Gould OS3000A 'scope failure

[1100 man]

I bought this second hand in 1987 and it has served me very well ever since. It went back to Gould in 1989 for a new EHT unit at some expense!
It still gets regular use for old TV fixing and gives a nice sharp trace.

Turning it on today, I was greeted with intermittent flashes on the screen and then a cloud of acrid smoke.

After some investigation, R934 & R935 (both little 1/4 watt things) had died a violent death!

Referring to the circuit below, the cathode of the tube is supplied via R934 (10K) from the -1500V Vk supply.

The tube heater gets 6.3Vac from a separate winding on the mains transformer.
R935 (100K), connects the floating heater to the -1500V cathode supply presumably to keep them at the same potential.

R935 (100K) has fared the worst. A fair amount of investigation didn't reveal any obvious cause. A replacement, unsurprising, poured smoke after a couple of seconds.

It's only connected to a floating heater so there should be no potential difference across it. I was starting to think it might be an internal tube fault or leaking heater winding but both seem pretty unlikely.

With R935 not connected so the heater floats at whatever potential it wants, I get a trace on the screen. However, the two neons flash brightly at random intervals. I only had it on long enough to take some voltage readings.

The EHT is correct at 9KV and the cathode supply, Vk is -1500V. However, the EHT unit in a metal box is squealing quietly as if it's either under stress or oscillating at the wrong frequency. This squeal can be seen superimposed on the trace.

So I'm not sure what's going on. Is the fault in the EHT unit although it is supplying the correct voltages or the CRT or the heater transformer or none of the above!

Any help greatly appreciated!

All the best
Nick

[high_vacuum_house]

I would first look at meggering the heater supply winding on the mains transformer as this looks like where the issue could arise. I know Gould were not the best in the transformer department and I have scrapped one analogue Gould scope because of a transformer breakdown.

If you don't have a megger then disconnect both ends of the heater winding from the circuit, replace the 2 resistors and try powering it again. Obviously there will be no trace as there is no heater supply but if the resistors survive then the 6.3V transformer winding has a breakdown in it.

If you understand the risks of high voltages if the above shows a transformer breakdown you could try running the CRT heater with a 6V battery instead of the transformer winding to finally prove it, obviously the terminals will be at a high potential

Christopher Capener

[turretslug]

Breakdown of the highly elevated heater winding to another winding? The demise of small transformers with a close-packed number of windings, some at very different standing DC potentials to others must surely be responsible for writing off many CRT 'scopes.... Like the Hameg 312/8 here a while back, a tiny and very busy mains transformer with no less than 7 secondary windings including a heater winding sitting at -2kV and a 470VAC winding feeding a quadrupler. Asking for trouble, IMHO.

[Ed_Dinning]

Hi Nick, heater winding breakdown is highly likely, good news is that it is normally the outer winding so should be easy to replace.

Ed

[1100 man]

Originally Posted by high_vacuum_house

If you don't have a megger then disconnect both ends of the heater winding from the circuit, replace the 2 resistors and try powering it again. Obviously there will be no trace as there is no heater supply but if the resistors survive then the 6.3V transformer winding has a breakdown in it.

Hi Christopher,

An eminently sensible method of diagnosis:- now why didn't I think of that! I will go and attend to that now and see what results I get.

Turretslug, It's obvious as soon as you say it but I had not appreciated that the heater winding would be at -1500V wrt ground! That makes transformer breakdown quite likely especially as the poor thing lives in the makeshift workshop in the garage!

Thank you both, I'll report back later.

Cheers
Nick

[1100 man]

Originally Posted by Ed_Dinning

Hi Nick, heater winding breakdown is highly likely, good news is that it is normally the outer winding so should be easy to replace.

Yes, of course you are all perfectly correct, the heater winding is only about 5 meg to chassis! Didn't even need the Megger to test that: it shows up perfectly well on a multimeter.

I'm seriously unimpressed with myself for not being able to diagnose something that obvious I even stated " It's only connected to a floating heater so there should be no potential difference across it." I still didn't twig that the reason the resistor was catching light was that the other end must be going down to ground! Ah well....

I've excavated the transformer and disconnected the wires from the tag strip on the top and it's definitely in the winding.

Unfortunately, Ed, it looks like the heater winding is the innermost of the secondary windings so not an easy fix without rewinding all the secondaries.

What's more annoying is that there were shed loads of 'scopes at Harpenden at the weekend but I didn't need one then. Most of them only went for a fiver!

It will have to go on the 'one-day-a-transformer-might-appear-for-it' pile.

Thanks for the help

All the best
Nick

[Argus25]

It might be possible, if there are some other low voltage windings about on the main transformer to hijack some power and to make a small heater transformer with the correct ratio (wouldn't need any fine wire) it would just need something like a one square cm cross section iron core to power just the CRT heater, and you could wind it with suitable insulation, and find a place for it somewhere physically in the scope.Then you could forget about the failed/leaky winding and not have to repair/rewind the original transformer.

[Ed_Dinning]

Hi Nick, try Argus's suggestion on a small bobbin. Note that it could be run from the defective heater winding as a 1:1 isolation transformer, or if you were liucky you may be able to get additional turns (well insulated) on the old bobbin.

Ed

[barrymagrec]

This used to be a extremely common problem on the Philips PM3210 scope - diagnosis was very quick and easy, just remove the tube base cover, observe the burnt resistor and order a new transformer.

Many years ago when something similar happened on an old Cossor 1035 I discoverd that a surplus place just off the Tottenham Court Road had Parmeko heater islolating transformers for pennies and the scope lived to smoke another day.

