Topic: 1978 CB750k charging diagnostics

[surveywaters]

Rectifier tester needs to provide more than .7 Volts during the test.  I don't know what tester you are using.  Modern DMMs have a diode test selector.  -|>-  (Text diode symbol)
 6 tests of each probe polarity.  Yellows to Red and yellows to green.

Was the ".7 Volts" a mis-speak ?

When performing a diode test, I am reading .42, my only questions are: Is 0.42 too low and, on the POSITIVE side of the rectifier, should the plate which the + side of the diodes connect to show any resistance to the + wire leading to the battery?  I don't know how to state this any clearer, it seems straight forward in my mind and I do not mean to confuse anyone. Perhaps if someone has a rectifier on hand to test I can post pictures with step by step instructions. I believe that my + lead is getting a poor connection to the + side of the diodes.

[TwoTired]

No disrepect or tone intended here but the rectifier I pictured looks exactly like every rectifier pictured on ebay for CB750's from1969-1978. The green wire has apparently been removed. 

Sorry, I don't recognize ebay as an official Honda source.   
If the green wire has been removed, you must use where ever it had been attached to test the diodes.

When using the diode function on your meter, what unit is used if not voltage? How do you test a diode by reading resistance?

As I explained, Ohms.
A good diode will exhibit low resistance to current flow with one probe polarity, and very high resistance with the other probe polarity.

Was the ".7 Volts" a mis-speak ?

No.  But, apparently it was information you cannot use.  Silicon diodes don't behave as simple resistors.  To operate and conduct they need a bias voltage of about .7 volts.  If the testing device does not supply this much voltage, the diode will read a very high number, with both polarity connections and pass no (or extremely little) current.  The user will then make the incorrect conclusion that the device tested bad, when in fact the test was not performed properly.
 

When performing a diode test, I am reading .42, my only questions are: Is 0.42 too low and, on the POSITIVE side of the rectifier, should the plate which the + side of the diodes connect to show any resistance to the + wire leading to the battery?  I don't know how to state this any clearer, it seems straight forward in my mind and I do not mean to confuse anyone. Perhaps if someone has a rectifier on hand to test I can post pictures with step by step instructions. I believe that my + lead is getting a poor connection to the + side of the diodes.

You will have to show or declare the model number and brand of your test meter.  I might be able to interpret the reading for you if you also state where you placed the multifunction selector knob, and the probe tips.

Do you understand what "polarity" means?  ...And that your testing device has this?

Also, you are NOT looking for absolute numbers, but a comparison of the two numbers taken with two probe polarity placements.  If your tests show the same number with both polarities, the unit is either bad, you are connecting at the wrong points, or your meter isn't capable of activating the diode and not giving us the needed information.

[surveywaters]

With my meter's + lead on the diode's anode lead and the and the meter's - on the diode's cathode lead I am reading 0.42v, which is indicating my barrier voltage drop. This is consistant for all of the diodes and all function properly, gating reversed polarity. Were I to use a meter which did not have a diode test function I would be reading Ohms. 

My question is: How does the voltage reach the + lead exiting the rectifier? The diodes appear to be connected to the isolated braket of the rectifier and the red lead I assume is suppose to have a resistance free connection to said plate. Is ths correct?

[74750k4]

I have used a multitude of DVM's over the years. I have never seen one in the "DIODE TEST" mode,  that gives resistance readings in OHMS. Diodes are tested with voltage, and give VOLTAGE readings, shown in VOLTS. If you're diode is good, typically it will read less than 1 volt in the forward biased direction. In the reversed biased direction it should read OL (OverLoad Volts) on  Fluke DVM's, and also an HP sitting here as well. Measuring that output voltage on another DVM, I get 7.5-8 volts for the Fluke. The HP only puts out about 3 volts, but as long as it's higher than a couple volts it will give you a reading of the forward voltage drop across a standard diode. One thing to be aware of is that it may not tell you if the diode is leaky at high reverse bias voltages. I have run into this many times. A leaky diode at higher voltages than the DVM puts out.

If you are trying to test a diode on the resistance scale, well, unless you get lucky, all bets are off, unless its's dead shorted.

[74750k4]

Diode Test

[TwoTired]

My DMM is about 30 years old.  See below.
  I do recognize there are thousands of DMM designs out there.  But, without knowing which ones we are discussing, further discussion is rather pointless, I think.

It appears, I'm unable to help with this thread.  Perhaps there are others that can help you.

Good luck!

[surveywaters]

I believe your inability to help may have been in the fact that you focused on semantics rather than the question posed.  I have no issue, nor did I indicate an issue in testing diodes. My question was solely based on the connection of the anode lead and the r/wh wire, I now believe they are in fact supposed to be connected via the isolated side of the OEM rectifier. I believe this to be my problem as I am reading high resisatnce between the two. I have decided to simply upgrade the existing regulator and rectifier with a modern and more efficient combination unit.  Thank you all for your help.

[74750k4]

My DMM is about 30 years old.  See below.
  I do recognize there are thousands of DMM designs out there.  But, without knowing which ones we are discussing, further discussion is rather pointless, I think.

It appears, I'm unable to help with this thread.  Perhaps there are others that can help you.

Good luck!

Wow, even Fluke isn't owning up to having made that it's been so long!   The old 8060A I have here (1982), looks very similar, http://parlos.tamu.edu/MEEN364/Fluke.pdf
but as the page from the manual above shows, it has a separate setting for diode test, as do more modern DVM's. I think I vaguely remember seeing some very early DVM's with the lower resistance scales including a diode test. Obviously the resistance test puts out some voltage to check resistors, but if it's reading out  in ohms, well, that's really kinda confusing, and certainly misleading, when reading voltage drops across diodes for sure!!! Time for a trade in?