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Re: Tek485 - a quick question on disabling/bypassing power supply protection
If you really have a short circuit from -5V to ground (eg a failed tantalum capacitor) you don't need to power up the scope, it should be obvious with a resistance measurement. Schematic <17> shows the power distribution between boards and you can isolate boards by pulling the multi-way 'combs' that are used for interconnection.
To be sure that your diagnosis of a -5V fault is correct, and assuming that you have another double beam scope available you can look at the low voltage power rails in pairs as the power comes on and check that each is heading for its intended value before the protection cuts in. Roger |
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Re: CRT Hood for 7000 series 'scopes?
I have one of these. I got it off eBay and it seems no-one else was interested, as I got it for the starting price of 99 pence (UK). They seem to be very rare items and bears a part-number 016-0154-00. I have not been able to trace this part-number.... I should probably say that I am in London, UK.
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Colin. -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of [email protected] Sent: 23 April 2019 09:01 To: [email protected] Subject: [TekScopes] CRT Hood for 7000 series 'scopes? I found this old thread (/g/TekScopes/message/152935?p=,,,20,0,0,0::Created,,crt+hood+,20,2,0,28557610) that showed the "batman" style viewing hood. While I can see the utility of this hood for looking at fast pulses on non mcp crts, did they ever make just a simple shade for the 7000 series that fits over the crt bezel to just keep room lights off? I've seen these (round) on 5xx series 'scopes. Can't seem to find a mention of such a thing in manuals. Thanks, Sean |
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Re: 7S12 with S-4 and S-53 troubleshooting
Hi Albert,
You're probably right, I didn't analyze the feedback/offset circuit in fine detail. My conclusion about the offset was mostly based on what I observed while probing around the head. My 7S12 suffered from poor contacts issue (mostly due to IC sockets), there is always a possibility that something like that interfered during tests. Anyway, I will try the equivalent test on my S-4 (remove Q55) that you carried out on your S-2. It would be worthwhile to reach a proper conclusion on this circuitry. However I will be able to try this only after the 1st of May holiday, will report back. Best Regards, Nenad |
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Re: 7L5 no video signal
Hi Ed,
Did you try switching off digital storage? If there is a fault in it (and it's quite a complex circuit), it would interrupt the signal path. My unit was such a case (was working only with DS off). When you switch DS off, internal MUXes route the detector output directly to the vertical amplifier (buffer). This path is much easier to diagnose, I'd try that first. Also check the Video Out plug on the front panel. Best Regards, Nenad |
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CRT Hood for 7000 series 'scopes?
I found this old thread (/g/TekScopes/message/152935?p=,,,20,0,0,0::Created,,crt+hood+,20,2,0,28557610) that showed the "batman" style viewing hood. While I can see the utility of this hood for looking at fast pulses on non mcp crts, did they ever make just a simple shade for the 7000 series that fits over the crt bezel to just keep room lights off? I've seen these (round) on 5xx series 'scopes. Can't seem to find a mention of such a thing in manuals.
Thanks, Sean |
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Re: Pacific Measurements or Wavetek 1038 System
Hans Boon
Hello,
I have a 1038-D14 and 1038-N10 combination, with 3 matching powersensors. Also have the operating and service manual for the D14A and N10 (already scanned), please indicate how to transfer these files to you. The major problem I have is the bad pushbutton switches on the N10, which prevent me from operating the units. I already unsoldered the switches, but I am still not able to control the N10 / D14 combination from the GPIB interface. Seems I would have to use the switches to put the N10 to remote contol mode. Any help on these problems is welcome. Regards, 73, Hans, pa2hbn |
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Interesting bit of trivia - the Fairchild/Dumont 766 oscilloscope
Whilst looking through my test equipment inventory a bit of trivia came to mind regarding a unit that I had acquired back in the late 90s off of surplus. It is still tucked away in storage and I had almost forgotten about it over the years.
