Class 66 DCC issue

[Dibs28]

I have dcc'd many locos over the last few years, but turned to 2 recently acquired class 66's R2652. These are DCC ready so simple, take out hornby blanking plug and plug in the decoder. Yip, but it over heated and blew. After some testing I am finding a short on pin 1 and 8, the orange and grey that are the motorb wires. So I snipped off the capacitor thing and I am still showing a short. I dont want to risk another decoder, but I tested my other unit which shows some resistence but not as much as this one

Prior to taking the motor out I should say I tested on DC and it worked perfectly.

A battery on the motor and it runs fine too.

Any ideas?

 

George

[Chrissaf]

Thank you for the invaluable information that you have provided stating that there is a short between pins 1 and 8.

 

Note that Pins 1 and 8 are the red and orange wires [or should be] not the orange and grey wires. The grey wire should be on Pin 5. If you have orange and grey wires on Pins 1 and 8, then the socket is mis-wired, but if this was the case, the loco would not work on DC. So I suspect the wire colours are actually on the correct pins and you have just miscounted them or just a 'typing error' in your post text.

 

So, if your text stating a short between Pins 1 and 8 is a typing error, and you meant to say a short between Pins 1 and 5. Then most of what I have written below is irrelevant and not applicable.

 

Note also, that the reference to colours in this post are referencing the colours of the wires on the decoder. The colours used at the factory on the actual socket within the loco chassis maybe completely different. They may even all be the same colour, possibly all black. They may even share some of the decoder wire colours but be terminated on different pins. Use the drawing below to confirm pin functions with your meter. But I suspect from reading your post that you are already familiar with the socket pin outs and their functions.

 

 

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I can confirm 100% that a short between Pins 1 and 8 or between Pins 4 and 5 is a wiring fault and seen regularly on DCC ready models. The short between Pins 1 and 8 or between Pins 4 and 5 have absolutely no impact when running the loco as a DC analogue loco as the DC Bypass blanking plate provides a direct metallic connection between these pin combinations anyway.

 

However, plug in a decoder and this Pin 1 to Pin 8 [or Pin 4 to Pin 5] short circuit connects the DCC track voltage [a 'peak to peak' voltage of around 28 volts] on the red [black] wire of the decoder to the motor control output circuit of the decoder on the orange [grey] wire. This cross connection although not affecting DC working, blows the sensitive decoder electronics.

 

You need to unscrew the 8 pin DCC socket from the chassis and look underneath it. The likelihood is that you will find a sliver of solder bridging Pin 1 and Pin 8 [magnifying glass recommended]. If the short is not actually on the socket PCB, it will be there somewhere. With nothing at all plugged into the socket. The resistance between Pin 1 and Pin 8 [and between Pin 4 and 5] should be infinite [a complete open circuit].

 

PS - You should expect to see a resistance meter reading across the orange [Pin 1] and grey [Pin 5] wires of about 50 to 120 Ohms give or take a bit. This relatively low resistance reading is measuring the motor winding coils of the actual motor, and is totally normal.

 

TIP: As a newbie poster on the forum, just be aware that the 'Blue Button with the White Arrow' is not a 'Reply to this post' button. If you want to reply to any of the posts, scroll down and write your reply in the reply text box at the bottom of the page and click the Green 'Reply' button.

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See also – further TIPs on how to get the best user experience from this forum.

https://www.hornby.com/uk-en/forum/tips-on-using-the-forum/

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[Dibs28]

Thanks Chris for your reply and advice re the site usage. 

My mistake it was pin 1 and the diagonal opposite corenr pin which is indeed 5 and not 8. I have taken the motor out and bare it reads 25 ohm between the 2 contacts and I am getting no readings between any pins on the socket now with the motor disconnected which is what I would expect. The question is 25ohm safe for a dcc decoder as most of my ringfield motors etc read no resistence between the connections.

The other 66 I have reads about 55ohms with the ferrite ring and other bits of circuitry in place on the motor.

I will recheck fully the socket solder side as this may sound like the issue too.

 

George

[Chrissaf]

I know very little about what is normal for Ringfield motors as I don't have any locos with them.

 

25 Ohms does seem a bit on the low side. The motor output of a DCC decoder uses PWM, which basically means that the output are pulses of 12 volts DC. Now Ohm's Law is not really appropriate for calculating current drawn by a motor because a motor generates a 'Back EMF' voltage that cancels out some of the driving voltage from the decoder. The effect of the 'Back EMF' is that it will reduce the current drawn by the motor to less than what a theoretical Ohm's Law calculation might indicate. So the following is just a purely indicative value of what the maximum current drawn by the motor might possibly be.

