The Pro-120 series are card controllers, open to the elements - and users. They are similar to the NCC series - but have several more sophisticated protection mechanisms built in.

The controllers are pretty well protected from the elements by a layer of protective varnish, which is good against normal humidity. The varnish does gives some mechanical protection - but never quite enough for some users!

So this page identifies some of the components that customers sometimes blow, with some other key components. This is not an invitation to experiment!.

The key components are indicated on the diagram below by a red arrow with a number beside it. The transistors are separately numbered on the circuit board silk screen and are shown also in the drawing here (this can be useful if we need to discuss transistor numbers via email) and the numbers cross-refer to the circuit diagram, Tr1, Tr2 etc.

The indicated components are as follow. The numbers are the same as those used on the NCC, so there are gaps!

1. 8v2 zener.
   This is deliberately chosen so that it has a tendency to fail short-circuit under any overload conditions. By failing thus, it protects other components and is generally good at limiting the damage. Type BZX55V 8v2 or any similar zener of the right voltage. Beware that if the zener is damaged, Tr 8 (item 24) also may well be damaged.
2. current source reference transistor.
   The zener diode is fed by a current source. This is the sensing resistor. PNP, type not critical, we use BC557C (centre base)
3. current source dropper transistor. Type not critical, we use BC557C, except on the 36 and 48v versions where it's a TIP42. On the 48v version, this is on flying leads and is mounted on the base plate. See NCC and Pro-120 series power and protection mechanisms.
4. current source 18R sensing resistor.
   If you have an accident which blows the current source, the 18R can turn black. Best replace it is if it's discoloured. And it's safest to replace both transistors.
5. Hiside driver (NPN) BC337
6. Hiside driver (PNP) BC327
   If you're into replacing these - you're getting deep!
7. Loside driver (NPN) BC337
8. Loside driver (PNP) BC327
   Can be blown if the MOSFETs go, if so, the 10R (CR16 size) gate resistors must be checked. We choose this size because they quickly fail open-circuit and rarely burn up. Check also the 9v1 gate clamp. Usually it's the PNP driver that goes, the NPN one survives.
9. Gate clamp 9v1. If this is blown, the MOSFETs are certainly dead and the 10R gate clamp resistors (to its right on diagram) will need replacing.
10. Relay drive (NPN) BC546C
11. Relay drive (PNP) BC557C
    The relay drivers are not at all fragile, but they can get blown if an uninsulated motor connection contacts the circuit board. Also, reversing the battery tends to blow both PNP drivers and the catching diode!
12. Relay drive (PNP) BC557C
13. Relay drive (NPN) BC546C
14. Relay catching diode 1N4148 - hidden beneath the relay.
15. Relay catching diode 1N4148 - hidden beneath the relay.
16. Pot fault transistor (PNP) BC557C
17. Brake catching diode 1N4002 - hidden beneath the relay.
18. Brake driver TIP42A
19. Overvoltage catching zener (47v on 12, 24 and 36v controllers).
20. The ramp capacitor. See Ramps - acceleration and deceleration.
21. Current limit resistor. 47K here gives a limit of around 55 amps. 22K gives zero current. The relationship between them is linear and is explained for the Pro-120 series (which use the same circuit).
22. Half speed reverse header. Link is shown in 'half speed' position.
23. Reverse-ignition diode. This is not present on the Pro-120 series.
24. The power relay drive transistor. May fail if the 8v2 zener is damaged.
25. Undervoltage cut-back resistor. This was 33K (giving cutback level around 20v) but was changed to 22k (cutback level around 15v) with issue 9 controllers. The change may be safely made to old controllers.
26. Undervoltage. With issue 7 and after, linking these two pins essentially removes Undervoltage cut-back. Not always desirable as if the supply voltage falls too low the relays will simply drop out and this is not safe.

Other pages with relevance to this subject

• Service Manuals - Index

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