Repair of a hp oscilloscopes 1725a and 1742a - Page 1
Forum section: electronics / general

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 01, 2012, 02:03:44 pm
content: HiYesterday I bought on ebay a hp 1725a oscilloscope for a relatively cheap Au$135 dollars.It is huge and heavy and looks great.ps. It was the one mint saw, and posted as a rant.I tried it out tonight and I am having serious trouble getting it to function.There are two problems, one I knew about which was the dual display didn't work in either alt or chop.The other one is a show stopper, I cant even get a stable trace of the calibration signal.I checked the leads and the calibration signal they are good. I used my even more pathetic cro (20Mhz analog that hasn't ever missed a beat).I went through the manual and put in the settings for the basic front panel test procedure. The first viewable result is the calibration output, I think it was 1kilohertz.The Y component of the trace (voltage) is perfect, the X component (time or sweep) is bad, it is often two white dots on the left side of the screen, as if the horizontal scan voltage just doesn't increase. But sometimes it scans across like it should but as if the auto trigger is firing randomly, so I cant make it stable.But on one particular timebase (0.5 milliseconds) I can make it stable and I see what I should see, then I flick it back to 1ms and cannot get a good view at all, across every trigger level. This is on a 3v square wave, normal or auto triggering.When I say I can't get a good view I mean I get one of the following depending on my trigger level.No trace: fine as it hasn't triggered.Unstable square wave.Stable square wave that looks like two traces super imposed.Two white dots on the left side of the screen. (Trigger but no sweep?)I think the two problems are related, and I think the fact that the sweep can actually work in certain circumstances, makes me think it is fixable.So what I am thinking is hand the cro back to my seller, who is local. ie. don't by other peoples problems.or ask for a discount and put it away for a rainy day if I can get a service manual. Does anyone know if it's easy to get in and work on these units, I can't open it now as I may not buy it.What should I do?

author: amspire
author_rank: Super Contributor
timestamp: May 01, 2012, 02:17:41 pm
content: It really helps if you can show a clear picture of the front panel settings that you are using. It saves a huge amount of questions from us.Just to check, You need the Auto/Norm button out. Int/Ext out.Int Trig on "A". "A" channel connected to the reference. Time per division on 1ms/div and delayed timebase off. Trigger level set to mid position.Not sure what other buttons you have that might affect things. Adjust the "A" channel amplitude for the best display of the waveform. Give us a photo for what you get.Richard.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 01, 2012, 02:51:53 pm
content: I actually packed it up but then posted this thread as an afterthought, I'll have a go at getting some photos tomorrow.I can appreciate that there a stack of buttons and many reasons to not get a trace, but I had a stable trace, changed time division, by one setting, and lost my correct trace across all trigger values. So I am sure this scope has a major problem.I'll post something tomorrow.

author: amspire
author_rank: Super Contributor
timestamp: May 01, 2012, 10:54:07 pm
content: All it takes is one switch setting to be wrong, and you will have exactly the symptoms you talk about. It is really easy to misunderstand the function of one of the trigger buttons.If you do have a problem, the details are important. For example if channel "A" trigger doesn't work, but external does, the problem may be as simple as a dirty switch contact.If you are prepared to post photos, the problem can be solved or identified pretty easily.Richard.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 02, 2012, 12:24:45 am
content: http://s1248.photobucket.com/albums/hh490/jonr0/HP1725/one photo is my old cro to show the lead and calibration output are good.the photos from the side are to show the button settings. One is out of focus but it may show some position that the others don't.except where specified all photos are on the same settings except time/div on the main sweep and the main trigger level.After I took the photos I noticed I had left the outside ring of the time/div at 0.1 u second, although I get similar results when it is switched to off. I thought I should mention this but I dont think it is the problem.What are those dots? surely they indicate problem? Also very occasionally I can get the two dots toward the middle of the time axis.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 02, 2012, 01:21:35 am
content: Well I left the cro on while I wrote that last post and did some other things for say around 40 minutes.when I walked back into my workshop, on the screen was a stable cro trace! Not one button had been touched.One thing I had neglected to mention was that the seller had told me that he hadn't really turned it on for 4 years.So It needed a good warm up? It took me 20 minutes to do the photos and it still hadn't come good.I think I will do some more testing, to see If I can get it to work without waiting for an hour.Another question, the optional multimeter that is attached on top seems to change output with the main time base, but it is set to DC volts. Anyone know how this is supposed work?The calibration trace is rock solid now, great. The alt/chop with two channels still doesn't work.

