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Help request please. Decoder replaced with blank plug but loco apparently not working on DC


highpeakman
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I have a Heljan Class 17 (1st series - 17001). I have been running it with a 8 pin decoder that I have previously fitted. I am about to sell the loco so have removed the decoder (but I did test it immediately before removing the chip and all was fine). I fitted an 8 pin blanking plug (which I assumed are generic?).

 

However the loco motor will not run on DC in either direction. Curiously the lights on the loco work correctly (red and white change correctly with direction) so DC is present. 

 

I have a number of blanking plugs which have been removed from locos of various makes when I have fitted chips to them. I have tried several different blanking plugs but with the same result. Another DC loco runs perfectly on the test track so no problem there.

 

I put the chip back in and everything is fine again but removing it produces the same result.

 

I am sure I am doing something obviously wrong which I can't see for looking, so if someone can point me in the right direction please?

 

(I did exactly the same with another loco yesterday and it worked just fine afterwards).

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I'm still being puzzled by this one.

 

There is full track voltage (dc) on the blanking plug pins 8 (red) and 4 (black) which are linked (on the blanking plug) to pins 1 & 5 (orange and grey) which should be the motor drive. 

As the motor functions with a decoder in place there does not appear to be a problem with it.

Now about to make an 8 pin header with connections so I can apply volts directly to pins 1&5 without the other pins in place and trace the connections to the motor but, before I do that, again, has anyone out there got any ideas please? Something on the Heljan PCB perhaps causing an issue?

Edited by highpeakman
Correction to pin numbers - silly errors
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On a 8-pin plug, 1/2 aren't orange/grey - motor feed.

1 is orange, but grey is pin 5 - bottom right if pin 1 is top left when viewed from above

4 is track/black  - bottom left

8 is track/red - top right.

 

(1/2 is orange/grey for a 6-pin decoder)

 

https://dccwiki.com/8_Pin_DCC_Plug

 

8-pin blanking plugs usually common 1/2/8 and 4/5/6. 

Turning it through 180 degrees will have no affect on how the blanking plug works.

Edited by newbryford
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If the motor works with a decoder fitted then there can’t be a problem with the pcb. My only thoughts are that the blanking plugs aren’t making positive contact in the 8-pin socket to feed the DC power to the motor. That the lights work would seem to indicate that some contacts are being made and that the blanks aren’t being pushed in too far and shorting out on the chassis underneath - it can happen.  Perhaps check all the connections on them, usually flooded solder joints, are actually connected, I’ve had ones that weren’t.

 

Izzy

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13 hours ago, newbryford said:

On a 8-pin plug, 1/2 aren't orange/grey - motor feed.

1 is orange, but grey is pin 5 - bottom right if pin 1 is top left when viewed from above

4 is track/black  - bottom left

8 is track/red - top right.

 

(1/2 is orange/grey for a 6-pin decoder)

 

https://dccwiki.com/8_Pin_DCC_Plug

 

8-pin blanking plugs usually common 1/2/8 and 4/5/6. 

Turning it through 180 degrees will have no affect on how the blanking plug works.

Thanks for pointing that out.

 

You are, of course, absolutely correct and I have actually been working on that basis. Looking at it again I can't work out why I typed those pin numbers but I wrote it after a very frustrating day (disconnected reasons as well) trying to sort this out and, being tired, didn't read it back which I normally do. My lack of ability to type correctly really isn't the same as my testing ability however and I have actually checked the correct pins.

Summing up.

 

I refitted the decoder and it works correctly on DCC. Blanking plug(s) fitted it will not work on DC.

 

Removing both the plug and decoder and measuring on the decoder socket pins:

 

Measuring voltage across pins 4 (black) and 8 (red) I see full track voltage.

 

I also applied dc power directly across pin 5 (grey) and pin 1 (orange) and the motor does turn normally.

 

I have tried several different blanking plugs - even one which only links between pins 4 to 5 and pins 1 to 8 (so not inc 6&2) - loco will not work.

 

I am aware that orientation of the blanking plug does not affect it.

 

Pic shows test track with voltage and one of the blanking plugs used connecting the pins as above. Can anyone confirm that the pic of a blanking plug is the original Heljan one please?

