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Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ed Breya
 

Yes Reinhard, if the bad caps aren't among the "easy" (to access) ones, you have to dig deeper, and possibly disassemble some things to get to them. Then having an idea of which supply and which sub-assembly is involved definitely helps to narrow it down, especially for the bigger, more complicated plug-ins. Murphy's Law would dictate that the bad caps will be the most inaccessible, which happens sometimes, but on single-wide ones, especially verticals, there aren't all that many Ta caps to begin with, and not many that are too hard to access. Horizontals tend to have more layers of stuff, but often can be quick-tested with simple disassembly if needed, or accessing from the back (board layout info from the manual helps here too) or other accessible points on the same nodes.

The quick test can actually be even quicker than I described, in cases where you know that all the caps are simply bypassing to ground, and not between supplies. I think this is almost always the case with 7K plug-ins. Instead of probing caps with two leads for the ohmmeter, one lead can be grounded, and the other one used to make two measurements - one from each cap lead. One will show ground continuity, and the other should be a reasonably high resistance, unless the cap or the line it's on is dumped to ground. You don't care which is which, only that one lead is ground, and the other far from it.

Ed


Re: TEK 465 S/N>25000 Power Supply Issue

Ozan
 

Hi Craig,
These voltages look OK for the given pin 7 voltage and the current through R1549 is very small at 7.2mA. All of this current has to come from U1524B to sink 22V supply. My best guess is collector supply of Q1544/Q1546 is not there. There is no value marked but should be above 17V.

Ozan


Oran,

Thanks for starting this discussion.
Here are the values you were asking about.
When everything is connected, what voltages do you see:
At 15V TP. 1.675vdc
At the emitter of Q1546. 1.683vdc
At the base of Q1546. 2.223vdc
pin 7 of U1524B 2.988vdc
base of Q1548B. 2.084vdc
Voltage across R1549 (this tells us how much current is pulled from Q1546).
0.008vdc

We are looking for a possible high current out of U1524B pin 7 (~ 10mA).

Craig


Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

n49ex
 

I agree in general, except in this case, given how densely packed the 7B92A is, it may be easier to know up front which power supply is the offender. And having to trace that power net through the unit using the schematic is a good learning experience anyway👿. And I would not be surprised if it is the entry filter cap, close to the connector.

Reinhard


Re: TEK 465 S/N>25000 Power Supply Issue

Craig Cramb
 

Oran,

Thanks for starting this discussion.
Here are the values you were asking about.
When everything is connected, what voltages do you see:
At 15V TP. 1.675vdc
At the emitter of Q1546. 1.683vdc
At the base of Q1546. 2.223vdc
pin 7 of U1524B 2.988vdc
base of Q1548B. 2.084vdc
Voltage across R1549 (this tells us how much current is pulled from Q1546). 0.008vdc

We are looking for a possible high current out of U1524B pin 7 (~ 10mA).

Craig


Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Tom Lee
 

The 7B92A (and NOT the 7B92) was indeed a masterpiece, brought to us by the eminent Bruce Hofer. He broke with the longstanding Tek tradition of a Miller integrator for generating the sweep and thereby solved many tough problems in one fell swoop. The non-A version marks the end of the old way of doing things. I thought my non-A timebases were all out of whack, but after studying the circuits, I realized that they were working as well as they could. It’s a great tutorial example of the limits of a Miller integrator.

Tom

Sent from my iThing, so please forgive typos and brevity.

On Feb 7, 2021, at 1:59 PM, n49ex via groups.io <n49ex=aol.com@groups.io> wrote:

No sense hunting around all over. I would suggest go to the 7B92's connector in the back and directly check the power pins to ground. They are on the A connector side: +50 net pin 19, +15 net pin 18 (and it's most likely short C972), +5 lights pin 9, +5 pin 8; on the B side pin 18 is -15 (with it's most likely C976). Once you identify the shorted supply there (something in the few ohms or less region), THEN start chasing it down on that power net. Schematic and full maintenance manual is on TEkWiki http://w140.com/smb/7b92a_sm.pdf

I've always thought the 7B92A was an absolute design masterpiece when you think about how much functionality they crammed into this space (for the time), and the astonishing analog triggering performance. One of my all time favorite Tek accomplishments, right up there with the (7904) distributed delay line CRT deflection system - and the roadrunner meep-meep cartoon on the first edition manual schematic of the vertical output!

