Dynatron FM2.LV Tuner repaired - Query - UK Vintage Radio Repair and Restoration Discussion Forum

RetroRichard · 17th Feb 2020, 12:11 pm

I have a query about the operation of a Dynatron FM2.LV valve tuner that I have owned since the late 60s. It was passed on by my father who bought it new. About 30 years ago I managed to get a just-about readable paper copy of the circuit diagram, which I have recently re-created in a modern file format. After working correctly for many years, the unit failed (no output) about 6 years ago and I put off repairing it.

I have recently repaired this 7-valve tuner which contained the original components, after getting some very helpful advice from Michael Maurice. I have also repaired the Dynatron Power Supply P1 and drawn out its circuit diagram. It correctly supplies 230v dc to the tuner via an EZ80 rectifier valve, but the 6.3v ac heater supply winding had failed so I put in a new separate 6.3v ac 20VA transformer to supply the heaters in the FM2.LV.

The tuner repair essentially consisted in changing all of the old Hunts capacitors (mostly 0.01µF & 0.04µF) for new 400v dc devices, except for the low pF value ones, plus replacing the one 50µF 25v electrolytic capacitor, plus changing most of the resistors that feed from the 230v dc supply line in the tuner because they had changed to much higher values over time. Only one valve appeared to have failed V1 (Z719 / EF80) and I purchased a set of new valves from Watford Valves.

The circuit diagram showed that the anode of the each of the valves links to the 230v dc line via an inductor and resistor, and most of the inductors show a 22pF capacitor in parallel across the inductor winding. When I opened the unit and carefully traced all of the components and drew location diagrams, it appeared that none of these 22pF capacitors were fitted across any of the inductors. Having replaced all of the other components and fitted a new valve, the tuner worked perfectly without any of these 22pF capacitors fitted.

My query is this: Why would the circuit diagram show those parallel 22pF capacitors across the inductors when none were fitted originally and the radio works perfectly without them?

In case anyone is interested, the output from the tuner goes to a Henry’s Radio P6-2A pre-amplifier and then via a Henry’s Radio MPA12+12 Amplifier (both purchased from Henry’s shop at 303 Edgware Rd in 1968) to feed LEAK Mini-sandwich 14Ω speakers

17th Feb 2020, 12:11 pm	#1
RetroRichard Triode Join Date: Feb 2020 Location: Maidenhead, Berkshire, UK. Posts: 12	Dynatron FM2.LV Tuner repaired - Query I have a query about the operation of a Dynatron FM2.LV valve tuner that I have owned since the late 60s. It was passed on by my father who bought it new. About 30 years ago I managed to get a just-about readable paper copy of the circuit diagram, which I have recently re-created in a modern file format. After working correctly for many years, the unit failed (no output) about 6 years ago and I put off repairing it. I have recently repaired this 7-valve tuner which contained the original components, after getting some very helpful advice from Michael Maurice. I have also repaired the Dynatron Power Supply P1 and drawn out its circuit diagram. It correctly supplies 230v dc to the tuner via an EZ80 rectifier valve, but the 6.3v ac heater supply winding had failed so I put in a new separate 6.3v ac 20VA transformer to supply the heaters in the FM2.LV. The tuner repair essentially consisted in changing all of the old Hunts capacitors (mostly 0.01µF & 0.04µF) for new 400v dc devices, except for the low pF value ones, plus replacing the one 50µF 25v electrolytic capacitor, plus changing most of the resistors that feed from the 230v dc supply line in the tuner because they had changed to much higher values over time. Only one valve appeared to have failed V1 (Z719 / EF80) and I purchased a set of new valves from Watford Valves. The circuit diagram showed that the anode of the each of the valves links to the 230v dc line via an inductor and resistor, and most of the inductors show a 22pF capacitor in parallel across the inductor winding. When I opened the unit and carefully traced all of the components and drew location diagrams, it appeared that none of these 22pF capacitors were fitted across any of the inductors. Having replaced all of the other components and fitted a new valve, the tuner worked perfectly without any of these 22pF capacitors fitted. My query is this: Why would the circuit diagram show those parallel 22pF capacitors across the inductors when none were fitted originally and the radio works perfectly without them? In case anyone is interested, the output from the tuner goes to a Henry’s Radio P6-2A pre-amplifier and then via a Henry’s Radio MPA12+12 Amplifier (both purchased from Henry’s shop at 303 Edgware Rd in 1968) to feed LEAK Mini-sandwich 14Ω speakers