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Davewantsone · 21st Jan 2019, 11:35 pm

I have recently acquired a honey coloured Roberts R66 in a awful state. The back had completely come apart including the rexine! The 3 knobs were fast on the rusted shafts. I had to saw the volume one off!
Well I have now got it working with a new volume control and several replacement components. I had to replace all the LT (3) capacitors C28-9-30 and resistor R14 which had a 5.6 ohm fitted but was reading 12 ohms. The diode bridge MR2 (listed as FSW1392A) was replaced with 2 1N4007. The reason for writing is that the heater voltage was now 1.72V This could be due to the less voltage drop produced by the 1N4007, however I did note an extra wire going from the C29 capacitor +ve back to MR2. This is not indicated on the two circuit diagrams I have (Trader 1273 and Radio Sevicing page232). When I connected this lead back to the defunct MR2 with the ground wire of MR2 also connected the LT supply dropped to 1.42V. Is the MR2 acting like a zener diode? Why no indication on the circuit diagram?

21st Jan 2019, 11:35 pm	#1
Davewantsone Hexode Join Date: Dec 2011 Location: Wigan, Greater Manchester, UK. Posts: 291	Roberts R66 diode MR2 I have recently acquired a honey coloured Roberts R66 in a awful state. The back had completely come apart including the rexine! The 3 knobs were fast on the rusted shafts. I had to saw the volume one off! Well I have now got it working with a new volume control and several replacement components. I had to replace all the LT (3) capacitors C28-9-30 and resistor R14 which had a 5.6 ohm fitted but was reading 12 ohms. The diode bridge MR2 (listed as FSW1392A) was replaced with 2 1N4007. The reason for writing is that the heater voltage was now 1.72V This could be due to the less voltage drop produced by the 1N4007, however I did note an extra wire going from the C29 capacitor +ve back to MR2. This is not indicated on the two circuit diagrams I have (Trader 1273 and Radio Sevicing page232). When I connected this lead back to the defunct MR2 with the ground wire of MR2 also connected the LT supply dropped to 1.42V. Is the MR2 acting like a zener diode? Why no indication on the circuit diagram?