Re: A bewitched voltage divider?
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?
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2445A / 2465A series - no beam finder (SOLVED)
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
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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
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Re: A bewitched voltage divider?
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
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Re: How to probe points on a pcb under another pcb on the 7B92A
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
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Re: A bewitched voltage divider?
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
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How to probe points on a pcb under another pcb on the 7B92A
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
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Re: Help! 2465A CRT Focus Troubles
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
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Re: Help! 2465A CRT Focus Troubles
Mark
Many thanks! This is the kind of information I need. Will put the order in with Mouser tomorrow.
Rusty
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Re: A bewitched voltage divider?
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
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Show quoted text
On 2/20/25 21:25, ed breya wrote: 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
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Re: A bewitched voltage divider?
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
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Re: Help! 2465A CRT Focus Troubles
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
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Re: A bewitched voltage divider?
Hi guys and thanks for your help. Here is a small update:
- TESTS - I disassembled the board, removed the critical components and checked them. I have three meters capable of measuring 10M. Two say the value is as it should be, one says +200K. I cleaned everything carefully and reassembled everything. I am now convinced that the components are OK. By the way, as far as I know, resistors increase in value over the years. In this case they should have changed a lot towards lower resistance. Also, if the 4051 was leaking, I would read a lower voltage, not higher.
- ANOMALY - Using R1355, I can easily set 1,600V on pin 4 of U1540, but I cannot go beyond 1,550V measured with the HV probe. This makes me think that, as you say, the problem could be in the HV probe measurement.
- ANOMALY? - That could be the point. Looking at TP1250, I see a 200mV oscillation at about 36 kHz, with a DC value of about -2.8V. From this, I can imagine that the -1600V has a significant 36 kHz ripple, and perhaps this could also be misleading the HV probe. This AC component is filtered by C1420, so the 1600V at the test point is accurate and very stable, but how can the HV probe behave in this case?
SO MY QUESTION is: should the cathode voltage be perfectly levelled? If so, my 11302 is faulty, otherwise I am looking for a problem that does not exist.
I forgot to say why I am checking all this. The 11302 seems to be working fine, but from time to time when I turn it on, the display is completely defocused and bigger than it should be. I turn it off and on, and most of the time the problem goes away. I could see that when this happens, the signal on the TP1250 becomes "spiky and noisy" as if the inverter has gone crazy (I upload a picture: on the left = inverter crazy; on the right = all OK).
Thanks again
Gianni
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Re: A bewitched voltage divider?
Gianni,
Ed B. has a good idea to isolate the divider from the mux. As you have soldering iron hot and ready to go, I suggest that you lift the resistors R1422 and R1100 at the -1.6V junction such that the 4051 mux (U1540) is not connected to the divider. Re-connect the resistors with a jumper wire and test the -1.6V point (low point of the divider).
If you need help to navigate the layout to determine which resistor ends to lift, I can post a layout map for the 11302A. I don't have the 11302 layout. Compared to the 11302, there were no significant changes in that area; so, the components should be in the same locations.
I am assuming that you have indeed verified that you have -1600V at the high end of the divider.
Clark
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Re: A bewitched voltage divider?
On 20 Feb 2025 9:29 am, Robert Gibson wrote: Have you verified the accuracy of the HV probe and "VOM (or whatever meter type in use)?" The resistors there may have drifted. Robert has an excellent point. Instruments and test equipment need to have their accuracy checked periodically. I only have one instrument which goes to the cal lab for an annual calibration. It is my 4-1/2 digit, true RMS. Keithley 179 bench DMM. I also own a Leo Bodnar GPS frequency standard. Those are my standards for checking the rest of my shop and field instruments and test equipment. An HV probe is intended to be used with a meter of specified input resistance, and the probe will only meet the accuracy specs with the specified input resistance. That input resistance is part of a voltage divider. A different input resistance reduces the accuracy of the HV probe, and unless you perform the calculation to compensate for the different meter resistance your measurements will not be accurate. It only takes a minute amount of contamination to ruin an HV probe. Handling the probe resistor with bare fingers is often enough to ruin that resistor. -- Dale H. Cook, GR/HP/Tek Collector, Roanoke/Lynchburg, VA
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Re: Help! 2465A CRT Focus Troubles
Bonjour WORGH, have used,and fixed 2465/7/B since 1990s.
I am very wary of any older SN, or no suffex 2465 or A.
Any vendor with "government" is also a flag.
