Oscilloscope. To repair or not? Fixed.

[clay1905]

Hi,
I have just joined this forum, because I have been gradually learning about electronics, by reading and teaching myself. I have gotten to a point where the multimeter just isn't enough for diagnosing things, so an oscilloscope seems to be the next instrument to get. I have been given one, but it doesn't work. The question is, should I attempt to get it working, or continue my search for another?
The instrument is manufactured by Kiksui, and was marketed as a Jayem Model 555. What I've read so far indicates that this instrument should be fairly good for simple things, as I'm likely to be engaged with for the foreseeable future.
So, here's what I have discovered by simple observation so far-
When I first switched on, no smoke got out (big relief there).
The power indicator light comes on, and if switched, the reticule lighting also works.
There is no time base trace, nor even a dot on the screen.
I read that one simple diagnostic is to turn the scope off in the dark and see if there is a momentary flash on the screen as this could indicate a possible source of the fault, but no luck there. I also looked as sideways as I can to see if the beam was striking the screen somewhere outside the mask. Nothing there either.
There are three valves (thermionic tubes) and all of them, along with the CRT have the heaters working. I haven't measured the heater voltages as yet. The valves are a 12AU7, and two 6DJ8's.
I can't see any obviously cooked components. Nor is there any smell of anything previously getting overheated.
There are some measurement points on the three circuit boards with stated voltages. Measuring these indicates that they are all approximately 20% below the printed numbers. 300V is about 250, 15V is about 12.5. And a trimpot marked '50V Adjust' was well below that between any leg and the chassis.
At the moment I'm guessing that the fault is something comparatively simple. Hopefully getting the voltages back to specs will move things along.
As I hinted above, I'm still pretty green with electronics, so maybe this is all a bit ambitious. However, with guidance from those more knowledgeable than me, I have managed some good repairs to valve operated things before today, and I'm willing to have a go.
So, is there some generous soul who would be willing to offer some guidance on what and where I should go next? I would very much like to succeed with this.
Thanks,
Clay.
P.S.
There are pictures associated with this thread. They are near the end in specific posts.

[tautech]

Welcome to the forum.

Of course it's worth spending some time on but first we need to positively identify it and get a Service manual or at the very least schematics.
Can you post up a pic of the front and rear panels to help with identification?

In the meanwhile here's a document worthy of study:
http://www.sphere.bc.ca/test/tek-parts/troubleshooting-scopes.pdf

[tggzzz]

I suggest your first task is to identify where the -2kV is generated and used - and then make sure you don't come into contact with it. In some scopes there is also +17kV.

The other voltages are non-trivial as well.

[clay1905]

Thanks tautech,

I have read on another forum that the service manual and schematics are a trick to get. One fellow managed to get a schematic for a different model from the makers, they're still in business, on the agreement to not share the document. Apparently it took a fair bit of begging, but at least they did eventually provide. Otherwise, it appears they were only available to agents. Attached is the front panel picture. The rear panel only has two banana sockets, one being a screen cathode, the other is a ground I guess. No labels or other data anywhere other than the front panel.

And thanks for the warning tggzzz,
I'm more or less aware that CRT's in general have high voltages. I tend to caution when working around the unfamiliar, and appreciate the reminder. I don't plan on getting any more intimate with the chassis until I know more about it.

[tggzzz]

... and appreciate the reminder....

You're welcome.

It is a pleasant change to come across someone on this forum that listens There are too many "I don't see/understand the problem => there isn't a problem" statements around! While I don't mind what such people do to themselves, I do object if they involve innocent third parties in their shenanigans.

[rstofer]

You have probably already done this but set the triggering to Line (either + or -).  This guarantees you will get a trigger if the system works.

Check the calibration outputs for the proper voltage with a DMM - just to get more info on the power supply issues.

Find an AC wall wart in the range of 12V.  Set the Volts/cm to 5 or 10, AC coupled (the switch to the left of the Volts/cm) and connect it up.  This will give you a known input that you can verify with your DMM.  You can try the position knobs.  Time/cm should probably be 2 ms/cm to display a full cycle, 10 ms/cm to get several cycles.

That's a neat old scope, it kind of reminds me of an old Dumont I had back in the '70s.

[rf+tech]

Hi clay1905,

This caught my eye:

The rear panel only has two banana sockets, one being a screen cathode, the other is a ground I guess.

I seem to recall that these older tube based scopes use a shorting strap between the intensity modulation post and the ground post. Without this strap in place, the beam may be cut off, if the cathode is expected to operate at DC ground. If the intensity modulation is AC coupled to the cathode, then the missing strap would make little difference.

The screen capture below is taken from the 556 manual and mentions a 10 Vpp signal required for intensity modulation, but does not provide detail as to whether beam cutoff is 0 Volts or 10 Volts, AC or DC coupled, etc.

Simple things first,

RF+ Tech

[ebclr]

Before attempt to fix this relique, think how much 20 hours of you time is worth, If the result is Higher than USD 250 buy one, and gift that one to a museum

[Kleinstein]

This is a nice old scope, but it is more a historic one than an really useful instrument these days. It's still of some use, but newer, even rather cheap ones instruments are usually much better in many aspects.

[tautech]

Hi clay1905,

This caught my eye:

The rear panel only has two banana sockets, one being a screen cathode, the other is a ground I guess.

I seem to recall that these older tube based scopes use a shorting strap between the intensity modulation post and the ground post. Without this strap in place, the beam may be cut off, if the cathode is expected to operate at DC ground. If the intensity modulation is AC coupled to the cathode, then the missing strap would make little difference.

The screen capture below is taken from the 556 manual and mentions a 10 Vpp signal required for intensity modulation, but does not provide detail as to whether beam cutoff is 0 Volts or 10 Volts, AC or DC coupled, etc.

Simple things first,

RF+ Tech

Quite right as shown in the manual for the 555 the OP has:
http://www.kikusui.co.jp/kiku_manuals/NUMERIC/555_E.pdf

[clay1905]

Wow. Lots of replies. Very welcome!

OK, I'll try to address all the suggestions.

rstofer:
Triggering. I have had that set at internal, both + & -   Sadly, no joy there. I've fiddled with the various knobs, and although I can't say I've experimented with every possibility, basically nothing, no matter how the controls are adjusted.
The calibration voltages are all low too. The 0.05V hardly got a mention on the meter. the rest, 0.5 & 5V are both low. Digital meters make it hard to get a number, but around that 20% mark. Without telling too long a story, I do volunteering work at the local hysterical village, so I'll get out a low voltage plug this afternoon and see what happens with that.

rf+tech & tautech:

Hi clay1905,

This caught my eye:

The rear panel only has two banana sockets, one being a screen cathode, the other is a ground I guess.

I seem to recall that these older tube based scopes use a shorting strap between the intensity modulation post and the ground post. Without this strap in place, the beam may be cut off, if the cathode is expected to operate at DC ground. If the intensity modulation is AC coupled to the cathode, then the missing strap would make little difference.

The screen capture below is taken from the 556 manual and mentions a 10 Vpp signal required for intensity modulation, but does not provide detail as to whether beam cutoff is 0 Volts or 10 Volts, AC or DC coupled, etc.

Simple things first,

RF+ Tech

Quite right as shown in the manual for the 555 the OP has:
http://www.kikusui.co.jp/kiku_manuals/NUMERIC/555_E.pdf

The strap is in place between the rear terminals. 10Vpp, is that being tested by the experiment suggested by rstofer with a wall plug? Looks like it to me. Sorry, but the knowledge is rather patchy. I've only picked up what is relevant to whatever the current project is along the way.

ebclr & Kleinstein;

Thanks for your thoughts. I understand that the scope is a historic relic but so am I, and I feel neither of us are ready yet for a museum. I have a soft spot for things that glow and get warm. Probably a strong nostalgic thing. And, I am hoping that I might learn something from the experience too. I'm one of those odd fellows who thinks knowledge itself is a prize most valuable, so time isn't that big a consideration.

I'll be back later this afternoon with results from the 12V ac input experiment.

Thanks one and all!

[tautech]

The bitch will be adapting to the input connector, IIRC it's an RF type.
Somebody will identify it at a glance.
You'll need to adapt it to BNC to accept standard probes.

