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Old 10th Jun 2018, 9:21 pm   #1
RichardHowells
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Default HP 175A Oscilloscope - Vanishing Trace

Hi All,

My aged 'scope has stopped working. When last used (some months back) it was fine.

Today I had no trace at all - or so I thought. Further fiddling showed that sometimes - it seemed random at first - I could get a trace. It would only appear shortly after switch on and it would only last a few (maybe two) minutes, before fading out. It definitely faded away over a few seconds rather than instant vanishing. Now it seems to be completely sulking. I can't get any trace at all and I cannot reproduce the fading behavior.

I have the manual with full circuit diagrams, but I am very wary of measuring round the HV circuitry. The rails round there are -2700DC and -2900DC. My highest voltage device is an AVO model 8 with a 2500DC range.

The EHT line is +9300, presumably making a handy 12KV between there and the -2700 rail. I really don't want to go near that.

So - are there lower voltage things that might be relevant culprits?

What are my safe options for measuring around the -2700Volt area?
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Old 10th Jun 2018, 10:16 pm   #2
factory
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Default Re: HP 175A Oscilloscope - Vanishing Trace

Have you checked all the obvious stuff like the main power supply rails, fuses and does the beam finder button do anything?

What is the first part of the serial number for your scope?
The early ones (prefix 235) and later ones (prefix above 344?) differ greatly in the physical design of the HV section.

You can see if the HV oscillator is running by looking to see if the filaments are lit on the four 1X2B valves (they may be hard to see as they are quite dim).
If they aren't lit you could start by checking the voltages around the regulator & RF oscillator valves, as well as checking (with power off) the high value resistors for the focus & intensity haven't gone open circuit.

I have three of these scopes myself, the early one still has the intermittent fault with the HV.
One of the two later ones had an open circuit CRT filament, I managed to find a replacement to get that one working and the other also had no HV when I acquired it, this was cured by cleaning the dirt from the HV section.

David
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Old 10th Jun 2018, 11:42 pm   #3
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Default Re: HP 175A Oscilloscope - Vanishing Trace

Quote:
Originally Posted by RichardHowells View Post
What are my safe options for measuring around the -2700Volt area?
Hv probe.

Various ones available. Mine run up to 7000v with the option to have that being 70 or 700v to the meter.
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Old 11th Jun 2018, 5:18 am   #4
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Default Re: HP 175A Oscilloscope - Vanishing Trace

HV probe definitely!

The 2.5kv range on the AVO takes too much current and loads down scope HT circuitry.

There are some very high impedances in the CRT bias networks. All the 175s had been replaced by 180s before my time at HP, but if they follow the usual practice, there may be dual2.7kv supplies, one for grid one for cathode so that a lower voltage amplifier can blank the trace when the timebase is not triggered.

David
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Old 11th Jun 2018, 6:35 am   #5
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Default Re: HP 175A Oscilloscope - Vanishing Trace

I have some 120M resistors if you want to have a go at making a 1000:1 EHT probe.

Andy.
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Old 11th Jun 2018, 3:04 pm   #6
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Default Re: HP 175A Oscilloscope - Vanishing Trace

ISTR it does indeed use separately generated CRT grid and cathode supplies, with the blanking circuit in the 0V return side of one of them- the sort of thing that makes you think, "hmmm, that'll be sketchy...." but Tek and H-P both made it work, and they're no fools.

You might be lucky and find that there's a leakage and loss of EHT through dust (and high-voltage circuitry attracts fine dust eagerly), you might be a bit less lucky and find that a capacitor is breaking down, less lucky still and have board insulation leakage or really damned and find that the HF generator transformer itself is breaking down. HF oscillator types don't have much oomf to them- in a way fortunate, as personal contact with EHT could be more survivable, but it does mean that it doesn't take much leakage to drag them down or quench oscillation. The initial fade-away, now nothing could mean that carbonised tracking has developed in something. I wonder if the old TV repairmans trick of holding a small neon bulb (the sort in a zillion mains appliances) near the EHT generator transformer would work?

If you come across a decent condition Simpson 269 meter with accessories, these have 100k/V sensitivity and scaling to 1,600V with a 240 Megohm screw-on probe that extends to 4,000V. Probably much more common in the US than here though- I got mine as NOS clearance at M + B Radio of Leeds many moons ago.
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Old 11th Jun 2018, 3:54 pm   #7
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Default Re: HP 175A Oscilloscope - Vanishing Trace

Come to think of it, as well as a few 100k/V meters, there were also various 50k/V multimeters on the hobbyist market from the likes of Jemco, Eagle and Micronta that could be used with a commercial or home-made probe for low loading. It can get difficult to transfer absolute traceability for high voltages with the same confidence as low voltage- but as mentioned, HV generators and circuitry have high impedance, so circuit voltages are usually either approximately right, or way off kilter.

