Tektronix 454 no more horizontal trace - Page 1
Forum section: electronics / repair

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 09:29:23 am
content: Hi all,I am pretty new on this forum and recently got an old Tektro 454.Surprinzingly this old scope worked after a long cleaning process and I started to discover joy of this vintage scopes (I was 7years when the scope bornt, vintage I am as well !)I struggled a little to use XY mode, but after reading the manual it worked as well and I could play with some "oscillo music".I escaped from my desk a few minutes and when coming back the horizontal (X) trace had disappeared. Only the vertical one was working.I went back to standard mode and ... nothing only vertical trace. So all the horizontal modiles didn't work (base time, horizontal position, ch1, ch2 etc...)I started to think that the horizontal amplifier was dead but where ?I attached another scope to the tektro and started to probe the H amplifier.Here are my findingsAll looks ok up to Q1058 Q1068 (green boxes) but not after Q1058 Q1068 (red boxes)But after these two transistors the signal disappears in "normal mode" but is still visible (saw teeths) when pressing the beam finder or the x10 one.All the voltages are ok +12, -12 and +75V...I have tested PNP transitors Q1058, 60 63, 68, 70 and 73 they seem ok (tested with multimeter in diode mode)I have also tested Q1084 and Q1094 these NPN seem ok as well...I am lost ! any help please ?

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 09:40:14 am
content: So here are the signals just before the output of the amplifierOn the left the signature of the problem ...Also when I press the beamfinder, the two dots skrink a little in vertical (normal behavior) and they move a little to the right also (normal as well)

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2025, 10:17:30 am
content: Thank you for taking the time to document your debugging, especially with the annotated schematic. That makes it worth my while to spend a little time on your problemYou note that the signals (and presumably visible trace) show horizontal deflection when either the *10 or mag buttons are pressed. That suggests the X deflection circuits and connections are fundamentally intact. It also suggests there might be a dirty switch or connector. Isopropyl alcohol (IPA) is one suitable cleaning agent. (N.B. for any gold finger contacts, the approved technique is to soak a strip of smooth paper in IPA, use the control to lift the finger, insert paper, drop the finger, and carefully pull the paper out)I'm a little surprised that your Metrix trace shows both signals moving in the same direction and of slightly different peak-peak amplitude, but that might be the Metrix channel matching and triggering.The X channel signals are differential signals. Hence it can be useful to look at, say, Q1058, Q1068 and then use add+invert to look at Q1058-Q1068.Any time a horizontal trace disappears in normal voltage-time mode, that suggests the trigger circuit might be faulty. That can happen if resistors drift over time so the tunnel diode bias becomes incorrect. If so, go through the manual's maintenance procedures.Good luck

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 10:38:29 am
content: thanks of your fast answer !At the base of 1063 or 1073 I can't see anything unless I press on the beamfinder.What do you mean with "add + invert" ?The horizontal trace disappears also in XY mode so I don't think that it is a trigger problem. I am not a specialist at all but I thought that in XY mode there was no triggering signal ? or at least replaced by the X signal ?And yes, you are right when pushing the beam finder , the vertical dots move a little to the right side, so the X deflection is still working.What surprises me is that the scope was working very well and when I went back (let's say 10 minutes after) the horizontal X signal disappeared. So I suspect a sudden failure of a component rather than a problem on a connector. I didn't touch anything while I was away from the scope and it worked just before I left !I have also tested these two diodes (soldered in place) with a multimeter. I can see 0.7V in both sides which seems normal ?I also suspect that there is a bug in the manual regarding voltage in A or F. They say 68V but I get 120V (in normal mode), which seems logical regarding the wiring to 150V unreg supply

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 10:45:30 am
content: I must say that I don't understand how the output stage works !The transistors 1058 and 1060 are wired in // with collector soldered to base... how do they work ?The service manual says this :Not obvious for me !

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2025, 12:18:02 pm
content: Quote from: freedom2000 on March 24, 2025, 10:38:29 amMost analogue scopes, including the 454, havea button allowing you to invert channel 2a vertical modes including ch1, ch2, alt, chop, addThat allows ch1 and ch2 to be combined to show the single trace ch1-ch2, i.e the differential.QuoteAgreed. But you may have two problems, conceivable coupled together via a power supply line.Have you measure the ripple on the PSU lines? Sometimes that is a better indicator of a marginal smoothing capacitor than the "DC" voltage across it.QuoteProvided that movement is similar to what you would see with a functioning scope, I agree.QuoteAgreed.Electrolytic capacitors are the usual culprits, especially tantalum beads. Tek and others had a habit of using them too close to their voltage limits. In the Tek 485 there is a "hidden" (i.e. not on the PSU diagram) 13V line with a 15V tantalum bead. It makes a mess when it goes short circuitQuoteYes. In fault conditions they limit the differential voltage applied to the later stages. In normal operation there should be <<0.7V across themQuoteTek has been known to make mistakes (e.g. capacitor silk screen identification in 2465), but I doubt the schematic is wrong to that extent.I suspect that voltage is a good indicator of a problem.I presume that feedback is used to hold the collector at the 68V, so you need to check voltages in the Q1063 Q1084 loop. Have a look at the 12V(dec) rail; is it decoupled by a tantalum?A broken feedback loop might cause problems, so check all resistors and connectors near Q1067/1073.

author: ondraN
author_rank: Regular Contributor
timestamp: March 24, 2025, 12:19:54 pm
content: Q1058 and Q1060 act as a current differential amplifier (current mirror) whose gain is given by the ratio h21e of both transistors. The emitter current of Q1060 is equal to the emitter current of Q1058 multiplied by h21e(1058)/h21e(1060). This ensures that the amplitude of the voltage change is very small and the parasitic capacitances of the transistors are much less affected.

