HP 1740A on the bench, help please?

[tautech]

http://www.sphere.bc.ca/test/hp-parts/hpparts7.html

There's another supplier on your side of the pond but their name escapes me ATM.
What has given me success is Google just the HP part # and you'll likely find a few of them.


WRT the screen pics, you have AC trigger coupling selected, not that it's likely to change the display....but it's something you rarely use. Dave did a vid on this not too long ago, might be worth digging out.

[bd139]

http://www.qservice.eu ?

[Specmaster]

I have also asked on the HP/Agilent users forum about this problem and a reply I had back from them said

"The signal is being differentiated, as if it is being coupled through a small value capacitor.  I would first look at the attenuator switch contacts on those particular v/div settings for an open or high resistance.  In other words dirty switch contacts."

Further looking through the SMM reveals that the only extra switch that is being used on those high ranges is S1D and contact D is common to all values from 0.5V to 20V and it also switches in a small cap of 0.47 along with the trimmer cap of 2.8, could this be the suspect? I have applied switch contact cleaner to all contacts but no change.

The other thing I have been thinking about is that I seem to remember that when I first brought this scope home and quickly tested it to see if I could get a trace on it, was that I was seeing odd traces on both channels, but now only on one. The only thing I have changed that is channel specific was Q3 on channel B, replacing that unknown T0-18 canned transistor for the correct MPSH81 which is a T0-92 plastic bodied transistor. I could swap out the Q1 (channel 1) for a new MPSH81 and see if that makes a difference as I did have concerns about the connections to the emitter and collector as these are terminated in socket tubes and looked like they might have been suffering from intermittent contacts when viewed under magnification?

[Specmaster]

Update time, I spent a few hours tonight going through the input and attenuation section and found what I a smoking gun but I have had to leave the culprit in circuit and bridge across it.

I discovered that R3 is only switched in on ranges from 0.5V and above and is supposed to be 990k which on channel B it is, but channel A was 11M so was O/C somewhere. As this appears to have a screening can soldered around it and grounded, I thought it was best to leave it in circuit for now and seek advice as it and the one the other channel are also labelled on the board component map as MP1 and I don't know what this means. I suspect that it might mean metal protected or maybe mains protection?

Anyway I soldered as temporary test, a 1m resistor in parallel and tried it, success, both channels now fully working just need to be calibrated to make it useful for my requirements.

If the weather is good tomorrow I'll take a break and spend a day photographing some lovely F15 fighter jets and maybe a Raptor or two at RAF Lakenheath.

So the question is now should I remove the old resistor and screen and replace screen over the 1M resistor or is it a special type of resistor?

Once thats done, I can then concentrate on sorting out the timebase knobs and replacement shafts oh and of course the obligatory 4 feet (missing on mine as they had all broken) then I shall have a nice decent 100Mhz scope on my bench.

Thank you one and all who has so far helped me with this project.  

Heres a round of applause for you

[bd139]

Nice job  ... If it's 1M, that's probably the gate resistor off the front end (hence 1M input). The shield, no idea. Might be a large guard ring or just an RFI shield. What's the resistance between it and ground?

[tautech]

The BOM offers few clues for A3R3 990K #0698-8622 resistor other than 5% .12WF.
What a bitch of a value but as it's 5% rated I guess you can trim any effect of tolerance out.
It's listed with others that are speced .125W so it could be the same or 1/2W, you should be able to determine the wattage from the size. Now carbon vs metallized film, one will be the better for this application but by how much ?

It could be screening and I think most likely when you consider how input stages are often in tin cans these days so it's more likely carbon composition as there's no metal caps on a carbon resistor.

[Specmaster]

Just under the resistor there is an isolation slot so I would suspect that the shield is to offer some protect in the event of someone stuffing a large voltage into the scope, the resistor would blow to offer a degree of input protection and the shield I guess is there to contain the blast?

bd139: this resistor is switched in on the higher voltage ranges only and then becomes in series with another 990k  and then a further 220k before it goes into the gate of FET, so yes it does form part of that resistance.

