Tektronix 475: No Horiz Sweep

[Anthony]

May I have some guidance on a fault(s) which hit my long-service 475 today. Before it all gets too serious I wonder whether anyone might recognise the symptoms? It just lost Horiz. Sweep and curiously the Power Neon does not come on. It is just possible the neon fault has been there for some time and I have simply not noticed it (?). Both Y-axis channels seem fine.

My initial attack on the formidable Tek manual have been worrying in that I cannot even locate the Power On neon on the circuit! But this is the first time I have looked at the service manual. It does not really make sense to me that the Sweep and Neon problems are related but I suppose the neon maybe driven from somewhere odd.

An additional fault, I suspect, is that the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen.

Clearly I am going to have to do my homework but I just thought to ask for some initial hand-holding before I actually do anything.

Many thanks. Anthony M0WWV

[ColinTheAmpMan1]

Originally Posted by Anthony

May I have some guidance on a fault(s) which hit my long-service 475 today. Before it all gets too serious I wonder whether anyone might recognise the symptoms? It just lost Horiz. Sweep and curiously the Power Neon does not come on. It is just possible the neon fault has been there for some time and I have simply not noticed it (?). Both Y-axis channels seem fine.

First off, is this a 475 or a 475A? I ask because there are differences. I have a 475A and the Service Manual.

My initial attack on the formidable Tek manual have been worrying in that I cannot even locate the Power On neon on the circuit! But this is the first time I have looked at the service manual. It does not really make sense to me that the Sweep and Neon problems are related but I suppose the neon maybe driven from somewhere odd.

I have looked in my manual and on the scope - there isn't a "Mains On" light; it is a "Low Line" indicator which indicates that the mains voltage is too low. It should only illuminate when that condition is true, not simply when the mains is switched on.

An additional fault, I suspect, is that the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen.

That is _the_ fault! This sounds very much like there is something wrong either in the sweep and trigger circuits, the horizontal amplifier, or any combination of the three. I would first try looking at the low-voltage power supplies and see if they are all within spec.

You could also tell us what your "Beam Finder" does (also when twiddling the "Horiz. Position control), whether the A- and B-Gate signals available on the rear panel are present, whether the time-base will trigger or not (check the "Trig" lamp) and what effect, if any, the "X10 Magnify" button has.

Good Luck, I doubt this is insurmountable.
Colin.

[Anthony]

Colin
Very grateful. It is a 475. `Ouch re: the `Low Line`- the neon is unlabelled on my front panel. Will get back to you.
Anthony

[ajgriff]

You may already have a copy but the Tektronix Troubleshooting Your Oscilloscope guide is a very useful document. It's widely available for free download including here:

http://www.sphere.bc.ca/test/tek-par...ing-scopes.pdf

Definitely start by checking power supply rails.

Good luck. I'm sure you'll manage to track down the fault.

Alan

[Anthony]

Alan.Thank you for the encouragement Yes I have that very helpful slim volume. Best wishes. Anthony

[MrBungle]

First thing on these is check the power supply voltages. 99% of the problems are power supply related.

And the problems are almost universally shorted tantalum capacitors! The blue blobs or striped ones.

[Anthony]

Mr. Bungle & confreres

That is my project for tomorrow. i just odo not want to rush it. Having taken off the `scope cover i am amazed at the component density!
Best
Anthony M0WWV

[Radio Wrangler]

Power supplies first

Then try to get the unit going in X-Y mode

Then try to get it triggering in 'norm' 'ext' trigger mode.

Then try to get the timebase going in 'auto' 'ext' trigger mode

Then try to get it 'int' triggering from Y amp signals.

Each of these steps brings in a bit more circuitry. Having the spot(s) far to the right with a timebase fault doesn't automatically implicate the X amp or the power supplies, nor does it exonerate them.

Simple stress factors make the power rails an order of magnitude less reliable than the things they power.

Dsvid

[Anthony]

David
Many thanks. I appreciate the graded approach. I think I'll pause until the alcohol levels have fallen.
Sincerely
Anthony

[Radio Wrangler]

One key thought to hold onto is that this timebase is designed not to keep running, but to run once and then stop, waiting for another trigger. Repetitive operation only happens in 'norm' if there are repetitive triggers from whatever source the switches select, OR from an added timer in 'auto' mode.... there's an extra timebase speed switch pole to select different capacitors for the auto-run self-trigger timer so the rate is appropriate to the timebase speed.

David

[Anthony]

Have a lot more in the way of clues but cannot integrate them into a single fault area:
All 6 PSU DC levels and ripple are depressingly well within spec. (surprised by the very low ripple checked with both a DVM and `scope ?odd).