The unit is a Dumont/Fairchild 766 oscilloscope with a 76-02 dual-trace and 74-03 time base plug-in. What was interesting that I found when I first opened it after receiving it was that the large panel immediately behind the CRT (a sub panel ahead of the actual rear panel that contains the power transformer, power transistors, capacitors and such that are mounted on the panel) was sprayed with a thick coating of either black paint or some other heavy substance that coves the entire panel including all of the components mounted to it. Aside from the paint job what I did find rather unique about this particular instrument was that it contains no pots for horizontal or vertical positioning of the trace but, instead, has a joystick on the panel for positioning. From the way the paint is applied it is obvious that it was a factory job being professionally applied prior to assembly of the scope. The edges of the panel are masked and there would be no way to spray the coating on the panel from the way it is installed in the instrument. I later learned that this scope was manufactured in the 1960s around the time Tektronix had filed a lawsuit against Dumont for a direct copy of the circuitry in one of their products. I never did find any evidence as to the outcome of the suit. I am wondering if this model oscilloscope was the victim of the Tek-Dumont/Fairchild battle and that a quick cover-up was made to circumvent any further penalties for the company in their product in a last-minute move. I can see no other reason to apply a heavy coating of paint or whatever to the innards of the scope. Tonight I did a little Googling and found that Fairchild/Dumont later came out with the model 766H ¡°Signal Analyzer¡± - no longer titled ¡°oscilloscope¡± which was still the very same oscilloscope with the same plug-ins that this unit in my possession has. This also raises a question as to why they did such a thing to the very same instrument. Greg |
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Re: Again a 7854 power supply beast to be repaired...
O.K Albert,
. I've found theDennis' documents you mentioned on how to build a complete dummy supply load for the 7104/7854 SMPS. I will buy the necessary resistors... :-/ BUT the document only mentions the connections to be made (using the dummy load) towards P82. IMHO the connections to P83 that performs the voltage sensing should also be wired in an appropriate manner. Otherwise the low voltage regulator (board A22, schematic 16) cannot work! Thanks G?ran for your suggestion regarding diode checking. I only checked them using my analog VOM Supertester ICE 680R in ohmic mode and no defects was revealed. I will try to measure them using the oscilloscope and differential probe. Max Mazza (from near Venice, Italy) |
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Tek485 - a quick question on disabling/bypassing power supply protection
My Tek485 has been sidelined for some months with a power supply issue - the inverter keeps cycling on/off. I've fixed similar issues a few times but this one has me foxed. Balance node shows a short on the -5V line (present intermittently in the past, now permanent) that I strongly suspect is caused by something in the cathode regulator circuit. Lots of component checking on the LV and HV regulation circuitry failed to turn up any obvious problem. Removing pin U3 hasn't helped so seems it's an LV problem. Other supplies come up normally. However, I've found it quite difficult working with the transient signals I get with just 30ms of start-up before the balance node circuit recycles - all the action happens in the first 10ms or so after startup. Wondering if there is any way of disabling/bypassing any part of the protection circuitry to allow easier tracking of what's shorting the -5V supply without risking damage to other parts. I suspect the answer is no but thought I'd seek some expert advice in case I'm doing this the hard way and there's a work-around. Haven't managed to find anything online.