[StillTrying]

Wow. Lots of replies. Very welcome!

I searched for the manuals but didn't find any.
Going by the number of visitors viewing very old dead web pages, I think there must have been about 8 of us searching for the manual.

[klunkerbus]

The bitch will be adapting to the input connector, IIRC it's an RF type.
Somebody will identify it at a glance.
You'll need to adapt it to BNC to accept standard probes.

The connector appears to be an SO-239, common in ham radio gear.  The mating plug is a PL-259, typically with an insert for the particular coax cable being used.

[tautech]

The bitch will be adapting to the input connector, IIRC it's an RF type.
Somebody will identify it at a glance.
You'll need to adapt it to BNC to accept standard probes.

The connector appears to be an SO-239, common in ham radio gear.  The mating plug is a PL-259, typically with an insert for the particular coax cable being used.

Thanks.
Further googling for "PL-259 to BNC adapter" has reminded me it's commonly called a UHF connector.
Adapters are easy to find, here's a selection:
http://www.ebay.com/bhp/bnc-to-pl259
https://www.amazon.com/CableWholesales-Female-PL259-Male-Adapter/dp/B000VGUCTM

[Back2Volts]

I know without the schematic it is going to be hard, but couldn't the low voltages measured be caused by failing capacitors in the PS ?   Those capacitors must be like 70 years old.

[Brumby]

Low voltages could be caused by several things - from high resistance in the PSU to high loading from the circuit.

A schematic would be very helpful - and one with voltages would be brilliant!

[FlyingHacker]

Before attempt to fix this relique, think how much 20 hours of you time is worth, If the result is Higher than USD 250 buy one, and gift that one to a museum

You don't fix a scope like that to save money. You fix it as a learning experience.

[alsetalokin4017]

Whoa there big fellas. Let's back up just a bit and make absolutely sure we've touched all the bases.

First, the issue of obtaining a trace. While the low PSU voltages may make it impossible to get a trace, let's make sure we have the scope set up properly in the first place. I know this may seem too basic and too obvious, but I have encountered many "non-working" analog scopes that were simply set improperly and were "dark" for that reason alone. So let's refer to the manual linked up above, pages 11, 12 and 13 for the proper initial setup to see a trace.
If you have the scope set up this way and don't see a trace, there is one more thing you can try. Note the current setting of the "DC Balance" pot under the Vertical Volts/CM switch. Then rotate that pot slowly throughout its range, looking to see if the trace shows up. If not, just return the pot to its previous setting, and start planning to look at power supply voltages more closely. Try the same thing with the "Stability" adjustment pot in the timebase section.

Next, the issue of the calibrator voltages. Measuring these with a DMM will read a lower voltage naturally-- because the output of these calibrator jacks is (supposed to be) a square wave at about 1 kHz. The DMM will give an average reading. You really need to look at these signals with an oscilloscope to see if they are working properly.

Next, the SO-239 to BNC adapter is pretty common, but it just so happens that the inner socket hole of the SO-239 is a good match for a standard banana plug. So you can use banana-plug patch cables to make your connections during testing. Be careful that you don't exceed the input voltage limits stated in the manual!

[clay1905]

Rightio, seems we have plugs in hand.

I should have put in the first post that I had the scope set up according to the handbook as a base position. I've slowly twiddled the pots as described above, but no change. Whilst I was there I also moved the position knobs through their ranges, along with focus and brightness. Silence is the stern reply.

Ah! but now the waters are muddied. A few days ago, I had not even one scope, now I have two. This new arrival is of absolutely unknown condition. I haven't even had time to power it up yet (That might be no bad thing). Perhaps there are tips and tricks to minimise the risks of frying parts before I go ahead?
This new to me scope is listed here:
http://www.radiomuseum.org/r/fairdumont_portascope_765h.html


What to do? Perhaps the portascope is the better long term choice. It has dual lots-a-stuff. But if it needs mending, can I do it? I suppose I should find out if it needs fixing before I get too far with that sort of thinking. I haven't looked yet for service info or diagrams, but I'll have a hunt later this evening. Opinions on what to do now I have two are solicited.

Thanks all,
Clay.

[rf+tech]

clay1905,

What to do?

Since you are newly learning electronics, let's focus on this older scope (and save the Portascope for another thread). Having only three tubes, it will be far simpler to work on. And you're already off to a good running start by wisely working through the setup procedure in the manual. 

Schematics are a definite advantage but the lack of schematics is not a serious obstacle. A few of us have many decades of experience and highly developed troubleshooting methodology. We'll hang in there with you and work through the issues in a logical manner, requesting photos and measurements to point in the proper direction.

For the baseline troubleshooting condition, the scope should be kept just as you have it set - a situation where a display "should" be present. Position controls mid range, intensity about 75%, triggering such that sweep should be present.

From this baseline, we can deduce if the horizontal and vertical amplifiers are working, with a few differential voltage measurements, and look at the CRT cathode-to-grid bias to see if it is being held in cutoff.

Before probing the CRT grid or cathode, it is imperative to determine whether this scope operates with a high negative voltage on these elements relative to the chassis, so as not to damage your - or yourself  . *Nothing* beats the adrenaline buzz of a 10cm mm arc reaching out to an approaching meter probe. 

If you are ready to dive in, let's see some interior photos. 

RF+ Tech

edited to correct metric units from cm to mm

[R005T3r]

Why searching for another one if you have the proper knowledge and skill to do that?
There's a channel on youtube that may help you if you are not familiar with calibrating the oscilloscopes:

There's the channel:
https://www.youtube.com/channel/UCU9SoQxJewrWb_3GxeteQPA

Remember: reparing analog oscilloscope is easy, but reparing modern digital stuff is something not possible in most cases...

[clay1905]

Thanks rf+tech,
I took the cover off the portascope, and took two steps back. I must agree, for the time being it can live on a shelf. The portascope is a bit out of reach for the time being.
As there is a pretty severe limit to the number of pictures I can attach to any post, I've uploaded the pictures to google cloud. The link is at the bottom of this post.
There are four boards in the scope. In order to label the boards I've made up this convention-  left & right hand refer to the chassis when viewed from the screen end, i.e. as viewed during normal operation. Top and bottom always as their normal position, no matter which way it's up at the moment.
One board is mounted vertically on the right hand side.
Another is vertical, and is at the back end of the chassis. It has the 6DJ8's on it. I can't get one clear shot of this board so there is a left and a right shot. I called this board the tube board.
The third lays flat beneath the CRT with the 12AU7, again, two shots to get all of the board. In taking the right hand shot of the horizontal board I had to unscrew the vertical board. Be aware that the two boards appear to be one where they meet. Couldn't get the vertical board far enough away for a clean shot, it's tied in place with leads.
The fourth is below the plug end of the CRT, and looks to me where the H.T. lives. Fat wires going from here up to the CRT plug.
The right hand side board and the horizontal board seem to serve both axes. The horizontal one even has a strip of aluminium to demark which parts are where. The vertical board seems to be more mixed up.
There are a few shots of the switch gear, but they look a bit confusing. There's a lot of stuff happening in there.
One more shot, of a pot on the back that was hidden by the cabinet. I'm guessing it has to do with either that screen cathode terminal, or something similar. Anyway, it's obviously only used during servicing or calibration. Oddly enough, it's labelled mystery pot.
Pictures of the insides are here:

https://drive.google.com/drive/folders/0B3JcpACaWCPlRVEyTzZiR2xxb2c?usp=sharing

I can add pictures to the folder as required.

Clay.

[tautech]

The limit for pics is relative to their size. Compress them to 100kb or less and you can put dozens in each post.
Then they're faster for you to upload and faster for us to view.
Although you might think it's only for your repair, do spare a thought for those that might stumble on this thread in the future and get the info they need for a fix too.


Like any repair break down the parts of the device into the important components, PSU is the most common point of failure and its correct output for a scope is crucial for operation.
Break this down again and you'll have some LV supplies, HV, EHT and PDA.
EHT usually ranges from negative ~1 kV down to negative 3 kV for scopes and it's applied to the cathode and grids. PDA on the other hand is a positive voltage, normally derived from the EHT with a diode/cap voltage multiplier to levels of 5 kV plus. It need not be measured as special equipment is required to do so.
If EHT is not present there will be no traces. It is sometimes supplied from a winding on the main PSU transformer but more commonly from a SMPS supplied by LV.

