HP 1725 Oscilloscope HT issue
 

My HP 1725 oscilloscope has not been used for a while, and when powered up was fine, but the trace was significantly dimmer than it should be (it has never been a bright scope especially at faster sweeps!)

Went through the Gate board, and everything checks out, except the brightness limit is not opperating due to insufficient brightness. Have had the scope running on and off for a few days.

I decided to check out the HT board, which meant removing the whole HT assembly including multiplier.

I "in circuit tested" some resistors and capacitors, and nothing untowards was found. No components were removed or replaced

However on re-assembly the scope was flashing over in the area of the 8x multiplier deep inside the PSU area

I stripped again and checked everything, but all checked OK, but on re-testing it is just the same.

It is not possible to see exactly where the flashing over is comming from but is definatly in the multiplier "box" area. All HT cables to the multiplier and from the multiplier look fine, and routed correctly.

There are no connections to the multiplier, with both input and output on leads into the potted multiplier and a soldered ground lead. Leads look OK. The flashing over is not comming from where the HT lead connects to the output socket, which is clearly visable away from the multiplier.

What can I do to investigate?

Could the flashing be caused by issues inside the CRT?

If I disconnect either the multiplier feed (3kV) or the output (17kV) would that be harmful for the scope? I want to try and isolate if the multiplier is faulty. I get a "sizling" noise between flashovers and want to isolate where this is comming from.

I will try and test outside of the scope, but I do not think this is possible!

Any suggestions how to move this forward?

It is a nice scope, if a little dated now, with a useful 275MHz bandwidth and dual sweep speeds.

Re: HP 1725 Oscilloscope HT issue
 

I've just had a similar problem with a Hickok scope, solved by intense cleaning of gunk ( it was covered in oil and dust, never did find where the oil leaked from tested caps at high voltage for leakage) then put thick plastic tape on the chassis as well as a liberal application of red nail varnish on all exposed metal/joints after rounding solder joints off so no spiky bits and lastly I covered one cap lead with two layers of thick heatshrink. This last was across the EHT and ground and was the last culprit to be tamed.

The result isn't pretty but my CRT dag runs at 8.5KV and was arching in the morning due to moisture I presume.

Andy.

Re: HP 1725 Oscilloscope HT issue
 

HP1725 oscilloscope EHT flashover.
Sounds like damp/dirt around the EHT components.

It will do no harm to operate with the multiplier 3kv feed or 17kv output disconnected for a time. Keep the brilliance down initially. The trace on the screen will be wrong size, probably much dimmer and smaller as this tube I expect uses scan magnification from the mesh/pda to the screen.

Throughly clean all around the EHT section, with isopropanol or even methylated spirits and a toothbrush.
I see it uses a sealed module for the multiplier, so you cannot check the HV capacitors.

Get as much of the EHT panel visible as you can. Then look at the unit in a darkened room. Any discharge will show up nicely.
But keep a torch in your hand so that you can see exactly where you are peering. It is very easy in the dark to get too close and put your finger, or nose into trouble. I once had a high voltage burn on a finger which took several months to heal _ nicely cauterised and all the cells sealed off.
Keep the scope in a warm room for a few days if it has been stored in the attic/garage.
Andy DiabolicalArtificer 's experience and tips are useful encouragement.
wme_bill

Re: HP 1725 Oscilloscope HT issue
 

Many thanks, will certainly get in there and clean everything off. Possibly fingerprints causing it. It was fine before I inspected it :'(

There are no components where it seems to be arcing. it just seems to be off the multiplier, either the 3kV input, or the 17kV output. No arcing along the 17kV lead to the tube, or at the ceramic connection pint, away from the multiplier.

There is no flashing from the High Voltage board, though the neons tend to flicker which they never used to do.

The leads from and to the multiplier are bent very sharply due to the proximity of the chasis panels, and touch the chassis plate at this point (no alternative positioning) so suspect this is where the arcing is, but when assembled impossible to see.

OK about safe to remove the 3kV and 17kV leads to aid diagnostics.

Going away for a few nights, so on my return will give it another go.

Thanks

Peter

Re: HP 1725 Oscilloscope HT issue
 

Found out where the flash over is taking place.

Right where the HT lead enters the ceramic connector (see pictures)

WARNING CONTAINS FLASH PHOTOGRAPHY

;D

I presume this is a damaged HT Cable. The gap has to be over 10mm. It appears the voltage needs to be 30kV to jump this, not the 17kV i should have

Everything was thouroughly cleaned with IPA, but no damaged observed.

Any suggestions how to cure this?

Also is it possible some issue with the tube could cause flashing over here, for example allowing the HT to build up too high?
Could the connector be causing this if it wasnt seated 100%. There seems to be a gap as though the connector is not fully home, but it clicks into place and I am using significant force to push together. Will have another look tomorrow when the HT has died away.

peter

Re: HP 1725 Oscilloscope HT issue
 

Could it be that EHT voltage feedback control has been somehow lost and the EHT generator is reaching for the megavolt gong, so to speak? High value resistors operating at high voltage have a tendency to go high resistance or open circuit over time, also could a resistor have been broken or a wire disconnected during dismantling/re-assembly?

