Caps in Valiant VFO

[stevef]

If they measure up okay, should I replace them anyway?  If so, where can I find 2% silver-micas??

Steve, KK7UV

[w3jn]

NO!!! Leave well enough alone if it's working OK. 

Why use 2%??  You would want to use NP0 (temp stable) with a N750 (or so) temp compensating cap.  The accuracy isn't that critical as there's a trimmer slug and cap to set the VFO freq to the dial.

73 John

[stevef]

The manual shows 2.5% Silver-Micas, and 2% ceramics.  I had run across one internet info source that simply said to replace them "because they WILL fail".  My post is fishing for other opinions.

I've never powered it up.  When I acquired it, it was very filthy and so I have dismantled and cleaned it up, and am now getting it back together, replacing any bad parts as I go along.

Thanks,

Steve KK7UV

[w3jn]

I haven't heard of problems with the caps in the Valiant VFO.  BTW the Ranger VFO is essentially the same.  Take all internet advice (including MINE!) with a grain of salt.

What WILL fail is the 18K resistor inside the VFO box.  It's under-rated for its duty and when it fails it causes all sorts of heat damage inside the VFO.  Do a search for "Valiant Chernobyl resistor" on this site; believe there's an article on amwindow.org as well.

It isn't important that caps in VFOs be an exact value, because there are trimmers to adjust the frequency to exactly track the dial scale.  What IS important is using high-quality caps of the EXACT same temperature coefficient.  I think Junkston used 2%ers because they were high quality and stable.

Symptoms of bad mica caps would include a dead VFO, inability to adjust the VFO to track with the dial, or sudden jumping of frequency.  And chances are if the VFO is dead, there are MANY other problems that are MUCH more likely to cause this than dead mica caps.  Sudden jumping of frequency is also usually caused by dirty wipers and/or bearings on the variable cap.

I'd do the 18K resistor mod, check it out, and leave it the hell alone if it works fine.

Good luck with that bad boy, sounds like a great project.  BTW the best way to get decent audio out of a Valiant is to replace the driver xformer with a high quality line to push/pull grid xformer and feed line audio directly to the modulator grids, thereby bypassing all the low level audio stages, that stupid clipper, etc.

73 John