn't when I took the photos with the 3340.
I remember that it was talked about in other topics a long time ago.

Anyway, this evening I will try some other experiments.

Max


Re: Help with diagnosis of Tek 2465 Power Supply Problem

Luca
 

On Sun, May 15, 2022 at 07:40 AM, <jimbert4@...> wrote:


From the Service Manual, Pg 6-17: A good way to test the microprocessor
address lines (and address decode) is to move Jumper P503 (diagram 1) to the
Diagnostic position. This will disable the Data Bus Buffers and force the
micro into a NOP loop, continuously incrementing the address lines.
I checked the addresses from A0 to A15. They decrease in frequency by half each time. I didn't check the timing of them relative to each other, only one line at a time. The frequencies seem correct.

Luca


Re: Help with diagnosis of Tek 2465 Power Supply Problem

Luca
 

On Sun, May 15, 2022 at 07:23 PM, Ozan wrote:

In the error light pattern are Ch1-4 lights off?
Correct. All of Ch1-4 lights are off when the Kernel is trapped in error.

If you push delta-V/delta-T/slope buttons all at once you will enter the test
menu. Then if you push "Trigger coupling" up button scope should run a small
diagnostic. I don't think EPROM checksum is executed from that menu but it is
worth a try.
All the 5 Tests passed.

I can enter the Exercises 01-04 and Exercise 04 report the checksums, that is how I got the checksum values, which all looked correct.

I wonder if it is some kind of power problem. When the CPU starts running, the power on the ROM is not ready yet. But once after a few seconds, the power is stable, and all is fine. I will be looking for a way to RESET the machine after it is powered up. If the kernel check passes, then it should be power related problem.

Luca


Re: Help with diagnosis of Tek 2465 Power Supply Problem

Ozan
 

On Sun, May 15, 2022 at 04:40 PM, Luca wrote:

...
I recommend checking connectivity between each address pin of U2378 and one
of the working EPROMs.

I checked the connectivity. It is all connected (the corresponding addressing
pin of U2378 is connected to other EPROMs).
In the error light pattern are Ch1-4 lights off?

If you push delta-V/delta-T/slope buttons all at once you will enter the test menu. Then if you push "Trigger coupling" up button scope should run a small diagnostic. I don't think EPROM checksum is executed from that menu but it is worth a try.

Ozan


Re: Help with diagnosis of Tek 2465 Power Supply Problem

Luca
 

I recommend checking connectivity between each address pin of U2378 and one of the working EPROMs.
I checked the connectivity. It is all connected (the corresponding addressing pin of U2378 is connected to other EPROMs).

Is this a CPU board with problematic SMD electrolytic caps?
All caps are through hole. I tried to desolder the capacitor C2486 near the PIN 28 of U2378. After removing it, it does not solve the problem. I checked the capacitor value, it is very close to spec (101nF actual compared to 0.1uF spec).

Thanks,
Luca


Re: 7A16A high frequency compensation.

 

On Sun, May 15, 2022 at 10:17 PM, Ozan wrote:


Same as above, but with selector on DC position:
https://groups.io/g/TekScopes/photo/262075/3430812
There's definitely a reflection there.
AFAIR, the OP mentioned that suddenly, things went wrong after being OK before, so I assumed what Ozan mentions:

In the setup coax coming from the signal source needs to be terminated to 50-ohms at the point it enters the plugin. Either a feedthrough 50-ohm
termination (ideal) or a BNC-T with one port terminated to 50-ohm is needed.
was already in the setup.

Come to think of it, I see a few more strange things in the photographs: The curves' slow start of the PG506, as shown on the PM3340, is surprising and the images inside the 7A16A seem very "rough". Neither look like my observations with my PM3340.
Together with Ozan's remark, could user error be involved?

Raymond


Re: 7A16A high frequency compensation.

unclebanjoman
 

Ozan:
yes, I'm always using a x10 50 ohm attenuator and a 50 ohm feed-thru termination at the BNC plugin. The same pieces I use with a 7A16 (not A) without problem.
Using a cable 50 cm more long, the width of the first step increases a little. Remember that I measure the waveform after R10. Measuring directly at the BNC pin of the plugin, the waveform changes, in the sense that the middle step is a the 80% of the final value, instead of the former 50%.

Raymond:
I did not check the continuity between the input BNC and R132. The form of the signal gives me strong suspicions that there may be something anomalous in the signal path even if, being the input selector au 10, V / div, all the "A" contacts should be closed ... giving a direct path from the BNC to the R130/R132 junction.
I'll check the continuity with a DVOM tomorrow.
And yes, I'm from Italy.
I already tried to reverse the Q150 since it's symmetrical. No changes whatsoever.
Unfortunately I only have the 500 ohm probe original from Philips, given together with the 3340 having type N connectors. I have a good 7A19 but not a suitable probe (I'm searching one at a reasonable price).
I've already tried to swap U350 and U450 some days ago, with no results, so I doubt it's the culprit.
I will try to unplug U350 and measuring the signal at pin 14. At this point we might as well try all the possibilities ...
I have tried also a 155-0078-01 pulled out from a 465 I use for parts, still with no noticeable results.

Max

Max


Re: 213 BATTERY MOD

tgerbic
 

Thanks. I will have to try that on mine.

Tony


FS: Tek 7854 at Dayton only (no shipping)

Tom B
 

Hello All,

A friend is selling his Tektronix 7854 at the Dayton Hamvention next week.  He will not ship this item.  It is in working condition and includes the keyboard, 7B85, 7B87, and two 7A24 plug-ins.  The price is $500.  Space number is 7736.

