Need T1 output voltages HP 5386A - Page 1
Forum section: electronics / repair

author: KA3YAN
author_rank: Contributor
timestamp: September 08, 2018, 07:27:03 pm
content: All, I have an HP 5386A with a bad transformer (T1) in the power supply. Would someone be willing to check the output voltages for the red, yellow and green wires? I doubt I'll be able to find a replacement transformer (HP part number 9100-4493), but I might be able to find a suitable replacement PSU...or make one.Also, I'm assuming that both yellow wires have the same voltage, and both red wires have the same voltage, but I'd like to know for sure.Any help would be greatly appreciated.Brad

author: ArthurDent
author_rank: Super Contributor
timestamp: September 09, 2018, 02:31:05 am
content: I think I got the correct information this time. Here is the description and the schematic of the power supply section that might help. The green wire is the CT that is fused by the 2 amp F1 to ground and if that fuse is open then the transformer would appear bad. The yellow wires are the full output and the red wires are lower voltage taps. There is a .25A primary fuse, F2, as well. both may be pico fuses about the size of a resistor. If they are open you need to find what caused them to blow. The diodes CR13-CR21 or large caps might cause these fuses to blow.I would suspect the transformer last unless you have some good reason to suspect it is defective.

author: KA3YAN
author_rank: Contributor
timestamp: September 09, 2018, 02:39:45 am
content: EDIT: looks like you edited yours while I was replying.ArthurDent, not sure that diagram is correct (see mine). The 5386A has a split primary (120/240VAC) and a split secondary. F1 is a 2A fuse and F2 is a 250mA fuse. Both fuses are good. I've removed T1 and tested out of circuit. Continuity is good on the primary side, but there's nothing on the secondaries. Not only that, but there's only a few milivolts output with 117VAC input.To be clear, there are four wires going in:- Black (Hot 120/240V)- Black/Yellow (Neutral 120V)- Black/Green (Hot 120V)- Black/Red (Neutral 120/240V)Leaving T1 are:- Yellow x2- Red x2- Green x1I don't know what the yellow, red and green voltages are. I assume green is neutral since it terminates through F1 to signal ground.Red goes into the full wave bridge rectifier consisting of CR13, CR14, CR16 & CR17.Yellow confuses me a bit. I should know what's going on, but it isn't a classic bridge rectifier circuit. Maybe two full wave rectifiers? This consists of CR18, CR19, CR20 & CR21.Bottom line is that I really need to know the voltages on the secondaries.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 09, 2018, 03:20:58 am
content: I edited my first post to have the correct schematic and description but reposted the same wrong schematic I had the first go around-damn. I meant to post the same schematic you have and the description I posted fits that one, not the one for the HP 5384 I posted by accident.It seems strange that you have no continuity on the secondary. I could see perhaps one lead opening but all of them seems highly unlikely. If there is only one open lead, and if you have the transformer out, you could apply an A.C. adjustable low voltage between those two (or any good secondary leads) and increase the voltage slowly until you get 120 VAC on one of the primaries and calculate what the secondaries should be from that. Probably a 1.5 amp secondary winding max, as a guess.With the CT, CR13-CR14 form a full wave negative and CR16-CR17 form a full wave positive rectifier, not a bridge. The other CR18 through CR21 are two full wave positive rectifiers, not a bridge. I suspect even though they both are positive and look redundant that that helps isolate them noise wise. There is a switcher involved and just using one positive full wave rectifier setup might allow noise coupling between the two output paths.

author: KA3YAN
author_rank: Contributor
timestamp: September 09, 2018, 11:28:34 pm
content: Well, I don't know what I measured the first time, but the secondary is fine. It's the primary that has no continuity.Yellow/Yellow = 4.8 ohmsRed/Red = 3.15 ohmsI could keep going with various combinations, but for the most part they range from 1.5 - 4.5 ohms.On the primary side it's an open circuit with any combination.That really narrows down where the problem is.BTW, the troubleshooting scenario you gave is quite good and makes a lot of sense. Reversing the direction of flow would've given me a good idea about output for the yellow and red secondaries. Too bad the primary appears to be the problem.

author: ArthurDent
author_rank: Super Contributor
timestamp: September 10, 2018, 12:43:08 am
content: There are two primary windings and it is unlikely that both failed. If you have the input switch set for 240 then the primaries are in series and any open lead would prevent a reading on both windings at the plug end but you could measure continuity right at the primary leads on the selector switch. If the selector switch is in the 120 position the primaries are in parallel and you should get a reading. It might also work for testing on 120 but if one primary is open but it probably isn't wise to run it that way.As long as one of the primaries is ok then you still should be able to test the transformer backwards to check the ratio and voltages as I previously described. With good secondaries you could also connect a 24V transformer yellow to yellow on the installed transformer and slowly increase the output with a variac until you get the correct outputs from the regulators, then add maybe 10%. This would be using the secondaries as an autotransformer. This would give you a good idea what the voltages should be or if it works and you can't find a replacement transformer, you could use an external transformer of the correct voltage across the yellow to yellow leads to use the counter until you might find a replacement.I have tried stuff like this myself but not with this counter so you'd have to decide if it works properly for you.

author: KA3YAN
author_rank: Contributor
timestamp: September 10, 2018, 11:05:03 pm
content: Just to be clear, I'm still looking for the secondary voltages if anyone has an HP 5386A that they don't mind popping open and probing.

author: KA3YAN
author_rank: Contributor
timestamp: September 17, 2018, 08:03:37 pm
content: I received a response from Keysight. The part is obsolete, but they gave me the specs. This will help me to work on a replacement part.