Reinhard





Re: TEK 465 S/N>25000 Power Supply Issue

Ozan
 

Hello,
I don't have a 465 but I briefly looked at the power supply schematic on sheet <12>.

Started trying to determine the possible issue and determined that I had no
+22vdc at U1524A&B input.
If this is the +22V at pin 8, it is needed for both 55V and 15V regulators.

So continued on going thru the +55 regulator and
determined that if I left pin 1 out of the U1524 base socket the +22 vdc would
show up.
This would disconnect +55V regulator feedback loop, without the feedback loop I expect +55V to rise to an out of spec voltage (not safe). I don't recommend disconnecting pin 1. I can't easily explain why +22V would show up, may be R1524 now supplies more current (see below).

So then re-socketed the pin 1 along with all others and it would drop
+22 again to 2.26Vdc. So I moved to the B section of chip U1524 and removed
pin 7 with all the others in the socket the +55 V would go to 54.9vdc.
What was the voltage at +22V when you disconnected pin 7? If it recovered to +22V you have either a short in +15V supply or a bad transistor Q1548. This is the theory of what is happening: If output of U1524B is pulled to ground it takes current from its supply (pin 8) which comes from +22V. This voltage is generated by R1524/VR1526 shunt regulator. It can't supply current above (55V-22V)/4.7k=7mA without going out of regulation.

Traced it down to the Q1544 transistor when it was removed the U1524 amplifier voltages
would come up to as the markings in the manual for voltages except for the
lead 6 of the U1524B.
Removing Q1544 cuts the current to +15V and also turns off Q1548. The issue still points to either a shorted +15V or bad Q1548. Lead 6 of U1524B is +15V supply, without Q1544 +15V can't regulate so it is normal to have lead 6 ~ 0V in this condition.

Swapped it out the Q1544 with no change. So now if I
just disconnect the base of transistor Q1544 +55 and voltage readings around
the U1524 a&b are as needed along with the +55.
I assume in this state +15V TP is ~ 0V

Thinking that the Q1546 is
causing the issue. As I have basically gone thru and checked all the
resistors, capacitors and diodes involved in the circuit down to the TP1548
+15VDC. Including the Q1546 and don't see any issue.
I also think you are looking at the correct area.

When everything is connected, what voltages do you see:
At 15V TP
At the emitter of Q1546
At the base of Q1546
pin 7 of U1524B
base of Q1548B
Voltage across R1549 (this tells us how much current is pulled from Q1546).

We are looking for a possible high current out of U1524B pin 7 (~ 10mA).

Ozan


Re: 576 serials

Mlynch001
 

Occasionally, the S/N is also written with black marker on the inside of the case or on one of the circuit boards.
--
Michael Lynch
Dardanelle, AR


Re: Spectrum analyzer Tektronix 7L13 on mainframe Tektronix 7603

Ed Breya
 

Hi Attilio,

Yes, keep your new crystal (52.5 MHz) in place from now on. For the first test, you want the setup you already were using at some point before, with the counter in high band, looking at the IF output, where it previously showed the LO frequency, and with the LO running from +15 V, which was the limit of your external supply, as I understand. This should show something around 2095 MHz on the counter, as before. The PLL should be running open loop without any IF feedback, which is what you want. Then you want to confirm that the PLL tune output voltage can rise and stay at somewhere around +17 V, which is about where I think it should go when open-loop. But, if the search oscillator kicks in, then it will need to be disabled. If all goes well, the tune voltage will go up and stay near 17 V.

Presuming the first test works as planned, the next step is to remove the external supply, and reconnect the internal tune voltage via P446, so now the LO is running near 17 V, and should be at a higher frequency, which should show on the counter. That tells you how high it can go without additional modifications, and it's hopefully well over the 2095 MHz result at 15 V. If it is, then there's a very good chance that when you close the PLL loop, it will run the LO up to the desired 2095 MHz and lock, and the tune voltage should end up somewhere near 15 V (where you got about 2095 MHz before), and be stable.