author: amspire
author_rank: Super Contributor
timestamp: May 02, 2012, 02:14:55 am
content: There may be a problem on the timebase - it looks a bit erratic.That bright dot at the start looks a bit like the delayed sweep selection, but the scope is set to main, so I wouldn't expect to see it. I think you have the delayed Time/Div ring set to minimum - try setting it to the same as the main sweep rate and see if the highlighted area expands.It looks like the trigger may be working, but the horizontal deflection is erratic rather then being perfectly linear. I get the impression that if the scope is faulty. it is a fixable problem. It could be that the time/div switch needs cleaning, but I just do not know.The horizontal amplifier looks like it is working perfectly, looks like the tube is good. You have two genuine HP probes with the grabber hook that on their own are worth a fair bit of money, so even if the scope needs some repair, you have still got a great price - particularly for Australia. You could possible sell it for a profit if you didn't want to fix it.If you do not want to fix it, there may be others in this forum who are interested.The service manual is on the Agilent site, so that is a plus:http://cp.literature.agilent.com/litweb/pdf/01725-90902.pdfStart up time shouldn't be longer then maybe 30 seconds. Is the 20 minute startup for all time/div settings, or 1mS/div in particular? What you want to do is set the trigger to Auto rather then Norm. If the trigger works, Auto will be a stable. Now go through all the different horizontal speeds and you should see the triggered edge of the reference squarewave . If some time/div positions work well and others do not, it would point to contact problems on the time/div switch. I do not know how easy the contacts are to clean on this scope.Richard.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 02, 2012, 02:55:47 am
content: HiI have seen similar problems on a few hp1741's and a 1740 and its most likely the time base switches that need to be stripped and cleaned (as Richard suggested),my guess is they are pretty much the same in the 1725.There is a grease which goes hard and prevents proper alignment of the switch detent's and also the actual contacts, (usually the switch feels stiff and does not 'click' into each position correctly, seems kind of mushy).Its not a difficult job, you need to get the PCBs that have the contacts on them out (basically there are collars on the shafts which need to be loosened and removed when taking out the shafts with the timebase knobs (the manual should have full procedures).Watch out for the fingers in the rotor part they are fairly delicate. The gold contacts on the PCB are more robust, clean both and lightly re grease with an appropriate contact grease. I have used Electrolube contact grease.Things to note that these old timers are pretty reliable with mostly just contacts in the switches giving problems due to age. Maybe the odd transistor failure (garden variety stuff) and a resistor going high causing a fault which looked like a failed vertical input hybrid (ie no vertical deflection at all!). Good thing is the custom IC's are usually not a problem.Good luckJohn

