7B92 with no repeating sweep


 

After freeing the EXT÷10 trigger source switch, replacing the broken rear plate (with one from a poorly treated 7B53A), cleaning the trigger, readout, and sweep boards with IPA to try to remove some kind of white residue (with limited success), and generally giving the entire instrument a close visual inspection, I decided that it looked good enough to try it in my 7633. I did verify that the white residue was not shorting anything out, though I have not figured out what the residue is or where it came from.

The good news is that it powers up and lights most of the lights. The bad news is that I can't get a repetitive sweep or trigger. I can get one sweep to happen when I switch the TIME/DIV knob between different positions, but that's it. I've tried in AUTO, NORM, HF SYNC and SINGLE SWEEP modes, with AC and DC coupling, using INT trigger source as well as LINE. Nothing gives me a repeating sweep. In SINGLE SWEEP mode the RESET button has no apparent effect.

I've read through the circuit description in the service manual, and did not find it particularly enlightening. I'm in the process of reading the Circuit Description Supplement, which is much more detailed (if not more enlightening for me). I'm tempted to just start testing circuit continuity and diode drops across diodes and transistors, in the hope of stumbling on the culprit (or eliminating a bunch of possible culprits), but I'd like to know if anyone has any wisdom or experience that might guide me. My plan is just to work my way through the internal trigger path and hope that I find a dead transistor or diode (regular diode, not a tunnel diode, which I don't understand how to check with a multimeter, though from reading old posts in this group, it sounds like I basically can't check tunnel diodes with a multimeter).

I would be very happy (and unsurprised) to discover that I have been setting the controls incorrectly (it's certainly happened before), but I have a feeling that's not what's going on here.

I'd really like to be able to get this 7B92 working, both because I'd like to have ALT horizontal mode on my 7603 and 7633, and because I just like the lighted delayed trigger controls.

-- Jeff Dutky


Dave Peterson
 

Good morning Jeff,

I'm chiming in because I've had somewhat similar experience with a 465 which suffered problems with the socketing of the https://w140.com/tekwiki/wiki/155-0049-00 sweep control IC. I found that reseating brought the sweep back - usually. Meaning that the chip was ok and it was the classic old Tek socket issue. I ordered some backups from the auction site and I recall the sellers listing saying that they were recovered from a 7B92. I see in the TekWiki doc that U310 is either a 155-0049-00 or 155-0049-01 based on SN. So your time base has that and is likely socketed.

Also I found https://w140.com/tekwiki/images/5/5d/068-0315-00.pdf very helpful in understanding sweep circuits and diagnosis.

Good luck with it!


 

Dave,

I should have thought of that! I'd been reseating transistors on the trigger board, but had not reseated any of the ICs.

It turns out, however, that it was NOT the sweep control IC, but a DM7474N (dual D-type FF) nearby on the sweep board. The 7474 had attracted my attention because it is in a full height socket, rather than the per-pin in-PCB sockets that Tek uses almost everywhere else, but I had not investigated it yet.

Not only are the legs of the FF black with tarnish, but the Vcc pin is broken off in the socket. That's bound to cause some problems.

I've got some 7474 variants lying around (74S74, 74LS74, and 74HC74), and I'm going through the data sheets to make sure that the substitution will work. I'll need to replace the socket, which makes this repair a bit more work than it would otherwise have been, but not too bad since the sweep board comes out easily. It's certainly better than what I feared: dead tunnel diodes. The tunnel diodes in this instrument are half soldered in, and the leads are really short, so I had no confidence in my ability to replace them without damaging the replacement with excess heat, even if I could find a suitable replacement.

-- Jeff Dutky


Roger Evans
 

Jeff,

74LS74 should drop in with no problem. 74HC does not strictly match 74LS logic levels but you can try it with no fear of causing harm. 74HCT is the proper CMOS match for LS logic levels.

Out of more scopes and plugins than I should admit to buying I have only once needed to replace tunnel diodes, all four of them in a 454. The most dangerous thing you can do is test them 'just in case' with your DMM on diode test.

