Chester DB7 speed control board? NVR switch doesn’t lock

[LainchyParticipant @lainchy]

Hi Forum,

Strange issue with my Chester DB7, in that the NVR switch wont lock down, but if held, the lathe runs perfectly. after it's warmed up a bit, the switch stays locked.

I thought originally that this'll be the NVR switch, so sourced another, yet the fault still exists. Coming from the NVR on the Chester, is a mains filer unit, then the control board – which is a BC2000-TB

Has anyone experienced this fault? Has anyone replaced the BC2000TB control board with a KBIC240? It's a little different but the same spec.

I'm going to try and ring Chester tomorrow, see what they say, but it's weird for sure.

Cheers

Ian

[LainchyParticipant @lainchy]

[Martin WParticipant @martinw]

Ian

If any of the interlocks are open or perhaps have not closed properly then the NVR will not latch as the interlocks are placed in series with the hold on coil in the NVR switch as a safety precaution. Check the chuck guard and make sure that you can hear the microswitch activate or better still check that it operates with a DMM or something that can indicate continuity. UNPLUG from the MAINS before trying any of the above.

Hope this is useful.

Martin

Edited By Martin W on 20/01/2020 01:27:44

[not done it yetParticipant @notdoneityet]

after it's warmed up a bit, the switch stays locked.

After what? The switch, the weather or the motor?

Have you tried it without a belt, if it has one? That may determine if it is the switch or the motor (or the weather).

[LainchyParticipant @lainchy]

…after the lathe has warmed up…by holding down the start. Picture below. Although the link to A1 on the switch meters ok… I'm going to remake those connections first I think.

[LainchyParticipant @lainchy]

[LainchyParticipant @lainchy]

Reverse switch shows 9 and 10 together as in, unswitched

[SillyOldDufferModerator @sillyoldduffer]

The symptoms suggest Martin is on the right track: either the NVR is faulty, or one of the interlock switches is intermittent, perhaps right on the edge of breaking the circuit. As it works when the NVR is held down manually, I can't think of a fault after the NVR in the filter / control box that would explain that. The NVR has an internal magnet operated switch that pops out when the mains input fails; they're ignorant of what happens to the output.

On my lathe the interlock switch protecting the gears is unlikely to go wrong. It's a solid reliable plug/socket affair, that either works or not. Not so the switch on the chuck guard. Closing the guard rotates an axle, which, inside the control box, turns a simple lever to close a microswitch. The microswitch and lever may need to be tweaked physically to ensure reliable make/break, but when mine misbehaved it turned out a loose guard wasn't turning the axle properly. Fixed in a few minutes by removing the guard, turning the axle manually into the 'GO' position, and then refitting the guard and tightening up the screws.

Not looked at my other interlocks, but don't trust any of them. I'd use a multimeter to look for a trigger happy interlock switch, or make sure the mechanical side of each is properly adjusted. I wouldn't spend any time inside the lathe until I was convinced the interlocks and NVR were all 100%.

Dave

[LainchyParticipant @lainchy]

Many thanks chaps. I've just metered the Live line from the plug, to 23 on the NVR (via all the interlocks) and that gives me a good reading of 00.2 ohm. Operating each switch returns to the same value. Interesting that the NVR doesnt care whats passed it, so that rules out the board and filter. It's got to be either a dodgy NVR (and replacement) or iffy supply then.

[Martin WParticipant @martinw]

Ian

I can, on my DB7, replicate the fault that you described above. With the chuck guard 'open' the NVR will not latch but if I hold the NVR down then the lathe will work. Your schematic shows the interlock in series with the mains feed to the NVR if this was the case then it would not be possible, certainly on my incarnation of the DB7, get the lathe to run by holding the NVR on with an interlock open.

The safety interlocks are normally wired in series with the 'Hold On' coil of the NVR and not as part of the mains power feed circuit to prevent overloading the interlock circuits and potentially leaving them live when the NVR has reset to OFF, a certain washing machine manufacturer put the full mains load through their interlock system and the result was a potential fire hazard.