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 02, 2012, 04:47:27 am
content: Thanks for the input guys, and thanks for the link to the service manual, normally I have to venture past unsavoury websites to find such things, I am glad HP -> agilent sees it differently.I noticed that some of the time divisions work better than others, so I think you're right the switch needs sorting first.Sometimes when I switch to a new time division the waveform stretches horizontally back and forth a few times in the first few seconds before it settles down.I guess it is switching different resistances through and the contacts are adding to the impedance. ( Yes/No ?)I will look in the service manual.QuoteThese symptoms exactly.Thanks again for your help, I will let you know if I get it sorted.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 02, 2012, 10:01:38 am
content: QuoteNo the startup time is less than 30 seconds.All I meant with the 20mins is that it took me about 20 minutes to get the photos, then I went away and 40 minutes (ball park figure) later I came back and everything seemed good. The trace was perfect and I hadn't touched anything.I have been fiddling around some more and the cro is once more in what I will call "failure mode".seems that the white dots are there in failure mode when the scope shouldn't be triggering. ie. when I set the trigger level to extreme and I set trigger mode to normal.The other strange thing I noticed, and it may be visible in those photos, it that there are actually two traces on separate but adjacent time bases, like the timebase switch is caught between the two position. Mechanically the switch does seem to find detents between the correct markings.I will clean that switch when I get a chance.Any good local agent for electrolube contact grease?I found this at RS for $20.http://australia.rs-online.com/web/p/contact-treatment/3375925/Do you think I should clean the other switches and pushbuttons as well, or just if they start playing up?Thanks for the advice

author: amspire
author_rank: Super Contributor
timestamp: May 02, 2012, 11:52:49 am
content: Extra resistance in the contacts definitely mess up the timing. If the contact resistance is not constant, the speed of the dot will go all over the place. The trace will always be too long, so at 1mS/div, you should see less then 10 cycles across the screen.You will find if a contact is good, the time/div will be accurate.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2012, 01:53:52 am
content: QuoteI'm having trouble getting the time base shaft out.There are four switches attached to the shaft, the middle two are socketed together, these I think are the main sweep.Theses seem to be attached to the shaft in some way whereas the first board/switch from the front, which is the delay slides freely.Ps. the manual doesn't say anything except pull the shaft out.You mentioned collars needing to be loosened, I was hoping you could elaborate on this as it is not obvious to me.Ps I pulled the flimsy metal retainer off the switch/board that needs loosening, it doesn't seem to do anything that I can see. And I have left it on the front one that is already moving freely.Any suggestions appreciated.Actually I just got it, it was all the gunk people had sprayed in had found its way onto the main shaft.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2012, 03:00:09 am
content: Well I didn't get far,I can't get the boards out???From the manual8-19 Delayed Horizontal Sweep Switch Assembly Removal.To remove Delayed Horizontal Sweep Switch Assembly A9, proceed as follows.a. Remove Time/Div shaft. (Done)b. Gently rock A9 (which is a pcb), while pulling upward to remove from socket on A8 (mother pcb).The problem is it doesn't want to come out.In the picture attached the pcb edge connector has a bit of black wire around one side, it looks like a zif lever.I have tried moving it around like you would a zif lever but it is just loose any way, and somehow the board is locked in.What I want to know is how to unlock the board.The socket says TRW on the side, I haven't found the exact part but I have found similar as "Edge Card Connector, gold plated CINCH/TRW".Any ideas.Ok I have them out now, the black wire is just an insulated steel wire that pokes in at either end to lock the boards in place, sort of in a long C shape, you just pull one end of the C to disengage the lock and likewise for the other end.Boards out, but they don't look that dirty.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 09, 2012, 12:27:36 pm
content: Cleaned the time sweep board switch contacts with metho. There was a lot of green gunk came off and you could see how it would probably cause mechanical interference with the many contacts. There also is a fair bit of scoring around some of the switches at different radii (dont use that word much). I guess the scoring is normal after many years.There is no other visual sign of damage on any component.Anyway I put it back together and viola the time sweep is ok again. Fixed.I didn't put any electrolube back on it, do you think I need to?It has gold finished contacts on the pcb and it looks like stainless spring contacts on the rotary switch.But I can still only get one channel at a time, so when I get the chance I will look into that.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 10, 2012, 12:49:42 am
content: Good to see its working again (at least in part!). The electrolube grease would help to maintain good electrical properties on the contacts and also lubricate any rotating sliding surfaces.Element 14 sells electrolube products, (of course if you can find a similar contact grease cheaper then go for it ).As well as the grease you can also get a spray can of 'contact cleaner and treatment' from electrolube (and others) which is handy for those push buttons etc.Be careful with how much you apply though.I would be even more careful if you need to get the vertical attenuators out for a clean.Keep plugging away, you will learn as well as gain a nice bit of old gear.John

