I've bought a broken scope haven't I ?

[aneng]

Hi Everyone !

This is my first post and the start of my venture into electronics repair.  Forgive the appallingly short introduction but I need to ask a question quickly !  I've just bought my first ever scope, a Trio 2100, 100MHz analogue.  It's just arrived and whilst I admit I don't really know my way around a scope properly yet, I think I've been sold a dud.

Basically, no matter what I do, I can't get it to display any waveform that extends higher or lower than the first line on the graticule...



That trace is with the probe clipped to the CAL terminal (which is supposed to be 0.3V pp @ 1KHz).  The scope is set to 5mV / division and 0.5ms sweep, DC coupling, Ch1 only.  I tried connecting a wall-wart to it to get some DC there to get the trace up higher, but it doesn't budge higher than 1 division, even set to 5mV / division and the wall-wart putting 3V onto the probe.  Also, the Y-axis trace position dial barely moves the trace at all.  I tried channel 2 and it behaves just the same.  Lastly, are scope beams supposed to be sharper than that ? - That is the most focussed I can get it, and that's with the focus knob turned fully clockwise.

I've been sold a dud haven't I ?   

[Ian.M]

Check the beam finder button isn't jammed.

[aneng]

The beam finder works OK - or at least how I expect it's supposed to....  I press it, the button lights up and I get a bright dot appearing on the screen, swooping in from one side.

[Ian.M]

So its got vertical deflection problems at or after the beamswitch.   Now you need the schematic.

[nctnico]

Aren't the 50 Ohm termination buttons pressed in? Then again the vertical position knobs should allow the trace to be moved all the way across the screen.

[commie]

I feel for you mate, you have done nothing wrong either, with what you have said it could possibly be one the Y-amps supply rails gone low, any difference with the lower channel?. Did you get any sort of guarantee with it or was it the dreaded 'sold as seen' scenario.?

[aneng]

It's off eBay.  It was advertised as 'fully working', so I'm covered.  I just wanted to be certain that it really is faulty and it's not just me being a noob and not knowing how to drive the thing.

Is there anything I can do to prove it's faulty, or have I already established that ?

[Ian.M]

You've proved its faulty, but I'd shoot a 1 minute video showing that the vertical position controls don't work properly, in case of dispute.

[nctnico]

Set the input coupling selectors (AC/GND/DC) to GND and see if you can move the trace using the vertical control knobs all the way across the screen. If that doesn't work then it is safe to say it's broken. And don't be ashamed: I have got burned by buying a defective scope a couple of times.

[aneng]

Do you mean the 'Position' knob at the very top left of the scope ?

[commie]

Do you mean the 'Position' knob at the very top left of the scope ?

Yes,yes and yes.The trace should sweep up and down with the knob.

[aneng]

It moves the beam total of 1.5 of the small divisions vertically over its whole range of movement... i.e. about 4mm.

Input coupling set to ground.  It does exactly the same thing on channel 2.

[commie]

It moves the beam total of 1.5 of the small divisions vertically over its whole range of movement... i.e. about 4mm.

Input coupling set to ground.  It does exactly the same thing on channel 2.

Yeah, it sounds as though one of the Y amp supply rails has gone down, definitely faulty though.

[aneng]

Right.  Thanks very much for that.  He's having it back.  I quite like the scope from an aesthetic point of view, but as I bought it to start my journey in electronics repair, I don't think a good starting point would be to try fixing a scope ! - especially with nothing more than a DMM !

Thanks for all the replies folks.

[commongrounder]

Just before you send it back, a long shot.  Did you work the 5X gain switches on the position controls (pull the knobs out to engage)?  If those are dirty/corroded, the Vertical gain could suffer.  Just a thought.

[KJDS]

I've still got a few of these left

https://www.eevblog.com/forum/buysellwanted/fs-uk-eu-philips-pm3217-oscilloscopes-analog-dual-channel-50mhz/msg781677/#msg781677

[Ian.M]

Incidentally the ibrightness appears to be far too high, which could be why the spot is blooming and appears as poorly focussed.  Does the grey A intensity control (below left from the CRT) actually work?  If you cant turn the intensity down, or if the focus is stii poor after turning it down, that's another fault to report.

If you are setting up an analog scope, adjust the intensity (brightness) towards the lower end of the acceptable contrast range *BEFORE* twiddling focus or astigmatism controls.  If its badly out of focus, you may need to further reduce the intensity after bringing it into focus.

[aneng]

Just before you send it back, a long shot.  Did you work the 5X gain switches on the position controls (pull the knobs out to engage)?  If those are dirty/corroded, the Vertical gain could suffer.  Just a thought.

Yeah, I tried that.  They did almost nothing - the trace moved a little bit but hardly anything.  Same with the x1 & x10 switch on the probe.  That's another thing... I'm p**sed off because I bought new probes specially as the scope didn't have any.  OK, they were cheapy ones but still...  Grrr. !

[lowimpedance]

These scopes are a real pig to get into for service, not well layed out for ease of access to 'modules'. So yeah send it back.
The new probes will most likely come in handy with what ever you replace it with.

[aneng]

I've decided to buy it off the seller anyway.  It originally cost me 60GBP (128$AU / 90$US) but I've agreed to pay him 20GBP (43$AU / 30$US) for the faulty scope as I love the way it looks (!)  I'm hoping I can repair it, but first I'm going to need another scope ! - and - more importantly, a service manual.