IMG_20201004_111613_resized.jpg

IMG_20201004_111704_resized.jpg

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13 hours ago, Izzy said:

If the motor works with a decoder fitted then there can’t be a problem with the pcb. My only thoughts are that the blanking plugs aren’t making positive contact in the 8-pin socket to feed the DC power to the motor. That the lights work would seem to indicate that some contacts are being made and that the blanks aren’t being pushed in too far and shorting out on the chassis underneath - it can happen.  Perhaps check all the connections on them, usually flooded solder joints, are actually connected, I’ve had ones that weren’t.

 

Izzy

Thanks for your answer. I have already tried a lot of that. I have used several different plugs with no difference in result. Also it always works first time with the decoder fitted no no dodgy connections have been seen - so far. Believe it or not I do have an electronics background although my practical training was left behind in the 60s and 70s. However I also remember that sometimes the b.......   obvious can be  overlooked by the best and that description doesn't apply to me! Thanks again.

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You report DC seen on the correct input socket pins.   And you report the loco runs if DC is applied directly to the output pins of the socket. 

 

So, try bridging the pins with bits of wire stuffed into them.  Do it one at a time - ie. left rail to left motor, and return the DC directly via the right motor pin.  Then swap to the right rail to right motor.   Then try both together.   Eventually something ought to show up, because the initial report of how it works at the pickup level, and works out to the motor suggests its simple.   (I'm wondering about a track partially broken on the PCB, which is dislodged/moved into contact when a decoder is fitted, but not moved the same when the blanking plug is used).  

 

 

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You can prove the blanking plug by mounting it in your decoder test rig and feeding DC from a battery or controller to the rig. The motor will respond to polarity and the lights leds will indicate.

 

You can also prove the loco by simply bridging pins 1-8 and 4-5 with wire links, small paper clips may fit as do those breadboard links.

 

The plug looks like a Hornby one to me but they are fairly generic

 

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1 minute ago, RAF96 said:

You can prove the blanking plug by mounting it in your decoder test rig and feeding DC from a battery or controller to the rig. The motor will respond to polarity and the lights leds will indicate.

 

You can also prove the loco by simply bridging pins 1-8 and 4-5 with wire links, small paper clips may fit as do those breadboard links.

 

The plug looks like a Hornby one to me but they are fairly generic

 

 Thank you very much for the suggestion. I had wondered about using the test rig like that and will now try it although, as I have used several blanking plugs, I don't really suspect them.

 

I asked about the plug in the picture as I was going to try to put the original one back in the loco but can't remember which one was there when I took it out!

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54 minutes ago, Nigelcliffe said:

You report DC seen on the correct input socket pins.   And you report the loco runs if DC is applied directly to the output pins of the socket. 

 

So, try bridging the pins with bits of wire stuffed into them.  Do it one at a time - ie. left rail to left motor, and return the DC directly via the right motor pin.  Then swap to the right rail to right motor.   Then try both together.   Eventually something ought to show up, because the initial report of how it works at the pickup level, and works out to the motor suggests its simple.   (I'm wondering about a track partially broken on the PCB, which is dislodged/moved into contact when a decoder is fitted, but not moved the same when the blanking plug is used).  

 

 

Thanks Nigel.

I didn't originally see any signs of an intermittent contact and, because the decoder worked OK, hadn't looked to closely. After a lot of finger poking there does seem to be something suspect around the decoder socket or the "bedding" of plug contacts into the socket although precisely where I am unsure at the moment. I think it has to be something like that.

 

Looks like I will have to remove the PCB and check, which I was reluctant to do, but now seems necessary.

 

Thanks for your help.

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2 minutes ago, RAF96 said:

Howes are offering this as a Heljan blank...

https://howesmodels.co.uk/product/Heljan-oo-8-pin-dcc-blanking-plug/

That looks like one of the blanking plates I have and have tried. 

I am currently suspicious of the socket on the PCB - either making contact with pins in a plug or a soldered connection underneath the socket. 

Thanks for taking an interest.

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This - may - be as simple as the holes in the socket being bigger than the pins in the blanking plugs and not making proper contact with the motor ones. Just taking one blank and bending the pins a bit will soon prove this one way or another. 
 

Izzy

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4 minutes ago, Izzy said:

This - may - be as simple as the holes in the socket being bigger than the pins in the blanking plugs and not making proper contact with the motor ones. Just taking one blank and bending the pins a bit will soon prove this one way or another. 
 