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 10, 2012, 01:17:08 am
content: Thanks for the input, both of you.I will get some of the grease then.re. The service manual.I should be pleased that it is so comprehensive, but I found it a bit wanting. (although I didn't go through the ammendments.)The manual describes how to remove the switch shaft but put the wrong part number on the surclip. So I wasted some time why the F*** I should remove this surclip when it was obvious that another surclip needed removal. It just seemed odd that it was just about the first part I removed.A better exploded view would have been good too. One that showed the boards and their different locking mechanisms.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 10, 2012, 04:05:33 am
content: The manuals can be like a godsend and also a PITA at the same time. I have experienced the same when servicing the 1741 with a manual that was shipped with the unit (an old work cro) and the manual did not reflect a major circuit revision that was clearly in the hardware in front of me. And of course none of the on line manuals had the update either.And no amendments I had alluded to it either (unless some were lost, which is not unusual where i work).John

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 12, 2012, 05:35:01 am
content: excuse post, dead keyboardchop/alt seems thermally dependant, starts ok when cool.might need ecl 'or' gates, where can i find alternative.U1 1820-0102 MC1013Pwww.americanmicrosemi.com/information/spec/?ss_pn=MC1013PU2 1820-0142 MC1004Pwww.americanmicrosemi.com/information/spec/?ss_pn=MC1004PU3 1821-0001 CA3046www.americanmicrosemi.com/information/spec/?ss_pn=CA3046expecting squarewave on u2 pin14 but cant see.

author: amspire
author_rank: Super Contributor
timestamp: May 12, 2012, 11:38:31 pm
content: I never realized how hard finding these IC's has become. The CA3046 is extensively used in oscilloscopes, and I thought it would be one of those IC's that would never become obsolete, but it is getting hard to source. Last time I looked for the CA3046 about 6 months ago, it was not nearly as hard to get, so someone has probably cleared out the stocks from the major suppliers. Luckily it does still seem to be available from ebay and Asian sources (like aliexpress.com).The CA3046 is usually very easy to fault find - just use a multimeter in Diode mode to find the junctions. The most common fault I have seen is open circuits on a pin, and a problem like that could easily be intermittent.The Motorola MECl look like they are a bigger problem. It is still available as a part number the HP part number from places likehttp://www.talonix.com/shop/category.aspx?catid=66\http://www.sphere.bc.ca/test/hp-parts/hpparts2.htmlhttp://www.etestparts.com/Search.aspx?Mfg=H-P&Page=63The MECL series I family are an odd choice to use as they were obsolete by the mid 1960's. The later generations of "04" type gates replaced the three complementary output gates with three single output and one complementary output gates so they are not a pin compatible replacement. I did find a good data book on the MECL family, but it does not say much about the Series I chips:http://ae6pm.com/Semidata_books/Motorola/DL122-D.pdfRichard.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 13, 2012, 08:42:20 am
content: These ECL chips were mentioned as I think a curiosity when I did my digital electronics course, I must admit that I have never run into a real one before.Apparently, from Wikipedia, they are faster than the TTL at the time because they never push the transistors into saturation, but they consume extra power.It's good to know I could just do diode checks on them as I didn't want to take any chances with them.I can't figure out why I can't get a square wave on U2 pin 14 at any time but for about one minute from turn on on a cold start I can get both the Alt and Chop working correctly.Tonight I will try to investigate further, and post any observations I make.My workshop is in a non heated part of the house and tonight it is 10 celsius. Although the CRO itself puts out a bit of heat.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 13, 2012, 09:26:11 am
content: I just noticed that the schematic of the vertical display switch board on page 121 is listed in the amendments on page 96.The only troubles are that the new schematic is harder to read than the old schematic.QuoteServes me right, I guess.I am not sure of the nomenclature on the or/nor gates those wings I guess are just the 4 inputs.Those two gates straight before U1 I guess are integral?At least pin 1 of U2 finally makes sense. In the last schematic it was supposed to be Vcc but the cro had a square wave between 3.5 - 4.5 volts, or somewhere around there.

