View Full Version : Heathkit HW8 Drift
KC9ERZ
06-24-2008, 09:21 PM
I have a Heathkit HW8 transceiver with a problem. I have a frequency drift problem on 80 meters. If I hold my Key down and transmit into a dummy load and listen on another receiver, the frequency drifts down until after 30 seconds or so, I hear nothing on the other receiver. Similar problem with 40 meters except the frequency seems to drift up (not down) just a little and but not nearly as fast as on 80 meters? The best I can tell 20 and 15 meters is okay.
Anyone Heathkit experts out there that care to take a stab at this problem.
Thanks,
Greg Johnson
KC9ERZ
WB7SKK
08-12-2008, 11:40 PM
I had a HW 8 for a while and one thing I tried to cure the drift was to take and make a dam around the VFO aira with tape and fill it with candle wax ,the wax keeps all the components at the same temperature,you may not want to do it ,but it did help the drift on my old HW8
Mike WB7SKK
Mike Bryce, WB8VGE publishes a book known as "The Hot Water Handbook". In it, stability of the HW7 and HW8 is discussed, and several fixes are presented. One involves better regulation of the VFO supply voltage; this is accomplished via a Zener diode.
Check with Mike to see if he still carries the book. Good stuff!
Thats a LOT of drift!
I would be looking for a improper compensating cap in the vfo, or some other
component failure before I attempted any mods.
Did you just get the rig? Or did this problem just appear suddenly?
Rege
Thats a LOT of drift!
I would be looking for a improper compensating cap in the vfo, or some other
component failure before I attempted any mods.
Did you just get the rig? Or did this problem just appear suddenly?
Rege
I'm thinking one (or more) of the diodes in the bandswitch area is suspect. It's not allowing the crystal oscillators for those band to stabilize. There's an HW8 schematic in my shack library; will dig it out and verify the circuit - and any potential failure modes.
Bryce's book does go into detail about this symptom, but I don't have it handy at the moment either...
The main modification for frequency drift is quite detailed, and if you would like, I can scan the page from the book and send it to you via e-mail.
The drift is approximately 150 Hz as the supply voltage varies over a rang of 10 to 13.5 volts. the cure is to install a zener diode where several resistors have been removed, and to instal a 7808 voltage regulator in another location.
Let me know if I can send the page to you. 73, Jim