The last test finally closes the loop by restoring everything to normal - putting the 2LO in from the 7L13, connecting the IF out to the PLL, and enabling the search oscillator. Now you're looking for a stable tune voltage around that 15 V, which would show that the PLL is in control, and the LO should be locked at 2095 MHz. The counter isn't necessary, but you can measure the IF if you want, with the scope probe and counter setup as you did before, tapping into the IF amplifier signal, but be sure to not interrupt the IF to the PLL.The IF should be at the new 105 MHz, or at 52.5 MHz after the divide by two circuit. Now everything should be running about right, if the 2LO BPF isn't so far out of adjustment that there's not enough signal to get a good IF level.

In reality, you could skip the first test, but it's good to start from a stable point you know, where you had it set up and measuring the LO frequency. Also, if the tune voltage driver does not operate as I expect, then that issue would need to be addressed before the next steps can work..Good luck.

Ed


Re: 576 serials

cnc_joker
 

Thanks, I can see where the sticker was, only the stickum is left.


Re: 576 serials

Michael W. Lynch
 

On Sun, Feb 7, 2021 at 08:54 AM, cnc_joker wrote:


i also have a 176 plug in, does that have
a serial number? I could not find one.
There should be a standard TEKTRONIX Serial # label on the 176. The one that I saw had this decal on the center of the lower front panel. See Tekwiki140 for a picture. Perhaps it has been removed?

--
Michael Lynch
Dardanelle, AR


Re: 549 transformer question

Roger M
 

Jean-Paul,
In your experience, can you tell us of any "newer" impregnation/sealing agents that have
good mechanical stability, do not alter coil Q, and also have very low permeability to moisture?
I might add that long shelf life is a plus.

Many Thanks,
-Roger


Re: 576 serials

Michael W. Lynch
 

On Sun, Feb 7, 2021 at 10:26 AM, Glydeck wrote:


Is there any other meta data we should submit? HV transformer type, etc. A
simple list of SN#s doesn’t seem particularly useful.
Certainly a good suggestion. I am documenting whatever anyone shares at this point. Some are sending IC date codes, others are sending CRT information. I would certainly appreciate any other suggestions of data that folks would like to see gathered. If anyone wants to add or change information regarding their instrument, just reference your S/N and add the new data, I will sort it out.

Thanks!



--
Michael Lynch
Dardanelle, AR


Re: Spectrum analyzer Tektronix 7L13 on mainframe Tektronix 7603

Attilio
 

Many thanks Ed,
I have read everything you wrote to me, I only have a couple of questions: when I reconnect P446 and the PLL commands the oscillator at 2072 MHz, the frequency meter where I connect it, considering that if I have to make the PLL work the IF output connector will be connected to the 110 MHz amplifier input ?
I could connect the frequency meter to the 110 MHz amplifier input.
When I connect the 2nd LO of the 7L13 (SA) to the 2nd LO of the TR503, on the oscilloscope should I see a line positioned at about + 15 Vdc?
Now I have the 52.5 MHz crystal on the PLL reference oscillator, should I leave it or put the 55 MHz one back on? I think the 52.5 MHz one should be left.

Tomorrow I'll try to carry out all the tests, then I'll tell you.

Thanks again.
-- Cheers
Attilio


TEK 465 S/N>25000 Power Supply Issue

Craig Cramb
 

Got a 465 that was somewhat functional but no trace with blown F1419. Went thru Groupsio file library and replaced the C1419 and that got the signal trace. Reviewed the Power supply and here are the original results.
-8 =7.79vdc, 265mvAC
+15 =13.13vdc, 2.26 mvAC
+55 = 53.7vdc, 1.8 VAC
+5 =4.88vdc, 156 mvAC
+110= 110.9 vdc 1.8 VAC

Decided to recap the unit with all input PS filter caps. Got this completed and then I had this result.
-8= .585vdc .4mvAC
+15=1.6vdc .3mvAC
+55=46vdc .7mvAC
+5= .788vdc .3mvAC
+110= 107.3 vdc , 6.2mvAC
Replaced the CR1541

