HP1743A Oscilloscope Rescue

[Diabolical Artificer]

Well done, glad you got it sorted. Most scope faults I've fixed have been one resistor or the odd cap, it's finding the bu**ers though : )

Andy.

[Radio Wrangler]

Originally Posted by MelJon66

Perhaps a big industrial customer demanded this feature and HP obliged. I can't think of a more likely justification for such additional complexity.

Sometimes a defence contract. The 54503A scope was crammed into a 1/4 rack width plug-in format module (sans CRT) to fit into a US navy test system that had run out of space. It went in the catalogue as 70703A

David

[MelJon66]

Hi All,

Some progress has been made but with a sting in the tail. I have reworked the LV power supplies so that all rails are performing well, at least when unloaded. Essentially I have reworked the hasty repairs that have been carried out by others in the past. Plus, I have replaced all the bridge rectifiers and the missing current limiting transistor on the +122V rail. All rail voltages are now spot on and ripple is well within specifications.

The sting in the tail is that when I fit the A14 connector assembly so that horizontal output amplifier A11 is connected the +122V rail falls to +12V. All other rails are fine. The +122V rail supplies the Gate Assembly A12, HT board A15 and the horizontal output amplifier A11. A12 and A15 are connected directly via a separate connector and the rail holds up with them connected on their own. When I disconnect A14 the +122V rail recovers so I assumed this meant that the problem was in A11. The +122V rail is current limited and appears to be hitting the current limit when full load is applied and there does not appear to be any way of increasing the current limit. This no doubt explains why the current limiting transistor was removed in the first place. It must be a small excess as it works ok if the current limiting transistor is removed, but I would rather deal with the root cause of the problem.

A11 works fine when the +122V rail stays up but just to make sure that there isn't something amiss I have stripped it and all components test well. There does not appear to be any form of bias adjustment so it must have a fixed bias circuit. There are two pots to adjust for trace linearity but they do not seem to address the problem. So, I now suspect the excessive current demand may be in the gate assembly A12 or HV board A15 and the extra load when A11 is added must be taking the +122V rail over its designed current limit.

There is a 3KV test point under the HV supply cover so I will dig out my HV tester and check that first before I jump in.

Before I delve any deeper into the workings of the A12 and A15 boards is there something obvious I have overlooked?

There is an intensity adjustment on the HV supply cover. If I adjust that down is it likely to reduce the current drain?

Are there any known problems with these supplies that result in these symptoms?

Any contributions welcome.

Cheers
Mel

[MelJon66]

Hi All,

Of course it could be that the +122V rail is not producing the current that it should or the current limiter is kicking in earlier than it should. I have attached a schematic of the circuit.

Q3 and R2 form the current limiter circuit and Q3 was missing so I replaced it with a 2N3904 (HP Part No 1854-0215). I also replaced the bridge rectifier CR1.

I checked all the resistor values and changed all those that had drifted even slightly with exact values, except R8 because I didn't have a good one and ordered them from RS not noticing that that were on backorder for March! I do have some cheap Chinese 0.25W rated ones (the ones with leads thinner than a human hair ) but I don't fit them into anything that I want to last. Maybe that was a mistake.

R8 should be 100k (10%) but it reads 114k out of circuit so it is out of tolerance. Would R8 being high affect the point at which the current limiter kicks in or affect the circuit in any other way?

Cheers
Mel

[Chris55000]

Hi!

R8 is a "startup shunt" resistor fitted across the emitter and collector of the series pass transistor Q2 – without this the regulator wouldn't start from first switch–on!

The current sensing resistor is R2 (8.2 Ω) in series with the negative h.t. line on the unregulated side of the circuit, make sure this item hasn't increased in value!

If the 8.2 Ω resistor is OK you'll have to look on the A11, A12, A14 and A15 assemblies for a fault, however before you get bogged down in that, it might be worth another read of the bookwork to see if the correct nominal load current from the +120 V h.t. line is quoted, there might be a suggestion for a suitable test load resistor – Dynamco, Philips and SE Labs all give suggested circuits for power supply test loads in their Oscilloscope books!

