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

[bd139]

I did it. She's going to murder me

[tautech]

Carefully check that jumper/connector bridge board (A12 ?) for cracked joints on pins and sockets and in the boards it mates with. A 1740A that I had some time back had numerous bad cracked joints on that PCB.

[Specmaster]

I did it. She's going to murder me

And will you be going for a Keithly from Germany? [emoji12]

Sent from my SM-J510FN using Tapatalk

[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).

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.

I used to work for Hewlett-Packard, and, while being generally impressed with their kit and people, some aspects of the 1740 didn't amuse me.

Two examples are the "floating" boards held in place by the timebase "axle" (shades of IBM PC ISA boards), and the timebase switch on the PCB - which abrades the PCB track when rotated. (Fixable by soldering the obvious wire, taking care not to allow solder to spread onto the contacts)

[bd139]

And will you be going for a Keithly from Germany? [emoji12]

Sent from my SM-J510FN using Tapatalk

Passing on that now

[Specmaster]

And will you be going for a Keithly from Germany? [emoji12]

Sent from my SM-J510FN using Tapatalk

Passing on that now

I might go for one, not sure, a bit rich currently as I'm unemployed. That might be about to change so maybe if another pops up?

Sent from my SM-J510FN using Tapatalk

[Specmaster]

Carefully check that jumper/connector bridge board (A12 ?) for cracked joints on pins and sockets and in the boards it mates with. A 1740A that I had some time back had numerous bad cracked joints on that PCB.

The jumper has been checked, I'll do the same on the boards it connects to as well. It actually looks inside as if has left the factory. It is spotlessly clean and the attention to detail on show is exceptionally good.

Sent from my SM-J510FN using Tapatalk

[bd139]

Never seen a single cracked joint on HP kit myself unless it has been repaired or an ECN applied by a chimpanzee. They're usually very well put together.

[tautech]

Carefully check that jumper/connector bridge board (A12 ?) for cracked joints on pins and sockets and in the boards it mates with. A 1740A that I had some time back had numerous bad cracked joints on that PCB.

The jumper has been checked, I'll do the same on the boards it connects to as well. It actually looks inside as if has left the factory. It is spotlessly clean and the attention to detail on show is exceptionally good.

These always are spotless inside which in a large way attests to their popularity.
I can't now remember all the issues with the one I had....jumper cracks and the vertical output PCB wasn't working.
Then I think the timebase stopped working.....nah can't remember much more.
It was saved from a rubbish skip and I had to get a donor unit from the NZ HP agent as I didn't know about Sphere way back then. It fixed up fine, a pair of new probes, manuals on CD and down the road it went, donor and all.
Now I have another to fix that donor could've come in handy. 

Never seen a single cracked joint on HP kit myself unless it has been repaired or an ECN applied by a chimpanzee. They're usually very well put together.

They crack all right, those jumper boards get put under heaps of stress prying them on and off. The factory solder joint balling could be described as miserly. Maybe they were short of solder that shift. 
I'd make a practice of reflowing them as a matter of course.

[Specmaster]

Carefully check that jumper/connector bridge board (A12 ?) for cracked joints on pins and sockets and in the boards it mates with. A 1740A that I had some time back had numerous bad cracked joints on that PCB.

The jumper has been checked, I'll do the same on the boards it connects to as well. It actually looks inside as if has left the factory. It is spotlessly clean and the attention to detail on show is exceptionally good.

These always are spotless inside which in a large way attests to their popularity.
I can't now remember all the issues with the one I had....jumper cracks and the vertical output PCB wasn't working.
Then I think the timebase stopped working.....nah can't remember much more.
It was saved from a rubbish skip and I had to get a donor unit from the NZ HP agent as I didn't know about Sphere way back then. It fixed up fine, a pair of new probes, manuals on CD and down the road it went, donor and all.
Now I have another to fix that donor could've come in handy. 

Never seen a single cracked joint on HP kit myself unless it has been repaired or an ECN applied by a chimpanzee. They're usually very well put together.

They crack all right, those jumper boards get put under heaps of stress prying them on and off. The factory solder joint balling could be described as miserly. Maybe they were short of solder that shift. 
I'd make a practice of reflowing them as a matter of course.

Yes, it would be better to do that while it's open, some of those boards are hard to get out.

Sent from my SM-J510FN using Tapatalk

[Specmaster]

Resistors have been replaced, all the soldered plugs / sockets as suggested have been checked on the boards that I have had to remove, ie. the power, the interconnecting and the vertical output boards, all are OK.

Reconnected and powered up now I have lost all vertical inputs and control so the flat trace cannot be moved up or down by the vertical position pots so naturally I retraced my steps thinking its something I've done wrong, all seems OK, measured volts on the vertical output board and everything there is OK apart from the schematic shows 0.5v on the collectors of Q1 and Q3 and I have 1.1v but have been advised thats OK.

Measured the incoming signal from the end of the delay line and it is constant at 11.4v regardless of the pot positions so my suspicions have swung toward the pre-amp and I discovered that the Q1 and Q3 on that board have their base soldered in a common joint with other parts but the collector and emitter are socketed (see photo) and on channel A the emitter and collector pins are a very loose fir in the socket tubes while on channel B, the transistor has already been replaced with another in T0-18 metal case, pin outs are in a different arrangement and emitter and collector again pushed into sockets but the leads have been bent to suit and were almost touching but loose in the sockets.

Surely this cannot be right? Is it ok to solder these in or not. Note that I will be replacing these anyway with new ones as I have no faith in them being matched. The one in the photo is capable of 100Mhz but cannot vouch for the other as it is crusty I cannot read the part number.

Thoughts please, could these 2 transistors be the root of my problem?

[tggzzz]

Reconnected and powered up now I have lost all vertical inputs and control so the flat trace cannot be moved up or down by the vertical position pots
...
Thoughts please, could these 2 transistors be the root of my problem?

Maybe maybe not. Maybe the BNC connector has become disconnected (I doubt it, but your tests haven't removed that possibility).

You have started at the output end of the signal chain and found the (differential) collector voltages don't change. Conclusion: somewhere between the BNC input and where you are measuring, there is a fault.

Now start at the input end and see where the (differential) collector voltages do and don't change. If the voltages change at a point, the fault will be between that point and the output.

[Specmaster]

The BNC connectors are connected just fine, I've decided to replace both these transistors as the B channel is not the same model as the A and also in trying to make the legs engage the socket better it has become broken not the one in the photo, its the T0-18 cased one. I can't see why the base leg would be soldered and leave the other legs pushed into badly fitting sockets, makes no sense to me. Surely if these transistors were prone to failure etc, that they would have had all legs socketed to allow for easy replacement?

Prior to replacing the resistors on the output board, I had working traces so the only thing I can think of is that I did go over the preamplifier stage with a multimeter checking for any caps short circuiting on the ohms scale and testing connections between any plugs/sockets so I must have disturbed these transistors in the process.

I have now located some of the correct transistors Motorola MPSH81 (HP had their own custom part number printed on these) and these have been ordered. I'll try and replace them in the same manner as they were fitted but the sockets only for 2 legs still mystifies me.

[tggzzz]

I've decided to replace both these transistors as the B channel is not the same model as the A and also in trying to make the legs engage the socket better it has become broken not the one in the photo, its the T0-18 cased one. I can't see why the base leg would be soldered and leave the other legs pushed into badly fitting sockets, makes no sense to me. Surely if these transistors were prone to failure etc, that they would have had all legs socketed to allow for easy replacement?

Personally I prefer to determine which component is faulty before starting to replace components in the hope they might make a difference. Start at the BNC socket and measure the voltages along the signal chain; RTexcellentM.

[Specmaster]

I've decided to replace both these transistors as the B channel is not the same model as the A and also in trying to make the legs engage the socket better it has become broken not the one in the photo, its the T0-18 cased one. I can't see why the base leg would be soldered and leave the other legs pushed into badly fitting sockets, makes no sense to me. Surely if these transistors were prone to failure etc, that they would have had all legs socketed to allow for easy replacement?

Personally I prefer to determine which component is faulty before starting to replace components in the hope they might make a difference. Start at the BNC socket and measure the voltages along the signal chain; RTexcellentM.

The reason I have done it this way is because as you will see from the schematic Q1 and Q3 are the start of the chain and with Q3 already being of the wrong type and broken, there is little to test, so my logic is to correct what I can see as faulty, and as it is effectively the start of the active devices, it will affect everything else downstream from that point? I can't believe that a resistor or cam switch on both channels would fail at the same time, but both of these transistors can be demonstrated to have either no connection or bad connections, depending on which channel you're checking?

[tggzzz]

I've decided to replace both these transistors as the B channel is not the same model as the A and also in trying to make the legs engage the socket better it has become broken not the one in the photo, its the T0-18 cased one. I can't see why the base leg would be soldered and leave the other legs pushed into badly fitting sockets, makes no sense to me. Surely if these transistors were prone to failure etc, that they would have had all legs socketed to allow for easy replacement?

Personally I prefer to determine which component is faulty before starting to replace components in the hope they might make a difference. Start at the BNC socket and measure the voltages along the signal chain; RTexcellentM.