HV is mostly used for electrostatic deflection at the X and Y plates, thereby deflecting the electron beam on its way from the CRT cathode to the phosphors on the face of the screen.
HV is normally 100-200 V and the neutral V value (no deflection) at the plates is usually under 1/2 of the HV supply.
This allows the plate output stages to drive the beam above and below that no deflection point.

There's much more members can offer but a little understanding of how a CRO works can give a few clues as what to look for.
End of lesson 1.

[rf+tech]

Clay,

The photos are a very good start. Interesting to see this scope is a tube-transistor hybrid. The 1968 marking on the 0.5 uF 1500 Volt oil filled metal can capacitor seems to indicate the year of manufacture.

An initial observation about the .02 uF 1500 Volt capacitor connected to the intensity modulation input suggests that the CRT grid/cathode are biased at about -1200 Volts. The silk screen legend "-DC High Voltage Out" lends support to this conclusion. RHhorizontal.jpg shows a silk screen legend -1300V. Nothing appears in any of the photos to suggest the presence of a Post Deflection Accelerator supply. I'm leaning toward this scope being a non-PDA type tube, similar to the 5UP1 used in some of the early Heathkit scopes.

A word of caution is in order as the 6.3 Volt filament supply will be biased at -1300 VDC to minimize the filament-to-cathode voltage differential. Knowing the pinout of the CRT socket will be advantageous. Is there a tube number marked on the base of the CRT? If not, then it may be necessary to extract the CRT from its shield to check for a label attached to the glass envelope. With the CRT part number, we can then locate a datasheet and thus eliminate some guesswork as to the connections and ascertain the grid-to-cathode voltage required for cutoff. Even without the datasheet, we can work out the connections by following wires from the CRT board & socket.

Once it is known which terminals connect to the grid and cathode, the voltage measured between the two (not to the chassis) can tell us if the tube is biased into cutoff. This measurement will need be made with some caution. Knowing the CRT is biased into cutoff will explain no trace - we'll just need to sort out why the blanking stage isn't unblanking the CRT.

Another observation - there does not appear to be a separate high voltage transformer. This suggests the main power transformer supplies the full HV AC

Expanding on tautech's comments about the deflection amplifiers, I find it more straight forward to measure the voltage differential across the deflection plates. By adjusting the position control, one can see the difference voltage decrease to zero and then know the beam will be very close to the center of the screen. Turning the position control either side of center should swing the differential voltage positive and negative, confirming the deflection amplifier under test is working. One can take this a step further by applying a 9 Volt battery to the scope input, and with the appropriate range settings, confirm the entire vertical channel is functioning.  The measured deflection plate differential voltage would change in response to the application of the battery and change in polarity, with the battery polarity.

Another three photos, one each showing the complete left side, top and right side "big picture" would be helpful.

If there's anything over your head that you are uncertain of, please ask.

It would be good to know what you are using for a meter - presumably a DMM that would be limited to about 600 Volts. If by chance you happen to have an equally old analog VOM, like a Simpson 260, some of those had a 1500 Volt range and special input jack.

RF+ Tech

[neko efecktz]

Hello all,
Just a thought,
Even though the valves are all glowing from the filament check them in the dark to see if there is also a blueish glow.
Sometimes a valve can have a fault but still have a working filament.
even then they don't always have the blue glow.

Just a thought
Bill.

[FlyingHacker]

Be sure to look up the specific tube, as some tubes have a blue glow naturally.

[rf+tech]

Hi Clay,

Now that *I* have taken a few minutes to R(most of)TFM, the CRT is indeed a 5UP1 and the datasheet is attached below. I've also attached the schematic for the Heathkit IO-18 oscilloscope, as a point of reference. This scope uses the 5UP1 CRT - the CRT bias circuitry and bias voltages found in the 555 should be very similar. Other similarites may be noted, possibly with the deflection amplifiers.

Referring to the IO-18 schematic, it shows the CRT beam blanking is capacitively coupled. The 555 may be the same in this respect. With the Intensity control full CW, the cathode to grid voltage difference would be essentially zero. It would be a good first step to check resistances throughout the CRT bias divider in the 555. It would not be unusual to find one or more out-of-tolerance resistors here. An open circuit in one of these resistors would result in an excessively bright and uncontrollable trace. The more likely cause of no trace may be loss of negative 1300 Volts.

Please compare the CRT bias divider in the 555 with this schematic and report the values actually found. From this information, an appliaction of Ohm's law will reveal what voltage to expect at the "bottom" tap in the divider. If this value is found to be less than 600 Volts, and can be measured safely, then by proxy we can determine if the negative 1300 Volts is present.

The following tests can provide insight to the functioning of the deflection amplifiers. A DMM will serve as a visual indication of what is happening to the CRT beam, even though the trace is not visible.

1. Connect DMM across Vertical Deflection Amp 6DJ8 pins 1 and 6, rotate Vertical Position through its full range. Note voltage swings through zero and changes polarity, with the CW and CCW extremes approximately equal magnitude.

2. Adjust Vertical Position for zero volts, set Vertical Volts/CM to 5. Apply 9V battery to Vertical Input (DC coupled) and note DMM reading is no longer zero. Reverse polarity of 9V battery, DMM should now read the same voltage magnitude with opposite polarity.

3. Connect DMM across Horizontal Deflection Amp 6DJ8 pins 1 and 6. Set Time Base to EXT. Rotate Horizontal Position through its full range.  Note voltage swings through zero and changes polarity, with the CW and CCW extremes approximately equal magnitude.

4. Adjust Horizontal Position for zero volts on DMM, apply 9V battery to External Horizontal Input and note DMM reading is no longer zero. Reverse polarity of 9V battery, DMM should now read the same voltage magnitude but with opposite polarity.

5. Set Time Base to 1 second/CM, Triggering to Line +, Level knob to Auto. If the sweep generator is working, the DMM should indicate steadily increasing voltage (polarity unimportant) that resets every 10 seconds.

From the photos, one 6DJ8 visible in RHtube.jpg should be the Horizontal Delfection amplifier, based on the presence of the SWP CAL pot just below. I've made an assumption based on this photo that the second 6DJ8, which serves as the Vertical Deflection amplifier, is on the left side of the CRT board.

While the above measurements reference the 6DJ8 plate pins, the cooresponding deflection plate pins on the 5UP1 may be used just the same.

Let's see how much of this scope is actually functional and what clues can be discovered from real hands-on testing.

RF+ Tech

[clay1905]

First, I'm having difficulty finding a method to compress images that have useful data in them to below about 1Mb each. I'm happy to put pictures here so others may be able to find them in the future, but I will have to continue the hunt for a compression method. Suggestions are welcome, but I don't run Windows.

I see I need a notebook to keep a record of acronyms. They're the steepest part of the curve for me. That was quite an avalanche of information. I spent some time last night reading. Mainly Wikipaedia pages. I didn't know about electrostatic deflection, my knowledge was limited to the basics of picture tubes and magnetic beam steering. There's quite a wealth of information there, but the pages, and especially the one on C.R.O's are a bit terse, and want a few readings.
Between your posts and other reading, I think I've got a reasonable picture of the goings on inside an oscilloscope.

Conventions. I'm seeing both high voltage and high tension used. Is the distinction the same as for cars? That is, high voltage carries current, whereas high tension is like an ignition spark, lots of volts but only a few milliamps or so?

SMPS. I see PS, so probably a power supply, but what are the SM for?

For now I've added three new pictures to the google folder. They are both sides- full shots, and one from the top.
https://drive.google.com/drive/folders/0B3JcpACaWCPlRVEyTzZiR2xxb2c?usp=sharing

My meter is an inherited Dick Smith rebranded Fluke DMM of about 25 years age. It's upper voltage is 500.

The CRT is a Toshiba 5UP1(F) so rf+tech's guesstimate was on the money there. Pin 1 is at the very top of the socket.

