Not completely lost...just need a little help. (Diodes)

[perieanuo]

your dmm is no good, maybe powered with aa/aaa batteries, the diode tester is off the chart
usually all zeners sustain 30V which I recommended with no problem
for example, first one from farnell says:
"BZV85-C27,113
Working voltage range:
nominal 3.3 V to 75 V (E24 range)"
if we are paranoiacs we limit at 20-25V, but if you got 27V zener it will never polarise, only in forward mode you will see approx 0.7V
if you gonna repair you need a correct diode tester, it's a must have

[perieanuo]

Re-Tested at 15v and there was no noticeable difference.

Great, all 3 diodes are good.

About the DMM, something is wrong with it.  The instrument's manual says the diodes are tested with "Open circuit voltage: about 1.5V", so without anything connected while in the diode mode the DMM should display about 1.5V, while yours display 0.794V.

(A DMM does internally about the same trick you did while testing with the 15V and the series resistor, just that the DMM is using a lower voltage for the diode mode, yours should use 1.5V according to the specs, not sure why it displays only 0.794V).

The AUTO mode is for resistors measuring, it means the instrument will select the appropriate range for resistor, so the number will fit on the display no matter how big or how small is the value of the measured resistor.

There is no AUTO mode for diodes in DMMs.

the diode mode in dmm's is some kind of automatic by default, you got the forward polarisation voltage for the tested patient, for zeners in reverse you don't get the correct voltage normally (you haven't enough voltage to reach the threshold in reverse polarisation cause you're applying only small voltage, in your case 1.5V)
BUT if forward voltage is ok for that zener (for example, like 0.7V), the reverse will be good too, haven't see a single zener good in forward and bad in reverse
seems all your 3 diodes are fine, retest with 15V/1K series and move forward

[RoGeorge]

I am dubious of D02

Maybe the DMM just needs new batteries, but one can never have enough DMMs, welcome to the club.   

However, D02 looks exactly like a Zener diode, and measures like a Zener.  No doubt about it, and no need to worry about that one.

That's a working 5 volts Zener diode.   
https://www.vishay.com/docs/85607/bzx85.pdf

[perieanuo]

- The measurements made with the 15V adapter show all diodes were good.
- The measurements made with the 40V looks like D02 and D03 are now damaged, or maybe something else went wrong with the 40V setup.

Do not test them at higher voltage.  Higher voltage will not get you "better" testing.  Higher voltage at most will just broke perfectly good diodes.  Let's hope it was just a problem with the wiring, or with the 40V power supply, and the diodes were not damaged in the process.

Try again.  Test at 15V only.  Do not repeat the tests at 40V.

"Better" is the worst enemy of just right.   



About the DMM indication on  (ohms), that is for measuring resistors.  Trying to measure diodes with the DMM set on  is irrelevant, that is why the DMM has a measuring mode made specially for diode testing.  Rotate the selector in the position for ohms/continuity/diodes, then press repeatedly the SEL button until a diode symbol appears on the display.  Do not apply any external voltage, and just connect the diode to the DMM, first in one way then in reverse.  When it is set on diode, the DMM will generate its own voltage for the diode.  For a good diode, in one way the DMM should say about 0.7V, and when the diode is reversed the DMM should display OL.

Same as described in the instrument manual, see the "Diode Testing" section
https://images.carid.com/performance-tool/items/pdf/w2971-owners-manual.pdf
---------------------
Maybe the DMM just needs new batteries, but one can never have enough DMMs, welcome to the club.   

However, D02 looks exactly like a Zener diode, and measures like a Zener.  No doubt about it, and no need to worry about that one.

That's a working 5 volts Zener diode.   
https://www.vishay.com/docs/85607/bzx85.pdf

BZX85C5V1: if you put 40V with 1K resistor with this diode, you got 40-5=35, 35V/1k==35mA.
The pdf test that diode at 45 mA and fig 8 will point breakdown won't happen even with 40V on it.
which confirmed by the tests done by OP
so the diode can't be damaged if tested like this

[RoGeorge]

Sure, except all the assumptions before drawing the conclusion that "the diode can't be damaged if tested like this" might have not been true.  For example, can you bet your head on that resistor being indeed 1k, as it was drawn, and able to dissipate 1.5W?  I wouldn't, but we are nitpicking already.

Also, there is no auto mode in a DMM diode testing mode.  No diode "range" is switched automatically depending on a measured diode, like the voltage range is switched automatically depending on the measured voltage, or like the resistance range is switched automatically according to the measured resistor.

Since the diodes probing was sorted out, any further details will just add confusion.

Have a nice ride with that bike!   

[archil]

can you show us photos of board ?

[Mace_Hacker]

To help resolve any problems moving forward...this is the new DMM I ordered and will have by tomorrow;

AstroAI Digital Multimeter, TRMS 6000 Counts Volt Meter

I am looking forward to using it. I did some research on it and for the money it is a good value. It should do everything I need it to do.

Again, I do appreciate all the input!

[Mace_Hacker]

can you show us photos of board ?

archil, if you look back on page one of this thread there are picture and in one of my posts there is a link to more photos.

[james_s]

The age of the transistors and ICs is irrelevant, they don't wear out unless something damages them. You are more likely to cause a problem by needlessly replacing components than prevent one. There are also numerous cases where a modern component does not have quite the same characteristics as a vintage one of the same type number and may not work in certain circuits.

[Mace_Hacker]

Update:
I have replaced my multi-meterwith a (AstroAI Digital Multimeter, TRMS 6000 Counts Volt Meter) and re-tested the diodes. They do in fact appear to be fine. (Remarkable what using the right tools can achieve.)
I ordered all new capacitors, mostly from Mouser, and I am still waiting on them to arrive. I did order 2 capacitors on Amazon, twice; because I did not like the way they looked. Too small, in my opinion. (see picture)
Lacking a better alternative I will probably use the first and last ones shown in the picture.
When the rest of my parts show up I will swap them out, test, and update post again.
Thanks again! 

Edit:
I just ordered 2 different capacitors from Newark to compare to the ones I got from Amazon.
We will see.

[james_s]

Modern capacitors are typically much smaller physically than older parts, you should not judge a capacitor by appearance. I would never order electronic components from Amazon, what you get is likely to be the cheapest generic Chinese junk available and a total crapshoot.

That said, in 30+ years of tinkering and repair I have *never* seen a ceramic capacitor fail except when it had been physically broken. I would absolutely leave the original in place unless you have good reason to suspect it is defective.

[bobbydazzler]

Ceramic caps usually fail short circuit you could test them for that if you don't want to change them, and I think you can still get the big disk ceramic caps pretty easy.

[Mace_Hacker]

Several of them are cracked, dry, brittle. 40 years of heat an vibration. I know, most of the time they should be fine but these aren't.
The original ones were only rated for about 15 years to begin with so, I think they have done ok.
The diodes, resistors and, transistors I think, will be just fine. Like you said; these usually are ok as well.

[Mace_Hacker]

This was why I opted to replace all the ceramic capacitors, as well as all of the other capacitors.
They were so brittle that they crumbled with little effort.
Everything is done now. I just need to reattach the wires, add some schmoo, and seal her up.
Fingers crossed.

[josh132]

diodes aren't likely to fail. What i would replace is capacitors and resistors and they can decay over time. Transistors can fail but also unlikely.