The trouble with discharges like this is that they may carbonise wire etc. insulation and set up a leakage path that becomes annoyingly persistent. Good luck with it, I had various 'scopes etc. that had gone high EHT over the years with work and I never quite got over the associated trepidation! One time, it was because I foolishly lifted the feedback in a Tek waveform monitor HV generator to check a symptom with its 300V supply, overlooking that EHT came from the same inverter- on switch-on, the HF transformer instantly and spectacularly became a bright blue fuzzy hedgehog with much crackling. Amazingly, and to Tek's credit, all was well once I'd resoldered the resistor.

Re: HP 1725 Oscilloscope HT issue
 

Long time since I looked at a 1725.

Probably the EHT regulation is done by a linear series pass transistor controlling the voltage supplied to a Royer oscillator inverter. Check the voltage against the service manual.

EHY reg will likely be sensed from the much lower (but still kilovolt) -ve going EHT for the grid or cathode. Usual HP practice was to have two EHT windings, one for grid one for cathode. One winding went to chassis at its cold end, the other went to the Z amplifier. This gave DC coupled Z axis mod and blanking.

David

Re: HP 1725 Oscilloscope HT issue
 

I think 30KV is highly unlikely. I once has an early Korting CTV with a small, shimmering picture. My Marconi EHT probe with 1041C indicated 30 KV. I switched off and returned it to customer. I had only just started "playing" with CTV, and I thought discretion was better part of valour!
But 1725. If your EHT- reads correctly, I think it is safe to say your EHT+ will NOT exceed the correct value. Years ago, Radiospares (and probably RS) supplied a grey substance, a bit like sealing wax, which you could melt around HT leakages to stop them. I think you can still get an EHT grease. I would not clean with IPA, and then switch on until plenty of time had passed, or even had a warm up with a hair dryer.Damp IPA will certainly allow tracking, and as has been said, once it starts, it likes to continue. The EHT grease may help.
Must get inside my HP1725 one of these days.
Les.

Re: HP 1725 Oscilloscope HT issue
 

Not sure how that EHT lead fits into the ceramic, but it might be worth stripping back about 1/8 - 1/4" of cable. I know it's not 100% kosher but try some tape on the side of the chassis and as a last result several layers of nail varnish thoroughly dried between coats- it works.

Andy.

Re: HP 1725 Oscilloscope HT issue
 

Thanks

Going to run it without EHT so I can be sure the HT PSU is working and regulating correctly.
I think I can actually test the regulation without running HT also, by pulling the oscillator transistor, feeding in some low volts DC to the feedback point (after the 30MEG resistor) and seeing if the volts/current at the oscillator transistor base changes.

Trying to keep away from HT and EHT!

If that all checks OK, then I will have to assume fault is EHT cable.

On closer inspection the connector is not ceramic, but hard plastic, moulded to the cable, so not possible to re-make connector.

Re: HP 1725 Oscilloscope HT issue
 

Made some progress today, but not solved issue yet.

Disconnected EHT x5 input, so just making -3kV.

Found one diode short to the -300V Grid (CR6). This might explain the issues I originaly had with low brightness as is part of the -300V rectifier.

Also found a 1.8k resistor (R4), just after the -3kV rectifier diode open. This will explain why the regulation is not working, as regulation is tapped off after the resistor.

I presume this resistor is current protection, or possibly limitting.

Replaced the resistor, and tried again. Sizzle sound then it blew open again.

My guess is something is shorting to ground in the -3kV section. Resistance measurement is in the Meg Ohm, so something must be breaking down under high voltage.

Will start by disconnecting the tube completely, and disconnecting the focus control to see if fault is on the board.

Should this HV PSU still work without the tube connected, or does it need some current draw?

Also discoverd HP drawings are not to be relied upon! Parts list and drawings do not match, and neither match what I have!

Re: HP 1725 Oscilloscope HT issue
 

That's interesting, sounds like your getting somewhere. Sorry can't help with your Q as the only HP scope I've worked on so far is a valved one. BTW saw a HP1725 on ebay recently if you need a donor.

Andy.

Re: HP 1725 Oscilloscope HT issue
 

Somewhere at the front of your manual it should state what range of serial numbers it covers. Placed inside, near the front would have been any update sheets to cover changes from the date of printing to the date of shipping of that particular unit the manual shipped with.

HP was punctilious that the manual included with an instrument had the right revision of information.

You must have either a manual that's been separated from its update sheets, or a later revision of instrument than the revision of manual you're looking at.

