Repairing HP 1740A - Page 1
Forum section: electronics / repair

author: Specmaster
author_rank: Super Contributor
timestamp: October 07, 2017, 05:09:37 pm
content: I have an old 1740A which was displaying some odd traces at times and a while ago I took the covers off for an inspection and discovered some resistors that been overheating and burnt as well as cracking. I had to order the values as I didn't have the correct wattage ones in my parts bin so today I stripped it down and replaced them. They were on the vertical output board. In the process I also took out the power board to inspect the smoothing caps. On unplugging the secondary taps plug, pin 4 (43v rail) showed signs of overheating as well and investigating this revealed the bridge rectifier had a dodgy joint.Resoldered the joint and also checked for other dry joints and reflowed those that looked suspect, caps all checked out just fine so reassembled everything and powered up, traces all appeared on the screen but now I'm unable to move them on the vertical position pots and there is no deflection of any sort of the traces in the vertical plane on the screen. Sweep seems to be working just fine.Checked all the voltages on the test points all were spot on so I pulled out the vertical output board and checked it over, all seems to be normal, new resistors are not overheating etc. Refitted the board and powered up again and checked the board for all supply rails to it, and all were found to be present and spot on again. The resistors that were overheating were R2 and R4 on the attached schematic and R3 was found to within 1 ohm and so was left in situ.Checking the voltages on Q1 and Q3 I measured 12.6V, 12V but the 0.5v measured 1.1v? I cannot get access to Q2 and Q4 as these sit behind a case strut. This 1.1v feeds directly into the hybrid A5A1 chip, has this been destroyed do you think with the voltage being doubled or can it withstand this as I see that the output voltage to CRT grids can vary between 5v to 30v depending on the vertical position pot.I attach the schematic for reference and would appreciate any advice or guidance on this please.

author: tggzzz
author_rank: Super Contributor
timestamp: October 07, 2017, 06:31:29 pm
content: Quote from: Specmaster on October 07, 2017, 05:09:37 pmThat last paragraph should be taken out and shot. I can't tell what you are asking, observing on the schematic, and observing in the circuit.First thing is to check all the PSU rails voltage and ripple; fix if necessary.You don't describe the oddness of the traces nor whether they are still odd.The voltages all along the deflection chain are differential, but are not symmetrical with respect to ground. Hence if the voltage on one side changes by +1V, the voltage on the other side should change by -1V. When the deflection is zero the voltages on each side should be the same (i.e. zero difference => zero deflection).The hybrid doesn't "care" about what the voltages are w.r.t. ground. All it cares about is the voltages across its pins.

author: tautech
author_rank: Super Contributor
timestamp: October 07, 2017, 06:47:27 pm
content: Same unit as this one ?https://www.eevblog.com/forum/testgear/hp-1740a-on-the-bench-help-please/Why did you start another thread ?

author: Specmaster
author_rank: Super Contributor
timestamp: October 07, 2017, 07:25:29 pm
content: Quote from: tggzzz on October 07, 2017, 06:31:29 pmThe traces are just a flat line as if the input is grounded, hence why I say that there was no deflection at all, not even if the vertical pots on either channel is operated, the trace stays exactly where it is.The supply rails are spot on and have not been checked for ripple at this stage as I'm trying to understand why there was a working trace prior to the removal of the power and vertical output boards and the replacement of those 2 burnt resistors. I can't see any connection between ripple and the disappearance of the vertical element of the display, especially when the horizontal works and can be moved on the horizontal position controls and the timebase is sweeping ok.The last paragraph was basically asking the question about the hybrids ability to survive having double the voltage stuffed into it as the schematic states 0.5V as the output from Q1 and Q3 but I'm measuring on the board that as 1.1v and I have used a more than 1 meter to double check incase my meter was lying to me. I was also wondering if the extra 0.6v could be causing the vertical problem?The reason why I'm leaning towards this board as being the culprit for loss of vertical deflection, is because no other boards associated with the vertical have been touched or removed. I did remove the 2 timebase switch boards for contact cleaning and these are working OK.

author: Specmaster
author_rank: Super Contributor
timestamp: October 07, 2017, 07:32:26 pm
content: Quote from: tautech on October 07, 2017, 06:47:27 pmYou are correct, it is the same one and I could not find the thread today as I thought it was the repair thread for some reasonPerhaps a moderator could merge the two into one?

author: tautech
author_rank: Super Contributor
timestamp: October 07, 2017, 07:49:38 pm
content: Quote from: Specmaster on October 07, 2017, 07:32:26 pmYour Profile info>Show Posts>TopicsTo get a mod to merge threads you'll need to bring it to their attention....report the thread and use that process to ask them nicely to do so.

author: Specmaster
author_rank: Super Contributor
timestamp: October 07, 2017, 08:12:31 pm
content: Reported, thanks for the info.