Help with an ailing 7D14

k6whp All Messages By This Member #187681 A while ago, I was querying here about a 7D15 freq counter that was on the fritz. It was a questionable cap and a bad solder joint and the good news is that has been repaired. (There was also some cockpit error.) Anyway, I wanted to mend a 7D14 I have that is flaky and wonder if I can get some help on narrowing down the problem. Below are the facts. (And, note, several years ago, David, W7PF, appropriately lectured me on a crappy job I did documenting the set-up. I will attempt to do better this time.) (1) Problem is that the 7D14 stops displaying the frequency (i.e., displays all zeroes) when the signal is sourced from a vertical amp. I have tested with a signal input from the unit's front panel BNC and it does seem to not fail. (2) The problem seems heat related. (3) The set up is as follows: (a) Scope is 7704A. (b) 7A26 in left vertical slot. (c) 7B92A in horizontal slot A. (d) 7D14 in in horizontal slot B. (e) Vertical mode LEFT. (f) Horizontal mode A. (4) Other settings per 7D14 manual troubleshooting/calibration set up. (5) Signal input to Channel 1 on 7A26 and adjusted to AC, set to display and trigger on channel 1., (6) 7D14 set 10 ms, Trigger Preset, 525 MHz, AC, TRIG SOURCE, and frequency is displayed correctly on CRT. (7) After about 1-2 hours, the frequency display becomes erratic and then displays all zeroes. (8) Removing the side panels on the scope and the plug-in and blowing it with cold air causes the frequency to be displayed properly. (9) Leaving the plug-in exposed (no side panels) causes it to run indeterminately longer without failing. (That is, I have let it run several hours without failure.) (10) Similarly, I have set it up to read a signal input from the CH A INPUT BNC in the 7D14 front panel and -- even when covered with the side panels -- it seems to run a long time and not fail. (Similarly, I have let it run several hours without failure.) In looking at the manual and reading the schematic and notes, I am guessing that the vertical input from the 7A26 arrives on pins 20A (+TRIG IN TO T870) and 20B (-TRIG IN TO T870) and, accordingly, I have begun to snoop through schematic <1> CHANNEL A SIGNAL CONDITIONING but sense that I may be a little off. Therefore, I am passing this along and asking for any ideas you all might have. Thanks in advance. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
Jeff Dutky Member Profile All Messages By This Member #187683 William, My first guess, just based on the obvious heat/time dependence, is that you have a bad solder joint in the chain from 20A/B, and I would try to do a close visual examination of all components in that chain. From the pictures on the TekWiki this counter doesn't seem too tightly packed, but it DOES seem to have a lot of socketed ICs. My experience with other instruments of similar age is that the socketed ICs are good suspects for bad connections. I would at least give them all a close inspection, if not remove, clean, and reseat every one of them (I'm not usually a proponent of shotgun techniques, but Tek had a specific problem with bad sockets at one point, and with oxide layers forming on IC pins even in the "good" sockets, so this is an exception). -- Jeff Dutky
Dave Seiter All Messages By This Member #187686 If the solder joints look good and resocketing doesn't help, you could also try very light applications of freeze spray on the components in the chain, as it could be a marginal component.? -Dave toggle quoted message Show quoted text On Wednesday, November 3, 2021, 08:43:15 PM PDT, Jeff Dutky <jeff.dutky@...> wrote: William, My first guess, just based on the obvious heat/time dependence, is that you have a bad solder joint in the chain from 20A/B, and I would try to do a close visual examination of all components in that chain. From the pictures on the TekWiki this counter doesn't seem too tightly packed, but it DOES seem to have a lot of socketed ICs. My experience with other instruments of similar age is that the socketed ICs are good suspects for bad connections. I would at least give them all a close inspection, if not remove, clean, and reseat every one of them (I'm not usually a proponent of shotgun techniques, but Tek had a specific problem with bad sockets at one point, and with oxide layers forming on IC pins even in the "good" sockets, so this is an exception). -- Jeff Dutky
k6whp All Messages By This Member #187695 Gentlemen, Thank you both for the suggestions. I will do so. But I am wondering if the fact that the external signal from the CHANNEL A BNC operating without the failure versus the internal signal from the 20A/20B connections failure indicates/isolates anything. (This is more or less rhetorical question.) I shall proceed with the socketed ICs, etc. There is a lot to learn -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
Jeff Dutky Member Profile All Messages By This Member #187699 Yes, the fact that external input works means that the switch must limit the location of the fault, or the switch is the fault itself. The switch appears, from the pictures on TekWiki, to be a wafer switch rather than a cam switch. Can you verify that? I would check the solder joints on the wafer, and on the PCB. It also implicates Q216 and Q218 (or associated passive components) at the bottom-front of the high frequency counter board. Are we lucky, and those two transistors are socketed? If they are socketed it may be as simple as cleaning and reseating the transistors, but I’d also have a close look at all the solder joints around them, and their associated passives. — Jeff Dutky
