[Heath] HW-8 Issue

[Mike Czuhajewski]

> When I adjust the receiver preselector to the left, it blows the 
> fuse. I am using a 2 amp fuse.


So use a bigger fuse :-)

This is obviously a bad preselector variable capacitor, with one or 
more plates bent somewhere. As you rotate it, the rotor (ground) and 
stator (hot) touch each other, shorting power to ground.

The stator of the rear section of the capacitor (C301B) is connected 
to the drain of RF amplifier Q1. Follow the path on the schematic up 
through the diodes, inductors and bandswitch sections and it ends up 
at the main power source. There is nothing to limit current except the 
low resistance of the coil and forward drop of the diode, so the short 
blows the fuse. A short on the other section (C301A, used on 80M only) 
would not do that since it has a 1000 ohm resistor in the path. It 
still wouldn't work right, of course, but it would not blow a fuse.

Your options are repairing or replacing the capacitor. Repair may or 
may not turn out well, and replacement can be expensive. Finding a 
replacement can be a problem but you can always get one from Oren 
Elliott Products. Located in the northwest corner of Ohio, that is the 
company that made Heathkit air variable capacitors (although under a 
different name at the time), and they are still in business. They call 
themselves "the largest domestic manufacturer of air-dielectric 
variable capacitors" and have been making them since 1925. While they 
probably won't have one of these in stock (it's a decades-old design 
with very limited demand), they can certainly make more although it 
won't be cheap.

The company URL is

http://www.orenelliottproducts.com

and you can find their catalog at

http://www.orenelliottproducts.com.//Docs/Catalog.pdf

To give you some idea of what it might cost, consider the HW-8 VFO 
cap, which is somewhat similar, and is listed on their web page at

http://www.orenelliottproducts.com/oep-capacitors/n-50-group/26-152-ns-51-heath-replacement

For one piece it's $51.00 (!). That drops to $21 each if you order 5 
or more. That makes sense, since the cost of setting up production and 
other overhead is the same no matter how many you order. Of course 
most people won't order 5 or more unless they are a dealer of some 
sort. I would expect pricing to be similar for the preselector cap.

(The Heath number for the VFO cap is 26-152 and the preselector cap is 
26-151.)

You might get lucky and find an HW-8 being sold for parts, but make 
sure it has the capacitor in it. If you're careful you might be able 
to adjust the plates so they don't touch. It will be tricky and you 
might ruin the whole thing--one possible outcome is having the plates 
pop off the shaft, which I have seen happen. (People who have done 
that  to the VFO cap have carefully pushed them back into place on the 
shaft and put on a bit of epoxy to help hold them in place and usually 
get away with it.)

If you aren't trying to keep the rig in 100% original configuration 
and just want to keep it working, you could always replace it with 
another dual section cap which has approximately the same capacitance 
spread. It might look ugly and probably require drilling some new 
mounting holes, and definitely rule out it ever being a museum piece, 
but at least it would work. (I don't know the capacitance range and it 
doesn't appear to be listed in the manual, but if I get a chance later 
I'll measure it.)

Wish I had some answers that are quick and easy and cheap and 
foolproof, but there aren't any that I know of that meet all 4 
requirements. Except maybe having someone donate a free HW-8 or 
capacitor :-)