Started trying to determine the possible issue and determined that I had no +22vdc at U1524A&B input. So continued on going thru the +55 regulator and determined that if I left pin 1 out of the U1524 base socket the +22 vdc would show up. So then re-socketed the pin 1 along with all others and it would drop +22 again to 2.26Vdc. So I moved to the B section of chip U1524 and removed pin 7 with all the others in the socket the +55 V would go to 54.9vdc. At this point decided to move onto the +15 section of the regulated supply. Went thru and disconnected jumpers, and plug connectors checked components as per the troubleshooting of the +15,+5,-8 in the manual. Rechecked with power applied and the voltages all went back to the low value readings. Traced it down to the Q1544 transistor when it was removed the U1524 amplifier voltages would come up to as the markings in the manual for voltages except for the lead 6 of the U1524B. Swapped it out the Q1544 with no change. So now if I just disconnect the base of transistor Q1544 +55 and voltage readings around the U1524 a&b are as needed along with the +55. Thinking that the Q1546 is causing the issue. As I have basically gone thru and checked all the resistors, capacitors and diodes involved in the circuit down to the TP1548 +15VDC. Including the Q1546 and don't see any issue.

Any thought are welcomed on this issue.
Craig


Re: 7L13 parts needed

Joel B Walker
 

I guess these are rarer than I thought. Surely there are some somewhere.............................


Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Ed Breya
 

Reinhard said: "No sense hunting around all over. I would suggest go to the 7B92's connector in the back and directly check the power pins to ground."

I believe this is an alternative to my suggested method. You can indeed go through all the steps methodically, but to me, it's a lot easier to quickly check all the Ta caps in a plug-in, than it is look through the manual, determine which supply has the problem, identify the parts, and trace the power buses through the actual circuitry. I've been there and done that - it's a PITA, which is why I came up with my random direct search method. You know in advance what you're looking for, and it's easy to identify and poke around and check dozens of caps in less time (at least for me) than it takes to search around the manual and figure out where to start.After you've found the culprits, then you go to the manual and figure out what's what for replacement parts. You don't even need the schematics, and you don't have to care about which supply is affected - it's all about simply and quickly finding the bad caps, which are mostly right in front of your eyes, only an ohmmeter check away.

Try it both ways and see.

Ed


Re: Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe

Jim Ford
 

I second the +/-15 V recommendation.   Had an S-4 sampling head with -15 V shorted by a bad Ta cap, and it brought the mainframe to its knees before I found and replaced it.  I'd recommend at least a 35 V rated cap for replacement.   Good luck!         Jim Ford                                        Sent from my Verizon, Samsung Galaxy smartphone

-------- Original message --------From: "Ed Breya via groups.io" <edbreya=yahoo.com@groups.io> Date: 2/7/21 11:41 AM (GMT-08:00) To: TekScopes@groups.io Subject: Re: [TekScopes] Tektronix 7b92a shuts down crt & graticule illumination on 7904 mainframe For 7K plug-ins, you can do it on the bench, just the plug-in alone, opened up for access. Probe around with an ohmmeter and check across each Ta cap for short or very low R, and mark any that show this. Also check for signs of overheating of series Rs and Ls in the power supply buses. Don't worry about which supplies and caps are which, or that you can't necessarily see or access every Ta in there. Go for the easy stuff first. After you've marked any bad ones, lift one lead - un-solder or clip close to the board (leaving some lead to tack it back) - and check the part itself, and at the board pads. Often, more than one part spot will show bad because there may be several caps on the same bus, and any one bad will cause all of the spots to show the same. Then you just figure out which ones are actually shorted and remove. Don't replace any until the plug-in can fire up and operate without crashing the mainframe. If you can't find any easy ones to blame, you'll have to dig deeper and look for more. In my experience, shorted Ta caps in plug-ins are almost always on the +/- 15 V supplies - that's where they're used the most. Also, don't be surprised if another cap shorts after fixing. Sometimes they chain react, where eliminating the shorted one allows the supplies to return, and the next weakest one pops, and so on. I remember once in an EIP counter, I had about half a dozen or more Ta caps on the +5 V supply light up and burn, one after another - after clipping the burnt one, the next would go within a minute or so.Ed


