RMweb Gold ianLMS Posted January 12, 2019 RMweb Gold Share Posted January 12, 2019 Good afternoon I have not come across tbis issue before. I have fitted a Zimo MX634 decoder to my kit built black 5. In reverse it runs perfect. Nice gentle acceleration. Going forward the slightest touch of the throttle and the loco goes off like a rocket. I have reset all the settings to factory, reversed direction and turned off dc in cv #29. Still not change. Any ideas? I dont have another chip to try that isnt already hard wired in. Ian Link to post Share on other sites More sharing options...
Nigelcliffe Posted January 13, 2019 Share Posted January 13, 2019 " Goes off like a rocket " - ie. does it appear to be engaging full speed regardless of throttle setting ? If so, double check wiring for potential shorts from pickups to motor feed wires, then re-try the factory reset. If no luck on either, then I start to suspect faulty chip, with part of the output stage damaged. - Nigel Link to post Share on other sites More sharing options...
RMweb Gold ianLMS Posted January 14, 2019 Author RMweb Gold Share Posted January 14, 2019 Hi Nigel, When reversing, the loco responds to throttle settings perfectly. When moving forward, it wheelspins and sets off like a dragster the moment I open the throttle and regardless of throttle setting. I have a new MX623 decoder on the way so will try that when it arrives to see if the chip is faulty. If the same thing happens with the new chip, then I know its the loco/pick-ups. The MX634 came with a stay-alive capacitor which I fitted so may remove that to see if thats causing the issue. Not sure how it would though. Thanks for responding! Ian Link to post Share on other sites More sharing options...
Nigelcliffe Posted January 14, 2019 Share Posted January 14, 2019 Hi Nigel, When reversing, the loco responds to throttle settings perfectly. When moving forward, it wheelspins and sets off like a dragster the moment I open the throttle and regardless of throttle setting. I have a new MX623 decoder on the way so will try that when it arrives to see if the chip is faulty. If the same thing happens with the new chip, then I know its the loco/pick-ups. The MX634 came with a stay-alive capacitor which I fitted so may remove that to see if thats causing the issue. Not sure how it would though. In the case of a loco doing something silly, such as a run-away, which might be due to a wiring fault in the loco, trying another decoder is not sensible. It might just destroy another decoder ! You mention stay-alive unit. With a Zimo that shouldn't be an issue if connected correctly, but temporary removal may be worth trying. If that fixes things, then there may be decoder CV changes which are relevant. I'd be going over the loco electrical connections incredibly carefully with a multi-meter, taking into account what happens when the body is refitted. There must be no possibility of a connection from either motor connection places and the loco pickups. If its a tender loco, those tests must include the tender as well as loco if there is any possibility of an electrical path. Once that is passed, the decoder must have no possibility of electrical contact to motor or pickups (which may include the body in a metal loco), so the decoder needs adequate insulation (only needs to be paper, but it has to be there). Link to post Share on other sites More sharing options...
RMweb Gold ianLMS Posted January 14, 2019 Author RMweb Gold Share Posted January 14, 2019 (edited) In the case of a loco doing something silly, such as a run-away, which might be due to a wiring fault in the loco, trying another decoder is not sensible. It might just destroy another decoder ! You mention stay-alive unit. With a Zimo that shouldn't be an issue if connected correctly, but temporary removal may be worth trying. If that fixes things, then there may be decoder CV changes which are relevant. I'd be going over the loco electrical connections incredibly carefully with a multi-meter, taking into account what happens when the body is refitted. There must be no possibility of a connection from either motor connection places and the loco pickups. If its a tender loco, those tests must include the tender as well as loco if there is any possibility of an electrical path. Once that is passed, the decoder must have no possibility of electrical contact to motor or pickups (which may include the body in a metal loco), so the decoder needs adequate insulation (only needs to be paper, but it has to be there). Much appreciated. I had ensured the stay-alive and decoder were suitably protected from touching the body. I will double check everything on the loco just to be sure. I have pick-ups in the tender and the loco but it still happens when the tender is disconnected so it must be somewhere on the loco. It also happens when the body is completely off the chassis and the decoder is flapping in the wind. It could be a short with the metal brake gear, but when I had previous shorts like that before, it tripped the controller, not sent the loco nuts. What I cant work out is why it only happens when the loco is going forwards, and not in both directions. Edited January 14, 2019 by ianLMS Link to post Share on other sites More sharing options...
Nigelcliffe Posted January 14, 2019 Share Posted January 14, 2019 Blown half of the output stage of the decoder could account for the behaviour. But why a decoder blew is another question, its usually because its been short circuited from pickups to motor terminals. But there is always a possibility it arrived faulty. I'd be very careful in installing and testing any new decoder. Once you decide to fit a new decoder, then test it responds to programming reads and writes on a programming track. Link to post Share on other sites More sharing options...
RMweb Gold ianLMS Posted January 14, 2019 Author RMweb Gold Share Posted January 14, 2019 I have a cheap Gaugemaster decoder which I am happy to sacrifice before connecting the expensive Zimo one. I will also try turning off Railcom which is something else to try. Before doing anything though I will follow your advice and test for any potential shorting issues. Link to post Share on other sites More sharing options...
Nigelcliffe Posted January 14, 2019 Share Posted January 14, 2019 The other thing to have is some sort of decoder tester. A minimal one is a cheap 12v motor with some sort of indicator disc fitted to the output so you can estimate its spin speed. Or there are nice commercial ones. Link to post Share on other sites More sharing options...
RMweb Gold ianLMS Posted January 15, 2019 Author RMweb Gold Share Posted January 15, 2019 So, last night I checked all over the tender and chassis for potential shorts using the multi-meter. After making sure all brake calipers, brake rods and other parts of the chassis had safe clearance I checked continuity and everything appeared ok. I re-adjusted the pick-ups to make doubly sure before I replaced the DCC chip for the Gaugemaster one. Quick test and it worked fine. Put the Zimo one back on and it shot off again. Its not picking up as well so running a little rough, but again, I am sure some adjusting of the pick-ups and it will be fine. I will pick up a tester at some point, but for now the Zimo decoder can be sent to the junk pile. Thank you very much for your help. I am sure it was a short somewhere which fried the chip in the first place, and now everything has been re-adjusted, the short has gone away. Ian Link to post Share on other sites More sharing options...
RMweb Gold RFS Posted January 15, 2019 RMweb Gold Share Posted January 15, 2019 Zimo decoders have short circuit and overload protection so I would not have expected one to fail in this way due to a short. What's the guarantee period for them? Lenz decoders have 10-year warranty and I had a similar problem with a 21-pin Silver decoder which had been working fine for several years. Then suddenly it started causing a DCC system short in reverse but was still fine going forward. It was replaced by Lenz free of charge, apart from return postage. Link to post Share on other sites More sharing options...
RMweb Gold ianLMS Posted January 15, 2019 Author RMweb Gold Share Posted January 15, 2019 I will have to look into the warranty and see if they will exchange it. Link to post Share on other sites More sharing options...
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