Just my experience, but I have also experimented with several Russian tunnel diodes in simple pulsers and never destroyed one (yet!).

Roger


 

Roger,

I used the 74S74 that I had lying around, as I found a data sheet from TI that listed the 7474, 74S74, and 74LS74 together, and it was clear from the detailed specs that they were basically interchangeable, though the 74S74 looks like it can operate at a lot higher frequency than the 7474 or 74LS74.

Thanks for the warning about testing tunnel diodes. I was already wary of them from other messages in this group, but I'd really like to understand a) why testing tunnel diodes with a DMM can harm the diode, and b) what the correct procedure for testing tunnel diodes (in circuit) should be. If I need to test them on a curve tracer I can arrange that, but I have questions about how this would be done when the diodes are soldered in rather than socketed.

I have not had to replace any tunnel diodes yet, and I've got a dozen different instruments with tunnel diodes in them (there may be three or four where I have not really evaluated the operation of the instrument enough to know if the tunnel diodes are good or bad), so my experience coincides with yours.

-- Jeff Dutky


 

I replaced the damaged 7474, but that did not fix the problem. It did introduce a (possibly) new symptom: when I engage or disengage the RUNS AFTER DLY TIME/DLY'D SWP TRIGGERABLE control I now get a single sweep. I also occasionally get a single sweep when disengaging the ALT horizontal mode control. I don't think it was doing this before I replaced the 7474.

I see that the RUNS AFTER DLY TIME/DLY'D SWP TRIGGERABLE switch connects to the base of Q908 on the delayed trigger board (A9), which connects to the base of Q910, which leads me to either U880 (a 741 op amp) or, tragically, CR920 (one of three tunnel diodes on the delayed trigger board). Both the op amp and the tunnel diode lead me eventually back to the sweep board through Q192, Q194, and Q184. The circuits surrounding those transistors on the sweep board are things I can check fairly easily.

-- Jeff Dutky


Tom Lee
 

Jeff,

A quick note about 74-series TTL:

74xx: The first of the family
74Sxx: These use Schottky diodes to prevent transistors going into hard saturation. That speeds them up considerably.
74LSxx: These also use Schottkys. The speedup allows a dialing back of currents reduce power consumption while retaining (actually, exceeding) the speed of the original 74xx.

As for TD testing, modern DMMs are unlikely to kill a TD. Those force peak currents in the range of a handful of mA (if anyone knows of an exception, please let me know), and you won't kill any tek TD with that low a current. Some older -- analog -- ohmmeters can push considerably more current than that on the low-resistance scales, though, and those will kill TDs (and other components as well). Examples include VTVMs.

All that said, you don't learn too much about the health of a TD from an ohmmeter measurement. A TD looks like a short under the conditions imposed by most DMMs, so you can only identify one fault (an open) unambiguously.

If you don't have a curve tracer, you can manually curve-trace with a battery, resistor in series with a pot, and a scope.

I used to test TDs with a simple LC circuit that was designed to oscillate at AM radio frequencies. That's adequate for a basic go-no go test.

Tom

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/16/2021 15:55, Jeff Dutky wrote:
Roger,

I used the 74S74 that I had lying around, as I found a data sheet from TI that listed the 7474, 74S74, and 74LS74 together, and it was clear from the detailed specs that they were basically interchangeable, though the 74S74 looks like it can operate at a lot higher frequency than the 7474 or 74LS74.

Thanks for the warning about testing tunnel diodes. I was already wary of them from other messages in this group, but I'd really like to understand a) why testing tunnel diodes with a DMM can harm the diode, and b) what the correct procedure for testing tunnel diodes (in circuit) should be. If I need to test them on a curve tracer I can arrange that, but I have questions about how this would be done when the diodes are soldered in rather than socketed.

I have not had to replace any tunnel diodes yet, and I've got a dozen different instruments with tunnel diodes in them (there may be three or four where I have not really evaluated the operation of the instrument enough to know if the tunnel diodes are good or bad), so my experience coincides with yours.