So, let me say how I'm seeing things, so you can see if I'm understanding this all correctly.

There are four major blocks, for want of a better word, making up a basic CRO.
There is power conditioning. This can include filtering, transformation and rectification.
There is a circuit that powers the CRT to produce a beam of electrons. There are sub-circuits to control intensity and focus. There is an amplifier associated with this circuit.
There is a circuit that can generate a pulse. The circuit can be made to repeatedly produce pulses, thus making what can be expressed as a sawtooth waveform. This circuit varies an electrostatic charge on two vertical deflector plates in the neck of the CRT. There are sub circuits that control the period of the sawtooth wave, and triggering controls that can determine events that cause one or more sweeps of the electron beam. This circuit controls the CRT's ability to block, or blank off, the electron beam. There is an amplifier associated with this circuit.
There is a circuit that is an amplifier/filter that generates a signal to control the electrostatic charge on the two horizontal deflector plates. The signal is an analogue of variations in voltage detected by the scope probe. There is provision for varying the ratio of input to output in orders of ten.
There is high voltage at the output of every CRT control circuit.
A high -ve charge is put on the cathode and grid, because if you want a stream of electrons, they've got to come from somewhere. Electrons find each other repulsive, so the high -ve grid voltage squeezes the flow into a beam. This charge is likely to be in the low thousands of volts. It is this voltage that  tautech refers to as possibly being not present.
High voltage is applied to all the deflector plates. The voltage difference between opposed plates causes electrons in the beam to change direction, the beam tending towards the more +ve plate. One can expect this voltage to be in the low hundreds.
A strong +ve field may then be applied to the beam to give it a little push along. This accelerator voltage is usually more than enough to burn your toast.
Some voltages required by a given circuit may be obtained by modifying part of the output of another circuit.

So gentlemen, does that seem to be about what's involved in making Lissajous curves?

[alsetalokin4017]

You don't use Windows... neither do I. But you don't say what you _do_ use.... I use Linux, Ubuntu kind.

The image editing package "gimp" is free and available for pretty much all OSs in common use. Here is an example, taking your photo that was previously about 1.2 MB and resizing it to 800 pixels wide and saving as .jpg at 90 percent quality. This brings the file size down by a factor of about 6. Many other image editing programs will allow you to do the same thing.

High voltage and high tension are the same thing. Don't be fooled by any illusions of "low current"... a low current power supply can still charge up a HV capacitor to lethal energy levels which can be discharged at high current if you give it a low resistance pathway -- like your body.

Acronyms do abound. SMPS means "switchmode power supply" which usually will not have a big heavy mains power transformer, but also may suffer from high frequency noise and ripple on the PS output.

If you are going to be working with tube circuits a lot, you might consider investing in a HV probe for your DMM, which will allow you to measure higher voltages safely.

Your verbal "block diagram" description of the oscilloscope is basically correct. However, generating Lissajous figures does not involve a triggered or timed sweep, it simply applies one external signal to the X deflection circuitry and another external signal to the Y deflection circuitry. Some analog scopes have essentially identical X and Y amplifiers, with the X part selectable to either external input, or time-base controlled and triggered sweeps.  Some scopes will use a separate input for the X dimension and others will use the two "normal" input channels, assigning one to X and one to Y when the X-Y mode is selected.

If I were in your shoes I think I'd be more interested in getting the Portascope working, as it will be a much more useful unit than the single-channel 555.

[rf+tech]

Hi Clay,

First-off, my apologies for overwhelming you with too much, too fast. I'll also try to optimize some of the terminology that differs between US technical English and UK technical English - e.g. ground = earth.

There are a few points in your understanding that could use some refinement:

Physically, the direction of effect versus the schematic orientation of deflection plates is 90 degrees. Deflection plates drawn with the broad side vertical deflect the beam at right angles to their broad flat surface, in this case - horizontally. When both plates of a pair have the same voltage/potential (balanced, no differential), the beam feels no pull and remains in the center of that axis.

Electrons from the cathode, having a negative charge, are attracted to positively charged plates. The electron beam will be more strongly attracted toward the plate with the higher positive charge, than the plate with the lower positive charge. It is this differential voltage between the opposing plates, as seen by the beam, that determines which way the beam will go.

The pulse circuit that generates the sawtooth waveform drives the horizontal deflection plates, which have their broad surface oriented vertically inside the CRT and as drawn schematically. This sweeps the beam from left to right. At the right end of the sweep, the beam needs to be returned to the left side. Just as in a television CRT, the beam is blanked during retrace by another pulse of much shorter duration from the sweep generator circuit and applied to the CRT grid. Visualize a sawtooth that rises linearly from zero volts to some positive voltage. The fast return to zero volts corresponds to the blanking pulse.

The signal from the scope probe is attenuated or amplified by the vertical amplifier to produce a much larger analogue of sufficient amplitude to deflect the electron beam in the vertical direction. This signal is applied to the vertical deflection plates and as described above, the beam favors attraction to the plate with the higher positive charge.

Your understanding of the electron beam is basically correct. Since electrons flow from the point of lowest voltage/potential to the point of highest voltage/potential, they don't really care about the absolute voltages as referenced to the chassis/ground/earth. To make deflection amplifier design a bit easier, the highest positive voltage is the nominal plate voltage of the tube/valve deflection amplifiers - something like +300 volts. The cathode is then biased to a much greater negative voltage. The sum of +300 Volts and -1300 Volts combine to create the 1600 Volts accelerating potential mentioned on page 5 of the 555 operating manual.

The typical operating conditions section of the 5UP1 datasheet gives us an approximation of voltages to be expected, and the maximum ratings section an upper limit. Further, the datasheet makes no mention of a Post Deflection Accelerator so there will be no positive high voltage supply present in the 555. This is further substantiated by the 1600 Volts accelerating potential mentioned on page 5 of the operating manual and the -1300 Volts legend silk screened onto the printed circuit board(s).

The high voltage applied to the deflection plates is about one fourth the high voltage applied to the cathode/grid. High is relativistic. As measured against the chassis/ground/earth, the deflection amplifier plate voltages are well within the measurement capability of your DMM.

RF+ Tech

[clay1905]

Easy stuff first. I use Ubuntu as well. I would never have considered simply reducing the image size like that. I would have *assumed* that too much information would be lost. Thanks for the tip.

A high voltage probe for the DMM is looking worthwhile. And a lot better than a wet finger is my guess. The portascope is standing on the workshop floor just now, calling to me. But it will have to wait for a while. First I need to learn more. It does indeed look a much more versatile instrument, and when my skills get to the right level, I'll have at it.

The avalanche of information was not all bad by any means. I'm with Richard Feynman on this, and derive a lot of pleasure from finding things out. When the knowledge is practical too, bonus!

Now, I see I've fallen into a trap for young players. After re-reading my last post and the comments arising, we're on the same page really. It's just that when I was talking about the deflector plates, the terms were 90deg. out. I used vertical to refer to the physical position of the plates, rather than the convention of referring to the beam axis a specific set of plates influence. I shall keep that in mind.

I will go and make the measurements re+tech asked for in a previous post this morning, and post results here. I'll also get cracking on the image reductions and make a post that contains them for future reference.

Thanks,

Clay.

[GK]

Hmm, I've got two of these 555 Kikusui scopes in my dumpster-dive pile. I've been hanging onto these for their CRTs; the model is of such poor overall quality (that's not really wholly conveyed in the photographs) that it's not something I'd personally bother restoring. The styling is an obvious Tektronix rip-off, but that is where the similarities end. Schematics appear to be unobtainable, but I did, surprisingly, find an operation manual (*.pdf) for free download on Kikusui's website a while back.

I remember these being essentially dead, but I just plugged one in and once the heaters warmed up it appeared to be fully operational for several minutes, but the trace slowly dimmed to darkness and eventually couldn't be revived. I'm only interested in getting these operational to the extent of being able to evaluate the condition of the CRTs. The design is pretty simple and it should be a relative doddle to trace the circuit out. Depending on how much time I have I might get around to now.........



 

[tautech]

What's the one top left Glen ?

[clay1905]

Aha!

