LMSFan72 Posted July 9, 2019 Share Posted July 9, 2019 Well, I was planning a little modification to the lighting circuit so that I have one end with directional lighting and one end with a switchable non-directional tail light. This would mean I could operate the lighting with a TTS decoder such that: 1. Top or tail a train with no carriage end light2. Have the loco parked with both tail lights on3. Be able to run light with a head and tail light showing Now, I have 4 different models and they have 3 different circuit boards! I thought I'd do this modification to the 2 I have that are the same, board 1621-X001R6. My plan is to identify the right place in the circuit to interrupt the tracks for F0f and F0r (pins 3 and 6 on J1) and wire the aux function pin 5 to the correct track for the second end tail light. Therefore, I have traced the circuit and identified the components to the best of my ability. However, I am not sure the circuit makes sense to me! I seem to recall reading about some main boards that were wired wrongly, and, I wonder if this is one of them. I might try to trace the other versions for comparison, but, I thought I'd ask the electronic experts! The main area I see an issue with is the common blue +ve connection to the decoder. Pin 4 of the socket (J1) seems to connect directly to terminal 2 of the end connections (ST1 and ST2). However, terminal 2 of the end connections connects to the common -ve of the lightboards, and that doesn't compute to me!! /media/tinymce_upload/5770eaf0351c5baf3962d61af9f96847.pngI'd be very pleased if anyone can help/confirm/point the error of my ways! Many Thanks, Andy Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 9, 2019 Author Share Posted July 9, 2019 I know I have missed the track connections from ST2 back to the socket. I have also now traced a second board, 1621-X009R1, and I will be uploading that image ..... Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 9, 2019 Author Share Posted July 9, 2019 Other board layout: /media/tinymce_upload/c3f504646795fdbcdaebca86d180d4ec.png Link to comment Share on other sites More sharing options...
96RAF Posted July 9, 2019 Share Posted July 9, 2019 Generally the function outputs are switched negative relative to the common blue and the diodes on the white and yellow wires to the socket support that flow.There have been some common negative boards but I think these were for the 21 pin sound models. Link to comment Share on other sites More sharing options...
Howbi Posted July 9, 2019 Share Posted July 9, 2019 LMSFan72.........in your diagrams I assume that J1 represents the 8 pin socket, if so, your pin numbers do not match the accepted format which is......./media/tinymce_upload/43b41dbfe578973cb2ad1e2322c6f38f.jpgMay affect how you evaluate the connections.........HB Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 9, 2019 Author Share Posted July 9, 2019 Thanks all. The limitation of using a library symbol for the socket is responsible for the pin numbering. I have colour coded the output wires to reflect the NEM652 standard.The light boards are definitely wired common negative and the transistors are used to switch the positive feeds to the lights with the negative outputs from the decoder. My question is really 2 fold: 1. Am I correct in my determination that the board as drawn has a trace fault that means the lights cannot function in the as built standard model? 2. I’d like to know the interaction between the different transistors at a more technical level. It’s a very complex circuit consideing it only has the 2 light functions on it! I am veering toward just stripping the components and using the mounting pads to link just the components I need under DCC - and cutting the traces I don’t. Link to comment Share on other sites More sharing options...
Rog RJ Posted July 9, 2019 Share Posted July 9, 2019 I think the circuit is designed in such a way as to maintain lighting levels to a set brightness even when running on dc as the track voltage varies with speed, which makes it more complicated. Link to comment Share on other sites More sharing options...