-- Jeff Dutky




Tom Lee
 

TD testing addendum: If you manually curve-trace, don't expect to see the negative resistance region. The circuit will be oscillating there. But that itself is a sign of a functioning TD.

--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu


Harvey White
 

A picky addition about 7400 series logic....

Each of the series, xx, Lxx, LSxx, Sxx and so on have different internal resistors.  There's also an H series as well.  What the designers did was to allow the chip to draw more current, allowed internal (and external) capacitances to charge faster, speeding it up.  The shottky diodes were placed so that the transistors did not go into saturation.  Getting a transistor out of saturation takes time, keeping the base drive low (by stealing it, see where they put the diode), keeps the transistor from saturating, and lets it run faster.

Because of the different internal resistor values, the various inputs load the circuit connected to differently on a per-family basis.  Normally, this may not make a difference, but if someone were messing with playing right at the edge of a design, then it could make a difference.  Ditto if someone tried to pull any number of tricks on a TTL input to make it behave differently.

So the bottom line is:  Substitute if you want, it may or may not make a difference, but don't automatically assume that *in all cases* (note emphasis here), it'll work.  There are bound to be exceptions.

To further complicate things, 74Lxx series logic is not pin compatible with 74xx series.  But 74S (I think), 74LS, 74xx, and the like are pin compatible.  I don't remember about 74H series (standard TTL with lower resistance values, like the 74L series has higher resistances.

Won't go into some of the more sordid details, but as mentioned, 74xx and 74LSxx series are perhaps the most compatible.

fun, right?

Harvey

On 10/16/2021 7:18 PM, Tom Lee wrote:
Jeff,

A quick note about 74-series TTL:

74xx: The first of the family
74Sxx: These use Schottky diodes to prevent transistors going into hard saturation. That speeds them up considerably.
74LSxx: These also use Schottkys. The speedup allows a dialing back of currents reduce power consumption while retaining (actually, exceeding) the speed of the original 74xx.

As for TD testing, modern DMMs are unlikely to kill a TD. Those force peak currents in the range of a handful of mA (if anyone knows of an exception, please let me know), and you won't kill any tek TD with that low a current. Some older -- analog -- ohmmeters can push considerably more current than that on the low-resistance scales, though, and those will kill TDs (and other components as well). Examples include VTVMs.

All that said, you don't learn too much about the health of a TD from an ohmmeter measurement. A TD looks like a short under the conditions imposed by most DMMs, so you can only identify one fault (an open) unambiguously.

If you don't have a curve tracer, you can manually curve-trace with a battery, resistor in series with a pot, and a scope.

I used to test TDs with a simple LC circuit that was designed to oscillate at AM radio frequencies. That's adequate for a basic go-no go test.

Tom


 

Tom,

Thanks for both the rundown on the 74-series TTL parts, and for the more detailed discussion of TD testing (and the dangers therein). That's exactly what I was looking for.

I do have a curve tracer (a 7CT1N) that may need some calibration, but is otherwise working. Would I be able to test a TD in-circuit with a curve tracer? Would I be able to test the TD with a pair of multimeter leads (several feet long)?

I have a bunch of Russian TDs, some of which I bought just to play with. I should really throw one of those on the curve tracer.

All of the TDs that I've examined in the 7B92 are soldered in by at least one lead, and I have no confidence in my ability to unsolder them without overheating them (the lead lengths are too short to grab with anything larger than a pair of dissection tweezers, which I doubt will act as a very effective heat sink). I'm still hoping that something less exotic than a TD is the faulty part.

-- Jeff Dutky


 

Not that I have any 74Lxx chips (that I know of), but I looked at the data sheet for the 74L74 and it looks like it's almost identical to the 7474/74S74/74LS74 (identical pin layout and power specs, but much slower set up and hold times).

The 74Lxx series are low-power TTL? Why would they need a different pin layout than 47xx?

I did notice that the 54xx DIPs have a the power and ground pins in the middle rather than at the corners, which moves everything else around as well. I'm pretty sure I don't have any 54xx chips either.

