[Test-Equipment] HP 8640B Fuse Question

[Barry]

Update:

I placed a 2A fuse in series with my ammeter across the fuse test points and the regulator board came alive.  The fan started working and the generator is putting out a signal (modulation works, attenuator works, controls are a bit sensitive, etc.).

Ammeter reads about 1.55A so it it made sense why the 1.5A fuse blew.  I wonder if it coasted along at something just under 1.5A and as various components aged and the line voltage went up in general, it finally blew.

Someone put a sticker on the inside that reads "KAPUT".  Sure was a pretty easy fix for someone to give up on it.  Probably was not someone's personal generator and they had no big reason to fix it.  Don't know.

The counter isn't reflecting the generated frequency ("000100" or something like that) so I have some work to do there, but at least most parts are proven to work.

Thanks for listening,
Barry - N4BUQ

----- Original Message -----
> From: "Barry" <n4buq at knology.net>
> To: "Test Equipment" <test-equipment at mailman.qth.net>
> Sent: Tuesday, February 3, 2015 9:34:16 PM
> Subject: [Test-Equipment] HP 8640B Fuse Question
> 
> I'm now the owner of another HP 8640B.  It appears to be a very early model
> (serial number 13xxxx making it a 1973 model).
> 
> The -5V and Fan Driver board (A18) is not working and I've discovered the
> on-board fuse is blown.  It is labeled "2A" on the board and is designated
> as a "2A 125V SLO-BLO" (GMW 2A); however, the specs I've seen for a GM 2A
> show this to be a fast-acting fuse.
> 
> What's odd is the board has a "GMW 1-1/2" in it and it appears this is from
> the factory.  The fuse is soldered in and there's no indication of
> desoldering and resoldering.
> 
> I'm wondering if I can jumper in an ordinary 2A fuse to see if the board will
> work again.  The fuse points are conveniently brought up to test points at
> the top of the board so this should be pretty easy to do.  If the fuse
> holds, then I can replace the GMW with another on-board fuse but I hate to
> waste any of these as they're pretty expensive.
> 
> I'm wondering, also, if the reason the existing fuse could be because the
> wrong rating fuse was put in at the factory and the circuit perhaps draws at
> or perhaps slightly above 1.5A causing the fuse to blow prematurely.
> 
> Anyone seen anything like this before in an 8640B?
> 
> As a side note, I was pleasantly surprised to see that the gears are in fine
> shape.  I expected that at that age, they would be in very poor condition
> but perhaps the early models used a different composition?  Also, I'm not
> seeing the little finger-style switches and, instead, just ordinary rotary
> switches.  Did the early models have a different switching system from the
> later ones?
> 
> Thanks,
> Barry - N4BUQ
> 
> ______________________________________________________________
> Test-Equipment mailing list
> Home: http://mailman.qth.net/mailman/listinfo/test-equipment
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:Test-Equipment at mailman.qth.net
> 
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html
>