Chrissaf Posted July 9, 2019 Share Posted July 9, 2019 However, I am not sure the circuit makes sense to me! I seem to recall reading about some main boards that were wired wrongly, and, I wonder if this is one of them..I very strongly suspect that you are correct and that the circuit for the first PCB represents one of the incorrectly factory manufactured PCBs. I don't know the detail with any surety, but AFAIK & IIRC the issue with the faulty boards related to incorrect PCB track connections regarding the Blue +ve on Pin 7 of the 8 pin DCC socket creating a short circuit..Looking at your blue coloured wire. That wire in the circuit is definitely -ve polarity and not +ve as it goes to the negative side of the bridge rectifier on the left hand side. Your blue wire also goes to the Collectors of the Q3 & Q4 PNP Transistors. These Q3 & Q4 Collector connections being PNP also confirms 100% that the Blue wire is -ve and not +ve. Therefore the Blue wire should not be connected to Pin 7 (your Pin 4) of the DCC Decoder socket..So, if a decoder was plugged into your first PCB the +ve output of the decoder would be short circuit to ground (negative) and blow the decoder. This is the issue that was reported with the faulty board, so it does look as if your first PCB image represents one of those early faulty boards..If you compare the same with your second image. Pin 7 (Pin 4 on your drawing) you can see that the same Blue wire now goes to the +ve side of the Bridge Rectifier. This by the way, is how the positive supply is created for the lighting on a 'DCC Ready Loco' with a blanking plug installed instead of a decoder. The bridge is used to provide the positive common instead..My conclusion, is that you should probably not try and utilise the first PCB in your model modification and bin the PCB instead. Make your own circuit as you have done in the past in previously posted threads.. Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 9, 2019 Author Share Posted July 9, 2019 Thanks very much Chris, tht is a really helpful analysis. I just checked my others. I do seem to have 3 of these boards. It’s very lucky that I hadn’t put the decoders in them yet! The one that I did use I had already built a custom circuit for without checking the original. Link to comment Share on other sites More sharing options...
96RAF Posted July 9, 2019 Share Posted July 9, 2019 Also trace the lighting sub-boards for interface with the main board. Link to comment Share on other sites More sharing options...
Chrissaf Posted July 9, 2019 Share Posted July 9, 2019 Looking closer at the schematic of the first PCB. It looks as if the factory error can be fixed by breaking the track of the Blue marked connection where it attaches to Pin 7 (your Pin 4) of socket J1. So that Pin 7 (Pin 4) of socket J1 is electrically isolated on the PCB..There are previous posts that state that if the 8 Pin Decoder plug is inserted 'back to front' on factory faulty Class 50 PCBs then the decoder doesn't blow. This fits in with my assessment, because if the decoder plug is inserted into the socket 'back to front' the decoder Blue wire then connects to Pin 3 (your Pin 5) on J1 which is shown disconnected. Reversing the decoder plug is not a proper fix because although the decoder doesn't blow, the 'Directional Lighting' and 'Motor Control' gets reversed too..I believe that if that track is broken at that location, then the rest of the PCB circuit will function correctly as designed when a DCC decoder is fitted..Note for readers: That suggested fix should theoretically (needs to be tested) be a perfectly valid solution to anybody else who is reading this post who has one of these faulty factory Class 50 PCBs which they want to update with a decoder to DCC. As described below, I believe the rest of the board will operate as designed for users who just want the factory fitted 'Directional Lighting' to work correctly under DCC control..I have researched the datasheet of IC U1 marked HT7530 (the black triangle) and it is a LDO (Low Drop Out) Voltage Regulator with a regulated positive 3 volt output. The output is on the bottom right connection with the mauve coloured wire connection shown on it. The regulator is rated at 100mA..Being an LDO regulator means the output is still regulated at 3 volts even if the input is very close to 3 volts too. Not essential for DCC working where the track voltage is always at maximum. But very useful when the PCB board is being used on a DC Analogue layout with a 'blanking plate / plug' inserted i.e when some controller types are turned down for slow running. A DC Analogue controller using a PWM output on the other hand shouldn't need the benefit of a LDO regulator as the PWM pulses are at full voltage. In effect the LDO ensures that the lighting LEDs maintain a designed brightness across a wide range of input track voltages..The LED lighting circuit is using common negative with switched positive outputs to turn the lighting on and off. The PCB board schematic shows that only 'Directional F0 Lighting' is supported on it. Thus