author: amspire
author_rank: Super Contributor
timestamp: May 13, 2012, 02:06:30 pm
content: Quote from: HackedFridgeMagnet on May 13, 2012, 09:26:11 amLooks like they have changed the IC used as the Chop Oscillator. It is now a dual 4 input OR/NOR gate (as it has two outputs). Possibly a MC1006P.Now the switched 3.4V to enable the oscillator on pin 11? is critical. If that 3.4V drifts upwards, it will stop the oscillator. You would detect this condition with a 1 on the non-inverted output and a 0 on the inverted output. Same thing would happen if the 3.4V was correct, but the supply voltage to the MECL chip was dropping with temperature.If you are wondering about the odd looking OR gate, the reason is that these diagrams are hand drawn in ink on Vellum using stencils for the logic components. The draftsman only has a stencil for for a 2 input OR gate, so some improvisation is required to draw a 4 input OR gate.Richard

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 14, 2012, 12:23:02 pm
content: Interesting about the draftsmen and the vellum. I can barely remember this sort of setup, but it used to be so common.When I started work in the 80s I think there were about as many draftspeople as engineers, I do remember the girls were called tracers, and drawing office was a nice place to work because of them.Well the good news is I found the problem.It is an open circuit intermittent track on the A8 board.Which is the biggest board and the one that all the other time base switch boards plug into, an therefore subject to the most mechanical stress.Once I managed to work out that the ICs were in fact 14 pin and not 16 pin as the first schematic said( it pays to count), I noticed pin 1 on the U2 Ic was always low, and was stopping both the alt and chop displays. Here instead of a logic low it should have been some of some sort of a square wave.I traced the fault back to the A8 board and the square was present.I did a continuity test on the suspect part of the board and found that by moving the board I could go from open circuit to short circuit. So it is something like a cracked board, I dont think it is just the connector itself, but possibly the joint where it connects onto the board.You can see in the images a red alligator clip that shorts the damaged track and when it is on, all the time sweep stuff seems correct.Now the bad news, I don't think it is going to be possible to fix this track easily as the fault is on the underside of the board, so I am going to do an ugly hack and short the bad track on the top side of the board and not even look at the back. Do worry I will solder it, I wont leave the alligator clip there. I really think if I try to get that board out I will break something else.Thanks for your help Richard and John, it really helped me fix this thing.Now I have just got to hope that nothing else is cracked.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 29, 2012, 08:52:45 am
content: Because I told people at work about the 1725a, somebody decided to give me a hp1742a that seemed to have some intermittent time or trigger based fault.So I took apart the time base rotating switches and I noticed that one is broken.One of the metal spring contacts has snapped.It is on the 3rd and last wheel from the front. Which only has one spring on one side and none on the other.Does anybody know a good work around?I can think of plenty of bad work arounds.

author: vk6zgo
author_rank: Super Contributor
timestamp: May 30, 2012, 12:27:55 am
content: Have a look at this guy's website,he has done some really neat fixes on HP switches, etc.http://jvgavila.com/

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 03, 2012, 11:19:38 am
content: Hi vk6zgoThanks for the link but I couldn't find the HP switches bit.Nevertheless tonight I got my Angle grinder and pliers out of the tool box.I proceeded to cut up and then shape a banana plug to replicate the spring contact.Put it all back together and it works. PerfectlyMostly.Actually I have a secondary bug on the vertical deflection, the position on both A and B channels wont go below the middle of the screen, I didn't check the vertical calibration, it's obviously going to be out of order too.Anyway I will put that part of the fix away for a rainy day.So if anyone wants their scope fixed by angle grinder just ask. My rates are very reasonable.

