Horizontal amplifier - intermittent problem

[Boulevardier]

I am about to embark upon repairs to a Farnell DTV 20, 20Mc/s oscilloscope. It’s going to be a difficult and frustrating job. I know that this isn’t exactly the best ‘scope ever produced, but it suits my humble needs, and I’d rather have a go at it than just drop it in a skip. It’s a very poorly documented piece of equipment, and the only service information I can find is the Mauritron manual (attached). I think I read somewhere that it’s a rebadged version of another make, but there seems to be no info on that around. The Mauritron manual (which is possibly Farnell’s own manual) is written in parts in unintelligible language, and provides no component overlays for the pcbs, no voltage indications apart from the main power supplies, and is generally a fight to find your way around, so it’s going to be a frustrating repair. Also, there’s the usual problem that it’s difficult to fault-find a scope without a scope!

Just looking for any general tips before I start – and I’ve only done some basic checks on how functionality is affected so far.
Problem is an intermittent one in the horizontal display), where the trace is cut short about 1/3 of the way across the screen (see photos of fault mode and working mode), and the trace is only visible with the Horiz. Position control fully clockwise. This can sometimes be cured by operating the X10 MAG switch. The position of the switch doesn’t seem to be the critical factor – just the act of operating it (which effectively disconnects and then reconnects the –12V feed to the amp’s input). I eliminated any problems in the switch itself by connecting the –12V supply directly – no difference. Because of this interaction with the X10 MAG switch, I’ve concluded that it’s a problem in the horizontal amplifier (page 42 of manual) rather than the oscillator.
The great problem is the intermittent nature of the fault – which presumably means that a component is intermittently breaking down, and won’t necessarily show up as faulty on any simple tests. Can anyone suggest the most likely culprits for this sort of fault before I start ripping things to bits, and working through the substitution of every component?

Many thanks for any help!

By the way, the mentioned manual is available on web, but too large pdf to attach.

Mike

[Boulevardier]

The service manual can be had here (Electrotanya) -

https://elektrotanya.com/farnell_dtv.../download.html

Mike

[Argus25]

Originally Posted by Boulevardier

Also, there’s the usual problem that it’s difficult to fault-find a scope without a scope!
Mike

With a two channel scope, you can use one channel to test and fault find the other, if at least one channel is basically working. It is important that the timebase is set on Chop (not Alternate) and one advantage is that the channel you are using is automatically synchronized to the other channels' timebase. But if there is a generalized timebase problem it is not as helpful, but it is ok for checking the vertical amplifiers in many cases.

In the ideal world you could attach the Ch1 scope probe into the Ch2 circuitry, go out the back for smoko and when you come back the scope would have repaired itself

[Argus25]

....The trace is either stopping short of completion and flying back too soon, but possibly, more likely, it is still there and being blanked out. It could be a fault in the blanking amplifier circuitry. You could connect the working channel probe onto the H deflection amp (or better the drive immediately to it where the voltages are lower) and see if there is a full sawtooth sweep voltage. If that is ok, find the blanking circuitry and look at the timing of the pulse waveform out of that circuit.

One other trick, if the scope has an Z (intensity) axis input you could feed a high amplitude signal in there, which might overcome the CRT's beam blanking and therefore show if the trace was still actually there.

Also, if you cannot see the left (start) of the trace, by moving the H position control, it could simply be an offset on the H position DC axis, meaning a DC offset. One common cause for intermittent faults in scopes is the preset potentiometers which go intermittent especially if they are not sealed types.Then there is intermittent switching. Both problems more likely than an actual intermittent component (unless it is a part in a socket)

[Diabolical Artificer]

" I think I read somewhere that it’s a rebadged version of another make" I think these were Scopex scopes re-badged. Doubt that'll help you much as Scopex scopes are not covered well.

Good luck with the repair, Andy.

[Argus25]

...ps: a quick way to check if there is a DC offset on the beam's H position is to switch the scope to X-Y mode and see if the spot centers on the H and V position controls, don't forget to turn the brightness down before you switch to X-Y mode, or it is a bit harsh on the CRT phosphor.

[dave cox]

It looks to me like one half of a push/pull stages has tied itself to a rail !

The wave form has squashed itself, assuming you didn't change the scope settings in between the two photos (they look the same, apart from h pos). With only one half of a push pull stage working the gain will be halved

dc

[dave cox]

My guess would be in the H-amplifier, since the sweep generator is single ended?
This is on the last page in the manual.

