Curious heater wiring

T1154M Ser. 86102 Restoration

T1154M Ser. 86102

This is the second of two ‘sad’ T1154Ms that I was presented with back in early 2010.

Although it may not appear so, this one was the better of the two, externally. Although not shown in the photograph, both meters, complete with Perspex covers were present, as were the 4 valves. However the ‘Front Door’ was missing and it had been modified ... Note the blue wire dangling out of the PA compartment and the screened wire hanging loose underneath!

The fuse holder was damaged and there was a curious pull-switch retrofitted onto the front panel in the PA compartment, to the left of the ‘Filament Resistance Mod’ label.

Apparently the previous owner, now a ‘Silent Key’ was known to have tinkered with the set and it seemed likely that he had ‘modded’ it in line with one of many ‘improvements’ that were published in the 1950s.

Although based on the AD67 and AD77 transmitters (Marconi’s designation for the T1154 was AD87D), and thus a genuine VFO-Controlled transmitter. The T1154 was actually intended to transmit on pre-selected ‘spot’ frequencies which were set prior to a mission, using the Uni-click or less common Multi-click selectors. These were then presumably written in chinagraph on the table on the ‘Front Door’ for quick reference by the W/O (Wireless Operator). With the exception of the MF (Yellow) range, crude fine tuning adjustment was also provided per band by way of a metal lever to the right of the respective VFO control. This was rendered inoperative when ‘FREE’ was selected; in which case the operator could select whatever frequency suited him.

The T1154 is renowned for drifting, and this is perhaps the reason for the curious heater wiring with the additional pull-switch, as in the drawing on the left. The T1154 did have a means of restricting PA heater current when in Standby (STBI) mode. This is R30/31. It has been suggested to me that this odd heater arrangement was conceived as a way of further controlling the heater current in the VFO (V1, ML6, VT105). Exactly how all this was supposed to work, I have no idea! ... So I set about rewiring the heaters as per the official diagram.