X-Y mode was interesting. Firstly I noticed that in both X-Y mode and 0.1/0.2 and 0.5 secs/div the spot goes off screen and needs Beam Finder to bring it back On all other sweep settings the spot is visible without BF.

Putting the Calibration signal into Ch.1 in X-Y mode (using BM to see trace) and selecting Ch. 1 there is a trace at 45deg with of about the right for the 300 mV Cal signal. If Ch.2 is selected the trace is horizontal.

It is as if Ch. 1 is feeding X and Y while Ch. 2 is feeding only X. if Ch. 1 is selected .? Red Herring.

Going to a sweep setting, will not trigger in Norm/Ext or Auto/Ext.

I looked at the A+GATE signal using a sort of working 465. On the 465 itself there are pulses but nothing comparable on the 745 under `repair`.

Am floundering at the moment and don`t feel I have localised to Trigger, Sweep or Horiz. Amp. Though the lack of the A+GATE signal suggests sweep (?)

Am floundering a bit (sorry for the fish metaphors!)

Thoughts

Anthony

P.S. David - understood.

[Anthony]

Should have said 475 not `745` also the lack of A+GATE signal could be either Triggering or Sweep -yes?
Anthony

[Anthony]

Proper look art manual indeed states CH2 to be selected for XY mode - ?why - so herring is indeed red on that point. This is a more complex instrument than I thought!
Anthony

[Diabolical Artificer]

" the Horiz. Trace control only moves the spot(s) on the right hand side of the screen and not the full screen." might be another red herring down a rabbit hole, but this to me indicates one half of the X amp is not working.

Another watery metaphor, sounds like your drowning here, flopping about all over the shop. After a PSU rail check, break the scope into sections (helped by the manual block diagram) and proceed in an orderly logical way, else you'll get lost; you need to be methodical.

For a start have you given it a good visual with a magnifying glass and checked all the tant's? Never owned one of these but the tantalum and dry joint issue is a well known issue with these scopes. Also check daft easy things first like connectors, iffy pots and switches.

Lastly post some schematics of the section in question, helps folks help you.

Good luck, Andy.

[Radio Wrangler]

I'm working on memories, I don't have a manual.

In X-Y mode it parks the timebase and only the X amp is in use, taking signal from one Y channel. This is a lot simpler than what's in play in the usual timebase mode! You appear to have an X amp fault, or maybe the voltage from the parked timebase is wrong. Whatever it may be, you need ti fix this first. 1) because it's easiest, 2) because testing/fixing all the rest needs it to be working.

This looks like a complex instrument, but it's really just an awful lot of simple enough circuits flying in very close formation. Slow, methodical, orderly work will get you there, anything else will get anyone lost and confused.

But before you start. Look at the schematics of the X amplifier. Spot all the places each power rail appears, notice any local filtering components and check the voltage is correct on all of them, and that it makes it through the filtering OK. Tedious? yes. but it can save more anguish later.

I used to design stuff of this complexity (and worse!) for the 'other brand' and this meant prototyping my designs and debugging them. When you do this, you don't even have the comfort of knowing that it once worked and you can't even expect there to be only one fault. This is the troubleshooting equivalent to extreme ironing, and it teaches you to be methodical, and just how precious certainty is.

David

[MrBungle]

Good advice. They’re not particularly intimidating really. They just look it!

The good bit with the X amp failing is it’s on the big interface board so it’s easy to trace out there, unlike the vertical which is all over the place and buried.

Last X amp failures I had were a shorted tantalum and the deflection plate wires were loose.

[Craig Sawyers]

As bungle says - check the power supplies before you do anything else. Apart from shorted tantalums, the key supply failure mode is Sprague twist-lock capacitors on the supply board. These have a failure mode that the internal connection foil to the can (so negative) rots through by electrolytic corrosion. So the capacitor just disconnects itself internally. These are a nightmare to desolder when one has failed, and someone at Tekscopes has gone as far as to layout an entire replacement board.

So check power supply voltages and ripple.

Another supply failure is the bridge rectifier that gives confusing symptoms. Tek's power supply parts list usually lists three or four bridge rectifier types while they arrived at a solution!.

Good luck. And join TekScopes@groups.io which is the community for all things Tektronix.

Craig

[RogerEvans]

One simple fault that hasn't been mentioned is that the push button switches become unreliable with age. It is worth cycling the horizontal mode buttons (A, mix, A intensified, B delayed ) many times. It might also add a clue if you describe what you see selecting 'B delayed' (A trig auto, B trig starts after delay).

