M38 Voltage Regulator

Post by **wesk** » Mon Nov 15, 2021 1:42 pm

Well at least Ron & I appear to have clear minds! For future reference Don when a document superseded previous editions the safest way to not become confused is to discard the old superseded document and pay attention to the current document. This is a standard operating procedure in daily life issues.

RonD2 · Post by **RonD2** » Mon Nov 15, 2021 3:01 pm

Hi Wes,
The second link Don posted was for a copy of TM9-2920-247-34, Prestolite Generator GHA-4804JUT.

Maybe I missed it, but didn't see a copy in the electrical section on the downloads page. For future reference it might be useful?

Here it is again:
https://books.google.com/books?id=0gAYA ... ut&f=false

Post by **wesk** » Mon Nov 15, 2021 10:54 pm

You missed it in the web sites "Links" page. No sense in loading it on our server when we can just provide the link!

Naugha · Post by **Naugha** » Thu Nov 18, 2021 8:13 am

The 1978 Prestolite Service Info. document contains three “Notes/ Special Instructions” which are indicated for the VBS-4004UT before 12X.

Does that mean the Prestolite Service Info. (1978) “Special Notes” should not be used when servicing my VBS-4004UT/ 12X-6Y voltage regulator?

The guys at the auto-electric shop will need detailed info to do the rocket science stuff. If the PSI 1978 Special Notes don’t work for my VR then I will move on to my next Prestolite Document Easter Egg hunt.

....or find a M38 VR that already works, or find an expert that restores Prestolite VR’s or find a M38 VR that has known service instruction documents that can be used by a technician. I would like to use a ‘real’ M38 VR if possible.

Contacted a service tech at ‘Prestolite Electric’ on the off chance anything was left over from the old days (VBC-4004UT VR Manual). The guy was nice enough to answer back. “Good day, but I am sorry to say, that we do not have any manuals for the parts that you are looking for. Be safe”.

Would a VBC-4004UT VR Manual have once existed?
Who keeps such arcane documents besides folks on this site.

RonD2 · Post by **RonD2** » Thu Nov 18, 2021 1:02 pm

Don,
Just my 2 cent opinion, but I don't think any really worthwhile old-school auto-electric shop that would take on a 65 year old 24VDC military VR in the first place would have any difficulty interpreting or using the 1978 Prestolite Service Information.

And if I'm reading the Prestolite Service Information correctly, the spring and gap settings you're wondering about for your VBC-4004UT are included in the Notes (rather than listed at the top for the other models).

Finding and then affording a known good M38-era VR (two is better) would certainly solve your dilemma --- temporarily, at least. Have you called MWM, AJP, BMJP, QTM, and the like for possible availability?

I'll keep my eyeballs peeled....

Vintage military 24VDC stuff isn't for the faint of heart (or wallet), or is it?

Post by **Ryan_Miller** » Thu Nov 18, 2021 1:39 pm

Contact Brent Mullins. He bought a lot of these as NOS service parts and puts them together.

The NOS stuff came from the late Calvin Ishmael down in Louisiana that amassed an extremely large collection of NOS parts and was at one time a large surplus dealer.

He was a good friend and straight forward individual.

Naugha · Post by **Naugha** » Thu Nov 18, 2021 5:43 pm

RonD2
“Vintage military 24VDC stuff isn't for the faint of heart (or wallet), or is it?”
You got that right.

Ryan
“Contact Brent Mullins. He bought a lot of these as NOS service parts and puts them together. The NOS stuff came from the late Calvin Ishmael down in Louisiana that amassed an extremely large collection of NOS parts and was at one time a large surplus dealer. He was a good friend and straight forward individual.”

Will do. Thanks.
Is Brent a willysmjeeps.com member? The member’s list is a little difficult to search because of all the nicknames etc.

OOPs, sorry .... Brent Mullins Jeep parts .... got it.

RonD2 · Post by **RonD2** » Thu Nov 18, 2021 6:48 pm

Hi Don,
For your reading pleasure, below is a link to a NOS bottom half of a military 24V VR. I can't see the data plate for what model it is.

Pretty sure the bottom half of these VR's have most if not all of the pure electrical components (rectifier diodes, capacitor filters, etc) whereas the top half is mostly all the mechanical stuff (springs, points, and such). It seems to me that a good portion of VR failures might be in the low half, and good luck finding those piece parts. It also seems to me that the bottom half could be common to all these VR's, but maybe not (close enough that a hybrid would work?).