-- Jeff Dutky


 

I wrote:
Why would they need a different pin layout than the 47xx?
I meant "a different pin layout than the 74xx" of course.

-- Jeff Dutky


Harvey White
 

Ok, I know where that came from.  SOME H series (74Hxx) chips, where available, have different pinouts.  Some chips don't have the standard edge power pins (7 and 14, for instance) but instead have them centrally located, 4 and 11 were the locations (the 7490 series counters have them there).  The reason for that location was that it was easier to bypass power and ground and eliminate spikes.

So you're quite right about the L series (a quick look at an old databook confirmed that).  The H series?  I think that the 74H01 was different, but the 74H00 was identical.  Just to make things different, some of the flatpack chips had the same odd power and ground pins.

I always thought that the 5400 series were the same as the 7400 except for the temperature range.  That doesn't stop them from having a different package and pinout, though.  I just normally might swap 54xx dips with 74xx dips, not that I had a lot of 54xx anythings.

Interesting.

Thanks

Harvey

On 10/16/2021 9:46 PM, Jeff Dutky wrote:
Not that I have any 74Lxx chips (that I know of), but I looked at the data sheet for the 74L74 and it looks like it's almost identical to the 7474/74S74/74LS74 (identical pin layout and power specs, but much slower set up and hold times).

The 74Lxx series are low-power TTL? Why would they need a different pin layout than 47xx?

I did notice that the 54xx DIPs have a the power and ground pins in the middle rather than at the corners, which moves everything else around as well. I'm pretty sure I don't have any 54xx chips either.

-- Jeff Dutky





Tom Lee
 

Jeff,

Testing TDs pretty much requires that you free one terminal. This requirement isn't unique to TDs, of course. Any time you have a bunch of stuff in shunt with any DUT, you're going to have difficulty sorting out what causes what. You'll be highly limited in what inferences you can reliably draw from measurements that involve more than the DUT's behavior.

Long leads will just lower the resonant frequency of the parasitic tank you'll be creating. That shouldn't materially affect the characteristic curve in the positive-resistance parts of the trace.

Btw, you may wish to take a look at this: https://w140.com/tekwiki/images/b/bb/Service_Scope_49_Apr_1968.pdf
Although it specifically uses the 454 as a sawtooth source and display, the basic setup is useful more generally. It's a simple way to perform a quick test of the TD's trigger point. You can get the sawtooth from a battery and wrist action on a pot (use a fixed resistor in series with the pot). And you have scopes aplenty.

If the risk of damaging the diode in the act of removing it is high, you might be forced to spend more troubleshooting effort proving to yourself that the problem doesn't reside anywhere else. I don't have a large database of TD failures from which to draw statistically significant insights. All I can say is that I've encountered few really dead ones, so I tend to look elsewhere first. And then second.

Cheers
Tom


--
Prof. Thomas H. Lee
Allen Ctr., Rm. 205
350 Jane Stanford Way
Stanford University
Stanford, CA 94305-4070
http://www-smirc.stanford.edu

On 10/16/2021 18:17, Jeff Dutky wrote:
Tom,

Thanks for both the rundown on the 74-series TTL parts, and for the more detailed discussion of TD testing (and the dangers therein). That's exactly what I was looking for.

I do have a curve tracer (a 7CT1N) that may need some calibration, but is otherwise working. Would I be able to test a TD in-circuit with a curve tracer? Would I be able to test the TD with a pair of multimeter leads (several feet long)?

I have a bunch of Russian TDs, some of which I bought just to play with. I should really throw one of those on the curve tracer.

All of the TDs that I've examined in the 7B92 are soldered in by at least one lead, and I have no confidence in my ability to unsolder them without overheating them (the lead lengths are too short to grab with anything larger than a pair of dissection tweezers, which I doubt will act as a very effective heat sink). I'm still hoping that something less exotic than a TD is the faulty part.

-- Jeff Dutky




 

Tom,

Thanks for the further explanation, and the reading material (that's a great issue of Service Scope, I also love the discussion of CRT resolution).

