HP 6274B DC Power Supply - Page 1
Forum section: electronics / testgear

author: ruffy91
author_rank: Regular Contributor
timestamp: January 13, 2016, 12:29:54 pm
content: Hello together,I've got a HP 6274B power supply for around 50$. It's a 0-60V, 0-15A analog type which uses thyristor preregulation.The supply was manufactured in 1977.The power supply is in working condition but has a few problems.First of all the Voltmeter sometimes hangs on 50V when increasing the voltage, it is a mechanical problem as tapping on the front releases the needle.Is it possible to fix this by cleaning and reassembling the panel meter or should I leave it as is, as I only will make it worse?Second problem, the output capacitors.These are 2 x 22mF, 75V electrolytics from Sprague, series 32D (about 8cm diameter and 14cm high). Is it possible that they are still in good condition or do I have to replace them?When replacing, can I replace them with 1 piece (f.e. Kemet ALS31 22000uF, 100V) they have an ESR of 13mOhm and a ripple current of 18A, so I can't see why it would need 2 of them, especially as they are wired with about 30cm of ~AWG16 wire, which has 8 mOhm by itself.They measure about 1.7mF with my U1241B multimeter, but I don't trust this reading as the meter can only measure up to 10mF.I have attached some pictures as reference, sorry for the quality of the pictures, the optical image stabilisation of my smartphone has some problems (picture is rock steady but as soon as I press the shutter it starts jerking around).Sincerely

author: PedroDaGr8
author_rank: Super Contributor
timestamp: January 13, 2016, 01:39:32 pm
content: Quote from: ruffy91 on January 13, 2016, 12:29:54 pmStatic charge build up on the face of the meter is the most likely cause. I can't remember the name off the top of my head, but there is an clothing anti-static stuff you can use to breakup the charge and it will stop this behavior.

author: ruffy91
author_rank: Regular Contributor
timestamp: January 13, 2016, 02:50:17 pm
content: Ah yes I read that you can open up the meter and apply some fabric softener (which is antistatic). But actually the meter sticks always at the exact same position, which makes me think that this is not the case.I never opened such a meter before, but is it possible to open them with some care and not damage them?I thought that maybe you can damage such a meter by just opening them, like some motors which magnets get demagnetized by just pulling out the rotor.If it is like this I will open it, clean the mechanics, lubricate with low viscosity oil and rub the cover with some fabric softener or antistatic cleaner.

author: Performa01
author_rank: Super Contributor
timestamp: January 13, 2016, 03:06:14 pm
content: Quote from: ruffy91 on January 13, 2016, 12:29:54 pmAs has been said already, make sure it actually is a mechanical problem. If so, it is possible to remove the front glass and fine adjust the needle with a pair of tweezers in order to ensure free movement. No cleaning should be necessary, but a careful blow with something like a bellow (as is used to clean SLR camera sensors) cannot hurt.QuoteWell, these are more likely to be the input reservoir caps, aren't they?First of all, this is a linear power supply, and even with the thyristor phase control modulator for the pre-regulation, since it all happens at mains frequency, particular low ESR isn't essential. In contrast, capacity does matter, so if HP thought they needed a total of 44mF (which is perfectly plausible for a PSU like this), you cannot make do with just half that value.Sprague caps from that time usually are extremely good and reliable, so I would be surprised if they'd actually need replacement. In general, it is not that easy to get a replacement of equal quality nowadays, even with 105°C, LL (low loss) and low-ESR ratings, simply because in the good old days the caps were allowed to be considerably bigger, hence they were more robust, had low losses by design and tended to exceed their nominal capacitance value by up to 80%. SO in general I wouldn't replace them unless you have proved they are actually bad.It isn't easy to find a DMM or even LCR meter capable of measuring capacities that high, but you can easily test them yourself using just a stop watch, a PSU and a voltmeter with 10Mohms input impedance. Set up an RC circuit by (dis)charging the capacitor over 1kohm and measure the time it takes to reach 63% of the applied voltage when charging (or 37% when discharging). Every second it takes corresponds to 1mF, so it should take at least 22 seconds for the caps in your device.

author: MrSlack
author_rank: Frequent Contributor
timestamp: January 13, 2016, 07:40:47 pm
content: Quote from: Performa01 on January 13, 2016, 03:06:14 pmThis. My Tek 453 had the original Sprague caps in it when it died in 2011 at the age of 44 and it was still spot on (cat piss incident, not self-induced failure)