HP 1741A oscilloscope, looking for some help

xbart

Super Member
I brought home a HP 1741A oscilloscope from a yard sale. The seller new nothing about it's condition, but for the $5 they were asking, the probes alone were worth that.
When I got it home I turned it on and it blew the fuse, it has a time delay fuse and it blew after a couple minutes.
I opened it up and it looks to be pretty clean inside. I pulled out the board that looks to have the power supply filtering caps on it, hoping to see something amiss. Visually everything on that board look ok.
I would like to try and fix this, but am going to need some help as my knowledge is limited..
I don't know how to check voltages if it is blowing fuses? If anyone would like to help me or have any suggestions, that would very much be appreciated.
I have the manual and schematics for it.
I figure I should ask you guys before pulling more board out!
-Brad

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Shouldn´t be to difficult to find a service/calibration manual. Not and owner manual--Service manual.
 
if it is possible to unplug all outputs from power supply board it would be a good place to start . without seeing it i dont have a clue if its at all possible .
 
Found the owners Service Manual here.http://www.keysight.com/en/pd-1741A:epsg:pro-pn-1741A/100-mhz-dual-channel-storage-scope?pm=PL&nid=-536900196.536896504&cc=US&lc=eng . Never worked on a scope before. Putting this up for the people who know more on repairing scopes. Check this thread, power supply problems.https://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/75/
Thanks, I too found the manual and have been looking it over. From the restoration post, It looks like they have issues with the rectifiers in the power supply, mine look fine, but I think one might have a short. I will pull and test it.
 
5$ with probes!! You must have done a good deed lately. I was/ am a 'metrologist' and have done mech. elect. calibration but if you have some decent standards to reference, considering 5% error most scopes will offer anyway these hp's are pretty robust that a comparative to a known scope is usually good enough.

Perhaps a short.. take your dmm and do a continuity check on all the traces toward the ac input side. just a guess but p.s. is always where all the action starts.
 
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Congratulations, Brad! That's a deal that can't be beat, and the fact that it needs repair means you're going to learn something new, always a win in my book.

What is your level of expertise? Are you familiar with basic electronics? Do you have a DMM, and do you know how to test semiconductors?

If memory serves, the red connectors at the top of the 2nd pic go to pass transistors mounted on the chassis. I'd check those, in addition to the caps on the board.

The fact that it takes a few minutes to blow the fuse means you probably don't have a dead short in the power supply. But as Binkman says the power supply is where all the action starts! So it's a good place to start.

Good luck!
 
Congratulations, Brad! That's a deal that can't be beat, and the fact that it needs repair means you're going to learn something new, always a win in my book.

What is your level of expertise? Are you familiar with basic electronics? Do you have a DMM, and do you know how to test semiconductors?

If memory serves, the red connectors at the top of the 2nd pic go to pass transistors mounted on the chassis. I'd check those, in addition to the caps on the board.

The fact that it takes a few minutes to blow the fuse means you probably don't have a dead short in the power supply. But as Binkman says the power supply is where all the action starts! So it's a good place to start.

Good luck!

My knowledge is limited, I do have a meter. Other than diodes, I don't know how to test semiconductors.
I was able determine that one of the bridge rectifiers was indeed shorted. I replaced it and can power it up without blowing a fuse.
I checked a few voltages and a couple were not correct. I'll see if I can trace these voltages back and check components in the path.
 
My knowledge is limited, I do have a meter. Other than diodes, I don't know how to test semiconductors.
I was able determine that one of the bridge rectifiers was indeed shorted. I replaced it and can power it up without blowing a fuse.
I checked a few voltages and a couple were not correct. I'll see if I can trace these voltages back and check components in the path.
Sounds like you know more than you think you do. Kudos for finding the shorted bridge rectifier!

Here's a tutorial by EchoWars on testing transistors:

http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Is the scope working now?
 
So you did indeed find a short. ic chips unless showing burning are probably good. I think it's more adjustments. 5$? might look for good boards. Pretty sure you need a variac for voltage measurements. i.e. if you are using house power that can vary all over and ground is ground.. (e.g. you do have a small output dc or ac supply regulated to ground to and that is already grounded. Just a faster way making reading and notes. just noting that mistakes to blow improper usage of the probes is when you disconnect earth ground and use a chassis test unit ground. that will require a plug without a ground lug or a switch.. soooo for all the higher switching stuff? was never that good at fancy mhz calculations so I shipped the stuff off as regular calibration repair as engineers didn't know how to calibrate them either without a standard? if that was in error? :D
 
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Sounds like you know more than you think you do. Kudos for finding the shorted bridge rectifier!

