HP1742A -no display- - Page 1
Forum section: electronics / testgear

author: tankoid1742
author_rank: Newbie
timestamp: July 15, 2021, 05:15:24 pm
content: noob here, I followed a few threads for the 1742, 1741 and 1740, and instead of grafting onto existing ones here is the quest :The unit is clean, it was given by a neighbor after he said that the trace disappeared one day.Tested the LVPS, board A16 and the only off number is TP6 at 4.78v instead of 120v. After discussing with a more knowledgeable tech, I replaced Q1, then Q2 with no changes. Our consensus is that the power transistor could be off... no way to test it at the moment... and may be somebody could give us some ideas may be a refnum for that power transistortest schem and pict of transistor, the Ti one on the left sideThank you

author: bob91343
author_rank: Super Contributor
timestamp: July 15, 2021, 10:40:26 pm
content: Perhaps a shorted filter capacitor. Tantalum capacitors have a bad reputation.Turn off the power and measure resistance across the 120V supply (after discharging any capacitors).

author: TurboTom
author_rank: Super Contributor
timestamp: July 16, 2021, 05:44:54 am
content: Input voltage to the regulator is present, and as per your measurements, there's no (significant) current flowing whatsoever. No drive signal to the pass thransistor / pre-driver as well. I'ld look for the problem around Q1/Q2. It may well be that a biasing resistor in the neighbourhood of these transistors has failed open, R6 would be my first suspect, maybe also R3. At least the voltage readings (if accurate) aren't a clear indication for a current limiting situation. To be sure, measure the voltage across R2. If that's close to zero (at least much less than 0.7V), then the current limit isn't active and the output rail isn't pulled down by an external fault (and vice versa).Is the 15V rail present? The 120V is referenced to the 15V.And by the way: Welcome to the forum!

author: tankoid1742
author_rank: Newbie
timestamp: July 17, 2021, 01:08:10 am
content: Thank you both for the useful tips: Bob, the cap is testing ok : resistance going up when testing.Tom, you nailed it... R4 a 10K is at 269K and R5 a 261K is at 37K (color bands rechecked to make sure that I did not misread the schematic). R8 is a tad high 130K instead of 100K, the others in that area are within 5%. Yes the 15v rail is good and so are the other test voltages.Thank you for the welcome and will post results when these 3 resistors are changed...Any other recommendations will be appreciated.

author: nixiefreqq
author_rank: Super Contributor
timestamp: July 17, 2021, 01:52:08 am
content: have three 1740 series in my pile o scopes.all have had problems around the full wave rectifiers. either the bridge is bad, or the solder joints have been overheated and gone open, or the pins in the connector have been over heated and gone open. look carefully at the solder joints around the pins or the connector and see if they look cracked or wonky. sometimes just reflowing with fresh solder will fix the problem.yeah.....it sounds crazy. but that is what happens.

author: tankoid1742
author_rank: Newbie
timestamp: July 28, 2021, 05:57:32 am
content: So I replaced the suspected wonky resistors and ... without removing the board from the chassis... no changes!I figured out how to pull the bezel of the on/off switch to pull out the rod from the A16 board: a pair of sturdy spudger from Ifixit on top and bottom to force the bezel out. Push on the switch and rotate counterclockwise, rod is free.I checked the bridge rectifiers solder, all are perfect. Reinstalled, no display.... fiddled with the timebase and the reset led shows a frequency proportional to the timebase...Redid some measurements, the voltage around Q2 is off -159v at the collector... so is the resistance of R4 still at 270K ohmsPOST EDIT: I messed up, swapped the wrong transistor in the parts bin... Need to place another order!

author: tankoid1742
author_rank: Newbie
timestamp: August 04, 2021, 01:32:40 am
content: checked the number 1 full wave rectifier... , solder at the base was perfect, no corrosion, but I still replaced it... no changesI removed the interface assembly A14 as described in troubleshooting to see if voltage would come back to 120, nada.. well at least voltage at TP6 bumped from 5 to 11v.