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SCB 83970GS / VRB 84132GS Parts List & Diagrams

Bassplayer1985

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Last Subscription Date
10/03/2019
I can't assume anything, but in the manner of how you removed the components, it is possible. I have the parts list posted on this thread, and my schematic only shows the circuit diagram, not which component goes where. C1, C2, C3, and C7 are part of the VR circuit
 

Facethewind

Registered
Ikaria73, one test that I found helpful is outlined on pg 79 of the B&S maintenance manual. You disconnect the wires from the brushes and remove the VRB from the system entirely. Then hook up a 12VDC battery to the brushes and start the generator. It seems that 12VDC applied to the rotor will generate about 103VAC at the output. For me, when I did this, the output was a solid waveform on my O-scope with absolutely no light flicker. This confirms that my issue with the flickering lights has nothing to do with the rotor/stator/engine, and 100% due to something wonky in the SCB/VRB voltage regulation boards (I'm still trying to figure out what the problem is!)

You could try to see what voltages you get when you do this. I got 103VAC at the power outputs, and 180VAC at the excitation winding (red #6 and blue #2 wires). If you still get high voltage at your output, I'd say something's wrong with your alternator wiring, since the SCB/VRB are now removed from the circuit.
 

Ikaria73

Registered
Thanks Facethewind. At this point I’m worried that if I messed up the board with my sub par soldering skills, it might not be worth the effort since I can’t get a new board.
 

Facethewind

Registered
Hi BassPlayer, in regard to my flickering lights issue, on the 83970 SCB I've now replaced:
- Q1, Q2, C7: No effect
- C1, C3, C4, C5: No effect
- D1, D2, D3: Stability improved, however now the AC output dropped to only 95VAC, even when the trim pot is turned up all the way.

For D1 & D2, I used Mouser 512-1N4148. For D3, I used Mouser 78-1N5234B. Could these be the incorrect components? I have a Rev T board.

The issue definitely seems to have been related to the Zener diodes, since the behavior changed so much after replacing them. I'm pretty good at soldering, so I don't think I was the cause!

Here's a link to what the o-scope output looks like after this latest mod:
https://www.dropbox.com/s/8l50nipx6ewo8m8/20180620_185043.mp4?dl=0
 

Facethewind

Registered
Also, your schematic shows two zener diodes in series, D3 & D15. I don't believe I have a D15 on my Rev T board. On my board, the anode of D3 connects to resistor R6 as shown in the schematic, and the cathode connects directly to the cathodes of D1 & D2. Could this be the issue? How do I correct this...just add another zener in series with D3 to make it match the schematic?

Here's a photo of my Rev T board, after all the mods I've made.
 

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Facethewind

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That was it! I put in another Zener diode (D15) in series with D3, so that my board matched your schematic. The output is now back at 120VAC, the output waveform is rock-solid stable with no glitches and the lights don't flicker anymore. I'm guessing the D3 that was originally in my board was spec'd differently than the Zeners that were spec'd in the parts list for your board schematic. So it needed two of them in series to get the proper voltage drop. The lack of voltage drop would have biased the transistor base too high, which would have made it trigger too often, thereby lowering the output voltage.

Here's a photo of the board, now that it works properly. D3 & D15 are now in series (going up above the board...ugly, but it works!)
 

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Ikaria73

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Bassplayer, can you confirm the part number for C2 for Rev U? The part on the original parts list shows a huge cap instead of a blue square component.

Does anyone have any spare boards they would be interested in selling? Or is anyone interested in repairing this board for a fee?
 

Bassplayer1985

Subscriber
Last Subscription Date
10/03/2019
Bassplayer, can you confirm the part number for C2 for Rev U? The part on the original parts list shows a huge cap instead of a blue square component.

Does anyone have any spare boards they would be interested in selling? Or is anyone interested in repairing this board for a fee?

C2 is a large cap for the SMD style board not the through hole. I dont recall the part at the moment. Do repair the boards for a fee through my LLC. I dont make a habit of soliciting business though here since there is a wealth of info on this thread for others to facilitate repair themselves. With the right desolder tools, electronics knowhow, and a lot of patience it can be done! :)
 
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Bassplayer1985

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Last Subscription Date
10/03/2019
PM'ed you Ikaria73

That was it! I put in another Zener diode (D15) in series with D3, so that my board matched your schematic. The output is now back at 120VAC, the output waveform is rock-solid stable with no glitches and the lights don't flicker anymore. I'm guessing the D3 that was originally in my board was spec'd differently than the Zeners that were spec'd in the parts list for your board schematic. So it needed two of them in series to get the proper voltage drop. The lack of voltage drop would have biased the transistor base too high, which would have made it trigger too often, thereby lowering the output voltage.

Here's a photo of the board, now that it works properly. D3 & D15 are now in series (going up above the board...ugly, but it works!)
***This is good information, and goes back to my original discussion that with the many revisions, the components may be slightly different.

My schematic and diagram is based on REV P, however the majority of the time I have come across REV U. I can count on my one hand how many times I have seen older revisions than REV P. Often the revisions change because certain components are available at the time of manufacture, and Generac redesigns the circuit slightly to accommodate. It leaves us DIY people in the dark of having to start over from scratch. Thankfully much of the research done on this thread (certainly not just me) is from the kind people on this forum helping others.

Best Regards,

Bass
 

Bassplayer1985

Subscriber
Last Subscription Date
10/03/2019
For those who have REV R:

Repairing one right now. The diodes are slightly different. for the VR circuit, the zener D3 is a 1N5242B, and the two other diodes are standard 1N4148. Low Oil and Idle control schematic is exactly the same as other revisions
 

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CEWall

Registered
The Rev R Board Bassplayer1985 was repairing just happened to be mine and I will say if you have a board that you need repaired, DO NOT hesitate to reach out to him in a Private Message for "Full Service" repair support.

It is a shame that these boards are no longer available and if you do find one they are crazy expensive. With someone like Baseplayer1985 out there willing to use / share his skills and talents, do not fear, he will literally get you back up and running faster than you could imagine.

My 7500EXL is 16 years old and with the repaired board, back in service running strong again.

I had been given the heads up upon the return of the board that I may have to slightly adjust the RPMs to get 60Hz and the Potentiometer to dial in the 120V and he was 100% spot on. I went from a VERY HIGH Voltage reading that adjusting the POT alone would not fix to everything running and adjusting as expected after the repair.
 

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mjf55

Registered
Bassplayer1985, I am starting to get my 4000EXL running again, and in preparation for the worst, I am requesting you send me your schematics for the SCB 83970 and if you have, the VRB 84132. I'll PM you my email.
I am not sure of the format, but would there be any need to have it in a CAD program (KiCAD) and maybe even generate a set of board files?
 

peterpiper50

Registered
Hello everyone,

I know this is an old topic but I was having trouble with my 7000EXL. Voltage above 145 volts. I reached out to Bassplayer1985 and he gave me some tips. Pulled out Q1 and C7 and they tested fine. Tested a few more things in circuit and things looked fine also. Turns out the female connector that plugs into the voltage regulator board had broken pins if you will. The pins are on the board but the female part in the plastic plug were broken not making contact. Like pulling it off and voltage goes high. Just wanted to let everyone know in case this happens to you. luckily I had some old equipment that had the same type plug.
 
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