author: vk6zgo
author_rank: Super Contributor
timestamp: June 04, 2012, 03:20:35 am
content: Shame you didn't do a videoJose' did repair some switches in one of the devices,but I can't remember which.It may have been a totally different type,anyway.I thought even if you didn't find anything of use on his site,it would probably encourage you to have a go at your switch.His site is a good read if you are a Tektronix or HP freak,with the added flavour of the Ham stuff.In my opinion,the more practical stuff along these lines on the 'Net,the better!Glad you had a good result!

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 04, 2012, 04:05:10 am
content: The hardest bit was shaping the remnants of the banana plug into a suitable shape.If I had had a bit of very thin flat spring steel I would've used that, it would have been much better and quicker.I was going to take a photo but my camera battery was charging. I haven't got a video camera.When I have a go at the vertical deflection fix I will do the spring again with something a bit thinner, I think my banana plug solution may score the copper in the long term.I will try and do a photo then.In the meantime I need to find some household item where I can get about an 8mm square of very thin steel.

author: lowimpedance
author_rank: Super Contributor
timestamp: June 05, 2012, 12:35:50 am
content: I think the bit about switch repair is in the HP8640b sig. gen. repair. (not a cro repair but similar style of switch if my memory serves correctly)

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: June 05, 2012, 01:04:27 am
content: Yes that does look similar although the mounting is probably different, it's hard to see.http://jvgavila.com/hp8640b.htmA relay contact sounds like it might be good, the steel should be the right sort, but I need about an 8mm square of steel, so they might be a bit narrow.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 24, 2013, 03:17:46 am
content: I got my HP1742a back out again and had a go at fixing it.One of the problems was the vertical deflection.The problem was no matter what you did with the voltage display levels or the voltage position that the trace remains steadfastly in the top half of the screen. This seemed to indicate that the signal to at least one of the vertical plates (or coils?) was wrong. This is simulated (after the fix) in one of the photos.Obviously one of the first stumbling blocks is finding a service manual, but I had been given a HP1740A manual and it seems to be very close.All the power rails were correct so the obvious troublemakers were either the Vertical pre-amp board or the Vertical output amplifier board.I noticed using my HP1725A that the two input signals to the output amp board behaved differently when I changed the vertical position. These inputs come from the preamp board via the delay lines (100ns). I don't know much about the delay lines but apparently you can make 100nS delay using about a metre of magic cable.To try and help isolate the problem board I swapped the delay line inputs around to see if the problem followed the preamp board or the output board.Same weirdness on the same input so the problem was with the output board. In the photos hopefully you can see the input lines, I swapped the yellow and blue around, the black is common.I wasn't sure what waveforms to expect on the output board but as both circuits looked similar but I was fairly sure one side was working so I decided to use the same input for both channels using an alligator clip.Immediately I could see that the biases were completely different between the channels.Luckily the culprit wasn't the fancy white unobtainable IC but was a 1210 ohm power resister.I replaced it with 3 x 390ohm and it started working properly.I must admit the fix took longer than the few minutes it takes to read this post.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 24, 2013, 03:57:15 am
content: the problem resistor was R4The question isHow closely do I need to match this to R2, I dont think it is very important but I am not really sure.I am using Futurlec 5% tolerance resisters, carbon I guess.I just measured them they are running hot, the middle one is 72C whereas R2 is only 61C.The outer ones are only about 62C.I guess this is why it failed, but no signs of scorch or carbon.

