Oscilloscope No Trace old Gould Advance OS3000

[TonyPh]

There's no waveform for anything in the EHT section. The closest is in the bright up board. Typical really as that would have been handy.

I will check through the board and makes replacements. I doubt I have substitutes for much of it so will likely have to be either creative or buy new. I know for certain I have no large wire wound resistors and this (R818) is one of the hotter components as the board works currently.

Maybe I will get a chance to make some measurements tomorrow.

I suppose I've been procrastinating on taking all the components off the oscillator board but it is inevitable that it needs to be done.

Only my broken DMM has a transistor gain checker. It was only a broken switch. My other two DMM don't have the feature, although I am waiting on delivery of a new DMM which has the function.

Edit: Actually one of the DMM I have here has the transistor checker, I've just never noticed or needed it it also has frequency

[tautech]

Just check the WW resistors only, likely to be ok.
Sorry the schematic you posted has no component values so I got the RC wrong for R818, C813

[TonyPh]

390 ohm and 0.01uF for R818 and C183.

[TonyPh]

Primary resistances.

Pos 13-16 - 1 Ohm
Pos 13-17 - 1 Ohm
Pos 16-17 - 2 Ohm

Pos 14-12 - 1 Ohm
Pos 12-15 - 1 Ohm
Pos 14-15 - 2 Ohm

Test leads together 0.5 Ohm

Confirmed on both meters.

Swapped all diodes for new.

Swapped R819, R820 for new.

Swapped TR812 for new.

Replaced C813, C814, C816.

Tried a D882 for TR813, it melted.



Checked all resistors for values.

Swapped TR814 and TR815 for new again.

Swapped TR816 - schematic calls for 2N3905, was BC212 fitted, for a 2N3906

Swapped TR811 for a 2N3904, schematic calls for BC107

[oldway]

You are loosing your time: bad HV transformer.
I have an HP1740A with the same problem and this happen often with Philips scopes too.

[TonyPh]

If the HV transformer is bad is there any way to recover the scope?

[tautech]

You could check secondarys for open circuit and all windings for isolation from each other and the core. Results will not be conclusive with a multimeter.
Also check secondary caps and pots for shorts.
I used to have some oscillator transformer specs (R&L) in a scope manual, but unfortunately now sold with that scope. From the best of my memory the inductance of primaries was in the uH range which suggests resistance would be low too.

[TonyPh]

Right.

Here's my findings.



Maybe not conclusive but there's no short there and the 100 Ohm and 4k suggest a lot of windings.

Maybe things would be different at high voltage though.

[oldway]

Generally, the problem is an internal short circuit in a few turns of the HV coil.
Only solution is to replace this transformer.
It's not possible to measure if the transformer is bad or not with a multimeter.

[tautech]

OK no OC. I have fixed one or two that had OC in a secondary that was in the first few turns, that I just removed and re-connected. Worked fine.
Sorry oldway, but I would not give up quite yet.

Not sure if I would disconnect secondarys and power up without a load just to see if it will oscillate, others might advise?
I have had to replace the HV caps in old stuff, they can be hard to source, but you can find them on ebay from China.

[TonyPh]

Might be worth a shot. Would I get anything on the CRT at all if just the VG hi VG lo section were connected. Obviously the other part is dependant on the winding that also goes to the tripler. I don't know much about CRTs and what they need to operate. Even a bit if illumination would be a clue as there is absolutely nothing at the moment.

The HV transformer is potted so any modification would be a challenge.

Still haven't made a HV probe. Would be interesting if there was actually some potential on the HV section.

Farnell do these 500pF disc caps.

http://uk.farnell.com/vishay-sprague/615r150gast50/capacitor-ceramic-500pf-15000v5u/dp/1311457

And they seem to have some 4700pF 6kV

Haven't checked all types but might be able to get them all, or maybe slightly higher capacitance or something. Farnell have a £20 minimum order though.

[tautech]

The point of that test is to find if there is too much secondary load that stops it from oscillating.
Again not sure if I would do it and have that HV unloaded for risk of transformer insulation failing.
I don't know how high voltages might go. We don't want to pop any thing more!

[TonyPh]

Hmm so it is the load of the CRT that keeps the high voltage in check?

[tautech]

Hmm so it is the load of the CRT that keeps the high voltage in check?

We do not know, process of elimination.

[TonyPh]

If I was to buy all the caps from Mouser (Farnell do most but not all values) it comes to a fair bit of money (£20+) by the time they throw their delivery charge of £12 on top. The diodes are harder to find though.

[tautech]

Try this seller: http://www.ebay.com/itm/5pcs-10KV-4700PF-472-High-Voltage-Ceramic-Capacitor-/161045275152?pt=LH_DefaultDomain_0&hash=item257f0be210

HV diodes should be alright if they pass a simple blocking and conduction test as in a previous post.

[TonyPh]

Thanks. Food for thought.

For the multipler I would need 3x 500pF 8kV and 1x 470pF 12.5kV, so to replace all then maybe I'd get a batch of 5 off 470pF 20kV rather than having to buy 5x 10kV and 5x 20kV. The 30 stray pF shouldn't make much difference, should it?

Maybe these from the US and a touch cheaper, and probably quicker than China. http://www.ebay.co.uk/itm/10-pcs-20KV-20000V-470PF-High-Voltage-Ceramic-Capacitor-/281102573138?pt=LH_DefaultDomain_0&hash=item417304de52

For the grid and cathode I would need 2x 4700pF 4Kv and 2x 0.02uF 1.5kV, so I'm thinking that 2x 10nF in parallel would do for each of the 0.02uF.

