BK 1602 HV Power Supply Question

[Dawn]

I recently picked one of these up, recapped it completely and went through cal with no problems. All appears to be working nicely.

There is however one minor detail that I need to know if it's normal operation or not.

Under no load, the milliammeter reads 0 at 0V output as it should. Raising the voltage to full 400V output causes a no-load reading of about 5ma.

This power supply uses a cammed shaft to actuate microswitches that select taps on the main power transformer as the voltage is increased by the front panel pot. As the switches engage to the next tap raising the unregulated voltage to the pass transistor in steps, the indication on the meter moves likewise in steps from 0 to 5ma as opposed to a linear increase as voltage is raised.

Is this normal operation of these units?

[johansen]

dunk the whole thing in IPA would be my advice.

unless you want to trace the schematic out and figure out which resistor or capacitor is leaky..

[Dawn]

I'd have appreciated it if you'd read my posting completely. The entire unit has been recapped with new electrolytic capacitors. I do have the docs and went through a complete calibration without a problem. There is no shunt in this unit for the milliammeter. There is a sense resistor to ground from the output of the final pass transistor that will develop a volatage across it up to 1V full scale for 200ma. The front panel milliammeter is really a voltmeter. Since the quiesent current raises in steps when the taps are selected rather then increase smoothly as the voltage is increased, this would appear to be somehow related to the pre-regulated part of the supply which has been recapped as well with fresh pair of 470mf/450V electrolytics. They both have a 47K resistor accross each cap. This may very well be the typical operation. I don't know and would like to hear from someone that owns one if it's normal.

[codeboy2k]

I don't own one, so take what I say just as my "thinking out loud"...

There's some leakage somewhere. I think that's what the poster was saying by dip it in IPA... he was supposing that there is dirt somewhere, maybe contacts, traces, whatever, and it's causing a leakage path to ground that you see as a stepped current increase as the voltage tap is increased.

about 400V , 5mA means about 80k resistance in the leakage path, which is pretty bad, I would say.
So, as a non-owner of this supply, I would say that's bad, and is not normal.

The leakage could be a cap, but you replaced them all. It could be dirt..so IPA it... it could be a leaky transistor, so check voltages across the at and across the leads of suspected transistors...

Can you post the schematic, or a link.. thanks!

[Dawn]

I've cleaned the current trip switch with deoxit 100 using the brush, not the spray w/lubricant and just enough to clean the contacts which were black with oxidation originally and not soak the phenolic. I've also used a hypodermic type dropper and put a drop on each side of the wipers on the standby and range switch and worked them. Most of the chassis was wiped out with 91% IPA to remove the dust sediment including the foil side. That's pretty much the extent of cleaning that I've done. Apparently, another user I've contacted reports the same problem and has also recapped, that 's the reason I'm wondering about this. These weren't that common and it's difficult to find anyone else that hase one to see if theirs is doing the same.

 Last production of these units is 30 years ago, but there are few other options if you need a solid state HV supply with current limiting. Links to the manual and the schematic are below.

Thank you for the reply CB2K



http://brianandgina.net/files/Reference/BK_Precision/B&K%20BK%20Precision%20Manuals/1602%20Solid%20State%20DC%20Regulated%20HV%20Power%20Supply/1602%20Schematic.PDF

http://brianandgina.net/files/Reference/BK_Precision/B&K%20BK%20Precision%20Manuals/1602%20Solid%20State%20DC%20Regulated%20HV%20Power%20Supply/1602%20Instruction%20Manual.pdf

[codeboy2k]

Thanks for the schematic... Nice supply.  I like it.  I took a long look at the schematic, it's well drawn and lots of voltages marked on there at no load and at 200mA load, for both AC, DC voltages. It's well done.. They don't make schematics like that anymore.

Coupled with the detailed operating description in the owners manual, I can see this is a normal operation. They even mention that they slightly drive the pass transistor in order to prevent non-linearities at low voltages.

So the rise to 5mA @ 400V that you report and the other owner reports is normal for this supply, but even still, they could have done it differently and prevented that current draw at higher voltages.

If you look on the schematic you will see R30 is a 100K, 2W resistor directly across the output B- and B+ (it is drawn from B- to the circuit ground, but this supply regulates the B- voltage, and B+ is the circuit ground)

And I said, based on your previous description, that there is about 80k leakage resistance somewhere. It's not leakage, it's by design, and it's R30, which should give about 4mA or so at 400V.  It may be off slightly, or the meter is off and reading high so you see 5mA.

[Dawn]

Thanks for looking at the print and pointing that out. The value may very well be 4ma and on the money. The first mark on the meter is 10ma and 5ma was just an eyeball interpolation from zero being about half way between on full output. I saw that mention about the output amp slightly driven, but this is very unlike BK not to mention the meter indication as normal. They used to make a point to note any idiosyncracies as you probably seen about the current trip in their product manuals.

These power supplies were made as I understand from about '74 thru '83. They must have sold very few supplies during that entire run. This unit is an '81 made unit marked all over chassis components and the chassis, yet the circuit board itself was made in 1974 with corrresponding date codes on the parts. They must have had only one run of circuit boards for the entire production run.

Thanks again for noting that CDBY2. I really appreciate it.

[codeboy2k]

You're welcome.

With the schematic, you could mod that so the meter shows only actual current drawn, not the internal bias currents needed.  That's what I mean when I said they could have done it differently.