 

Ohm's Law Current = Voltage divided by Resistance

Ohm's Law Current = 12 volts divided by 25 Ohms

Ohm's Law Current = 0.48 Amps = 480mA

 

Now you haven't indicated what decoder brand and model has been fitted to the model. But if it is a Hornby decoder, the maximum consistent current rating is 500mA. But if you draw anything close to 500mA for an extended period of time, then it will heat up. If cooling of the decoder is inadequate, then failure is quite possible.

 

Now as I have roughly indicated with the indicative Ohm's Law calculation. Your 25 Ohm motor could potentially draw up to 480mA which is close to the 500mA limit.

 

The question I can't answer is whether the 25 Ohm reading is totally correct for the motor in your Class 66. Personally, I think it might be on the low side. This seems to be borne out by your statement that your other Class 66 reads 55 Ohms. If the Ohms Law calculation was replicated on the 55 Ohm figure, the maximum indicative current would be 218mA. Around 200mA would be considered perfectly normal for a modern loco. Plus the 55 Ohm value fits in with my 50 to 120 Ohm motor resistance estimate in my last posted reply..

 

If we assume hypothetically for a moment that the 25 Ohm reading is totally correct for this model, Then a longer term solution may be to uprate your decoder to a non Hornby brand that is specified as having a 1 Amp rated motor current capability. You said the loco runs on DC as an Analogue loco with the 'Blanking Plate' fitted. Personally I would perform a 'Stall Test' to actually measure the maximum current that the loco will actually draw and then you have got a figure to use as a basis for selecting a suitable high current decoder. This would also give you a baseline figure that you could then compare with another 'Stall Test' performed on your other Class 66.

 

However that said, your other Class 66 is 55 Ohms which (and you said it is another Hornby R2652 presumably with the same motor too) would infer that the 25 Ohm motor is actually faulty in some way. This reduced resistance in a motor could occur if one of the windings in the motor has a break through in the insulation between adjacent windings, thereby making the length of a motor coil shorter by a length equal to the missing 30 Ohms [55 minus 25].

 

Given that it has already burnt out a decoder, something is obviously wrong somewhere with the motor. My theory offers a rational explanation to explain what might be wrong. It is up to you what you do about it....... such as:

 

1. Do you send the loco back for replacement after trying to restore it back to its original as sold condition? This might be difficult as you have removed suppression components such as the capacitor.
2. Do you continue ownership of the model and try to source a replacement (spare) motor at your cost?
3. Do you just uprate the decoder to a 1 Amp decoder model?

 

Over to you ....... to make a decision.

[ColinB]

I have done a load of ringfields, they seem to be about 30 ohms so you should see some resistance. That was on my Fluke. I had issues with my Hornby class 66, the capacitor across the motor went short circuit whilst it was running, took out the TTS decoder, so perhaps Hornby had a duff batch of capacitors when they built these. If you are really worried there is a guy on EBay that sells decoders with short circuit protection on the decoder, if the motor current goes above the decoder max it cuts out and if they are fitted, flashes the front and back lights. He charges £15.00 for them, so they are worth a try. They are rated at between 1 and 2 amps, you set the max value on a CV. I bought one for my Triang because I knew it drew a lot of current. They are Taiwanese according to Manufacturer ID, but who cares if you are just using it for testing. The same guy does decent 8 pin DCC sockets, not like Hornby's, decent PCB with big gaps between pins, and they are about £1.00 (Hornby charge about £4.50).

You say the loco works fine with DC, so perhaps you could measure the current when it is running on DC, that will give you a rough idea of motor impedance. The other thing is most multimeters are not that good on low resistance values, so measure the resistance of the motor a couple of times just in case.

[Dibs28]

Thanks Chris and Colin for your replies.

Did a few tests today and DCC plugged my 2nd Class 66 which worked all good. So the motor in my first one seems to have a fault. 

A stall test revealed 480 as you said Chris, my decoder is 1amp so shouldn't have been an issue. However, a resistance check on the bare motor using crocodile clips for a better reading revealed 16 which is really low and it does cause a short on my district in DCC. However, under DC the motor runs great but it is clearly malfunctioning under DCC conditions. I am not sure what prolonged use of the faulty motor would be under DC only?

Hence, I have splashed out on a new replacement motor for this unit which should solve the problem for me and only cost £13 delivered so wasnt too bad.

Thanks again for the friendly advice and guidance.

George

[Chrissaf]

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