HP 6299A Power supply upgrades?

Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
I recently picked up a 2nd HP 6299A power supply. these are nice 0-100V .75A power supplied with adjustable current and voltage regulation. they also can be wired in master-slave series mode for tracking Bi-polar operation. perfect for designing amplifier stages! as these supplies are now 40+ years it is time for some maintenance and upgrades! All the electrolytic caps will have to be replaced. As capacitor technology has gotten better and smaller. I have an opportunity to upgrade the capacitors. But the questions is, where and by how much?

The owners/service manual can be found here.
http://cp.literature.agilent.com/litweb/pdf/06299-90001.pdf


Looking at the main bulk supply capacitors. They all appear to be pretty small. 3 main supplies, all half wave rectified. The main 150V supply has a 710uf cap, the 125V supply has a 200uf cap and the reference 27V supply which I would think is pretty critical, only has a 100uf cap! I would think all 3 of these supplies would benefit from a decent increase in uF!! Now as Overkill is my nature, my first thought is to stuff as much uF as I can in there. Putting overkill aside for a moment, How much of an increase would be of benefit?



And what about R49 (and R11)?? here we have a 2.2K 20 watt resistor across a 150Vdc line just idling away dissipating 10 watts of heat! and they stink! burning off 40 years of lab and attic dust and dirt! scrubbing them in alcohol helped but... why is it there? all I can think of is that it is to quickly drain C14 at shut off so there is no errata out of the supply after power off. To me it just seems a waste to be burning up 10 watts of heat for no reason. why not add a relay to the 120V line that would connect the resistor across the supply at shut down? what am I missing here??
 
I think it goes without saying you have to be careful upgrading remember these power suppliers are basically adjustable output VERY HIGH precision amplifiers. Depending on the topology it can be VERY easy to drive them into oscillation by messing with the circuit. Also, too much capacitance in a lab supply can be a VERY bad thing, especially on the output. It messes with the current and voltage control circuits, it can cause huge voltage or current spikes, delays the response time of the power supply, etc. These are all things that don't matter as much in an audio supply but can be really critical for a lab power supply. In a bench/lab power supply, seldom is more uF better (as opposed to audio power supplies where usually more uF is better).

The 2.2k 20w resistor is likely because they power supply isn't stable without a minimum load. I can promise you, the HP engineers back then were REALLY intelligent. They would not leave a huge expensive part like a 20W resistor in there for no reason. These guys designed the 6114A/6115A supplies, a precision power supply which to this day you would have to pay around $5k to get similar features and performance.
 
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infinia

Member
2005-05-15 9:51 am
SoCal
I have DC supplies using the same design running on original 60's era bulk caps, all the other elctros where replaced with the same values some 2 decades ago and still has low noise. The higher the voltage like yours, the more strict replacement policies apply.
yea sure lab linears aren't known for low heat, consider a custom fixed design . these have low noise and other features, and they are proven reliable.
 
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Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
Well this is exactly why I asked! and I am only looking to upgrade the bulk, pre regulator supplies. the rest of the caps I will replace with as high a quality cap I can find in the same values.

The 710uf main cap is going to have to be replace with a 1000uf cap which is probably close enough but, if there is reason to, I can fit a much larger cap in that space.

The manual does state a larger output cap could be used but had some warnings. I'm ok with keeping the values of everything in the reg and post reg stages the same values.
 

infinia

Member
2005-05-15 9:51 am
SoCal
Id go for 50% value increase and possibly 20-30% higher voltage. and still be 1/4 of the size. LOL
FWIW axial VS radial and stable mounting are the issues IMO.
the only design upgrade I did was adding a small electro and 0.1 uF ceramic across the output binding posts
 
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Zero Cool

Member
Paid Member
2004-09-20 6:10 am
MN
the biggest cap is a screw can type, 2 of the caps or the old metal can axial type with 4 post's. I can find snap in caps that fit those locations. but the rest are all axial and yes hard to source new parts. HP used all Sprague and I have found new Spragues on eBay but they might be just as old for all I know. Nichicon seems to be the newest maker of Axial type caps so I may have to change brand. I am not one to kluge in radial cans with wires and zipties and crap to try and get them to work. I try and keep things as close to the original design whenever possible.