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Tek 453 resistor specs -- Horiz amp
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The diode addition is around Q664, half of the slope comparator pair of transistors. (B-time base) There is also an extra resistor. The soldering looks very neat, as if it was done at the factory, but I suspect someone else added these.
I could post a photo, or a sketch, but maybe a brief description would be enough to jog someone memory. The extra resistor is placed along R665, one end connected to R665, the other end to R653. The two diodes are connected in series, One end of this series is connected to R665, the other to D653 and R653. D653 has a fine wire coiled around it and soldered to each lead on the diode. One of the two extra diodes has the a mark in the middle of the body, but no cathode mark is visible; it is glass-bodied. The anode of the other one is connected to R665. Any ideas on why this extra circuitry is there? The slope switch for the B-time base works as expected when the B-Sweep Mode switch is in the "Triggerable after Delay Time " position., but not when it is in "B-Starts after Delay Time" position. The manual is not specific on what position the B-Mode switch can be in (works both ways or not), but my guess is that the slope should change in either position using the Slope switch. Same goes for the level knob (for B) -- it only works while in the "Triggerable after Delay Time" setting. Unless there is a good reason to leave it in, my plan is to lift the leads and see what happens. I'll leave the fine wire around D653 there. |
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Hello Barry,
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Metal film could be better, I agree, and I am not committed to carbon film. As I recall, carbon comp had lower inductance than the other two types, but of course they can drift in resistance over time and with heat. I will do some more studying on the advantages and disadvantages of the three types before I pick. The two resistors to replace don't look critical (they were all 5% in the original design), so the type probably does not matter that much. On 4/1/2022 7:42 AM, n4buq wrote:
Just curios why a carbon film. Wouldn't metal film be as good or, possibly, a better choice for stability, etc.? |
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Just curios why a carbon film. Wouldn't metal film be as good or, possibly, a better choice for stability, etc.?
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Thanks, Barry - N4BUQ ----- Original Message -----
From: "Mike Merigliano" <mmerig@...> Hello Mark, |
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Thanks Ozan -- I will look into R1003 and 3F? too.
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The last time I checked (a while ago) all of the power supply voltages were good. But when I open it back up I will double-check. Except for the odd shift and intensity problem in the Ext Horiz, position, everything else works fine. My profession is plant ecology, and although there are some plant physiology parallels to circuits, fixing a scope is a bit out there for me. But it's a nice change, and I would rather do that than crossword puzzles. I also have a lot of old audio gear that needs maintenance once in a while, and I have built some things, like a preamp. On 3/31/2022 8:13 PM, Ozan wrote:
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Hello Mark,
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Thanks for the advice -- I will try it soon. I will probably put in 1/4W 1% carbon film to replace the broken ones. That might be over-kill, but the cost difference should be small. I usually order from Mouser. The broken lead was on R631. Also R638 -- I should have said two resistors in my earlier message. In the midst of my repair, a wire came out of the intensity pot (the wire did not look broken, looked like a cold solder), and I started chasing down problems in the Z-axis circuit until I went back and looked at that pot. There was a slow-blow fuse for the Z-axis (should be fast-blow), so while checking the HV oscillator, I switched that out. This scope did not work at all when I bought it used years ago -- just a dot on one channel, and then only with the beam finder pressed. There's a strange diode addition in the B-delay sweep circuit, and some other obvious repairs, so whoever had it before me changed some things in that circuit for some reason. Some of the transistors were plugged in the wrong way. Broken horizontal deflection transistors were a key problem. I will re-visit that diode addition. It looked superfluous, or maybe helpful, but that diode could be part of the issue. On 3/31/2022 7:34 PM, Mark Vincent wrote:
Mike, |
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On Thu, Mar 31, 2022 at 05:22 PM, Mike Merigliano wrote:
After switching to Ext Horiz, the trace was way off to the left, so I have to----- For horizontal shift Mark's reply is most likely. If it doesn't fix the shift confirm that you see 0V at connector CJ of sheet <10> with Ext Horiz grounded, and Ext Horiz mode selected. Intensity could be because of shifted R1003 on page <14> or dirty contact 3F in the same sheet. Are all supply voltages at their target levels? Ozan |
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Mike,
Which resistor did the lead come out of? As for replacing or new build, I use 1% Xicon 1/2w. I have found that the Chinese sellers on "that auction" site have excellent 1-5W 1% types and are of the proper physical size. The temperature drift of these is 100 ppm/deg.C while the Xicons are 50 ppm. Using 1/4W 1%, I will use in places where the power is low enough. I like the low drift, very low noise and accuracy of the 1% types. This is my personal choice from using them long enough in enough things to decide to use these. Carbons are alright in some places. Replacing a carbon with a film type is usually better. By the dot being off that much, it sounds like your "mag register" pot need adjusting. That is done by putting in a signal, setting the horiz. pots to centre, setting the sweep to 10x then getting the centre of the signal in the centre of the screen. Using the 1kc test signal to the input will give ten waveforms on 1ms/sec. in the normal position. You would want the centre of waveform five in the centre at 10x setting. This should get the dot in or near the centre when in XY mode. Moving the position knob some is likely. Mark |
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After checking through most of the circuit involved with the Ext Horiz function, and finding the signal going through all the way to Q824, and still no trace, I figured out what was wrong.