The reason I have done it this way is because as you will see from the schematic Q1 and Q3 are the start of the chain and with Q3 already being of the wrong type and broken, there is little to test, so my logic is to correct what I can see as faulty, and as it is effectively the start of the active devices, it will affect everything else downstream from that point? I can't believe that a resistor or cam switch on both channels would fail at the same time, but both of these transistors can be demonstrated to have either no connection or bad connections, depending on which channel you're checking?

If there are bad connections, fix that. But you said it was working earlier, so why has the connection become bad? (Q1 & Q3 don't need to be matched; they are independent emitter followers)

If you apply 1V to the BNC, what's the voltage at Q2A gate (or Q1 base) and TP9 for various input attenuator switch positions? Do the voltages vary as expected when you change the input voltage to 2V?

[Specmaster]

I'll check that out later but perhaps the reason for the failure is that I did go over the preamplifier stage with a multimeter checking for any caps short circuiting on the ohms scale and testing connections between any plugs/sockets so I must have disturbed these transistors in the process?

It is clearly something that I've done to cause it to fail. I have not removed the board or poking around the panel controls at all but I did check the caps on the component side of the board.

[tggzzz]

I'll check that out later but perhaps the reason for the failure is that I did go over the preamplifier stage with a multimeter checking for any caps short circuiting on the ohms scale and testing connections between any plugs/sockets so I must have disturbed these transistors in the process?

It is clearly something that I've done to cause it to fail. I have not removed the board or poking around the panel controls at all but I did check the caps on the component side of the board.

Randomly checking for caps shorting is something that should only be done after you've observed a specific signal voltage that doesn't vary in a way that might be expected if a cap has shorted.

What's the maximum voltage your multimeter outputs when testing components? How does that compare with the Q2 FETs' max ratings? Could ESD have damaged the FETs. They might be a pain to replace, since they are a dual matched FET.

[tggzzz]

If you apply 1V to the BNC, what's the voltage at Q2A gate (or Q1 base) and TP9 for various input attenuator switch positions? Do the voltages vary as expected when you change the input voltage to 2V?

I've just got out my copy of the service manual, and p8-18 (the one before the schematic you've included) describes how to setup that test.

It is a good manual; use it.

[Specmaster]

Using another scope to observe the waveforms? I thought that page was about checking the calibration?

[tggzzz]

Using another scope to observe the waveforms? I thought that page was about checking the calibration?

Use your imagination, to glean all the info you can; it is telling you what the circuit does when it is working correctly. What more do you need?

Reduce the frequency to 0.01Hz, and use a multimeter.

[Specmaster]

Ok, heres the latest, ordered up some replacements MPSH81 transistors as Q1 has been replaced at some point with another that the part number is not quite readable but looks like BVC 96305 (metal can type TO-18) and the correct one should be a TO-92 package. Anyway, I'm unable to find out any data on it, plus it did not seem to be making good contact with the socket tubes, partly because the legs had been screwed round to try get them in the correct order to match the circuit.

Checking the pre-amplifier board in more detail against the schematic that there was a 12v +&- supply that was not apparent on the power board. This is derived from the +&- 15v rails on the pre-amp board and the -12v was reading -1.89v caused through Q17 (2N3134) deciding that it has more in common with a diode than a transistor. It is listed as obsolete as are the equivalents that I have found for this.

Does anyone know of a suitable replacement , or a supplier at reasonable cost for this?

[bd139]

Assume this is just a TO5 canned pass transistor based on the application and characteristics (I haven't looked at the schematic). 2N2905A is a good sub for them. CVC sell them for a very reasonable £1.90 for two: http://www.ebay.co.uk/itm/262085968304

I just had a dig around in my bags of transistors and I've only got complimentary 2N2219's typically or I'd send you one. Need to buy more transistors!

Edit: just had a look at the SMM and a 2N2905A will be fine. There's only 2v across C-E junction by the look so it'd take a lot of current to blow that. Check that whatever is on the -12v rail is not pulling it down to -1.89v otherwise that will drop quite a few volts and go phut! In circumstances like this I tend to pull the transistor and stick some clip leads across the test points (TP4 and TP3) and connect it to a bench supply with current limit and bring it up slowly just to make sure it's not a problem further up. Otherwise you end up blowing a lot of silicon fuses very quickly. This isn't heavily loaded by the looks so whatever blew it may be present still.

[Specmaster]

I did a check with my multimeter and the resistance on both +12 with the other complementary transistor in circuit is the same as the -12 with the duff transistor pulled, 660 -ohms, so there's no problem on the remaining circuit except for the fact that I have pulled out the BVC one ready for the new one to be fitted when they arrive next week.

I had already checked with CVC as they list the 2N3134 for £4.50 BIN or offer plus £2.50 economy postage. Seeing as they are only 16 miles from me I offered £3 and collection, they countered at £4.50 and collection which I accepted and paid. Checked my Paypal receipt and they still took postage!
I messaged them about a refund of postage, if they wont, then I'll ask for a couple 2N2905A collected at the same time, otherwise they can cancel the order and I'll somewhere else.

Thanks for your very kind efforts.

[bd139]

They're pretty good fighters in that department. I spent a week arguing with them over a single FET

Good luck

[tggzzz]

...Seeing as they are only 16 miles from me I offered...

When I'm not sure whether to have something posted, I calculate the travel cost as 20p/mile. That's ~12p/mile for petrol, and 8p/mile for depreciation and insurance. On that basis, 32 miles = £6.40.

[CJay]

...Seeing as they are only 16 miles from me I offered...

When I'm not sure whether to have something posted, I calculate the travel cost as 20p/mile. That's ~12p/mile for petrol, and 8p/mile for depreciation and insurance. On that basis, 32 miles = £6.40.

Yeah, that and the cost of the 'ooh, that's handy' factor which always doubles the spend when you visit somewhere and see their stock.

[Specmaster]

...Seeing as they are only 16 miles from me I offered...

When I'm not sure whether to have something posted, I calculate the travel cost as 20p/mile. That's ~12p/mile for petrol, and 8p/mile for depreciation and insurance. On that basis, 32 miles = £6.40.

True, but economy, that could take 5 working days, I wont to crack on with it, bad enough waiting for the MPSH81's coming from Hong Kong.

I got to thinking that maybe I did this when testing for shorted tants using the Keithley 160 so I did voltage check on it in the resistance ranges and I can say that wasn't the cause of the failing, it only gives out on the lowest range 1.3V and on the 1000M range, a staggering low value of 1.4mV. This then got me thinking out of the box and checked my other bench meters and the results are amazing.

Kikusui 1502              Low range 430mV   High range 215mV

Iso Tech IDM-203       Low range 400mV   High range 200mV

Philips PM2521           Low range  3.8V      Auto  1V

HP 3466A                   Low range 4.6V      Auto 1.1V

So the Iso Tech and the Kikusui seem to be  the best ones in circuit testing.

I wonder how the Fluke 8505A will compare, when it finally arrives, it still sits in Heathrow according to the tracking info.

[Specmaster]

...Seeing as they are only 16 miles from me I offered...

When I'm not sure whether to have something posted, I calculate the travel cost as 20p/mile. That's ~12p/mile for petrol, and 8p/mile for depreciation and insurance. On that basis, 32 miles = £6.40.

Yeah, that and the cost of the 'ooh, that's handy' factor which always doubles the spend when you visit somewhere and see their stock.

To be honest, thats one of the reasons I like to collect if possible, very often these places will have special deals in the shops / stores that they can't advertise because of limited stock etc. Always a chance of picking up a reaL bargain while there. 

[Specmaster]

She lives again, Q1 replaced to remove the incorrect and unidentifiable transistor as shown in q1 picture which had its legs criss crossing in all directions to try and make it fit into the socket and was failing most of the time and also replaced Q17 which reduced the -15V to -12V, or should I say it was supposed to but I got 1.89V on test.

The main power supply rails were rock solid and that was confusing me and after much head scratching and searching about on the various boards I noticed that there was a particular requirement for a + and - 12V rail that was only applicable to vertical section and thats when the lack of the -12V was detected.

The calibration is a mile out however so I'll tackle that another day, but to give you an idea, 1v test square wave will give a 1 Cm division, not on the 1V as expected but on the .1V on both channels so it looks like gain issues there.

I still have the thorny problem however with the undoing of the grub screw on the delay timebase shaft, the actual knob has broken away from the brass boss. This needs to be resolved and also a new knob as well. I suppose in the mean time until I can get it sorted I could use it without, as if it never had a delay feature.

[tautech]

The calibration is a mile out however so I'll tackle that another day, but to give you an idea, 1v test square wave will give a 1 Cm division, not on the 1V as expected but on the .1V on both channels so it looks like gain issues there.

Which is what you'd expect when using 10:1 probes, are you ?

[Specmaster]

Yes I was, I was so engrossed with the repair that I forget scope basics   

[tautech]

Yes I was, I was so engrossed with the repair that I forget scope basics 

Not uncommon user mistake....fingers working faster than brain. 

So really it mightn't be far out of Cal at all, if everything's OK then you only need to verify accuracy not a full Cal that can lead to more gear needed that you probably won't need again.
Traditionally voltage, frequency and mark space references are needed but these can be sourced from a modern AWG albeit not to exacting standards but good enough for the hobbyist.