Tom Bryan
N3AJA


Tek 2465A Frequency readout

Nick
 

Hi
I fixed my 2465 over a number of years, replacing the ROMs etc and fighting my way through the CAL process despite not having the best kit.
My question relates to the frequency measurement readout. While I can get reasonably accurate 1/dt frequency readings with the cursors, the auto measurement function tops out around 10Mhz.
I cant find any useful info in the manual on what its capabilities are , so don't know for sure if its faulty and I've only ever had access to my own scope.
Should the frequency measurement work for the full bandwidth of the scope?
If so, is this likely to be connected to the final parts of the CAL which rely on the PG506, which I have never had access to, and which I had to "fudge " as best as I could with what I had.

If not, any tips on where to focus to clear up the problem.

Nick


Re: 7A16A high frequency compensation.

Ozan
 

On Sun, May 15, 2022 at 12:52 PM, unclebanjoman wrote:


Just replaced all 1000 pF caps, C132, C142, C144, C156. No luck. No relevant
changes detected.
While changing caps, I measured R132 = 467.5 kohm.
R130 reads practically 1 Megaohm

Then, following Ozan's suggestion, i checked the input waveforms. Results:

Waveform immediately after R10 (right side of R10 on the schematic) with the
input selector on GND position:
https://groups.io/g/TekScopes/photo/262075/3430811
Same as above, but with selector on DC position:
https://groups.io/g/TekScopes/photo/262075/3430812
Waveform on the left side of R132 (junction of R130/R132):
https://groups.io/g/TekScopes/photo/262075/3430813
Waveforms look like reflections coming from the cable between signal source and the plugin. If you double the length of the coax you use, does the place where first step happens in the middle picture change?

In the setup coax coming from the signal source needs to be terminated to 50-ohms at the point it enters the plugin. Either a feedthrough 50-ohm termination (ideal) or a BNC-T with one port terminated to 50-ohm is needed.

Ozan


Re: 7A16A high frequency compensation.

 

On Sun, May 15, 2022 at 05:07 PM, unclebanjoman wrote:


I don't really know what else to try
Hi Max,
Currently, I'm not where I have my equipment, doc and parts so I'll do without:
I'm pretty sure that Q150 is an often-used FET (151-1032-00?), so it would be easy to replace. I'm almost certain I have a few. You're in Italy, right? I'm in NL, so replacing it wouldn't be a real problem, even if you don't have a replacement. I can check if I have one this Tuesday.
Since these FETs are basically symmetrical, you could swap both by rotating the case 180 degrees, depending on pinout, just to try, also depending on how the case is connected.
R133 isn't in my schematics, so I didn't know where it was until now. It's not surprising that it doesn't make any difference re. your problem.
The DC voltages in the circuit seem fine.
I agree with you that the red diodes and - CR130 - are for protection only.
Are you sure that continuity from the BNC input to R132 is OK?
You can't really trust your 500 Ohm 'scope probe input at the gate of Q150A. Still, I don't quite understand your different observations at the gate of Q150A with Q150 in vs out, although of course, there's Q150A's gate capacitance.

I have a feeling the problem isn't in the area around Q150.
What do you see at U350 pin 14 if you first pull the IC? Again, don't worry, easily replaceable IC, not expensive. Carefully lift each pin (spider leg) a little in sequence and go around until the IC comes free. Do you have another 155-0078-xx available for testing?

Sorry, can't be of much help ATM.

Raymond


Re: 7A16A high frequency compensation.

unclebanjoman
 

Just replaced all 1000 pF caps, C132, C142, C144, C156. No luck. No relevant changes detected.
While changing caps, I measured R132 = 467.5 kohm.
R130 reads practically 1 Megaohm

Then, following Ozan's suggestion, i checked the input waveforms. Results:

Waveform immediately after R10 (right side of R10 on the schematic) with the input selector on GND position: https://groups.io/g/TekScopes/photo/262075/3430811
Same as above, but with selector on DC position: https://groups.io/g/TekScopes/photo/262075/3430812
Waveform on the left side of R132 (junction of R130/R132): https://groups.io/g/TekScopes/photo/262075/3430813

I must admit that the last waveform is somewhat lousy.

I'm more and more confused.

Max


Photo Notifications #photo-notice

Group Notification <noreply@...>
 

The following photos have been uploaded to the 7A16A step response (with 7854) ( https://groups.io/g/TekScopes/album?id=262075 ) album of the TekScopes@groups.io group.

* input_signal at_R10C10_DC_position.jpg ( https://groups.io/g/TekScopes/photofromactivity?id=3430812 )

*By:* unclebanjoman <mmazza@...>



---

The following photos have been uploaded to the 7A16A step response (with 7854) ( https://groups.io/g/TekScopes/album?id=262075 ) album of the TekScopes@groups.io group.

* waveform_at_junction_of_R130R132.jpg ( https://groups.io/g/TekScopes/photofromactivity?id=3430813 )

*By:* unclebanjoman <mmazza@...>


213 BATTERY MOD

Miguel Work
 

Battery mod to avoid battery discharge when is off

I use a new 4 amp smd mosfet with 2 volts gate VT

https://groups.io/g/TekScopes/album?id=275120


Re: 7A16A high frequency compensation.

Ozan
 

On Sun, May 15, 2022 at 08:07 AM, unclebanjoman wrote:

...
I measured voltages in the source follower stage.
I posted a screenshot of the partial schematic with measured voltage values
written in red.
All the DC voltages look fine. Unless there is a subtle failure mechanism that doesn't show up in DC operating point JFET looks good.

If you probe left side of R132, is rising edge clean? If left side of R132 looks good R132 may have went up in value. If you don't want to lift one side of R132 to measure you can short its right side to ground and measure resistance of parallel combination of R132 and R130 =~ 320k.

Stephen's suggestion of checking caps is also good.

Ozan

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