Restoration This Tektronix 485

2020/2/23

I got this buty 4 years ago. Back that time, I barely know anything about scope, and thus, I think this awesome device is nothing wrong.

Trace Wobbling

Two year later after this Tek 485 got home, I notice this bothering glitch. The trace wobbling horizontally. This is not like a trigger problem ,which the waveform might flicker but the beam trace itself.

Obviously the wobbling is related to the horizontal drive circuits. The video clip(righ one) catch the sawtooth waveform on one of the horizontal drive arm. This convinced me, that was where the problem came from.

Horizontal driver is a symmetry circuits contain 2 arms. And most of transistor in this amplifier is working in current mod, the base, collector, emitter voltage is fixed.

-5V lock the emitter/base voltage of Q1154, meanwhile Q1172 BE voltage lock the Q1154 collector voltage.

So the Q1154 working as a current amplifier, which is gain the bandwidth. This is essential to understanding how this amplifier works, especially, how the DC working point is settled.

I found one of the arm's sawtooth voltage is wobbling alone with the beam trace. Very likely this cause by Horizontal amplifier somehow, maybe some component too old or damp.

I was on this way about 2 years, take it out, trying to find which component is culprit, from time to time.

I narrow down the problem scope, I hope I did right. Only one of the arm sawtooth wobbling - the upper arm, and it's disappeared while exceed the Q1374.

Pull out the connector D is a easy checking.

The most excited symptom, while I pull the connector D, leave only last stage of the the Horizon amplifier, the spot on screen is stabilized !

I then suppose the Q1374 is defective.

The fact is merciless -- It's not the culprit at all, the wobbling continued!

Fix wobbling Round 2

It have been one year since I replace the Q1374. This hit me hard, I put it away for a whole year. Today, I take it out again, trying to fix it. I remember this cope is good while I got it. And the I put it near the air conditioner in summer. This might be the reason?

Some Electrolytic capacitor is damp? I decide try keep 485 running for several days to see what gonna happen.

It's stable After 72 hours burning!

Guess what, Power if off just 2 minutes, turn on it again. The wobbling still there. I trying to figure what really going on there.

This really frustrating. I then pull myself together, trying to identify which exactly component defected. The way to do so is cool the component down by pure alcohol. The idea is brilliant --- but I can not find any candidate. What i get is -- some of the component is more variable cause the trace shifted horizontally, but I figure it's just because that position is front stage of the Horizontal Amplifier - which affected the trace more.

I do have 2 spare Tek 485, I decide trying to change the whole SWEEP board. And then, I trying 2 sweeping board, but none of them make the things any better.

It's time to clear my brain, it's so messed up!

Fixing number one rule: Checking the Power supply ! I did before, all of them seems pretty well... This time I decide to calibridge the power : the -15V/+15V supply is a little bit out of alignment.

+15/-15V is unregulated power supply. Seem the only way to adjust is this +59.4V POT. While I touch this POT, the power supply flickering- It must be voltage protected, shutting down the power supply, but immediately come back again.

The good news is while I wiggled this PO--R1940 , Wobbling symptom does relieve, bad new is I can not confirm that, maybe never. This POT is definitely defected.

Fix wobbling Round Final?

I replace R1940 with a good one - from spare Tektonic 485. Does it fix the Wobbling? NO. But the +-15V power supply does get better -- much stable. With new R1940, turn it back and force won't cause the power shutdown again. And the power quickly settle down around 15V -- with in several mVs error.

What's next I could do for this? Wobbling is not cured, It's just not that serious as before, and not so that often as before. Which is actually quite bad, It's became much difficult defect !

What a surprise! It actually been fixed!

Before replace R1940, 15V/-15V drift slowly after power on, about 10-50 mV !!!, Now it's settle down with in seconds!
Wobbling ensued while cold start, but now , cold start is steady !
I can tell it's good now, for few days observation.

Attenuator module Contact Issue

Another issue is attenuator. It's flickering a lot in both 50 ohm and 1 Meg ohm band. I got attenuator knob off the front panel, this bothering me, and won't clean them for years.... just lazy.

And of course, got to take off bunch of screws.

Such problems all cause by these 2 little attenuator box, x10 and x100.

The deepest reason of these problem is one or 2 leads of the attenuator boxes are dry joints. Just get them all re-soldering could resolved all of contact problem. There is a very connerd joint, please use thinner tips to protect the plastic.

One Mega band is totally recover! And 50 ohm does not yet. 50 ohm attenuator don't have any attenuator boxes. And clean up the outer gold plating joints don't resolve the problem.

Nothing but disassemble this totally, flipping the 50 ohm attenuator board, and get it clean! Be careful, while pull it apart and put them together, the gold plating contacts near the front could easily bend to destroy !

Broken B trigger

B sweep works just fine except the B trigger not working at all. I decide stop here. Most of circumstance, It's enough.

2020/2/25

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