I just got a good bargain on an oscilloscope off ebay- a 250MHZ Tektronix 475A dual trace for $70 :lol: The way I see it, I can sell my other 20MHZ scope that I got for $80 and cover my costs :wink:

There is one problem with the scope though that can probably solved easily. If not I'll just resell it. The beam is stretched out, I don't know any other way to explain it. In one picture, the beam is not moving at all, so it should be a dot, not a line like you see. In the the other picture you can see the distortion of the waveform caused by the beam being stretched out.

So how do I fix this?

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I'm no electronics god, i just talk too much.

 

Dither in the horizontal can be caused by power supply ripple or low level oscillation in one of the amplifiers. U can use the other scope to trouble-shoot.

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see my website: www.geocities.com/russlk

 

Thanks, I'll look at that. Just have to figure out how to get the stupid thing apart first :lol:

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I'm no electronics god, i just talk too much.

 

Zach,
I concur with Ruslk re the first photo.

In the second photo, it looks to me as though either the Horzontal amp is lacking in gain or the Delayed Sweep is faulty. Does the X shift move the trace?

Was the delayed sweep on when you took the photo?

If so, does the Delayed Sweep delay control move the trace?

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Len

 

Len, if you look at the sweep control knob, he has it in X-Y mode. I think this is intentional, as he should get a dot on the screen with no inputs, as he says.

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Ron

 

Ron,
I was using a friend's computer and had limited time. If both photos are supposed to show only a dot, then there must be ripple in the Y amp and ripple or noise in the X.

Zach,
Do you have a Service Manual? Or a circuit diagram?

If not, there is still much you can do.

1. do a visual examination looking for electrical, mechanical or chemical damage, eg. there may be hairline cracks in the PCB, broken components, overheated components or corrosion (partic if it has lived near the sea) etc.

2. check all of the electrolytic capacitors, they tend to fail after a few years. If you can (ie. have the equipment) measure their capacitance, leakage resistance and ESR. Low cap or high ESR can cause ripple in the supply lines. If not then replace the electros that are used for power supply filtering.

3. Measure the supply rail voltages. Also examine the supply rails with your other scope using AC coupling and see how much ripple (60 Hz or 120Hz) there is. If you don't have a circuit, you will have to guess. Steer clear of the high voltage section. Your problem(s) don't appear to be HV related anyway.

4. note the effect of all of the controls, eg. does the Y shift actually make it move vertically? Is there any limit to the movement, eg. will move to the top of the screen but not to the bottom.

The X amp in my HP scope failed recently (the latest in a long line of faults) and so the trace was only visible at the left side and could not be moved very far to the right with the X shift. A resistor had gone high and 3 others were trying to go high.

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Len

 

Ron is right, I had it in x-y mode and I should be getting a dot, not a line.
Here's what I found when I took the thing apart-

1. I hooked up my scope to the + side of about 6 electrolytic supply caps, which I'm guessing is the supply rails as well. I found a triangle wave in the neighborhood of about 1 volt riding on the supply rails. The frequency was 125hz- because of the bridge rectifiers correct?

2. I measured the ESR of the capacitors with a ohmeter and they had resistances ranging from 10k-70k. Of course they were in the circuit and i have a sneaking suspicion i might be reading the bleeder resistors because the capacitors wouldn't keep their charge after i turned it off.

3. I found an index card that said that the ch. 2 x10 attenuator was repaired in '56! I think I should probably replace those electrolytics.

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I'm no electronics god, i just talk too much.

 

How does the scope perform when it is not in the X-Y mode?

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Len

 

To get the 475A out of the case, put the front cover on the scope, flip the tilt-handle out of the way of the front panel and stand the scope on its face. If you don't have the front cover, leave the scope on its feet and work from there, although the process will be a heck of a lot more difficult because of the effects of gravity.

Remove the four feet on the rear using a #2 Pozidriv (or less effective, a #2 Phillips) screwdriver. Remove the two other screws top and bottom center. Remove the plastic ring that holds the cover on. Lift the cover by the tilt handle axles straight up and off the scope.

Installation is the reverse of removal. But when you get the cover down into place, you'll have to tap the top and bottom edges with your fingers to get it to slip into the groove on the front casting.

Yes, 1956 for a repair is impossible. The 475 came out around 1974 and the 475A around 1980 or so. The 475 and 475A are nearly identical -- the extra bandwidth comes primarily from tweaking up the output amplifier. So if you need a "hangar queen" for parts, a 475 should work about as well and be more readily available as more of them were made.

I'd suggest getting your test scope probe into the horizontal amplifier with the 475A in the X-Y mode and take a peek along the signal path and see where this spurious signal starts and stops to isolate the defective stage. It may be nothing more than the replacement of a decoupling cap in one of the supplies.

Troubleshooting help for the scope can be found on the Yahoo! Tekscope forum at http://groups.yahoo.com/group/TekScopes It's a good spot for both troubleshooting help and parts.

Dean

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Dean Huster, Electronics Curmudgeon
Contributing Editor emeritus, "Q & A", of the former "Poptronics" magazine (formerly "Popular Electronics" and "Electronics Now" magazines).

R.I.P.

 

Does anyone know of a similar site for HP scopes?

I have a 1740A and may, one day need spare parts.

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Len

 

I think you can send it in for repair but I don't know what it will cost.
I had a broken one and ended up throwing it out because there was noone to fix it locally,but you can mail it out to be fixed.

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Reality

 

I'm sorry, when I said ESR, I didn't mean ESR. I just measured the cap with a ohmeter to see if it was shorted. How do you measure ESR?

The scope works fine when it's not in x-y mode. It's like looking at a waveform made with a caligraphy pen held horizontal. I hope it's just one of the supply caps because I really have no idea what I'm looking at, there are a lot of boards on the scope. I'd have to get a repair manual.

I wonder why I found that index card taped in there with those words on it :?

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I'm no electronics god, i just talk too much.

 

With an ESR meter! - and I'm not taking the pi**!.

An ESR meter measures the impedance (NOT the resistance) of the capacitor at 100KHz.

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PIC programmer software, and PIC Tutorials at:
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Do a search for "esr meter". You'll get several hits that show you how to do it without buying another meter.

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Ron

 

ESR = Equivalent Series Resistance.

My understanding is that it is resistance of the connections to the plates which can change with age due to (I assume) the aging of the electrolyte.

Here is a chart of the maximum ESR for normal caps (taken from my ESR Meter that I built as a kit)

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Len