Author Topic: HP 1741A oscilloscope - restoration project  (Read 98044 times)

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Online lowimpedance

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Re: HP 1741A oscilloscope - restoration project
« Reply #150 on: April 23, 2021, 12:25:14 am »
Thats quite some bbq-ing there  :P, I assume its now clean though !. The manual on this site is ok and has the Horizontal op circuit.

https://elektrotanya.com       ,scroll down for the search window. Read the supply circuit descriptions etc.

Given the damage you really need to isolate the LV power supply from the rest of the unit to fully debug it first as the 120 and 48V rails are
referenced to other supplies and if those are not fully functional things will go bad quickly. So best to trouble shoot in isolation then when working
look for other damage (beyond the horizontal OP PCB as well).  So just to confirm you only get 26V across C5 and 188V across C3 ?. Does that change when
the supply PCB is isolated from the res of the instrument ?. BTW be mindful of the voltages your playing with in the 1741 and follow best safe practices.

The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #151 on: April 23, 2021, 06:16:14 pm »
Thank you very very mutch, this manual is very useful, i was searching it for days but couldnt find it, whithout the manual it was very hard to understand the working principle, now I vill start measuring all the resistors and all components one by one, I really want to get this boy up and running. Thank you very mutch.
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #152 on: April 26, 2021, 05:19:20 am »
Can someone recomend me some good modern day replacement tranzistors for the A11 board? Only one of the tranzistors survived there, and I cant find any information for those Iwas thinking about BD439,Bc547 and their PNP brothers but Im not sure that those will be fine.
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #153 on: May 02, 2021, 07:41:30 pm »
Now my three main supplyes are good, I had to make some little adapters for the LM723 from DIL14 to TO100package, I couldn't get TO100ones, But my +120v and +48v rails are busted, I'm measuring +186volts at the +120v and 26volts at TP5 across C5 I have 78volts, but betwwen the negative side of C5 and GND I'm measuring -52 volts. Across C3 I have 186volt, and betwwen the negative side of c3 and GND I'm measuring -7.5volts. These measurments does not change id I disconnect the power tranies, the power supply is disconected from the rest of the circuits. All the transistors seem fine, what can cause this problem?
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #154 on: May 03, 2021, 06:48:18 pm »
I finaly managed to fix the power supply. All the voltages are oke, when the LVPS is isolted from the rest of the scope, but when I connet it I see no trace, the LEDs are working, and the 48v rail drops to zero. Can I get in contact whits someone for help, or some tips, what can cause that? Thank you.
 

Online lowimpedance

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Re: HP 1741A oscilloscope - restoration project
« Reply #155 on: May 04, 2021, 02:43:43 am »
I finaly managed to fix the power supply. All the voltages are oke, when the LVPS is isolted from the rest of the scope, but when I connet it I see no trace, the LEDs are working, and the 48v rail drops to zero. Can I get in contact whits someone for help, or some tips, what can cause that? Thank you.

 So the whole power supply is working when isolated from the rest of the chassis?.  And only the 48V rail collapses when reconnected, all others remain in spec.?.
OK, try loading the 48 V rail with suitable values of resistance to see if it can deliver a decent amount of current (within its current limit of course, ~400mA ? i guess) without the rail collapsing.
That will test its operation and thus if any transistors/components etc are still faulty. Check the other passive are in spec.
 If it is working correctly there is a short somewhere in the scope on that rail. best to use the interconnecting diagram
in the manual (page 8-48 to 8-50) to see where the 48V rail is routed to and then isolate those modules and measure for shorted components, ie transistors, cap's etc.
Did you repair the Horizontal O/P stage.
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #156 on: May 04, 2021, 08:12:34 pm »
I found out thet there is problems whit A7 as well as A17. On the A7 board U2 (that shiny gold one) is getting warm, removing it does not  change the current draw. But I cant find any schematic for thet board to trace out what can be problematic, any ideas? Later I will move on to A17

« Last Edit: May 04, 2021, 08:15:40 pm by jeno »
 

Online lowimpedance

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Re: HP 1741A oscilloscope - restoration project
« Reply #157 on: May 05, 2021, 12:05:40 am »
I found out thet there is problems whit A7 as well as A17. On the A7 board U2 (that shiny gold one) is getting warm, removing it does not  change the current draw. But I cant find any schematic for thet board to trace out what can be problematic, any ideas? Later I will move on to A17



Ok A7 from the interconnect diagrams refers to circuit diagram numbers 7,9,11,12 as seen on the bottom right of the manual schematics , ie multiple schematics make up the overall A7.
A17 is circuit diagram 3.
 Does the 48V rail work ok with an appropriate resistive load ?.
Be mindful that there is a number of custom HP parts (eg U2 ) and you should use proper handling procedures when out of circuit. Hopefully none of those are broken. BTW that IC U2 is running off the 5V rail.
 Use the interconnect diagrams to find which schematic sections use the 48V rail and work through likely failures.
Remove A17 and check the 48V rail (possible A1 output amp failure  which uses 48V).
The odd multimeter or 2 or 3 or 4...or........can't remember !.
 

Offline jeno

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Re: HP 1741A oscilloscope - restoration project
« Reply #158 on: May 24, 2021, 06:58:49 pm »
I think I managed to repair A1 and A17 (not 100% sure) now all my voltages look Ok when the LVPS is connected to the rest of the components, I also managed to get somthing in the screen, it's all green, none of the buttons doind anything, On the vertical preamplifier I measure 9.23volts at TP1, What can be now the problem?
 


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