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Old 18th Sep 2020, 10:19 am   #1
Diabolical Artificer
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Default Tektronix 545B restoration Timebase Horizontal Display.

So I've started trying to find why there isn't a trace, all we have a glow or momentary blip. I started with the Time Base A Generator I made sure all controls were as set in the instructions however I'm uncertain about whether I should have a sinewave IP. In the manual it refers to a calibration jig, I have none.

I started looking for a waveform at V135A grid, pin 2 C131, there is none, that leads over to the time base A Trigger and Q45. There is 110v on it's collector, I also wiggled it in it's socket, took it out and tested it, it tests ok. I presume i jump over next to V24 looking for a sinewave on pin 7. As you see the ciopy I have has dire waveforms on it.

Any ideas what else I should be looking for? Iffy diodes?

Andy.
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Old 18th Sep 2020, 11:51 am   #2
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

It's worth having an input signal, then you can follow it through the trigger processing circuitry. Vollum (Tek) invented the triggered timebase. Without triggers, it doesn't run but sits ready to go and spot blanked. Auto run circuitry is an add-on.

Follow the trigger.

Find the run/not run flip flop function and see if it makes sense for the state it's in.

David
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Old 19th Sep 2020, 7:22 am   #3
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

Thanks David. I scoped V24 yesterday, there's no waveform as per the schematic with the controls as prescribed, however I do get a waveform if the trigger is set to line. If I put a signal in should it stay as a sinewave all the way through the trigger?

"Find the run/not run flip flop function" would that be V24? The SW function diagram is confusing as hell. I'll test Q34/5 today, they're in holders, not the best idea in my books and i'll test all the diodes which are germainium point contact types by the looks of it.

Andy.
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Old 19th Sep 2020, 8:28 am   #4
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

The run/wait flip flop is the function that the dreaded tunnel diode does in later scopes! In this case it is implemented with three valves

V135A, V135B, V145.

You will see a forwards path via V135B and some resistors. But notice there is a hidden feedback path via the connected cathodes of V135A and V145.
This circuit is a cathode coupled multivibrator. It looks quite different to the traditional multivibrator circuit, but it's easier to make this sort fast. You can also look at it as a sort of schmitt trigger. V135B is a bit of an enhancement to the bare bones circuit, to allow a bit higher load resistance in V135A anode to get more gain there.

OK, the trigger circuit takes in a sample of the signal from the Y amplifier, or switches in the input from the ext trig socket on the front panel. Whichever is chosen is either AC or DC coupled to a differential amplifier V24. The other input to that diff amp is the DC level from the 'TRIG LEVEL' front panel control. The inputs of the diff amp get swapped or not by a switch that sets the trig polarity (trig on rising voltage, or trig on falling voltage... at the voltage set by the aforementioned front panel pot)

Q35, Q45 are an emitter-coupled Schmitt trigger (see the similarity to V135A V145? there's a forwards path, and then there's positive feedback via the shared emitter connection). Again, it's not the trad Schmitt circuit. This one is built for speed.

The trigger schmitt generates a pulse as the trigger signal voltage passes the trigger level pot voltage. At this point the trig signal ceases to be a sinewave... well ceases to be a replica of whatever wave you are inputting and is a pulse. The output of this schmitt goes via a small capacitor to V135A and the shortened pulse from the capacitor timeconstant kicks the V135A/B V145 circuit into operation.

The heart of the timebase is the timing capacitor (switched in value by the big time/div switch)

V145 via the diodes V152 bosses the timing capacitor circuit around. When the timebase is not running, it forces the capacitor voltage. When V145 says the timebase is running, it frees the capacitor voltage and the capacitor charges up, making the ramp. The ramp gets buffered off of your posted diagram bit and goes to the X amplifier. There is also somewhere another Schmitt monitoring the ramp voltage on the timing capacitor. When it reaches the end voltage it triggers and sends a pulse back via a delay circuit (the hold-off timer) to R135/D131 to reset the trigger flip flop.

In fixing a scope timebase there is an optimum ploy:

1) Check the X amplifier is working. Select external X and put some signals in. Check you can get this signal to move the spot in the X direction. Check you get a spot. Fix the X amplifier, fix the CRT supplies/Z modulation amp etc as needed. Don't try to go beyond this until it's sorted.

