HP/Agilent/Keysight 6114 Repair

[echen1024]

Following tips from robrenz and PedroDaGr8, I recently picked up a HP/Agilent/Keysight 6114A ultra precision PSU off of eBay for $60 shipped. It was in poor condition, with a dent in one corner. When I powered it up, the OVP crowbar tripped immediately. Researching the service manual, I found that the OVP circuit could be disabled in this case by cutting the pin of R34 on the main A1 board. I cut it, and lo and behold, the PSU works perfectly. I have now started trying to isolate the problem and fix the OVP circuit. I soldered R34 back on, and no surprise, the issue was still there. Measuring voltages according to the table provided in the manual proves that test points are within tolerance, except for test point 18, on the A2 reference board. Where it should be reading 6.2V, it is reading -6.2V. checking VR4, it measures fine on my multimeter. I have also checked CR20 with my DMM. and it reads fine as well. Any suggestions?

The front panel


A look-see from the top


The big A1 analog board


Absolutely gorgeous A2 board with the voltage reference


The problem with VR4. 6.2V reading -6.32. And yes, I checked, my polarity is correct.

[robrenz]

The overvoltage trip point is adjustable with that screwdriver slot pot on the front panel.  Where is it adjusted to?

[echen1024]

all the way clockwise. Basically limited to full voltage

[BravoV]

Assuming the OVP adjustment pot is fine, just because of the OVP is kicking, disabling it using the hard way is not recommended imo.

It may have a serious problem even though it seems like its working with OVP disabled by force. 

If I were you, I'd dismantle the front panel and the components nearby like the pots and boards that are installed near or mechanically attached to the panel, and inspect & test them throughly.

There are possibilities that the hard impact that caused the huge dent at the front panel might affect the components integrity.

Remember, this is NOT ordinary adjustable bench PS, it more like a high power adjustable voltage reference to me  , so treat it carefully and with great care.

Edit : Posted the big and "stitched" HP 6114/6115 schematic -> HERE

[quantumvolt]

The negative 6.2 V you are measureing is correct. The table in the manual you refer to: Table 5-2 on p. 5-12, has two errors:

In line 4: meter common shall to test point 16 (not 17), and in line 6: meter common shall to test point 17 (not 18). Trust the schematic.

On the same page in the manual there is a troubleshooting guide for the crowbar. I would start with that.

[echen1024]

The negative 6.2 V you are measureing is correct. The table in the manual you refer to: Table 5-2 on p. 5-12, has two errors:

In line 4: meter common shall to test point 16 (not 17), and in line 6: meter common shall to test point 17 (not 18). Trust the schematic.

On the same page in the manual there is a troubleshooting guide for the crowbar. I would start with that.

OK. I will try that at home tonight.

Assuming the OVP adjustment pot is fine, just because of the OVP is kicking, disabling it using the hard way is not recommended imo.

It may have a serious problem even though it seems like its working with OVP disabled by force. 

If I were you, I'd dismantle the front panel and the components nearby like the pots and boards that are installed near or mechanically attached to the panel, and inspect & test them throughly.

There are possibilities that the hard impact that caused the huge dent at the front panel might affect the components integrity.

Remember, this is NOT ordinary adjustable bench PS, it more like a high power adjustable voltage reference to me  , so treat it carefully and with great care.

Edit : Posted the big and "stitched" HP 6114/6115 schematic -> HERE

The manual instructs me to disable it the hard way. It seems to be working correctly, even under load.

[echen1024]

OK. CR20 measures 100 ohms between the Ground and the unmarked pin. There are 3 pins, one labeled G, one labeled A, and one unmarked. Could someone with a 6114 check this and report back on what you find?

[robrenz]

G to blank  31.3Ohm either direction

A to G or A to blank with neg. probe on A is 1.6M Ohm

A to G or A to blank with pos. probe on A is open circuit

[echen1024]

For me CR20 is completely screwed. For me, G to blank is around 110Ohms, and G to A with the neg probe on A is 14kOhms. I will be looking for a 2N4102 SCR diode.