Testing H7864 (MicroVAX II) PSU With No Load

wulfman wulfman at wulfman.com
Sun Nov 1 12:34:14 CST 2015

its a switcher.

On 11/1/2015 10:37 AM, Robert Jarratt wrote:
>> -----Original Message-----
>> From: cctalk [mailto:cctalk-bounces at classiccmp.org] On Behalf Of Paul
> Koning
>> Sent: 01 November 2015 16:52
>> To: General Discussion: On-Topic and Off-Topic Posts
>> Subject: Re: Testing H7864 (MicroVAX II) PSU With No Load
>>> On Nov 1, 2015, at 7:12 AM, Robert Jarratt <robert.jarratt at ntlworld.com>
>> wrote:
>>> Some of you may recall I have a faulty H7864 PSU, which failed a while
>>> ago with a loud pop, but no obvious physical damage. I replaced the
>>> blown transistor (on the primary side of the large transformer), but
>>> when I power it on,  the transistor does not switch and there is no
>>> output, so clearly there is still a problem somewhere.
>>> I have been spending some time drawing schematics for almost the whole
>>> thing. I am now at the point where I intend to compare it with a
>>> working one, probing each one side by side, to see up to where it
>>> appears to be working. It would be awkward to have two dummy loads,
>>> just for lack of suitable equipment. Does anyone know if it is safe to
>>> run these PSUs with no load? Would my testing be valid without a load?
>> If it has a transistor at the primary side, and a high frequency
> transformer, it's
>> a switching supply.  Those want a load, usually.  Especially the older
> ones.
>> If it has a laminated iron transformer, big electrolytics for filtering,
> and
>> transistors at the secondary side, it's probably a linear regulator
> supply.  Those
>> don't need a load.  You may want to do full testing under load to observe
>> correct regulation, but for initial testing you can test them without.
>> Linear supplies are older; I think by the time of VAX you're likely to see
>> switchers (with the possible exception of first generation stuff like the
> 780, I
>> don't remember if those big supplies are linear or switchers).
>> 	paul
> Hmmm.... Until your email I was pretty confident this was a switcher. But
> your description of a linear PSU seems to fit. I have posted three photos of
> the main board here: http://1drv.ms/1KQkTBp can you tell from that what kind
> of PSU it is?
> The big TO-3 on the left is the one that I found had failed. The replacement
> does not appear to be switching, but I don't know why.
> Regards
> Rob

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