[Specmaster]

Channel B seems to be OK so far but I'll do more tests tomorrow. Channel A on the other hand displays the 1v test signal as a perfect square wave with input set to .2V and display uses 5 divisions which is spot on but moving onto .5V, 1V etc the trace changes as can be seen in pics. I'm guessing this is a capacitance issue?

Now I know how to take the time base shaft out, I could perhaps remove the shaft and drill the grub screw out if I can then source a replacement knob I could then reinstate the time base knobs (just have to suss out how the knobs react to each with the rachet type of unit that sits between the main and delayed TB knobs. I'll be doing more with this tomorrow.

[tautech]

AC coupling. 

[Specmaster]

AC coupling. 

Yes both channels are set on AC coupling?

[tautech]

AC coupling. 

Yes both channels are set on AC coupling?

This one's not:

[tggzzz]

Traditionally voltage, frequency and mark space references are needed but these can be sourced from a modern AWG albeit not to exacting standards but good enough for the hobbyist.

It is worth comparing the spec of an AWG+DMM with the scope's spec. Which do you think will have wider tolerances?

[Specmaster]

It is, the control is on AC coupling, volts/division is on .2 so the screen shows 5 divisions from the 1v test point, which shows the vertical calibration to ok.

[tautech]

Traditionally voltage, frequency and mark space references are needed but these can be sourced from a modern AWG albeit not to exacting standards but good enough for the hobbyist.

It is worth comparing the spec of an AWG+DMM with the scope's spec. Which do you think will have wider tolerances?

Scope of course. We are talking about CRO's aren't we ?

[Specmaster]

Traditionally voltage, frequency and mark space references are needed but these can be sourced from a modern AWG albeit not to exacting standards but good enough for the hobbyist.

It is worth comparing the spec of an AWG+DMM with the scope's spec. Which do you think will have wider tolerances?

The scope should have the wider tolerance I would have thought?

If you're referring to calibrating it using a AWG & DMM, my AWG is only capable of 25Mhz.  I do however have a Heath kit oscilloscope calibrator.

[tggzzz]

Traditionally voltage, frequency and mark space references are needed but these can be sourced from a modern AWG albeit not to exacting standards but good enough for the hobbyist.

It is worth comparing the spec of an AWG+DMM with the scope's spec. Which do you think will have wider tolerances?

Scope of course. We are talking about CRO's aren't we ?

Precisely - and by a good margin too. Hence I think an AWG would be more than sufficient for any such use, not just hobbyist use.

[tautech]

It is, the control is on AC coupling, volts/division is on .2 so the screen shows 5 divisions from the 1v test point, which shows the vertical calibration to ok.

OK, so the 0V position (no waveform) is where ? In the middle (0V) position on the display ?

If they're all AC coupled then only the first image (0.2V/div) is OK. At .5 and 1V/div there's something wrong.
Houston, we have a problem.

Try the same with DC coupling....which is mostly what one uses anyway.

BTW for amplitude checks I'd just use a known DC source, the SM specifies what's needed.
BTW 2, you shouldn't use the probe cal for amplitude or frequency checks and especially not for internal adjustments, why, well put a counter and another (in cal) scope on it and you'll see why. Probe cal is only for the fast square edge to adjust probes with, the frequency and amplitude are only set to nominal values, albeit that in some scopes it can be adjusted in both amplitude and frequency close to intended spec.

[tautech]

If you're referring to calibrating it using a AWG & DMM, my AWG is only capable of 25Mhz.  I do however have a Heath kit oscilloscope calibrator.

Of greatest value is mark space checks, these are for the timebase accuracy...the part where a scope has its most precision. Ideally you want one that can go close to the BW of your scope in Pulse mode were very short markers at defined periods allow checks of timebase accuracy and subsequent adjustments ...or not.

I used a term a few posts back....verify. That's all I'd do for now as it's probably bugger all out.

Watch this to see Dave do a few minor tweaks on this Hameg:
https://www.eevblog.com/2013/08/03/eevblog-502-19-hameg-analog-oscilloscope/

[Specmaster]

With it set on DC coupling there is a significant offset of the display on both channels as you would expect.

With coupling set to ground then the trace position will vary according the type of coupling, AC with square wave centralized vertically then the trace is on the centre 0 line, on DC the trace when set to ground is on the lowest part of square wave, again as I would expect.

All of the photos are of channel A, the 1st is with volt/div on 0.2V, 2nd photo is with v/d on 0.5V and 3rd is v/d on 1V. The voltage at the test point has been verified with a couple of DMMs to 1V so the screen display is about correct in that case as far amplitude goes.

[tautech]

With it set on DC coupling there is a significant offset of the display on both channels as you would expect.

With coupling set to ground then the trace position will vary according the type of coupling, AC with square wave centralized vertically then the trace is on the centre 0 line, on DC the trace when set to ground is on the lowest part of square wave, again as I would expect.

All of the photos are of channel A, the 1st is with volt/div on 0.2V, 2nd photo is with v/d on 0.5V and 3rd is v/d on 1V. The voltage at the test point has been verified with a couple of DMMs to 1V so the screen display is about correct in that case as far amplitude goes.

That all sounds good but the 0.5V and 1V/div waveforms seem wrong even for AC coupling but it's a while since I used a 1740 or any CRO for that matter. Am I that out of practice ? 

Have you found timebase knobs ?
Sphere wreck these, they must have some tucked away for sure.

[Specmaster]

With it set on DC coupling there is a significant offset of the display on both channels as you would expect.

With coupling set to ground then the trace position will vary according the type of coupling, AC with square wave centralized vertically then the trace is on the centre 0 line, on DC the trace when set to ground is on the lowest part of square wave, again as I would expect.

All of the photos are of channel A, the 1st is with volt/div on 0.2V, 2nd photo is with v/d on 0.5V and 3rd is v/d on 1V. The voltage at the test point has been verified with a couple of DMMs to 1V so the screen display is about correct in that case as far amplitude goes.

That all sounds good but the 0.5V and 1V/div waveforms seem wrong even for AC coupling but it's a while since I used a 1740 or any CRO for that matter. Am I that out of practice ? 

Have you found timebase knobs ?
Sphere wreck these, they must have some tucked away for sure.

No you're not out of practise as these 4 shots will show you, the 1st is B channel DC @ 0.5V, 2nd is B channel DC @ 1V, 3rd is A channel DC @ 0.5V and the 4th is A channel DC @ 1V. This time I have taken the whole fascia so you can see which channel is in use and the settings of the Volt/Div and the coupling mode. Like I said, B channel does seem to be to working almost spot on as far as this test goes, there is some movement on the screen between settings which I can eradicate later by doing the calibration procedure as there are different gain settings etc for different ranges internally that I can tweak.

Whats puzzling me is the odd waveforms on channel A, regardless of the coupling mode, which appear on the higher voltage inputs.

I have seen that video by Dave before but the only adjustment he made was to the trace rotation, the rest was cleaning of contacts and repairing the Auto/Normal trigger button and the tightening, although he did not show it on screen of the vertical verniers as both had become inoperable and he pulled out the B channel knob and shaft. Thats something I can relate to with Hameg scopes as I have a HM408-1A that I'm attempting to repair (both of those had come undone due to broken flexible couplings breaking) but that is a little bit more of a sick puppy compared to the 1740A.

[Specmaster]

No I have yet found a source of the larger of the 2 time base knobs yet, that is the delayed one that the shown brass boss has become detached from, infact the actual knob split into two while trying to undo the bloody grub screw and I just cannot find a screw extractor that goes down small enough to get a bite on the screw head.

I have heard people mention Sphere before but I have no idea where they are located or contact details for them, do have any information on them please?

I really do appreciate your help with this as well by the way, I'd be much further away from completion without your assistance thats for sure. 

[bd139]

Sphere is a bit of a buggering. They’re in Canada. The last thing I got quoted by them which was an HV multiplier for a Tek 465b it was cheaper to buy another unit off eBay for £70 by the time you add shipping, import and Royal Mail to it. The entire scope worked better than the one I was repairing so parts mule duty got swapped over.

This is why when I buy a scope I buy two of them

[Specmaster]

Sphere is a bit of a buggering. They’re in Canada. The last thing I got quoted by them which was an HV multiplier for a Tek 465b it was cheaper to buy another unit off eBay for £70 by the time you add shipping, import and Royal Mail to it. The entire scope worked better than the one I was repairing so parts mule duty got swapped over.

This is why when I buy a scope I buy two of them

I can't buy 2 of them, they are lining up on my workshop (boxroom) floor already and the Hameg (also sick) is in my sons bedroom, SWMBO would be showing me the door if was to a buy another with first getting rid of at least one of the others 

[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.

[bd139]

It's a trade off really. £30+fuel for parts stock worst case. £30+fuel for working scope with trivial problem best case. Usually it lands somewhere in the middle.

The timebase control looks in relatively good condition however. You can tell a munted one because the first thing to break is the vertical channel knobs. I've seen them damaged on a lot of HP scopes I didn't bother bidding on. I can't see any bent knobs in the picture. In fact it looks like all the knob caps are even present which is a rarity for HP kit.

The tape holding it together looks like someone lost the screws which means it may have had it's innards mucked around with.

It's probably cheapest way to get spares though. It's literally a 50/50.

If that was a Tek 475 I'd probably buy it for the spares.