Three resistors "out of range". They form the part of the voltage divider connected to pin 4 on the CRT.
The focus pot measures infinite between any two legs, and each of the resistors connected to the outside legs of the pot are also open.
I would like have some idea as to why this failure occurred before I cook another set so the parts will be waiting on the bench until I hear back.

Attached is a little sketch of the values of the divider circuit. It doesn't seem to be quite the same as the Osborne though.

Where the diagram is marked 1300V, that isn't strictly true. In the LHhoriz picture, you can see where the board is marked -1300V, and quite close by is a red cotton covered wire connected to the board. The other end of the wire connects to one of the outside legs of the 100K pot in the divider.
The centre leg of the intensity pot is connected to the RH-HTboard. You can see a white/red stripe wire connecting next to a blue PVC covered wire. This board is marked High Voltage, and the hole for the red/white wire is marked 'out'.

Clay.

[alsetalokin4017]

@GK:  Heh... I see an HP180A mainframe in there with the 1801a 2-channel vertical amplifier in the left plugin slot, but I don't recognize the right-hand timebase plugin.
This is a great old scope... although I don't think I've actually turned mine on since I got the Rigol DS1054z....
I'd still like to find a spectrum analyzer plugin for it.

[StillTrying]

It's quite common for high voltage, high resistances to go even higher over many years of use, without there being a fault anywhere else.

[alsetalokin4017]

It's quite common for high voltage, high resistances to go even higher over many years of use, without there being a fault anywhere else.

Yep. I'd just go ahead and replace the pots and resistors (and the capacitor as well.). This could be the whole problem with the scope, right there.

[tautech]

Aha!

Three resistors "out of range". They form the part of the voltage divider connected to pin 4 on the CRT.
The focus pot measures infinite between any two legs, and each of the resistors connected to the outside legs of the pot are also open.
I would like have some idea as to why this failure occurred before I cook another set so the parts will be waiting on the bench until I hear back.

Attached is a little sketch of the values of the divider circuit. It doesn't seem to be quite the same as the Osborne though.

Where the diagram is marked 1300V, that isn't strictly true. In the LHhoriz picture, you can see where the board is marked -1300V, and quite close by is a red cotton covered wire connected to the board. The other end of the wire connects to one of the outside legs of the 100K pot in the divider.
The centre leg of the intensity pot is connected to the RH-HTboard. You can see a white/red stripe wire connecting next to a blue PVC covered wire. This board is marked High Voltage, and the hole for the red/white wire is marked 'out'.

Clay.

AS StillTrying explains this is a common failure in CRO's, more so older ones and related to the quality of the carbon resistors from back then. New components sourced from any of the big suppliers will give most likely better than new service. I have seen them that "baked" that the values are only obtainable from the schematic or BOM. I hope that's not the case for you.
For the pot you could attempt to source a wire wound that will give better service again and some manufacturers used these to ensure long service.

[clay1905]

Hi Gents,

I went down to Jaycar and got baffled with bull. Came away with resistors an order of magnitude small. They don't do a 1meg 5watt resistor, so it was down to building a series, 470K+470K+68 should have done it, but I didn't look and the lad serving gave me 4.7K's instead.
I think part of the problem is I believe I'm coming down with whatever the latest flu release is, and the brain is getting a bit too fuzzy. I was getting all ready to solder in the new bits, and something in the back of the head was saying "something here isn't right". I'll go back in the morning and get some more resistors, but I'm not too keen to be messing about in unfamiliar territory until I feel clear headed enough to be confident. All that high voltage, you know.
Hopefully this will be one of those 24 hour 'flu things and I'll be back on deck soon.

Thanks,
Clay.

[tautech]

Hi Gents,

I went down to Jaycar and got baffled with bull. Came away with resistors an order of magnitude small. They don't do a 1meg 5watt resistor, so it was down to building a series, 470K+470K+68 should have done it, but I didn't look and the lad serving gave me 4.7K's instead.
I think part of the problem is I believe I'm coming down with whatever the latest flu release is, and the brain is getting a bit too fuzzy. I was getting all ready to solder in the new bits, and something in the back of the head was saying "something here isn't right". I'll go back in the morning and get some more resistors, but I'm not too keen to be messing about in unfamiliar territory until I feel clear headed enough to be confident. All that high voltage, you know.
Hopefully this will be one of those 24 hour 'flu things and I'll be back on deck soon.

Thanks,
Clay.

4k7 is Jaycars highest value 5 watt resistor, so good luck getting higher values from them.
IME it is not normal for this resistor chain to be made of 5 watt resistors, 1 watt is normal and should suffice.

Grab one of their catalogues they're handy to have.

[alsetalokin4017]

A little bit of Ohm's Law calculation will show the necessary power ratings of those resistors and potentiometers.

P=I²R and V=IR

[clay1905]

Hi again,

I was pretty fuzzy yesterday, as you can tell from the confusion about power figures etc.
I do like the idea of making me work to learn. It's good to be pushed.

OK, so I set about trying to get the figures to apply Ohm's law. The obvious starting point is the data sheet, which gives all the voltages. However, I don't see anything about current draw. Then I found the data sheet attached. It's a bit of a cheat as all the wattages are included in the "typical circuit" diagram.  I still remember the advice my 1st grade teacher gave the class:   "There's no such thing as cheating in the real world. The only thing that will count is the result."
The result is that 1/2Watt resistors should be good. 1 watt would give a nice margin and Jaycar have these in the right ratings.
Rhubarb! Jaycar aren't open today.
On another note though, they do appear to carry the correct probe and an adapter. Seeing there's the potential for success with replacing these few resistors, I thought it wouldn't hurt to start looking about in this direction.

So, I've made a little shopping list of catalogue numbers, and will splurge tomorrow. Fingers crossed, I'm hoping that replacing the dead resistors won't just be revealing something else wrong further along.

Thanks for all the words of advice so far. This is quite an interesting and fun project, and I'm looking forward to knowing why things like the oscillator attached to the 555 chip isn't doing exactly what the book says it should. The timer is a project to replace a failed circuit in a coffee mill, but that's another story altogether.

I'll report back as soon as I have some sort of result.

Clay.

[clay1905]

The pictures.

Here is the beginning of the pictures. They will be in this and the following three posts.

Clay.

[clay1905]

Four more pictures.

Clay.

[clay1905]

And another four.

Clay.

[clay1905]

The last two pictures.

Clay.

[rf+tech]

Hi Clay,

Good progress, you're learning fast.

I would like to share a few more points and theory to further your understanding.

First, note the 5UP1 datasheet "Examples of Use of Design Ranges" shows two values for maximum G1 voltage for visual cutoff: -45 Volts for an Anode 2 voltage of 1000 and -90 Volts for an Anode 2 voltage of 2000. You are correct in that the -HV at the HV board is higher, however, it will only be higher by something between -45 to -90 Volts. Anything greater would be of no benefit and at worse would serve to limit the useful range of the intensity control by driving the CRT into deeper cutoff. This knowledge implies the -HV from the HV board would be something in the vicinity of -1350 Volts.

With a cathode ray tube (or most any device), the current through the tube is established by the biasing and load resistances/impedances. When given in a datasheet, the values are intended as either absolute maximums or typical, and the latter can be quite broad.

As mentioned by others, a first approximation of the current through the CRT bias network (not the CRT itself) can be obtained from 1350 Volts divided by the sum of the divider resistances to chassis/ground. The current drawn by the CRT from the bias network does have an effect on the actual voltage divisions, and further, the current drawn by the CRT varies with intensity.

I did note in your drawing of the bias network for the 555 CRT that the ground connection is not labeled. How will the current through the bias network return to the -HV power supply? I suspect the ground connection may be the junction of 1M and the 47nF capacitor, or there is another resistor to ground from this junction. Given your illness, it is an understandable oversight. Reverse engineering is an art in itself.

Thanks for sharing the quote from your 1st grade teacher. This reminded me of a quote from one of my grade-school teachers: "Knowledge isn't what you know, knowledge is knowing where to look for information and how to apply it."

Hope you are feeling better quite soon.

