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Re: Strange Tek2440 issue
On Wed, Sep 2, 2020 at 2:07 PM Vince Vielhaber <vev@...> wrote:
I had actually done that last nite, but didn't have the parts layout Uh, this makes no sense to me. The base of Q870 should be pulled toward +15V unreg through R864(? hard to read on the schematic), and the two op amps shunt it towards ground when they detect too high voltage or too high current, respectively. Whichever of pins 1/7 is lower is the op-amp in control of the output. ... time passes ... Yeah, still doesn't make sense to me. I'm going to guess that either the op-amps are (pins 1/7 are) reversed on the schematic, or that your channel designations are. Even so, it still doesn't make sense, as the voltage-control op-amp starts dialing more voltage (Channel 2 normal at 5.54V rises to 16.94V), while the current limit op-amp goes on the limit (Channel 1 normal at 16.76V dips to 6.16V). What doesn't make sense here is that the current control op-amp is dialing a voltage that exceeds the voltage control's usual/normal voltage. I haven't done the maths, but as there's a bias on the current limit op-amp's feedback input, it might be that it's simply kicking in because the output voltage has sagged. I think you need to look further afield, how's the +8V rail doing, the +-15V rails, raw and regulated? How is the -5V rail doing? Any other rails you can find... If you can't find a bad rail, start looking at the biasing for Q870 & Q879, have the resistors drifted? If R864(?) or R478 have drifted significantly upward, that'd explain why the transistors can't meet the current demand. If the biasing is OK, you need to look at the transistors themselves. If their gain is down, that'd also explain things. You can measure the base current to Q879 indirectly by the voltage drop over its base resistor. Even looking at what's happening on Q870's emitter under collapse - is that transistor going to saturation, indicating that Q879 is dozing off on the job? Good luck! |
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Re: Strange Tek2440 issue
I had actually done that last nite, but didn't have the parts layout handy to see which probe was on which pin (I had them on the diodes>.
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Here they are with voltage measurements: Thanks! Vince. On 09/02/2020 01:25 PM, Siggi wrote:
On Tue, Sep 1, 2020 at 10:06 PM Vince Vielhaber <vev@...> wrote:Both the voltmeter and the 2430A. There's not much garbage on it but IHere is a shot of both sides of R873 on the 2430A. At power off, both --
K8ZW |
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HAPPY 20th BIRTHDAY TEKSCOPES!!!
Michael Dunn started TekScopes 20 years ago on August 31, 2000. There was
just one post that day from Walter Shawlee who joined that same day and he has been an active member ever since. To Walter: Thank you for 20 years of contributions to TekScopes and for all the parts and instruments you have on your web site! * Three days later Stan Griffiths W7NI (RIP) joined. * Three more members, including Tom Gootee, joined in October, 2000. * By the end of 2000 there were 13 members. * A year later it was clearly important: there were 68 members and >150 posts each month. John Miles and Morris Odell joined TekScopes in that first year. To John and Morris: Thank you for your regular contributions over the 19 years you have been members! * I was a late comer. When I joined in mid-November 2002 there were already 250 members. I was looking for something like this for a long time and I jumped right in with my first post that same day. Apparently I have been busy ever since I joined - I just noticed this will be my 5,000th post since I joined. Today we have over 8,000 members worldwide and there are 500 to 1500 posts per month. Thank you, Michael Dunn, for realizing the need for a Tektronix support group, for all you did to keep it going the first 17 years, and for being there for me when I need occasional help and/or guidance as I continue to moderate it. Dennis Tillman W7pF |
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Re: Strange Tek2440 issue
On Tue, Sep 1, 2020 at 10:06 PM Vince Vielhaber <vev@...> wrote:
Both the voltmeter and the 2430A. There's not much garbage on it but IHere is a shot of both sides of R873 on the 2430A. At power off, both traces are together. The top one is the at the junction of R873, R876, I guess next up is to capture U870 pins 1 & 7, see what's up there when your rail collapses. On one hand, you could be hitting the current limit, which could be because of actual overcurrent, or because the resistor network on U870A pin 2 has drifted. Also the -5V rail could be compromised, as the current limiter is referenced to that. Also check all the other rails, as the +5 needs the +8 to be up and sound as that's where Q870 draws from. On the other hand it could be that Q870/Q879 have degraded and can no longer do the lifting required. |
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Re: TM500-series back-plane connectors
Hi Colin,
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If the contact is not one of the ones dedicated to providing power or connecting to the pass transistors then it may never get used and you can put a note on the top of the TM504 saying that you removed it. The odds are extremely low that you will ever need that particular contact on that slot to interconnect a signal between plugins. If that pin is being used for something important then I have the entire backplane from a prototype TM501 with the transformer and all the parts. It has the EDAC connector on it. You can do what you want with it. You are welcome to it. Let me know off list (dennis at ridesoft dot com) if you can use it. Dennis Tillman W7pF -----Original Message-----