Roger

[Anthony]

I am very grateful for these suggestions. Yes, I am intimidated by the 475. It has numerous features which I never used. The only equipment at all `similar` to this which I have worked on is HP of this era - is that David`s `other brand` - which I have almost always, after some effort, been able to debug. The 475 will take some getting used to and I must be ever so humble and RTFM + Tek troubleshooting guide again and again. Will update;Anthony

[Anthony]

I would be grateful for further advice. Not least with David`s help pointing me in the direction of XY mode, I have narrowed a problem down to the Z-Axis amp causing the beam to be blanked all the time.

The reasoning is that a spot will only appear in XY mode when Beam Finder (BF) is activated. BF overrides the Z-Axis amp and sets the Amp O/P to about 25 Volts (from 6V) which then goes into the DC Restorer (please see CRT circuit and Z Axis Amp circuits attached; apologies for quality but the best I have). This is confirmed by simply putting -8V on the junction of R1342 and R1346; the spot reappears. X and Y position controls work fine with BF on.

The operation of the Z-Amp is described as follows in the manual:-

Z-Axis Amplifier
The Z-Axis Amplifier circuit controls the CRT intensity level from several inputs. The effect of these input signals is to either increase or decrease the trace intensity, or to completely blank portions of the display. The input transistor Q1338 is a current-driven, low input impedance amplifier. It provides termination for the input signals as well as isolation between the input signals and the following stages. The current signals from the various control sources are connected to the-emitter of Q1338 and the algebraic sum of the signals determines the collector conduction level.

Q1344, Q1352, Q1354, Q1358, and Q1362 compose a feedback amplifier stage. R 1369 is the feedback element, C 1352 provides high-frequency compensation. Q1344 is an emitter follower that provides drive to the output complementary amplifier made up of Q1352, Q1354, and Q1358. Q1358 is a device with higher frequency characteristics than Q1354 and is used to improve the overall frequency capabilities of the Z-Axis Amplifier. On the fast positive going output signal peaks, Q1358 depletes the charge on C1358. Then, on the negative-going signal peaks, Q1362 is pulsed on to renew the charge on C1358. CR 1367 provides protection to the Z-Axis Amplifier circuitry in the event of short duration arcing in the CRT High-Voltage Power Supplies.

My problem is, I think, that I do not really understand the circuit! When I rotate Beam Intensity I see no change in the voltage on the emitter of Q1338 (even with all other inputs (Q1332 etc.) disconnected. The voltages on Q1338 are Ve -2.498, Vb -1.729 and Vc 0.467 so Vbe is ~0.75 forward biaseed. Perhaps this always pulls Ve down when the Intensity Control is changed? This my lack of understanding.

If CR1341 is lifted as per the troubleshooting instructions below I do get the specified change in Vc of Q1338 which swings from +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise

Troubleshooting the Z-AXIS Circuit (CRT Circuit). Set TIME/DIV to 1 ms (lock knobs) and set TRIG MODE to SINGL SWP. Disconnect and lift one end of CR1343. Check the voltage at TP1364 for approximately +6 volts (OK). Press the BEAM FINDER pushbutton and check the voltage at TP1364 for approximately +25 volts (OK). If these voltages cannot be obtained, check the voltage across VR1362 for approximately +6.2 volts. Remove Q1362 from its socket and check the voltage at the base of Q1352 for approximately +1.3 volts. Check the base of Q1344 for approximately +0.6 volt.

If the above-mentioned voltages can be obtained, disconnect and lift one end of CR1341. Check the voltage swing at the collector of Q1338 for approximately +7.8 volts to –1.3 volts while turning INTENSITY control from full counterclockwise to full clockwise. (OK). If these voltages cannot be obtained, check the voltage at the base of Q1338 for –1.75 volts. Remove Q1332 from its socket.
Disconnect J1333 and J1334 (coaxial-type end-lead connectors) from their sockets. Repeat the voltage swing check at the collector of Q1338 as performed previously. Connect J1333, J1334, R1343 and CR1341 to the circuit board. Install Q1362 and Q1332 in their sockets.

Replacing all parts and confirming initial voltages around Q1338 I see no response to the Intensity Control at Ve of Q1344, the emitter-follower driving the Z Amp. That is surely wrong?

This places the fault around Q1338/1344 but I cannot find a problem. Junctions of the two test OK. The only obvious fault about the board is some discolouration of CR1360 but it still rectifies.

I apologise for the density of this email but any help much appreciated. I have no more hair to tear out.

Anthony