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7A16A high frequency compensation.
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Hi all,
I've recently purchased a 7A16A, working, looks new, S/N B138015. Probably the previous owner has tried to calibrate the HF response in a badly manner: risetime was 3 ns with very rounded front corners. Suspecting a strong misadjustment of HF compensation, I did a full calibration procedure. Problem: I'm unable to obtain a decent compromise between risetime and flat-top and square corner. I'm using a PG506, a Type 111 and 7854 mainframe. 7A16A in the left compartment, no extender. Following the procedure as per manual, I spent 4 hours trying to calibrate the best, but the various settings interacts with each other and I'm unable to obtain a decent rising edge. Sometimes the rising edge is distorted in the middle of the front. Other time there is an auto-oscillation. Touch one trim and the waveform gets better. I touch another one and the waveform gets worse. I heard that the calibration procedure was tricky, but I have never encountered a difficulty like that. All resistive and capacitive trimmers are O.K, perfectly functional. Does anyone know a better procedure than the (fairly simplistic) one outlined in the manual? Moreover, in the manual there is not even a photograph of some typical waveform that can be used as a reference. Does anyone know where to find a screenshot of a typical 7A16A step response? Cheers, Max |
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Hi Max,
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My 7A16 (not A) manual shows a picture. The 7A16A performance is the same or better I guess. I did the same setup with PG506, 7854 and "as is" 7A16A, Nice smooth step with rise time 1.55 ns, followed by just one overshoot 6.5% and one undershoot 3.5%. After this negligible aberrations from flat. With a 7A19 rise time was 1.1 ns. Albert On Sat, Mar 20, 2021 at 01:01 AM, unclebanjoman wrote:
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Hi Albert,
thank you for the response. After a further two hours, and comparing with the more detailed explanations given in the 7A16P manual, I was finally able to calibrate the step response in a decent manner. I could not appreciate any variations by turning the last trim, C770. When I got the plugin the first time, it was almost completely unscrewed. I screwed in some turns to prevent the screw from falling out! I posted some screenshots showing the final result. Photos are in /g/TekScopes/album?id=262075 Album title:"A16A step response (with 7854) What do you think about? All comments are welcome. Furthermore, I noticed that my PCB is very recent: it shows a S/N 670-2323-08 (see photo). There's a trimmer that does not appear in the schematic. I've marked in the photo with a red arrow. Green arrow points to the LF compensation trimmer R436 which is present in the schematic. The other one is marked LF (on the PCB) but is not present in the schematic. I presume was an additional trim point for the same purpose of the green one, so I adjusted it in the same manner as R436. Max |
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Hi Albert,
thanks! Yeah, it's that aberration "up" in the 10 ns point I can not eliminate. I didn't think it was the cable. I generally observe the "dribble up" phenomenon. Do you think is the cable ? I also have a 7A16 (not A) but I don't think it has that reflection. Maybe because of the smaller bandwidth? Anyway, now the response of my 7A16A is much better than before. Initially was horrible. I think the previous owner tried to calibrate it but then gave up. Really a tour de force: more than 4 hours fiddling the pots! What do you think about the trim pot not shown in the schematics? May it be the very latest revision? I did not find more recent schematics. Max |
Yeah, it's that aberration "up" in the 10 ns point I can not eliminate. IThe 7A16A input causes reflection anyway. When the cable impedance doesn't match the PG506 output impedance then the reflection from there can be observed at the CRT after twice the cable travel time. I wouldn't bother too much about not perfectly flat, in my opinion the step response is very nice. (How did you calibrate the PG506?) What do you think about the trim pot not shown in the schematics? May it beThat must by R436 in the LF filter leg between pins 2 and 3 of U450. Has always been there according to parts list. Is present in my 1973 unit. Albert |
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Hi Albert,
to check my PG506 I used 3S1/3T77A pair, connected with a 20 cm GR874 airline. Measured a risetime of 750 ps and aberrations as per service manual. So I did prefer touch nothing. :-) R436 is the one pointed by the green arrow in my photo and it's O.K. The unknown R is the one pointed to by the red arrow. See /g/TekScopes/photo/262075/3 Max |
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Hi Albert,
you made me doubt about the PG506 now :-) I would not like to have calibrated the 7A16A trying to compensate for the aberrations of the PG506 itself. In the weekend I'll check the fast rise output of my PG506 using the 3S11/3T77A and also with a 2 GHz Philips PM3340 that I received a month ago (fully functional, like new). I'll post some photos as soon as possible. Max |
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Hi Max,