[tggzzz]

It's a trade off really. £30+fuel for parts stock worst case. £30+fuel for working scope with trivial problem best case. Usually it lands somewhere in the middle.

The timebase control looks in relatively good condition however. You can tell a munted one because the first thing to break is the vertical channel knobs. I've seen them damaged on a lot of HP scopes I didn't bother bidding on. I can't see any bent knobs in the picture. In fact it looks like all the knob caps are even present which is a rarity for HP kit.

The tape holding it together looks like someone lost the screws which means it may have had it's innards mucked around with.

It's probably cheapest way to get spares though. It's literally a 50/50.

If that was a Tek 475 I'd probably buy it for the spares.

All sensible, but be aware that at the end you might still have one almost-working 1740 (plus a working 1740). Only the TEA/GAS-afflicted purchaser can define what they might or night not do thereafter

[bd139]

Well that's obvious, you buy another one! 

[Specmaster]

Yep, I could end up with another working scope, minus the timebase knobs and shafts, because if they are good, they will be transplanted into this one here. Then it would be a case of buy another one to bring the second one into play and so it begins again, the circle of life. 

[Specmaster]

Just what the hell is up with these scopes, it seems almost everyone you come across that has the stupid little allen key grub screws on the knobs, that once you disturb them and take them off, they never ever bite back on the shaft correctly, especially the ones that a series of switches on them like the timebase for instance ARRGHHH 

[bd139]

They're a pain in the arse aren't they.

One thing that relieve the pain I found is one of these: http://uk.rs-online.com/web/p/screwdrivers/8759369/

Yes it's completely the wrong tool for the job but it works much better than the prescribed ones! 

[Specmaster]

They will fit into these allen key headed screws ok and you can make them bite onto the shaft again with that can you?

[bd139]

That’s correct. They bite on better than the Allen keys do as well. If the slot is stripped in the grub then you’ve got a battle on either way. I’ve got a few stripped ones out with these with some difficulty.

[Specmaster]

I reckon that I was spot on about that scope being 120V configured because I see that the seller has changed his wording
to read as
"Tested earlier in the day and it turned on, showing line, however when tested just now, would not turn on. Also slightly taped at side as case has seen better days!
Comes with 2 pin (US) power cable," notice the change from 2 pin Euro to 2 pin (US)?

If that is true, just how much internal damage has been done do you think. would any of the circuit boards and electronics survive being served twice their normal voltage? I suppose thinking about it, the damage might be contained to the voltage regulators and possibly the transformer?

At least he did change the warranty to include a 14 day refund in case I did discover anything that I was not expecting? 

Whatever he has done with it, it can't have damaged the parts that I'm currently primarily interested in   

 

[bd139]

Well two possibilities:

1. He's full of shit, didn't check the line voltage selector before he plugged it in. Probably killed it dead instantly. I doubt it showed a line; that's just fishing for a sale. In this circumstance, I've had a look at the power supply schematic. That probably took out the pass transistors at the very least and popped the fuse. If you're really lucky it didn't toast the transformer before it popped. The deadly bit is the 15v unregulated line. That floats around 20v. I doubt anything on it would take 40v. If the pass transistors went short then everything on the rail is toast. Probably still good as a parts mule however. If you want the mechanical parts and some of the components you're probably good as long as you pull and test them first.

2. There is also the possibility that he has no idea what he's talking about and it's a schuko plug and the fuse just popped because one of the PSU caps was duff. US mains cables don't just appear here in the UK randomly very often.

Best bet is get it home, check the line selector, fuse state, power supply board for physical damage, plug it in and see what happens.

If it's been blown up then raise a claim for £20 back. He's a new seller so won't want to shaft a basic rating.

[Specmaster]

Thats what I thought, assuming it has a fuse fitted and not been bypassed, that best case is its blown the fuse fast enough to prevent anything else occurring and the 15v unregulated supply just feeds the HV oscillator so any damage there will be contained to the HV anyway.

[tggzzz]

Almost forgot...

You'll get your HP1740 so that it is working, but the risetime is too slow on both channels - say 10ns whereas it should be 3.5ns. Do not start fiddling with pots, caps etc.

Instead disconnect it from the mains, wait for things to discharge, stick you hand right into the middle from several directions, and gently massage and fondle the delay line. Seriously.

Remove hand, then look at the risetime

[Specmaster]

Thanks for the tip, but currently rise time looks ok, but sometimes I can see the fly back line faintly. Still have that resistor to replace and calibrate as the vertical is out on some ranges, others there is offset to bring back into line and I think the timebase needs tweaking as well.

[oldway]

Almost forgot...

You'll get your HP1740 so that it is working, but the risetime is too slow on both channels - say 10ns whereas it should be 3.5ns. Do not start fiddling with pots, caps etc.

Instead disconnect it from the mains, wait for things to discharge, stick you hand right into the middle from several directions, and gently massage and fondle the delay line. Seriously.

Remove hand, then look at the risetime

http://www.hparchive.com/Bench_Briefs/HP-Bench-Briefs-1983-01-02.pdf

[bd139]

Interesting failure mode and solution.

[Specmaster]

Interesting failure mode and solution.

Can think of better things to fondle

Sent from my SM-J510FN using Tapatalk

[Specmaster]

Picked up the parts mule and first glance seems it is / was set for 240v so further investigation to see if it's in good shape or not. Might be better then the one I'm working on, soon find out though, I'll put it through the variac and bring up nice and gently.

Visually it is a right turd, all 8 case screws are missing, hence all the tape along the centre rail, all 4 feet are smashed off (seems par for the course with these HP scopes), it does however have the badge left on the fascia but boy, does it need cleaning badly?

It does however have the complete set of control knobs and interestingly, the screen/CRT appears to be a yellow one?

Lets see what happens when I take the covers off and look inside before applying power then 

[Specmaster]

So glad I took the cover off first, the seller claimed that when he switched it on, he got a line on the screen. The vertical output board is missing so is it at possible that he could have got a line with it?

The thing is full of wooly dust all over the voltage regulators on the back panel so its been in a garage or similar, stood on its base and the dirt has gotten in and settled, so tomorrow it goes out in the garden gets blown with an air pump.

Given that its not all and the fact that it was sold for spares or repair, do you reckon I could get a partial refund or would that cheeky?

[bd139]

It's possible he got a line. If the horizontal is working then it will sweep the beam across, but there will be no vertical control in the plates.

I'd try and hit him with a partial refund on the basis that "there is no way that could have ever shown a line" or something so even as spares/repair it was poorly described and dishonest. Go on about rot or something. I'd ask for £15 back myself. Works quite often.

Yellow tube might be long persistence. You might be able to shift that on ebay on it's own as they're quite rare. Check the PN on it (be careful with the HT!)

[Specmaster]

I paid him cash on collection though, I doubt that he would send me the £15 without me driving back to bloody Harpenden for it 

If the tube is rare is it worth swapping it over (assuming it works) or is it not worth the hassle?

[bd139]

Ah I'd skip it then. At least you got a healthy stock of parts out of it. I've paid a lot more for worse.

I wouldn't swap it TBH - requires a full recal if you change the tube. Someone might be interested however. I sold an HP tube a few years ago for £75 for a 180C scope which is very similar to the tubes in the 1740A (I think it was the only thing they kept between the models). Not sure you'll get that now but it might cover your outlay.

[Specmaster]

Yeh, that makes sense, well if I do sell it on Ebay, I'll leave it in the case then and then they can remove, not only that but it would the best form of protection in transit for it surely? I'll extract all the boards and other bits and bobs first though. I could also extract that input resistor I need complete with its shield and transplant it.

Already extracted the timebase knobs with their shafts and just about to perform an organ transplant with them into my working one after giving them a though clean up without separating of course to retain the integrity of the mechanism between main and delay controls   

[tggzzz]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

[Specmaster]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

I did, its the antique dealer in me looking for the "Real Deal" 

[tggzzz]

Simple question: did you get what you needed for a fair price? If so, don't worry; move on.

I did, its the antique dealer in me looking for the "Real Deal" 

When I was backpacking around India, many decades ago, I went to a market to get some food. Just outside the market I found a girl on the pavement with a few satsumas in front of her. I bargained with her to get 5 pieces for, IIRC, 2Rps (~2p).

About 5 mins late, while walking back to the hotel, I realised what I had done, and went back and gave her 5Rps. She was rather surprised and happy.

Lesson learned.

[Specmaster]

The CRT wasn't a different, it just had the blue diffuser missing from the front so I was looking directly at the CRT 

[bd139]

Ah well keep it

[Specmaster]

Donor timebase shafts and knobs, removed, cleaned up and refitted to working scope, nice and easy. Now to read the manual and calibrate it a bit as it is slightly out on some ranges and also the flyback can be seen on certain ranges so perhaps tweaking here and there may cure it. After that its a clean up of the fascia and I reckon after that its a keeper. 

[bd139]

Nice work!

[Specmaster]

Nice work to you as well because you and a few others also helped with loads of useful advice so it was teamwork.

[Specmaster]

Murphy has struck again, replaced the 990k input resistor on Ch A, all was working nicely, perfect square wave etc on both channels volts/div was reading spot on, nice steady trace, so put covers back on the night and switched off for dinner.

