A very interesting thread has sprung up on the "Fox Tango" Yahoo group prompted by an Italian amateur with an interest like mine in classic hybrid radios - in his case a Yaesu FT-901.
Apparently his radio developed a couple of faults, in particular the failure of the 6V regulator which feeds (inter alia) the DC supply to the VFO circuit.
Not surprisingly the original IC regulator is obsolete, and having nothing else to hand he replaced the faulty one with a bog-standard 7805 unit, and even though the previous 6V supply is now only 5V he claims that the VFO is now "rock solid".
You will realise from my www.g4fui.com web site that old radios are also an interest in mine, and it so happens that over the last couple of days I have been looking at the performance of my own FT-902DM and in particular the VFO stability (or rather the LACK of VFO stability) of this particular set.
I had been musing upon the supply regulation to the VFO as it appears that the real problem is that the VFO drifts as the set warms up, and as it is a bulky, heavy radio it really NEVER warms up properly (not in a reasonable operating period anyway), consequently it NEVER stops drifting.
It drifts so much that you couldn't use it sensibly for a mode such as WSPR.
I had come up with a theory that the original regulator, which claims to be a "low drop-out" device (ie it will work with the input and output voltages relatively close together, really was working just TOO close to the wire with only a theoretical 8V input being regulated to 6V out, such that the thermal changes in the environment were affecting the output volage to a significant extent.
Having a bunch of 7805s kicking around in my junk box, I just might give this a try. OK the output voltage may be significantly lower, but assuming any static frequency change can be trimmed out, the extra volt of dropout voltage, coupled with a more modern device may well make a noticeable difference to the performance.
Another contributor to the same thread suggested putting a diode in the common lead of the 7805 to lift the output voltage, but I would be tempted to try it without as (a) it would reduce the dropout voltage and (b) I suspect that the temperature coefficient of the diode would exacerbate the drift problem. Having said that, it would be very interesting to find out!
Another alternative would be to fit a 7806, and possibly take the input voltage from somewhere else, eg a 12V or similar rail in the radio.
Another item added to my "to do" list, so watch this space!
5 hours ago