RF+ Tech

[clay1905]

Phew,

That's a lot to digest ATM. This bug has a better grip than I first thought. I haven't really attempted to trace the circuit back any further for the while. I just stand there looking while my head swims.

I did find that the second pot, intensity I think, is actually a ganged double. It was dead too, both parts. I have noticed that the ganged pot connects to some whopping capacitors underneath. They even have little insulators sticking up like miniature power station transformers. They're giving me the willies. Any suggestions on how to ensure they're properly discharged before I go sticking my fingers in there? Little ones, up to say 0.2uF I'm not too worried about shorting with an old screwdriver tip, but these have a certain keep away look to them.

Thanks,
Clay.

[tautech]

Purists use something like 100  but most of us just zap them with a driver. 

Do be aware some caps will regain some charge after a short while so either leave a jumper across them of driver them again a time or two.

[BradC]

Purists use something like 100  but most of us just zap them with a driver. 

I use a 60W incandescent bulb. I already have the ES socket with a couple of flyleads and croc clips for when I need a dim bulb tester, so it's handy for the odd cap discharge.

Second the comment about putting something across the cap. I use cheap croc-leads. I've been bitten once or twice by HV caps that have regained some bite over time. I now keep them firmly clipped off. Remember to remove the clips before you power up. That's a mistake you'll only make once.

[alsetalokin4017]

Cheating is all well and good, you might think ... as long as you have a cheat sheet. But life does not usually come with a cheat sheet. It is better by far to learn the principles and facts you need to accomplish your goal.
 
The voltage divider resistor values shown on the CRT data sheet are a starting point, and are useful certainly. But in order to understand better the reasons for the original power handling capacity that the scope's designers may have used... and to make sure any substitutions will work for more than a few minutes... you really should try to understand the principles involved, rather than relying on a "cheat sheet".

In other words... that first-grade teacher was wrong, and did you and everyone else in the class a disservice by telling you all basically that it's "OK to cheat" to obtain your desired result. Try cheating on your GRE for example... it only works if you don't get caught.

[clay1905]

Spent a bit of time this morning tracing the divider circuit more thoroughly. I do wish some businesses would buy more different coloured wires.

Anyway, here it is. A rough sketch showing casual disregard for drawing convention. The two 100K pots are ganged.

Oh boy, the arithmetic is a problem. I'm getting weird answers in attempting a current calculation, but I suspect I'm missing something fundamental here. In the calculation, I chose to halve the values of the pots, because that's the 'normal' condition, but that can't be correct. The divider uses both the full value of two of the pots, as well as tapping off variable voltages. (I'm pretty woeful at arithmetic. When I was studying chemistry a long time ago, the class was tasked with calculating how many molecules were in a litre of a sample. Typical answers took the form NX10^26. I got 2. ) For this calculation, I got 0.000008824, which I think would be Amps if I got it right, but the number looks pretty meaningless to me.
I think I need a bit more guidance in this.

Thanks,
Clay.

[StillTrying]

I make it about 1300V/1.7M = about 7.6470588235294117647058823529412e-4 = about 0.76 mA, about twice as much as I would have guessed, about.

[rf+tech]

Clay,

The important detail is that although the pots are mechanically ganged and operate in parallel, they are electrically connected in series.

Since everything is in series, one simply adds up all of the resistor values. Depending on which rotation extreme of the left pot, the total resistance varies from 1.73 MOhms to 1.63 MOhms.

Current through the divider varies between a minimum of 1300 Volts divided by 1.73 MOhms, to a maximum of 1300 Volts divided by 1.63 MOhms.

Using the maximum current from above, the maximum power dissipated by each resistor is I squared divided by R.

Try working out the answers yourself and post back the results. For extra credit, solve for the voltages at each point in the divider.

We'll check your math and set you on the right track.

RF+ Tech

[clay1905]

Thanks for the reply rf+tech,

I was despairing of getting a result. This project has grown into something far more important to me than just repairing the 'scope, and any help would be enormously appreciated.

Somehow I'm not yet seeing the gaudily coloured elephant in the room. I will knuckle down this arvo and see what I can do. I have this Ohm's circle chart which I've been trying to use but so far the figures seem so far out that I'm rather confused. Maybe I'm misunderstanding the results though. I'll run the numbers through the mill again and post what I'm getting. I'll try to show working so you can see where I'm going off the rails, if indeed I am.

Thanks again,

Clay.

[alsetalokin4017]

Clay,

The important detail is that although the pots are mechanically ganged and operate in parallel, they are electrically connected in series.

Since everything is in series, one simply adds up all of the resistor values. Depending on which rotation extreme of the left pot, the total resistance varies from 1.73 MOhms to 1.63 MOhms.

Current through the divider varies between a minimum of 1300 Volts divided by 1.73 MOhms, to a maximum of 1300 Volts divided by 1.63 MOhms.

Using the maximum current from above, the maximum power dissipated by each resistor is I squared divided by R.

Try working out the answers yourself and post back the results. For extra credit, solve for the voltages at each point in the divider.

We'll check your math and set you on the right track.

RF+ Tech

Err.... um..... no.

http://info.ee.surrey.ac.uk/Teaching/Courses/ee1.cct/circuit-theory/section1/powerdis.html
and many other references....

and basic Ohm's Law: P = I x V, where V = I x R, so P = I² x R 

[clay1905]

OK, here come the sums.
Can't get the formatting for the divisions quite right.
 
163Mohm                                                    173Mohm

        0.000797546                                          0.000751445
  1300 )1630000                                           1300 )1730000

0.08mA                                                         0.075mA


Working left to right in the diagram I posted earlier, and using P=I^2*R  :

I^2 =                                                           I^2 =
0.0008^2* =                                               0.00075^2 =
0.00000064                                                 0.000000563   

&                                                                  &

100,000*                                                     100,000*
0.00000064                                              0.000000563
0.064                                                           0.05625

the 100Kohm pot = 0.064Watts.            the 100Kohm pots  =  0.05625Watts each.

330,000*                                                      330,000*
0.00000064                                              0.000000563
0.2112                                                          0.18579

the 330Kohm = 0.2112Watts.                       the 330Kohm = 0.18579Watts.

200,000*                                                      200,000*
0.00000064                                              0.000000563
0.128                                                            0.1126

the 200Kohm = 0.128 Watts.                        the 200Kohm = 0.2112Watts.

1,000,000*                                                  1,000,000*
0.00000064                                         0.000000563
0.64                                                             

and the 1Mohm = 0.64 Watts.                      and the 1Mohm = 0.563 Watts.

So the suggested wattages are, at least for the final resistor in the chain, a little on the low side, and the 1 Watt rated are an acceptable choice.

Voltages. Using V=I*R, I calculated the drops thus:

100K Ohm     80V                                               100K Ohm      75V
330K Ohm   264V                                               100K Ohm      75V
200K Ohm   160V                                               330K Ohm     247V
1M Ohm      800V                                               200K Ohm      150V
                 _____                                                  1M Ohm      750V
SUM         1384V                                               _______________
                                                               SUM                         1297.5V

Considering I didn't do enough rounding, I think the final figures aren't too far from actual values. They look quite reasonable, at least to me.

How did I really go?

Clay.

[alsetalokin4017]

OK, here come the sums.
Can't get the formatting for the divisions quite right.
 
163Mohm                                                    173Mohm

        0.000797546                                          0.000751445
  1300 )1630000                                           1300 )1730000

0.08mA                                                         0.075mA

Some decimal point trouble here.

First, you have 163Mohm and 173Mohm when you should have 1.63 Mohm and 1.73 Mohm.

Then your long divisions are actually correct.

1300 / 1,630,000 = 0.000797546
1300 / 1,730,000 = 0.000751445

But now when you divide by 1000 to produce the answer in milliAmps you've misplaced the decimal.
To divide by 1000 simply move the decimal point three places to the right:

0.000797546 A = 0.797546 mA
0.000751445 A = 0.751445 mA

so with rounding and discarding false precision we have 0.80 mA and 0.75 mA as the max and min currents.

Working left to right in the diagram I posted earlier, and using P=I^2*R  :

I^2 =                                                           I^2 =
0.0008^2* =                                               0.00075^2 =
0.00000064                                                 0.000000563   

Correct. You have used the correct values for the current here.