Came back after dinner, and thought that maybe I'd look at the offset on some ranges so I switched on and no trace, pushing beam find button had no effect either, using the vertical position controls then operate as screen illumination controls and absolutely no horizontal activity at all.

Back to basics reveals that the power rail -15V is down to 1V, pull of the A14 power bridge board and power on, check -15V and its back. Replace A14 and power, -15V is still up, check the 12v rails, both are fine then trace is back again and is looking fine .... Sweep seems to be working just fine again ARRGHHHH. 

I'm pulling handfuls of hair out wondering just WTF is going on here, do I have a Tant playing up on the -15v rail?? 

[bd139]

That’s an interesting one. Possibly. However 1v is suspiciously high for a tant failure. When mine went, it dropped to about 400mV at the output of the regulator all three times one went.

I think if it’s working now it’s probably an idea to run it for a few minutes, power down and disconnect line then have a feel of the caps on the power supply board and see if anything is getting warm.

[tautech]

Don't forget about checking for failed bridge rectifiers like the one Dave did a vid on.

[Specmaster]

Seem to have a dodgy contact somewhere around the timebase switch boards bt wriggling and push pulling on the delay timebase control, sometimes the trace will go off or come on and sometimes the trace brilliance level changes. Can also repeat this by using the well known and scientific method of tapping the case sharply with an instrument can cause the display to flicker, go dim or bright.

[bd139]

Sounds like the switch needs dismantling and cleaning. The switches on these are notorious for swing dicky. The one I had dropped out if you pushed the timebase knob.

[tggzzz]

The timebase switches are easy to dismantle, but make sure you don't lose any of the internal fingers.

Carefully check the PCB tracks where they leave the switch: the edge of the switch abrades the tracks. If that has happened, simply jumper across the gap with a single piece of thin wire, being careful not to allow solder to creep onto the switch contacts.

I was not amused when I saw that switch design.

[CJay]

That’s an interesting one. Possibly. However 1v is suspiciously high for a tant failure. When mine went, it dropped to about 400mV at the output of the regulator all three times one went.

Just be careful around Tants, always have  colleague still has the scar on his thumb from when one went off, he was prodding around a power supply that was 'singing' as the load changed, he worked out that if he touched this particular tantalum cap the singing changed.

Unfortunately it spontaneously disassembled itself when he prodded it to demonstrate the source of the noise to another colleague.

[bd139]

Yes they're pretty vicious if you get them in the wrong way round. Make sure you check the polarity at least 5 times!

[Specmaster]

Lesson learned the easy way, thanks guys, if I feel the need to check on tantalum caps for temperature I think I'll stick to the good old infra red thermometer, not a bad thing anyway especially on HV circuits.

[Specmaster]

Update.
Certainly glad I got that parts scope, took the time base boards from it, cleaned up the board as it was carpeted with filth, almost as deep as some carpets to  took the switches apart and gave them a really cleaning with Isopropanol, checked for wear on the tracks and also where the traces meet the switch contacts, looked like they have seen very little wear, reassembled the switches and swapped the boards over.

I now have a nice and steady trace with no hint of a bad connection, when the tap test is done, trace remains

[bd139]

Excellent - good news 

[Specmaster]

 :wtf:It went off again

[bd139]

It's only doing that because it wants some attention. Sounds like something is failing. I bet it is tants or the filter caps.

[Specmaster]

Update.
Since it went, I left it off for a few minutes, upon switching on, everything was back to normal and has now been running OK for about 90 minutes so I tempted fate and switched off and then on again, no trace and no trigger and no UNCAL light on the timebase despite the vernier being in the UNCAL position.

Switched off for about 5 seconds and back on and everything is working fine and is still working fine after 15 minutes. Its as if there was a built in timer to prevent the scope from working straight away following a switch off and on again without giving it a few seconds delay? 

None of my other scopes behave in this fashion, any ideas?

[Specmaster]

It's only doing that because it wants some attention. Sounds like something is failing. I bet it is tants or the filter caps.

The supply rails all measure within spec when the display goes.

[bd139]

Are any LEDs still on?

If they are then it's likely the HT side of things. Check the oscillator is running when it goes.

[Specmaster]

All the leds are on with the exception of sweep UNCAL one. The two leds for the input verniers work just fine.

[Specmaster]

Oh, I forgot to mention, the 2 timebase boards I took from the parts mule, cleaned up etc, when I put then in, the flyback lines that were visible on certain settings have all disappeared and I noticed that they had been partially recapped at some point before being left to gather dust, all neglected and left alone in the corner

[Specmaster]

Are any LEDs still on?

If they are then it's likely the HT side of things. Check the oscillator is running when it goes.

I'm not too sure how to do that. Are you talking about one in the HV box above the PSU? I don't have anything to measure high voltages.

[tggzzz]

Switched off for about 5 seconds and back on and everything is working fine and is still working fine after 15 minutes. Its as if there was a built in timer to prevent the scope from working straight away following a switch off and on again without giving it a few seconds delay? 

I had exactly that phenomenon in a Tek 485. It was a b*gg*r to find since I could only do one test every 6 hours. The switching PSU needed to be kickstarted into operation, and that was done by a small inconspicuous electrolytic capacitor. Replacing that permanently cured the problem.

It looks like the only part of the 1740 drive by the SMPS is the CRT, so you could test that hypothesis by looking at the Y-output while it is not working.

[bd139]

You shouldn't need to touch the HT itself to see if it's running or not. I don't know where TP3 is in the following diagram but popping a scope on that should indicate if it's running or not. That is the primary side so is peaking at 15v.

You should be able to work back from that to the oscillator and/or feedback or the HT side of things:

[Specmaster]

Ok, I'll give it go if it happens again, switched it off at the moment while I have my dinner. TP3 is in the HV enclosure, its within the dotted lined area which is the HV enclosure. If its working I should get a sawtooth wave, if not, check C1?

[bd139]

Mot sure what sort of wave you will get, probably a bastardised square wave of some description on that. Main thing thing is to check it is actually doing something. 

C1 is possible.

[Specmaster]

The manual shows a triangle wave at TP3, scope to be set at 10v/div and 10uS/div.

[tautech]

You shouldn't need to touch the HT itself to see if it's running or not. I don't know where TP3 is in the following diagram but popping a scope on that should indicate if it's running or not. That is the primary side so is peaking at 15v.

The simplest way to check if the EHT oscillator is working is to wave a probe close to it, ahla near field style.
They run at 20-40 KHz. The feedback circuitry can give problems as can the secondary side caps and DC restorer circuit. In a CRO it's one of the areas under some stress, all the high value resistors should be checked, they drift and can go OC.

[Specmaster]

Oscillator is not running when the display vanishes, TP3 is not marked on the board at all but there is a testing post that is unlabelled and visually tracing the circuit through, that is indeed TP3. TP1 is a 3Kv one accessible with a probe though a rubber grommit but no point in probing that with my AVO as just holding a scope probe near the transformer confirms when it is running without any need  for direct contact to be made.

I have just noticed that there is occasionally a light buzzing sound emanating that quarter of the scope, it could be I suppose transformer laminations buzzing at times. I turned off the lights in nicely darkened room and looked for any sign of tracking / arcing when the sound is audible but nothing is visible.

[Specmaster]

Update.
Been checking the HV board & it is slightly different to the schematic as it is one of the areas that they made changes to and the thing that I have discovered is that Q1 and Q2 checked while in circuit with a transistor checker are reported as double diodes? Q3 seems OK but on a quick check earlier tonight to see if it was still firing up, shows that there is no HT at all at the moment, all the LEDs light up ok. I'll check this tomorrow evening and make sure I have not accidently disconnected anything.

[bd139]

Ok next thing to do is check the HV disable isn't jammed on. If you check the collector of Q1 that shouldn't be close to ground. HV disable shuts the whole show down and kills the oscillator. If there are any out of spec or dodgy components (Q1/Q2/R30-R32) it'll just shut itself down instantly. If it's ok, next thing is check the feedback loop. Debugging this can be a dick but it's great fun at the same time

I'm starting to wonder why I do this because this week I have spent the entire week debugging hideously broken asynchronous threading stuff built by the lowest bidder so fixing scopes is taking repair work home 

[Specmaster]

Why does a transistor tester indicate that Q1 and Q2  are double diodes, I just don't understand why that should be?

[bd139]

Did you check them in or out of circuit?

If it’s one of those Chinese transistor testers they can be right dumbasses sometimes. That’s what a transistor looks like to a naive algorithm.

[Specmaster]

In circuit tested ,same for Q3 which showed up as a transistor.

[tautech]

Yep, testing active components in circuit can be a crap shoot even with the best of gear.

[bd139]

Yep. You should be able to spot if it’s working by measuring voltages in circuit.

[Specmaster]

Ok, just ran a quick test (have to take Mother in Law out shortly to East London), switched it on there was this light buzzing as mentioned earlier coming from the rear quarter, took all the covers off, the oscillator is not running and the buzzing is definitely coming from mains transformer, sounds different to most transformer hums, maybe because this one is fully screened?

Checked main power rails, all present and correct, will probe the primary side of the HV section later today on my return.

[bd139]

If it's buzzing it's under load which suggests a short somewhere.

I'm wondering if this is something else. Check the rails on the HT board coming in as there's an opamp on there if I remember correctly.

