HP 6266B (40V/5A) Crowbar repair and tear down pictures

[HighVoltage]

This HP6266B Power Supply was a really good hunt, almost free, advertised as broken and the repair was simple.
It seems the only thing that happened was that the crowbar of the Over Voltage Protection circuit got burned up, causing resistor R102 (0.125 Ohm 5W ) to burn up. Replacing the fuse and R102 repaired the power supply.

I did not have a 0.125 Ohm 5W resistor, so I use a 1.5 Ohm 10W instead to repair the crowbar circuit.
This seems to be a very good DC power supply for up to 40V and 5A.

My question to those who know much more about this power supply:
Is there anything else that I need to check?
I did test the CR56 Si Diode (100V 40A) and it works perfectly.
Is there anything else I need to watch out for in these power supplies?
Thanks for any advise.

Here are a few tear down pictures and the schematic of the crowbar circuit.

[HighVoltage]

More pictures

[HighVoltage]

More pictures

[Shock]

Generally you want to check the surrounding circuits components and semiconductors for open and shorts, then check across the supply rail and then make sure nothing further has failed in the power supply area.

It would be best to give that vintage of equipment a graceful power on with a variac especially if it has been sitting around forever. If you want to go to the trouble of checking and reforming caps you can go down that path as well.

[MadTux]

Lol, that happened to one of my HP 6274Bs too. The resistor got fried even worse and burnt itself into the PCB. Used a Dremel to remove all the crunchy black PCB material and soldered a new resistor onto the remains of the copper traces. Finally used UHU glue to putty the resistor back in place. Not nice, but works fine.

Things to learn from this: Be careful when using HP-62xx PSUs or any other PSU with overvoltage crowbar to power devices that can act as power sources, such as batteries, DC motors with big flywheels that can act as generators etc. If the crowbar trips, the source will fry the resistor

[HighVoltage]

Generally you want to check the surrounding circuits components and semiconductors for open and shorts, then check across the supply rail and then make sure nothing further has failed in the power supply area.

It would be best to give that vintage of equipment a graceful power on with a variac especially if it has been sitting around forever. If you want to go to the trouble of checking and reforming caps you can go down that path as well.

All other components are good and the rails are perfect as well.
Thanks for the hint to power through a Variac, I have done just that and it all seems to work.
I went through the full calibration procedure and everything is in alignment with the manual.

[HighVoltage]

Things to learn from this: Be careful when using HP-62xx PSUs or any other PSU with overvoltage crowbar to power devices that can act as power sources, such as batteries, DC motors with big flywheels that can act as generators etc. If the crowbar trips, the source will fry the resistor

It happened to me before as well, when I charged a SuperCapacitor bank (27V 300F) with another Agilent PSU.
This PSU did not have a protection resistor installed
The SuperCapacitor was hooked up and the PSU went to Over Voltage Protection and big smoke came out of it.
Turned out, that the big PCB traces for the protection burned away.
I am using this PSU now without the OVP.

Really, I think Agilent should have a big warning on the outside of the PSU, explaining the Crowbar installation, especially when there is no protective resistor installed. It seems they rather kill their own equipment and protect the DUT.

Yes, I also learned a great lesson at that time.
 

[HiTech]

I did not have a 0.125 Ohm 5W resistor, so I use a 1.5 Ohm 10W instead to repair the crowbar circuit.

Is that 1.5 ohm a typo? That's a big difference between the two values! If it is 1.5 ohms indeed, it should be affecting the trigger point for the crowbar circuit and or the over-voltage adjust range. At least try to find something closer to the original value like .2 ohm

[HighVoltage]

Is that 1.5 ohm a typo?

ooops .... that was a big typo
I used a 0,15 Ohm resistor that actually measured 0.142 Ohm
May be I will order a correct replacement resistor in the future.

The PSU works absolutely flawlessly and I can only highly recommend these older HP power supplies

[Leo Bodnar]

While you are in there (you still might be - who knows!) replace caps in the mains input filter, they will most probably explode sooner or later. 
There seems to be a problem with moisture ingress with RIFA caps so it's a question of time, not amount of use.

There is RIFA appreciation thread here https://www.vintage-radio.net/forum/showthread.php?t=69128

Leo

[HighVoltage]

Leo,

Thanks, this is really good advise and I do replace old RIFA caps or Schaffner filters all the time as a precaution on older instruments.
I had two Philips counters with failed and exploded schaffner line filters. (May be they use RIFA caps in there)

This HP 6266B has been working perfectly alright since I repaired it in 2014.

But the most important thing I learned from this repair is to look out for a CrowBar being turned ON on any PSU in the lab, before I use it.

[SeanB]

Schaffner did use RIFA capacitors, and then potted them in resin as well, or in bitumen in some units. Does not help with age, as the resin also cracks, and the bitumen evaporates out the volatile components and then cracks, plus now has a lot of carbon nanotubes as a conductor in it as well. When the caps go nuclear now they also have added smoke generation capable material to do so, plus it catches fire and burns internally to the case till the solder melts off.

[HighVoltage]

Good to have confirmation that the Schaffner filter used RIFA caps.

I had one Schaffner filter that not only exploded but the bitumen inside started to burn and destroyed the complete instrument. After I came back from lunch, it looked like the lab was on fire and it totally scared me.

Since then, old Schaffner filters get replaced by me on old instruments right away.

[Wbbev2]

Does anyone know if there are replacement meters or brackets available?

[HighVoltage]

If you want original meter replacement, you best bet would be to buy a broken PSU and use those.
Or you find something similar and make it mechanically fit the instrument.