[Johnson] Johnson Viking "Pacemaker" FS

[Richard Knoppow]

----- Original Message ----- 
From: "Roy Morgan" <k1lky at earthlink.net>
To: <Johnson at mailman.qth.net>
Sent: Saturday, January 04, 2014 12:26 PM
Subject: Re: [Johnson] Johnson Viking "Pacemaker" FS


On Jan 4, 2014, at 11:16 AM, Ken Simpson, W8EK 
<kenw8ek at gmail.com> wrote:

>
> E F Johnson Viking "Pacemaker" Transmitter For Sale:
> …    It will need to be brought
> up slow on a variac, and probably will need caps replaced.

Ken,

If the buyer “brings it up slowly on a variac” and fries the 
power transformer in the process, I’ll send him the name of 
a good transformer rewinder.

If anyone wants to know why I object to the practice, I’ll 
send them my diatribes on the topic.

Roy

     FWIW, I agree with you. I think this is just a way of 
fooling yourself.
1, If the transformer has a short already this won't save 
it. Disconnect the transformer from the rectifier and then 
bring it up using either a lamp or a Variac with an amp 
meter.  If it starts to draw heavy current shut it down. 
That won't save it but will prevent an evil smelling mess.
2, Bringing the voltage up slowly on a power supply with 
vacuum tube rectifiers does not bring the DC voltage up 
slowly, at least not in proportion to the AC voltage. The 
rectifiers need to have their filaments going fully so the 
output voltage is likely to come up faster than you think.
3, If you suspect a bad electrolytic filter caps they should 
be tested while disconnected from the power supply.  There 
are cap checkers which will supply current limited voltage 
to re-form the cap but usually bad caps will not re-form or 
will not be reliable if re-formed. Just bite the bullet and 
replace them.
4, A choke or transformer with marginal insulation may look 
OK at first and then short suddenly after its been running 
for a while and gets hot. A Hi-Pot test will catch some of 
these. Real Hi-Pot testers are not too common but they are 
essentially ohm meters that apply fairly high voltage to the 
tested item, typically 300 to 600 VDC. Some ohm meters, for 
instance a Hewlett-Packard 410-B or C or the 412-A will 
measure very high values of resistance and can spot weak 
insulation without applying high voltage.  BUT, note that 
the problem is break-down when high voltage is applied. 
Something may look perfectly good with low voltage and 
arc-over when high voltage is applied.
    In much equipment replacing the filter caps is a wise 
precaution. Keep in mind that non-operation is deadly to 
electrolytic caps, they last much longer in constant service 
than in storage.  Oil filled paper caps also go bad but for 
those continuous use generally degrades them. due to the 
effects of the voltage on the dielectric.
    The problem with power supplies is the amount of current 
they can put into a short when something goes bad.
    My understanding from others on this list (like Carl) is 
that the transformers in the Valiant are marginal to begin 
with so treat it gently.


--
Richard Knoppow
Los Angeles
WB6KBL
dickburk at ix.netcom.com