The close I get to 990k the less calibration I'll need to do as it the equivalent resistance is currently around 880k and the difference on screen between the trace on the 2 channels together is enough to skew measurements out.

[tautech]

Just under the resistor there is an isolation slot so I would suspect that the shield is to offer some protect in the event of someone stuffing a large voltage into the scope, the resistor would blow to offer a degree of input protection and the shield I guess is there to contain the blast?

Maybe but it also provides a ground path should the resistor insulation break down. Either way an experienced user wouldn't expose their scope to suck risk.

[bd139]

Idiots blow up scopes regardless of the protection!

Did some digging. This is Nato Stock Number 5905-01-160-9273 which is apparently Vishay Dale metal film. HMF 1/8W which is milspec epoxy moulded metal film 0.5% (not 5%). Made of pure unobtainium! Last seen in a Vishay Dale catalogue in 1973!

990k is the resistance you get in a perfect 100:1 divider when combined with a 10k resistor. This used to be a common value but alas no longer.

You can use a 750k and a 240k in series. Get 0.1% ones if you can. 1% is probably fine really. I'd go with 1/4W as well as the milspec ones were usually 1/4W commercial ones that were derated to 1/8W.

RS sell RN60D resistors for between 12-50p each (including delivery via parcel force next day). If you're lucky they will have both values. They are milspec and well overrated for this job but are cheaper than buying big packs of MRS25's for example.

[Specmaster]

Ok thanks, so does that mean that this is a scope that was made for the military then? if so that might account for that transidtor that had the BVC reference because I seem to remember from years ago that used to be a military code didn't it?   

[bd139]

Not necessarily. HP used good quality parts in most of their kit.

The numbering rarely implies originator. For example I’ve got a few thousand transistors which are CV numbered (common valve). These were ex GPO stock. Need to sell them really.

Edit: get on the A414 quick https://www.ebay.co.uk/i/292294801766

[tautech]

The numbering rarely implies originator. For example I’ve got a few thousand transistors which are CV numbered (common valve). These were ex GPO stock. Need to sell them really.

We know you'll keep the BD139/140's but is there anything special in your stash ?

[Specmaster]

Thanks, offer made 😉

[Specmaster]

Not necessarily. HP used good quality parts in most of their kit.

The numbering rarely implies originator. For example I’ve got a few thousand transistors which are CV numbered (common valve). These were ex GPO stock. Need to sell them really.

Edit: get on the A414 quick https://www.ebay.co.uk/i/292294801766

How about CVC they buy up stock?

[bd139]

The numbering rarely implies originator. For example I’ve got a few thousand transistors which are CV numbered (common valve). These were ex GPO stock. Need to sell them really.

We know you'll keep the BD139/140's but is there anything special in your stash ?

Lots of ex GPO CV numbered parts. Mainly BC109/BFY51/BSX20 from Philips with CV numbers on. Nothing special but popular. As for special stuff I’ve got a ton of 2N4416, 2N2219, 40673, MPF102 and J310’s as well which I’m going to palm on eBay once I’ve worked out how many I’m going to need until I’m dead.

CVC won’t give you a lot for them. People are selling 40673’s at up to £8 a pop on eBay. I’ve got about 80 of them. I’m waiting until they’re a little more scarce as they are magic fairy transistors for the amateur radio bods.

[Specmaster]

Just think of that test gear porn you could buy with the proceeds 

That scope by the way, I offered him £30 for as it switched in the morning and then later the same day it didnt. Said it came a 2 pin plug, could be 120V one hence came on briefly until the damage was done, mentioned that to the seller but he said he couldn't go below £45 collected. Hell I paid less than that for the one I got and that did have life so I won't be getting that know because used up all my offers chances by countering him. The photos are shit as well, so I could be buying one with the same timebase knobs broken, who knows?

I've got a search set up on Ebay now for anymore that pop up.

