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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Wed 13, 2015 6:02 pm 
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Tube Radio wrote:
If the schematic lists enough voltages you may be able to find the current the HV CRT supply is drawing then you can compare it with the schematic voltages and see if the current is higher than what it should be.

At this point I'd strongly suspect a CRT issue.



The CRT showed up and it is definitely a NIB 5UP1. And... it's definitely not a CRT problem.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Wed 13, 2015 6:54 pm 
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Mike,

I have been following the saga and have just skimmed back through, but still may have missed something.

The thought just occured to me that your traces (and I'm assuming they're still "fuzzy" with the new tube) look an awful lot like something in the vertical input path is going into oscillation.

I couldn't find where you had replaced the ceramic coupling capacitor that you talked about at one point. Not sure if that cap would contribute to oscillation though unless it upset the bias of the following stage. Any bypass caps woulld be likely suspects if oscillation is in fact occuring.

All of this advice (mine only) may be worth what you have paid for it.

I shall be continuing to follow with interest.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Wed 13, 2015 7:03 pm 
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Chuck - Since the NOS tube looks as bad or worse I'm inclined to think I have a cap or two that is wonky. I didn't mess with any of the disk caps. It might be worth taking some of the more likely suspects out and testing them.

Something is definitely oscillating. I'm glad that you said that. With no connection and Gain turned up a little there is a wide band. I can't get a spot out of it no matter what I do.

As Chief Dan George said in The Outlaw Josey Wales- we must endeavor to persevere. And the rest goes

Quote:
We thought about it for a long time, "Endeavor to persevere." And when we had thought about it long enough, we declared war on the Union.


So maybe I'll declare war on this thing and fix it :)

Onward thru the fog but we'll get there.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Wed 13, 2015 9:06 pm 
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Mike6158 wrote:
With no connection and Gain turned up a little there is a wide band. I can't get a spot out of it no matter what I do.


Have you grounded the inputs, while testing?

David


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Wed 13, 2015 9:18 pm 
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dholland wrote:
Mike6158 wrote:
With no connection and Gain turned up a little there is a wide band. I can't get a spot out of it no matter what I do.


Have you grounded the inputs, while testing?

David


No sir but that sounds like a really good idea. I guess I'm too used to newer stuff... I forget the obvious with this one...

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Thu 14, 2015 1:01 am 
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Get a can of freeze mist. If you don't have access to any, get a can of "duster" and spray it holding the can upside down. Freeze mist will come out that way. Your fat trace is due to some component heating up in the Hi-voltage circuits. It is not oscillation. It is simply not focusing due to improper voltages to the CRT. Turn the scope on and let it get to the point of not focusing, then spray suspect components with the freeze mist one at a time and watch for any changes. Since the intensity and focus controls do not have the expected effect on the trace, it may be worth checking them as well. The controls themselves may be bad or dirty. At the high voltages needed for proper CRT operation, dirty grease or lube inside the pots may bleed off your voltage. The high voltage in old oscilloscopes has a very tiny current, maybe a milliamp or two, and it takes very little to kill it.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Thu 14, 2015 11:45 am 
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Try removing the focus and intensity controls from the front panel then isolate them from the chassis and see what you get. If you adjust the pots while they are not mounted to the front panel be sure your hands do not touch the pots directly.

If the trace now looks more proper then mount one control at a time to see which control is leaking to its case.

Once you find the suspect control, replace it with a Clarostat/Honeywell pot. I used their pots in a Dumont 304H oscilloscope and have been well pleased with them.


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 15, 2015 2:45 am 
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Tube Radio wrote:
Try removing the focus and intensity controls from the front panel then isolate them from the chassis and see what you get. If you adjust the pots while they are not mounted to the front panel be sure your hands do not touch the pots directly.

If the trace now looks more proper then mount one control at a time to see which control is leaking to its case.

Once you find the suspect control, replace it with a Clarostat/Honeywell pot. I used their pots in a Dumont 304H oscilloscope and have been well pleased with them.