Chris Williams

[MelJon66]

Thanks Chris,

The 8.2 ohm resistor had drifted high but I replaced it and it is spot on 8.2 ohms now.

I have started to investigate A15 and I have a very bad feeling about it. I was getting a good trace before I did the work on the power supply but now there is no HV at test point 1. I removed the cover and checked all the incoming voltages were good and they were. See attached section of the schematic to follow what comes next.

There is 1 amp slow blow fuse on the +15V unregulated input (22V measured) which had blown. It feeds the chassis mounted HV oscillator transistor Q1 and that tests ok. I looked around for signs of problems and spotted that the board mounted transistor A15Q1 was missing. This is part of the HV disable circuit so that has not been working. Why on earth would anyone do that? What an almighty bodge . Someone has disabled the current limiter on the 122V rail by removing that transistor and has also made the HV disable circuit ineffective. Cowboys!

When I replaced the missing current limiting transistor I have inadvertently created a situation where the 122V rail voltage could collapse due to current limiting without the HV disable circuit being there to protect the CRT. I don't fully understand the potential repercussions of this but I know it could be very bad news indeed.

Are there checks I can do to ensure that the CRT and HV transformer are still good before I bother replacing the missing A15Q1 and the blown fuse?

Cheers
Mel

[MelJon66]

Hi All,

The missing -3kV is back! The blown fuse was a bit odd as the helical fuse wire was still intact so it must have broken under the cap. I was hopeful that it may have been a faulty fuse and the failure was not an indication that there was a short somewhere. On close inspection a possible cause revealed itself; an unsoldered joint on the input terminal to the HV transformer. See attached photo of the vcc connection at the end of the Sprague cap. It looks like the transformer had been changed at at some point and the VCC terminal had not been re-soldered. It was only making contact with the edge of the though hole due to the spring in the connecting pin. More shabby work by someone. Now re-soldered, with a new fuse and HV disable transistor Q1 fitted the 'scope is producing a nice bright trace again - vertical deflection only at this stage.

The cathode voltage should be between -2970V and -3030V and I measure -2850V with an Avo 8 Mk 5. Using a DMM with a 100M Ohm resistor in a makeshift probe extension I read -257V which I think equates to -2825V so the readings are very close. I did try a 30kV probe with a 1440M Ohm resistance but I couldn't get a consistent reading between DMM's so the Avo wins. The cathode voltage is not adjustable and the manual says it will vary due to the tolerances on a couple of resistors, which may have drifted slightly over the years so the current reading seems fine.

I still have some problems with the low voltage supplies but on the positive side I have discovered the problem with the current limiter kicking in on the 122V rail. I'll report back on that when some parts have arrived and I have fitted them.

Cheers
Mel

[MelJon66]

Hi All,

Still plugging away on the 1743A when time permits.

The problem with the current limiter operating on the 122V rail was due to too much current being drawn by the horizontal output amp board A11. This was down to the stupidity of two idiots. I had ordered some 47K5 resistors to replace two that had drifted high. I did check them before fitting but wasn't observant enough to note that the 47.5 displayed on my DMM didn't have a k after it. So, one idiot had sent the wrong parts and another (me) had fitted 47R5 resistors instead of 47K5. Not surprisingly the A11 board must have presented a challenging load to the 122V power supply. Even when I re-checked all the component values on A11 I didn't check the 47K5 resistors as I had only just fitted them and "knew" they were good. Luckily I had got the 122V rail current limiting circuit working before I switched on and it did its job. The 122V rail is now happy when A11 is plugged in and ripple on all the rails is well within specification.

In a way it has worked out well because it made me check the HV board and I discovered the missing current limiting transistor and bad transformer connection there. I am sure these would have caused some serious problems at some point in the future.

