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 Post subject: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 3:52 am 
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I've got a flakey HP 6202B power supply that I'm contemplating restoring. The scan of the manual on the Agilent site is pretty poor:
http://cp.literature.agilent.com/litweb ... -90001.pdf
If I get serious, I'll have to invest in a scan from Artek or a paper manual. Anyway, I've been puzzling over the schematic and finally came to the realization that most of the supply voltages (labeled things like +6.2V, -6.2V, -4.4V) are actually floating from ground and are instead referenced to the power supply positive output voltage! And all the little flags that I first took for ground or negative voltage rail symbols are actually flags indicating a connection to the positive sense voltage!

I'm no power supply wizard, but this is the screwiest power supply design I've ever seen.

I've read allusions to the Harrison supplies not being up to the quality of the rest of HP's test equipment line. Looking at this kludge of a design, I'm inclined to agree.

Anyway, anyone have any tips for working on these? Any typical failure modes? It's basically alive, but the voltage metering circuit is out of calibration and the current metering just bounces around from meter stop to meter stop as I adjust the current limit.

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 4:28 am 
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stevebyan wrote:

I've read allusions to the Harrison supplies not being up to the quality of the rest of HP's test equipment line. Looking at this kludge of a design, I'm inclined to agree.



Conventional Wisdom in the three Cal Labs I've worked in was that HP bought Harrison Labs to improve their Power Supply offerings. I've used, owned and repaired several Harrison power supplies, and always thought them to be superior to Lambda, Kepco and Sorensen.

I've never seen a Harrison power supply that failed on it's own - most were subject to abuse by negligent techs or sheer ignorance.

Just my experience,

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 5:58 am 
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I believe the legend states that power supply voltages are referenced to +5 volt.

When working on these supplies it is important to make sure the reference voltages are correct, then go from there.


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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 12:45 pm 
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I have a 6202A which has always worked well, but no manual.

I don't recall if H-P even made transistor power supplies before buying Harrison.


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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 1:14 pm 
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HP/Harrison supplies are excellent designs. I have quite a few different examples. In the day of tubes Lambda ruled the roost but as semis took over they fell by the wayside for some reason. I suspect they felt as did many, that they had no competition so why change.

Quote:
I don't recall if H-P even made transistor power supplies before buying Harrison.
Weren't these little single meter supplies pre-Harrison? I think the 7XX series were all before the Harrison acquisition.

Image

THis is the first HP supply I remember.

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Last edited by Mikeinkcmo on Aug Tue 11, 2015 1:21 pm, edited 1 time in total.

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 1:20 pm 
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The little triangles with "+5" do not connote the common point of the circuit. They are the 5-volt reference bus, and everything with that symbol is tied together. Where to place the meter common or return depends on where you are in the circuit. The troubleshooting section of the manual gives more details (but none too many). The fact that there's no simple common point means that they really did their homework; having neither the positive nor negative side of the output is referenced to chassis ground is essential for control of low level noise. This power supply had a 200-nanovolt ripple spec at full output. Not the easiest thing in the world to make, even today.

For a product introduced in 1966, this is an extremely advanced design. It can be tough to troubleshoot due to the multiple feedback loops employed, but when they are working right, these are very stable and reliable power supplies. Good news is, if nobody "cooked it" and the resistors and semiconductors are still good, you will probably find that it should only take some new electrolytics to get the unit back on its feet. That's been my experience with these anyway. Harrison used the best electrolytics they could at the time, but after 50 years or more, any that still work can reasonably be expected to be on their last legs.

And more than a couple of engineers and techs who were there at the time said they thought the quality of Harrison power supplies actually declined after HP acquired the company!

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Tue 11, 2015 4:22 pm 
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Location: Pocasset, Cape Cod, MA
Quote:
In the day of tubes Lambda ruled the roost but as semis took over they fell by the wayside for some reason.
Not for quite a while. These linear supplies were very popular and built like tanks:
Attachment:
Lambda.jpg
Lambda.jpg [ 45.69 KiB | Viewed 1658 times ]

But they were expensive and eventually people started buying Asian-made designs.

Quote:
I think the 7XX series were all before the Harrison acquisition.
I just looked in the H-P catalogs. The 721 appeared in 1959, and the 722 and 723 in the 1961 catalog. Harrison's 6000-series models appeared in 1963.
Attachment:
Harrisons.jpg
Harrisons.jpg [ 36.32 KiB | Viewed 1652 times ]


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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Wed 12, 2015 3:33 am 
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Johnnysan wrote:
I believe the legend states that power supply voltages are referenced to +5 volt.

Chris108 wrote:
The little triangles with "+5" do not connote the common point of the circuit. They are the 5-volt reference bus, and everything with that symbol is tied together.