Still quite an expense. I'd love to fix it but what are the odds that it would work? Could be that pesky HV transformer is faulty and I'm just wasting my time and money.

Actually I'm wondering why there are 4kV caps in the grid and cathode section when the output voltage is a mere -1580v. I wonder if I could 'get away' with 3kV caps, in which case I could source from the UK for £1.30.

Looking at the EHT section though. Those capacitors are rather large. Would any cap, provided it meets the capacitance and volt rating do ok or are these things exotic in construction and required to be used, or is it that manufacturers are now better at making caps than in the 70s.

[tautech]

I would dismiss HV multiplier at this stage.
Close as you can for pF values and near or better for KV values.
Component list will provide info of component type(ceramic, polyester etc), just match that and physical size is a secondary consideration.

Maybe more research on HV transformer specs or another post seeking info on R's & L's.
Maybe a little test project to connect transformer to, to check its function at a lower voltage. Say 5V input @ 5 KHz that should give you 25% of in scope voltages.

[TonyPh]

Thanks. There's no info on the transformer specs I've seen thus far but I will see what I can dig up.

I'm sure a 5v/5kHz sine wave ish signal shouldn't be that difficult to make.

I managed to score myself a working 20Mhz scope off eBay yesterday for £27 so that'll help.

[tautech]

Yep, while you old CRT scopes, you will always need a spare one to fix the other!!!!!
BTW what brand/model did you score?
If its a good runner, you could put your fixit money towards a new DSO

[TonyPh]

Ah it's just another Gould, a slightly newer model OS300.  I've seen them go for between £50 and £70 or more depending on how well it was marketed and whether there are probes bundled and if delivery was offered. It caught my eye because it was collection only and they usually go for cheaper. I asked if I could send a courier in and he said it was fine. Always a bit risky that I will get a pile of oscilloscope bits delivered though.

I have a wein bridge sine wave oscillator circuit 20Hz to 20kHz that I built and a push pull audio amplifier that is good for a few watts that I can hook up together to feed into the HV transformer.  I will dig them out and see if I can get it oscillating as sometimes it's a bit of a fiddle as the potentiometer I used seems to drift a touch. I should be able to use the PCBScope to verify the frequency and voltage before attaching to the transformer.

What  would I be looking for when doing this experiment? Would I be loading down the secondary with something to see what effect it has?

[tautech]

Just try the oscillator alone and scope transformer primary(16 & 17) and secondary. Maybe 50 ohm in series with the primary to protect your oscillator. Say a 1M secondary load for starters.

Scope oscillator output OC and then connected to primary to see what happens to waveform under primary windings load.

By the info you posted it was ~1 : 50 step-up for the grid/cathode windings.

You will want to see ~50x the primary on each of the grid and cathode secondarys and an appropriate step up for the multiplier secondary.

We only want to check for transformer action on all secondarys. ie. step-up
This type of transformer is optimized for a certain frequency, IME ~20-30 KHz but I doubt you can drive it that fast with enough current for this test.

Watch out for ground loops through your scope ground leads. I know it is a LV low power test, but you may not get a result if it is not set up right.

[TonyPh]

So I made this thing of beauty today. The marking is just in case I forget what it's for.



20x 10MOhm resistors and a 1MOhm resistor in series. Miliatry grade heatshrink plu 15kV Silicone sleeve plus another 6kV silicone braided sleeve on top, all encased in some tubing. Depending on the range I'm getting around a 220x reduction in voltage.

So here's the meaurements that I'm getting right now with the few substituted components still in place.

-Vk = -128v
Vg Hi = -128v
Cath (on bright up board) = - 126v
Anode around 700v

So it's around 10% of the normal output but it is definately stepping up, just not enough.

I suppose looking at the plots I got from PCBscope the other day which were only partially oscillating this makes sense.

[tautech]

Well, if you have that other scope coming, you will be able to do some comparative measurements to isolate the fault. :-)

[TonyPh]

I received the new oscope. It works! 

Here's some of the waveforms. Someone on another forum recommended doing some tests with TR814 and TR815 disconnected so these are with them removed and all wiring on primary and secondary attached as normal.

This before connecting, an unloaded signal. The signal, by the way, is off a 555 wired up as a 20kHz PWM, with roughly 3.5v peak. I don't know how much power it provides but it's coming off pin 7 so don't think it's that much really.



On wires 13, 17 and 16.



On wires 12, 14 and 15 (14 is marked as 11 in the schematic).



I don't really understand what the four primaries (or are they centre taps) are doing in the circuit but the top half is totally loading down my signal to nothing while the bottom half is not affecting it so much.

Now with the scope powered up....

This is the oscillation on Position 13. It's a lot slower than I expected. The amplitude is so small I have to use AC coupling to see it. The oscillation is 100Hz so is probably power supply ripple or something.



And position 14



It's stable like this and only wobbles a very small amount. Nothing is heating up.

Swapped in a new (probably old stock) MJE520 at TR813 (to replace the doofah BD435) and there's no change to oscillation speed or amplitude.

Looks like power rail ripple, it's at 100Hz and 0.4v. I wonder if the main reservoir cap is doing a good job.

I will connect up TR814 and TR815 tomorrow and take a look at the waveform then.