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Re: Little ceramic trimmer caps that always break... A fix.
Chuck Harris
It is becoming a touchy subject for me. All my life I have
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been a very fast touch typist. I spent a lot of time writing programs, and later writing documentation, and could always type quickly and effortlessly... But in my 60's, I am noticing coordination problems in my typing, I "slur" my shift control, and swap adjacent letters. FG504 can become FG540, Chuck can become CHuck... I even do a little weird thing where small common words, like "the" come out backwards "eht", and other words always gain an extra letter, such as "ratio" always getting typed "ration". I always find it amusing when I type a long treatise on some subject or other, and the only comments that get made are about a misplaced comma, or a typo. I am getting older. I can't see as well as I once did, I cant type as well as I once did... And I don't like it. I am working through a couple of FG504's. If I can help, just ask. -Chuck Harris Colin Herbert via groups.io wrote:
No, the typo doesn't matter in so far as that is what it was. I was just curious because I am trying to get a FG504 working correctly and not doing too well at the moment. I didn't mean to insult you or anything like that; please forgive me if that's what it came over as.
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Re: A possibly sick 7904A
Chuck Harris
Your description is pretty accurate.
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The more modern scopes have compensation circuitry that changes the focus voltage with the instantaneous intensity setting.... and changes the instantaneous intensity settings for the different beam uses... So, the intensity is separately controllable for the A horizontal plugin, the B horizontal plugin, the readout, and in some cases, the "intensified" horizontal A vs B settings... but FOCUS, and ASTIGMATISM, each have only one user control. Each of these settings result in a different beam current, which require a different focus/astigmatism voltage. For the 7904A, which was one of the earlier adopters of this enhancement, they have a circuit called Auto Focus, and in the Performance check and Adjustment procedure, page 5-12 of my manual, they explain how it gets adjusted. The do the main focus adjustment with the beam very dim, and then they adjust the Auto Focus circuit with the beam at full intensity control position. -Chuck Harris Roger Evans via groups.io wrote:
I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.
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Re: Little ceramic trimmer caps that always break... A fix.
Stephen
On Wed, Aug 19, 2020 at 01:22 AM, Chuck Harris wrote:
I make typos all the time. Especially with multi spellcheck in different languages. It writes whatever it wants... If I don’t double check... Don’t worry, it’s okay. I understood you were talking about the FG504. But maybe Colin did believe an actual FG540 existed and was confused when he did find it. Mistakes happen.
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Re: Little ceramic trimmer caps that always break... A fix.
Colin Herbert
No, the typo doesn't matter in so far as that is what it was. I was just curious because I am trying to get a FG504 working correctly and not doing too well at the moment. I didn't mean to insult you or anything like that; please forgive me if that's what it came over as.
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Colin.
-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Chuck Harris Sent: 19 August 2020 13:22 To: TekScopes@groups.io Subject: Re: [TekScopes] Little ceramic trimmer caps that always break... A fix. Honestly, does it really matter that I mistyped FG504? I am talking about ceramic trimmers that exist in hundreds of Tektronix instrument models. -Chuck Harris Colin Herbert via groups.io wrote: Do you mean an FG504? I can't find any FG540 on TekWiki.
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Re: Little ceramic trimmer caps that always break... A fix.
Chuck Harris
Honestly, does it really matter that I mistyped FG504?
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I am talking about ceramic trimmers that exist in hundreds of Tektronix instrument models. -Chuck Harris Colin Herbert via groups.io wrote:
Do you mean an FG504? I can't find any FG540 on TekWiki.