Jeff Dutky Member Profile All Messages By This Member #187700 It might also be the dual FET (Q130), just above and to the front of Q216 and Q218, clearly socketed in the board image on TekWiki, or the solder joins or passive components between FET B and Q218 (e.g. C222 or C223, or maybe R227 or R228?) I don't know how likely this is, but if the fault is INSIDE the FET then you can diagnose that, I think, by reinstalling the FET "backwards" in its socket (so the tab faces the opposite direction), and seeing if the fault moves from the internal trigger signal to Ch A external trigger input. -- Jeff Dutky
k6whp All Messages By This Member #187703 Jeff, Sadly, the Q130A/B double FET is firmly soldered in and I and loath to attempt to free it. If that DOES prove to be the problem, I will "screw [my] courage to the sticking place" as Lady Macbeth advised and give that a try. I still have loads of exploration to do on the unit to understand its workings. And, during this exploration, I might pull, clean, and re-seat the socketed ICs. I agree that it would be interesting to drive the internal trigger source "to exhaustion" and and see what presents itself aftr Q130A/B. Thank you for your suggestion. I am not sure what is presented at the wiki page (will traipse over there when I post this response), but I did create an entry on my blog with some detailed pictures to document this. You are, of course, free to visit there. And, yes, the switch is A wafer switch. Q218 and Q216 are socketed. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
k6whp All Messages By This Member #187705 ..sorry, I neglected to add: -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
n4buq All Messages By This Member #187706 I will be interested in following this. I have two 7D14s - neither of which work properly. I thought the problem with my first one might be with U329 and when I received the second one, I tried swapping those with no change. It's possible that both ICs are bad but I think it's more likely that I just haven't found the actual problem. Keep us posted! Thanks, Barry - N4BUQ toggle quoted message Show quoted text ----- Original Message ----- From: "k6whp" <k6whp@...> To: "tekscopes" <[email protected]> Sent: Thursday, November 4, 2021 11:35:05 AM Subject: Re: [TekScopes] Help with an ailing 7D14 ..sorry, I neglected to add: -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
Raymond Domp Frank Member Profile All Messages By This Member #187707 On Thu, Nov 4, 2021 at 05:34 PM, k6whp wrote: Sadly, the Q130A/B double FET is firmly soldered in Looking at your photograph, Q130 doesn't seem soldered in. Its pins are individually socketed in "Berg" sockets, AFAICS. Raymond
k6whp All Messages By This Member #187712 Trust me, it is! -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
n4buq All Messages By This Member #187713 That's odd. I looks like it's socketed to me as well. Are we all talking about Q130 (somewhat left and down from center in image 10 on your web page)? Thanks, Barry - N4BUQ toggle quoted message Show quoted text ----- Original Message ----- From: "k6whp" <k6whp@...> To: "tekscopes" <[email protected]> Sent: Thursday, November 4, 2021 1:50:50 PM Subject: Re: [TekScopes] Help with an ailing 7D14 Trust me, it is! -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
Raymond Domp Frank Member Profile All Messages By This Member #187714 On Thu, Nov 4, 2021 at 07:50 PM, k6whp wrote: Trust me, it is! Sorry, still not convinced, to say the least. On your image 20211104-072656_Burst01.jpg I clearly see 2 (or 3) socket pins under Q130's leftmost pins. Maybe if you try and wiggle Q130 a bit, you may find out. Raymond
k6whp All Messages By This Member #187715 Barry, Raymond, Yes, we're talking about Q130A/B and, well, now I stand corrected. It is socketed. At first, it would not budge so I thought it was soldered in. But after looking closely and prying it ever so gently, it "wiggled" a bit. That said, I am off to determine what happens to the signal at the gates of Q130A and Q130B. I presume they are switched with the INPUT SENS switch where the TRIG SOURCE position makes the internal signal from 20A/20B on the connector appear on the gate of 130B while the other positions make the signal from the CH A INPUT appear on the gate of Q130A. Therefore, Jeff's suggestion to "reverse" Q130 would make sense. (Please hold the sniggering down to a minimum out there, lads!) -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
k6whp All Messages By This Member #187739 Well, thought I had fixed it. I traced the signal through to Q136 and, after spraying that with a shot of cold air, saw the counter started working again. Q136 is a common ordinary 2N3904 -- socketed, no less! So, after replacing it with one from my stash, I smugly began to clean up and call it a night.. ..when, just before I returned indoors, I noticed the frequency display wilt down to zeros again! (Of course, I would have known all this if I had taken the time to read the DETAILED CIRCUIT DESCRIPTION CHANNEL A SIGNAL CONDITION part of the 7D14 and realized that Q136/Q1236 are it's part of the bandwidth selection circuit!) sigh Well, I am reasonably certain that it is a part from around that vicinity and tomorrow is a new day! -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