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It's nasty that the PG506 trigger output is not usable as pretrigger. Also (e.g. in my PG506) there is too much between-trigger jitter to view the fast edge of the next pulse. That's why a 7M11 delay line is used in the calibration section. But the 7M11 is not ideal. Hence the PG506 signal output waveform is compared with a faster TD pulser waveform. The PM3340 only does sequential sampling and requires signal delay.. BTW I found that the fast-rise rise time depends a lot on the amplitude, something like 0.75 ns for low to 1 ns for max amplitude. Albert On Tue, Mar 23, 2021 at 10:23 PM, unclebanjoman wrote:
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Hi all,
I've just uploaded two screenshots: /g/TekScopes/photo/262075/4 /g/TekScopes/photo/262075/5 It's my PG506's (+) fast rise output directly coupled to a PM3340 2GHz Philips oscilloscope. High quality, 80 cm RG223/U with Uber-Suhner connectors (BNC to N) cable was used. 10x attenuator was used to keep the PG506's output level over 100 mV. A nice 632 ps rise time with no aberrations. This implies that the calibration of my 7A16A is practically perfect, as Albert also said Cheers, Max |
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Hi all,
I reopen this thread because a few days ago suddenly the step response became very bad. I've added a new photo here: /g/TekScopes/photo/262075/3428607 I tried to tweak the trims but without success. Seems that the waveform at 50% suddenly slows down with some distortion and ringing. The power voltages are fine. I checked the response of my mainframe (a 7854) with the calibration fixture 067-0587-01 and the step response is fine. I own a 7A16 (not A) and its step response is fine also. I'm very puzzled and I don't know what to check what. A quick check of the signals with a P6106 probe and my 7A16 shows no significant deformation, but I think I don't have enough bandwidth to detect that aberration... I've tried to swap Q820/Q840 and Q860/Q880 with no success. My plug-in use all 155-0078-10 ICs. I've tried to swap a couple of these (U350, U450). Result: signal always distorted, no changes. Any suggestion is welcome. Max P.S. my 7A16A is one of the very latest, S/N B138015. Some components are not the same as in the manual. |
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Max,
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Some plugins of that era had small ceramic trimmers that went open circuit. See if one of the trimmers does not seem to have the effect implied by the calibration procedure. --John Gord On Wed, May 11, 2022 at 10:40 AM, unclebanjoman wrote:
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Hi John,
All ceramic trimmers have some effect when turned. So, I assume none of them are open. It seems strange to me that after a year of use the plugin suddenly exhibits such behavior. Can a capacitor/ceramic trimmer break so suddenly, without external stress? I can try to change them all, but I have to order them from mouser... I also have a 2 GHz oscilloscope, Philips PM3340 but only with a 10:1 passive probe (500 ohm at the probe tip). I am afraid that the impedance of the probe is putting too much load on the circuit to be tested... In any case I will try to trace the signal starting from the 7a16A input, tomorrow, using the Philips. Could a possible cause be the dual fet Q150 damaged from overload ? Max |
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A simultaneous failure of all trimmers is highly improbable. Your problem is elsewhere.
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Possibilities include: Bad connection at input BNC, poor contacts at attenuator socket pins, etc. So, your plan to signal-trace from the input forward is the right idea. To narrow the suspects, carefully map which attenuator settings exhibit the problem. That will tell you whether it's a problem downstream of the attenuator, or with a specific attenuator or group of attenuators. Good luck! --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 5/11/2022 13:39, unclebanjoman wrote:
Hi John, |
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Thanks Tom,
the problem already arises in the 5 mV and 10 mV/div setting, where all attenuators are off. Surely I'll trace the signal from the input toward the dual fet unity gain buffer (Q150). A strong suspect is a dirty contact closure but the displayed (still distorted) signal is stable. I'm somewhat skeptical. Tapping on the attenuator box has no noticeable effect. Max |
the problem already arises in the 5 mV and 10 mV/div setting, where all attenuators are off.If you didn't already, I'd check, resistors, especially the low-ohm carbon composite ones. They tend to drift up, sometimes in jumps. You can measure in-circuit, where necessary by comparing with what you have in the good unit or (not quite as reliable) the other side of a differential stage. In one of my 7904A's, one of the 10 Ohm 0.125W resistors in the delay-line compensation circuits jumped up by almost 50%, resulting in a sudden and serious deformation of the trace. Raymond |
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Since the problem shows up with no attenuators engaged, there are very few possible sources. Raymond's suggestion is an excellent one. Carbon comps should always be eyed with some suspicion, especially ones of this vintage.
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-- Cheers, Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 5/12/2022 04:04, Raymond Domp Frank wrote:
the problem already arises in the 5 mV and 10 mV/div setting, where all attenuators are off.If you didn't already, I'd check, resistors, especially the low-ohm carbon composite ones. They tend to drift up, sometimes in jumps. You can measure in-circuit, where necessary by comparing with what you have in the good unit or (not quite as reliable) the other side of a differential stage. |
Raymond Domp Frank
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