author: SeanB
author_rank: Super Contributor
timestamp: February 24, 2013, 05:56:20 am
content: Replace both of them, with the correct value metal film units. Use a 1W unit there, 2 in series as they dissipate up to 1.5W, so you will need to look for a pair of 1W metal oxide units to make up the value. 2k2 and 2k7 1W metal oxide resistors from Panasonic (RS parts 747-3262 and 747-3272) in parallel will make it up and have the power rating and be quite stable. You may have to call and ask if you can get them in small quantities though...... Dirt cheap as each, but there is a MOQ.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 24, 2013, 12:31:31 pm
content: Thanks I will get some next time I do an order. I notice they are only 5% tolerant. Would series values be easier to get in 1W?I have a few queries for anyone game: please point out any mistakes.I guess you would call this part of the circuit a DC coupled input stage, PNP, common base, Is that right?Vbase is set, Vemitter is set, so the dc bias current is roughly proportional to r2 and r4.So as long as the bias doesn't move too far away from the original operating point the the gain shouldn't be affected.If there were any changes in gain, they would show up as calibration errors, of the Volts/div.But the change in bias current will affect the output voltage so the whole wave form on the display will be shifted slightly. Just like moving the Pos control.WikipediaQuote

author: SeanB
author_rank: Super Contributor
timestamp: February 24, 2013, 06:06:02 pm
content: Tolerance does not matter ( much) provided they are from the same lot and were made at the same time. They are going to match well then, and should track well with both time, temperature and voltage. The parallel connection was closest with just 2 resistors, while still providing a good power balance between the resistors making up the composite unit. Using 4 resistors would enable you to use 301R resistors, which then could be 3/5W 1% types.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 25, 2013, 12:34:41 am
content: While it was open I thought I better fix the leaf spring that I had previously fixed with a banana plug.I was always worried about it scratching away the copper contacts.I had some spring metal that a colleague had given me for this job but it seems too thick.So I found an answer in an old Australian PSTN connector. It was a very good match, the only differences being the thickness and the type of steel. But I think it is close enough to work sucessfully long term.Sorry about the photo quality, you can't see much of my workmanship (or lack thereof) but it seems to work in a similar fashion and I think it will outlast this old scope.Tools used bull nose pliers, angle grinder and calipers.It's all back together and working now, I will replace the resistors when I get some, probably only a 20 min job as I don't have to remove any boards.

author: lowimpedance
author_rank: Super Contributor
timestamp: February 25, 2013, 03:29:57 am
content: Heh, those resistors in that bias chain do dissipate quite a bit of power and over time they drift high. As I noted in my other post (page 1) where I had a problem with the vertical deflection on a 1741 the culprit was R3 (on the schematic you posted, different P/N in the 1741 manual but still the same arrangement!), which went a fair bit higher in value totally changing the bias to the preamp hybrid output and preventing any input signal from getting through.I would check the others in that cct while your at it. I used some metal film power resistors in parallel to get the value within tolerance of the original. Note that each model scope may have different values for the resistors in the bias cct. (the 1740 and 1741 are different).Good to see you have got the oldies up to speed.LowZ

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: February 25, 2013, 06:53:01 am
content: the other resistors all measure correctly (in circuit).One thing I found strange was that rather than one side of the differential amp being independent of whats going on in the other, they were actually slightly dependent. ie that resistor to the 43v is common.I was thinking that this may be a way of compensating for some non linearity that he field between plates has.Yes it's good to have 3 working scopes albeit all old and analogue. My 20Mhz scope works well enough and I use it if I think something might pose a danger to my good scopes.Also luckily I have a job coming up where I might justify the expense of a digital cro.

author: alfon
author_rank: Newbie
timestamp: November 05, 2021, 10:37:40 pm
content: "The service manual is on the Agilent site, so that is a plus:http://cp.literature.agilent.com/litweb/pdf/01725-90902.pdf"This link is now dead, but the manual can still be found - this time in the Keysight site:https://support.keysight.com/s/?language=en_US. just enter 1725A in the search box and you should get the link to the manual.