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Re: Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal
Roger Evans
Luis,
Since both channels are equally affected the fault must be either in the beam switching circuitry (not very likely) or in the Y amplifier stages following the beam switching. There is a fairly simple test on the beam switching that you can do with just a DMM. If you locate test points TP364 and TP 374 on schematic <3> and the example waveform shown in <21> and <22>, you should be able to replicate the DC values ( +1V and -1V) by setting the horizontal mode to ALT and the timebase to its slowest setting so the voltage flips every half second or so. Set both traces to somewhere near the middle of the screen and trigger to AUTO. This is not a definitive test but if these voltages are wrong the diodes may not switch correctly. I only just now noticed you have Trig View option on the 465. Try feeding the calibrator signal to Trig In, set Trig Source to Ext or Ext/10. Push the trig view and you can move the trace up and down with the Trig Level control. Does the vertical height of the displayed trigger signal change with position in the same way as do the Ch1 and Ch2 signals? If Q322 and Q324 are socketed then remove and test them. The Service manual schematic <3> shows various voltages around Q322 and Q324, set up the scope as best you can to the conditions defined in the manual at the start of the Schematics section under the heading 'DC Voltages'. See how well your measurements match the figures in the manual. Farther down the Y amplifier chain there was a change from discrete transistors to an IC based Y amplifier. Which one do you have? Regards, Roger
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Re: Little ceramic trimmer caps that always break... A fix.
Colin Herbert
Do you mean an FG504? I can't find any FG540 on TekWiki.
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-----Original Message-----
From: TekScopes@groups.io [mailto:TekScopes@groups.io] On Behalf Of Chuck Harris Sent: 19 August 2020 05:15 To: TekScopes Subject: Re: [TekScopes] Little ceramic trimmer caps that always break... A fix. Just to clear up some confusion: This note is about subminiature ceramic disk type trimmer capacitors. Tektronix equipment from the 1980's onward is littered with these little timebombs. It is a "modernization" of the old style ceramic trimmers that had a silver plated half circle on the lid that was soldered to the adjusting screw. In this case, the silver plating is inside of the trimmer, and the ceramic disk is held on by very temperamental magic. The moving ceramic disk sticks to its ceramic base, and the magic disappears. -Chuck Harris Chuck Harris wrote: Hi All,
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Re: A possibly sick 7904A
Roger Evans
I am not an expert on this, trained in physics not electronics. When you look at a low rep rate signal at 500ps/div the beam is on a very small fraction of the time, about 5ns every msec so one part in 200000. The readout is being triggered automatically and runs around 1kHz or so, I don't remember the actual clock speeds. So for the same brightness the signal trace has to carry maybe 1000 times the current compared with the readout, this means a higher density of electrons and the space charge can affect the beam focussing compared with a low current beam.
I tried putting a fast rise square wave into my 7904 at 1kHz and qualitatively I see what you see. I have to turn up the brightness a lot to see the trace at 1ns/div and it does defocus compared with the readout. I can improve the focus slightly and that worsens the focus of the readout significantly. Maybe if we get some more comments we can decide who is unrealistic and who is easily satisfied. I have no idea what the factory specifications were. Regards, Roger
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Re: Little ceramic trimmer caps that always break... A fix.
Chuck Harris
Just to clear up some confusion:
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This note is about subminiature ceramic disk type trimmer capacitors. Tektronix equipment from the 1980's onward is littered with these little timebombs. It is a "modernization" of the old style ceramic trimmers that had a silver plated half circle on the lid that was soldered to the adjusting screw. In this case, the silver plating is inside of the trimmer, and the ceramic disk is held on by very temperamental magic. The moving ceramic disk sticks to its ceramic base, and the magic disappears. -Chuck Harris Chuck Harris wrote:
Hi All,
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Re: Little ceramic trimmer caps that always break... A fix.
Chuck Harris
Hi John,
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This is normal stuff when you calibrate an instrument for the first time in decades. These little subminiature trimmer caps break all the time. Every time I try to turn one for the first time, my jaw clenches up, and I give it a gentle turn. Either the ceramic part breaks free and the cap turns, or the screw breaks free of the ceramic disk. It is fixed, and the generator impedance is now adjusted. I am about half way through the calibration adjustments... All is good. I just like to put out repair notes as my local reality reminds me of them. -Chuck Harris John Ferguson via groups.io wrote:
Wow, Chuck, it was worse than I thought, but it sounds like you have prevailed.
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Re: Little ceramic trimmer caps that always break... A fix.
Chuck Harris
Trimmer caps is short for trimmer capacitors.
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-Chuck Harris keantoken via groups.io wrote:
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Re: Little ceramic trimmer caps that always break... A fix.
