Testing H7864 (MicroVAX II) PSU With No Load

Paul Koning paulkoning at comcast.net
Sun Nov 1 12:42:58 CST 2015

> On Nov 1, 2015, at 12:37 PM, Robert Jarratt <robert.jarratt at ntlworld.com> wrote:
> ...
> 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.

I agree with Wulfman that it looks like a switcher.  There's too much circuitry in there for a linear.  Also, that pile of small electrolytics is typical for a switcher.

That said, it's possible that there may be an auxiliary linear regulator in there.  That's common for additional outputs with modest current needs, especially in older designs where the switching circuitry takes a lot of individual components.  A TO-3 can isn't what I would expect to see in a switching-only supply.  In other words, that particular transistor might be the pass transistor of a linear regulator.


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