[Specmaster]

If its got a short then its going to drag one or more of the rails down, but they are all spot on within 0.1V of their specs?

[bd139]

Depends if there is any decoupling on the rails. If there is a tant after a resistor then the resistor will just get hot

[Specmaster]

Just got back and checked the voltages in the HV section and in doing so discovered that the schematic for that board has been changed but fully documented, attached is the schematic for that board and is the only one that I can find on line. I purchased a manual from Artekmedia and they have made impossible to print a page as a pdf or to copy a page and that manual gives some of the changes but the schematic and board overlay is still the old version so I have taken a photo of the listed mods.

All the incoming supplies to the HV section are on spec and the voltages measured on Q1 and Q2 are as follows

Q2
Base         +16.8v
Emitter     +15v
Collector    0v

Q1
Base         0v
Emitter     0v
Collector    +7.27v
 

[Specmaster]

Just got back and checked the voltages in the HV section and in doing so discovered that the schematic for that board has been changed but fully documented, attached is the schematic for that board and is the only one that I can find on line. I purchased a manual from Artekmedia and they have made impossible to print a page as a pdf or to copy a page and that manual gives some of the changes but the schematic and board overlay is still the old version so I have taken a photo of the listed mods.

All the incoming supplies to the HV section are on spec and the voltages measured on Q1 and Q2 are as follows

Q2
Base         +16.8v
Emitter     +15v
Collector    0v

Q1
Base         0v
Emitter     0v
Collector    +7.27v

Oops forgot to post the drawings I will send them when I get home again.

Sent from my SM-J510FN using Tapatalk

[bd139]

I hate it when people send protected PDFs. Quick tip:

1. Open it in Google Chrome.
2. Print it to a PDF.
3. All copy/paste/print protection gone!

If that doesn't work, qpdf (available on sourceforge) does it.

[Specmaster]

I hate it when people send protected PDFs. Quick tip:

1. Open it in Google Chrome.
2. Print it to a PDF.
3. All copy/paste/print protection gone!

If that doesn't work, qpdf (available on sourceforge) does it.

Me to so annoying, I'll try that out later, good tip, thanks [emoji3]

Sent from my SM-J510FN using Tapatalk

[Specmaster]

The pdf's are in the post

[bd139]

Cool. I reckon Q2 is wonky. Should be max 0.6-0.7v between base/emitter.

Looks like they added some hysteresis to the old design based on the Zener and it’s getting stuck in one state.

[Specmaster]

Thanks, I'll have a look and see if I have an equivalent that I could try.

[bd139]

Make sure you have two because if you replace it and it blows up there’s another problem

[Specmaster]

I've got a bulk lot of transistors as a experimenters kit where you typically get something like 50 of a type, so I probably have about 1,000 small power / signal T0-92 type transistors

[Specmaster]

The BOM shows the HP part number for Q2 as being 1853-0066 (PNP SI PD=200mw) and that comes as being an equivalent to a 2N3906 (I have plenty) which translates to a NTE159 which has a PD of upto 625mw.

Seems to fit the bill according to the data sheet, lets give it a burl shall we?

[bd139]

2n3906 is the part that came to mind looking at the schematic. Go for it.

[Specmaster]

Q2 replaced, no difference. the volts are the same and no oscillator running.

[bd139]

Ok that eliminated then! Is anything happening on the feedback winding of the transformer?

[Specmaster]

In what way do you mean, the voltage I'm reading on the feedback winding before CR1 is 7.9v and after its 7.26 which is also being seen on the base of Q3 and collector of Q1.
Is the feedback circuit shutting down Q3 to stop it oscillating? Looking at circuit, I take it that if the voltage on the base of Q3 that normal service should return?

[Specmaster]

The feedback circuit should only kick in if the 120v rail drops to 100v, but it currently sits at 122v

[bd139]

Is it at 122v when it fails?

Unfortunately had to take SWMBO out this evening so haven’t had time to play or look at this

[Specmaster]

Yes it is at 122v even when it fails (I take you mean, no HT)

Gone back to basics again as still not getting any HT despite the voltages supplying the board are well within their spec and also replacing Q2 to no avail.

Rail voltages on the main power supply, again all are well within the spec.

Started checking voltages on the pre-amplifier and input attenuation board and the supplies to the board are again within spec, but however the +5v rail is playing up. At TP2 I'm getting +5v but at TP6 +0.5v and at TP5 its +2.8v. previously these were all within spec. 

Will re-look at this area tomorrow but surely should have no effect on the HV not working or vice versa? 

[Specmaster]

Pre amp board voltages all look normal, after due consideration it was down to various switch positions and operating these switches restored the readings, so that was a rabbit hole that I followed far too long. However, as I always do as I go around checking, I always ensure that the plugs etc are firmly plugged in and I was noticing that the -15v rail was disappearing, even from the source, the power supply was reading as low -1v. Off I go again and discovered that the A14 bridge board was the culprit, somehow shorting out?

Backing this plug board off a bit from the boards it was connecting to restored the supply again??

Now once again all voltages are back to spec but not the HV, normal supply volts are still within the spec but can't get the oscillator to kick in again??

[Specmaster]

If I pull the HV board from the parts mule and swap them over, there's nothing to be worried about with regard to charged caps etc on the faulty board is there?

Its been disconnected for a couple of hours and beside the oscillator is not running so surely in reality they should discharged by now?

I've never done anything before that was higher than 415v so I'm just being cautious.

[bd139]

It’s worth shorting both ends of each of the Hv caps then each lead to the chassis. The connection between the multiplier and the board is the most likely to bite you.  While it most likely won’t kill you, it’s not something particularly fun

I do this, on tek scopes anyway, with a 4mm banana lead plugged into the earth hole in the front.

[Specmaster]

Ok, I chickened out on that for the time being, instead I have been trying to work out just how this oscillator is supposed to function and what starts it oscillating and what voltages on the base of Q3 enable the HT or disable the HT output and the how the OP amp interacts with it. I have to say regrettably I can't get my head around it, I'm still really sort of learning this again, it was about 50 years ago when I last did electronics so I've forgotten a lot. 

[bd139]

It’s very difficult to understand so I don’t blame you. I tend to extract bits of the circuits and put them in LTspice and see why they do what they do and divide the whole thing into little blocks. This takes hours. If you look in the picture of the bench above you will see about two hours worth of scribbling and thinking trying to work out something. To the left is a pile of paper stuffed under everything (like Bob Pease’s office) which contains scribblings

Oh also beer. Beer helps!

Not much help that but never give up

[Specmaster]

Yes I did notice, I have LTspice but yet to learn that as well  I have to many things on the go at anyone time to find the time but I guess I'm going to have to make some time.  That is how I thought it would be be of assistance to me by drawing up a circuit and then run it and observe voltages and waveforms etc without having to breadboard everything first. Not used my breadboards yet either 

Anyway, some good news late last night I popped the question about HV oscillator on the HP/Agilent group and someone confirmed that the voltages on the transistors is correct apart from the 7.2v on the base of the HV OSC Q1 which should be less then 1v. They reckoned that the socket on that transistor may not be making proper contact, hence the high voltage. I removed it and dosed it with contact cleaner, plugged it back together and unplugged, repeated it a few times to clean the pins and tried it. The oscillator is back running again and the traces are back on the screen. 

So now I can crack on with the calibration process and correct some of the biases that are present on some ranges 

Thank you for your sterling efforts and lovely tips, you never know, you might make an electronic engineer out of me yet 

Its now 2.20am so I'm off to bed. 

[Specmaster]

Trace on this is not correct, the total length is about 2 to 2.5 times the width of the screen on all ranges and if you pump a square wave into it and get it to display what you think is one cycle, move the horizontal position to the right and the left side is twice the length of the right side (when moving trace to the left on the horizontal position control) is there an internal control for this somewhere? Strikes me that HP seem to use a language all of their own, I mean since when has LINE meant power switch or ON/OFF??

I'll post a vid of this later if that helps to describe what I'm talking about, they say a picture says a 1,000 words, lets see how many a vid says shall we? 

[bd139]

Interesting. Possibly horizontal amplifier balance or problems with the 10x magnification. Definitely post a video.

LINE is American terminology. Everything I have is labelled POWER or just ON OFF.

[Specmaster]

Its nothing to do with the x10, on that time base setting all I get is one long half of a square wave if I press the x10. Yeah, the americans do call power, line and live, hot etc so I was wondering if they had labelled any of the internal adjustment trimmers by ambiguous names as well 

After dinner I'll take a vid and post it via a link to my flickr account.

[Specmaster]

Here is the link to the video which shows the odd width of the horizontal trace and the linearity of it and you can also see some of the flyback lines at times. On some of the slower sweeps they can be seen to double back behind the main trace, but this I think is due to the massive length of the trace in the first instance or I have I gotten that wrong??

https://www.flickr.com/photos/158191416@N07/24056629268/in/dateposted/
 

[bd139]

Interesting. Id concentrate on the fly back / blanking first. That might yield some other clues.

Are very slow speed sweeps linear or do they slow up or speed down during the sweep?

[Specmaster]

https://www.flickr.com/photos/158191416@N07/26134122199/in/dateposted/

This shows the slower sweeps and I don't think it varies during a sweep but it does show the flyback very clearly.

