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Who's up to some remote troubleshooting?
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Lynn Lewis
I plan to get back to this after lunch today. Any suggestions would be
appreciated. It's not life or death, just my idea of fun. Item: 7A26 with serial number B2.... Setup: This 7A26 is in a 7704 with a known good 7B53A. I am feeding both channels, not necessarily at the same time, from the 1KHz square wave calibrator. I started at .04V but have worked up to 4V. I'm using a 7904 to see the waveforms. The 7B53A is set to use the internal trigger. Symptom: Can not sync through channel B using internal trigger. If both channels are receiving square wave, it syncs. If channel A only is receiving square wave, it syncs. If channel B only is receiving square wave, it will not sync. Knowns: 1. I have another 7A26 that works perfectly in that same bay so it's not the scope and it's not the 7B53A. 2. If I feed the square wave only into channel B, I can use the other scope and follow it right up to U2750. The input pins to U2750 have the square wave (fuzzy but square). The output pins do not (fuzzy and flat). (Note: If I feed the square wave only into channel A, I can see it on both the input and output of U1750.) The DC voltages around U2750 are very close to those indicated on the schematic and appear to me to be reasonable. I did resistance checks on the resistors in the input network to U2750. 3. I have tried: a. replacing U2750 twice b. replacing U1750 c. replacing Q920, Q940, Q960, and Q980. Suggestions: |
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Craig Sawyers
I don't have a copy of the 7A26 schematic, so this is a *real* stab in the
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dark. How long are the triggering events that you're looking for? I fooled myself for a day recently while trying to fix my HP counter when I was convinced that the input section wasn't working I was feeding it with a 1kHz square wave, and could see nothing at all on the bus lines coming from the differetial line driver on that card. The reason? The output was a 5ns duration ECL pulse at the trigger transitions of the input wave (as set up by the trigger level controls). Once I triggered on the event, and then sped the scope sweep up - there it was! Interestingly (and very tackily) this is generated by using three ECL NORs in a loop, and the output pulse duration is generated by the differential pulse delay of two gates - or about 5ns for 10xxx series ECL. Now since many of the y-amp 7000 series plug ins use custom silicon (as well as the timebase), and much of this fast custom silicon is ECL-based, are you sure that there isn't a triggering event there but of a non-intuitive time duration? Cheers Craig -----Original Message----- |
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Hello Lynn,
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How about Trigger Source Selector (or whatever is name of that mux controlled by a rotary switch) being stuck to Channel A. I think that the selector proper are two ICs that get alternatively disabled by a voltage input; that might be even broken wire (or trace) coming from the rotary switch, so select input is in default position. Regards Miroslav Pokorni ----- Original Message -----
From: "Lynn Lewis" <mrzuzu@...> To: "TekScopes@yahoogroups. com" <TekScopes@...> Sent: Tuesday, January 22, 2002 1:52 AM Subject: [TekScopes] Who's up to some remote troubleshooting? I plan to get back to this after lunch today. Any suggestions would beto see the waveforms. The 7B53A is set to use the internal trigger.receiving square wave, it syncs. If channel B only is receiving square wave, it willthe scope and it's not the 7B53A.scope and follow it right up to U2750. The input pins to U2750 have the squareIf I feed the square wave only into channel A, I can see it on both the input |
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Lynn Lewis
I'm ashamed to admit I have not checked that. Will do so in the A.M.
