开云体育

Mark

So the HV board that is in the scope now is not the original but rather a "new" board from QService. I had hoped this would be a known good board. Perhaps that is not the case. The original board had a cracked HV transformer. Do you feel strongly that I should start replacing resistors on this "new" board?

I have the luxury of being able to compare resistance values across both of the HV boards that I have albeit this testing is in-circuit and with one of the boards installed into the scope (connected to power supplies, etc.). My measurements are similar between the 2 boards.

My A1 board does have the carbons for R710 and R910. Both test about 2.3k in-circuit. Do you have a 2465A and could test some of the resistances across these resistors? Don't get me wrong. I can start swapping out resistors but would hope there is a decent likelihood of that solving the problem.

This is not what is seen with an IC failure right?

Any point to checking more voltages at the CRT socket?

Rusty

On Fri, Feb 21, 2025 at 01:13 AM, Joe wrote:

You do not have to buy expensive precision parts for that experiment but
select almost exact values from a dozen or so cheap 10%

does anyone make, or sell 10% resistors?
1% resistors are cheap. (like 0.10USD per @ 1/4 watt)
They are used a lot.


--
Roy Thistle

I was going to recommend soldering test wires onto the solder tails of the sockets on the bottom of the board, assuming they were through-hole.? Glad you got it fixed.
BTW, thanks for mentioning this plug-in, as I had the 7B92 on my test equipment wishlist (you do have one of those, right?), but a look on tekwiki for the 7B92A showed it had improved the 7B92's sweep aberrations.? So I've updated my wishlist after the visit to tekwiki (always a pleasure for test equipment addicts like us!) to verify the 7B92A exists.? I'd not heard of it before.
I did have a note in the wishlist that the 7B92 has that HF sync circuit that works better than the direct trigger.? Triggering being a bugaboo of mine on these vintage 1970s scopes, I'm sure I will appreciate the HF sync when I get my hands on a 7B92A.? The nice thing about the plug-ins, as opposed to the scope mainframes, is that they are small enough and cheap enough to slide under the nose of SWMBO (She Who Must Be Obeyed) without detection!
A 7B92A would I'm sure be great in my 7904 or 7104.? Although I really want a 7B10 and a 7B15 and a couple of 7A29s or 7A29Ps for the 7104.? Of course, getting one or two (or more!) 7A29P(s) would really lead to a 7854 or a 7912HB, and down the rabbit hole we go!
The 5CT1N is also on my wishlist, as I have a a 5103N D10 mainframe (working) and a 5113 parts mule that got mangled in shipping.? Also a 7CT1N if that pops up first.? And a 576, but the disadvantage there is the size.? I'm in Southern California where space is tight (and it would be difficult to obtain a 576 without SWMBO knowing about it!)? I do know a guy who has a 576 just collecting dust....
Ya, I'm salivating like one of Pavlov's dogs now LOL!
Well, best of luck to you, Benjamin, with the 7B92A and all that other great vintage gear you have!? At least we have a chance at fixing the old stuff, designed before the suits took over and messed everything up.? HP >> Agilent >> Keysight.? Tek is still called Tek, but it's of course nothing like it was.? And Broadcom Corp >> Broadcom Ltd/Inc./Avago, Hughes >> Raytheon Company >> RTX, etc., etc.? Those last 2 sad progressions are where I've worked for the last 19 years, unfortunately.
Hopefully somebody knowledgeable chimes in about subbing a 2N3906 for an SPS6765.? Can't help you there.? Maybe Tom Lee can?
Jim FordLaguna Hills, California, USA On Thursday, February 20, 2025 at 10:58:23 PM CST, Benjamin Badrakh via groups.io <benbadr2009@...> wrote:

Hello people of TekScopes,

I have a Tektronix 7B92A that needs repairing. It has a very severe horizontal deflection off to the right of the display, but the sweep is also jittery and not stable.

I've probed the sweep generators and I've confirmed they are not the problem as the sweep waveforms are beautiful.

I'm pretty sure the problem is in schematic No. 6, the output amplifier for the plug-in. The only problem is, this part of the circuit is on a section of the A6 main board that is under the A7 logic board. Because of this I can't reach any of the points I need to test.

I've tried removing the logic board and probe the A6 board that way, but that stops the plug-in from sweeping, which is useless for testing points on the output amplifier.