n4buq All Messages By This Member #187747 Is it possible that the relays in the CH A INPUT are malfunctioning? Have you checked the signal at R111 before and after it fails to ensure that the signal is making it through all those contacts? Also, does exercising the COUPLING and INPUT SENS switches change things at all when it fails? BTW, I presume you meant Q136/Q236, correct? Thanks, Barry - N4BUQ toggle quoted message Show quoted text ----- Original Message ----- From: "k6whp" <k6whp@...> To: "tekscopes" <[email protected]> Sent: Thursday, November 4, 2021 11:09:20 PM Subject: Re: [TekScopes] Help with an ailing 7D14 Well, thought I had fixed it. I traced the signal through to Q136 and, after spraying that with a shot of cold air, saw the counter started working again. Q136 is a common ordinary 2N3904 -- socketed, no less! So, after replacing it with one from my stash, I smugly began to clean up and call it a night.. ..when, just before I returned indoors, I noticed the frequency display wilt down to zeros again! (Of course, I would have known all this if I had taken the time to read the DETAILED CIRCUIT DESCRIPTION CHANNEL A SIGNAL CONDITION part of the 7D14 and realized that Q136/Q1236 are it's part of the bandwidth selection circuit!) sigh Well, I am reasonably certain that it is a part from around that vicinity and tomorrow is a new day! -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
k6whp All Messages By This Member #187813 Barry, I have been indisposed and only recently gotten back to this. In essence, I did what you mention by looking at R222 (rather than R111) before and after failure. Basically, it involved letting the 7704A cool down over night and then setting a scope up to measure the trace at R222 and letting the scope "cook" until the 7D14 frequency display zeroed out. The internal input from the 7A26 at that point did NOT diminish when the 7D14 failed. Repeating the test, I discovered the signal loss seems to disappear after the Q130A and Q130B dual FET. I have conducted a flurry of experiments and am collecting and analyzing the results to make some sense of them. I am disinclined (for some reason) to believe that Q130A/Q130B has failed. Although I have come to discover that replacements are available. When I have completed this phase,I will return and announce it here plus post pictures and narrative over on my blog -- where I can intermix the picture and the description of the problem and possible resolution. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
n4buq All Messages By This Member #187814 William, My suggestion was just a long-shot. I have two 7D14s, neither of which work properly so will still be interested in following your thread. Thanks, Barry - N4BUQ toggle quoted message Show quoted text ----- Original Message ----- From: "k6whp" <k6whp@...> To: "tekscopes" <[email protected]> Sent: Sunday, November 7, 2021 11:41:35 AM Subject: Re: [TekScopes] Help with an ailing 7D14 Barry, I have been indisposed and only recently gotten back to this. In essence, I did what you mention by looking at R222 (rather than R111) before and after failure. Basically, it involved letting the 7704A cool down over night and then setting a scope up to measure the trace at R222 and letting the scope "cook" until the 7D14 frequency display zeroed out. The internal input from the 7A26 at that point did NOT diminish when the 7D14 failed. Repeating the test, I discovered the signal loss seems to disappear after the Q130A and Q130B dual FET. I have conducted a flurry of experiments and am collecting and analyzing the results to make some sense of them. I am disinclined (for some reason) to believe that Q130A/Q130B has failed. Although I have come to discover that replacements are available. When I have completed this phase,I will return and announce it here plus post pictures and narrative over on my blog -- where I can intermix the picture and the description of the problem and possible resolution. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
k6whp All Messages By This Member #187815 Would you mind sending me a PM? I am good in QRZ. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"
k6whp All Messages By This Member #187963 I have done a lot of snooping and testing and RTFMing and am getting to know this plug-in quite well. But, alas, not well enough, apparently! I cannot seem to figure out why the display fails, i.e., measures correctly for about an hour or two and then displays zeroes. That said, for those who are interested, I have found -- with the help of a good friend -- that it is possible to replace the Q130A/Q130B dual FET with two matched J310s. Also, Q218 can be replaced with a good quality 2N3906. I am pretty sure that these replacements will work "under normal circumstances" because the plug in with the original parts last only about 20-30 minutes before it fails while with the "new parts" its endurance is consistently over two hours before failure. But that's not the point. Succinctly, I am wondering if -- and how -- a failed relay might be causing this problem. During testing, I found that the K106, K101, and K100 units were very hot to the touch but K103 was not in the TRIGGER SOURCE setting. Could this be a/the problem? If it is, might there be a replacement for these relays aside from the usual "parts or not working" unit that one might come across? I posted an update on my blog of my efforts to date so I am just not dumping the question on the group and fleeing. Appreciate your forbearance. -- William, k6whp "A friend told me to cheer up, things could be worse. So I cheered up and, sure enough, things got worse!"

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Help with an ailing 7D14