[bd139]

Yes that's the idea. Waiting for the opportune moment to shift them

Fair points there. Probably a turd then! Good luck. I'll let you know if I see any more via other channels. The one I grabbed the other week; the seller had no idea how to test it so was selling it spares/repair. I got there and played with it and couldn't get it a trace out of it mainly because I've never used a 1740A until then and it was in his porch in the dark. He said it's probably broken. I didn't but I was going to go with that assertion if he offered it!. So with some negotiation I halved the value to £40 which was plan A anyway. Got it home; RTFM for a bit, cleaned off the crud and it worked fine!

I was slightly disappointed if I'm honest. At least this 475 is giving me some stick

[Specmaster]

Well you did say I, if I remember it correctly, that they are very good scopes and that you regretted selling it but there you go, you were at the cross roads with the 475 in your sights. So maybe you won't be dragging the 475 to your grave after all 

The overall condition of my 1740A is really pretty good cosmetically and now electronically as well so perhaps it might be coming with me when I go down that road 

[bd139]

Hahaha. I'm winning the 475 battle so far. One broken knob and some z-axis slowness at high speeds and we're done: https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

It's a good scope. Definitely keep it. TBH the only reason I got rid of mine was that my bench is absolutely tiny so it was too deep and I feared for the knobs on the front. The Tek scopes are a lot smaller!

[tggzzz]

Hahaha. I'm winning the 475 battle so far. One broken knob and some z-axis slowness at high speeds and we're done: https://www.eevblog.com/forum/repair/tektronix-475-repair-assistance/

It's a good scope. Definitely keep it. TBH the only reason I got rid of mine was that my bench is absolutely tiny so it was too deep and I feared for the knobs on the front. The Tek scopes are a lot smaller!

Not the 464, which is noticeably longer due to the analogue storage capability.

[bd139]

Fair point. Never got my hands on a 464. Have avoided storage scopes to be honest because of the relatively short tube lifetime. We had a couple when I worked for an MoD contractor and they had been wrecked by daily use.

[Specmaster]

I noticed that too, it only just went on my work desk/bench and also its so heavy, but that means 2 things to me, 1 its well constructed so will withstand a few knocks and the second, which I love, it has a traditional linear power supply, so much easier to work on and power in reserve.

[bd139]

Yes. I certainly prefer the linear supplies as well. I've had a few Philips scopes and those seem to universally have a switching supply. Some clever units have a linear supply that gets 28v DC out (defence/avionics standard) and then a switcher after that. Usually a very tetchy explodey one. Also Philips being Philips they decided to implement the power supply themselves using discrete parts and lots of RIFA X2 capacitors which decreases reliability considerably and results in lots of small interesting fires.

[tggzzz]

Fair point. Never got my hands on a 464. Have avoided storage scopes to be honest because of the relatively short tube lifetime. We had a couple when I worked for an MoD contractor and they had been wrecked by daily use.

Analogue storage scopes always were a pain; digitising scopes were always better in that domain. Relatively recently, affordable digitising scopes have become as useful as analogue scopes, but the latter can still be used in the right circumstances.

[Specmaster]

Yes that's the idea. Waiting for the opportune moment to shift them

Fair points there. Probably a turd then! Good luck. I'll let you know if I see any more via other channels. The one I grabbed the other week; the seller had no idea how to test it so was selling it spares/repair. I got there and played with it and couldn't get it a trace out of it mainly because I've never used a 1740A until then and it was in his porch in the dark. He said it's probably broken. I didn't but I was going to go with that assertion if he offered it!. So with some negotiation I halved the value to £40 which was plan A anyway. Got it home; RTFM for a bit, cleaned off the crud and it worked fine!

I was slightly disappointed if I'm honest. At least this 475 is giving me some stick

Just got an email from that chap with the 1740A after he turned down my offer last night, apparently if my offer of £30 still stands, he'll accept it if I collect, not sure what to do his photo is not that clear, decisions decisions   

Edit.
I'm off down the A414 tomorrow, fingers crossed all the knobs and shafts, and the feet are in good condition.