Izzy

You may well be correct. I just noted that if I push carefully sideways on the blanking plate then it will make contact and the loco runs. That could be an intermittent contact on the base of the decoder socket or, as you rightly point out, I had wondered already as the pins on the decoder look to be a bit bigger than those on the blanking plugs and that would make sense as to why it works but the plugs don't. I was a bit puzzled as I would have thought that at least one of my 6 blanking plugs that have been tried would still have worked. I was just about to make a "jig" out of another old decoder plug but will try bending pins on one of the plugs to see if that works first. 

 

Thanks.

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I tried bending the pins slightly on one blanking plug and that made no difference.

 

If I "twist" the plug in the socket - and not lightly - it does connect and the motor works. So it looks like it may be a bad connection on the socket into the PCB.

 

It still bothers me though that the decoder chip plug just fits in and everything works as the same connections must be used.

 

Need to remove the PCB I think.

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There's interesting!

 

Haven't had time to take the PCB out yet but if I leave the decoder plugged in but put DC on the track it works perfectly happily! (Obviously the decoder is set to allow DC running).

 

Can it still be a pin/connection problem? The decoder plug makes thing work correctly but any normal blanking plug doesn't work unless pressed hard in one direction.

 

Driving me mad!

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Yes, of course it will work on DC or DCC with the decoder in place.  You established that right at the beginning.  The output of the decoder to the motor doesn't fundamentally change when you put DC or DCC onto the track.   

Every bit of information presented points to a poor connection around the socket.  Either the socket isn't properly connected to the PCB (dry joint, fractured PCB track, etc..), but adding a decoder moves things enough to make contact, or the pins in the socket are not connecting on the blanking plug. 

Yes, to fix it, the PCB has to come out of the loco. 

 

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Thanks Nigel. I have just removed the PCB and am about to start examining it. I will reflow the joints around the socket.

 

I think that what was getting to me is that if it is a bad joint/broken track then I would expect to see some sort of connection problems with the working decoder plugged in as well. But touching around it and the board has no intermittent effect on it, it's rock solid. I have now plugged that decoder in around a dozen times - always the same - works perfectly immediately.

 

However I have just plugged in another decoder of exactly the same type (with the same type of plug and which was verified working using the ESU tester) and that doesn't work either (lights work but not motor)! 

 

Looking at the PCB I have found one point where a couple of resistors are placed on the board and the joints don't look good but also they are covered in insulating tape to stop them shorting on the chassis but they have broken through the tape. Don't think that's the fault but it needs looking at nevertheless and may mean other things are quite right. This model is first generation of this model and I think there were other problems with the model and Heljan offered replacement chassis? This one wasn't refitted.

I have to do other things now but will get back to it later.

Thank you for your suggestions.

Edited by highpeakman
"type of plug"
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I had something similar happen when I was doing some work on a friends DCC loco and i wanted to test it on my DC layout.  Turned out to be the  internal pin connection.  Found it by doing the jiggle test.  As i remember I sort of flattened the pin on the blank so that it was a tighter fit in two planes.

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Also forgot to mention that when I first started doing work on my Friend's DCC locos (bear in mind I have DC) I had exactly the same experience you describe and like you I tried different blanks.  Turned out I was not pushing the blank all the way home.  Now I make a pint of visually examining if the blank is all the way home.  Several times even though i know what to do I have seen there is about a 0.5mm gap and a fairly forceful push is required to get them all the way home.  

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34 minutes ago, Theakerr said:

Also forgot to mention that when I first started doing work on my Friend's DCC locos (bear in mind I have DC) I had exactly the same experience you describe and like you I tried different blanks.  Turned out I was not pushing the blank all the way home.  Now I make a pint of visually examining if the blank is all the way home.  Several times even though i know what to do I have seen there is about a 0.5mm gap and a fairly forceful push is required to get them all the way home.  

 

Thanks for the suggestion although I have  tried that.

I do have track voltage on the pins at the topside of the blanking plug but it is not reaching the motor tabs on the PCB. Curiously it is reaching the loco lights so it looks like a probable PCB fault. 

 

I have "reflowed" the solder pins on the PCB under the socket but no solution yet.

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Have you got a different blanking plug to try. (I’m still at a loss as to why the decoder should work, no problem, if the issue is beyond that and on the PCB).

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3 minutes ago, BoD said:

Have you got a different blanking plug to try. (I’m still at a loss as to why the decoder should work, no problem, if the issue is beyond that and on the PCB).

 

As noted above, about 6 different blanking plugs of differing types!

One decoder works but an identical one (tested good) doesn't! I'll get to the bottom of it but need a bit more time to work on it at the moment. :scratchhead:

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