Please Help Identifing Diodes In Old Boat Motor Rectifier

[barryg]

I have a boat motor I restored from 1966, it was one of the first with electronic switchbox for the ignition. Mercury thunderbolt brand. the rectifier is bad, these old parts are hard to find and expensive. the rectifier is in a small metal case that grounds through the case. it has three connection points other than the case for ground. two connections come from the stator/alternator and one feeds dc to the battery/ignition I don't believe there is a voltage regulator in the circuit but it does charge the battery I believe. I took out all the old epoxy and have all the parts out. I have included a rough drawing with pictures of the components. two diodes are grounded at the case and connect to the stator leads(yellow) another component is connected from the stator leads to the 12 v positive output terminal but they have no markings for identification, I assume they are diodes also and there is a capacitor connected from the case to the 12v positive also. the stator is rated at 8 amps at full speed.
my questions;
why does the rectifier need the diodes from the case to the stator input wires
are the unmarked components in the middle diodes also
why does the circuit need the capacitor
can I replace these components with new diodes of what specific kind
what does full wave rectifier mean

 

[Minder]

If you google bridge rectifier you will see loads of examples, it is a way to rectify the positive and negative going wave forms,
The diodes are connected to case as this is the negative termination for the DC.
Look on Digikey, you will see many examples for the diodes.
The cap bleeds off any high voltage spikes that may occur.
There is normally a regulator as a alternator can quickly output a high voltage at relatively low rpm.

M.

 

[barryg]

thanks very much for the reply
I will check out digikey
any idea what the component in the middle is ? diodes?
could I replace all these components in the same case with something more simple?
case is approx. 2 inches by 31/2 inches
not sure but there may be a regulator in the switchbox
I will probably open that one up next if the epoxy does not give too much trouble
again thanks

 

[davenn]

any idea what the component in the middle is ? diodes?

yes a pair of diodes ... just different style from the other 2 stud mount type

 

[Minder]

Rectifiers are fairly flexible when it comes to selection, as long as the DC voltage maximum is well covered and the current is sufficient, it doesn't matter if you oversize a bit.
The two in the centre look like solder end, similar to the press fit ones in current auto alternators.
M.

 

[shrtrnd]

I think it would help us to help you if you would tell us what part numbers you can read on each device.
If they're no longer available we can try to cross-reference them, If they're still easily available we might be able to
tell you where you can get them cheaper.
If there is a Model Number on your 1966 Mercury Thunderbot switchbox (I don't know if they were just produced under that brand name, or if they have a model number on them), somebody might even be able to find a full schematic for your circuit(?)
We'll help where we can.

 

[barryg]

I think it would help us to help you if you would tell us what part numbers you can read on each device.
If they're no longer available we can try to cross-reference them, If they're still easily available we might be able to
tell you where you can get them cheaper.
If there is a Model Number on your 1966 Mercury Thunderbot switchbox (I don't know if they were just produced under that brand name, or if they have a model number on them), somebody might even be able to find a full schematic for your circuit(?)
We'll help where we can.

Already helped a bunch
I see now that what is in the case is exactly what is in the diagram posted just mounted a different way
I think I'll find the correct solder in diodes and mount a copper stand to attach them to and use the stud holes to screw the plate down, might be easier to service
Maybe find an inline voltage regulator to mount on the battery wire externally
Really appreciate the help
I will find switchbox part numbers, a schematic would be excellent
If I could rebuild it maybe save about $600

 

[barryg]

Already helped a bunch
I see now that what is in the case is exactly what is in the diagram posted just mounted a different way
I think I'll find the correct solder in diodes and mount a copper stand to attach them to and use the stud holes to screw the plate down, might be easier to service
Maybe find an inline voltage regulator to mount on the battery wire externally
Really appreciate the help
I will find switchbox part numbers, a schematic would be excellent
If I could rebuild it maybe save about $600

Mercury thunderbolt switch box original part number 332-2986

 

[kellys_eye]

If you're not too concerned over originality then you could replace the unit with a modern bridge rectifier such as this:



They can be found in many voltage and current ratings up to 100's of amps, bolt straight to a piece of metal (for heatsinking) and use standard spade (crimp) connectors. The higher amperage devices have 'bolt/screw' fittings.

 

[73's de Edd]

Sir barryg . . . . .

Your best rebuild quality / result would be to use the already suggested metal bathtub FWB unit, as your original one was not to well-balanced in its using the two different diode sets.

We know that the cathodes of the two central Motorola diodes mounted to the copper bar would go to the battery. An anode-cathode connection goes to one of the alternator stator winding while the other anode-cathode connection goes to the other stator winding. That leaves the anode pair going to the ground frame.

The original Motorola diodes had two leads, so that PCB mount was possible.

You have one lead broken off, but that still leaves the existing lead to go to your disc ceramic capacitor. The diodes cases were additionally flash silver plated, but you see the time related AgO2 onset, but you might still, barely be able to read their OEM part numbers.

I also see that your much heavier stud rectifiers are a variant . . . in that the stud case is commonly the cathode, but in this case it is the anode.

Their reasoning . . . . .The use of two different types lets you have all the studs mounted to a shared heat sink without two of them shorting out.

Use the suggested procedure and put that saved $595 BUX back in your wallet . . . .Ka CHING !



Your Line Drawing . . . .Marked up . . . .

73's de Edd

 

[barryg]

Understood, I will use the rectifier style posted for running purposes, on this system the case is a heatsink but also the ground feed, will this type work if the ground post is just wired to the case? the part number KBPC2504 is shown would that be the recommended one for this application? the voltage is 12 the output from the stator is supposed to be 8 amp but i'm sure this can vary with speed any recommendation is appreciated
next question ;
in order to maintain originality I would like to rebuild the original rectifier also by using 4 similar axial diodes and a new capacitor I looked on digikey and the assortment was astounding I got confused please recommend an axial diode for this purpose. I don't understand the ratings listed for each device
last question
the axial diodes have a stripe painted around them is this the same meaning as the bar on top of the arrow for a diode schematic? anode or cathode?
really appreciate your guys help

 

[kellys_eye]

The KBPC2504 is rated at 25A and 400V - higher than your requirement but 'higher is better' and nothing is lost by using it - in fact having a (very) wide margin of safety is a good thing with electronics.

Same will apply for the replacement axial diode(s) you seek. Providing you meet or exceed the requirements then you will be fine with any device that physically fits/matches.

Take care to ensure the anode/cathode (polarity) is the same as some devices have them reversed.

 

[Minder]

You have shown the alternator output to the rectifiers, but the rotor is usually wound and has two slip rings with brushes for the field which then goes to a regulator to maintain the field current and consequentially the AC/DC output.
I can't see it being wide open with no regulation.
Maybe more info on the generator itself or more pic's.
M.