HP 6200B Power Supply repair - help needed. - Page 1
Forum section: electronics / repair

author: VictoriaGuy
author_rank: Contributor
timestamp: March 10, 2015, 10:11:05 pm
content: I've read some discussions here on 6200B repair, but I thought it would make more sense to start anew, as my PS has some different problems.eBay purchase -AsIs - so I expected problems.Board has a couple of seriously toasted areas and evidence of a previous repair.I found a couple of 'trouble spots' - replaced the reference 6.2v zeners and a couple of resistors.The voltage goes 'up and down' as it's supposed to, but there's no current output with a load.Meter is also erratic on anything but 50V monitor setting.My first question is pretty basic: The schematic has voltage measurements but I can't figure out what they are referenced to in some cases. (Circuit is isolated from chassis ground.)The manual hassomespecific instructions for troubleshooting, like attach ground probe to S+ and voltage probe to Test Point 'x'....that I can follow.Then the next section says 'Measure voltage between x and y. If positive, go to step..., if negative go to step...' Is there a grammar convention I'm missing?On some points in the schematic, it's straightforward - a vertical line with arrows between two points. At others, it can be +6v and a short distance away a -4v specification.Are all the + voltages referenced to the - output and vice versa?I want to get some data and some specific questions before I come back for help!Thanks!

author: Kevin.D
author_rank: Frequent Contributor
timestamp: March 11, 2015, 11:01:27 am
content: Hi. Yes all voltages are referenced to S+. The ones that arent then the refs points are given by the arrows.You need to clarify the fault symtoms a bit. I think your saying without any EXT load (there is always a small internal load present) then Output V measured at output terminals adjust normal. But then at a specific load current the output Voltage collapses ? .If so then perhaps it's simply your current limit is stuck low.I would check the +/- 6.2V lines are present and they are not being pulled down by excessive current draw.

author: VictoriaGuy
author_rank: Contributor
timestamp: March 11, 2015, 06:25:38 pm
content: Thanks, Kevin.I've made a bit of progress on this project.Thanks for the info on voltage measurements. Once I printed an enlarged copy of the schematic, I figured out that a lot of the voltages can be measured from the (nicely marked PCB) test points.The power rails look OK with more-or-less correct voltages (within a volt) of spec, and some (100mV or so) ripple at the filter caps (just checked with the AC range on the bench meter). Ripple on the output looks to be <1mV, so that seems fine.The voltage controls and current limit seem to be working properly.Part of the problem was that I didn't fully understand the interplay between the CV and CC in this supply ('user error'), being accustomed to a BK supply with an LED to let me know whether CC or CV was limiting. A second 6200B unit arrived in the mail yesterday and it is working OK, so I could figure out how things are 'supposed to work' by comparing the two units.I need to go through the rest of the 'performance checks' in the manual, as much as I can without a pile of calibration and test gear.But first:It looks like it's mostly a meter circuit problem now as the 1st priority.I 'cured' some of the erratic meter action with contact cleaner into the meter switch.However I can't get the voltage meter to read the full (40v) output voltage even with the adjustment pot fully cranked. Plus a couple of other oddities. I'll report back after I have some more data.Thanks again for the help.I'm sure to be back here, as troubleshooting transistor circuits is new to me (a simple tube amp I can handle....).

author: VictoriaGuy
author_rank: Contributor
timestamp: March 11, 2015, 06:48:07 pm
content: A few pics of the PCB - at some point in the unit's history the 'Self Protection' features didn't work!Most of the visible repairs were done before I got the unit.

author: dacman
author_rank: Frequent Contributor
timestamp: March 12, 2015, 10:12:45 pm
content: Check Q6 & Q7 and the diode across them. Also, I'd check any aluminum electrolytics in-circuit with a capacitance meter.