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2025, 12:24:23 pm
content: Quote from: freedom2000 on March 24, 2025, 10:45:30 amA key realisation is that the signal amplification chain is differential current mode, not voltage mode. That's why the differential voltages are small, and D1054/55 should never conduct.QuoteThe key phrase is "diode connected transistor". That's usually done with JFETs.QuoteDon't worry about that! There are many subtleties in advanced scope circuits

author: tggzzz
author_rank: Super Contributor
timestamp: March 24, 2025, 12:29:52 pm
content: Quote from: ondraN on March 24, 2025, 12:19:54 pmI thought of a current mirror, but Q1060 base would not be connected to its collector, and Q1058/60 emitters would be connected together. That's confirmed by the circuit description text stating "acting as limiters".However, I suppose they could be operating in an inverse mode, but I don't see why they would do that.https://en.wikipedia.org/wiki/Current_mirrorhttps://en.wikipedia.org/wiki/Diode-connected_transistor

author: ondraN
author_rank: Regular Contributor
timestamp: March 24, 2025, 01:06:25 pm
content: When I watch the changes in current from the emitter of Q1058 through the other transistors, it makes perfect sense. Increasing the emitter current of Q1058 causes an increase in the voltage at the collector of Q1084. The feedback there is purely current. But, I'm afraid I'm not able to describe it in a simple and understandable way.The current mirror has both collectors connected to both bases probably because the input and output voltage levels are almost the same and it probably increases the transient frequency or stability in some way.

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 03:42:34 pm
content: Thanks for all these informations : I am learning a lot !Meanwhile I have tried to clean contacts on my old Metrix to have a more reliable triggering.Not yet perfect (as the rotary switches cannot be accessed... I have just droped isopropylic alcool inside...)And I have redone the readings :Q1014 and Q1024Shape seems to be fine although amplitude a little weak (should be 6V peak to peak. I have only 4V)Now going to QIO34 QIO44 the signal is not that nice...Then signal is lost after these points as before.I have also double checked the 68V mentionned in the manual. I have 128 and 120V instaed of 68 and 62Worth to note is that the "beam finder" decreases these values to around 66VBUT the MAG button has no effect... Probably because MAG signal is inserted before the output amplifier which doesn't work ? While beamFinder is inserted on the right side (+75V inserted instead of +150)

author: freedom2000
author_rank: Contributor
timestamp: March 24, 2025, 03:44:30 pm
content: All the green voltages are okHow could it be 68V as it is attached to 150V (top right of the picture) ? Is it the amplified sawteeths which are pulling this down ? (and as they don't work I can't see this regulation ?)

author: ME
author_rank: Frequent Contributor
timestamp: March 25, 2025, 05:05:29 am
content: If it was me i would start checking all caps for capacity and esr.

author: ondraN
author_rank: Regular Contributor
timestamp: March 25, 2025, 06:17:38 am
content: And what about DC voltages at emiter Q1058,1068 and Q1060,1070 and emiter, colector Q1063,1073. All voltages with and without Beam finder pressed.It looks like the output transistors Q1084,1094 are almost closed.P.S.So are the green oscillograms OK or not? In the waveform photos, we don't know how many mV/div are set.

author: ondraN
author_rank: Regular Contributor
timestamp: March 26, 2025, 09:41:28 am
content: I've been thinking about this problem for a while. Since the signal is symmetrical in both halves of the differential amplifier, but completely shifted outside the operating point, and according to the thread author's hints, the green oscillograms are also not quite correct, I think the problem is somewhere between the transistors of the first or second pair of differential transistors. I would take a close look at the first pair Q1014,1024. Especially the -12V power supply part (R1016,C1016,R1017) and then again at the part between the emitters Q1034,1044 (+75V,R1036,R1046) and finally at the trimmer R1053 and the power supply on it (-12V)

author: freedom2000
author_rank: Contributor
timestamp: March 26, 2025, 10:06:22 am
content: I have checked values of tantalum capacitors on the power supply board : all ok

author: freedom2000
author_rank: Contributor
timestamp: March 26, 2025, 10:39:55 am
content: Quote from: ondraN on March 26, 2025, 09:41:28 amhere are the voltages just measured ![ Specified attachment is not available ]

author: ondraN
author_rank: Regular Contributor
timestamp: March 26, 2025, 11:22:33 am
content: Is it measured in X-Y mode and with the X input terminated to GND? Measure all the values ​​that are marked in the schematic. It looks like the problem is somewhere in the first pair of differential transistors (Q1014,1024).

author: freedom2000
author_rank: Contributor
timestamp: March 27, 2025, 07:39:34 am
content: no it was measured in normal mode.Screen doesn't change between normal and XY mode.Should I redo measurements in XY and inputs grounded ?BTW I have also checked C1016 cap and it was ok !

author: ondraN
author_rank: Regular Contributor
timestamp: March 27, 2025, 09:15:26 am
content: As described in the schematic, DC voltages are under the condition of No signal applied. This is most easily achieved by switching to X-Y mode and terminating the X input. Otherwise, the voltmeter readings will be affected by the DC component of the signal.

author: freedom2000
author_rank: Contributor
timestamp: March 27, 2025, 12:03:21 pm
content: ok thanks, I will redo the measures in XY mode inouts grounded!

author: freedom2000
author_rank: Contributor
timestamp: March 27, 2025, 01:06:24 pm
content: And here is the result in XY mode X and Y grounded

author: freedom2000
author_rank: Contributor
timestamp: June 19, 2025, 03:48:12 pm
content: A friend of mine found the culprit...It was R1035 which was broken... I couldn't believe it