HP 1740A low level power supply - Page 1
Forum section: electronics / repair

author: john2021
author_rank: Contributor
timestamp: May 17, 2021, 04:41:06 pm
content: Hello everyoneI hope someone can help me with troubleshooting an old HP1740A low level power supply.I am not an expert electronics guy, but I know the basic stuff.As of now I managed to get all the right voltages across the main caps as well as the test points TP1,2,3,4,5 with the same negative pin on the board but not the 120v on TP6 (only 8v).I get the 120v on TP6 with the negative lead on the negative of cap C3. I guess I don’t get the negative side for the power supply.Any help would be very much appreciatedJohn

author: lowimpedance
author_rank: Super Contributor
timestamp: May 17, 2021, 11:32:47 pm
content: What value is R2 ?, (the 120V rail current limit sense resistor). Any burning or signs of thermal stress on any components?.Note if ok then the schematic has operational voltages noted for the transistors in the 120V regulator and others.Also be careful poking around in this scope when powered.

author: john2021
author_rank: Contributor
timestamp: May 18, 2021, 01:50:01 pm
content: Thank you very much for the response, I do appreciate it.R2 measured resistance correctly and there are no signs of heating, I get about 150v on it.When I test in diode mode (continuity) on Q3 I get a short on all legs also with reverse leads.Could this be the problem?Is it possible to test Q3 on board, or do I need to pull it off of the board?I have a PN2222A transistor at hand could it be a suitable replacement?Thanks againJohn

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 18, 2021, 11:33:06 pm
content: Are we talking about a HP 1740A scope? if so I had a similar problem on a HP 1727A scope, if not then disregard everything I am going to say. On mine the 120V was low, about 20V, I had to replace a couple of transistors on in the power supply including Q3. You should be able to replace Q3 with a PN2222A, that is what I put in my scope, and it has held up fine so far.I should warn you that the rectifier bridges in these scopes are prone to failing, I have seen several instances of weird intermittent issues caused by the diodes, including my scope, although I have not replaced them yet I will soon.You should get one of those cheap component testers on amazon or ebay, they are about 20 bucks and can come in their own acrylic case, worth their weight in gold in my opinion.

author: lowimpedance
author_rank: Super Contributor
timestamp: May 18, 2021, 11:41:37 pm
content: You should measure the 10K B-C and C-E and 8.2B-E , so it looks like Q3 is toast. As Alex said the PN2222 will be ok. Check Q2 and in particular its connector.edit : the original was a GP NPN 2N3904 for ref.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 12:28:47 am
content: I had to replace Q3 and Q4 on my scope, which were PN2222A (or 2N3904) and MPS A42 respectively. Not sure about Q2 though.

author: john2021
author_rank: Contributor
timestamp: May 19, 2021, 04:21:15 pm
content: Thank you all you guys for your immediate and correct responses.Success!!!I’ve changed Q3 and got it to power up. I still need to test it for a longer period of time to see if it holds and doesn’t heat up.Thanks again, much appreciated.John

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: May 19, 2021, 10:21:55 pm
content: Great!. I wonder why Q3 always goes in the first place?

author: lowimpedance
author_rank: Super Contributor
timestamp: May 19, 2021, 11:26:47 pm
content: Excellent, and while your there as Alex said cast a critical eye on those bridge rectifiers too and their solder joints etc.

author: john2021
author_rank: Contributor
timestamp: June 03, 2021, 12:04:17 pm
content: Dear forum membersSorry to bother you again with a problem with my HP 1740a.The low voltage power supply gets the correct voltages on all test points.Now I noticed a shorted & burned diode near C2 that I can’t find on the schematics (see photos).Can you please identify it? it has 23082 possibly written on it.Another issue is, I get a beam only when I press the beam locator, and not a constant running trace as it is supposed to, when powering on. Is this connected to the diode issue?Thank you all in advance for any helpJohn

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 03, 2021, 02:00:54 pm
content: It looks like it is VR4, on the board layout picture VR4 is in a different spot than it is on your circuit board but I believe it is the right one. According to the schematic VR4 is a 4.65V zener diode and it makes up part of the flood gun power supply. I am no expert but I believe that if the flood gun does not have the right voltage it would cause problems. Before you replace the diode I would make sure that neither Q12 or Q11 is shorted, that diode could have failed due to time but you never know.I noticed while looking at the pictures you attached that a couple of the bridge rectifiers have been replaced, I recently replaced the ones on my 1727A with ones that actually fit right, do you want the part numbers?

author: john2021
author_rank: Contributor
timestamp: June 03, 2021, 04:09:21 pm
content: Thank you very much Alex for the quick replay.You were right about VR4 as I now see that the HP part number in the manual is 19023082.As for Q12 and Q11 they don’t seem to be shorted.Now the question is what a suitable replacement is?I have on hand 1N4742 12V 1W Zener Diode and some others, but I don’t know what the important parameters are, in regards to choosing the right one?The rectifiers have been replaced before by someone who messed up the trace on the board, so I don’t want to risk a breakdown, while they still work.Thanks againJohn

author: lowimpedance
author_rank: Super Contributor
timestamp: June 04, 2021, 12:01:16 am
content: Looks like the LVPS has gone through quite a few revisions as your PCB layout is completely different to mine and just to confuse the paper manual I have is different to both of ours.Anyway I took a guess at which zener on my 1740 was the likely one and measured it at 4.7V and as it turns out the other diode in that area of the circuit is just a standard signal diode !.I suspect a completely revised circuit was done for the floodgun drive, (which BTW is only for the scale illumination!), between your version and mine. So bets guess is a 1/2 to 1 W 4V7 zener will be fine.Have you measured the volts across it or is it open cct ?.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 04, 2021, 03:01:05 pm
content: I recently replaced several zeners on my 1727a LVPS, they were still OK but they had browned the PCB beneath them and were a little crusty. I Think the originals were 1W but I went with 5W and elevated them off the PCB a little in hopes that they would dissipate heat better. It might be worth going for a bit higher wattage since it seems like the zener broke down from heat stress, I could be wrong though.So the FG is for illumination? that's good to know, the 1727a is a storage scope and I am still trying to figure out what all of the many many parts of my storage CRT do,

author: lowimpedance
author_rank: Super Contributor
timestamp: June 05, 2021, 02:30:48 am
content: On my 1740 LVPS version none of the zener's show any heat stress, not to say that other components in the unit don't get toasty. The TO-39 transistors with the heatsink pressed on for example most certainty do need it.The floodgun drive on the 1740 is part of the scale illumination. On the storage CRO's their tubes are different and there may other factors being considered. Or maybe not.

author: Alex_Baker
author_rank: Frequent Contributor
timestamp: June 05, 2021, 03:27:15 am
content: Quote from: lowimpedance on June 05, 2021, 02:30:48 amAs far as I can tell those storage CRTs are a giant mess of magic and wizardry!I will stop hijacking this thread now..

author: lowimpedance
author_rank: Super Contributor
timestamp: June 05, 2021, 02:17:15 pm
content: Quote from: Alex_Baker on June 05, 2021, 03:27:15 amDo offer any hints or hard won advice as these HP CRO's generally don't get as much love here as there contemporary TEKTRONIX versions do..