I could just solder a tiny wire to every point I need to check and do it that way, but some of the points I need are on socketec transistors and I don't want to risk solder filling up a socket.

Any tips?

Benjamin

On 2025-02-21 9:52 AM, Craig via groups.io wrote:

This App note explains: Or just search on resistor
VCR.

A very interesting read! But I was hoping he would explain a mechanism behind the variation. Also, he describes using pulsed measurements in order to minimize heat buildup and therefore the confounding influence of Tcr. I wonder how much of the resistance reduction could be due to capacitive effects - that would indeed lower the effective resistance on a short pulse.

Steve Hendrix

One should always pay attention to resistor VCR specs when using resistors in high voltage circuits. Can be significant even at 2 kV.
This App note explains: Or just search on resistor VCR.
Craig

On Fri, Feb 21, 2025 at 6:10?AM James55 via groups.io <james_55=
[email protected]> wrote:

It briefly flashes some LED's before going to sleep. No beam finder, no
scale illumination etc.

If the scope is dropping into the diagnostic monitor due to a power on self
test (POST) failure, the 2465A should leave a front panel LED lit. The LEDs
flashing on the front panel (FP) are indicating POST progress, so you can
tell what's happening and where it's getting stuck. There is a table
somewhere in section 6 of the service manual that correlates FP LEDs to
tests *one I always fail to find when I look for it).
If the scope is failing a POST, then you should be able to drop out of the
diagnostic monitor by pressing the A/B TRIG button, at which point you
should be able to mess with the CH1/2 V/DIV controls, hear relays clicking
and such.
If you report which test is failing (I'm going to bet on the 05 tests), we
can probably point you in a profitable direction.
The scope can also fail earlier tests, the "Kernel tests" (IIRC), which
would also leave breadcrumbs on the FP LEDs, but those indicate more
serious A5 failures.

My 2467 suffered a problem on the A5 board that collapsed the -1.25 and
1.36 reference voltages, which led to no trace, no on-screen-display (OSD)
and 05 POST failure. If you have another A5 board to drop in, my bet is
that'd resurrect traces and OSD.

Good luck,
Siggi

:) :) :)
In Italy it was "Vita da Strega"

On Fri, Feb 21, 2025 at 8:03?AM Gianni Becattini via groups.io
<Giovanni.becattini@...> wrote:

P.S. Who is Samantha?

Main character in a 60's TV series called Bewitched. She was the
witch wife played by Elizabeth Montgomery.



David

Thanks again, your help is valuable. I understand the suggestion regarding the resistors. Provided I check the value, could I use normal resistors even if they are not specific for high voltage?

Since I disassembled, cleaned, reassembled the critic resistors and set the correct 1.60 voltage on U1540 pin 4, I haven't had the problem again (but it's too early to say I've won). The previous HV setting was too high (1.65V), so maybe the inverter was a bit overstressed?

Before I do anything else, I want to take advantage of the opportunity offered to me by a friend of this group: he also lives here in Florence and has a wonderful 13302A in mint condition. Next Sunday I will go to him to read/measure his oscilloscope, which I am sure is working fine, and compare the results with mine.

I will keep you informed. Thanks again Gianni

P.S. Who is Samantha?

Just a quick one which might help anyone whose scope boots fine but then has no display..

Obviously there are several reasons this might happen, however whilst working on one of these last week, this problem was created by a faulty U950, Z-Axis Hybrid chip. 155-0242-00.

Swapped the chip and the display came back perfectly.


James

Hi all,

I have a 2465A scope here which fails to boot.

It briefly flashes some LED's before going to sleep. No beam finder, no scale illumination etc.
The PSU had the Rifa's replaced and the voltage test points are all fine and within spec.

Obviously I'll be going through the manual, however thought I would post here first, on the off-chance that someone might already know what might cause these symptoms?


Thanks in advance,

James

My experience is that measuring voltages in a high resistance circuit is hardly as trivial as it seems first sight. And I've even seen intermittent changes of values of "precision" resistors randomly.
Why don't you try new resistors? You do not have to buy expensive precision parts for that experiment but select almost exact values from a dozen or so cheap 10%s. If the behavior of the instrument does not change you will know the resistors are not the cause of the problem.
Regards, Joe

I no longer need advice for this issue as the 7B92A has been repaired.