I pulled all but 4 ceramic caps and tested them. I'll pull the last 4 and test them tomorrow. If they had all checked good I would stop.

I had one 470pf 500V cap that is on the cathode of the tube that is in the Horizontal Gain circuit (V6B - 1/2 of a 12AU7). It was 62% high but wasn't leaking so probably not a factor in the current problem. I'm going to replace it because it's so far out of tolerance. It was the only out of tolerance cap so far.

There were 3 that leaked at around 350V (none were exactly the same)

C12 is a .02µF 1600V cap. Tolerance was good. It starts to leak around 350V. It connects pin 2 on the CRT to the Z axis. It's probably not the cause of the problem I have now.

C28 is a .01µF 500V cap. Tolerance was good. It starts to leak between 300 and 350V. It's on the Horizontal Amp / Freq. Selector Input from the power transformer. There are 4 more on the selector that I haven't pulled yet. It's kind of suspect. It's on the front end of the Horizontal Gain stage.

C33 is a .02µF 1600V cap. Tolerance was good. It starts to leak around 360-ish volts. This is in the blanking circuit just before the CRT It's on the main board and it's a long way from the CRT ( i wonder if it would be better to mount it to the CRT socket instead).

When the replacement caps come in and it's all back together I'll test it with the switches pulled.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Mon 18, 2015 11:59 pm 
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Update. There's good news and there's bad news :)

The good news is that the ceramic caps came in. The other good news is that while I was recapping I found a... uh... cough... wiring error. I think it's possible that the wiring error was the root of the problems. I left off a .25µF 500V cap that goes from the end of the capacitor string on the H. Amp / Freq. Selector to ground. It's just ground. No big deal. That's like saying dirt right :shock: Anyway, all of this is the good news. It's good because replacing the caps won't hurt, especially the out of tolerance and leaky ones, and finding the (hopefully) last major error should let the scope do "scope things".

And now for the bad news. When I laid the oscilloscope on it's side V10 (6C4. I'm not quite sure what it does) fell out. I probably bumped it. Working on this thing with the CRT and boards mounted can be a challenge. Anyway... It landed on the bench, rolled off, and fell to the carpet. When I reached down to pick it up I could see that it was broken? The glass was almost perfectly cracked all of the way around at the base. I rummage through my small stock of tubes and there was no 6C4 to be found. So I looked again and it still wasn't there. I looked for the "other" numbers listed on the tube sites, nada. It'll be here later this week.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Tue 19, 2015 12:51 am 
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You could drop a 6C4 from a ten-story building and not hurt it. It was probably cracked before.

It's odd that every cap would start leaking at 350V, but perhaps that's an artifact of the measurement circuit. The tester doesn't show leakage with no cap connected, does it?


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Tue 19, 2015 1:08 am 
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Location: Weimar, Texas
Alan Douglas wrote:
You could drop a 6C4 from a ten-story building and not hurt it. It was probably cracked before.

It's odd that every cap would start leaking at 350V, but perhaps that's an artifact of the measurement circuit. The tester doesn't show leakage with no cap connected, does it?


The tube had a white haze around the inside of the base so I'm inclined to think your right. All of the tubes are what came in the oscilloscope and the oscilloscope sat in my barn for at least 10 years plus however long it sat in the previous owners barn before the estate sold it. I'm glad that's the only one that has failed... so far.

The tester is my recently overhauled from stem to stern Eico 950. It does not show leakage with no connection (just checked).

I had my Telohmike on the bench last night and I recapped it. While I was recapping it I saw some resistors that were bubbled. The power supply electrolytic caps were shot. They used two 12µF 450V caps in series and then a pair of resistors in series in parallel with the caps. I'm going off of memory but I think there was a 2W resistor in the B+ line that the paint had bubbled on . Anyway, it was late and I didn't feel like digging in to resistor issues so I tested it smoke leaks and then buttoned it back up. I'll look at it closer this evening. The Eico does a good job but a working TO6 would be nice. I'll re-check the ceramics when it's working right. It does not measure cap value worth a flip but I have other instruments for that. I want something to show me leakage current and the TO-6 does that pretty well.

