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CRT scope high voltage supply problem?

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CRT scope high voltage supply problem?

Postby Adam Inglis » Sat Feb 01, 2014 10:44 am

The display on my 40-plus year old scope has gone dark.
I think the problem is in the high voltage (they refer to it as "EHT", extra high tension) supply. All the other power rails check out OK, however the -1000v line is only giving me around -185v or so.
I've repaired bits and pieces in this scope before in the low voltage circuits, but I have no HV experience. I believe the circuit is similar to that of an old TV flyback circuit, where an oscillator and a transformer generate a high frequency, high voltage AC that is rectified and filtered.
The schematic is here...
Any suggestions where to start fault finding in this old circuit?
Obviously I'm dealing with lethal voltages here and so being extra careful...!
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Re: CRT scope high voltage supply problem?

Postby ef37a » Sat Feb 01, 2014 11:32 am

Hi Adam.
Well, unless you have a pacemaker or a history of heart trouble, those EHT supplies won't kill you. They will however bloody hurt!

Your first problem is measurement. Most DMMs only go to 600V dc, 1000V tops so you need to make up a
very high resistance attenuator. 9 meg Ohms (8.2M for jazz) in series with 1meg gives about 10:1 if the DMM has a 10meg input R. (yeah, not NPL I know but will do for an old scope!)

Then, if the 3kV and 1kV supplies ARE low the first suspect would be leaky caps IMO (if the diodes were short I would expect the osc' to collapse? ) Problem here is that whilst decoupling cap values are not usually critical, so long as they are big enough, in circuits such as this they will "tune" the oscillator and bigger caps might actually result in less volts or the osc' may fail to start at all.

Next check high value Rs, especially that 220k.

Of course the CRT MIGHT be low emmision? Get those high voltage readings before going too far.

I think Will (Folderol) has had a lot more experience than I on test equipment?


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Re: CRT scope high voltage supply problem?

Postby sonarbell » Sun Feb 02, 2014 6:29 pm

Checking C80 would be my first item.
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Re: CRT scope high voltage supply problem?

Postby Folderol » Sun Feb 02, 2014 9:20 pm

Very much inclined to agree with sonarbell. If that has gone seriously leaky it'll load the transformer enough to drag everything right down.

I'm also a bit surprised at C78 & C79. They are rated at 1kV and running at 1kV. I would have expected to see 1.5kV caps there.

C75 & C81 should also come under suspicion along with D16 and D19. These four components form a voltage doubler and if any of them has gone bad the result is likely to be excessing loading of the transformer.

Rather than measuring the very high voltages directly, I'd be inclined to (carefully) measure across C80. although standing at -1000 there should only be about 80V across it. As well and checking the cap is OK it can act as a 'canary'. If it seems about right the others are likely to be as well.
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Re: CRT scope high voltage supply problem?

Postby ef37a » Sun Feb 02, 2014 9:58 pm

Hi Will,

There are quite a few lowish voltages marked and it might be an idea to measure those and work out some current flows? This might give an idea as to whether you are looking for a short/leak or an OC somewhere?
The old guys at the British antique radio soc' might be able to shed some light.

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Re: CRT scope high voltage supply problem?

Postby Folderol » Sun Feb 02, 2014 10:03 pm

ef37a wrote:Hi Will,

There are quite a few lowish voltages marked and it might be an idea to measure those and work out some current flows? This might give an idea as to whether you are looking for a short/leak or an OC somewhere?
The old guys at the British antique radio soc' might be able to shed some light.

Dave.
Well spotted that man. I had ASSumed the O/P had already checked these :blush:
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Feb 03, 2014 5:57 am

Thank you all.
The -50 and +150 voltages are present and correct. The +18 is actually around 15 - probably close enough.
I'll have a look at the drop across those caps,
cheers
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Feb 03, 2014 7:33 am

Re-checking the lower voltages, I'm getting -13 volts at the -11.5 test point, and 16v at the 18v test point. Not sure if that's significant.
I'm getting around 155 volts across C80 when there should be about 80v.
With the unit off, in-circuit diode testing of D16 and D19 isn't giving me a reading (these are big chunky things, can't see the markings), although the associated disc caps 75 and 81 seem about right value.
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Re: CRT scope high voltage supply problem?

Postby ef37a » Mon Feb 03, 2014 9:17 am

Where does that +18V come from?

The value suggests the source is a 7818 regulator? If so it should be right and so either the reg' is faulty or the total load current is too high.

I can't explain the near twice the indicated voltage on C80 at the moment but circuits have been known to be wrong!

Those diodes look a bit special (and old!) they probably won't read on normal DMM test voltages. Do you have access to an old Avo 8 with the 15V Ohms range?

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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Feb 03, 2014 9:33 am

For my own reference, I've uploaded the description of the HV circuit from the SM here....

On the low voltage side of the transformer, I've measured voltages at the transistors
Q42 Collector +14v Base -15.4v Emitter -11v
Q41 C 0v B +6.1v E -15.7
Q40 C +6.3v B -13v E -23.8v

Presumably the big power transistor Q42 is operating saturated, but the amplifiers Q41 and Q40... are those voltages workable?

Dave, will just 1/4 watt resistors do for the HV probe, or do I need something bigger?
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Mon Feb 03, 2014 9:41 am

ef37a wrote:Where does that +18V come from?

The value suggests the source is a 7818 regulator? If so it should be right and so either the reg' is faulty or the total load current is too high.