Anyhow, the vendor is Pete DeBella of Pete DeBella Jeep Parts in Riverhead, NY. One of our known good military jeep parts vendors (I should mention him more often).

He's got 3 available at $75 each. Something to think about. Give him a call.

https://www.ebay.com/itm/175001174477?h ... SwVmphfaBW

Naugha · Post by **Naugha** » Fri Nov 19, 2021 6:30 am

“... NOS bottom half of a military 24V VR”. Ron

Transplant surgery. Interesting. That’s how I kept 400 computers humming at my high school.

Create a ‘perfect computer’ with all the software issues solved and test it out using an idiot teacher. Refine the look and feel of the setup.

Then CLONE that sucker, keeping several backup units in storage... don’t tell the boss what you are doing or he will take away your backup units.

When something goes down, grab any important doc files or backups, have one of my student assistants wheel a cloned unit to the classroom and bring back the corpse for examination & repair by me or another assistant.

My only request to the assistants, “Please don’t make me get out of this soft chair, come down to Building Five and listen to Ms. Unhappy tell me how it wasn’t her fault.” I also had an assistant who was specially trained to handle phone requests from irrational teachers/admins.

The less time you spend talking to people who can’t help .... the more time you have to fix stuff.... and everyone stays happy without knowing what you are doing.... because if they find out, someone in authority will stop you from doing it.

When you work for educators, that gentlemen, is called ‘leadership’.

RonD2 · Post by **RonD2** » Fri Nov 19, 2021 6:39 am

Interesting story of your life experiences Don, but try to stay focused?

The data plate on those NOS bottom half's might just be VBC-4004UT. Probably not tested recently, but the price is very nice. Just trying to help. I can stop if you prefer.

P.S. If that's leadership, I'm glad I never worked for "educators". Leadership and life experiences in the Marines were somewhat different.

Post by **Ryan_Miller** » Fri Nov 19, 2021 7:26 am

That is Peter Debella in Riverhead NY. I don't know if it will be cheaper to call him directly.

RonD2 · Post by **RonD2** » Fri Nov 19, 2021 7:30 am

RonD2 wrote:Anyhow, the vendor is Pete DeBella of Pete DeBella Jeep Parts in Riverhead, NY. One of our known good military jeep parts vendors (I should mention him more often).

There must be an echo in here.

Naugha · Post by **Naugha** » Fri Nov 19, 2021 11:05 am

I have found the following Prestolite VR at a good price but no warranty. It looks new and the ad describes it as new. VBU-4002UT (Not VBC)

Is this model in the realm of M38 VR’s that are seen in motorpool resto’s?

Condition: New

Superseded Part Number:
MS13805, 2920-00-640-7518, 1118644, VBU-4002UT, SAW-6407518, FAK10505B, 8376514

Manufacturer Part Number: MS13805
Modified Item: No

Other Part Number: 2920-00-640-7518

Country/Region of Manufacture: United States

Brand: Prestolite

Custom Bundle: No

Fitment Type: Direct Replacement

Interchange Part Number:
MS13805, 2920-00-640-7518, 1118644, VBU-4002UT, SAW-6407518, FAK10505B, 8376514

Manufacturer Warranty: No Warranty

RonD2 · Post by **RonD2** » Fri Nov 19, 2021 11:27 am

Hey Don,
Yup, saw those on fleabay being sold by RFJP I believe. Tempting price at $80 isn't it? Never mentioned them because I don't think they get you anywhere.

With a little research, I can't determine what they were used on. Even the seller can't tell you. Maybe M274 MULE, the M422 MIGHTY MITE, or a small military generator?

Pretty sure they aren't M38 and never were, even during service life in the motorpool. Why? Because the data plate on it says 18 amps and the M38 generator is 25 amps.

Just my 2-cent opinion that is....

.....asking a voltage regulator rated for 18 amps to regulate a generator rated for 25 amps is asking for trouble (a.k.a. smoke and fire).

RonD2 · Post by **RonD2** » Fri Nov 19, 2021 11:53 am

In case anybody wants to see a VBU-4002UT voltage regulator:
https://www.ebay.com/itm/224627047760?_ ... BM3p6Uladf

And this listing provides an interesting clue. Says it's the solid state replacement for the one listed above, and used on a M250RVP Davey Compressor.
https://www.ebay.ca/itm/2920-00-640-751 ... 0180607460

Here's a copy of the military tech manual if you need it:
https://www.ebay.com/itm/363137551468?h ... SwhBtfcyrU