After switching to Ext Horiz, the trace was way off to the left, so I have to turn up the intensity knob to 3 o'clock and the Horiz Position knob to 1 o'clock. Normally, In A sweep, the Intensity knob is around 11 o'clock, and when in A Intens B or B delayed, the Intensity is at noon. Is the low intensity and shifted position while in Ext Horiz normal? I did not see anything in the manual about this. Also, the phase shift a 1 MHz is 42 degrees. Is this excessive? The shift negligible/invisible at 100 KHz. Also, I broke one resister when unsoldering a lead to check the resistance. The lead came out of the body, and I think it was because i did not have a heat sink on the lead. I cut open the resistor and it looks like a carbon composition resistor -- filled with a black carbon paste of some sort. Does it matter performance-wise if I replace the carbon comp resistor a with carbon film one? Thanks for all of the help! |
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Thanks to all for the advice, and I am not offended. I measured them in-circuit, and thought they were isolated enough to do that when viewing the schematic. My plan was to find replacements in my stash of resistors and do the final checking as I un-soldered them. If I needed carbon comp resistors I would have double-checked (lifted a leg) before ordering. I did not know that resistors are unlikely to drop in value.
The B trigger works -- the B-channel works fine unless I switch the horizontal display setting to Ext Horiz. Then the signal and sweep disappear. I get a signal at the B-Gate too whatever the setting is. There is also 1.3 volt DC at the base of Q643, with no signal at any input. As many know, on the 453 a small network is substituted into the input amp (Q824) when the Ext Horiz is switched-in, so I am concentrating on that part of the circuit. I am using the calibration signal al the 1 volt level at an input for signal tracing, either into channel 1 or the Ext Horz input when its switched-in (top-right BNC). I am using a tek 475 for the measurements. My goal is to get the XY function to work. The signal level dropped way down (to 5 mV instead of about 1 V P-P) at the output of R645 (at CJ). I get the same at the base of Q824 (G). The gain on the original Q824 was a little weak, so I replaced it awhile ago with a new, good one, and that did not make any difference. SW801A is between CJ and G, so the switch seems okay. The signal goes away when I switch it away from EXT HORIZ, as expected. Same thing when I switch the B-source away from EXT. The signal drops even further when this switch is set at EXT/10, so that makes sense too. According to Stephen's test, R634 is okay. R644 measures at 10K when EXT HORIZ is switched-out, so that one is okay too (thanks again Stephen). I will do some more checking -- especially R646. I get 15K ohms when CJ and CK are disconnected, and this should put R646 in series with my meter. It is supposed to be 820K ohms; I will unsolder it to make sure. But does anyone know if the type of resistor matters (carbon comp, film,etc.)? I am still curious about that. Thanks |
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Does "B Trigger work"? Is the voltage at the base of Q643 1.0 Volt as the schematic specifies?
If so, I wouldn't jump to blaming R634. The schematic shows a coax connection to SW801A. Disconnect it and remeasure the other 2 resistors. Also, see if your R644 reading varies with the "Ext Horiz Gain" setting - and fpr R646. Stephen |
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Are you measuring these resistors in circuit???? I have never seen resistors like these decrease this significantly in value unless they have been "cooked"?(burnt).? ?Your data does not make a bit of sense.? Don't mean to offend you but I have 64 years of experience along these lines so what do I know about basic electronics???? ? ? ?Terry On Wednesday, March 16, 2022, 01:03:26 PM CDT, Mike Merigliano <mmerig@...> wrote:
The EXT HORIZ function of my Tektronix 453 does not work, and I found some resistors that were way-out of specification. They are: R634 (reads as 550, should be 11K) R644 (reads as 4k, should be 10K with 1% precision) R646 (reads as 1.37 K, should be 820K) The signal diminishes as it enters the resistor network. Everything else looks okay. i know the part numbers, the power ratings, and the precision, but originals are hard to find. I assume that carbon film or metal film would work okay, Any recommendations as to the kind of resistor to use? ] |
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Hi Mike,
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It is unusual for resistors to go down in value. Did you make these measurements with (at least) one leg unsoldered from the circuit? --Tom -- Prof. Thomas H. Lee Allen Ctr., Rm. 205 350 Jane Stanford Way Stanford University Stanford, CA 94305-4070 On 3/16/2022 11:03, Mike Merigliano wrote:
The EXT HORIZ function of my Tektronix 453 does not work, and I found some resistors that were way-out of specification. They are: |
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The EXT HORIZ function of my Tektronix 453 does not work, and I found some resistors that were way-out of specification. They are:
R634 (reads as 550, should be 11K) R644 (reads as 4k, should be 10K with 1% precision) R646 (reads as 1.37 K, should be 820K) The signal diminishes as it enters the resistor network. Everything else looks okay. i know the part numbers, the power ratings, and the precision, but originals are hard to find. I assume that carbon film or metal film would work okay, Any recommendations as to the kind of resistor to use? |
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