HP 1740A on the bench, help please?

[Specmaster]

The 1740A has just made it onto the bench as it has a few issues that require sorting out. It had power issues and like Dave's it was in the area of the full wave rectifier, bad joints, now remade and testing OK, all caps have been checked and all passed OK on the power board.

Before putting the board back, I noticed on the vertical amp board 2 burnt resistors, one of which is cracked so I pulled out that board as well, pics attached. The resistors in question are R2 and R4 which actually check out as being OK on a DMM but the fact that their getting hot concerns me so I started checking components on the board and discovered that C1 was reading strange when I pulled it out of circuit to test it. the pic "cap" shows the results when tested on my digital bridge, it also shows the "cap" itself, the likes of which I have not come across before, looking more like a resistor and as can be seen by the pics, behaves as one at 1Khz test frequency, a cap at 100Hz and an inductor at 7.8Khz?

What kind of capacitor is this?

Also attached is a pic of the vertical amp board with the R2 and R4 highlighted (vertical one has the crack) and the location where C1 should be and the associated schematics.

The hybrid A5A1 also has what appears to be a crack on it, although this is not visible on the sides and cannot be felt unlike the crack on R2 which can be felt with ease.

Do you think it is a crack or a mark on the hybrid itself? I still have the rest of the parts on this board to test yet.

Would you change the R2 and R4 for new ones, despite reading perfectly OK

[tggzzz]

Would you change the R2 and R4 for new ones, despite reading perfectly OK

Yes, and I would also change R3: if too low it will cause it would cause excess power dissipation from the 43V line (check it is 43V, no ripple, of course).

[bd139]

Agree. Mine don’t look burned like that for ref.

Those hybrids scare me.

[CJay]

The 'crack' looks like someone has dragged a component lead across the top of the ceramic and left a deposit of tin/lead on it.

If you can't feel a crack with your fingernail try rubbing it with a pencil eraser to see if it changes?

[Specmaster]

Do I take it then that as nobody has mentioned the C1 capacitor and the results observed that you consider this to be OK? It looks like a short fat resistor to me, I have searched through loads of references on capacitors and can find nothing that remotely looks like it?

[bd139]

I suppose the question is, what happens when you stick it across an ohm meter?

I'm going to take mine to bits quickly and photograph it and you can compare ... give me 5 mins

[Specmaster]

On an ohmmeter that goes upto 60M it is open circuit. I suppose it is reacting differently at frequencies shown to filter out ripple and other unwanted frequencies and only becomes a true capacitor at 100hz, is that about right??

[bd139]

Quick analysis based on the observation in mine.

If we take it at face colour code value the resistance is 109 10% -> 10Gohm so there's no way to measure it without specialist equipment. It's too small to be a capacitor of anything close to that value. I suspect this is a "hack".

On the SMM it's marked up as part A5C1 which is across the delay line input and is marked as 1pF which is very low for a capacitor.

Ergo, I think this might be a selected high ohm value resistor that happens to show up on an impedance bridge as around 1pF. As far as DC is concerned it looks open circuit but due to parasitic capacitance it may show up as 1pF. Probably was cheaper than a 1pF rated cap or performed better

Board from mine (also note no burn )



Usage analysis: If this is around 1pF, the reactance (Rc = 1/ (2 * pi * f * C)) at full bandwidth of 100MHz is only 1.5k so this might be to terminate the delay line properly and stop reflections or oscillation above the rated bandwidth.

[Specmaster]

Thank you so much for checking against yours, I was  as the SMM shows it as a cap, and I'd already worked it out as a being (if it was a resistor) 10Gohm and I've never seen anything that high before hence the confusion. I was wondering if someone had been in before me and buggered about although it showed no signs apart from a resistor being shown as a cap. 

Checked all the other parts with the exception of the hybrid as I have no idea on how to do this, and everything else is fine, including R3 which tggzz suggested checking, so I guess its just the R2 and R4 to change out for 1w ones (I think, still trying to find them on the parts list to make sure of the right rating), reinstall the "C1" and reinstalling the board. 

[bd139]

The hybrid works or the hybrid doesn't 

Those resistors are driven pretty hard. To get the transition frequency and driving ability to drive the plates at that speed requires quite a high standing current. Wondering if there's a calibration step that keeps these in check and not smoked. Not 100% sure. Worth checking.

Going to be honest; mine is barely used by the looks so I reckon it just hasn't had time to cook the resistors yet.

Incidentally it's only mine until Sunday where it is going to another home to make room for more things.

[Specmaster]

Cool, if I'd known that you was selling it I would have been interested in it at the right price. According to the parts list they are 0.25w rated.

I love the way that this scope is engineered, it seems to well designed from a mechanical point of view with a good strong chassis and the parts seem to come apart with relative ease once the little secrets have been learned.

[bd139]

I sold it for the wrong price to the person who could afford to pay it

It's quite nice, especially compared to some Tek kit. My heart really sinks when I have to deal with a 4xx scope. They're lovely machines but total nightmares to get to half of the parts.

