ST2 generator losing residual magnetism after every use

[MQuinn]

I have a second hand ST2 lister generator, 6kVA. I routinely start it with a starting handle but it comes with the option of starting via motoring the generator under decompression (I have been using a 12V car battery although I noticed online that others seem to use a pair of batteries giving 24V).
My knowledge of electronics is very rudimentary, but when I bought it, I upgraded some of the supply wiring in the top box, replaced the absent starter relay, added trip switches to the live outputs, and added an hour meter. I used it regularly for many months with this arrangement without issue.
However, a few months ago I began to notice that, after switching on the power after the generator had reached full speed, the voltage appeared to ramp up from nothing and take a few seconds to reach its normal operating output (lights increasing from off to dim to full brightness). This continued to worsen until no power would be produced if I did not first ‘flash’ the generator by motoring it using the push start and battery arrangement under decompression. (I took this to mean that residual magnetism was being lost after every use, despite no change in operation and removing the load before shutdown.) This proved adequate, if not ideal, and I used it in this way for several more months.
However, this practice no longer seems to be working as despite using the battery to motor the generator first, the voltage output remains unusably low (the outside LED strip lights I routinely switched on first to test now flicker or do not light). It was working adequately the day before.
I investigated for a source of the problem among the wiring I was familiar with, and everything seems intact. However, I am no expert and there are several components that I do not recognise and have no knowledge of. I have photographed these and labelled them A-E. It is the first time I have removed the side panels.
•   Image 1 – left panel components (I do not know what these are, and although all wiring is intact and secure, it would appear that the component marked B is damaged?)
•   Image 2 – component B up close
•   Image 3 – top box with components C and D. Component D shows evidence of heating (dark area) and I do not know if this is normal.
•   Image 4 – right panel component E. I believe this is a rheostat and see no obvious fault with it.
•   Image 5 - view of brushes and associated wiring from right
•   Image 6 - view of brushes and associated wiring from left
I would be grateful if you could outline what the components are and suggest or identify a likely problem with any of them from what you can see. If you suggest replacing any of them with their modern counterparts, details of the specifications and exact wiring guidance would be appreciated.
Many thanks,
Matthew

Unfortunately it is not letting me attach any photos - try the link below or contact me privately if you can help. Thanks.

https://drive.google.com/file/d/1cSaeVwzhgLZ5HjchoS4kJpvE3WS9-53Z/view?usp=drive_link

[alexm]

Hi,

I have an ST2 as well. Do you have anything permanently connected to the wiring when it's turned off?

For example, I had something to read the ammeter wired in permanently, and, after a period of time, it always takes an exciter 'jolt' to spring it into generating mode since I had that wired in (it's modified to start with a starter motor and to give it an exciter voltage when it it first starts up).

Alex

[ajaffa1]

Good day MQuinn, Sorry I didn`t get back to you sooner, been in hospital.
I suspect that the issues you are experiencing are due to the old selenium rectifier failing. When they do fail they usually release a very toxic chemical that smells slightly like garlic. This rectifier converts some of the AC current generated into DC current which then flows through the field windings to produce the North/South poles that the armature spins inside. Without this energisation of the field windings your generator will produce no power except that relating to residual magnetism.
Behind the two covers on your generator head there are two rheostats and the old bridge rectifier. The rheostats adjust the load/no load output from the generator. Test continuity across them with a resistance meter, I would expect to find resistances of between 1 and 20 Ohms. Check the condition of the slip ring brushes and that they are free to move inside their brass sleeves. Check for continuity between the two slip rings with a multimeter, I would expect to find a resistance of less than 100 ohms between them, if they are open circuit the armature windings have burned out and will need re-winding. Check for continuity in the field windings (two wires that run from the bridge rectifier to the electromagnets mounted inside the outer perimeter of the casing), again I would expect to find resistance in the 10s of ohms, open circuit will require a re-wind. If all the above checks are OK then replace the old selenium rectifier with a modern silicon bridge rectifier. These are readily available on Ebay, ideally get one rated to 1000 volts with screw lugs and a heatsink.
I attach a couple of pics showing the two rheostats and the selenium rectifier.
let me know how you get on.
Bob

[MQuinn]

UPDATE

Hi Bob,
Many thanks for your reply. Today I replaced the old selenium bridge rectifier with a modern one (100A 1600V, Full Wave, 4 Screw Terminals) and an aluminium heat sink. I also replaced the damaged wire-wound resistor across the AC rectifier terminals with a modern 5ohm 50W one. After some teething issues (it transpired that I had misidentified and subsequently mixed up the original DC+ and - rectifier terminals) the generator is back and functioning as normal...long may it last!
Many thanks for your input...greatly appreciated

[MQuinn]

Labelled photos of the terminals of the old selenium bridge rectifier attached

[alexm]

Labelled photos of the terminals of the old selenium bridge rectifier attached

Glad you got it sorted. I managed to miss the fact that you hadn't replaced the selenium rectifier yet! 

I wonder whether the red/black rubberised tubes were originally on every ST2 rectifier, because I've only seen them like ajaffa1 and mine!