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Thanks. Lynn -----Original Message-----
From: Miroslav Pokorni [mailto:mpokorni2000@...] Sent: Tuesday, January 22, 2002 4:56 PM To: TekScopes@... Subject: Re: [TekScopes] Who's up to some remote troubleshooting? Hello Lynn, How about Trigger Source Selector (or whatever is name of that mux controlled by a rotary switch) being stuck to Channel A. I think that the selector proper are two ICs that get alternatively disabled by a voltage input; that might be even broken wire (or trace) coming from the rotary switch, so select input is in default position. Regards Miroslav Pokorni |
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Stan or Patricia Griffiths
Hi Lynn,
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I'll be the first to admit I don't know much about troubleshooting stuff with IC's in it but here is what I would do, if it were mine, and I had the stuff that you have. It sure sounds like U2750 to me . . . and to you too, because you have replaced it twice with no luck. To be certain it is not U2750, I would like to see the original U2750 removed from the bad 7A26 installed in the working 7A26, and verify that it works there. I always worry that something might be blowing every U2750 that I plug into the bad 7A26. If that were the case, you could indeed change U2750 twice and still not get the 7A26 to work properly . . . and you would have 3 bad IC's after that. I would want to see those IC's work in a good 7A26. The next thing I would do is check to make sure the power supplies and grounds are getting to U2750. Could be a bad IC socket. Just a couple of ideas . . . Stan w7ni@... Lynn Lewis wrote: Knowns: |
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Lynn Lewis
Thanks, Stan. That's sound reasoning. I thought about it early on but having
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never pulled or installed a Tek IC, I was reluctant to mess around with the good 7A26. Now that I've done a few, I don't think it will be a problem. I have "anti-static finger cots" that should keep me from blowing a good IC. I will definitely try that. Lynn -----Original Message-----
From: Stan or Patricia Griffiths [mailto:w7ni@...] Sent: Tuesday, January 22, 2002 11:59 PM To: TekScopes@... Subject: Re: [TekScopes] Who's up to some remote troubleshooting? Hi Lynn, I'll be the first to admit I don't know much about troubleshooting stuff with IC's in it but here is what I would do, if it were mine, and I had the stuff that you have. It sure sounds like U2750 to me . . . and to you too, because you have replaced it twice with no luck. To be certain it is not U2750, I would like to see the original U2750 removed from the bad 7A26 installed in the working 7A26, and verify that it works there. [remainder of quote deleted] |
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JOSE V. GAVILA (EB5AGV/EC5AAU)
Hi Lynn,
I have a service manual for the 7A26 and could scan any schematics part you need, if that helps. Regards, JOSE ---------------------------------------------------------------------- 73 EB5AGV / EC5AAU - JOSE V. GAVILA La Canyada - Valencia (SPAIN) EB5AGV Vintage Radio Site: European Boatanchors List: |
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A word of caution. The U2750 might have been blown in such a way to damage
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circuitry in working plug-in. Personally, I would use this test as a last resort, after verifying other things, like pulling out U1750, sending signal through channel B and checking where does it get lost (or finding out that it was U1750 that was at fault for not releasing commong lines). Regards Miroslav Pokorni ----- Original Message -----
From: "Stan or Patricia Griffiths" <w7ni@...> To: <TekScopes@...> Sent: Tuesday, January 22, 2002 9:58 PM Subject: Re: [TekScopes] Who's up to some remote troubleshooting? Hi Lynn,with IC's in it but here is what I would do, if it were mine, and I had the stuff thatyou have. It sure sounds like U2750 to me . . . and to you too, because you havereplaced it twice with no luck. To be certain it is not U2750, I would like to seethe original U2750 removed from the bad 7A26 installed in the working 7A26, and verifythat it works there. I always worry that something might be blowing every U2750that I plug into the bad 7A26. If that were the case, you could indeed change U2750twice and still not get the 7A26 to work properly . . . and you would have 3 badIC's after that. I would want to see those IC's work in a good 7A26. The next thingI would do is check to make sure the power supplies and grounds are getting to U2750.Could be a bad IC socket. Just a couple of ideas . . .it's not the other scopescope and it's not the 7B53A. Ifand follow it right up to U2750. The input pins to U2750 have the square inputI feed the square wave only into channel A, I can see it on both the thoseand output of U1750.) The DC voltages around U2750 are very close to indicated on the schematic and appear to me to be reasonable. I did |
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