Stephen
On Tue, Aug 18, 2020 at 03:21 PM, keantoken wrote:
I believe he’s talking about precision adjustable capacitors, not trim pots... 🤷♂️
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Re: Little ceramic trimmer caps that always break... A fix.
John Ferguson
Wow, Chuck, it was worse than I thought, but it sounds like you have prevailed.
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john
On 8/18/20 9:50 PM, Chuck Harris wrote:
Hi All,
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Re: Little ceramic trimmer caps that always break... A fix.
keantoken
You mean these trimmers?
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https://www.mouser.com/ProductDetail/?qs=HHuYZtQ5iDWVDisM2QQAHw%3D%3D I had them fail in 2 ways. The caps became loose so the wipers had no contact, or they get dead spots which don't seem to be reversible at all. I want to replace these with sealed Bourns trimmers as possible. My FG504 came very clean and new looking and with a calibration sticker, like it had been used in a climate controlled building. In my 465B scope, one of them had the cap break off in the middle where the shaft is. I didn't have a replacement so I wedged something between the trimmer and some wires in that area to keep it in contact. And there it remains to this day. The brown cap is plastic, not ceramic. They seem to get brittle over time, perhaps accelerated when exposed to chemicals. I wonder how long yours will last after this treatment.
On Tuesday, August 18, 2020, 08:51:04 PM CDT, Chuck Harris <cfharris@erols.com> wrote:
Hi All, Just a note. As happens to all of us, one of those little ceramic trimmers in an FG540 was broken. The screw turned and turned, but the ceramic disk on top didn't. It was frozen solid. Bother... I didn't have a spare, so I removed the trimmer, and found the ceramic disk frozen solid to the ceramic base... no trimming going on here! First, I put the capacitor into some IPA to soak for a bit. There was more than enough stinky flux on the cap, so I think the flux was gluing the cap stuck. After it soaked for a while, I was able to turn the disk with my fingers... don't use tools, it will break! I put it in for some more soaking, and twisting, until it was good and clean. Next, I took a small dot of cyanoacrylate glue, and spread it around the screw. A small dot, not a flood. [If you can't control your glue bottle, put some CA glue on a piece of plastic, and use a toothpick to bring a dot to your trimmer.] Then finally, I gave a rag a squirt of "Kicker", a CA glue accelerator, and put the rag near the trimmer... The kicker's fumes are enough to harden the glue almost instantly. If you don't have any kicker, just let it set a while, it will harden from the moisture in the air. I gave the screw a twist, and all was back to normal. -Chuck Harris
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Little ceramic trimmer caps that always break... A fix.
Chuck Harris
Hi All,
Just a note. As happens to all of us, one of those little ceramic trimmers in an FG540 was broken. The screw turned and turned, but the ceramic disk on top didn't. It was frozen solid. Bother... I didn't have a spare, so I removed the trimmer, and found the ceramic disk frozen solid to the ceramic base... no trimming going on here! First, I put the capacitor into some IPA to soak for a bit. There was more than enough stinky flux on the cap, so I think the flux was gluing the cap stuck. After it soaked for a while, I was able to turn the disk with my fingers... don't use tools, it will break! I put it in for some more soaking, and twisting, until it was good and clean. Next, I took a small dot of cyanoacrylate glue, and spread it around the screw. A small dot, not a flood. [If you can't control your glue bottle, put some CA glue on a piece of plastic, and use a toothpick to bring a dot to your trimmer.] Then finally, I gave a rag a squirt of "Kicker", a CA glue accelerator, and put the rag near the trimmer... The kicker's fumes are enough to harden the glue almost instantly. If you don't have any kicker, just let it set a while, it will harden from the moisture in the air. I gave the screw a twist, and all was back to normal. -Chuck Harris
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Re: Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal
Greenpitu@...
Thanks Chuck for the detailed explanation!
Much appreciated. Regards Luis
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Re: Tek 465: CH1 No signal, CH2 signal reads a fraction of input, V Pos distorts signal
Greenpitu@...