I have not replaced a single cap on this as yet, do think it is likely to a be high ESR or something else altogether.

All I have done so far is extensively clean the timebase switches, (stripped them down to parts, cleaned lubricated and reassembled, replaced those 2 burned resistors on the vertical amp output, repaired the +3V rectifier connections and the plug connection as well, replaced the 3 transistors and cleaned up the socket on the HV oscillator transistor.

Checked all caps on the beast for shorts to ground, also tested the ESR of power filter caps, remade and cleaned all connections at the plug / socket joints in case of bad connections, replaced the timebase control shafts and generally cleaned the whole scope up as it was a bit on the grubby side when I got it.

[bd139]

Ok definitely hit blanking next. The time distortion might be an artefact of that. I think blanking is the most common scope breakage so fingers crossed should be relatively ok to debug.

[Specmaster]

Thanks, will do.

Looking at my scopes tonight, its easy to see how the inputs can get toasted as they don't all have the same capabilities, the Hitachi V-252, Rapid 7025A and the Goldstar OS-9100D only go to 5V per division, the Iwatsu SS-5710 goes to 10V and the HP 1740A up to 20V. So anyone used to working with a 1740A at high voltages who then has to jump onto any of the others could end up overloading the input stages by 2 or 4 times. There ought to be a standard for these things?

[Specmaster]

Thanks, will do.

Looking at my scopes tonight, its easy to see how the inputs can get toasted as they don't all have the same capabilities, the Hitachi V-252, Rapid 7025A and the Goldstar OS-9100D only go to 5V per division, the Iwatsu SS-5710 goes to 10V and the HP 1740A up to 20V. So anyone used to working with a 1740A at high voltages who then has to jump onto any of the others could end up overloading the input stages by 2 or 4 times. There ought to be a standard for these things?

[bd139]

It's possible that can happen easily. The Tektronix 465/475 have thin film attenuator modules which get toasted pretty quickly in this circumstance. Also difficult to get replacements for!

I'm slightly scared every time I put my Rigol near anything more than 100v as they are bastards to repair!

[qu1j0t3]

I'm watching this thread with interest as I have a 1703A, 1741A and 1746 with various problems (the 1741A is almost fully functional though).

I have a copy of the 1740A Operator's Guide (see attachment). If you PM me your address I can post it to you.

[Specmaster]

I'm watching this thread with interest as I have a 1703A, 1741A and 1746 with various problems (the 1741A is almost fully functional though).

I have a copy of the 1740A Operator's Guide (see attachment). If you PM me your address I can post it to you.

Thats very kind of you but I already have the owners / operators manual and also a copy of the workshop service manual. It was very nice of you to offer though. If I or anyone else here can assist you with getting yours up and running then I'm sure we will do our best.

[Specmaster]

Ok definitely hit blanking next. The time distortion might be an artefact of that. I think blanking is the most common scope breakage so fingers crossed should be relatively ok to debug.

I'm looking at the A7 board (Horizontal output) and I tried to remove the cooling hat for the transistor Q7 which actually came away with the transistor body and left the legs in the PCB 
So I need to replace this now and the manual shows this as being HP part number 1853-0380 and described it as Transistor PNP SI TO-92 PD=350mw which it clearly isn't as TO-92 is plastic encapsulated and Q7 is a T0-18. Got any ideas as to the equivalent for this?

I should also point out that this is not from the from but the parts one I got and I'm going through those boards at the moment looking for obvious faults so I can perhaps try swapping them for the others to see if there is any improvement and then work on those ones.

Which of the boards would you attack first looking for the blanking issues, Gate Amplifier, Main Sweep, Horizontal sweep or Horizontal output. I cannot locate any reference to blanking in the manual at all but as it effects the horizontal I assume that it is part of one or more of the listed boards / sections.

[bd139]

Oh nice - I hate it when that happens. If you have another one on that parts mule, I'd try that first. If not it's forensics time. 1853-0380 comes up as an MPS-H81 for me which is a PNP 600MHz RF transistor. Suspicious!

Blanking usually works as follows. The horizontal timebase in a scope usually has the following states:

ARM -> TRIGGER -> UNBLANK -> SWEEP -> HOLDOFF -> BLANK -> FLYBACK in a loop.

The blanking might be marked up as "Z-axis" or intensity. When the scope is sweeping the intensity is high. When it's blanked the intensity is low. So I'd start by pumping a signal into the scope's external trigger and getting it to sweep on that. This is usually piped out to a z-axis amplifier from the timebase where it is combined with various signals from delayed timebase etc so there is a composite intensity signal. This is used to modulate one of the CRT grids to control brightness. So you want to start at the horizontal timebase and try and find the output intensity or blanking signal and follow that entire chain to the CRT. Usually if these are broken it tends to be the final transistor which controls the CRT grid or some resistors that are bonked or some of the cables between the two sections.

If you're sneaky you can actually use the scope to debug itself. Connect the cal output to ext trigger via a probe. Then connect the channel 1 and channel 2 amps to things inside the scope. Sneaky but it works. If it's on auto you can also trigger on the timebase as well.

[Specmaster]

Haha, it was the parts mule board that I was working on, thought that I could work my way through them, checking for faults like broken connections, duff resistors, caps etc and refurb them, then swap them over for the ones that are currently in the scope and take it from there. That way the blanking etc. might be resolved as I did each board, recapping etc as I went through them, at least that was the theory. 

The transistor most certainly is not a MPS-H81 as that one I already have replaced on the scope and have a spare, that is also a T0-92 package (nice one HP). I discovered that a few people have been asking the same thing on the web going back years and never got an answer to that either, so that board will not get refurbished just yet, thats for sure.

There is a Z axis input on the rear panel, never had occasion to use that on any of my scopes yet (never really understood what it did and what it was there for TBH).       

The timebase and trigger all sits on A7, A8 and A9 boards which is the area that I was suspecting for the odd length of the trace, all my other scopes are all on or very slightly longer than the 10 divisions on the screen but this one as you saw on the vid is more like 20+ divisions.

On a tantalum what frequency should they be tested at, my LCR meter has 100hz, 1Khz or 7.8Khz options and the choice of frequency will make a difference to the ESR value so the wrong choice would mean that a cap could be left in circuit that had a undesirably high ESR figure.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

[tggzzz]

On a tantalum what frequency should they be tested at, my LCR meter has 100hz, 1Khz or 7.8Khz options and the choice of frequency will make a difference to the ESR value so the wrong choice would mean that a cap could be left in circuit that had a undesirably high ESR figure.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               

For tants that are used as power rail decouplers, I wouldn't worry too much.

The simplest sanity check would be to make measurements of old and new tants, and see how much variation there is.

For tants used elsewhere, e.g. in the timebase sweep circuit, ESR might be more important. However, the capacitance vs DC bias voltage might be even more important, since that would affect the sweep linearity. (MLCC ceramic capacitors are astoundingly bad in that respect!)

[Specmaster]

I notice that on some parts of these schematics it gives a voltage rating into a section and another when the voltage leaves that area, for instance, it might show +15V on part of the circuit and at the end it reads +15VF, particularly on a custom hybrid it shows the supply rails as having the VF suffix. Does that mean the supply rail has been passed through an extra stage of filtering as the hybrid is susceptible to ripple etc? 

[bd139]

Genuinely have no idea at all. There’s usually a sheet of conventions etc before the schematics in HP manuals that may (or may not) have the answer.

With respect to tants they seem to have a binary failure mode I.e open or short circuit. The open ones are harder to find than the shorted ones. I like the shorted ones to be honest as long as they don’t blow up a power supply or something.

[tautech]

So I need to replace this now and the manual shows this as being HP part number 1853-0380 and described it as Transistor PNP SI TO-92 PD=350mw which it clearly isn't as TO-92 is plastic encapsulated and Q7 is a T0-18. Got any ideas as to the equivalent for this?

HP X references:
http://www.sphere.bc.ca/test/hpparts.html#cross

[Specmaster]

So I need to replace this now and the manual shows this as being HP part number 1853-0380 and described it as Transistor PNP SI TO-92 PD=350mw which it clearly isn't as TO-92 is plastic encapsulated and Q7 is a T0-18. Got any ideas as to the equivalent for this?

HP X references:
http://www.sphere.bc.ca/test/hpparts.html#cross

Thanks for this it has helped a bit. Why are these HP manuals so hard to fathom out?

Turned out I was looking for the wrong item as the Q7 transistor I was looking for is located on board A11 not A7 as previously thought, a glimmer of hope I thought and armed with the fact that the transistor bore the new shortened code of 3-0232, which means the HP part number is 1853-0232, yes, I thought I was in luck... Turns out it suffers the same fate, not listed as a spare or any known equivalents. Back to the search again maybe another dead 1740A may yield a working replacement but that can wait a while as it was only a spare board that I broke.

Oh well back to sorting out the blanking and timing issues.

[Specmaster]

Here's where we are at the moment. After reading the manual a bit more (heavy going) I thought before I start pulling boards and then pulling legs out on the parts in order to check their values without others interfering, I spotted a bit about timing controls on board A8 and so set up the calibrator and went through the adjustments.