I have just done some work on the main sweep board A8. The symptoms were that the trigger hold off was very long on sweep speeds of 0.5msec or slower. On the slowest 2 second setting the hold off between sweeps was over 1 minute! It turns out there are a group of hold off capacitors (C16, C17, C18) that were associated with the low speed ranges and were obvious suspects. The manual says these are TA types which I assume is tantalum. The capacitance values of these were good but ESR was between 4.9 and 7 ohms on some. I swopped C17 and C18 with some spares that came in the box with the 'scope. They measured better with ESR's of 0.23 ohms (C18) and 2.3 ohms (C17). I didn't have one for C16 so I left that in although its ESR is 4.9 ohms. Nevertheless it seems to have done the trick as the trigger holdoff now seems fine on all ranges. At least I know I was on the right track with the changes I made.

I also checked some other caps while I was there and C11 (100uf/20V Tantalum) had an ESR of between 5 and 7.3 ohms (it kept varying) so I changed it for what I think is a standard electrolytic (RS brand 100uf/63V) that was also in the spares box. I think C11 is the wet tantalum capacitor I saw Radio Wrangler referring to in a recent post. I am not sure swopping it for a standard electrolytic is a good move but it does work.

So, things are much improved with the main sweep but I suspect it could be even better so it raises a few questions:

What ESR should I expect to see on tantalum capacitors? Is up to 7 ohms acceptable?

Do tantalums have to be replaced with tantalums (expensive) or will modern (cheap) electrolytics work well enough in this application? Is C11 the one critical capacitor that must be tantalum?

While I had the A11 board out I also checked all the resistors. Many of the small chocolate coloured resistors (carbon composite?) had drifted high so I changed all of them.

So, good progress but is there more I should do before I move on to the vertical section?

Cheers
Mel

[MelJon66]

Hi All,

HP1743A Main Sweep Generator Board A8

As stated in my last post with an electrolytic cap installed in place of the original A8C11 100uf/20V tantalum the sweep seems to work fine. With the original C11 tantalum in hand I have done some more testing on it. The capacitance value reads consistently fine on a number of meters. The only equipment I have to test ESR is a cheap Chinese transistor checker. The first test after discharging is consistent at 96uf and 1.5 ohms ESR. I was reading up to 7.3 ohms with one lead unsoldered while it was still on the PCB. After that I get all sorts of strange readings, sometimes it says it's a N D Mosfet which is odd because I have only got two leads connected (it reports the non connected lead as the Gate). At other times it sees it as a diode. I can only assume that the readings vary based on the state of charge of the cap during repeated tests so I assume the first one is the most accurate.

Anyway I have removed the electrolytic and put the tantalum back in. Nothing has changed and the improvements in the holdoff remain so I will leave it at that and move on to other boards. If things deteriorate in future at least I know where the likely suspects are.

If anyone has any comments on whether modern electrolytics or dry tantalums are acceptable alternatives to wet tantalums in this application I would like to hear it.

Cheers
Mel

[MelJon66]

Greetings All,

I have been struck down with Gout for 8 weeks so have done very little on the 1743A. It was too painful to even sit at my bench. I am back in service now.

I have done some cleaning up of the front panel and serviced all the switches so many of the gremlins have been sorted out. One problem that remains is that the trace shifts to the right at fast timebase speeds and the start of the trace is lost. The trace can be brought back on centre using the position control but it is shortened so I suspect the start of the trace is being delayed or lost.

The A8 board partial schematic is attached. This is from a 1740A manual but looks the same. It's not very good but I cannot take copies of pages of my good 1743A manual. Also attached - Photo 1 shows a 10MHz trace on the 0.2usec timebase setting and photo 2 on the 0.05usec setting to illustrate the problem.