There is some psychological benefit to putting new ceramics in. Less than there was before I found the missing cap to ground but I won't question the ceramics at all now.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 1:29 am 
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The 6C4 replacement arrived. On cold startup the scope had a very nice trace. As it warmed up the trace became wider and now, after about an hour, it's a furry line again. Dang it...

On the upside, the retrace was only visible at the highest H Amp / Sweep setting, which is 600KC.

As I was typing this I remembered that I needed to take the Intensity and Focus pots off of the front panel to see if either of them are leaking.

:/ Well... When I powered the scope up after pulling the pots, the trace looked good. I turned around to type this and when I looked back the trace had doubled in amplitude and was very sharp. I didn't touch the scope. I just changed for the better.

I reached for my camera and when I turned back around the trace amplitude was 1/2 of what it was and the trace was fuzzy. That's where it has stayed.

I strongly believe that there is a temperature sensitive component or components in the scope. Maybe in the retrace circuit? Intensity never has varied intensity and that hasn't changed. It just moves the trace. Maybe something there too?

I guess I'll get back to basics and check voltages again.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 2:12 am 
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What I'd suggest doing is using an alligator clip lead ground each pot separately while the scope is operating and see if there is any effect on the trace. If so that pot is leaking to the case and will need to be replaced.


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 1:47 pm 
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About now I'd grab the can of freeze spray and try the resistors in the CRT circuit.


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 2:09 pm 
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Location: Weimar, Texas
Tube Radio wrote:
What I'd suggest doing is using an alligator clip lead ground each pot separately while the scope is operating and see if there is any effect on the trace. If so that pot is leaking to the case and will need to be replaced.


Will do

Alan Douglas wrote:
About now I'd grab the can of freeze spray and try the resistors in the CRT circuit.


I tried freeze spray (upside down can of air) but it's definitely worth a try again. I probably missed something.


I'll put it back on the bench sometime today and give these two ideas a try.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 5:50 pm 
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Did you replace all resistors or just the out of tolerance ones?


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 5:58 pm 
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Tube Radio wrote:
Did you replace all resistors or just the out of tolerance ones?



Just out of tolerance but I used a very narrow criteria so I replaced some that were still in tolerance (about midway between the value and max were replaced).

Freeze spray had no affect and grounding the pots with a jumper had no affect.

The trace starts off thin and gradually widens as the scope heats up.

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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 8:33 pm 
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Quote:
The trace starts off thin and gradually widens as the scope heats up.
Just to be absolutely sure, with no sweep, do you get a focused spot on the screen, which enlarges as time goes on?


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 10:31 pm 
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Be sure to keep the intensity low if you try Alan's suggestion. It's fairly easy to burn the screen with this type of scope. The safest way would be to keep the intensity all the way down then turn it up periodically to see what's going on.

It's always better to use only enough intensity to get a clear trace. These old scopes never had the brightness you see with modern ones.

RRM


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 Post subject: Re: Knight Wideband Oscilloscope
PostPosted: May Fri 22, 2015 10:37 pm 
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Location: Weimar, Texas
Yes. As it warms up I have to turn the focus control clockwise. Eventually I get all the way to the end of the control and the spot is a fuzzy ball. Not a large ball but at least 2 to 3 times bigger than the original fine dot.

To get a dot I have to put the attenuator on .1 or .01, H. Gain fully counterclockwise, and V. Gain full counterclockwise.

I just found this- If the attenuator is at 1 and the input is open when I increase H and V Gain 25% or so I get a small waveform that won't settle down.

When I ground the input I get this artistic expression of photons (the previous owner added a BNC jack where V input is and moved the V input jack down to where the ground jack is so V input has two places to input a signal. That's why the ground lead is connected like it is) There is a new cap to ground on the input but I think I'll change it just to make sure something hasn't affected it:

Image


Input section of the schematic:

Image


RRM: That's another problem. Intensity doesn't affect the intensity. It just slightly moves the dot or trace...

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