there is no support that I can see for things such as 'Cab Lights' or other decoder function outputs..As far as I can tell, the left hand output connector ST2 looks to be for lighting connections to the front of the loco, whilst the right hand output connector ST1 is for the loco rear..How the circuit works (basic overview):.Q3 is switched by applying a negative from the decoder white wire (Decoder Pin 6...which is also your Pin 6 on the schematic). If a blanking plug was fitted, then -ve track polarity on LH Track (Pin 4 on the J1 Decoder socket [your Pin 7]) would switch Q3 instead. Q3 switched on switches Q1 which puts out a +ve onto the Front White LED 'Head Code' and 'Head Marker' and Rear Red Tail Marker terminals at about 3 volts (off load) give or take..Q4 is switched by applying a negative from the decoder yellow wire (Pin 2 on the J1 Decoder socket [your Pin 3]). If a blanking plug was fitted, then -ve track polarity on RH Track (Pin 8 on the J1 Decoder socket [your Pin 2]) would switch Q4 instead. Q4 switched on switches Q2 which puts out a +ve onto the Rear White LED 'Head Code' and 'Head Marker' and Front Red Tail Marker terminals at about 3 volts (off load) give or take..Because of the factory PCB manufacturing fault. The suggested break in the PCB track for Decoder Pin 7 (your Pin 4 of J1) positive output of the Decoder (the decoder Blue wire) means that the common positive output of the decoder is isolated and unused. This is not an issue because the positive is being created within the PCB circuit via the D3,D4,D5.D6 bridge rectifier in conjunction with the U1 MT7530 voltage regulator. Thus the decoder blue wire (Pin 7) has no function in this loco..Andy (LMSFan72),This hopefully should give you enough information and guidance to progress your original query further. Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 10, 2019 Author Share Posted July 10, 2019 Thank you Chris, that's exactly the sort of explanation I was hoping for, and, confirms what I was thinking. It's going to be pretty difficult to interupt the transistor circuits to feed with the extra function, so, I may have to cut the board "upstream" of the resistors on the positive feeds. However, I expect that this will leave the brightness with an imbalance to the other end as I would expect the resistors around the transistor circuits will reduce the overall current, so, perhaps trial and error to increase the resistor value. Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 10, 2019 Author Share Posted July 10, 2019 So, the simplest modification to reach my goal, is:1. Remove R10 and R11 so that the headlight and headcode on End 2 are open circuit (I could simply detach the +ve connections at ST1 but I will re-use one of those)2. Cut the track (blue wire) between C2 and NEM652 pin 3 (my schemantic pin5)3. Remove R9 to create a break in the positive feed to End14. Solder a jumper wire between NEM652 pin3 and the End1 side pad where R9 used to be to create the positive feed to the tail light5. Solder a jumper with between the currently unused NEM652 pin - for the aux function - and the resistor of the correct value that I will then solder back to the solder pad at the ST1 end for R11 and switch the wire from the tail lights -ve to ST2 pin 5 I will upload a schematic! Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 10, 2019 Author Share Posted July 10, 2019 The additional ST connectors are just to show where the solder pads from removed resistors are. A new resistor, R13, possible of 1K ohm will be soldered to a pad at one end and glued in posiiton with a wire to the other (I may drill through the board to help secure the "free" leg). /media/tinymce_upload/0274b0b8c5272e7ca3bc2206ff04afa3.png Link to comment Share on other sites More sharing options...
Chrissaf Posted July 10, 2019 Share Posted July 10, 2019 2. Cut the track (blue wire) between C2 and NEM652 pin 3 (my schemantic pin5).Do you not mean NEM652 Pin 7 (your schematic Pin 4)?.Less haste.......more speed. Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 10, 2019 Author Share Posted July 10, 2019 Do you not mean NEM652 Pin 7 (your schematic Pin 4)? That's exactly what I meant Chris! Schematic is now up Link to comment Share on other sites More sharing options...
LMSFan72 Posted July 12, 2019 Author Share Posted July 12, 2019 OK, final post on this I think. It was a simpler mod to the board as the trace from NEM652 pin 7 (picture pin 4) doesn't need ot be cut if I cut the trace on the top of the board to ST2 pin 2.2 schematics updated. The original board but with the trace showing the path closer to route on the actual board and then the revised modified schematic. Simply 1 cut of the trace and, 3 resistors removed, 1 link with resistor added and light board terminationns on ST2 switched/removed:OriginalEND1/media/tinymce_upload/94b5525f3a387169c22862fad13173a1.pngModifiedEND1/media/tinymce_upload/43a5631ebc7cefbd60199150ae343d8d.png Link to comment Share on other sites More sharing options...
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