I think there must be a ground point missing on the schematic between Q809 & Q810 ...

dc

[RogerEvans]

If your sweep speed goes slow enough, ideally 1sec/div, you can just watch the voltages change with an analogue meter. Most scope horizontal amplifiers end up with antiphase (or is it paraphase) stages where the voltages just mirror each other - as one goes up the other should go down by the same amount. You also should be lucky enough to be able to do the same measurement in working and non-working states and find the first place where they differ.

Roger

[Boulevardier]

Many thanks all.

• I have tried the X-Y mode suggestion (while it's in the fault condition), and trace still offset to left.
• Yes, the scope settings were unchanged between the two photos. The scope flipped from 'non-fault' to 'fault' condition while completely left alone - no connections or physical disturbance of any sort.
• I've looked at pictures of Scopex stuff on web, and nothing I've found has the same front panel layout.
• I haven't tested (yet) the four output transistors and their voltages, since I was unsure about expected scale of voltages here. Can I assume that any voltages here will be well within the range of my Fluke (1,000V max)? I don't have an EHT probe. Although highest supply voltage here is 160V, I note that there are a number of caps rated at 500V.
• "It looks like one half of push/pull stage has tied itself to a rail" - yes, looked like that to me too. Because they're cheap, I have ordered replacements for the four diodes (not available from any main UK supplier, so warily ordered from UK traders on the Bay). If it's safe to use my Fluke throughout this part of circuit, I can check some voltages on o/p transistors

.
For convenience, I'm attempting to attach a pdf scan of the relevant bits of schematic - the quality of the original isn't good.

Thanks for bearing with me. I will fairly soon get to the point where repairs become uneconomic, and it would be more productive to seek out a replacement scope. I doubt if this one is worth much more than £10-15! But it seems a shame to scrap it if it will only take a few quid's worth of bits.

Mike

[Ed_Dinning]

Hi Mike, the tube and PSU circuitry would form the basis of a decent homebrew scope.
Plenty of circuits that could be used in ETI/ Elektor etc back numbers

Ed

[TonyDuell]

Looking at the circuit you posted, the maximum voltage in the X amplifier would appear to be the +160V rail. So your Fluke meter should be fine for checking voltages.

I wonder if the emitters of Q809/Q810 should be grounded. It's hard to see where the emitter current would end up if not.

[Boulevardier]

Yes, davecox also made that point. I'll have to investigate - may be an error on schematic, since it has always worked in past...

Mike

[buggies]

Why has no one suggested giving it a thump or two first?

Connectors, pcb joints...

[Boulevardier]

"Why has no one suggested giving it a thump or two first?"

Actually, I have been round the board tapping (hard) all the connectors, potentiometers, etc. Not a flicker on the screen.

Mike

[buggies]

I don't know what the plate connectors are like on that tube but I have had problems in the past with a single plate connector making poor contact.

In general, with X/Y mode (i.e. no timebase), a meter across the x-plate connections should read zero with the position control centred If it does not then you have a known symmetry in the amplifier to compare voltages to see where the discrepancy occurs.

[Boulevardier]

I've just tried thoroughly "agitating" the tube-base connector - no effect at all. But nevertheless a few on/off clicks of the 10X MAG switch brought back full screen - for now!

[dave cox]

As Roger suggested, a very slow sweep should allow to see what is going on with a meter. I would look at X+ and X-. Given the trace pulls to the left (from front of scope) either the left plate gets stuck around +160V or the right plate is stuck around 0V.

That should narrow it down a bit!

Also, voltage across R825 and R826 should give the bias current in the 2 halves.
(I'll work out the expected current if nobody beats me to it)

dc

EDIT 5mA / 1.7V

[Boulevardier]

I really thought I was getting there last night. I decided to take voltages along the Q811-Q809-Q810-Q812 output chain. I noticed a couple of times that when I touched test probe to collector of Q810 it restored normal trace from restricted one, and this turned out to be reproducible every time. Looking at the pcb joint under magnification, it looked distinctly iffy, so I got out the wick and solder and improved it along with the other joints on those transistors. Trace was then normal and stayed so for over an hour without relapse, and I thought I'd fixed it - even celebrated with large whisky at 2.00am!

But, turned it on this morning, and the fault was back. Measuring voltages again got strange and new results, and found that Q810 was showing as just two diodes on my tester (the others seemed OK). I think I must have let my probe slip very slightly while testing those tiny, close pads and shorted to something nasty. So, current state of things is that I have a new Q810 on order, and I'm wondering if the dying one will have taken any other diodes or transistors to the grave with it.

I'm trying to be philosophical about all this - not being helped by traces on pcb starting to lift with the stress of getting leads in and out of very tight holes in board.

New transistor may arrive tomorrow (from Langrex - who seem trustworthy), and will then persevere.

Mike

[dave cox]

Q812 should protect itself as its wired as a current source!