Hello Roger,
I am glad to inform you that I have made good progress after following your advice with CH1. Thanks so much! I am still, however, facing some challenges. Armed with ISO and strips of paper, I followed the instructions to clean-up all possible levers I could possibly reach. The task is cumbersome but (strangely) fulfiling when you pull the paper out and feel that bit of resistance against the gold plated contacts. In addition, I pulled out the 10X and 100X attenuators from CH1 attenuator boards, and cleaned all their pins, before plugging them back onto the board (few times). I then turned the power back on, but CH1 was still deaf. In addition, I now realized CH2 was deaf, too! I wasn't expecting this. Got me thinking for a while as I was certain I didn't touch CH2 circuit at all. I left CH1 for a minute and focused back on CH2. I was upset I would be losing that one, even though we know it wasn't 100% working either when it broke. In any case, after some pulling and pushing and whatnot, I realized that, if I pushed CH2 BNC adapter - where the probe meets the female connector - just at the right angle with the right pressure, then the 1/3 of my input signal came back to life again. There was hope! That was a lucky and easy one. It appears that C53 and R53 soldered in series with the BNC pin have a soft spot. Pushing the connection around a bit got the beam showing the calibrators 1KHz signal once again. I sense I need to solder the contacts once again, but I park that one for now. Still, back to 1/3 of the input signal. I went back to CH1 and pushed around the equivalent C3 and R3 in series with CH1 BNC pin, in the hope it would get the channel back to life. And it did, but temperamentally so. The beam showed up as a heavily under-compensated signal on the screen. I noticed the problem was more noticeable on the .2mV position when a particular cam-operated contact was closed. I cleaned that contact once again with more ISO and voila.. stable square signal on CH1! Now we got to this point where both, CH1 and CH2, show the 300mV 1KHz square signal, but just 100mV i.e 1/3 of its amplitude! Plus, the bean on both channels now equally distorts and expands from 100mV to 150mV when the V Pos controls are moved all the way up (or down)... there is consistency! Any thoughts about what else to try next, please? Does it appear as if the signal is not being amplified as it should? Regarding ADD, ALT and INVERT, all three appear to be in working order: I hooked two probes to the onboard calibrator, and then one each on CH1 and CH2. The ADD adds them, the ALT alternates between the two (albeit per V Pos above they distort greatly when moving off centre) and the ADD + INVERT on CH2 substracts both signals, leaving me with a nice, flat, focused beam at the centre of the screen..... I am afraid, I won't be able to get hold of a second oscilloscope at least within the next 3 or 4 weeks :( Any further suggestions much appreciated! Best Luis
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Re: 7854 display set up procedure without the calculator keyboard
There are a bunch on eBay. Search for Tektronix 7854 Calculator.
-Bob N3XKB On Tue, Aug 18, 2020 at 10:38 AM Martin Whybrow <martin@thewhybrows.co.uk> wrote: My 7854 has issue with the readout, the characters are lines (no Y
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Re: SC502 With Display Issues
Stephen
On Tue, Aug 18, 2020 at 08:11 AM, Dave Wise wrote:
I’m not educated enough in electronics to understand why that would matter, but I’m sure you’re right.
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Re: SC502 With Display Issues
Dave Wise
I looked at the schematic and the parts list, and the Common Design Parts Catalog.
Q930 switches off the power. Q935 and Q965 handle current limiting. All three only need to handle about 10mA. During normal operation, they are off. One would think they can be anything. However, Tek specifically chose 151-0341-00 instead of 151-0190-00, even though the latter is both cheaper and more reliable. My guess is they picked the 2N3565 because it is slower than the 2N3904, fT = 40MHz vs 300MHz. This doesn't matter for Q930, but you may want to stay slow on Q935 or Q936 just to be on the safe side. Dave Wise ________________________________________ From: TekScopes@groups.io <TekScopes@groups.io> on behalf of Stephen via groups.io <stephen.nabet=gmail.com@groups.io> Sent: Tuesday, August 18, 2020 11:09 AM To: TekScopes@groups.io Subject: Re: [TekScopes] SC502 With Display Issues On Tue, Aug 18, 2020 at 07:02 AM, Dave Wise wrote: While nobody is making the 2N3565 anymore, PN3565 by Central Semiconductor isThank you Dave. It’s in the Low Voltage Supply part of the circuit.
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