&                                                                  &

100,000*                                                     100,000*
0.00000064                                              0.000000563
0.064                                                           0.05625

the 100Kohm pot = 0.064Watts.            the 100Kohm pots  =  0.05625Watts each.

330,000*                                                      330,000*
0.00000064                                              0.000000563
0.2112                                                          0.18579

the 330Kohm = 0.2112Watts.                       the 330Kohm = 0.18579Watts.

200,000*                                                      200,000*
0.00000064                                              0.000000563
0.128                                                            0.1126

the 200Kohm = 0.128 Watts.                        the 200Kohm = 0.2112Watts.

1,000,000*                                                  1,000,000*
0.00000064                                         0.000000563
0.64                                                             

and the 1Mohm = 0.64 Watts.                      and the 1Mohm = 0.563 Watts.

So the suggested wattages are, at least for the final resistor in the chain, a little on the low side, and the 1 Watt rated are an acceptable choice.

All good. However, you should consider being conservative here. Myself, I'd use 2 Watt resistors out of an abundance of caution, at least for the 1M, and I might even use 1/2 Watt for the pots.

Voltages. Using V=I*R, I calculated the drops thus:

100K Ohm     80V                                               100K Ohm      75V
330K Ohm   264V                                               100K Ohm      75V
200K Ohm   160V                                               330K Ohm     247V
1M Ohm      800V                                               200K Ohm      150V
                 _____                                                  1M Ohm      750V
SUM         1384V                                               _______________
                                                               SUM                         1297.5V

Considering I didn't do enough rounding, I think the final figures aren't too far from actual values. They look quite reasonable, at least to me.

How did I really go?

Clay.

Close enough.  Also, if there is room, you can mount the resistors standing up off the board by their leads a bit to allow for better cooling.

[clay1905]

Thanks for the comments,  alsetalokin4017.

You'll notice the errors were all about moving from actual numbers, like 1,630,000 to a shorthand version, like 1.63M. It's a bit of a blind spot for me. I seem to be more able to get to base units from the abbreviation.

Now I can see where in the divider circuit I can safely put my probe to measure actual voltage. This might help to figure out why there is still no image, even though all the failed resistors I found in the divider are replaced.

The two fixed value resistors are flying, so as to speak, and 1M connects to a Bakelite tab strip, thence to ground. the 330K just loops across behind the front panel. They're both inside a plastic tube like the originals.

(Pretty pleased I got it mostly right.)

Clay.

[tggzzz]

You'll notice the errors were all about moving from actual numbers, like 1,630,000 to a shorthand version, like 1.63M. It's a bit of a blind spot for me. I seem to be more able to get to base units from the abbreviation.

With all SI calculations, it is best to use the base unit plus exponent notation. Thus:

• 1630000ohms = 1.63*10⁶ = 1.63e6
• 2.5cm = 2.5*10^-2m = 2.5e-2m
• 2.5cm = 25mm = 25*10^-3m = 25e-3m
• 0.0000000015s = 1.5e-9s = 1.5ns

Additionally, if you are working in an environment where a decimal point might be lost, e.g. on a drawing or when making handwritten notes, then don't use a decimal point but use the multiplier. Thus 1.5nF would be shown as 1n5, 0.15nF as 0n15 or 150p, and 15nF as 15n.

And never use "S" as the unit of time, e.g. nS. "S" means Siemens, i.e. conductance. "s" means seconds, i.e. time.

[clay1905]

Thanks, but no.

I work in and around various fields, using different units. Mostly in machine shops where millimetres are the rule, or decimals of. In any case, it's deriving that's where I seem to go wrong most, so working with long decimal strings yield the better results.

Thanks though,
if everyone used exponents I'd probably be better off, but that's not the case.

Clay.

[clay1905]

OK,
Started to look at where in the divider I could put a DMM probe to measure, but soon realised that if the 1M resistor drops the voltage by 753V then anything before it is out of reach, as my meter can only go up to 500V

So, it looks like I need to get another resistor, maybe around 500K off the top of my head, to interpose between the ground end of the 200K resistor and the chassis.
I've added what I mean to the diagram, as attached. The extra resistor and a dial face indicate where I believe the voltage should be taken, and how. Please excuse the hand drawn resistor and dial.

If things are OK, I should find about 375V using a 500K as above.

Before I let some smoke out, I'd like this confirmed please. It's a bit of a commitment based on newly learnt maths.

Added later-

Was sitting on the verandah when it occurred to me, parallel resistors is what I'm proposing. So I did a bit of reading and saw that wasn't the way. The voltage across parallel resistors is equal, and would fry the DMM.

Thanks,
Clay.

[clay1905]

It's happened again.

Went to visit a friend who asked me what I've been up to lately. I told him of my adventures trying to repair the 'scope, and how I'm now pushing through the mental block concerning maths.
Long story short as they say. I'm now the proud owner of a Philips Type 5650 oscilloscope. A tiny unit with a 70mm screen. Another one on the shelf for later. I did download a schematic, and this 'scope produces a fuzzy trace. There's hope for it, but not yet.
I think it's time I kept quiet about this. I'm running out of space.

Clay.

[Brumby]

It's happened again.

Went to visit a friend who asked me what I've been up to lately. I told him of my adventures trying to repair the 'scope, and how I'm now pushing through the mental block concerning maths.
Long story short as they say. I'm now the proud owner of a Philips Type 5650 oscilloscope. A tiny unit with a 70mm screen. Another one on the shelf for later. I did download a schematic, and this 'scope produces a fuzzy trace. There's hope for it, but not yet.
I think it's time I kept quiet about this. I'm running out of space.

Clay.

Too late.  You've begun a journey that will occupy every available bit of storage space you have - and then some.  All filled with a combination of working gear, repair projects and sources of parts.

[StillTrying]

That's nonsense, come up with another measuring plan.

[tggzzz]

I work in and around various fields, using different units. Mostly in machine shops where millimetres are the rule, or decimals of. In any case, it's deriving that's where I seem to go wrong most, so working with long decimal strings yield the better results.

The evidence you provided indicates the results are unsatisfactory! Given that, only a software "engineer" (and I use the word loosely) would repeat doing the same thing in the hope it would be better next time.

But out of curiousity, given that you prefer "long decimal strings":

• which of these capacitors can be purchased/made/modelled: 0.00000001F, 0.000000001F, 0.00000000001Fm 0.0000000001F?
• if you have a probe with a 15pF tip capacitance and a 150nH (i.e. 6") ground lead, what is the resonant frequency?
• what is the impedance of a 10Mohm/15pF scope probe at 100MHz?
• if you use that probe to measure a 100MHz 1V signal from a 1kohm source, what voltage will the scope show?

Once you start to deal with those very real-world calculations, you will see the advantages of the base-unit-plus-exponent method.

[alsetalokin4017]

No fair !

For someone who is not very fluent with arithmetic, the use of scientific (aka exponential) notation should wait a bit. That's why I showed my calculations using ordinary decimal notation, that can be punched into a calculator as-is and the results read off as-is. 

Certainly scientific notation is "easier" when you are dealing with 9 or more decimal places, and once a person is used to using it --- and getting correct answers --- no one will argue that it shouldn't be used. But that isn't the problem here.

[alsetalokin4017]

OK,
Started to look at where in the divider I could put a DMM probe to measure, but soon realised that if the 1M resistor drops the voltage by 753V then anything before it is out of reach, as my meter can only go up to 500V

So, it looks like I need to get another resistor, maybe around 500K off the top of my head, to interpose between the ground end of the 200K resistor and the chassis.
I've added what I mean to the diagram, as attached. The extra resistor and a dial face indicate where I believe the voltage should be taken, and how. Please excuse the hand drawn resistor and dial.

If things are OK, I should find about 375V using a 500K as above.

Before I let some smoke out, I'd like this confirmed please. It's a bit of a commitment based on newly learnt maths.

Added later-

Was sitting on the verandah when it occurred to me, parallel resistors is what I'm proposing. So I did a bit of reading and saw that wasn't the way. The voltage across parallel resistors is equal, and would fry the DMM.

Thanks,
Clay.