[Specmaster]

Yep, the Tek kit seems to be the industry standard through and they have a very active user group but  they always seem to be breaking down judging by the amount of posts on their forum.

[CJay]

Yep, the Tek kit seems to be the industry standard through and they have a very active user group but  they always seem to be breaking down judging by the amount of posts on their forum.

Nah, it's *REALLY* reliable, the problem is it's now *REALLY* old

[bd139]

I don't think the old Tek analogue stuff was that reliable when new. I worked for a company that had a lot of high end 7000 kit that was 5/6 years old at the time and it was constantly wonked. The only stuff there was literally never a single problem with was HP kit.

[Specmaster]

You made a tactical error there then, should have sold the Tek and kept the 1740A, you'd probably have got a better price for the Tek too. Always seems to go for more on Ebay as well, suppose its the perceived image as I think the name has been around longer as others have been swallowed by bigger groups like Phillips and Fluke have and then relaunched under a different name.  You would have more money to reinvest in more test gear and beer 

[tggzzz]

Yep, the Tek kit seems to be the industry standard through and they have a very active user group but  they always seem to be breaking down judging by the amount of posts on their forum.

Breakdowns = failure rate * number of items. There are a large number of (old) Tek scopes around.

There are two principal reasons there are a large number around: they were the best at the time (so many were sold) and they have good reliability (so, unlike other scopes, they haven't been junked along ago).

[bd139]

I think there are so many around because people like me nicked them out of the skips and sold them on yahoo auctions back in the day and they've been going round in circles since

You made a tactical error there then, should have sold the Tek and kept the 1740A, you'd probably have got a better price for the Tek too. Always seems to go for more on Ebay as well, suppose its the perceived image as I think the name has been around longer as others have been swallowed by bigger groups like Phillips and Fluke have and then relaunched under a different name.  You would have more money to reinvest in more test gear and beer 

The better price was why I sold the Tek kit. A limping 465 is worth as much as a shiny 1740A

Philips stuff is generally quite good (once it has exploded a couple of times and all the RIFAs worked out of it).

[CJay]

Breakdowns = failure rate * number of items. There are a large number of (old) Tek scopes around.

Hence the reason why I'm one of the worst people to ask 'what's the most reliable...' anything, my stock answer is 'no idea, I only ever see faulty ones'

[tggzzz]

Breakdowns = failure rate * number of items. There are a large number of (old) Tek scopes around.

Hence the reason why I'm one of the worst people to ask 'what's the most reliable...' anything, my stock answer is 'no idea, I only ever see faulty ones'

An excellent example of stats skewed by "sampling bias"!

[Specmaster]

Yep, the Tek kit seems to be the industry standard through and they have a very active user group but  they always seem to be breaking down judging by the amount of posts on their forum.

Breakdowns = failure rate * number of items. There are a large number of (old) Tek scopes around.

There are two principal reasons there are a large number around: they were the best at the time (so many were sold) and they have good reliability (so, unlike other scopes, they haven't been junked along ago).

Having not ever used or worked on a Tek, I can't really say. Only ever worked and used ones that I own or owned, those being Rapid (Pintech), HP, Hameg, Goldstar (LG), Iwatsu and Hitachi and the general construction of the HP really impresses me.

[Specmaster]

A5 board has 2 1.2k resistors, R2 and R4, that have been getting hot and cracked, new ones on their way, A8, A9 and A11 boards removed and checked for faulty parts, and all seems to be OK. A7 low voltage power supply board board pulled out and checked, pin 4 of the power plug to the bridge rectifiers had been getting hot, bad solder joint on the corresponding rectifier remade and checked, all seems to be OK at the moment. That just happens to the 43V rail that feeds the 2 burnt resistors!

Once the replacements arrive and are fitted will be the moment of truth when all of this goes back in. Trouble is, its been so long since my first look at this, I've forgotten why I took it apart, other than to check R2 and R4 for the burning. Who knows, maybe it will be OK now....

[bd139]

Fingers crossed. Worst case is they'll burn again.

I keep meticulous notes in A4 squared school exercise books because my memory is crap. Highly recommended. Only problem is I tend to forget where I've left them

[Specmaster]

Yep, I seem to remember it was giving some strange display on the screen at certain settings, hence why I lifted the bonnet and discovered the over heating resistors, can't remember if I did a voltage check or not now, probably did, oh well soon find out.

Thought you was getting some new toy(s) and was going to post photos the other day? Hopefully I'll have a new toy or 2 very soon.

[bd139]

Yeah unfortunately had to cancel that. The eldest managed to bust a toe so I spent the night down the hospital instead of collecting it. The window of opportunity has passed now.

More toys = good.

I'm weighing up an Agilent vs GW Instek meter at the moment. SWMBO is out tomorrow so if I opt for Saturday delivery I'll be able to get it in unnoticed