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space_charged · 31st Jul 2018, 4:11 am

Been away for a while but back now and been fitting the 100pf silver mica caps to all the IFTs. I've devised a way to remove the built in caps in the top paxolin plate. The IF gain is dramatically improved. I'll post some photos of what I did shortly.

My 770R had a long standing problem of frequency drift which I thought could be drift in the IF strip but much more likely in the local oscillator. I had already replaced all the components in the oscillator and the valve, with no improvement. I did notice that slightly moving the valve would produce big changes in frequency so I was seriously suspecting the valve base. The base has a mixture of normal pins and special ones that go directly to the tuning capacitor. One of these sets of pins goes to the variable capacitor via what Eddystone call a "built in" 40pf capacitor. That translates to a mica plate and some insulating washers between the two parts. I removed that in order to get the valve base out. The mica turned out to be in poor condition, possibly having been sprayed by someone in the past with switch cleaner. Not me I hasten to add.

I used an ultrasonic bath to clean as best I could the valve base. The pins were tightened up including the "special" ones and checked to make sure they now made good contact. After re-assembly with new mica the oscillator functioned correctly and showed no sign of the irritating drift.

31st Jul 2018, 4:11 am	#51
space_charged Hexode Join Date: Dec 2014 Location: West Midlands, UK. Posts: 315	Re: IF fault on Eddystone 770R Been away for a while but back now and been fitting the 100pf silver mica caps to all the IFTs. I've devised a way to remove the built in caps in the top paxolin plate. The IF gain is dramatically improved. I'll post some photos of what I did shortly. My 770R had a long standing problem of frequency drift which I thought could be drift in the IF strip but much more likely in the local oscillator. I had already replaced all the components in the oscillator and the valve, with no improvement. I did notice that slightly moving the valve would produce big changes in frequency so I was seriously suspecting the valve base. The base has a mixture of normal pins and special ones that go directly to the tuning capacitor. One of these sets of pins goes to the variable capacitor via what Eddystone call a "built in" 40pf capacitor. That translates to a mica plate and some insulating washers between the two parts. I removed that in order to get the valve base out. The mica turned out to be in poor condition, possibly having been sprayed by someone in the past with switch cleaner. Not me I hasten to add. I used an ultrasonic bath to clean as best I could the valve base. The pins were tightened up including the "special" ones and checked to make sure they now made good contact. After re-assembly with new mica the oscillator functioned correctly and showed no sign of the irritating drift.