UA78H05 hybrid VR tech help

KPW66

Well-known member

Donor 2018, 2024
Joined
Jan 2, 2018
Messages
1,212
Reaction score
720
Location
Foster City, California
Need some technical help to educate me on this TO-3 hybrid 5v voltage regulator. I'm not completely sure how this part is meant to behave. Here is the situation.

I am replacing this part in my Super Don Quixote power supply but the entire circuit and heat sink is missing and all Universal schematics suck so I'm winging this a bit.

I fabricated this a bit by using a spare heat sink I had from a roached Williams PS but voltages (+5v) are still not right.

I purchased the VR off eBay, actually bought two, and they are NOS. In circuit both act the same, both output 11v when they are meant to be fixed 5v. The input is 12v

I already built my own breadboard 5v regulator circuit using a 7805 and it works fine so it's this UA78H05 and associated circuit that I need to sort out.

I did find that the original application calls for two tantalum caps and a ceramic resistor also to be in circuit (see image), but I have not added those yet so perhaps that's my issue.

Question 1: will installing these components bring the output voltage down to 5v?

Question 2: does the VR need a load to bring the output voltage down to the fixed 5v?

I want to ensure I have this circuit correct before connecting any boards and I also want to make sure that this VR is not bad. They do look legit.

Thanks in advance.
 

Attachments

  • IMG_5849.jpeg
    IMG_5849.jpeg
    994.6 KB · Views: 7
  • IMG_5740.jpeg
    IMG_5740.jpeg
    129.2 KB · Views: 7
  • IMG_5848.jpeg
    IMG_5848.jpeg
    559.9 KB · Views: 8
  • IMG_5720.jpeg
    IMG_5720.jpeg
    353.9 KB · Views: 7
You will need the caps for stability.
But that doesn't explain the output voltage being too high.

The 3.9 ohm resistor is likely pulling the output up if you included it in the circuit. You'll need at least 1.5 A load to drag it down to 5 V with the resistor in place.
 
You will need the caps for stability.
But that doesn't explain the output voltage being too high.

The 3.9 ohm resistor is likely pulling the output up if you included it in the circuit. You'll need at least 1.5 A load to drag it down to 5 V with the resistor in place.
Thanks. So do you suspect it's a bad part or it needs the load? I'm just surprised it isn't putting out 5v as-is, but not sure I understand it clearly.
 
The board needs more current than the 78H05 can supply, so the rest comes through the shunt resistor.
Without enough load the +5V will read high since the reg is designed to source current not sink current from the resistor.

In any case your life will be a lot easier with a switching power supply.
 
The board needs more current than the 78H05 can supply, so the rest comes through the shunt resistor.
Without enough load the +5V will read high since the reg is designed to source current not sink current from the resistor.

In any case your life will be a lot easier with a switching power supply.
Thanks, Mark, but considering the rarity of the game I'm striving for originality.

That said, I can certainly test it with a less important board to ensure the 5v really regulates before plugging in the SDQ board.

I'll install the caps and resistor and then test it.
 
The board needs more current than the 78H05 can supply, so the rest comes through the shunt resistor.
Without enough load the +5V will read high since the reg is designed to source current not sink current from the resistor.

In any case your life will be a lot easier with a switching power supply.

Thanks, Mark, but considering the rarity of the game I'm striving for originality.

What Mark said about the linear regulator is right. But about using a switching supply instead? That's just an opinion You could go in either direction. But kudos for keeping it as original as possible!
 
Thanks. So do you suspect it's a bad part or it needs the load? I'm just surprised it isn't putting out 5v as-is, but not sure I understand it clearly.

Sorry…replying in reverse order because…phone.

The regulator is probably working fine.
If you want to test it remove the 3.9 ohm resistor. It should regulate to 5V with little or no load. If in doubt, use a 1k resistor to give it a small load.

Edit:
Mark basically already said it.
 
Sorry…replying in reverse order because…phone.

The regulator is probably working fine.
If you want to test it remove the 3.9 ohm resistor. It should regulate to 5V with little or no load. If in doubt, use a 1k resistor to give it a small load.

Edit:
Mark basically already said it.
Right now the VR has neither the resistor nor the two caps installed and it's putting out 11v (no load), that's what made me concerned.

When I found the schematics for another Universal game which used a similar setup, that schematic showed the caps and resistor so I started thinking that was why my output voltage was high.

But now you're saying if I remove the resistor which isn't there the voltage may drop.

Here's a pic, right now it's just direct wired to the rest of the PS with the bridge rectifier and cap.
 

Attachments

  • IMG_5725.jpeg
    IMG_5725.jpeg
    524.5 KB · Views: 10
  • IMG_5722.jpeg
    IMG_5722.jpeg
    630.4 KB · Views: 10
Try adding a 1k resistor from 5v to ground for testing.

Other thoughts:

Those caps are critical. If the output is oscillating the "DC" voltage reported your meter will not be accurate. Try measuring AC volts. That should be very close to zero.
eBay components are always questionable.
 
Try adding a 1k resistor from 5v to ground for testing.

Other thoughts:

Those caps are critical. If the output is oscillating the "DC" voltage reported your meter will not be accurate. Try measuring AC volts. That should be very close to zero.
eBay components are always questionable.
I installed the two caps and resistor in circuit and then tested the +5v with and without a 1k resistor and it stays high (12v)

I also tested the AC with and without the 1k resistor and it's mostly 0.00 but also shows a spike of both 6v and 71v.

I'm leaning towards these eBay parts are just crap.
 
Back
Top Bottom