Better to play it safe and simple, and get yourself a high voltage probe for your DMM.

[rf+tech]

Clay,

Although others disagree about your proposed 500 kOhm measurement resistor idea, let's apply some math to the problem to see why.

0.5 M divided by 1.73 M equals about a 29% increase in total resistance. How will this affect the measurement results?

If the amount of added resistance is limited to 1% of the total, would this be better or worse?

Can you see the advantage of using a 100:1 ratio?

So two 33 kOhm wired in parallel (to approximate 1% of 1.68 MOhms - the average), and the combination added in series between the bottom 1 MOhm and ground will suffice. The objective is to insert a sampling port that provides a safe measurement method, while minimizing disturbance of the bias network. The reading obtained will show if HV is present and represent 1% of the actual value, just as if a HV probe were used.

If he above technique seems too much of a risk to some, to each his own. To those who are open-mined and willing to learn - this technique is sound.

The limitation here is that the other voltages in the bias network cannot be measured. For those measurements, and future HV measurements on your growing collection of CROs, a HV probe would certainly be a worthwhile investment.


@tggzz - thanks for the sanity check in my previous post. I actually looked at an Ohm's Law pie chart to be sure, only to have a neuron misfire and flip a bit in the following few seconds on the keyboard.

RF+ Tech

edit: Clay,

Was sitting on the verandah when it occurred to me, parallel resistors is what I'm proposing.

I envisioned the 500 kOhm in series and think that is what you had in mind. Those pesky misfiring neurons ...

[tggzzz]

No fair !

For someone who is not very fluent with arithmetic, the use of scientific (aka exponential) notation should wait a bit. That's why I showed my calculations using ordinary decimal notation, that can be punched into a calculator as-is and the results read off as-is. 

Certainly scientific notation is "easier" when you are dealing with 9 or more decimal places, and once a person is used to using it --- and getting correct answers --- no one will argue that it shouldn't be used. But that isn't the problem here.

Very soon he will need (and I hope want) to do calculations similar to those I mentioned. Doing it properly from the beginning means it is easier in the long term, and there's less chance of someone being discouraged by simple arithmetic errors. There's enough subject matter to learn without also having to worry about arithmetic errors.

But I agree; it is probably better for the OP to get an HV probe for his DMM.

Plus, ideally, an HV scope probe, so he can observe a scope's blanking waveform on the grid Yup, been there, done that, got the pretty pictures of the CRT's internal fault, and haven't go any scars. Yet.

[clay1905]

Well, that cruelled it.

I haven't attempted anything like this for over 45 years. For me maths equals black eyes, blood, scars and welts. I had hoped that the scars were more healed than this but it''s all back, just like yesterday. I can't deal with the pain.

I'm going away to think about where to go, but right now I feel like just dumping the scopes and walking away. The veil has dropped, and that moment of clarity is gone.

Very sad today.

Clay.

[clay1905]

OK, I've had a think. There's a few things to say.

First, and most important, nobody intended any hurt. So there should be no finger pointing, guilt or any other negative stuff.

I'm surprised by how I reacted this morning. I can see more than ever how the legend of Pandora's box came about. I thought all that stuff was dead and buried. Not so. I'll have to find a way to deal with it, but this is not the forum for such things.

I've decided that I want to carry on and finish what I've started. However, perhaps a few days to get equilibrium back? If you gentlemen will be so kind as to bear with me, I would appreciate that more than I can say. Since I joined here I've achieved what feels to me like a miracle. And I'm so grateful for that alone.
Please though, let me do things the way that works for my muddled mind. I  know the results look patchy, but for the time being at least, it's better to do badly over doing nothing.

Meanwhile, I'll read and re-read your post rf+tech, and see what I can digest.

Thanks all for your kindness and care. You're making an immeasurable difference.

Clay.

[rf+tech]

Clay,

Thanks all for your kindness and care. You're making an immeasurable difference.

Sir, you are most welcome. It is a pleasure to share my knowledge and skill with someone who appreciates this. Never stop learning.

RF+ Tech

[pricemp]

Hi Clay
I am in a similar situation. Ie limited experience but lots of interest and have been given a Jayem Model 555 oscilloscope.
Mine is showing a trace but not functioning as designed. I have been watching a youtube video of restoring an old scope by Mr Carlson see link attached. This is useful in terms of approach although a different make/model. https://youtu.be/RF7CRqb54XM
The good news is that I have an original operating manual (A4 size) for the Jayem model 555. It contains 20 pages in 4 sections - 1) general characteristics and specs, 2) initial setting up procedure, 3) measurement procedures and 4) maintenance and alignment procedures.
At the back there are 9 detailed schematics as appendices:
Power supply & CRT circuit
Vertical amplifier
Time-base trigger
Time-base generator
Time-base timing switch
Horizontal amplifier
Ext. Horizontal amplifier
Voltage calibrator
High voltage rectifier

I am happy to share this with you. Maybe we can colloborate to get our scopes up and running.
Let me know if you are still interested in restoring yours.
Cheers Phil

[clay1905]

The 'scope has been diagnosed and repaired. I'll briefly cover the main points-

The faults:

All the resistors in the voltage divider were open, including potentiometers.

Some (most) of the resistors in the intensity control circuit were open, others were out of range. These resistors are in an area on the horizontal board where a red, and a white with red trace wire are connected, rear R/H corner, and cover an area about the size of a matchbox.

The high voltage was chronically low in some circuits, but not others. This was quite a mystery until a sneaky 0.02uF 1500 volt capacitor linking cathode (CRT pin 3) to the external cathode input terminal was checked. Follow the yellow cloth covered wire. This capacitor is in the area for the voltage multiplier, under the socket end of the CRT and directly behind the banana sockets on the rear.

The resistor from cathode input to ground was shorted. This was at first thought to be the shorting link, but tested faulty when disconnected.

There is a 200K ohm resistor inside a transparent sleeve that is connected to the centre leg of the front panel intensity pot, alongside a white with red trace cloth covered wire. I would strongly suggest testing this resistor as well. All the failed resistors were in high voltage areas, and this resistor is as well.

Schematics will be posted on the web soon. As soon as I have the U.R.L. I will publish here.

Once all the faults were attended to, a trace appeared. Wobbly at first, but over time has settled down to a nice crisp straight line.

Thanks to all those who constructively commented on this subject. Learning the basics of diagnosis has been one steep, but very enjoyable learning curve.

Regards,
Clay.

P.s.
If you are inexperienced around high voltage, be very, very careful. I blinked at the wrong moment, and I'm up for a new DMM. 

[tautech]

Congrats.
Sorry to hear about your DMM, that was bad luck. In the future sharpen the points and even consider sleeving the shank just leaving the point uncovered.
Your experience on this scope with the failed dividers and other EHT part is very typical of old CRO's, they're some of the parts of the scope that are under the most stress.

[John Carlaw]

Good to see it is working.
Just some history.
These were common as university undergraduate lab project instruments in the early 70s,
Pretty sure it was the Jayem insignia, there were a couple of similar ones.
At least at UNSW, a dozen in a lab for physics/engineering labs, not top of the line but not bad until they let you loose on the expensive gear  .
In the late 70s , around 1977 ?? most were replaced and auctioned off.
Got one cheap at auction for $100  that is $530 in 2016 money and it seemed a decent price by comparison to new gear at the time.

[Housedad]

Wow,  I remember all the surplus stuff back then.  That was when (1978 or so) That I got my first oscilloscope.  A surplus Tektronix 535 with a dual amp drawer and a rolling stand.  I think I paid around $50 for it.   Ah, memories.

[Gadgetman]

Hi pricemp. I know, this topic is old and I am not sure you will even read it. I am in the process of trying to restore the 555 Oscilloscope too. I was given it for free and it is in much worse shape than shown in the photos attached by Clay1905. A lot of dust inside and slightly rusted outer cover. I cleaned most of inside - at least to a workable level. It turns on alright, but no trace as yet on the CRT. I replaced already dead 12AU7 tube with the spanking brand new one. The trace is still not there, so I have to hunt further. I tried to locate a schematics for it (I have the user manual but without schematics). You mentioned that you have it. Could you P L E A S E make it available?