The connections to the NVR in your schematic show terminals A1 and 14 to be linked, I would expect these terminals to be connected to any safety interlock circuits. It might be worth checking again how the wiring your machine is configured with regard to this. If I recall correctly the reversing switch does interrupt the mains feed circuit and is intended to stop the lathe from being switched directly from forward to reverse instantly.

Martin

[Martin WParticipant @martinw]

Ian

If you activate the chuck guard interlock by having the guard in the up position and then press and hold the NVR switch and the lathe starts then the interlocks are not in series with the mains feed to the NVR.

Martin

[LainchyParticipant @lainchy]

Hi Martin, this is definitely the way mine is wired… Live feed via all 3 interlocks, then to the switch. with any of the interlocks open, there is absolutely no power available, held or otherwise.Interesting then, because it sounds like they are wired differently, and if so… why? I know someone has been in the NVR before, as it's been fudged, so I wonder if they've done something else with the wiring. (it is all brown coloured though and heat shrunk, so maybe not) For some reason, the main quick blow fuse has gone…probably having all the wires out while testing, so that's the first bit now. I wonder if It's worth re-wiring while it's all out? I might drop Chester a mail to see if they have the wiring diagram.

Cheers for now

Ian

[Martin WParticipant @martinw]

Ian

Had another look at your wiring diagram and it looks as if the 'Hold On' coil has been wired incorrectly. Your diagram shows contact A1 wired to contact 14 and therefore the hold in coil will have no voltage across it as it is effectively connecting mains neutral IN to mains neutral OUT across the hold in coil. This assumes mains neutral is applied to contact 13 and mains live to contact 24 that said connecting the mains the other way round will make no difference to the operation of the NVR. If you move the link so that A1 is connected to terminal 24 then it should function as intended as the mains will applied across the hold in coil when the switch is active/on.

Martin

[LainchyParticipant @lainchy]

Hi Martin,

Always worth a double check, so thank you. I did just re-check the diagram on the side of the switch… It's a KJD 17B

The diagram on the side of the switch does show a connection between 14, and A1. If you do a quick google image search for the switch, you can see the diagram. (Although there are a few) I'm sure that 24 to A1 you give exactly as you say. Bear in mind also, that this did work and has gotten worse. My Live comes in on 23, and out on 24, N in on 13 and out on 14.

I have spoken to Chester also, and Mark over there sent me a diagram, but unfortunately, it's nothing like what I have and shows a 400v brushless motor (I think – 4 cores), BUT, he did say I could send it up… they'll collect for £35.

I'm absolutely convinced the interlocks are OK, but this evening I'm going to wire a light bulb up to the switch, and have a dabble. It's a shame I have nothing on it now though. Has the main board failed? I have no idea now. I'd like to see the chuck and gear case interlock on the A1 link, and the emergency stop switch on the Live feed. I'll see what I can find out tonight though. Annoyed, but I've made my beginner mistakes on this lathe, taking far deeper cuts than I should have, with chatter, and stalls…. so my thoughts are check all I can, then let Chester replace what they find. Take the hit on the cost. It's a great little lathe that owes me nothing

[Martin WParticipant @martinw]

Ian

Going back to your first post you said that if the NVR was physically held in then the motor would run and that the fault was that the switch was not latching. Is this the case still? If it is and the motor responds to the speed control then there is nothing wrong with the drive electronics or the motor.

If the switch has the A1 contact then to activate the latching mechanism this must be taken either directly or via normally closed interlocks to contact 24. The other version of the KJD 17 NVR doesn't have the A1 contact and the coil is internally wired between contacts 14 and 24. In both cases the mains live and neutral can be connected either way round to contacts 13 and 23 without affecting the function of the switch or latching operation.

If I am teaching my granny how to suck eggs then I apologise.

I agree with you that they are cracking little lathes and seem to be nearly bomb proof; famous last words.

Martin

 

PS. With regard to that circuit from Chester, when they had a forum it was shown there until I suggested that it wasn't correct. They removed it from the site but evidently they didn't follow through to check where else it might raise it's ugly head. Unfortunately they closed the forum probably as it was not used a great deal however it was a good source of information regarding some of their machines especially some of the smaller lathes.

Edited By Martin W on 21/01/2020 17:21:48

[LainchyParticipant @lainchy]

Martin, bang on! Somewhere along the line, I've got mixed up… what's thrown me off trail totally is the new switch is DUFF!

