Topic: 1977 cb750a - Charging issue

[SmallTownStunt]

Hello everyone! 👋🏻
I’m brand new to this forum and pretty dang new to these old sohc bikes. I’ve never messed with one before but got a good deal on a 1977 cb750a cafe for $200. The guy I got it from originally had the bike running and riding but then sat for over a year because one of the ignition coils started smoking and wouldn’t produce spark. After getting then bike, I replaced the coil, went through the wiring and made sure all connections were good and clean, (Grounds too!) cleaned the carbs, put new fuel in and got the bike running good, but after taking it for a short ride and coming back home, I realized it wasn’t charging the battery. It got so low that it didn’t have the power to start again. I charged the battery up good and then checked volts at the battery while it was running and they were dropping very quickly. Instantly thought that maybe the stator was bad so I removed it and the field coil and made sure everything was good and clean. Put it back together and now it still drops volts but very very very slowly. Stator reads .1ohm on all three wires and the field coil reads 3.4ohms. Since the reading is so low on the field coil, would that be the cause of the slow voltage drop? Is the field coil slowly dieing? Or could it be the stator? I have another stator but it looks to be in worse shape and reads the same as the one currently on the bike. The bike also has a different voltage regulator that was hardwired in. Sorry for the long post, just wanna make sure to give all the info I know. Any help is much appreciated!

[raymond10078]

First thought - at what engine speed are you checking for output?

The spec is 14.5 volts at 3,000 rpm.

I've read elsewhere on this site and in Hondaman's book (I recall anyway) that the charging system does discharge at idle.  Need higher engine speeds to charge. 

I recall that if you ride around town at very low RPMs (~1500 rpm or lower) - you'll discharge the battery.

Otherwise, the field coil resistance does not sound too low to me.  I looked around for a resistance spec, and couldn't find anything quickly.

[raymond10078]

Read through this, too:

http://forums.sohc4.net/index.php/topic,80472.0.html

[SmallTownStunt]

**UPDATE** I replaced the field coil with a different one, got everything back together and my bike still wasn’t charging the battery. It would hold a solid voltage without dropping (maybe drop slightly) but still no charge. Did some more extensive research on this resourceful forum and found a procedure to also test the voltage regulator. I’ve previously tested both field coil and stator on the bike and both seemed to be good. Stator was .1-.2ohm on all three connections and field coil was 6.7ohms. After basically bypassing the voltage regulator, the volts jumped up so in theory that should mean I have a bad regulator. New one should be delivered in less than a week and then I’ll post another update. Any comments or input is greatly appreciated! It’s been a huge joy learning about this vintage bike and I definitely couldn’t have done it without help from this forum. 

[05c50]

It sounds like you're on the right track, but unlike many voltage regulators, the 750a regulator is mechanical instead of electronic and can easily be tested and adjusted if needed.

.........Paul

[HondaMan]

Put the first field coil back in. The 750A has a special field coil of 3.8 ohms resistance that puts out more alternator power than the standard 7-ohm version does.

This whole post sounds like a bad voltage regulator, to me...

[SmallTownStunt]

Okay I changed out the voltage regulator but it’s still not charging! When I rev the engine, the volts go up maybe .3-.8. I’m so lost and confused on what to do. The bike doesn’t have the original voltage regulator so could it be the wrong one? When I bought the bike the previous owner had changed it to a different one. Also HondaMan said I should change my field coil back, could that be the cause now? Please answer asap, I just wanna rideeee. Hahaha. Thanks.

[raymond10078]

what output voltage readings are you getting (and at what (idle, rev, etc)?

[SmallTownStunt]

Anything under 2500 the volts wouldn’t change, maybe lower .1-.2v very slowly but over 2500rpm it holds 11.8-12v. Kinda fluctuates. But my battery was pretty dead so I charged it up for a few hours and then the bike would hold 12-12.4 above 2500rpm. Could it be that my battery is bad or just not fully charged, causing the charging system to not bring the volts higher?

[05c50]

You need a fully charged battery to properly diagnose a charging system.

....Paul

[SmallTownStunt]

I’m going crazy over here trying to get this bike to charge. If I bypass the voltage regulator it will charge the battery but obviously overcharge because of the bypassed regulator. The voltage regulator is also brand new, so idk why it wouldn’t be working. I’ve went over every single wire about 100 times and trust me, it’s all good. The damn bike just won’t charge. It’s a 77 cb750a so it has the two in one rectifier/regulator. Not separate. I’ve even unplugged the headlight and taillight and still made no changes.

[dave the welder]

like 05c50 said you need a good fully charged battery to test.if the old battery is discharged,sulfated,or just old it will not show a proper charge

[SmallTownStunt]

The battery is as new as it gets. Just bought it the other day specifically for figuring out why the bike isn’t charging, so unless I have a bunk battery, then I don’t think that’s the issue. I have swapped out the stator, field coil and voltage regulator and still have no change in charging. I even tried another battery that is only a month old. Can someone explain or have something that shows how the voltage regulator should be wired up? And like I said previously, if I bypass my brand new voltage regulator, my volts will shoot way up so I definitely know I’m getting a charge. Seems like the charge just isn’t making it to the battery like maybe some wiring is incorrect or something.

[dave the welder]

can you post a pic of your reg and wiring going to it ?maybe that will help with diagnosis

[SmallTownStunt]

These are pictures of the voltage regulator and then the wires from connected to the wires that run into the engine. Keep in mind the wires look like a mess because I’ve been trying to test everything and figure this out. Once I figure out my issue, I’ll solder and heat shrink everything like it originally was. Thanks for the help!

[raymond10078]

Have you checked your rectifier? 