Turns out Q938 was the issue. Funny thing is I tested all the transistors on my 5CT1N and they all tested fine at the time.

What I did was I removed the logic board and only tested voltages rather than following the signal until something weird happens. Base of Q942 was at -8.5V instead of -6.9V so I did a little simulation in falstad and turns out if I remove Q938 the base of Q942 gets a higher magnitude of voltage.

Taking Q938 out and testing it on the 5CT1N, it showed no curves. It was entirely dead. Strange that it worked before when I pulled all the transistors and tested them. Must explain the jittery issue then I guess.

Question now is, will a simple 2N3906 transistor be a sufficient replacement for the SPS6765?

Thanks,

Benjamin

A little comic relief- Because the circuit is "Bewitched", try doing the
nose twitch that Samantha does. Who knows, it might help?!? :)

??? Brenda Gentry, KA2IVY

Ha ha! Excellent Brenda!
Try it Gianni. What have you to lose?
What you don't know how to do it? I don't either, ask your wife.
Renaud

Hello people of TekScopes,

I have a Tektronix 7B92A that needs repairing. It has a very severe horizontal deflection off to the right of the display, but the sweep is also jittery and not stable.

I've probed the sweep generators and I've confirmed they are not the problem as the sweep waveforms are beautiful.

I'm pretty sure the problem is in schematic No. 6, the output amplifier for the plug-in. The only problem is, this part of the circuit is on a section of the A6 main board that is under the A7 logic board. Because of this I can't reach any of the points I need to test.

I've tried removing the logic board and probe the A6 board that way, but that stops the plug-in from sweeping, which is useless for testing points on the output amplifier.

I could just solder a tiny wire to every point I need to check and do it that way, but some of the points I need are on socketec transistors and I don't want to risk solder filling up a socket.

Any tips?

Benjamin

Rusty,

The 150,000 ohm: 71-CCF60-150K-E3, 180,000 ohm: 279-H4P180KFDA, 332,000 ohm: 71-CCF60-332K-E3, 442,000 ohm: 71-CMF60442K00FEWF (12 week delay) or (2) 660-MF1/2CC2213F (in series), 430,000 ohm: 279-H4P430KFZA, 100meg: 708-HVA12FA100M or (measure in mm the hole spacing of the original carbon then select the on that will drop in which will be larger in wattage than original). If yours has the two 39meg 1/4w carbons on the bottom board, use two 594-MBE04140C2005FC1 in series. These will be R710 and R910.

Mark

Mark
Many thanks! This is the kind of information I need. Will put the order in with Mouser tomorrow.
Rusty

A little comic relief- Because the circuit is "Bewitched", try doing the nose twitch that Samantha does. Who knows, it might help?!? :)

??? Brenda Gentry, KA2IVY

The noted symptom on-screen is consistent with the divider giving too high an output result - this causes the HV to drop, so the display blooms oversize. Also, that "spikey and noisey" TP condition during the fault is probably what you'd expect when the regulation loop goes open from loss of feedback or something hitting a limit. The problem I think is that something is wrong with one of the parts, causing intermittent jumping of the apparent divide ratio. It can be one of the resistors' elements or connections going bad, or a bad solder joint, or the mux.

Remember the 4051 is a bunch of CMOS transmission gates that are actuated by changing gate voltages internally. Being leaky can cause different troubles depending on the power supplies and signals used. If it's a fully bipolar switching arrangement, a fault leakage current may be of either polarity, and would actually cause an offset, not a scale change - but you don't know yet until getting some measurements. Since the signal is negative, the Vss would have to be negative and bigger, and there's probably a plus Vdd analog supply and plus 5V logic (or one in the same), so faults could go either way. I'd recommend again that if you have not eliminated the 4051, to do so and be sure.

I'd still suspect possible resistor or contact oddities, but since you've done a lot already to eliminate those, that leaves I think only the mux, or maybe something downstream from that. The CMOS parts have pretty good transient voltage protection, but they can still randomly fail, and that signal is part of a HV environment, and you don't know what sorts of things may have happened in there before. Good luck.

Ed

Rusty,

R1853, R1854, R1871, R1872 and R1880 are known problems. Replace them with 1W 1% types. When any of these go bad, focus will not work. R1991 should be replaced with a Vishay or Ohmite type.

Mark