An old dinosaur bench top power supply - help needed

[Kg74]

Re: An old dinosaur bench top power supply - help needed

Well the parts arrived from China and are now installed into the modified unit.

Attached are the photos - First is from the Top View displaying the incredible complexity of the modification. It was rather straight forward and I followed the advice given on how to keep the analog needle meter. I did pull the LED off the board and ran a jumper wire to the front panel.

The second photo shows the front view. It was just a matter of using a drill, Dremmel tool and file to make the modifications to the panel.

I agree the unit was some sort of kit/home brew as the PCB board has professional markings. Regardless I have an operational unit now and the cost was under $15.00 with shipping, including a spare module.

Thanks to all.

[eccerr0r]

Re: An old dinosaur bench top power supply - help needed

I'm only thinking it was homemade because it seems poorly balanced, and I suspect the "indicator lamp" over the power switch looks like a hack job.

Rectifiers are 1N4003 series 1A rectifiers... okay.
2.5mF bulk capacitor... a bit oversized for 1A (funny looking, probably two series 25V 5mF capacitors heatshrinked into one?).
little tiny heatsink... will go nuclear if outputting 5V@1A

Lots of improvements possible...

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I have this homemade PSU that I picked up from the trash, it has a 26.8V 1A transformer, a bridge rectifier, an undersized but reasonable 500uF bulk filter capacitor, a LM317T, a meter, a DPDT switch to switch the meter between current and amperes mode, and misc "glue" components. This thing looks hideous and the LM317T is mounted on the only metal there is - the front panel. A chipset heatsink is attached via some pressure sensitive double sided tape, must be some huge thermal resistance between the LM317T and that heatsink...

whether I should mod or redo this PSU I don't know, but people probably could say the same to my "real" homemade PSU... (CC/CV dual meter, 1.2V-14V, 0-2.5A, using LM317T and pass transistor, and yes I've fried that pass transistor multiple times also due to insufficient heat sinking...really insufficient...)

[redwire]

Re: An old dinosaur bench top power supply - help needed

The original was LM309K 5V 1A competing with Fairchild uA7805K 5V 1A back in the day of 8080 home computers with TO-3. In the late 1970's the TO-220 package was good enough at those power levels and everything quickly went that way, much less hassle to assemble things. The LM323K was 5V 3A TO-3.

OP could just keep the original PCB and wire up a LM317T. The sad thing is the new chinese kit will have crappier quality parts.

It's a good basic linear PSU for experimenting and bench use. If the LM317 has the two protection diodes in the circuit, they last years. I would add a grounded power cord, ground the enclosure.

[stj]

Re: An old dinosaur bench top power supply - help needed

when i worked in the arcade industry 7805's running 5a in a TO3 can where everywhere.
heatsink like a shoebox!!!

by around 86 switching psu's became cheap enough to replace them!

[eccerr0r]

Re: An old dinosaur bench top power supply - help needed

I still wonder who made that "78P05", and whether or not it's a fake ...

Find datasheet? Never heard of "New Era Electronics" ...

Don't think there's a 13.8V version, you'd probably can add a drop on the GND terminal on one of these if it really can handle 10A.

[sam_sam_sam]

Re: An old dinosaur bench top power supply - help needed

https://www.cricklewoodelectronics.com/78P05.html

i actually have 3a and 5a 7805's in one of my old parts drawers.
dont ask how big the heatsinks were on them back in the mid 80's!!

Thanks for posting this information I did not know there was this version of this type of regulator

I have an application that I could use this type of regulator do you know if there is a 13.8 fix regulator in a 10 amp format

Thanks

[Kg74]

Re: An old dinosaur bench top power supply - help needed

The power supply is from a company that is no longer in business. It was founded in 1983 so the unit is no older than 38 years. Like in the title - an old dinosaur. As the picture shows the design is simple and primitive. I must have bought in 20 years ago at a garage sale - never worked. Based on the posts it appears it is of a poor design heat sink wise.

After looking up the price of a replacement 7805 and seeing how rickety the wires and pc board is I opted to go with the $9.00 eBay solution.

I will update when the package comes from China and I get the unit working.

Thanks all for the input - I appreciate the comments and learned a good deal.

[eccerr0r]

Re: An old dinosaur bench top power supply - help needed

Correct. This just means the in-out differential can be higher for the K versions but amps can't be that much higher - need beefier pass transistors to do this whether it be internal or external.

[sam_sam_sam]

Re: An old dinosaur bench top power supply - help needed

Nonstandard 7805...

Just like LM317T vs LM317K, the "K" version is no more current - they want you to buy LM350's and LM338's ...

Your are right but I think that they can dissipate the heat better than the regular lm317 or the lm7805 could at least that what I found

[eccerr0r]

Re: An old dinosaur bench top power supply - help needed

Nonstandard 7805...

Just like LM317T vs LM317K, the "K" version is no more current - they want you to buy LM350's and LM338's ...

[stj]

Re: An old dinosaur bench top power supply - help needed

https://www.cricklewoodelectronics.com/78P05.html

i actually have 3a and 5a 7805's in one of my old parts drawers.
dont ask how big the heatsinks were on them back in the mid 80's!!

[eccerr0r]

Re: An old dinosaur bench top power supply - help needed

7805's are no more than 2A or so whether it's TO-220 or TO-3, just determines how much heat it can dump. Swapping and rewiring for a LM317 is a good idea but keep in mind that you will still heat thermal limits fast with lower voltage outputs.

This looks like a homemade versus a mass produced PSU? Not sure.

oh boy that ebay ad pisses me off, do not say "step down" and especially not "buck" with linear regulators...aaah... (though marking a SMPS with "LM317" as search bait is equally annoying.)

[stj]

Re: An old dinosaur bench top power supply - help needed

that's actually a 3a or 5a regulator,
7805K

but heatsinking it to the case will be tricky because you need an insulator-kit AND a plastic cover to stop the case of the regulator touching anything on the table and linking to the chassis - instant 5v!

[Kg74]

Re: An old dinosaur bench top power supply - help needed

Excellent. Thanks again for your help RJ. I will circle back in a few weeks when the board comes. Appreciate your and redwire's answers and guidance.

[R_J]

Re: An old dinosaur bench top power supply - help needed

The analog meter is just connected across the output, so as long as it is still connected there it will still work. So there is no problem using both.

[Kg74]

Re: An old dinosaur bench top power supply - help needed

RJ - Thanks again. I will place the order. It will be weeks before it shows up.

Any idea how to get the analog meter to also work?

[R_J]

Re: An old dinosaur bench top power supply - help needed

Yes, that would work, I would remove the old analog meter and install the led readout in the hole.

[Kg74]

Re: An old dinosaur bench top power supply - help needed

RJ - I performed the test. VOM negative to heatsink and VOM positive to negative leg of the gray vertical 100uf 50v cap. As I turned the pot the resistance would increase and decrease.

I did the same for the really big 25000MFD cap and the resistance would vary, but not in a linear fashion.

[Kg74]

Re: An old dinosaur bench top power supply - help needed

Would I be able to use this eBay board? https://www.ebay.com/itm/LM317-Step-...4AAOSw-itXrY-G

Take out the existing board, mount the eBay board to align with the existing pot control, cut a hole for the LED Voltage display, remove onboard LED power light and run jumper wires to the panel and keep the analog scale and wire it somehow?

Turn the dinosaur into something modern, but keep the old school look and gain some function.

[R_J]

Re: An old dinosaur bench top power supply - help needed

It could be that the ground circuit is open, check the resistance between the large capacitor negative and the regulator common(can/heatsink) then vary the control and see what resistance you measure. The pot might be open?