Date   

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


Re: TDS540 repair; drifting vertical position at start-up

Ozan
 

On Sun, May 15, 2022 at 07:46 AM, visitslovenija wrote:
I have read in various threads about these ‘scopes that washing the pcb’s
thoroughly was recommended, but mixing water with a complex board just made me
queasy! So I’d stuck to using IPA and a foam cleaner I have used
successfully before. However, after reading your post I decided to give it a
There is some chemistry reasons (I don't remember the details) for some deposits not dissolving in alcohol. I also found these boards to have so much electrolyte residue it was difficult to completely rinse it off without running water. When I tested the residue with litmus paper it was very basic so the reason for vinegar to neutralize somewhat. After running tap water I use distilled water before drying off with air (I use a shop vac in reverse).

try. So have given both main boards the vinegar, dishwasher treatment.

So far though, I can’t see it has had any effect. Having seen the corrosion
under two of the 074’s so far, I’m not really surprised.
At least it should prevent further corrosion.


Mind you, I have managed to short two output pins on one of the multiplexers
(now busted), so there is a chunk of circuit not working at the moment (idiot)
and will have to wait until the spare parts arrive before I can be sure how
things now look. Sigh.
As long as fixes are more than damage it is still progress.

Ozan


2430 parts needed

Robert Putala KE8UW
 

I have a 2430 in need of two socketed IC’s;
U440 (165-1215-01) & U450.
Channel 2 appears to work ok, Ch 1 has these two parts missing. Seems worth a try to just replace them and see if it comes back.


For Sale Tektronix TM515 Power Module Mainframe and K217S cart

David Sidrane
 

For Sale

- Tektronix TM515 Power Module Mainframe $75 + SH
- K217S cart $200+SH

Location is Palm Desert CA

Contact me via PM or email in interested.


Re: 7A16A high frequency compensation.

SCMenasian
 

Perhaps one of the capacitors, C142, C156 or C158 has gone open or nearly so. Try paralleling them with known good capacitors (with short leads) of the same value.

Stephen Menasian


Re: 7A16A high frequency compensation.

unclebanjoman
 

Hi Raymond,

I measured voltages in the source follower stage.
I posted a screenshot of the partial schematic with measured voltage values written in red.
There are also some diodes added by Tek, not present in the original schematic. I draw these extra diodes in red also.
I was unable to recognize the type.I think they are for protection purposes.

The schematic with my annotations is here: https://groups.io/g/TekScopes/photo/262075/3430589

I tried also paralleling R133 with a 82 ohm resistor to bring it back to the correct value of about 16 ohms but with no visible effect. I tried to short completely R133 but also without noticeable effects. I really don't know its purpose.

Just for curiosity I measured the signal on pin 14 of U350. It is visible here: https://groups.io/g/TekScopes/photo/262075/3430590

I don't really know what else to try

Max


Photo input_sourcefollower_schematic .jpg uploaded #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_sourcefollower_schematic .jpg ( https://groups.io/g/TekScopes/photofromactivity?id=3430589 )

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


Re: TDS540 repair; drifting vertical position at start-up

RJ
 

Hi Ozan,

I have read in various threads about these ‘scopes that washing the pcb’s thoroughly was recommended, but mixing water with a complex board just made me queasy! So I’d stuck to using IPA and a foam cleaner I have used successfully before. However, after reading your post I decided to give it a try. So have given both main boards the vinegar, dishwasher treatment.

So far though, I can’t see it has had any effect. Having seen the corrosion under two of the 074’s so far, I’m not really surprised.

Mind you, I have managed to short two output pins on one of the multiplexers (now busted), so there is a chunk of circuit not working at the moment (idiot) and will have to wait until the spare parts arrive before I can be sure how things now look. Sigh.

Ralph


Re: Help with diagnosis of Tek 2465 Power Supply Problem

jimbert4@...
 

On Sun, May 15, 2022 at 01:59 AM, Ozan wrote:


C000 EPROM looks correct with code 1626-04 and start address of C000. All the
data bits are exercised too (bitwise or gives 0xff), only remaining
unexercised connection is the address bus. Perhaps one of the address lines is
disconnected and default address logic state maps the header correctly. I
recommend checking connectivity between each address pin of U2378 and one of
the working EPROMs.
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.


Re: 7603 CRT tick

Neil Barlow
 

Mark,

Thanks. Your input is appreciated.

Neil

2841 - 2860 of 195635