Wired a plug up, and a table lamp to the old switch…. it was occasionally latching as before, put the new one on… not even the slightest hint of a latch. Nothing. Metered the lathe out to make sure nothing was adrift, old switch back in, lathe turns on by holding the on side.

So, I'll get back in touch with whee I bought the switch from and get a replacement.

Blown fuse…. no idea. I've spent a couple of hours faffing in there tonight.. came to turn the fire on first thing, no fire, so not only had the fuse gone in the rear of the lathe, but the shed ring main MCB had tripped 😀 I just hadn't noticed in my disappointment.

Another day, a fresh pair of eyes.

Thanks SO much for the help.

Result.

[Martin WParticipant @martinw]

Ian

Good news and I hope that they will replace the defective switch without too much faffing about.

Cheers

[LainchyParticipant @lainchy]

Just in case anyone needs this… I've uploaded to the album. new switch on the way Martin

[Russ BParticipant @russb]

I’m picking up a faulty DB7S tonight, so this might well come in handy, thanks!

He said the motor won’t run, but he seems to think someone’s tested the motor and it checks out ok, and i’m guessing he’s checked all the internal fuses as he said there were a load, I wonder what mysteries I’ll find!

[Russ BParticipant @russb]

Well, I finally got round to looking at this machine. Can anyone help me confirm a few things?

The main plug had a 3 amp fuse in it, which after testing & sorting a few things…. blew as soon as I pressed go, but it did spring to life very briefly and all appeared fine! Since the machine is (should be) protected by the small internal fuses, I think the main plug fuse can be 5 or 10 amp to protect the cable alone, however……. as below…

I notice it has a pair of 10 amp fuses in the back, should these be 5amp fast acting, there is no information in the Chester Manual or on the parts diagram about any of the fuses rating or types – good old Chinese manuals!

I'm reluctant to try a 5 amp main fuse till I know what the control board and motor fuses should be.

I found a couple of faults, the side panel interlock was smashed, and the chuck guard interlock wasn't quite positioned correctly.

I also found that the neutral line (which runs though the stop switch and interlocks not as per above) had been connected to a terminal called F- which is right next to L2….. a mistake perhaps? It runs though some sort of separate EMI filter first that is glued to the bottom of the enclosure.

I tested the NRV and that works and latches as it should when the control board is disconnected. Looking at the wiring, someone has been at this in a big way which is never comforting, either that or the Chinese are fond of sticking insulated crimp terminals in various lines to and from the various switches, board, motor etc etc. I tested the pot and apart from going though a dodgy almost open circuit patch just before full speed, it checks out exactly as it should.

When I tried to start the machine, I had to pot turned to minimum. I have now disconnected the drive belts and will try 9v and 12v across the motor while measuring current to quickly assess general motor health/cleanliness.

[kevin laxonParticipant @kevinlaxon28008]

If  the motor rateing is 750watt or less then a 3A fuse should be fine especially as its a variable speed machine & starts at low speed.

 

Edited By kevin laxon on 16/11/2021 18:09:34

[Russ BParticipant @russb]

Fuses aside for a moment….. motor testing didn’t check out, I think there may be an issue with the brushes or commutator which will require investigating thoroughly.

[Russ BParticipant @russb]

After a good clean, the motor works perfectly and spins smoothly on the bench from a little 9v battery, and consumes around 200ma on 12v.

I went on to do the light bulb test on the motor output and found constant 110vdc with a 50hz flicker which was a big clue….. as I see it with my very basic electrical knowledge, the board uses 3 diodes and 2 SCR's, one SCR for each half of the rectified AC sine, and one of them was stuck closed, allowing the 110v flicker as half the AC flowed constantly.

New parts ordered sub £10, I think I'll change the pair and keep the old one as a spare, time will tell if my diagnosis is accurate but I did test the SCR with the 2v given by the diode test function on my meter (spec sheet says it should switch at half that) and one opened and closed nicely, the other was just constantly passing current no matter what I did to try and shut it.

Hopefully nothing else is damaged. All the diodes played nicely, nothing on the board appears burnt or damaged.

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