HP 1740A Repair

[Cigarsnob]

Hi,

I recently purchased an HP 1740A oscilloscope off of ebay for $55. This is my first big repair job, well attempting at the moment. I saw a thread by Tekfan about his restoration project for his HP 1741A and that really got my juices flowing:
https://www.eevblog.com/forum/testgear/hp-1741a-oscilloscope-restoration-project/

When the scope arrived, it would not power on and then I noticed that it wasn't set for US mains. So I switched the switch and probed all the voltages on the power supply board and they were all good. I injected a test signal (1kHz sine wave) on channel A and did not notice anything. Turned the intensity knob, made sure it was triggering correctly (well I think it's set up correctly) and still nothing. When I push the Beam Find button, I do see a trace and I actually see my sine wave as well. While pressing the Beam Find button and adjusting the volts/div or times/div, I do see a proper change in the scaling.

However, this is where I'm currently stuck. I'll read more of the service manual (http://cp.literature.agilent.com/litweb/pdf/01740-90909.pdf) when I get home tonight from work, but thought I would post here to see if I can get any hints/tips before tonight.


Thanks and I really hope to get this beauty up and running like a champ as it's a sexy scope 

[peterpan]

Hi,

"but thought I would post here to see if I can get any hints/tips before tonight." A few good tip
http://sci.tech-archive.net/Archive/sci.electronics.repair/2004-06/2059.html
http://www.electro-tech-online.com/repairing-electronics/109071-hp-1740a-oscilloscope-repair-help.html

[lowimpedance]

Make sure you strip and clean the old 'hardened ' original grease of the timebase switches (use IPA and cotton tips and BE CAREFUL with the contact fingers they are delicate), re grease with some good electronic lube and put back together.
The old grease gums up the mechanism making it harder to index the switch into known positions as well as affecting the actual contact of the fingers to the PCB gold contacts.

[Cigarsnob]

peterpan - Thanks for the links...I'll definitely check them out.

lowimpedance - Good call on the on cleaning the timebase switches. I sprayed it with some cleaner and after following the "displaying a signal" from the service manual, I got the thing up and running. I still need to go back with some electronic lube, but at least I got the signals to display.

My next issue, which I have yet to tackle (I'll go at it tomorrow evening), is the preamplifier for channel A. Channel B seems to be working just fine. Well at least I think that's the problem or maybe it's another contact switch issue. Probably should check there first. Anyway...Here are my symptoms: When I have the volts/div knob set to say 50mV/div with a 150mVpp sinewave, I read 200mVpp. The same is true for other settings. For example, 200mV/div with 500mVpp sinewave, I read 1.48Vpp. Below is a line of my notes that I took last night showing various volts/div and it's corresponding values and compared with a rigol scope.