gener-raco: Third filament supply.

Examination of the Sound Level meter 1551-C reveals it has two filament supplies, 20mA and 40mA using two pins on the Jones connector or the two D size 1.5V cells. All the batteries must be removed before installing the PSU as the connections are shared. The third filament supply on pins 5 and 6 isn't used. It's floating unlike the other two filament supplies.

Examination of the PCB reveals that the HT supply winding and the other two filament supplies share a common ground / 0V connection. Actually the two used supplies use a common full wave rectifier and centre tapped winding, with only the filtering and selenium shunt regulator diodes being separate.

Perhaps the PCB or entire unit was used for another product or earlier model.

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The third supply has about 9V DC at the rectifier with the RX5 selenium rectifier as regulator removed. Thus it can't be for much more than about 18mA. Also the DVM measures the winding at about 78 Ohms. This seems high, so perhaps the DVM is confused by the transformer inductance. I'll compare using an AVO analogue meter later. The AC is about 8V.

Disassembly of the transformer shield suggests the centre tapped LT winding and the HT do share a common 0V, pin 5. However this is under the mounting panel so the windings might not be tied together inside the transformer but under it.

• Pins 5 -> 6: 950 Ohms*, 71 V AC.
• Pins 7 & 8 are a centre tapped winding, each 13 V AC to pin 5. 40 Ohms + 40 Ohms*
• Pins 9 - > 10: 8 V AC, isolated. 78 Ohms*
• Primaries, 1 & 2: 115 V AC,  3 & 4: 115 V AC. Resistances are 700 & 800 Ohms. Tie pin 2 to 3 for 230V.
• Transformer shield and pin 5 (common 0 V) are connected to mains earth wire. USA green, white and black mains lead. There is a fuse soldered on one of the mains switch terminals, not shown on the schematic. It's not clear who fitted it.

A pair of series 1N5408 would seem to be a suitable replacement for the selenium rectifiers as regulators, giving 1.35V on point A2 (Jones 2) and 1.4V on point A1 (Jones 1). A pair of 1N4007 in series is a lower voltage and lower power rating. The actual fitted C2A, C2B & C2C is 1000uF., 2000uF & 1000uF rather than the values on the schematic (6V rating). C3 is 100 V, C4 is 15 V and C 1 is  15 V. The voltage ratings are not on the schematic or parts list.

The HT  selenium rectifier was replaced by a 1N4007, no load HT was 79V.

The Electrolytic capacitors all "reformed" after a few minutes, evidenced by the rise in voltage.

* The winding resistances may be inflated by inductance due to how the DVM measures resistance. A confirmation test with an analogue meter is required.

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