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Old 4th Feb 2019, 10:06 pm   #23
Uncle Bulgaria
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Join Date: May 2016
Location: Cornwall, UK.
Posts: 2,315
Default Re: Ferrograph 4A: Restoration advice needed

The head has a Cyberman hat that screws on the top.

I can't see how to get the base off. It appears to be riveted, but there are wires soldered to the top of the rivets and to the screws holding the coil cores down. I'm loath to tamper with this as it looks pretty major.

The pins were soldered to rings formed in the wires. The 'dummy head' (something to do with adding stereo) plugs into a socket.

I've finished covering the case, so pictures will be forthcoming.

I've been investigating the PSU filter/reservoir capacitors as they are probably out of whack after all this time. the 16+8mfd reservoir/filter dual can will probably be replaced with a modern 16+16mfd, and the same with the oscillator dual can. I haven't got a high voltage capacitor tester and haven't powered it up.

I was interested in the circuit so was calculating what changes would happen to the filter response if I changed the LC capacitor value, and this led me to checking the anode limiting resistance on the EZ80. There aren't any limiting resistors, so I was interested to see what limiting resistance was being provided by the transformer. I think I've done the calculation wrong so would be grateful for someone to point out the error. The datasheet where I got the calculation from is attached.

Rt = Rs + n2 x Rp where Rs is 1/2 secondary resistance, n is 1/2 ratio of primary to secondary windings [1 : (n+n)] and Rp is total primary resistance. As I understand it from that diagram.

I have:

Rp = 7R

Rs = 420/2 = 210R

n = (420/7)/2 = 30

From this I get Rt to be 6510, which is rather different from the 2 x 175R to 2 x 300R the datasheet plots show.
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