Here's a tutorial by EchoWars on testing transistors:

http://audiokarma.org/forums/index.php?threads/bipolar-junction-transistor-testing-basics.43186/

Is the scope working now?
Thanks for that link on testing transistors. It turns out 2 of the bridge rectifiers were bad, that is why I didn't have any voltage readings at one of the power supply test points.
I created 2 temporary bridge rectifiers out of 1n4007 diodes, till I get proper replacements.
Now, I have good voltage readings at the power supply and the display comes on.
Not a lot of time to work on it today, I spent a couple hours after work chasing down one of our steers that got out.

Here is the first bridge rectifier I cobbled together out of 1n4007 diodes.
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I had similar scope - the bridge rectifier shorted, overheated, melted its solder, and fell out of the PCB. I thought it to be a handy self-servicing aid.

I'm afraid that once you get used to repairing to old HP equipment, you'll never want to go back to consumer audio gear. That HP stuff of that era was absolutely top-notch in all aspects...
 
I don't have much in the way of experience working on anything like this but, I thought it was a bit strange that there were no signs of failure on the board, that I could see.
 
I don't have much in the way of experience working on anything like this but, I thought it was a bit strange that there were no signs of failure on the board, that I could see.

In many cases,equipment such as this HP scope have sat unused for many many years,which means that all of the electrolytic caps have lost their ''form''. When you first power up,they appear as a dead short and can take some time to charge up.And don't just count the main filter caps: there is often a bucket load of smaller decoupling caps scattered throughout the unit as well contributing their own little sustained surges.

All the while this is overloading the rectifiers, and this often results in their failure.I have found this failure mode quite common in older solid state gear.

Consider yourself lucky,the failures in the old Tektronix tube scopes (the ones I collect!) are far more spectacular.And smelly! Boy,when those old caps let go :confused:
 
In many cases,equipment such as this HP scope have sat unused for many many years,which means that all of the electrolytic caps have lost their ''form''. When you first power up,they appear as a dead short and can take some time to charge up.And don't just count the main filter caps: there is often a bucket load of smaller decoupling caps scattered throughout the unit as well contributing their own little sustained surges.

All the while this is overloading the rectifiers, and this often results in their failure.I have found this failure mode quite common in older solid state gear.

Consider yourself lucky,the failures in the old Tektronix tube scopes (the ones I collect!) are far more spectacular.And smelly! Boy,when those old caps let go :confused:

Do you typically replace or just reform the electrolytic caps? I don't have a way to test for leakage and most of them in the power supply show a wide range in tolerance ( +75%& -10%) in the schematic. I'm hesitant to replace them as I don't have the equipment to recalibrate the scope. It might be ok to replace them in the power supply.
 
In solid state gear,the need to shotgun all of the caps doesn't seem to be required as often as in tube gear. Generally,replacing an electrolytic in a scope will not require recalibration,as they are usually only used in power supply filtering and decoupling applications.

I'd let it run for lots of hours and keep an eye on cap (especially the larger caps in the power supply) temperatures using the old calibrated fingertip method.Do the same with the rectifiers.Please make a point of avoiding the high voltage areas

Do keep in mind,most equipment of this nature was meant to run continuously on test and assembly lines in order to maintain stability,so nothing ought to be getting too hot.

Most (all?) HP equipment was built using top quality components,so the likelihood of the caps reforming on their own is pretty high.

Just as an example,I have a Hewlett Packard 200BR oscillator from the early 1940's that's all original,except for a couple of tubes. I still use it a couple of times a month to keep it from feeling lonely:)
 
In solid state gear,the need to shotgun all of the caps doesn't seem to be required as often as in tube gear. Generally,replacing an electrolytic in a scope will not require recalibration,as they are usually only used in power supply filtering and decoupling applications.

I'd let it run for lots of hours and keep an eye on cap (especially the larger caps in the power supply) temperatures using the old calibrated fingertip method.Do the same with the rectifiers.Please make a point of avoiding the high voltage areas

Do keep in mind,most equipment of this nature was meant to run continuously on test and assembly lines in order to maintain stability,so nothing ought to be getting too hot.

Most (all?) HP equipment was built using top quality components,so the likelihood of the caps reforming on their own is pretty high.

Just as an example,I have a Hewlett Packard 200BR oscillator from the early 1940's that's all original,except for a couple of tubes. I still use it a couple of times a month to keep it from feeling lonely:)

Thanks! I will do as you suggest.
 
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