I've managed to get my hands on the service manual for the CS-2070, which is the 70MHz version and looks identical visually to my CS-2100 with the exception of one coupling selector for the B channel trigger.  I don't know whether the internals will be similar enough for this to be useful though.  I have been searching extensively for the correct service manual and have found several sites that all look a bit dodgy to me, that all claim to have it.  Being a suspicious type, I did some Googling to try to establish the integrity of these sites and all I could find were people moaning that they had not received their manuals etc, so I'm steering clear of them.  I know I would be protected, allegedly, by the 'Paypal guarantee' but I suspect that may be fraught with problems trying to claim, or some clause in the small print that said 'goods delivered electronically are not covered' etc !!

This leads me to my next question...  Where do you guys get your manuals from ?  Can anyone recommend particular sites ?

Thanks in advance !

[Ian.M]

Open it up CAREFULLY (the tube EHT will bite!) and compare the boards with the manual you've got, paying particular attention to the PSU and vertical deflection sections.  You'll soon see if the boards are similar enough for it to be usable.   You can probably fix it without another working scope.

First check the power supply for missing voltage rails.  Don't worry at the moment about +/-10% differences as that could be the different model.  If you find one or more missing rails, dont just fix the obvious fault, look for another fault on the boards they feed that could have overloaded them.

Once you ave all the rails up, find the beamswitch and test the voltage after it going to the deflection amplifier.  With the scope in single channel mode and the input grounded, it should respond to the vertical deflection control with a swing greater than the p-p amplitude of any test signal the manual gives for that section.  With the timebase set to a very slow sweep and the channel selection on ALT, it should flip between two voltage levels set by the two vertical defection controls.   Once you've found a good vertical deflection signal, its just a matter of tracing it through the vertical amp all the way to the CRT.

Get it basically working and try it on some known signal sources  e.g, a CMOS 555 running from a stable supply, or a sinewave signal from a PC soundcard buffered and amplified by an OPAMP, with the level checked on a good multimeter.   Once you are satisfied its basically working, you can use it to check its own power rails for excess ripple to see if it needs new caps in the PSU.

DO *NOT* twiddle any presets, unless you have the full service manual with complete alignment and calibration procedure + all the jigs and test equipment required.

[aneng]

Wow ! - Thanks for the help mate, much appreciated.  However, you're gonna have to be a bit gentler with me on the instructions !  I bought the scope to start my venture into component-level repair, so this is a bit of a sudden learning curve.  I've repaired stuff at component level before, usually with a helping hand from someone on a forum :-)  I know my way around the various components but, for example, I don't know what the 'beamswitch' is or where I could find the deflection amp on the schematic (unless it was labelled !).

Talking of schematics.... here's the service manual ;-)  ...https://www.dropbox.com/s/ikw5aa81hvlm5ut/trio_kenwood_cs-2070_70mhz_oscilloscope.pdf?dl=0

I'll be back once I've had the lid off over the weekend !

[Ian.M]

The beamswitch is where the two Y channels combine to get a single signal to feed to the vertical amp.  The vertical deflection amp is everything that steps up that signal to drive the tube's vertical deflection plates.  There's probably a delay line between the beamswitch and the vertical amp as that allows the trigger signal tap-off from the Y input channels  to be a little earlier than the displayed waveform so the trigger point isn't lost off thel eft edge of the screen.  Find the block diagram and all should become clear.  Then you've got to look at the detailed schematics and try to correlate them with the boards in front of you.   If the component designators differ (because its the wrong model) you will have a hard time of it without an experienced analog 'scope man (or other high grade eGeek) beside you.

[Ian.M]

The block diagram is on the eighth page (2-4) with the delay line top right near the CRT.  Block level circuit description is one page earlier.  After checking the PSU, work forwards and backwards from the delay line, (a fancy coil of cable, 2/3 of the way back on the left hand side of the chassis).

[NF6X]

While aneng's scope probably has a different issue, I thought I'd share my similar tale of getting a lovely analog scope with a deflection problem.

A while back, I bought an AN/USM-488 scope on eBay. This is a militarized version of the Tektronix 2235. In this case, I think the militarization just consisted of changing a label or two to show the US military nomenclature. Just like aneng, I bought it as much for its looks as its features. My specific application for it was to use it as a modulation monitor for my great, big T-368C/URT radio transmitter. An analog scope is the right thing to use for that application, the 2235 is a good size to sit on top of the transmitter next to the outboard MD-239A/GR FSK modulator box, and the military markings are icing on the cake.

When it arrived, I was disappointed to discover that one side of the horizontal drive was not working. I could have returned the scope, but I really wanted one with the military markings on it. So, I decided to take it apart before contacting the seller, and I'm glad that I did. In this scope, the deflection plate connections are made to individual pins sticking out of the sides of the glass envelope, rather than through the multi-pin connector on the back of the electron gun. The connections are made with individual wires that have female contacts installed on the ends. Once I dug far enough into the scope to see the bottom of the CRT, I discovered that one of the two horizontal deflection wires had simply fallen off the pin during shipping. I plugged it back in, and the scope began working perfectly!

Most repairs aren't that easy, but sometimes you get lucky. Good luck to you, aneng!