UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Eddystone S640. Mains primary open circuit.

turretslug · 18th Mar 2021, 1:06 pm

Oh yes- the mica grid coupling cap to the output valve grid. Is it a thick square (almost cube!) dark brown 10nF component with classic strangely thick lead-outs? Both the 670A and 750 here featured one of these in this part of the circuit (in the 670A, retained in place with a natty little plated brass clamp- very Eddystone). I glared at it initially, sure that a 10nF Bakelite-case capacitor would be one of the notorious "Micamold" paper types. However on removal, they felt strangely heavy (mica/silver sandwich?) and proved impeccable on a 500V tester. I changed them anyway, on the grounds that they were nearly 70 years old and a 10nF 630V polycarbonate cap was much cheaper than a UL41 or a N78.

Eddystone were a funny outfit, using some rather rubbish passive components, interspersed with a few very good ones- I suspect that the latter may have come from surplus sources, the micas in question may even have been transmitting or some other arduous spec. types.

18th Mar 2021, 1:06 pm	#4
turretslug Dekatron Join Date: Nov 2011 Location: Surrey, UK. Posts: 4,400	Re: Eddystone S640. Mains primary open circuit. Oh yes- the mica grid coupling cap to the output valve grid. Is it a thick square (almost cube!) dark brown 10nF component with classic strangely thick lead-outs? Both the 670A and 750 here featured one of these in this part of the circuit (in the 670A, retained in place with a natty little plated brass clamp- very Eddystone). I glared at it initially, sure that a 10nF Bakelite-case capacitor would be one of the notorious "Micamold" paper types. However on removal, they felt strangely heavy (mica/silver sandwich?) and proved impeccable on a 500V tester. I changed them anyway, on the grounds that they were nearly 70 years old and a 10nF 630V polycarbonate cap was much cheaper than a UL41 or a N78. Eddystone were a funny outfit, using some rather rubbish passive components, interspersed with a few very good ones- I suspect that the latter may have come from surplus sources, the micas in question may even have been transmitting or some other arduous spec. types.