My immediate thought was that there was a problem on the Main Sweep Assembly A8. There are a set of integrating capacitors C6, C9, C10 and C11 but C6 (up top in the diagram) seems to be the only one in circuit at the fastest settings. It should be 100pf +/- 5% but it measured high at 112pf. I changed it for a good quality mica cap but the new ones also read high so I suspect my meters are reading high and the original was fine really. Anyway, it made no difference. There also a set of hold off capacitors C13 - C18 for most timebase selections but non appear to be in circuit at the fastest timebase speeds so there was nothing to check there. I have checked a few resistors in the relevant areas and they are spot on. The Hold Off pot R9 (far right) reads virtually zero ohms when in the off position where it is set under normally conditions.

It occurs to me that the problem could be elsewhere and perhaps the trace isn't being unblanked fast enough but I can't work out where that is done.

Any contributions welcome.

Cheers
Mel

[WME_bill]

HP1743.
I suspect the trouble is slow bright-up. You can confirm this by looking at the actual start of the trace. Increase the brilliance, does more of the trace appear. Move the trigger position. Does the start move along the visible trace or is it hidden before the bright-up.
There is a speed-up capacitor C11 on the bright-up feed in the Gate amplifier ( I quote from sheet 3 on HP1740 manual). Check this and the high voltage transistors Q1 to Q4.
The manual gives typical wave forms. Expect a steep front to the pulse on the gate output TP1.
I have found many fauts of this sort are failure to the transistors in the bright-up circuit. Although they will appear to be operating within ratings, high voltages just don't seem to suit them.
If not there, then you will have to look at the diodes and isolation on the EHT supply. This is all a bit uncomfortable, but the first part is at low voltages, so much easier. I suggest draw a line across the circuit diagram to separate the low from the high voltage bits. C4 and C5 are the high voltage isolation (confirm with the parts list ratings), so keep your testing to the left. Read the manual on how this works (para 4.91,4.92 of the HP1740 manual). It is not immediately obvious.
wme_bill

[MelJon66]

Thanks Bill,

Increasing the beam intensity doesn't show any more of the start of the trace. In fact it probably dims a bit. The flyback trace does show up very brightly as the attached photo shows. I have positioned the trace to the left in this image and that's why the start seems nearer to the left side.

Also, adjusting the trigger level does reveal a bit more of the trace, one rising edge, but it is very dim.

I'll investigate the gate amplifier to see if I can work out what might be happening there.

Cheers
Mel

[dave cox]

In my experience ESR varies a LOT with capacitance and it is intrinsically linked to test frequency. With my 'peak' ESR meter is see several ohms for a few uF and 10's of milli ohms for really big OR low ESR types. Tant caps generally lower ESR but the tech has got better in the last 50 years so electrolytic may be better than the original (but might only last 20 years!).

dc

[MelJon66]

Hi All,

Following up on Bill's suggestion that the fault could be in the Gate Assembly A12 I have probed some waveforms at a couple of the points shown in the manual. There are a couple of Test Points TP1 and TP2 where TP1 corresponds to sample waveform 4 given in the attached sheet. That fooled me for some time as I expected TP1 to correspond to waveform 1 and I wasn't getting what I expected. I got there in the end.

Photo 1 - Gate Drive - Pin 91 (Sample Wave 1). The vertical height of the trace is correct but the horizontal proportions are wrong. The negative half of the square wave is more than twice as wide as the positive half. In the sample waveform they are equal. Is there some significance to this?

Photo 2 - Gate Output - TP1 (Sample Wave 4). The spot above the waveform is the peak of the spike shown in the sample waveform. The spike widens into a square wave as the delay time is increased. Note that the spike is well to the left and not in the centre of the horizontal line as shown in the sample waveform. Should it be?

The sample waveforms appear to be hand drawn so they may be for indication only and not that accurate. I imagine quality images and photos were reserved for marketing brochures back in the 70's so manuals got simple sketches. Should I expect to see exactly what the sample waveform shows?

I will probe the other sample points when time permits.