author: VictoriaGuy
author_rank: Contributor
timestamp: March 13, 2015, 02:56:51 am
content: Quote from: dacman on March 12, 2015, 10:12:45 pmThanks.Checking the caps with an ESR meter is something I can do, though figuring out which terminal is which under the cans is a problem. The low ripple on the output is reassuring, but I'll have another shot at checking the caps.I'm completely ignorant about transistors - How do I 'check' Q6 & Q7 ? (I've attached that section of the schematic.)Checking the diode - just (with power off) check the resistance 'in both directions' with the meter and should get short one way, and open the other? And 0.7 volts across it when the circuit is powered up?Thanks for your help.

author: Paul Moir
author_rank: Frequent Contributor
timestamp: March 13, 2015, 09:49:38 pm
content: Yes, that's good for an alive/not alive check, which will turn up the vast majority of failures. However sometimes transistors, particularly earlier model transistors, fail in subtler ways. The only decent ways of checking them is to test the circuit voltages and figure out if they're "doing the right thing", or to pull them and check them with one of those transistor checkers.Given that they blew the bridge rectifier, they likely blew the transistors Q7 & Q6 as well. My first check would be to see if they've been replaced with the wrong type.Have you measured the meter output with a multimeter? Is the supply putting out 40V and the meter reading under? Have you cleaned the controls with a blast of tuner cleaner? Are the jumper connections on the rear terminal strip good and the screws nice and tight? That's a very common issue with these.For load tests, you can get creative. Hunt around for a heater (ie, a big resistor) that suits your desired load. Or something in parallel or series that will work. When I first checked my 6294A I used a hair dryer as part of the load bank. Mikeselectricstuff has a video where he uses some nichrome wire submerged in water in a bucket for a load, which is I guess a little more professional.PS - I'm pretty sure that some of the calibration steps are really impossible, particularly some of the constant current stuff.https://www.eevblog.com/forum/repair/old-hp-6294a-1a-60v-psu-calibrationrepair-question/I did a lot of head scratching on that, and in the process got to understand the supply very well. My meter circuit was working fine though after cleaning up the pots & switches with tuner cleaner.

author: dacman
author_rank: Frequent Contributor
timestamp: March 14, 2015, 01:37:13 am
content: You should get 0.7 volts across the diode when powered down using a DMM with a diode check function that displays voltage. The diode should not read 0.7 volts when powered on in-circuit, because it is reverse biased. Q6 and Q7, I would assume, would be TO-3 transistors on heat sinks in the back of the unit. They are in parallel, to a degree, and at least one would need to be removed to check them both. Get a DMM with a diode test function and check across each transistor junction (B-E and B-C). They should drop about 0.6 Vdc one way and be open the other. The same goes for the diode. (Note that germanium junctions drop about 0.3 Vdc.) The transistors should be open C-E both ways, although in circuit, the diode would come into play since it is across C-E of Q6 and Q7. Also, do some Easter egg hunting (check each diode and transistor and each resistor and each electrolytic capacitor). Make sure the wipers of the adjustments are not open. (Note: I would lubricate the trimmers with LPS-1.)The reason I mentioned Q6 and Q7 is that they are the series regulator transistors. There is something wrong with this circuit, either they are blown or they are not being turned on. You need to start somewhere.Also, I don’t trust ESR only meters. You need to know what the ESR should be (IMO).

author: VictoriaGuy
author_rank: Contributor
timestamp: March 14, 2015, 02:09:26 am
content: First - thanks to you all for your help.Dacman- I've printed out those transistor checking instructions and will practice with devices I have on hand here.Paul - The pass power transistors on the heatsink look to be original as they are identical to the devices in a second 6200B that arrived this week.I will do some more checking around, but I'm pretty sure the supply we're discussing is working OK.It seems to be delivering the current and voltages I expect and the current and voltage limit front panel controls seem to be working properly on this supply.Several things seem to have made the difference:Replaced VR1,VR2 zeners which brought 6 v rails back to working order.Sprayed the trimmer pots and meter selector switch with contact cleaner - there was some immediate improvement, then after a day or so, when I re-trimmed, everything in the meter action calmed down - no more erratic meter excursions.It also took a while to figure out (operator skill) how the current and voltage limit control interact on this supply, so I would know when it was working properly.Next up on the bench - another 6200B which doesn't seem to be putting out max voltage and current. I'll report back later on that one.Thanks again for the help.