From: [email protected] [mailto:[email protected]] On Behalf Of Colin Herbert via groups.io Sent: Wednesday, September 02, 2020 6:36 AM To: [email protected] Subject: [TekScopes] TM500-series back-plane connectors I have a TM504 which has one of the springy brass-coloured contacts on the back-plane mangled. I have tried removing one of these contacts from a spare back-plane connector with no success - they just get destroyed. I think if I could obtain a good contact, I might have some luck in fitting it to the damaged back-plane. This might mean that I will have to destroy a plastic back-plane connector to obtain one, unless someone has a broken back-plane connector that has loose contacts. It seems that these connectors were originally made by EDAC, but he modern ones don't have the same contacts - they are nickel-plated brass and are a different size. Similar connectors are made by and for JAMMA, too, but they are also not like the originals. Does anyone have any ideas or some loose (and correct) contacts? TIA, Colin. -- Dennis Tillman W7pF TekScopes Moderator |
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Re: Tek 7854 intermittent issues
Hi Albert -
On Tue, Sep 1, 2020 at 03:02 AM, Albert Otten wrote: You probably tried AQS because I pointed to that. Something very strange isAre you doing single-sweep Acquisitions with the 7B87? Single-sweep uses the 7B87's internal sample clock, and I just checked on mine and confirmed that at .5mS, the acquisition works fine, and beyond that, it's clocking too fast to fill up the memory. If I do a single-sweep acquisition at .2mS, I'll see the wave, but I have to hit stop for it to complete, and it beeps with an error. I've got a seemingly working (knock on wood!) 7854 w/ a 7B85 & 7B87, so y'all let me know if you need assistance checking measurements or behavior. cheers, Paul |
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TM500-series back-plane connectors
I have a TM504 which has one of the springy brass-coloured contacts on the back-plane mangled. I have tried removing one of these contacts from a spare back-plane connector with no success - they just get destroyed. I think if I could obtain a good contact, I might have some luck in fitting it to the damaged back-plane. This might mean that I will have to destroy a plastic back-plane connector to obtain one, unless someone has a broken back-plane connector that has loose contacts. It seems that these connectors were originally made by EDAC, but he modern ones don't have the same contacts - they are nickel-plated brass and are a different size. Similar connectors are made by and for JAMMA, too, but they are also not like the originals.
Does anyone have any ideas or some loose (and correct) contacts? TIA, Colin. |
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Re: Strange Tek2440 issue
Both the voltmeter and the 2430A. There's not much garbage on it but I did hang another cap on it and it made no difference.
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Here is a shot of both sides of R873 on the 2430A. At power off, both traces are together. The top one is the at the junction of R873, R876, Q879 E. The lower one is on the C494 side. This was caught just when the sweep was turned to 100ns/div. I quickly hit Save when it happened. Thanks! Vince. On 09/01/2020 08:55 PM, Siggi wrote:
On Tue, Sep 1, 2020 at 8:15 PM Vince Vielhaber <vev@...> wrote:Ok, Q879 collector is solid. When things go sour, it stays. The base --
K8ZW |
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Re: 1S1 sampling unit; (also 7B53AN & 7A18 plugins)
David,
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Hopefully there is an active HP group where you can get more informed assistance on your 1740A. I will say this: Many years ago,? I purchased a used HP all vacuum tube oscilloscope, which was initially a fine instrument. However, after a week or two, it stopped working. I opened it up and discovered that the deflection amplifier output tubes, which ran quite hot, were in sockets which were attached directly, and only, to a large printed circuit board. Those tubes were literally falling through holes in the PCB which were the result of the heat from the tubes dessicating all of the resin from the board over how many years old (Probably 10 to 12) it was. Fortunately for me, I had purchased the it from a reputable dealer who, upon return of the HP o'scope, allowed me to apply 100% of its purchase price towards a used Tektronix 561A, along with a variety of plugins. I still have the 561A. It now lives in my closet, along with the plugins, having been supplanted by 46x, 5000 and 7000 series instruments. It still works and I had to dig it out last year to use as my 5000 required repair. The 561A did yeoman work once again, helping me to repair its somewhat newer brethren. I guess the moral of this story is that HP was capable of, and did, build some excellently performing oscilloscopes, but their idea of quality construction wasn't even in the same league as Tektronix. This is why you see very few classic HP scopes. Worldwide, aerospace and military buyers prized the reliability and longevity of Tek o'scopes so, over the years, they purchased them by the hundreds of thousands. Also, because Tek o'scopes were so long lived, many survived long enough to be retired due to obsolescence, not failure, and surplussed out. Regarding the 1740A: Just as the much older model of my experience, it is not exceptionally solidly built. Beware the Schadow push button switches. They are commercial grade parts and notorious for having corrosion build up on the contacts. They can be serviced but its not a congenial process, requiring the use of some somewhat nasty solvent based chemicals and a tiny bit of VERY fine abrasive powder which must be completely removed once its done its work. Thomas Garson Aural Technology, Ashland, OR ------------------------------------------------------------------- On 9/1/20 3:55 PM, David Collier wrote:
My mistake: HP1740A. |
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Re: 1S1 sampling unit; (also 7B53AN & 7A18 plugins)
Lots of vintage stuff pricing is quite high these days. I also collect 8 bit computer junk and for example a digital doorstop like a Commodore 16 is priced in the 400-500$ US range these days.