I was wondering if you'd lost one/more diodes; preventing full output.

[SmallTownStunt]

It’s a combined regulator/rectifier. From what I was informed, the 77 750a didn’t come with a separate regulator and rectifier.

[HondaMan]

It’s a combined regulator/rectifier. From what I was informed, the 77 750a didn’t come with a separate regulator and rectifier.

That's essentially correct: the rectifier was integrated into a large finned heatsink, and the solid-state regulator was on a small circuit board in it, buried in silicone potting compound.

You can test your field coil and alternator setup by tying the disconnecting the field coil wires from your present regulator and tying one to +12 volts (Black wire, ignition power) and Green (bike's electrical ground). This is full-charge mode with no intervening control devices. It should start charging at about 2000 RPM then, increasing with engine speed to 5000 where it reaches max until redline (but, don't rev the bike above 4000 in Neutral, not real good for it...).

[05c50]

The 1977 750A was manufactured with a voltage regulator (part #31400-300-035) and a rectifier (part #31700-371-000), I suggest that you contact the manufacturer of your combo unit to verify correct wiring. The factory shop manual has instructions for testing the original equipment (regulator and rectifier) as well as the correct wiring.

......Paul

[SmallTownStunt]

Everyone is giving me great information and I’m very thankful for it! HondaMan, I’m a little confused though on how to do the test you are describing, sorry about that. I’m quite new to the electrical side of bikes but definitely learning a lot! Any feedback is appreciated and I will go back out tomorrow and run more test on my bike. Praying I can get this issue resolved ASAP so I can get back on two wheels!

[raymond10078]

Everyone is giving me great information and I’m very thankful for it! HondaMan, I’m a little confused though on how to do the test you are describing, sorry about that. I’m quite new to the electrical side of bikes but definitely learning a lot! Any feedback is appreciated and I will go back out tomorrow and run more test on my bike. Praying I can get this issue resolved ASAP so I can get back on two wheels!

Here's what I would do:

• At the engine connections, temporarily disconnect the white and green wires.
• Connect the green to a good ground.
• Have another wire already connected to a 12V positive source.
• Start the bike.
• With the bike running, connect the 12V positive wire to the white (kind of cream in your pic).
• Measure the output voltage of the generator (rev the engine, too).
• Disconnect the white wire/shut off the bike.

What is the model/part number of the rectifier/regulator you are using?

[HondaMan]

Everyone is giving me great information and I’m very thankful for it! HondaMan, I’m a little confused though on how to do the test you are describing, sorry about that. I’m quite new to the electrical side of bikes but definitely learning a lot! Any feedback is appreciated and I will go back out tomorrow and run more test on my bike. Praying I can get this issue resolved ASAP so I can get back on two wheels!

There are 3 Yellow wires that come from the alternator stator (they are usually dirty and slightly other colors by now, but started out as Yellow). In your picture it looks like those are wired up already. The other 2 wires can vary, depending on which field coil you have installed, but are often White and Black, or White and Green, or White and (some color other than Yellow or White) for the field coil. The White one goes to the regulator and its mate goes to the bike's Green (ground) wire circuit.

This white wire receives the current from the regulator, and it makes the field coil act like an electromagnet, setting up a magnetic field in the center of the alternator where the field coil is mounted. When the battery is low, more current is fed to this field coil, making a stronger magnetic field. The metal 'vanes' of the rotor (on the end of the crankshaft) alternately block and pass (i.e., switch 'on' and 'off') the magnetic field as the crankshaft turns, supplying an on-off-on (etc.) magnetic field across the alternator windings. This makes them pulse power out to the rectifier. The rectifer turns this into DC power to charge the battery back up.

[SmallTownStunt]

Well The previous owner definitely wired the voltage regulator wrong. To my knowledge, I’ve rewired it correctly now and we will see if there is any changes tomorrow because it’s currently too late to start the bike right now.. I also invested in a new multimeter because mine was a little janky and old. The field coil checked out good but when I test the stator, all yellow wires read 0 ohms. Does this possibly mean my stator is bad? I have a spare stator and it reads the same.

[SmallTownStunt]

Here's what I would do:

• At the engine connections, temporarily disconnect the white and green wires.
• Connect the green to a good ground.
• Have another wire already connected to a 12V positive source.
• Start the bike.
• With the bike running, connect the 12V positive wire to the white (kind of cream in your pic).
• Measure the output voltage of the generator (rev the engine, too).
• Disconnect the white wire/shut off the bike.

What is the model/part number of the rectifier/regulator you are using?

I did the test you described for me to do and I was getting 13v+ just letting the bike idle and then when I would rev it, the volts would jump to 17v+ And I’m not sure where the part number for my voltage regulator is. I could probably post the link to eBay, from where it came.

[raymond10078]

Here's what I would do:

• At the engine connections, temporarily disconnect the white and green wires.
• Connect the green to a good ground.
• Have another wire already connected to a 12V positive source.
• Start the bike.
• With the bike running, connect the 12V positive wire to the white (kind of cream in your pic).
• Measure the output voltage of the generator (rev the engine, too).
• Disconnect the white wire/shut off the bike.

What is the model/part number of the rectifier/regulator you are using?

I did the test you described for me to do and I was getting 13v+ just letting the bike idle and then when I would rev it, the volts would jump to 17v+ And I’m not sure where the part number for my voltage regulator is. I could probably post the link to eBay, from where it came.

It appears to me then, that your regulator is not providing sufficient current to the field coil.  Assuming that all connections are/were correct, all the grounds are good (including the case of the regulator (some need it)), then I'd buy a Hondamatic specific regulator.  I know Vintage CB750 has one.  I don't recall what brand I bought - too long ago.