Heathkit 2718 Power Supply Upgrade - Page 1
Forum section: electronics / projects

author: xrunner
author_rank: Super Contributor
timestamp: May 01, 2015, 10:23:45 pm
content: I've had this since I was a kid, and it's time to make it a little better. I don't want to put too much into it but a few minor updates would be nice. A few years ago I had already converted the analog meter backlighting from bulbs to LEDs, and that meter is really nice looking.After a little research I've decided to upgrade four of the filter caps to larger values. The two variable supply caps go from 2200 uF to 4700 uF, and the negative sub supplies go from 10 uF to 1000 uF.The other issue was the analog meter. I wanted to go digital and found some nice small blue LED meters for cheap. The problem is mounting them. One thing I don't want to do is destroy the case or analog meter. I thought a neat idea would be to mount the LED meters behind the analog meter's whitish back, but unfortunately it was so thick (1/8") that the digits were just not legible. Too bad.So I will just look for a good place right on the front panel to stick them. Not totally ideal but it will still look pretty good. The LED meters can use a supply which is not the supply being measured, so that you can go all the way to zero. For that supply, I will tie them into the 5V power supply.

author: kripton2035
author_rank: Super Contributor
timestamp: May 02, 2015, 07:44:46 am
content: just over the supply A and B potentiometers ? there seems to be some room for the display there.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2015, 09:22:49 am
content: If you use the internal 5 V supply for the digital meters, the outputs will be somehow connected. So this is not a good idea. The right way would be getting the supply somewhere from the unregulated part of the output it is used for. Here it depends on the type of regulator used.The alternative would be a small additional Transformer with 2 separate secondary windings.

author: xrunner
author_rank: Super Contributor
timestamp: May 02, 2015, 07:04:45 pm
content: Quote from: kripton2035 on May 02, 2015, 07:44:46 amCould be, We'll see what happens.Quote from: Kleinstein on May 02, 2015, 09:22:49 amTrue it would require the negative outputs to be tied together, but that's an option anyway if you don't care about floating outputs. But I will examine some alternatives.

author: Kleinstein
author_rank: Super Contributor
timestamp: May 02, 2015, 07:25:23 pm
content: From the schematics I found, this is a simple linear regulator. For each channel a simple 7805 from the main filter cap would do the job, though it may get rather hot, depending on the current consumption. A little better would be going directly from behind the fuses with own diodes and a resistor before the cap - this reduces the voltage to the regulator and load on the transformer a little.Having the 2 channels separate can be handy getting a +- supply for audio amplifiers or operational amps.

author: tytower
author_rank: Contributor
timestamp: May 02, 2015, 10:13:54 pm
content: I did the same with my old ETA power supply and took power from the 12 V rail I notice there is a voltage over-read on my digital display of at least a volt compared to the meter reading

author: rdl
author_rank: Super Contributor
timestamp: May 02, 2015, 11:22:39 pm
content: I think you should stop right there. The lighted meter dial looks very nice.I'd like to do something similar to the CPS250 I recently got. I think the meter faces on it are metal though, so nice back lighting won't be possible.I've always preferred an analog meter for amps. I like to see the needle moving and in my opinion it's more informative at a glance as to how the load is acting than flickering digits. Digital is nice for voltage though, since it doesn't change much. Maybe you could add the volt meter in a small box that could sit on top or something.

author: xrunner
author_rank: Super Contributor
timestamp: May 02, 2015, 11:45:38 pm
content: Quote from: Kleinstein on May 02, 2015, 07:25:23 pmThanks for the tips.The little meters can actually use up to 30V for their supply power (not the measured power). I checked the voltage at the main filter caps and it was +36V, so I was unlucky as I can't tap onto that directly. But I could use a higher regulated voltage other than 5V. That's a possibility. I'll check what's available in the higher ranges in regulators.QuoteYep, I haven't messed with anything needing +/- supplies in a while, so I might temporarily gang the negative terminals until I get a better solution, because I want to see it work soon.Quote from: rdl on May 02, 2015, 11:22:39 pmWell, the good news is I'm keeping the analog meter, for the reasons you said. I couldn't destroy something like that.I'm still struggling with how to mount the LED meters. It won't be with screws so I cut off the little wings. I think were looking at double sided tape, but I don't want it on the back because the back has circuitry and also the cal set pot and I don't want sticky gunk all over that stuff. To be continued ...

author: matt6ft9
author_rank: Contributor
timestamp: May 05, 2015, 03:37:52 pm
content: Nice looking supply.Maybe attach a translucent blue enclosure to the side of the front panel and then put the digital meters in the enclosure?An enclosure like this:http://www.hammondmfg.com/dwg2XXTBU.htm