ZD-981 Soldering Station 78L05 issue

[0b01010011]

Just a heads up to anyone who may have a ZD-981 soldering station, and possibly any of the other similarly designed soldering/desoldering combo stations (as seen here):

http://www.allthingsrc.com.au/image/cache/data/tools/soldering/soldering-station-60w-zd981-white-900x600watermark.jpg

Mine had an issue recently where it would just keep resetting every 5 minutes - not really a big deal in the scheme of things and considering the thermal inertia of the soldering iron, but it was pissing me off and I suspected it could get worse or fatal if I didn't do anything about it.

I decided to look at it today and immediately suspected the 78L05 as the less scrupulous manufacturers seem to abuse these.  Anyway I powered it on for a few minutes and the 78L05 was roasting hot.  As you would need to remove the through-hole LCD to desolder it, I just cut it off the IC part, leaving the three legs and made up a separate board with a heatsinked 7805 (from an old CRT colour TV) on it, then ran wires to the legs. 

I could not use the metallic base of the unit for heatsinking as the negative rail is isolated in this design and I didn't have any isolation pads/washers.

Works perfectly now, and may be worth doing if you have one of these as catastrophic failure would likely cause damage - the incoming unregulated DC to the 78L05 is 16v.  Of course this would be OK if the current was low, but it looks like the 78L05 was driving the display backlight as well as possibly some of the logic.

[Hagis2k]

That is nice to hear you could also replace the entire 7805 with a LM2596 switched voltage regulator sure you need to hook it up first to a dc source and set it for 5V before hooking it
up and i know some cheap chinese products are terribly designed.

[0b01010011]

Will certainly consider doing this if I ever have to replace it again, as I have some small buck regulator ebay DC-DC boards hanging about somewhere.

Cheap Chinese design is odd really - in some ways you have to wonder why in a cheap as piss soldering iron like this they don't just have a switching PSU or whatever just to cut down on weight if nothing else.  Yet for the iron element they have a triac feeding a bridge/capacitor linear PS arrangement then to the iron element.  Can't see why it would give a toss about being fed DC or AC really.

It's not like this because it needs the DC for control - there is a separate winding for this that drives a small half-wave rectifier/capacitor/78L05 (now replaced of course).

However it does seem to work and now the power supply is stable the temperature is a stable, to within a few degrees of set.  Well that's what the firmware says anyway and we know that will be reporting 100% truthful measured temperature 

[EJP]

I bought one of these a year or so ago and the design apepars to have been modified from what is described here. The 78L05 on the display/logic board is now a surface mount component and doesn't appear to run hot. The regulator on the 24V heater supply is now a TO-220 with a heatsink. Worth checking the soldering on this latter board, especially at the connector pins.

EJP