2) Try to get the timebase (we're talking the main timebase here, there will be a second 'delayed' timebase in most comprehensive scopes) running with a signal applied to the "Ext Trigger" socket on the front panel. Don't worry about internal trigger from the Y amp, that might be faulty but we'll get to that later, we're fixing the timebase right now and we need the timebase to check/fix the internal trigger circuit.

3) So you've got a timebase that runs properly on an external trigger. Time to check it still works on the internal trigger waveform sniffed off of the Y amplifier. It's not much further circuitry.

Done randomly, scopes are nightmares and everything seems so complicated. People get lost.

Done in a logical order, you don't have so much to handle at once, you can see where you are and you can see progress

David
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Old 20th Sep 2020, 8:59 am   #5
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

Thanks David. I did get lost yesterday, I traced a sig in from the vertical amp to the trigger then into TB A. Replaced a diode and cap, got the trace a little further then got bogged down after taking voltage readings around V173, there's over +200v on V173 cathodes, should be - 5v ish.

First off "Check the X amplifier is working. Select external X and put some signals in." What sort of signal? Same question re Ext trigger?

I've been using the 545A manual partly as the waveforms are much clearer in it and the copy I have is better quality than two 545B manual;'s I downloaded.

Andy.
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Old 20th Sep 2020, 12:54 pm   #6
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

You should be able to put a low frequency sinewave into the ext X connector and watch a spot move to and fro on the screen with simple harmonic motion. Winding the Y shift up or down will give you something like a scope display rotated 90 degrees with time going upwards.

David
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Old 26th Sep 2020, 6:41 am   #7
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

Dropped back onto this yesterday and did as suggested with mixed results, trying to trace an ext signal through the trigger, it disappears/isn't correct when it reaches the base of Q34. I checked all caps for leakage and all R's for out of spec tolerance - all ok. Unlike on a lot of vintage gear I've worked on the components are top notch. For instance I've just worked on an AC precision meter and ended up with about 50 out of spec carbon resistors and a handfull of leaky electrolytics. Not so with Tek scopes of this era, which leads me to wonder if all the valves are ok ( I tested them) what's to go wrong?

So far all DC voltage readings are in spec, give or take a few %, trannys check out, only waveforms are off.

So with that approach unsuccessful I tried to be a bit more logical. I only get a trace with standard settings if I move the TB x1/x5 SW to half way between the two settings, therefore SW problem?

I can also get a trace (it's still static though) if i set the TB A SW to X1/X10 Ext, if not set to these settings I get no trace and the Hor vernier DOESN'T work.

With the TB selector SW set to B and by messing about with the time/div SW I briefly get a blip which shoots off to the right, but doesn't appear again, however the time is all wrong, EG it only runs at one speed.

I was hoping writing this would clear things up but still in the dark a bit.... it leads me to think I have a blanking error, TB B won't free run, some possible SW contact issues (tried cleaning them) and probably a few other issues.

Andy.
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Old 26th Sep 2020, 9:04 am   #8
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

Andy
I know you have checked transistors but I remember finding some dodgy transistor sockets In both my 544 mainframe and a plug in. Also had the odd failed one too. Sockets introduce a possible failure mechanism but makes it easy to remove the transistors for testing.
Keep going!
Ian
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Old 27th Sep 2020, 6:57 am   #9
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

How did you diagnose a dodgy socket Ian, be interested in how you test them. Some of these in this 545B are buried and hard to get at, still, I will as you say keep going. Thanks for the cheering on.

A.
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Old 27th Sep 2020, 9:58 pm   #10
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Default Re: Tektronix 545B restoration Timebase Horizontal Display.

It's a while since I worked on my 544 but I think I measured electrode voltages (ebc) with a DMM. Then did a bit of transistor waggling with finger. Then removed them one by one if necessary and tested on one of those cheap universal transistor testers. The odd one was faulty. Then cleaned the wire leads with some contact cleaner and plugged in. Not sure if yours has the tunnel diode in the trigger circuit but I had to replace one to get a stable trigger.
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