David

Re: HP 1725 Oscilloscope HT issue
 

HP1725A.
I suspect your copy of the manual is taken from that in the HP archive. Covers serial 2025A, and the yellow update sheets covering versions 2034 to 2510A have surprisingly been copied and are at the end of the HP archive version, not with the update sections in the middle of the manual. I have always found the proper HP documentation first class for all mods and later models.
Your apparent short to ground from 3kv supply requires your ohmmeter. How is the 30M R10 feedback resistor and the capacitor in parallel 1500pf C7 and the smoothing C6. How is CR1 between the grid and cathode rails. If CR6 to the grid rail failed, it may have affected that one also.
I believe these scopes use a form of auto-focus, so check with the manual.
I am always suspicious of the bright-up driver transistors. They don't seem to like high voltage transients.
wme_Bill

Re: HP 1725 Oscilloscope HT issue
 

Thanks

All coments about the manual noted, and thanks for the pointers WME_Bill

All the mentioned components check out good with OHM meter, and capacitor tester, but I think I will need to get my high ohms meter out to check at 1000V, as just before the resistor burns out, i can hear a sizzle sound, which I suspect is some component breaking down at high voltage.

I have a paper version of the manual, as well as the manual from the HP archive I posted a page from, both manuals appear to be identical.

As an example, R12 and R15 are reversed in value..ie R12 is 20k pot, and R15 is 150k 1/4 watt. (parts list is correct, but drawing is incorrect, both manuals)

However on my scope the 20k pot is connected as a variable resistor, not a potentiometer and the feedback taken directly from R10

My paper manual, is for an earlier version, and does have a printed changes section in it (no mention of the errors), as well as a yellow changes document stapled to it. This document does cover my instrument serial, but does not mention any of these differences to the HT board, nor the missing R9.

Not a big issue, but everything needs checking before assuming!

Re: HP 1725 Oscilloscope HT issue
 

Quick question!

On the schematic is a diode, CR7 (not clear but diode between -2950 rectified and smoothed, and -300V before the rectifier, so 300VAC (TP1 and CR6)

This diode is fine, but it is only 400V. 750mA rectifier diode.

Surely volts across it are in excess of 3000V (absolute minimum is 2650VDC)

How does it survive? What does it do? Surely it shorts the two rails together at DC

Just pulled the Cathode connector from the oscilloscope tube, and R4 blew imediatly.

Beginning to think this PSU needs loading in some way for the regulator to start working

Could the neons pull enough current to blow the resistor? When powered up they are flashing away, presumably due to over voltage condition.

Re: HP 1725 Oscilloscope HT issue
 

Hi!

I'm just in the process of getting a better manual and I'll be back with repair hints when I receive it!

Chris Williams

Re: HP 1725 Oscilloscope HT issue
 

Many thanks.

Tonight I have a plan 8-)

Start from basics and check the regulation circuit works, as I think issue might be over voltage. Still curiouse about CR7 though!

Re: HP 1725 Oscilloscope HT issue
 

Even better--- glue a big piece of Teflon (if you have it) or Perspex, covering the spot the "zap" goes to, with plenty of margin.

I used this trick with an arc from the discharge resistor to the chassis on an NEC FM Transmitter.
The resistor was OK, except for having an exposed bit where the arc came from.

The FM Station was a community one, part of of my employer's "Good Corporate Citizen" policy, & its owner had to "save up their bawbees" to afford a new resistor.

Gluing a nice big piece of Teflon, on to the "zapping" spot of the chassis with superglue (I was told it wouldn't stick, but it was still there years later), sorted the problem.

From memory, they never ended up buying the resistor.
We were a bit overstretched on the "nice guy" approach, so didn't offer to give them a similar one from our spares.

Over on eevblog, there have been quite a few threads on restoration of both HP & Tek 'scopes of this vintage.
Here's one:
https://www.eevblog.com/forum/testge...n-project/125/

It's about a HP1741A, but may be helpful (I'm pretty sure there is one on the 1725, but that website isn't always the easiest to search)

Re: HP 1725 Oscilloscope HT issue
 

I have thouroughly insulated the point where the UHT spark comes from, but not yet tested it.

It seems my issue is a resistor burning out in the HT (3000V line), before the regulator tapping point, so effectively allowing the HT to rise well above its set level.

I need to locate the short, blowing the resistor, before thinking about the EHT, which might not be an issue when the HT is regulating.

Put my Megger on the HT line, and no issues, reading 30Meg Ohm to ground ( I actualy blew C8 during the test as board was disconnected and I had no voltage on the +15V line, which allowed voltage across C8 to raise to 1000V :dunce: With C8 out of circuit I get Infinity resistance at 1000V.

The regulator circuit on the A14 board works perfectly, giving a nice inverse swing to Q1 as the volts on the feedback line increases (from about -0.2V to +0.5Volt (according to the drawing the running voltage should settle at 0.2V)

So it is re-build the HT board again, checking diodes and resistors again and re-test