Lister LT1 Markon genny under voltage!

[Welderherup]

Hello all. I found the forum from doing google searches. I’ve a workshop in Lisburn. I’m an amateur engineer. Welders, oxy, lathe. Play with racing bikes mostly. So I’ve wanted a diesel genny incase of Armageddon etc and got a wee single lister. She was working fine and then one day the voltage dropped off. Checked it all out over. Few wee things were obvious. Give it a service, cleaned the rings, freed up the brush arms and increased the engine speed and voltage came up nicely. Then the next day I started it it was at 300v! I turned engine speed down and it was at 220 then 200 then 100. Now even with her banging away at 3600rpm she’s only making 100v. There is an old circuit board with a transformer beside it that I assume is voltage regulaton. On this board there are two components that I believe are capacitors which have clearly been damaged. Can anyone help me work out if I can repair this board or if I can replace it with an avr? Maybe this isn’t even the problem? I’m unfortunately a half wit with electronics but I’ve no choice but to have a go. I’ve no money to replace it and Santa robs me blind rather than brings me goodies so it’s up to me.

 

[Barking Mat]

Quick google found this:

Don't know if this helps or not.

Welcome to the forum.

Maybe try cleaning the board, and inspecting the components.

You say the caps are dead, try cleaning them to identify values ?

 

[Welderherup]

Hi, yes I found that also, it’s not clear I can replace it with that. Also, maybe I just need to replace the damaged components. I’ve tried to identify their value but I can’t find any chart that gives me a value for these.

 

[eddie49]

Hello and welcome!
From the spiral burn pattern around both components, I think that, rather than capacitors, they are resistors.
Can you read anything from the main Markon label, and also the part numbers of the 16-pin integrated circuit and the transistor ( round metal can ) on the board?

 

[Welderherup]

I will go out tomorrow and clean the paint off it to get the proper model number of the plate on the genny. I will try to get the wax off the board also, been avoiding it incase I damage it. The two large capacitors at the bottom of the power head where giving me weird inconsistent readings so I don’t know if they are part of the problem. I’m hoping to repair it. The windings are good and these old engines go forever so if I can fix it I will have it for years. Hopefully I can get the numbers off it tomorrow and at least work out if I can replace the board with that newer typw

 

[Barking Mat]

Worst case scenario there are companies which repair / replace these boards.

 

[DSR]

Is that the only electronics on the generator ? There doesn't look to be anything on that board capable of handling the power that will be needed. Think car alternator, there's usually a fair bit of aluminium in there somewhere to get rid of the heat from the semiconductors, this is much bigger and higher voltage but the principles are the same. I'd expect there to be some power semiconductors somewhere to control the field current.

 

[Welderherup]

Worst case scenario there are companies which repair / replace these boards.

Cool. Can you recommend any? Have you used one before?

 

[Ubique]

The AVR doesn't carry the output current, only the excitation current - the voltage regulator in a car alternator of, say, 200A @12.5V (~2.5kW) isn't a particularly big thing, nor does it have a high thermal output.

 

[Welderherup]

Is that the only electronics on the generator ? There doesn't look to be anything on that board capable of handling the power that will be needed. Think car alternator, there's usually a fair bit of aluminium in there somewhere to get rid of the heat from the semiconductors, this is much bigger and higher voltage but the principles are the same. I'd expect there to be some power semiconductors somewhere to control the field current.

There are no other obvious boards or components, it’s all in the photos. Perhaps there are others in the belly of the rotar housing. I need to get the proper number for this model and then I can maybe find a detailed diagram for it.

 

[DSR]

The AVR doesn't carry the output current, only the excitation current - the voltage regulator in a car alternator of, say, 200A @12.5V (~2.5kW) isn't a particularly big thing, nor does it have a high thermal output.

Yes agreed but the excitation current will still be quite high - there's no component on that board (apart from the fried wirewound resistors) that looks like it could dissipate more than 1W - the transistor looks to be about the size of a BFY50 - 800mW - with a heatsink. The chip - could be an LM723 - again a few hundred mW, normally used to control a power device.

Without a circuit diagram and proper service manual, we're stuck really as to what that board does, it does look like it is controlling the excitation current, especially with the size of the wires coming to it, it could be doing it with purely passive components but without the circuit diagram and tecnical documentation, it's difficult to tell what and how it's actually doing.

 

[GRW]

Markon became part of New age Stamford (now Cummins Power Technologies).
The common off-load excitation voltage of this brand is 12vdc. The wires off that PCB should be marked up. If you can identify what the connections are, then
1. It'll be able to be tested & proved that the rest of the machine is fine using a 12v battery connected to Excited +ive & -ive (though with 300vac it's almost certainly all grand)
2. If the OE board is unrepairable or obsolete (almost certainly) then an SX460 or similar might well be viable as a replacement (the SX460 offload excitation output is 12vdc)

 

[weldvandal]

Welderherup if thats the type of markon that i have then it's winding's produce 110v each joined to give you 220v so the first thing is to check all the connections from the output windings chances are there's a bad connection i think i have a manual somewhere i will try to find it ,as for the components on the board they are capacitors axial leads unless they have fallen off i would not get too worried at this point

 

[weldvandal]

this should be right for your alternator
high voltage equals 220v
low voltage equals 110v

 

[Welderherup]

Great, thanks for all the very helpful information. That’s me home from work so I’m going out to look at this now. I will report back with proper serial number and voltage reading of each winding. I think I checked it before and the voltage was way way too high. Those big caps at the bottom of the power head were giving me 320 at one point and I was getting more than 50 of the brushes so i thought my windings and connections must all be good. But maybe it doesn’t mean that at all.

 

[Welderherup]

So I’ve got an sg 21 j
Voltage starts at 270 and falls very fast down to 50. I can’t find L3 L4

I can find x, xx, x and xx. I can also find L1 L2. ZZ, Z1, Z2, Z3

across L1-L2 I’m getting 136v ac steady. Going from 136.2-136.8

there is another board that I didn’t see before. It has the ZZ labels. It is below the transformer

 

[Welderherup]

The black chip has the number
MC14069UBCP
M 78-48

 

[Ubique]

The second board is the rectifier & surge suppressor as shown in the circuit diagram above.

 

[Welderherup]

The second board is the rectifier & surge suppressor as shown in the circuit diagram above.

Yes cheers for that. The good voltage would suggest the windings are ok? It’s starts off well so the right part is excited but then falls away because the circuit that excites is not maintained?

 

[GRW]

X & XX on these is exciter DC positive & negative.
It's vital that they ate connected the correct way around on any replacement AVR.
It's only 4-wire which is a basic shunt excited alternator (as you'd expect on a small older alternator - so it's quite likely that there are a number of AVRs that might do instead...
I'd need to be in work to try and find out.
As for Frequency yes you need to be at 50Hz, but with AVR Regulation the voltage should stay as set-up. Only capacitor controlled alternators have the AC voltage rise with increased revs & frequency.
At 300vac, it's most likely that the voltage control part of the AVR has failed - so a healthy AVR is most likely all you need.