HP audio oscillator

[Magnus Danielson]

From: Neil Jendon <neil at newcontrol.com>
Subject: HP audio oscillator
Date: Fri, 15 Dec 2000 16:04:51 -0800

> Hi, gang;

Hi Niel!

> New to the list, and this is a long shot, but maybe you can help:
> 
> Recently found an old HP audio oscillator (big heavy tube thing). It works
> great until it warms up, then the tone fades off and sputters. Any theories
> as to where to start?
> 
> I have new tubes on route, but I was wondering if there was anything else I
> could check in the meantime.

One of the things I would look at with that old gear is electrolytic
capacitors. When they age they will be shorted with an ever increasing
conductance, thus resulting in a heavier and heavier load on the
powersupply.

So, to check this theory, measure the internal powerlines, and if they
first rises and then slowly drops, it is no wounder if your output
amplitude drops along with it, is it?

Then, check powersupply caps for leakage... if you can see a steady DC
current into a cap, then it is pulling current besides it's capacitive
stabilizing (which should manifest itself as AC). If the decoupling
caps have a high DC current relative to the total DC current of the
supply, then you really should suspect it and try to replace it.

There migth be many other explanations, but this one is among the
first things I would check. Maybe it will just give you a reason to
check some other things and put you on track on where the fault is.

Cheers,
Magnus