On the Tektronix side I've noticed far fewer letter series plugins these days. Sampling and spectrum analyzers are rarely seen now IME. I guess all the ones worth snapping up have been snapped up, certainly by one person (who could it be) who has a closet full of them and never uses them... |
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Re: Strange Tek2440 issue
On Tue, Sep 1, 2020 at 8:15 PM Vince Vielhaber <vev@...> wrote:
Ok, Q879 collector is solid. When things go sour, it stays. The base How are you measuring the collector voltage? The switching frequency is ~25kHz, and it's going to be ugly as heck if the capacitor is out, so the instrument you're using to measure this and its bandwidth are quite important. The average or RMS voltage there just isn't super important :). What happens if you tack on extra capacitance from Q879 collector to ground? You probably want a capacitor that's fairly low ESR, the ripple current is going to be something wicked if the average current is 3A (see below). Under normal conditions there's aThat looks sane according to the voltages annotated on the schematic. That's a 3A nominal load, though. U870A/B are the current/voltage controls, respectively, or-ed together through the diodes. You can tell which one is in control by the voltage out of pin 1/7. Going to take a closer look at Q879's feed.Have fun! |
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Re: Strange Tek2440 issue
Assuming a VBE drop across the sensing resistor, then you're drawing roughly half an amp.
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Harvey On 9/1/2020 8:15 PM, Vince Vielhaber wrote:
Ok, Q879 collector is solid.? When things go sour, it stays.? The base voltage is moving, as is the emitter.?? So it's being told to compensate for the change.? However, under normal conditions, the collector voltage is lower by a few hundred mv than the schematic calls for while the base and emitter are within a few mv.? Under normal conditions there's a 300mv drop across the shunt. |
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Re: Strange Tek2440 issue
Ok, Q879 collector is solid. When things go sour, it stays. The base voltage is moving, as is the emitter. So it's being told to compensate for the change. However, under normal conditions, the collector voltage is lower by a few hundred mv than the schematic calls for while the base and emitter are within a few mv. Under normal conditions there's a 300mv drop across the shunt.
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Going to take a closer look at Q879's feed. Vince. On 09/01/2020 10:03 AM, Siggi wrote:
On Mon, Aug 31, 2020 at 8:36 PM Vince Vielhaber <vev@...> wrote:The 5V jumping around is the 5V regulated that C494 is on. There itOh, interesting. I assumed you were talking about the 5VD, which is the --
K8ZW |
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Re: 1S1 sampling unit; (also 7B53AN & 7A18 plugins)
David Collier
My mistake: HP1740A.
I get the impression vintage TEK scopes are ten times the price they were ten years ago, and now rare. My HP1740A was from a recycling centre for next to nothing, just needed switch cleaner. And thanks for the recent information: very useful. Regards David Collier |
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Re: Tek 7854 intermittent issues
Hi Nick,
It doesn't help you any further, but in a rather tricky way I managed to create a horizontal gap in AVG waveform acquisition. And indeed a (dotted) linear interpolation line between the dots at the ends of the gap was shown. Since the vertical digitizing in your video seemed to miss or add a sign bit during part of the trace it seems that both vertical and horizontal digitizing suffer from DAC errors. Albert |
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Re: 4-pin Lemo connector used on end of cable for 4041 PD keyboard
Raymond: I only need 1 Lemo 4-pin plug. How much would it cost me to have
you send me one? How would I pay for it? Here is my address: Gary Bosworth 139 E. Shrode Street Monrovia, CA 91016 Cellphone: 626-803-8336 e-mail: grbosworth@... On Mon, Aug 31, 2020 at 10:09 AM Raymond Domp Frank <hewpatek@...> wrote: On Mon, Aug 31, 2020 at 06:04 PM, Gary Robert Bosworth wrote:--through Gary Robert Bosworth grbosworth@... Tel: 310-317-2247 |
Gary Robert Bosworth
Shaun M
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