I've gotten a much closer look at the main trigger board now, and I checked most of the transistors on the board (they all seem to be good). all the TDs on the front of the trigger board have one leg in a socket, so I can try those on the 7CT1N using grabber hook leads, but, as you say, I am going to try to exhaust every other possible fault before I finger the TDs. I have a fear that U660 (155-0061-02, trigger amplifier) might be bad, as it is the earliest point at which all three trigger inputs come together (and I have verified that trigger source makes no difference in the symptom I'm seeing).

There's also the suggestively named "sweep disable" signal coming from the sweep control IC (155-0049-01), through Q755 and Q754, then down through CR752 to drive the cathode of CR740 (one of the tunnel diodes). That sounds like it might be a fruitful avenue of investigation.

At some point I'm just going to have to put this plug-in on an extender and look at live voltages, but I'm trying to postpone that as long as possible.

-- Jeff Dutky


Dave Daniel
 

Add to that the fact that National sometimes shipped 74Sxx parts marked as 74LSxx parts because they didn’t worry that they were shipping parts that were too fast. This became apparent to me in the late 70s when a crystal oscillator design that used 74LS14 inverters failed to oscillate because National sent us 74S14 parts marked as 74LS14 parts.

DaveD

On Oct 16, 2021, at 20:09, Harvey White <madyn@dragonworks.info> wrote:

A picky addition about 7400 series logic....

Each of the series, xx, Lxx, LSxx, Sxx and so on have different internal resistors. There's also an H series as well. What the designers did was to allow the chip to draw more current, allowed internal (and external) capacitances to charge faster, speeding it up. The shottky diodes were placed so that the transistors did not go into saturation. Getting a transistor out of saturation takes time, keeping the base drive low (by stealing it, see where they put the diode), keeps the transistor from saturating, and lets it run faster.

Because of the different internal resistor values, the various inputs load the circuit connected to differently on a per-family basis. Normally, this may not make a difference, but if someone were messing with playing right at the edge of a design, then it could make a difference. Ditto if someone tried to pull any number of tricks on a TTL input to make it behave differently.

So the bottom line is: Substitute if you want, it may or may not make a difference, but don't automatically assume that *in all cases* (note emphasis here), it'll work. There are bound to be exceptions.

To further complicate things, 74Lxx series logic is not pin compatible with 74xx series. But 74S (I think), 74LS, 74xx, and the like are pin compatible. I don't remember about 74H series (standard TTL with lower resistance values, like the 74L series has higher resistances.

Won't go into some of the more sordid details, but as mentioned, 74xx and 74LSxx series are perhaps the most compatible.

fun, right?

Harvey


On 10/16/2021 7:18 PM, Tom Lee wrote:
Jeff,

A quick note about 74-series TTL:

74xx: The first of the family
74Sxx: These use Schottky diodes to prevent transistors going into hard saturation. That speeds them up considerably.
74LSxx: These also use Schottkys. The speedup allows a dialing back of currents reduce power consumption while retaining (actually, exceeding) the speed of the original 74xx.

As for TD testing, modern DMMs are unlikely to kill a TD. Those force peak currents in the range of a handful of mA (if anyone knows of an exception, please let me know), and you won't kill any tek TD with that low a current. Some older -- analog -- ohmmeters can push considerably more current than that on the low-resistance scales, though, and those will kill TDs (and other components as well). Examples include VTVMs.

All that said, you don't learn too much about the health of a TD from an ohmmeter measurement. A TD looks like a short under the conditions imposed by most DMMs, so you can only identify one fault (an open) unambiguously.

If you don't have a curve tracer, you can manually curve-trace with a battery, resistor in series with a pot, and a scope.

I used to test TDs with a simple LC circuit that was designed to oscillate at AM radio frequencies. That's adequate for a basic go-no go test.

Tom




Dave Peterson
 

Jeff,

Didn't realize you were doing this sans extender. Ouch. That does make this a bit more challenging.


From your description it sounds like this time base is not free-running, and so is an issue with the sweep generator, not the trigger? Not to say the triggering is not working - it may well be - but don't you need the sweep working in "auto" mode first? Or at least?


