HP 6034A Repair

[HalFoster]

Just throwing this in there - I'm having to say this from memory - but while you are checking things, I believe there will be 2 1.5K (I think) 5W resistors under the fan; I've seen several of this series where those overheat.  Just something to check.

Hal

[anewmanx]

Thank you so much for the tip! I hadn't checked anything under the fan. I will tonight.

[anewmanx]

Those resistors ended up being fine, but thanks again for the tip!

Update:

I did make some progress tonight. I tested both MOSFETS very thoroughly and they were functioning properly. I decided to repopulate the half of the driver circuits that feeds Q2. I repopulated CR11, R68, R21, & Q4. System turns on and nothing blows up this time.

Next to fix the other side and see if we get the same result.

[anewmanx]

Fireworks again.

Replacing the components leading to Q1 resulted in blowing the entire set again. A spectacular display of smoking resistors and transistors.

CR11 punched through & Q4 did not die. R22 blew. R 68 did not. R23 died.

CR12 did not punch through, but Q3 died. R14 & R67 both blew. R16 died.

At this point I'm at a loss. At least I believe I have narrowed it down to something in the circuit that leads to the Q1 mosfet.

R20 appears out of spec now when I'm testing. It's showing 0.33 instead of 1mohm. Perhaps that could be causing the problem. I'm in over my head at this point on what to do. I don't want to be wholesale ripping parts out of this board and replacing them all. The traces lift too easily even with the soldering iron set to a lower setting.

Every other part around Q1 appears to be in spec. Even T3 which I tested with a function generator and oscilloscope.

[anewmanx]

Between other obligations I tinker with this thing.

Well, today we made some real progress!

Q1 is definitely broken and that explains everything blowing up when Q1 gets hooked up. I'm showing 20 ohms resistance gate to drain out of circuit.

I believe I will replace them both while I am at it, just for good measure. Now to find a suitable replacement! All the manual says is that they are FETS, with part number 5080-2017 and I have seen online that this part was replaced by 1855-0547 on one website.

This website seems to have information on the one that replaced the original: https://aerobasegroup.com/part-number/1855-0547_01-275-8265 Still bloody expensive used on ebay. I need to find another solution.

Any suggestions?

I may have found an inexpensive site for the replacement FET, if the site can be trusted. Has anyone heard of this site before?

https://www.transistormosfet.com/index.php?route=product/search&search=1855-0547

[bsdphk]

Spend a moment looking at other similar PSU's from HP, they might share the same primary side and provide different info about parts.