Re: Spectrum analyzer Tektronix 7L13 on mainframe Tektronix 7603

Ed Breya
 

OK, Attilio, here you go:

I looked at the LO drive circuits and circuit descriptions and calibration info in the TR503 and TR502 manuals. I think you've been hung up on this 2LO BPF adjustment issue, where you're manually driving the LO with an external supply. Forget about that for now. You don't need to redesign the circuits to get higher tune voltage - it's already capable of reaching around 17 V, maybe a little more, which hopefully is plenty enough. At the plus extreme, U365A's output should rail at somewhere around +18 V or so (you should look up the NE5558 specs to see just how high it can get), since it's powered from +20 V. Q420 and the current limiter should drop about a volt, so the output will be able to get to around +17 V, and intrinsically limited there. Q445 and Q430 form a search oscillator to make sure the tune voltage sweeps around until phase lock is achieved, but apparently only kicks in if the tune voltage drops below +3 V. In the BPF tuning procedure with external supply drive, it says something like "do not exceed 20 V," so the capability of the circuit is within the limit.

In your test setup, you already have the PLL open-loop since you've run the IF into the counter, and the PLL tune voltage is disconnected. So, using your current setup, with the LO powered from the +15 V external supply, and measuring about 2095 MHz on the counter, check the PLL output tune signal with a scope - it should be either stuck high near 17 V, or sweeping up and down between there and some low value around a couple volts.

If it's stuck near 17 V, then that's perfect, just what you want for the next step. Now shut everything down and disconnect the external supply, and reconnect P446 so the PLL will run the LO. Fire it up and observe the frequency on the counter - it should be stable and well above 2095 MHz. Continue to monitor the tune voltage on the scope, and make sure it's still up around 17 V, not sweeping. If it is sweeping, then disable the search oscillator by grounding Q445's collector. You should now see the maximum tuning frequency available, on the counter. Hopefully, it will have a comfortable margin, maybe 2100 MHz or higher. If so, then shut down again, and hook everything back up to normal, and connect the 2LO from the 7L13. Continue to monitor the tune voltage. Fire it all up, and see if the PLL locks, indicated by the tune signal being constant, somewhere near +15 V. If it is, then you've lucked out, and the 2LO BPF is close enough to get a good signal through, the IF and PLL are working, and the LO is running around 2095 MHz. The rest is then a matter of perfecting the changes and tweaking it up. At this point it would be good to shut down, and solder an 18 V, 1 W, 5% Zener diode onto the LO's power supply/tune connection, from there to ground, to serve as an absolute clamp around +18 V. It will also provide reverse voltage protection. The physical location can be at the module (best) or wherever convenient, as long as it's on that line, and can't be disconnected accidentally. This will give some protection against accidents during any situations where external driving is used - later during tweaking - or if a failure in the tune driver (like if Q440 were to short) lets it go over-voltage.

Whether this all works, or doesn't, please report the results, and we'll see what to do next. Good luck.

Ed


Re: Help for DC503A universal counter repair

Attilio
 

Hi everyone,
I have isolated the "Blanking" output from U1520 (pin 11), obviously the non-significant zeroes remain on (visible on the display), but in this way the frequency meter works well and the display remains on. But there is a second problem, FREQUENCY, all AVG functions and TIME MANUAL work OK while instead the PERIOD B, WIDTH B and TIME A - B functions do not work.

Do you have any advice for the repair.

Thanks for the attention.
-- Cheers
Attilio


Re: 576 serials

Eric
 

Mine was a change from -02 to -03. The transformer cot warm over current
and the power supply collapsed do you know if I register change is
nessessary from 02 to 03?

On Sun, Feb 7, 2021, 5:17 PM Dave Brown <davebr@modularsynthesis.com> wrote:

The 576 mod summary shows the new 120-0612-03 transformer was implemented
for serial number greater than B089999. It required two resistors to be
changed. The field mod notice is dated June of 1983.





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