I can't explain the near twice the indicated voltage on C80 at the moment but circuits have been known to be wrong!

Those diodes look a bit special (and old!) they probably won't read on normal DMM test voltages. Do you have access to an old Avo 8 with the 15V Ohms range?

Dave.

There aint no regulator chips in this old thing!! There are a variety of zener/pass transistor v reg circuits around the other side of the machine. I'll have to re-check them.

I have an analogue VOM as well as my DMM - what do you use the 15V ohm range for if I have it?
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Re: CRT scope high voltage supply problem?

Postby ef37a » Mon Feb 03, 2014 9:52 am

"what do you use the 15V ohm range for if I have it?"*

The old Avos used a 15V battery to give a high resistance (x100) Ohms range. Those diodes probably need more than the usual 1.5V to read the forward conduction.

*(kids!)

You have probably reached the point where you just need to replace components en bloc starting with the most likely, caps. I would also check out the reason for the low 18V supply. You can use modern 78/79 regulators in old kit and replace chunks of less reliable circuitry.

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Re: CRT scope high voltage supply problem?

Postby Folderol » Mon Feb 03, 2014 6:58 pm

Wow! Almost twice the grid bias supply. That would certainly make things interesting. It would suggest virtually no beam current. If it weren't for the fact that the -1kV line is low I'd say it was R121 open circuit (worth checking anyway). I would however, want to make an alternative check on that supply.

Do you know your meter's resistance? Digitals are usually 10M. If yours is the same you can make a high voltage probe by putting 9x10M resistors in series inside a filleted biro, then divide whatever reading you get by 10 - this only works for DC. I recommend using 1/2W carbon resistors simply because you want nice long resistor bodies to reduce the possibility of tracking. When soldering the resistors together try to only handle the wires for the same reason.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Tue Feb 04, 2014 10:32 am

The fault seems to be intermittent - while probing the -1000v area with my analogue VOM the voltage came up, my CRT screen lit up, and... I cooked my VOM! Not sure why, as it was on the 1000v DC range. It's rated at 20kohm per volt. Anyway, it only took out a resistor and a diode so I've fixed the VOM. I'd better get that HV probe sorted before I do any more damage...
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Re: CRT scope high voltage supply problem?

Postby Folderol » Tue Feb 04, 2014 3:55 pm

In a sense this is (potentially) good news. It points to a bad connection somewhere rather than a faulty component.

I suggest you make a very close examination of PCBs with good light and a magnifier, particularly in the vicinity of plugs and sockets. Check all connectors as well of course.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Wed Feb 05, 2014 6:59 am

Just to be clear, I'm simply making a high-resistance probe here, and not adding a voltage divider, as suggested here?

The resistors I bought are high voltage 15W ones, and so too big to fit into a biro. Electrical tape and heat-shrink perhaps?
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Re: CRT scope high voltage supply problem?

Postby Folderol » Wed Feb 05, 2014 7:57 pm

Adam Inglis wrote:Just to be clear, I'm simply making a high-resistance probe here, and not adding a voltage divider, as suggested here?
Their justification for a potential divider setup is safety, but at the voltages you are working and with the series resistance this really isn't a problem. Absolute worst case a 90M probe at 1kV will pass 11 micro amps, which if you are in contact with the bottom end would feel slightly unpleasant but would be no risk at all. In this situation we are specifically trying to minimise the loading of the high voltage supply, and a shunt resistor would tend to detract from that. When plugged in to the meter, the meter provides the lower part of a potential divider and if you use a nice insulated plug you won't be able to come into contact with it at all. You should, of course securely connect the 'cold' end of the meter to 0V before trying to use the probe.

The resistors I bought are high voltage 15W ones, and so too big to fit into a biro. Electrical tape and heat-shrink perhaps?
Those are rather big resistors! Electrical tape would be likely to produce a noticeable leakage path and I wouldn't be too trusting of heatshrink if you touched the probe part way up. The idea of the biro tube is to provide physical support as well as insulation. However, any rigid plastic tube would probably do. The joint with the lead to the bottom resistor should be well inside the tube and finger proof. The hot end of the top resistor should be turned into a probe point, and anchored to the tube in some way - a dollop of epoxy would do the trick.

Finally, you can calibrate the probe by using it to measure a known lower voltage. For example, the 50V supply (as measured with your meter directly) should read 5V via the probe. Up to 20% difference is acceptable. A dramatic difference needs investigation.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Wed Feb 12, 2014 5:35 am

OK thanks Will.
I've managed to make a safe probe, I estimate it to be about 91.5 Mohhm from segmental readings I took before I epoxied it up, so its pretty close to ideal. Checking it on the 150 and 50 volt supplies confirms it divides by 10.
So now checking the high voltage points shows I'm getting 1.8kvolts where there should be 1kvolt, and 4.8kvolts where there should be 3kvolts - probably why I cooked my VOM!!

Any ideas?

schematic link
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Re: CRT scope high voltage supply problem?

Postby Folderol » Wed Feb 12, 2014 9:07 pm

That's very interesting. All the supplies except the 3kV one are about twice what they should be, and that one is far too high. That suggests to me that the voltage control circuit isn't working correctly. My first suspicion would be C72 going leaky.
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Re: CRT scope high voltage supply problem?

Postby Adam Inglis » Thu Feb 13, 2014 12:55 am

OK, that might explain why those three transistors have weird voltages on them.
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