Repair can be PS, or HV, and control board,A5 CAL,RAM, SMD,lytic corrosion
CRT damage is rare, unless it,was dropped or mispackaed in shipment
The very,severe defects appears more,than a PS, or just bad HV.
Even for very experienced 2465 veterans, with best CAL equipment, it can take hours, days weeks...
One,dog I have had on the bench since 2018!
You should get the full service manual for the model and serial, follow the excellent flow charts and check each board and subassembly in sequence
Alternatively just write it off as,a parts,donor, and look out for a,100% working 2465B.
Bon courage
Jon
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Re: A bewitched voltage divider?
Is it still connected to the 4051 during measurements? Try without any other connections and of course be aware of the DVM load effects. If the 4051 is leaky, it could cause grief. Also try getting a reading from the output of the mux while the divider output is shorted to ground.
Ed
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Help! 2465A CRT Focus Troubles
Hello
I am at a standstill with figuring out why the CRT of my 2465A is blurry and won't focus.
I purchased this unit from Government Planet last summer with the idea that I would probably have to troubleshoot it. I turned it on and it made a screeching sound and had no display. I thought the sound was coming from the fan but no it was the high voltage board. Checked the voltages on J119 and they were all good with the exception of the -15V rail which had high ripple. Decided to recap the LVPS with a kit from Condor Audio. This fixed the -15V rail. Turned it on and there was a blurry dispay but intermittent screeching from the high voltage board. Looked closer at this and discovered that the ceramic of T1970 was cracked in several places. Purchased a new high voltage board from QService and installed it. Now no screeching but the display remains blurry with the Read Out letters wider and taller than they should be and the same for the traces. I have uploaded 2 pictures to the file Amberwave 2465a that shows what I'm talking about. I checked voltages to the CRT socket at pins 8(31V), 12(-108V to -186V) and 13(-192V to -224V) and they look reasonable. DAC reference is right at 10V. When I input a signal into the scope I can just make out that it is a sign wave.
I just don't know where to go from here. I have invested a lot of time and dollars in this scope to just give up. It would seem that this isn't a HV board issue as both boards gave a blurry image. Could this be the CRT tube itself or one of the IC's? I would really appreciate some guidance on where to go from here from someone more experienced with these scopes than me
Rusty Hilliard
W0RGH
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Re: A bewitched voltage divider?
Gianni,
According to the schematic, cathode voltage is supposed to be -1600 V. At 1000:1 divider would give 1.60 V, which it does. You say the input is -1510 V. Have you verified the accuracy of the HV probe and "VOM (or whatever meter type in use)?" The resistors there may have drifted.
--Robert
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Show quoted text
On 2/20/2025 2:49 AM, Gianni Becattini via groups.io wrote: Hi Guys,
I am trying to fix some problems on my 11302, but I am stuck on a problem that seems very simple, but which I cannot overcome: a simple voltage divider. I am probably doing something wrong, but I cannot guess what (I added an album among the photos with the simplified schematic diagram).
The 11302 "reads" the CRT cathode voltage using a voltage divider, selected by a 4051 analog multiplexer. It should be simple: the voltage divider is made up of a 20 M ohm and a 20 kohm resistor (the former is actually 2 x 10 Mohm resistors). So the output should be one volt for every 1000 volts in the input (well, 0.999).
For me this is not the case: the divider output is significantly higher than it should be (with -1.6 V in the output, the input is actually -1.510 V).
Easy, you say, one of the resistors has changed its value over time, but it is not so. I desoldered one extreme of them and they are all very precise. Another hypothesis was that the CRT mesh was causing some problem, but it is not: I disconnected it and nothing changed. I neglected the C1100 and C1420 capacitors because they could only lower the voltage, not make it greater.
The last thing I didn't check yet is the dirt on the lower part of the board, but before disassembling it I wanted to hear your suggestions, if you like.
I will be very grateful for any help, I am getting crazy. Thanks Gianni
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Re: A bewitched voltage divider?
On Thu, Feb 20, 2025 at 4:49?AM Gianni Becattini via groups.io <Giovanni.becattini@...> wrote: I will be very grateful for any help, I am getting crazy. Thanks Gianni
This is how I felt until I discovered that my 485 had a voltage dependent "precision" resistor in the circuit that derives 13V from the 50V rail. See /g/TekScopes/message/113493. I have no idea why this happened, nor how common it is, but apparently it can happen. In your case I guess it could be as simple as some kind of contamination on or around the 10M resistors, which doesn't affect your DMM measurements, but appears at the working voltage?
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James55
Joe
Benjamin Badrakh
Dale H. Cook
Jean-Paul