This is why I need to invest in a better scan or a paper copy in place of the scan on the Agilent site. I too at first thought the little triangles contained "+5". But then I tried to figure out where the negative feedback was coming from. Eventually I noticed that the positive sense connection on the output terminal strip was connected to a little triangle with "+S" (not "+5") in it. So it's not a +5 volt rail, it's the positive sense line from the output! And it goes everywhere - there are at least seven connections to it, and at least three of them appear to be feedback loops for the voltage regulator.

As a result, puzzling out the operation of the circuit from the schematic is a real brain-twister. I'm glad HP produced such good manuals. I'll have to spend some quality time with the theory of operation section.

Chris108 wrote:
Where to place the meter common or return depends on where you are in the circuit. The troubleshooting section of the manual gives more details (but none too many). The fact that there's no simple common point means that they really did their homework; having neither the positive nor negative side of the output is referenced to chassis ground is essential for control of low level noise. This power supply had a 200-nanovolt ripple spec at full output. Not the easiest thing in the world to make, even today.

Interesting, I hadn't looked at the specs. Thanks for cluing me in.

Chris108 wrote:
For a product introduced in 1966, this is an extremely advanced design. It can be tough to troubleshoot due to the multiple feedback loops employed, but when they are working right, these are very stable and reliable power supplies. Good news is, if nobody "cooked it" and the resistors and semiconductors are still good, you will probably find that it should only take some new electrolytics to get the unit back on its feet.

There's a crispy section of the PC board underneath a power resistor that's obviously been replaced. And there's flux on a lot of connections on the bottom of the PCB. Someone either re-soldered a bunch of joints or replaced a good many components. But since the output is basically alive, I don't think there's very much wrong with it. It needs a good cleaning first; the voltage and current range switches might just be dirty.

Thanks to everyone for the education on Harrison's engineering. I should have read the manual before forming an opinion :oops:

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Aug Wed 12, 2015 3:53 am 
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Quote:
This is why I need to invest in a better scan or a paper copy in place of the scan...
I always purchase original paper manuals for every piece of gear I have, it's just part of the purchase price. I've found that when it comes time to sell I get my money back and then some. Paper manuals are quicker and easier to use on the bench than messing with computer PDFs.

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Jul Mon 08, 2019 9:25 pm 
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To recap, some time back, I got a bit of an education on Harrison power supplies via this thread.

Image
I was trying to puzzle out the 6202B from the poor scan of the schematic in the manual on the Agilent (now Keysight) web site.

The multiple feedback loops make deciphering the operation of this power supply pretty challenging.

Image
The initial problems were that the meter was missing the bezel that holds it to the front panel, ...

Image
... the voltage metering was out of calibration, and the current metering just bounced around from meter stop to meter stop as I adjusted the current limit.

Image
However, the supply had output and responded to the controls, so it was basically alive.

Image
The interior was initially filthy with some kind of oily grime. (These are “after” pictures.)

Image
A thorough cleaning with spray cleaner left this residue.

Image
I followed it up with a scrubbing by a paintbrush with isopropyl alcohol.

Image
After cleaning, everything worked fine!

Image
The 6202B is remotely programmable through this row of terminals and jumpers on the back.

Image
Fortunately the meter bezel, though cracked, was still rattling around inside the cabinet.

Image
So, a little plastic solvent and the bezel was usable again.

Image
The bezel slides in from the front of the panel, and then the meter clips into the mounting ears.

Image
All back together.

Image
I haven’t verified the performance against the specs, but it’s working well enough for my purposes at the moment. It could probably use some new caps.

Image
Simon reports that he needed to replace the filter cap on the reference on his 6202B.

Image
I don’t have a photo handy, but I rigged up a 10KΩ ten-turn pot to control the voltage via the remote voltage programming terminals. It lets me dial in a precise voltage when I’m using it to calibrate meters against my HP 3468A.

Image
I gave it some new feet, since the old ones were torn up.

Image

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Jul Tue 09, 2019 12:17 am 
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Findm-Keepm wrote:
I've used, owned and repaired several Harrison power supplies, and always thought them to be superior to Lambda, Kepco and Sorensen.

I've never seen a Harrison power supply that failed on it's own - most were subject to abuse by negligent techs or sheer ignorance.
Hi Brian,

My experience matches yours.

Many decades ago I worked for a metrology shop that serviced all of those brands and similar.

We always felt the Harrison supplies were better-designed than any other make.
Harrisons were also IMPOSSIBLE to troubleshoot due to complex feedback loops.

- Leigh

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 Post subject: Re: HP/Harrison 6202B power supply
PostPosted: Jul Tue 09, 2019 5:03 am 
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Steve, I'm sure you know this, but HP and TEK made some of the best service manuals ever printed. If one reads the theory of operation section while reviewing the schematic, there will be no problems troubleshooting any of their equipment.

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