[buggies]

Going back a good few years, we used to get heater boost transformers from radiospares to keep a tube going - so maybe just a separate little 6.3v mains transformer (assuming good insulation).

[1100 man]

Hi George,
I would doubt if a standard 6.3V transformer would be built to withstand 1.5KV on it's windings. Possibly a transformer used to power a TV CRT when it had a heater cathode short might be OK, but even then, it would have no need to withstand KV's!

I have very kindly been offered a whole OS3000 for spares but it is not known whether the transformer is any good in that. It's a fair distance from me so I need to see if the FCS can help!

Hi Ed & Hugo,
I like the idea of a small 1:1 isolating transformer. How easy would this be to construct? How would I work out how many turns it would need etc? Also, in TV's, transformers used for CRT's with heater cathode shorts need very low capacitance. Would this be an issue with a scope?

I don't think there is enough room to wind another heater winding on the outside of the existing transformer bobbin. How would I estimate the number of turns required?

Many thanks for the suggestions.

All the best
Nick

[turretslug]

Something a little like this problem crops up from time to time, and I've long wondered if a modern manufacture transformer with split plastic bobbin would actually be usable here. ISTR that part of the certification is ability to withstand 4kV primary-to-secondary and windings-to-core- it's probably short-duration in order to withstand transients rather than an expectation of continuous operation (the safety and transformer folk here will know better than me!) but it wouldn't surprise me if such a transformer would be just fine at half the secondary-to-chassis/earth voltage, i.e. 2kV, continuously. It might not score an official safety approval stamp as such, but for informed use by a competent owner would be fair enough.

I'd actually considered doing something like this with the Hameg mentioned- unusually among many 'scopes, there's lots of space inside, certainly enough for 2 transformers of around 25VA instead of the original hopelessly crammed one of approx. 40VA, with the two nasty HV windings well insulated from each other on one transformer, everything else on the other one. Another rountuit...... Perhaps if the spares Gould's transformer is a good 'un, you could fit a supplementary 6VA or so plastic bobbin type dedicated to CRT heater alone anyway to head off a recurrence of the problem,

Colin.

[1100 man]

Originally Posted by turretslug

I've long wondered if a modern manufacture transformer with split plastic bobbin would actually be usable here. ISTR that part of the certification is ability to withstand 4kV primary-to-secondary and windings-to-core- it's probably short-duration in order to withstand transients rather than an expectation of continuous operation (the safety and transformer folk here will know better than me!) but it wouldn't surprise me if such a transformer would be just fine at half the secondary-to-chassis/earth voltage, i.e. 2kV, continuously.

Hi Colin,
I've just looked at the spec for the 'RS Pro' range of transformers on their website.
They quote 4KV between primary to secondary, 2KV between windings and core and then state 'primary/secondary/core 500V dc.

I assume this means the maximum DC there should be on the windings continuously is 500V? Whether it would stand 1500V continuously is debatable!

Cheers
Nick

[turretslug]

I see what you mean- I suppose that primary-to-secondary has two thicknesses of winding insulation (I probably shouldn't refer to it as "enamel" nowadays) plus an appreciable thickness of bobbin barrier in order to protect delicate little semiconductors (and, with Class II, delicate little people....) from the rough-and-tumble world of incoming mains, hence the 4kV rating here.

I'm tempted to think that an oversize core-plus-primary (say 12VA for 6VA requirement, etc) with a generous few turns of appropriate tape added to the secondary bobbin before winding start would be good for a few kV- but that's getting into the territory of the difference between what one would be prepared to risk for one's own use and what would stand up in court! Maybe high isolation secondary bobbins are actually available for specialist purposes- highly elevated secondary applications may not be run-of-the-mill, but neither are they unknown,

Colin

[Chris55000]

Hi!

Another thing you can try is to get a small isolated d.c/d.c. converter module of about 6W rating, with one output of 9V or 12V, and guaranteed 4kV isolation, with one o/p lead connected to cathode, the other o/p lead to one side of the heaters, and the heater pin left over returned back to cathode via a low value dropper resistor covered in heavy p.v.c. sleeving. (You'd have to calculate the dropper value from the 'scope tube's datasheet, but most are 6.3V @ 0.6A)

The input leads of the converter can be taken back to the oscilloscope's main positive LT. supply for the transistor stages. You can buy these isolated converters in a variety of voltage ratings, so you should be able to find one to suit the OS3000's LT supply.

These converters weigh only a few grams and can be directly wired to the heater tags on the CRT base with no fear of damaging the base or CRT itself.

Chris Williams

PS!

This mod also can be used with those Gould 'scopes (OS4000 etc.,) that have a 12 pin CRT with heater/cathode common on one pin, but you would have to suspend the heater dropping resistor directly in one of the converter o/p leads itself, making sure it can't contact adjacent tags, wiring, metalwork in the 'scope, etc.

[1100 man]

Hi Chris & Colin,
As I've got a few 6VA 0-6, 0-6 split bobbin transformers, I decided to try one. I can't believe it won't withstand 1500V and at the moment it is quite happy.
Unfortunately, the rest of the 'scope has other problems:- the trace gradually faded away
Metering the grid supply resulted in a very bright trace so the EHT unit is working and supplying the correct cathode & grid supply.

I discovered that the 150V rail was in fact 180V due to a shorted regulator transistor. This may have damaged other things so I need to do some more in -depth fault finding.

I think the fault is probably on the 'bright -up' board which drives the tube, but there is a lot of circuitry in this 'scope- all discrete transistors!

Cheers
Nick