As mentioned by Andy, there's also the option of a divider probe into a DMM- there'll be plenty of guidance on this and other forums,

Colin
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Old 11th Jun 2018, 4:18 pm   #8
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Default Re: HP 175A Oscilloscope - Vanishing Trace

HP175 and EHT probe.
A high voltage probe is much easier than you might think. You are not looking for the long term stability and accuracy of a commercial one.
I have posted about this before, but for now, connect a number of 10Mohm resistors (or whatever other high value in your spares box) in series. Put them in a plastic tube from a felt pen, or even better, the lower part of a car wing mounted radio aerial, a nice 10" long, 3/8 diameter hard plastic tube. Fix a point on one end with araldite.
Select the values and number of resistors to get the ratio you want.
Aim to keep the meter test current load down below 20ua at the test voltage. These CRT EHT supplies are only expected to give a few hundred microamps at most.
I have two. One is 200M, and gives me 10kV at 50ua or 12.5kv when plugged into the 2500V socket on my Avo8.
The other I use with a digital voltmeter. As the input resistance can vary, I have 100M string and then a 100k at the bottom. Connect the DVM across this 100k resistor. This gives me about 1000:1. Obviously, clip the bottom of the string firmly to ground.

But the secret, which I read somewhere in a Tektronix or maybe HP manual, is to calibrate against a known high voltage elsewhere in the scope. Usually the X Amplifier feed. Measure it with and without the probe, and calculate the ratio. Note the ratio. Or play around with the bottom 100k resistor to get it closer to what you expect.
Get your calculator ready and calculate the expected actual readings then go ahead. Do this every time you use the probe.
The sparking/fuzzing as you probe the high voltage points will make you quite careful enough. I usually wear kitchen gloves as well. Gives me confidence, though I doubt how effective.

The resistors are not likely to change in value under voltage stress with occasional use. The formal voltage rating of high stability carbon, metal film or metal oxide is around 200V. The old 1watt carbon is usually 500v.

The Philips VR37 metal glaze are rated at 2kv. Use these ideally for the EHT chain across the tube. HP and Tek use the right ones. Telequipment tended to use any old ones in the D83 /D75 /DM63 scopes, so these are the first you should replace on these models.
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Old 11th Jun 2018, 7:29 pm   #9
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Default Re: HP 175A Oscilloscope - Vanishing Trace

If you build a HV probe out of a load of 10 meg resistors... Remember to check them first! If they're old carbon ones in your junk box, they may have gone high and turned into just that - junk!

It may be less trouble to buy some modern resistors.

If the cathode's independent supply is not good (leaky C, leaky insulation or a high resistor) then that leaves the grid a lot more -ve than the cathode and the CRT becomes cut-off.

You may be able to check the current inthe bias strings to get an indication if something is off.

David
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Old 11th Jun 2018, 8:43 pm   #10
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Default Re: HP 175A Oscilloscope - Vanishing Trace

If the trace fades over a few seconds, it could be poor heater continuity, dirty base pins or similar. It could be just that easy.
Les.
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Old 11th Jun 2018, 10:25 pm   #11
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Default Re: HP 175A Oscilloscope - Vanishing Trace

There's quite a lot of possible causes. They won't start to converge until some testing is done.

Remember these scopes have DC coupled blanking, so the blanking amp can get in on the act.

I don't think thee can be much progress before some voltages get measured,, then we might home in fairly quickly.

I might have an HT section for the lter 140 scope, but thats transistorised, along with a few CRTs sitting in their mag shields and face-holding castings. Unfortunately, I think they're all 8x 10cm rectangular ones for the later 140.

David
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Old 15th Jun 2018, 2:49 pm   #12
RichardHowells
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Default Re: HP 175A Oscilloscope - Vanishing Trace

Thank you everyone for your suggestions. I am not able to get on to this problem for a couple of weeks now. I will report back when I do.
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Old 16th Jun 2018, 12:36 am   #13
radiograham
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Default Re: HP 175A Oscilloscope - Vanishing Trace

Quote:
Originally Posted by Diabolical Artificer View Post
I have some 120M resistors if you want to have a go at making a 1000:1 EHT probe.

Andy.
More of an anecdote than advice,I bought a load of 10meg resistors from Maplins a few years back to make a high voltage probe for a multimeter.Cant remember the amount,Made this up in a plastic tube and blew the meter up!. I had made the mistake of not checking the resistor values they were totaly wrong value,100k I think.Took them back but couldnt find my receipt.They tried charging me again even after checking their stock all in the wrong box. I cant print my reply!.
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