Constant Voltage board

That to me looks like a 5 volt board set.

I believe you had an engine mounted board set. So the easiest thing to do with the new board set if it fit, would be to keep it there. If it didn't fit, you need to get creative. The tether was the right solution. Just be careful what you connect it to.

You could contact a tech like GGG, and ask if a engine MTH CVB board is available. He also could look at that 5 volt board set for you and see what works on it and advise where to go next.

If you used a 10 wire tether, there maybe enough wires to get the whole job done. You might loose individual control of some lights though. You'd have to switch to LEDs to handle the extra loads too.

That's if you got the PS3/2 board set kit for steam? or did you use the diesel set?

Joe is right, that's a 5V PS/2 package, there is no CV board in that locomotive. 

FWIW, what I do with these is add a couple of connectors to the headlight output and drive them all from that.

I have a couple of the CV boards if you need one.  These were in some PS/1, and also many Weaver and Williams brass locomotives.  There are many form factors, but all seem to have a common design as far as the switching power supply.  They all take track power (left hand white connector), and offer up either 6VDC or a current limited 6V for LED markers/class lights.

The easiest route would have been to get a PS32 board with 5V connectors and swap that in. I did that exact repair on a Premier FEF for a friend locally. He got the board set from a tech and I swapped it in for him. Kept the original lighting functionality of the locomotive intact. 

Now I've done a few locomotives for myself that had no CV boards. Like John I just run all the lights (usually 6-8 3mm LEDs) off the headlamp circuit. 

True Lou, but he's already converted the tender, so he's in the middle of the bridge.

I have an 0-8-0 steam switcher with the 5 volt board in the tender that has no CV board in the engine. The only difference, I notice in operation, is the smoke unit heating elements turn off and the fire glow goes out, when the engine is thrown into reverse. It still puffs smoke but only for a few feet til the elements cool down. It does have a tender reverse light that comes on when the fire glow goes out. At first I thought it was all a malfunction but have come to believe it was a sacrifice MTH had to make because of the lack of room in the engine for the CV board.

@Dave Zucal posted:

I have an 0-8-0 steam switcher with the 5 volt board in the tender that has no CV board in the engine. The only difference, I notice in operation, is the smoke unit heating elements turn off and the fire glow goes out, when the engine is thrown into reverse. It still puffs smoke but only for a few feet til the elements cool down. It does have a tender reverse light that comes on when the fire glow goes out. At first I thought it was all a malfunction but have come to believe it was a sacrifice MTH had to make because of the lack of room in the engine for the CV board.

That is 100% a malfunction and should be checked out. Going in reverse should not cause the smoke heater and firebox light to shut off.

Lou is, of course, correct Dave.  Something is VERY wrong when that happens.  My guess is one of the diodes that reconstitutes the PV voltage is bad, a simple fix.

To explain, in order to save a wire between the tender electronics and the locomotive, MTH uses the two motor leads and a pair of diodes to "regenerate" the PV voltage in the locomotive.  If one of those diodes is bad, in one direction the PV voltage will go away.  Since PV is used for the smoke heater and lighting, those will go away as well.

Thanks John and Lou. My mistake. I really thought the tether was limiting the functions the board is capable of through the tether, hence the need for the CV board. So, is it correct to say, if mzenft's loco had pick up rollers on the tender, where he mounted the board, he wouldn't need the CV board in the engine? John, would this be simple enough for me to repair by splicing a diode into the motor leads, or do they need replaced on the board which is more difficult?

The diodes are not on the board in the tender, they're in the locomotive.  Look on the PCB for the tether connector.  Here's a diagram of the simple RK locomotive with no mux or CV board.  The diodes will be discrete 1N4003 diodes in the locomotive.

This is excellent info for those of us doing upgrades. I am a rank amateur and haven't seen this issue yet. But I won't be surprised to see it in the future. Now I feel prepared.

Thanks.

Don Merz

For the upgrade kit, the diodes are on the tether connector PCB.  However, for some non-MTH upgrades, they don't fit into the space for the connector.  In that case, I remove them and wire them direct from the motor and the PV wire in the locomotive.

I opened the engine and found the diodes and tested them on the board with an ohms meter. They both were closed in one direction and open with the same slight resistance in the other.  I thought I would find one that was open both ways. I'm assuming their ok but do they have to be tested removed from the tether board?

Make sure the anode of each diode (no stripe) goes to the two motor leads (one per diode).  The diodes are combining the PV that switches between the two motor leads into one voltage.

Your issue certainly screams out missing diode.

Yes John, anodes are correct. I put it on the tracks without the shell and noticed the firebox glow does not go completely out, it just dims down quite low. So the bulb and heating elements are both starving for power when in reverse. I'll change out both diodes if I have them. They both read a resistance of 2300 ohms in one direction with an analog meter, if I read the meter correctly.

UPDATE: I found a bad solder connection with one of the pins on the tether board. Resoldered it and all is well. Thanks for your help John, if I wasn't poking around at those diodes, I would have never found it.

Great, it had to be something right there.  I'm surprised the pin only affected one direction, must have been a motor lead.

Yes it was for the yellow motor lead.

Amazing that the motor ran at all with that loose.