Dave

On Saturday, October 16, 2021, 11:51:35 PM PDT, Jeff Dutky <jeff.dutky@gmail.com> wrote:

Tom,

Thanks for the further explanation, and the reading material (that's a great issue of Service Scope, I also love the discussion of CRT resolution).

I've gotten a much closer look at the main trigger board now, and I checked most of the transistors on the board (they all seem to be good). all the TDs on the front of the trigger board have one leg in a socket, so I can try those on the 7CT1N using grabber hook leads, but, as you say, I am going to try to exhaust every other possible fault before I finger the TDs. I have a fear that U660 (155-0061-02, trigger amplifier) might be bad, as it is the earliest point at which all three trigger inputs come together (and I have verified that trigger source makes no difference in the symptom I'm seeing).

There's also the suggestively named "sweep disable" signal coming from the sweep control IC (155-0049-01), through Q755 and Q754, then down through CR752 to drive the cathode of CR740 (one of the tunnel diodes). That sounds like it might be a fruitful avenue of investigation.

At some point I'm just going to have to put this plug-in on an extender and look at live voltages, but I'm trying to postpone that as long as possible.

-- Jeff Dutky


Roger Evans
 

Jeff,

Have you spotted the P309 jumper for using the 7B92 in slower mainframes? The manual says it is only important for sweeps faster than 10usec/div.

Thinking this through properly will take me some time but it is worth trying HF sync mode since this uses different sweep logic. It requires a signal around 100MHz or faster which should still be OK for the 7633.

If it works it would give you a quick check on triggering and also a clue as to which parts of the sweep logic are functional.

Roger


n4buq
 

Jeff,

Not sure if you meant you don't have an extender or whether you just haven't employed one for this. I fall into the first category. It's pretty difficult trying to troubleshoot a plugin with it in the bay!

Thanks,
Barry - N4BUQ

----- Original Message -----
From: "Dave Peterson via groups.io" <davidpinsf=yahoo.com@groups.io>
To: "tekscopes" <TekScopes@groups.io>
Sent: Sunday, October 17, 2021 10:12:09 AM
Subject: Re: [TekScopes] 7B92 with no repeating sweep
Jeff,

Didn't realize you were doing this sans extender. Ouch. That does make this a
bit more challenging.


From your description it sounds like this time base is not free-running, and so
is an issue with the sweep generator, not the trigger? Not to say the
triggering is not working - it may well be - but don't you need the sweep
working in "auto" mode first? Or at least?


Dave


On Saturday, October 16, 2021, 11:51:35 PM PDT, Jeff Dutky
<jeff.dutky@gmail.com> wrote:

Tom,

Thanks for the further explanation, and the reading material (that's a great
issue of Service Scope, I also love the discussion of CRT resolution).

I've gotten a much closer look at the main trigger board now, and I checked most
of the transistors on the board (they all seem to be good). all the TDs on the
front of the trigger board have one leg in a socket, so I can try those on the
7CT1N using grabber hook leads, but, as you say, I am going to try to exhaust
every other possible fault before I finger the TDs. I have a fear that U660
(155-0061-02, trigger amplifier) might be bad, as it is the earliest point at
which all three trigger inputs come together (and I have verified that trigger
source makes no difference in the symptom I'm seeing).

There's also the suggestively named "sweep disable" signal coming from the sweep
control IC (155-0049-01), through Q755 and Q754, then down through CR752 to
drive the cathode of CR740 (one of the tunnel diodes). That sounds like it
might be a fruitful avenue of investigation.

At some point I'm just going to have to put this plug-in on an extender and look
at live voltages, but I'm trying to postpone that as long as possible.

-- Jeff Dutky










 

Barry,

I have a rigid extender (third party, bought on eBay), and an overcrowded bench. I meant that I haven’t employed it yet. I also prefer to avoid poking around inside powered equipment as long as possible because I’m a greenhorn and have been known to make things worse through clumsiness and I experience.

— Jeff Dutky