Sweep length is still a bit long but that might be another adjustment and if I read the manual again, I may spot it. The traces are now much improved and are about 1hz out on the screen and if I can adjust the trace length then it should be possible in conjunction with the trimmers on A8 to get it within spec again. I'm getting the impression that someone has been in here before me and screwed the settings up,

Anyway, as a result of my tinkering tonight, the blanking seems to be working correctly, I get no hint of flyback lines or the trace doubling back on itself so it seems that we're getting there.

I have noticed however that while adjusting these trimmers, the A8 and also the backplane A7 flex a lot and on the slower sweeps the display can flicker and or at times grow dimmer. Gently tapping, or pulling A7 towards me (as I look at the right hand side with screen on the left) with a plastic prodder gives a temporary fix for this. Problem is that there are about 7 plug & sockets as well as 14 switches to be checked and heaps of solder joints that may have to remade to track it down.

So I think that my best plan now is to see if any of the linearity trimers on the horizontal output have any effect and then work from there.

Just as I was writing this report, I turned the scope off (been on for about 2 hours) and switched it back on again, no beam and no sweep vernier CAL light either. Switched it off and left it for approx a minute and switched on again, beam is back, CAL light is off so this may be a duff LED, and everything works as it did before switching off. Does the HV side have a thermal cutout? But why does not switch off when its on? It only fails if I switch it off and back on again?

More work to do another day thats for sure but the light at the end of the tunnel is glowing ever brighter. 

[bd139]

Sounding good! You'd be surprised how much people can f*ck up a scope just whacking the trimmers in it. Does my head in when you get one of them. I think my 465B has been played with in that way.

Unknown with the HV side of things. Might be the feedback network in the inverter that's doing it. I had a similar problem after power cycling on another thing I actually put together myself that happened randomly when it was turned on and off. Very annoying. Turned out to be not enough decoupling.

[Specmaster]

After a couple of days chasing rabbits down holes, I gave in and swapped out the A7 board (main trigger) for the one from the parts mule after I spent a few hours going over it with meters looking to see if I could detect anything out of spec and it all checked out as being reasonably OK.

Powered it up and I now have a trace that is just about there, marginally longer then 10 divisions, 10.5 to 11.5 depending on the range and with my calibrator connected I can my 1 cycle per 10 divisions  and also the 10 cycles, 1 per division on the screen, result. No flyback visible and lots of brightness, which was one of the problems with the other board, it would dim down a lot, poke the board a bit and it would come back for a few seconds. Also after a few minutes, some of the slower speeds would become unusable and only send a sweep every so often, again poking the board would provide a temporary repair.

Over the weekend I'll attempt to calibrate and adjust the input capacitance etc and then this particular scope can out of my repair queue and be put to work on the duty roster along with the others so I can get into the Fluke 8505A a bit and check that out.

I'll post a couple of photos of the 1740A when shes done.

[Specmaster]

Ok the end is here (nearly, more on this later) as you can see from the photo she is performing beautifully a nice steady and sharp trace on the screen, all in cost, includes, scope, a parts mule and other spares £75.

Heres the later bit, the manual is very misleading and following it has got me a trace that is still slightly by 1 division wider then the screen but I now know how to get that back to 10 divisions, if I can be bothered, later. I may just elect to leave where it is, like bd139 said, its good enough considering its age and its probably had a hard life. 

Heres to the next item on the bench, it might be the Fluke 8505A for some tweaks, for some reason, it has a slightly difference of opinion on a voltage depending on if have the black or the red lead connected to ground, only a few millivolts but a difference nonetheless. 

[bd139]

Looking good to me! Nice work

That’s a very sharp trace you’re getting there, arguably better than you get out of a similarly aged tek.

[qu1j0t3]

Heres the later bit, the manual is very misleading and following it has got me a trace that is still slightly by 1 division wider then the screen but I now know how to get that back to 10 divisions, if I can be bothered, later.

Can you describe the problem in the manual for posterity and people finding this thread in future?
Thanks

[Specmaster]

Heres the later bit, the manual is very misleading and following it has got me a trace that is still slightly by 1 division wider then the screen but I now know how to get that back to 10 divisions, if I can be bothered, later.

Can you describe the problem in the manual for posterity and people finding this thread in future?
Thanks

The manual, page 5-11 section 5-52 speaks about setting the beginning of the trace on the far left graticule (far left line) and then using the 2nd graticule line as a reference point for the small highlighted dot during calibration, I took it as meaning the 2nd vertical line, which with 10 columns, or 10 divisions, the 2nd vertical line happens to be the 1st graticule line and the 10th graticule is the last vertical line on the right of the screen.

Given that this is how you set the trace width. You have to set delay on .2 which sets the highlight dot in the first instance 2mm from the far left line, then position this dot, using the horizontal control, on the 2nd graticule, with the delay vernier set on 1.

Then you set the delay to 9, then adjust A7R93 until the dot is on the 10th graticule line (far right), then you set delay to 1 and repeat the process again until delay on 1 set the dot on the 2nd graticule and then setting delay on 9 moved the dot to the 10th graticule. Once done you can move onto other settings etc.

So unless you remember that 10 divisions actually has 11 graticule lines and compensate for that by actually using the 3rd vertical line for delay position 1 setting, you will lime end with 11 divisions in your sweep like I did 

Not really the end of the world because you can compensate using the sweep vernier but you have to remember to this each time your want to approximate a value of the screen (this where digital boys gain over us analogue boys since their scope automatically take frequency readings. I just wonder if any of their scopes will still be working after some 30 plus years, I doubt it. 

I'm going to open her up again and readjust the sweep length.                                             

[Specmaster]

here she is, adjustment this time was spot on and the sweep width is marginally wider, I guess that might be added automatically to allow for drifting slightly? as you can see from the attached photos with various frequencies fed in I was able to get a perfect single sine wave on screen, pixel perfect. The video shows just how much extra wriggle room the scope allows.

I'm extremely happy with the end result, a worthy addition to my scope line up, maybe I'll bring the Hameg 408 back onto the bench and try to resolve its problems? 

https://www.flickr.com/photos/158191416@N07/37942213376/in/dateposted/

[bd139]

Perfect

Go go go with the 408. They're rather nice. Cursors and everything!

[Specmaster]

I know, thats why I got it 

I need to find a cheap source of their header cable though as many of them have suffered and and are not really long enough now to reach.  It had power issues when I got it, its a SMPS and despite many attempts to cure it I couldn't, recapped it, rechipped it etc, nothing doing. Brought a reconditioned PSU for it from their German repair agents, still nothing doing, so I guess there's a real chance that I now have 2 PSU's for it as well.

Its in pieces here at the moment waiting for me to get on with it at component level on the bench to see if I can find any reason for the PSU not firing up, they believe that there's an issue with the feedback circuit preventing it from switching on.

Just been playing around with my other recent acquisition, Hitachi V-525 50Mhz scope which also has cursors and it has this special on Ch1 where it will display either the trace and cursors or the trace as normal and also 3 divisions below, the trace at Mag10. Its a lovely machine, almost new, pristine condition, with its own protective bag and a set of unopened probes, all for £25 and it was local to me, 3 miles. I just did the same test as I did with the 1740A and its spot on spec. 

[Specmaster]

I also checked my Iwatsu SS-5710, no cursors though but does have 2 + 2 channels, a very nice scope actually and it too was spot on as well.

[Specmaster]

Perfect

Go go go with the 408. They're rather nice. Cursors and everything!

This should make you happy, today I ordered some cable that I hope will be what I need for the 408-1A.

[bd139]

Excellent 

Loosely on the subject of Iwatsu, I was looking at Japanese oscilloscopes the other day and Yokogawa have by far the best form-factor I've ever seen for oscilloscopes: http://cdn.tmi.yokogawa.com/DLM2000_Front_image_lg.jpg

Search for more toys continuing here. Nothing yet

[Specmaster]

Wow, that looks great but I bet its a pig to work on.

Talking about working on, I decided that I'd tackle a flickering trace on my Goldstar CRO this morning and when I took the cover off I discovered that a 500 ohm trimmer has fallen apart so thats at the head of the queue for now,  parts have been ordered, thought I had some but they turned out to 5K ones.

[bd139]

I think it's likely disposable if it breaks.

I like the Goldstar/LG units, particularly the OS-9020. Dead simple, really easy to fix and they're slow enough so you can stick pretty much any transistor you have lying around in any hole! Really a design marvel as they used crap tolerance components throughout and still got pretty good performance.  I used one exclusively for about 3 years until I got my hands on a broken Tek 453 in about 1999.

[Specmaster]

Yeh, its all plugged together but some of those plugs are pigs to get to with big hands  mine is the OS-9100D, 100Mhz so is a little bit faster and therefore a bit more fussy over its parts, its in really good condition cosmetically and internally as well and has a party trick that it can become a 3 channel scope and uses the trigger control as the vertical position control for the 3rd trace. It also has time delay and you can separate the delayed trace from the main trace as well, neat. My overall favorite though is the Iwatsu, 4 channels (2 are only up to 1v) but it has so many controls and it too can separate the delayed trace from the main trace. It has one main issue, still useable, and that the timebase and delayed timebase are as always a dual concentric control, but this one is a triple concentric control as it as it has a variable delay pot as well and like the 465 / 475 with exposed controls, this one sticks right and is the thing that gets hit and bent. To use the delay on mine is a 2 hand job, 1 to hold the pot still and the other to click through the fixed settings.