Cheers
Mel

[WME_bill]

Mel,
Low corresponds to trace blanked, the waiting time of the timease when it is not scanning. High is the brightened trace during the scan.
At the end of a scan, the trace is blanked while the timebase resets ready for the next trigger input. This may be a much shorter time than the trace period, or much more, depending upon the TB speed.
The ratio of Low (off) to High (trace visible) is of little significance. The timebase holdoff control will alter the ratio also. Your spike in photo 2 will move with the delay 10turn pot and is the greater brightness when operating the A-brightens-B for the delaying TB. That is what the little peak is, which alters as you alter the delayom the 10 turn pot or the A timebase speed.
The HP manual is showing bright-up operation with both Main and Delayed timebase operation and using a separate square wave generator is perhaps a bit confusing. The waveform drawings look excellent for the stated test conditions.

I suspect you are trying to operate the delayed timebase feature at the same time.
I think you would find it preferable to avoid confusion by switching the delayed timebase off, as then you will only have to measure and consider the bright-up operation under the simplest conditions.
wme_bill

[MelJon66]

Thanks Bill,

I did follow the manual instructions to the letter so a delay time was set while I was capturing the waveforms. Your explanations are very helpful. Adjusting the delay 10 turn pot does indeed enable me to move the spike from left to right and when centred the captured waveform is exactly the same as sample waveform 4. I am very happy with that.

While looking at board A12 to work out where to probe next I have spotted some bad solder cracks around the pins of header P1. This connects mainly to the LV supplies and pin 10 is the Gate Output. Peering in from the side I can see cracks on pins 1, 2 and 3 but there may be more. The logic board A17 sits right over the top of many boards on the 1743A, including board A12, so access isn't as good as on a 1740A. I will have to do some disassembly to check more thoroughly and re-solder the joints.

It will be next week before I can do more so I will report back on what I find.

Cheers
Mel

[MelJon66]

HP 1743A – Board A12

Having re-soldered all the connections to pin header P1 the problem with the start of the trace being missing at fast timebase speeds still remains. I didn’t think I was going to be that lucky.

Referring back to the sample waveforms I posted in #34 I have attached photos of actual traces to compare with sample waveforms 2 and 3.

Waveform 2 – Chop Blanking Input. The height and frequency of the actual waveform looks reasonable versus the sample but the shape is all wrong. The rise and fall times seem too slow and the peak ends up like a sharks tooth rather than rounded. Maybe this doesn’t matter because its just about timing but I would like to be sure that this acceptable. It may not have any bearing on the issue as this is in chop mode and the problem is not related to chop mode.

Waveform 3 – Delayed Gate Input. The actual waveform looks good apart from the fact that the height is around 7 divisions versus 4 divisions in the sample. The volts per division on the measuring scope was correctly set on 0.2V/Div just as recommended in the manual. When no delay is set the spikes disappear as expected.

These departures from the sample waveforms may not be related to the problem at all but I would like opinions on whether these waveforms are acceptable. If they are good I will move on to check the HV board.

Cheers
Mel

[PJL]

Focus on the apparent bright-up delay which seems to be taking 50+nsec to go from blank to full brightness. The waveforms are all likely to look roughly OK at slower speeds as it is the rise time that is the problem. It would be worth using say a 1MHz square wave to see if you can see the rising edge of the waveform when triggered? Also look at the rising and falling edge of the 'main gate' which I assume is the critical blanking.

[MelJon66]

Thanks for the input PJL,

Further checking revealed that at the fastest timebase speed (50 nsecs/div) the first 1.5 divisions of the trace are missing even when displaying a flat line with a grounded input. I had interpreted that as indicating it is not rise time related but must surely be a problem in the gate assembly or HV board. I have visually compared the rise times with a 20MHz square wave input versus what I see on my Iwatsu 'scope with the same input. The slopes of the rising and falling edges are the same

I am now checking components on the HV board. If anyone thinks I should look elsewhere please shout.

Cheers
Mel

[PJL]

The gradual increase in intensity does suggest a rise/fall time problem with the gate.
Have you checked the rise/fall times of the gate output from the A12 board?
Checking it's progress through the HV board is not so easy given the 3KV kicking around...