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Re: 577 Curve tracer ringing CRT and noisy step generator
In cases where you need to measure a transistor (or other device) at
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extremely low currents, electrical noise or hum pickup and inter-electrode capacitance can become so great it completely swamps the curves you are trying to see. The filter solves part of this problem. The other part of the solution is to switch the Collector Supply Polarity from NPN or PNP, which sweep the beam at a 60Hz rate, to +DC or -DC respectively. Then you sweep the beam manually at such a slow rate that you eliminate any capacitive coupling due to inter-electrode coupling. The filter eliminates electrical noise or hum pickup that might be causing you problems. Of course, it helps to have the D1 Storage version of the 577 when you are sweeping the beam manually, otherwise, as you sweep the DC voltage, how can you capture the current curve that results? One solution that comes to mind is to use a grease pencil to trace it on the screen :) Storage is one good reason why a 577 is a better solution than a 576. The other one is the 577 is almost 30lbs lighter than a 576 so it doesn't take two people to pick it up onto a desk. Dennis Tillman W7PF -----Original Message----- -- Dennis Tillman W7PF TekScopes Moderator |
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Re: Pacific Measurements or Wavetek 1038 System
Hi,
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The 5000 series mainframes were, in part, designed to be low cost products. As such they were very attractive to other manufacturers as the display portion of the product they developed. Often, Tek would have provided a way to OEM (Original Equipment Manufacturing) products like this to other companies at a substantial discount provided they were purchased in quantity. In these cases the company would or could private label them with their own logo. This is a common practice in the electronics industry. Dennis Tillman W7PF -----Original Message-----<SNIP> Interestingly, after acquiring the Pacific Measurements 1038 system, I -- Dennis Tillman W7PF TekScopes Moderator |
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Re: Again a 7854 power supply beast to be repaired...
You need a load of about 50W, possibly equally shared between positive and negative outputs. Either on the raw voltages or after the regulator board. You need to check all caps and diodes on the rectifier board, if you have not done this before. You can check diodes by making differential measurements across the diode. I found two bad power diodes this way.
G?ran |
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Re: Again a 7854 power supply beast to be repaired...
Hi Max,
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Well, I didn't say that the load MUST be distributed. For me reasons to do so were that I could use available 5 W and 10 W resistors and that I wasn't always sure about the maximum current an output could deliver. Your 10 W resistor will survive start attempts, but when an error has to be detected which tends to occur after several minutes then you have a problem. I am really surprised that about 11 W was sufficient. IIRC in a 7904 the HV circuits are less easily to disconnect so maybe these formed a load also. At this moment I don't remember all the details, but I think 11 W would not even be enough to maintain the 7.5 zener voltage inside th IC. Anyway, over 50 W was my experience. Albert
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Re: Again a 7854 power supply beast to be repaired...
To G?ran:
OK, shorted pin 13 to ground. Still same result: the SMPS refuses to starts and ticks herratically. I've checked the signal across A and K of Q30 (the PUT): it fires up every 20 msec (I live in Italy) but there are some spurious pulses in between. The pin 9 shows a steady +0.6V with sparse negative pulses going to approx 0 V. To Albert: I really don't know how a dummy load must be connected to raw outputs! Some people told me to load +17V was sufficient, some others the +108, now you and others say that the load MUST be distributed across the raw outputs! I'm really confused now... And yes, I've disconnected the supply to HV unit obviously... since the SMPS block sits on my workbench, alone. Some times ago (several months ago I suppose), a friend of mine fixed a 7904 SMPS; he told me that as a load he had connected a 20 ohm resistor on the +15 (regulated output) and nothing else and the smps worked ... |
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Re: Again a 7854 power supply beast to be repaired...
Hi Max,
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From my own experiments with a 7704A SMPS I remember that it needed a much heavier load for proper operation. I tried with loads on the HV outputs and also with loads on the raw rectified outputs (LV disconnected). Other members (including G?ran) found similar results. Dennis in his recent 7104/7854 .docx file uses 70 W total, distributed over the LV outputs. So why do you think about 35 W is enough? I guess you also disconnected the supply to the HV unit? (Some other readers might not know that the HV unit is a separate unit here, not buried deep inside the smps box.) It has been noted that the documentation of the control IC is not very trustworthy. [BTW your 10W resistor will be glad that the smps is not working continuously!] Albert
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Re: 577 Curve tracer ringing CRT and noisy step generator
Thanks for the tip!
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On 22 Apr 2019, at 21:23, Ed Breya via Groups.Io <edbreya@...> wrote: |
G?ran Krusell
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